The Tectonics of North America 

A Discussion to Accompany the 

Tectonic Map of North America 

Scale 1:5,000,000

GEOLOGICAL SURVEY PROFESSIONAL PAPER 628

The Tectonics of North America 

A Discussion to Accompany the 

Tectonic Map of North America 

Scale 1:5,000,000

By PHILIP B. KING

GEOLOGICAL SURVEY PROFESSIONAL PAPER 628

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1969 

Second printing 1970

Third printing 1970 

Fourth printing 1978

Fifth printing 1981

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract..._______________________________________

Introduction __ ___ __ ____ ___________

Tectonic maps defined ___________________________

Historical sketch __ ______ _____________

Appraisal of existing tectonic maps ________________

Representation of platform areas_________________

Representation of foldbelts_____________________

"Tectonic Map of United States"_____ ^_ 

"Tectonic Map of Australia".________________

"Tectonic Map of U.S.S.R. and Adjacent Areas," 

and "Tectonic Map of Europe"_________

"Tectonic Map of Canada"___________

"Tectonic Map of Mexico" and "Tectonic Map 

of U.S.S.R."___________________________

Representation of subsea areas.______________

"The Tectonic Map of North America".-_____________

Compilation of the map_________________________

Specifications of the map____________________

Platform areas.______________________________

(A) Platform areas within the Precambrian. ___

(B) Platform deposits on Precambrian basement _________________________________

(C) Platform deposits on Paleozoic basement. _

(D) Platform deposits on Mesozoic basement. _

(E) Volcanic rocks and associated sediments of

North Atlantic province.________________

(F) Icecaps________________________________

Foldbelts of Precambrian age_________________

Stratigrapbic classification._________________

Tectonic classification. ______________________

Foldbelts of Canadian Shield_________________

Foldbelts of Greenland________________

Foldbelts south of Canadian Shield. __________

Precambrian of northern South America______

Phanerozoic foldbelts ___________________

The tectonic cycle_______________

Terminology _________________________

Geosyncline___________.._____________

Orogeny______________________

Names of orogenies..._____________

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"The Tectonic Map of North America" Continued Pw 

Phanerozoic foldbelts Continued

Sedimentary units______________ ________ 4C

Geosynclinal deposits.... __ 46

Eugeosynclinal deposits _ _ 47

Miogeosynclinal deposits. _____________ 47

Metamorphism of geosynclinal deposits..__ 4P

Deposits of successor basins....__________ 4P

Younger basinal deposits.__ ______ 50

Thick deposits in structurally negative

areas._________________ ____________ 50

Volcanic units_______________ _ 50

Plutonic units____________ ______ 51

Granitic rocks.______ _ __ 51

Mafic plutonic rocks_____ 5?

Ultramafic rocks______________________ 5?

Description of foldbelts____________________ 5f

(K) Innuitian foldbelt____________ 

(J) East Greenland foldbelt_________

(L) Appalachian foldbelt..______________ 56

Northern Appalachians.__________ 56

Southern Appalachians.____________ 5f

(M) Ouachita foldbelt____________ 61 

Structural systems related to Ouachita

foldbelt-..._____________. 64

(O) Cordilleran foldbelt____________ 64

Northern Cordillera___________ 65

Central Cordillera. __ __________ 6r

Southern Cordillera_______.___ 7?

(P) Pacific foldbelt______________ 74

Alaska. ____________________ 7ft

Washington and Oregon. 7ft

California and Baja California.__... 76

(Q) Antillean foldbelt_____________ 77

(N) Andean foldbelt______________ 7f

Subsea areas______________________ 80

Fault zones______________________ 81

Glossary.____________________________ 8f

References cited______________________ 87

Index of units on "Tectonic Map of North America"

which are referred to in text_______________ 9f

m
IV CONTENTS

ILLUSTRATIONS

Page

FIQTJRE 1. Structural map of Bighorn Mountains uplift, Wyoming and Montana, by N. H. Darton___________________ 2

2. Structural maps of the Jura Mountains, Switzerland, after Albert Heim____________________________ 3

3. Tectonic map of the Alps, by Rudolf Staub____--_-____- __ _______________________________________ 4

4. Tectonic sketch map showing the chief belts of folding in the American Cordilleran system, by Hans StillS__ 6

5. Maps of eastern Tennessee and western North Carolina showing three methods of representing the geology.__ 13

6. Tectonic map of north-central Nevada showing "structural stages" in an area of complex superposed rocks and

structures.-_____-_--__-_-_-_-------_---------------------_------- _____ __ ___ 14

7. Map of Gulf Coastal Plain in Texas, Louisiana, and Mississippi showing structure contours on two horizons in the

strata of the platform cover_________________-________-__-_-_____-_--__-___________ ____________ 26

8. Map of North Atlantic Ocean and adjacent lands showing plateau basalts of North Atlantic province. ______ 28

9. Map of the Canadian Shield showing provinces that contain Precambrian rocks of different ages.______________ 34

10. Map of North America showing inferred extent of exposed and buried provinces of Precambrian rocks__ 39

11. Map of northeastern Greenland showing inferred extent of the late Precambrian Carolinidian foldbelt._______ 55

12. Tectonic maps of the two larger areas of exposure of the Ouachita foldbelt_________________________________ 62

13. Map of northern Nevada showing extent of Antler orogenic belt of middle Paleozoic time___________________ 71

14. Map showing transverse structural features in the southern part of the area of the "Tectonic Map of North

America," and in adjacent regions_______________-_-_---_---_--__-_____________________________ 82

TABLES

Page 

TABLE 1. Comparison of various classifications of the Precambrian-__.__________________________ ___________ __ 31

2. Classification of orogenies in Canadian Shield_______-_____--_--_-_-___---_-__-__-_- __ ___ 33

3. Summary of Precambrian rocks and events south of Canadian Shield______________ , ______________._ 40
THE TECTONICS OF NORTH AMERICA A DISCUSSION TO ACCOMPANY THE TECTONIC

MAP OF NORTH AMERICA, SCALE 1:5,000,000

By PHILIP B. KING

ABSTRACT

The "Tectonic Map of North America," on a scale of 

1: 5,000,000, has been compiled by the United States Geological Survey in collaboration with other national geological surveys, and with the assistance of various individuals. The 

compilers made use of tectonic maps of some of the countries 

maps that have been published or are in process of publication. 

In addition, many other basic maps and reports were consulted.

North America is divided technically into foldbelts of different ages and platform areas where flat-lying or gently tilted 

rocks lie upon basements of earlier foldbelts. The two most 

extensive platform areas are those with Precambrian basement 

in the central craton, and those with Paleozoic basement in the 

Atlantic and Gulf Coastal Plains. Configuration of the upper 

surface of the basement beneath the platforms is shown by 

contours on a 500-meter interval.

The foldbelts include three of Precambrian age whose principal 

exposures are in the Canadian Shield, but which also emerge 

in various outlying areas. The foldbelts of younger ages lie 

nearer the edges of the continent, and include four that are 

mainly of Paleozoic age, two that are mainly of Mesozoic age, 

and two that are mainly of Cenozoic age. Each foldbelt was 

formed during a geotectonic cycle many geologic periods in 

length, beginning with a geosynclinal phase, passing through 

a time of orogeny, and ending with a postorogenic phase.

On the "Tectonic Map of North America" the foldbelts are 

distinguished by different colors according to age; where several 

significant times of deformation occurred within them, these 

are represented by tints of the prevailing colors. The different 

kinds of rocks which make up the foldbelts are shown by patterns of these colors.

In the foldbelts, the principal sedimentary rock units on 

the map are those which formed in the eugeosynclinal and 

miogeosynclinal areas. In some foldbelts, deposits are preserved 

which were laid down in successor basins during or shortly 

after the main orogenies. In the foldbelts of western North 

America various subdivisions are also shown in the Cenozoic 

sedimentary rocks and terrestrial volcanic rocks.

Among the plutonic rocks of the foldbelts, granitic rocks 

are most extensive; they form large to small masses mainly 

in the eugeosynclinal areas. More mafic and more alkalic varieties are separately indicated. Ultramafic rocks are important 

in places, especially near the Pacific Coast and in tine Caribbean 

region; most of these were emplaced in their present positions 

by tectonic rather than by magmatic processes.

INTRODUCTION

This discussion is a companion to the "Tectonic Map 

of North America," which is being published separately 

by the U.S. Geological Survey. It is intended to aid the

use and understanding of the map, by providing explanations more lengthy than could be included in th^ 

legend of the map itself. Such a discussion is the mor> 

desirable because the map embodies innovations derivedpartly from techniques of tectonic mapping that hav 

been developed during the last few decades by geologists 

in other countries, and that may not as yet be well 

known, or be well understood in North America.

TECTONIC MAPS DEFINED

A tectonic map portrays the architecture of the upper part of the earth's crust that is, the features prc - 

duced by deformation and other earth forces and represents this architecture by means of symbols, pattern1" 

and colors. Such a map differs from the more familir.r 

areal geologic map, whose primary aim is to represent 

the surface distribution of rocks of various kinds and 

ages. Nevertheless, most tectonic maps contain some indication of the ages and kinds of rocks from which th^ 

structures were made, and areal geologic maps contain 

some indication of the structures of the rocks represented, so that distinctions between the two kinds of 

maps are not absolute. Tectonic maps are nearly synonymous with structural maps, just as the subject of teutonics is nearly synonymous with that of structural 

geology. Nevertheless, geologists commonly make a 

vague distinction between structural geology and structural maps, which deal primarily with the description, 

representation, and analysis of structures, mostly on a 

restricted scale, and tectonics and tectonic maps, which 

synthesize these data over greater areas, inevitably with 

a larger amount of interpretation.

Many other kinds of maps are being made which

show earth features, some of which resemble and some

of which differ from tectonic maps as here defined:

1. Paleotectonic maps show geologic and tectonic

features as they existed at various times durir^r

the geologic past, rather than the sum of the tev

tonics as it exists today. Most of the paleotectonic

maps that nave been made portray in much detail

the sedimentary facies and the thickness of strata

in the cratonic areas, but show few of these details

in the more intensely deformed areas.
THE TECTONICS OF NORTH AMERICA

2. Neotectonic maps are a kind of paleotectonic map in 

that they represent the tectonic features produced 

during one part of geologic time, in this case the 

Quaternary or at most the Quaternary and latest 

Tertiary. Because of the epeirogenic nature of 

much of this latest deformation, neotectonic maps 

emphasize the broad upwarps and downwarps of 

the crust.

3. Paleogeographic maps show the probable extent of 

lands and seas as they existed at various times during the geologic past. They thus resemble paleotectonic maps, but involve a much greater element of 

interpretation.

4. Paleogeologic maps show the areal geology of a surface of unconformity that has been covered by a 

younger body of strata. This buried areal geology 

has tectonic significance, but it is generally not 

feasible to represent it on a tectonic map; tectonic 

maps and paleogeologic maps should supplement 

rather than duplicate each other.

5. Geophysical maps show the instrumentally determined values of gravity, magnetic intensity, or 

other physical properties of the earth, generally 

by means of contours. The contours express numerical values produced by the summation of many 

earth processes, not all of which are known. The 

data are not themselves tectonic, although many 

of them have ultimate tectonic causes. Interpretations of these geophysical data are frequently 

helpful in making tectonic maps.

HISTORICAL SKETCH

Geologists have been making structural and tectonic 

maps since the early days of the science. Even some of 

the early structural maps showed folds, faults, and 

structure contours in much detail, but most of them 

dealt with rather small areas on large scales. On the 

other hand, the earlier tectonic maps, covering larger 

areas such as whole countries, continents, or the world, 

were on small scales and were primarily intended to portray the tectonic or the historical-geological predilections of their authors. Only in recent decades have tectonic maps approached the scope and refinement of 

areal geologic maps.

In the United States, many excellent structural maps 

appeared during the first part of the century in the 

folios and professional papers of the U.S. Geological 

Survey; among these, the structural maps by N. H. 

Barton of various areas in the Western States are 

classic (fig. 1). Since 1916, many excellent structural 

maps of small to large areas have also been published

in the "Bulletin of the American Association of Petroleum Geologists." A parallel evolution of structural 

and tectonic maps occurred in Europe, where notable 

maps have portrayed, for example, the folds and faults 

of the Jura Mountains (fig. 2), and the superposed 

nappes and structural layers in the Alps (fig. 3).

FIGURE 1. Structural map of Bighorn Mounts ins uplift, 

Wyoming and Montana, by N. H. Darton (Darton and 

Salisbury, 1906, p. 13). Configuration of uplift shown by 

contours on base of Madison Limestone. Dashed lines show 

approximate configuration where all sedimentary rocks 

have been removed by erosion. Heavy lines are faults. 

Stippled, areas are covered by Tertiary deposits.
HISTORICAL SKETCH

FIGURE 2. Structural maps of the Jura Mountains, Switzerland, showing folds and faults: A. Part of a 

detailed map. B, Generalized map of the whole area. Copied from Albert Heim (1919, pi. 20 and fig. 103). 

Longitude is in degrees east of Paris.
THE TECTONICS OF NORTH AMERICA

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HISTORICAL SKETCH

One of the most ambitious of the early tectonic maps 

was the "Carte tectonique de 1'Eurasie" on a scale of 

1:8,000,000, that was compiled and hand colored by 

fimile Argand. He presented this at the 13th International Geological Congress in Brussels in 1922 as a companion of his epoch-making treatise on "Le tectonique 

de 1'Asie" (Argand, 1924). A colored reproduction of 

this map on a scale of 1:25,000,000 was published by 

the 13th Congress in 1928. The state of tectonic knowledge at the time is suggested, however, by the large 

blank areas which were left in various parts of the map.

Argand's map had a great seminal influence among 

geologists, as it created a desire for comparable maps 

of other areas or continents. Thus, in 1922 the Committee in Tectonics was organized in the Division of 

Geology and Geography of the U.S. National Kesearch 

Council, and the first report of its chairman, Rollin T. 

Chamberlin (1923, p. 3), stated: 1

One of the great needs in our field is a series of tectonic maps 

of the different continents. An important advance in this direction has recently been accomplished by Argand, whose magnificent structural map of Eurasia was one of the outstanding 

exhibits at the International Geological Congress at Brussels 

last summer. Our Committee voted to commence work on a tectonic map of North America which would bring out the trends 

of folding, the principal lines of faulting, the axes of doming, 

and related structural features. Messrs. Willis and Mansfield 

are to undertake this very important project

Later, the Committee on Tectonics realized that there 

were still insufficient tectonic data available to represent 

all of North America, and decided to restrict its objective to preparation of a tectonic map of the United 

States. The task of compiling this map began in 1934 

when Chester R. Longwell assumed chairmanship of 

the committee. Early aspirations of the committee in 

regard to the map are suggested in Longwell's 

prospectus (Longwell, 1934, p. 3):

It is suggested that the map represent the following features: 

trend-lines in folded belts, with axes of individual major folds 

so far as the scale of the map permits; direction and degree of 

important overturning of folds; cross folds or important 

changes in pitch of major fold axes; direction and degree of 

regional dips; all important faults, with appropriate symbols 

and figures showing, so far as known, direction and degree 

of dip, amount of throw and nature of displacement whether 

normal, reverse or strike-slip; major thrusts (with a special 

convention or color to designate overthrost masses); belts of 

en echelon faults; important areas of metamorphic rock, with 

strike and attitude of cleavage so far as it can be shown; 

areas of Precambrian rocks related to erogenic zones, as discussed recently by Bucher; all major igneous masses; swells and

1 For the subsequent actions of this committee, see the a«""n1 reports 

of the Division of Geology and Geography of the National Kesearch 

Council. The history of the committee has also been narrated by Longwell (1944a, p. 1767-1760), from whose account the succeeding paragraphs are largely abstracted.

basins in areas of unfolded rocks (possibly by structure contours) ; salt domes and anticlines; monoclinal folds.

Time relations should be indicated, so far as practicable, by 

conventional patterns or colors. If conventions are chosen judiciously, they may be superposed in areas that have experienced 

repeated diastrophism. It may even be feasible to indicate important vertical movements in folded belts, such as the lat? 

uplift of the Appalachian region. Considerable ingenuity will 

be required to represent all the complex disturbances in som° 

western areas, even where adequate information is available. 

It does not seem practicable to show the geologic ages of individual faults. However, faults that are recognized as 'active' 

can be distinguished from those supposedly 'dead'; and it may 

be desirable to indicate that faults in an important group an 

essentially contemporaneous.

Many of these aspirations were realized on the "Tectonic Map of the United States" as it finally evolved, 

but some had to be discarded as infeasible. Nevertheless, 

representation of most of the items listed has been attempted on various tectonic maps made subsequently. 

Final specifications for the "Tectonic Map of the United 

States'? developed as the work of compilation pro 

grossed, and were discussed and adopted by members 

of the committee during periodic meetings. The worJof compiling the map was divided among the committee 

members, each assuming responsibility for one part of 

the country; in the end, 14 different parts were thus 

compiled. Compilation of the map was largely completed by 1939, but there were inevitable discrepancies 

between the results of the various compilers so that the 

results required review and editing} this was done b^ 

Philip B. King under Longwell's direction. Drafting 

and printing of the map were delayed by the exigencies 

of World War II, and it was not published until 1944.

Completion of the "Tectonic Map of the United 

States" prompted the making of the comparable "Tectonic Map of Canada," which was compiled by a committee of the Geological Association of Canada under 

the chairmanship of Duncan R. Derry (Derry, 1950). 

On this map, many of the same specifications as thos^ 

for the United States map were followed, but they included some innovations required by the differing tectonic features of that country.

In Europe, in the meantime, Hans Stilte had been 

developing his philosophy of geotectonics and his 

classification of tectonic features in a lengthy series of 

publications, which were mostly illustrated by tectonic 

sketches (fig. 4). A desire was felt among many of 

Stille's European colleagues to represent his and other 

tectonic concepts in the form of more precise and elaborate tectonic maps, and in this effort leadership wa^ 

assumed by the Soviet geologists. A. D. Arkhangelsk:7 

(1941) had developed "an historico-morphologioal 

method, which enables the earth's crust to be tectonic * 

ally zoned according to the degree of completion of th«
6 THE TECTONICS OF NORTH AMERICA

ATLANTIC

Age of climax of alpinotype deformation

1. jjyuu Neocimmerian (Nevadan)

2. JHIMUL Subhercynian (Andean) 

9 ' '»' ' Laramide 

4. faoeg Late Tertiary 

.-.-.'. . . Continental shelves

FIGURE 4. Tectonic sketch map showing the chief belts of folding in the 

American Cordilleran system, by Hans Stille (1936, p. 138).

processes of folding of the geosynclinal regions undergoing transformation into a platform." - He prepared a 

"Tectonic scheme of Eurasia" on a scale of about 1:40,- 

000,000 that was published in the "Proceedings of the 

17th International Geological Congress" (Arkhangelsky, 1939, pi. 2, p. 304). It remained for N". S. Schatsky

2 Brief summaries of the Soviet work on tectonic maps are given by 

Spizaharsky and Borovikov (1966) and by Yanshin (1966a), from which 

most of this and succeeding remarks are taken.

to elaborate these methods and make possible the 

preparation of tectonic maps on larger scales,

The first map by Schatsky, published in 19£3, covered 

the Soviet Union and adjacent areas in color on a scale 

of 1:4,000,000, but in a very generalized manner. This 

map was preliminary to a more detailed compilation 

that was made in collaboration with N. A. IMiaevsky, 

A. A. Bogdaiioff, and M. V. Muratov, with the aid of 

41 contributors. The resulting map was published in
APPRAISAL OF EXISTING TECTONIC MAPS

1956, in color on a scale of 1:5,000,000, and represented 

not only all the Soviet Union, but also those parts of 

neighboring countries that lay within the area of the 

map base.

At the 20th International Geological Congress in 

Mexico in 1956, a Subcommission for the Tectonic Map 

of the World was organized within the Commission for 

the Geologic Map of the World. Preliminary to preparation of the world map, the subcommission encouraged 

the compilation of tectonic maps of the various continents. One of the first products was the "Tectonic Map 

of Europe" on a scale of 1:2,500,000 prepared by geologists of many European countries, which was issued 

in 1964 (Schatsky, 1962); its specifications closely followed those of the tectonic map of the Soviet "Union 

and adjacent areas of 1956, but with elaborations.

Ait the 21st International Geological Congress in Copenhagen in 1960, the U.S. Geological Survey agreed 

to assume leadership in preparing a tectonic map of 

North America, in collaboration with other national 

geological surveys in North America, and with the cooperation of various research institutions and interested 

individuals. To facilitate this collaboration a committee 

on the map was established, with George V. Cohee as 

chairman. Philip B. King was designated as chief compiler. Work on the tectonic map was begun by the compiler in June 1961, and was carried to completion in 

December 1966. The completed map was exhibited at 

the 23d International Geological Congress in Prague 

in 1968, and printed copies were available in 1969.

Tectonic maps of many other countries or continents 

have now been published or are in preparation, complete listing of which would be tedious here. Also, work 

by the subcommission is now far advanced on the longplanned "Tectonic Map of the World" (see Bogdanoff 

and others, 1966).

APPRAISAL OF EXISTING TECTONIC MAPS

The specifications of the tectonic maps previously 

mentioned, and others not listed, differ from each other 

in many particulars. These differences reflect evolving 

and diverse methods of tectonic mapping, as the methods have not been stabilized like those used for conventional areal geologic maps. To some extent these variations are useful; little future progress can be made in 

tectonics if the subject is to be ruled by a single set of 

dogmas, nor can progress be made in tectonic mapping 

if the maps are forced to adhere to a single set of specifications. Some of the variations reflect special conditions in the different regions, not only as to the kinds 

of tectonic features to be represented, but also as to the

amount and quality of the information available. Qthr^ 

variations indicate wide divergences in the objectives 

of the compilers. To evolve specifications for the "Tectonic Map of North America," all the varieties of exis*- 

ing tectonic maps were reviewed, in an effort to find an 3 

emulate the better features of each.

The basic components of all tectonic maps are two 

fold: Those of the first order indicate the nature of 

the rocks from which the structures are made by means 

of patterns and colors. Those of the second order repr**- 

sent the structures themselves (folds, faults, and tl <* 

like) by means of symbols. On all tectonic maps there 

is a further subdivision (either stated or implied) of 

the first-order components between those of the platform or cratonic areas, where the surface strata are 

gently tilted or warped, and those of the foldbeltwhere the rocks are much deformed and intruded. This 

subdivision appears on the early tectonic map t y 

Argand (1928), where Eurasia is subdivided into "pays 

tabulaires" and <4pays pliss&s" by means of color 

patterns.

The following examples indicate some of the ways 

in which first-order components are shown on tectonic 

maps:

1. Some color patterns are used, mainly in the foldbelts, and the remaining areas, mainly in the 

platforms, are uncolored. Example, "Tectonic 

Map of United States," 1944,1962.

2. Color patterns show stratigraphic units that have 

some rudimentary tectonic significance. Examples, "Tectonic Map of Canada," 1950; "Teutonic Map of Australia," 1960; "Geologictectonic Map of Northern Venezuela," 1962.

3. The platform areas are colored with layer tint"; 

the foldbelts are colored according to a few 

widespread epochs of climactic orogeny (for 

example, Caledonian, Variscan), the rocks of 

the foldbelts being subdivided in turn in+o 

tectonic units ("structural stages") that formed 

prior, during, and after the orogenies. Examples, "Tectonic Map of U.S.S.R. and Adjacent Areas," 1956; "Tectonic Map of Europe," 

1962; "Tectonic Map of Eurasia," 1966.

4. Colors in the foldbelts represent primarily the 

ages of folding of the respective units, the^ 

ages being more minutely subdivided than in 

(3). Example, "Tectonic Map of Canada," 19f>.

5. Colors in the foldbelts show rock sequences claprnfied according to "geotectonic cycles" the a^e 

spans of the "geotectonic cycles" differing frcTi 

one foldbelt to another. Examples, "Tectoric 

Map of Mexico," 1961; "Tectonic Map of 

U.S.S.R.," 1967.
8 THE TECTONICS OF NORTH AMERICA

Representation of second-order structural features, 

shown by symbols, offers fewer problems in tectonic 

mapping than representation of first-order components. 

Most of the features to be represented and the symbols 

used for them are reasonably standardized, so that they 

are much the same from one map to another. New or 

unusual symbols for special structural features, to suit 

the desires of the compiler are not objectionable or 

confusing, because these symbols can be identified in 

the legend.

REPRESENTATION OF PLATFORM AREAS

The objectives in showing platform areas are similar 

on all tectonic maps, however much they may differ in 

appearance namely, to express the gentle tilting and 

warping of the covering strata, the different structural 

layers of which they are composed, and the configuration of their basements.

On the two versions of the "Tectonic Map of the 

United States (Longwell, 1944b; Cohee, 1962), platform areas are largely left uncolored, except for inliers 

of Precambrian rocks and for narrow bands of color 

which indicate the edges of structural layers that are 

significantly unconformable on the layers beneath (for 

example, bases of Pennsylvanian, Cretaceous, lower 

Tertiary, and upper Tertiary). Among other purposes, 

these colored borders differentiate the platform of the 

Atlantic and Gulf Coastal Plains from the platform of 

the central craton. Configuration of the strata is indicated by brown structure contour lines at a uniform 

interval of 500 feet, an effort being made to select contoured horizons that can be extended as widely as possible ; on the 1944 version of the map 19 horizons were 

used, on the 1962 version 36. In selecting horizons for 

contouring on the first version of the map, the highest 

horizon that would give good results over a wide area 

was chosen; this was due to the state of knowledge at 

the time, when configuration of the deeper horizons was 

largely speculative. This consideration was abandoned 

in the second version, on which some of the areas were 

contoured on the top of basement rocks rather than on 

strata in the cover. On the two maps, portrayal of the 

platform areas is effective for detailed study, but the 

general absence of colors detracts from their usefulness 

as wall maps.

A different method of representing platform areas 

was introduced on the "Tectonic Map of the U.S.S.R. 

and Adjacent Areas" (Schatsky and others, 1956), and 

has been closely followed on many subsequent maps. 

Contours are drawn on the surface of the basement rocks 

at 500- or 1,000-m intervals, and the whole area filled in 

with layer tints that indicate the depth to basement. 

Basements of Precambrian and Paleozoic ages are

shown on the map referred to, and are distinguished by 

layer tints of different colors; basements of o*her ages, 

in other colors, are shown on subsequent maps The configuration of strata within the sedimentary cover is 

shown by colored contour lines that are superposed on 

the basement contours and layer tints. This method of 

representation produces a very effective wall ir ap, as the 

configuration of the base of the sedimentary cover is 

visible at a glance, leaving the configuration of the 

strata within the cover to be ascertained by more detailed study.

Besides the features discussed, many recert tectonic 

maps differentiate "foredeeps" or "marginal homoclines" at the edges of the platforms by means of special 

color patterns. Many maps show the "foredeeps" by 

means of stripes of the same color as the adjacent foldbelt, alternating with stripes in the same color as that 

of the underlying basement; this is intended to indicate 

the historical significance of the "foredeep." Nevertheless, "foredeeps" and "marginal homoclines" are actually parts of the platform, in which the strata and their 

basement have been deeply downwarped or steeply tilted 

against the adjoining foldbelts. The present reviewer 

questions the need for distinguishing these foatures by 

separate color patterns. The method might be helpful in 

regions of scanty information, but where much information is available their nature is evident from the contouring of the basement and the higher strata.

REPRESENTATION OF FOLDBELTS

"TECTONIC TMTAP OF UNITED STATE"*"

On the two versions of the "Tectonic Map of the 

United States" (Longwell, 1944b; Cohee, 1962) the miogeosynclinal and other areas of folding and faulting 

are illustrated mainly by structural symbols in black, 

without colored overprint, the strength of the deformation being suggested by the relative crowding of the 

symbols. Other tectonic features in the foHbelts are 

indicated by color patterns: (1) Precambrian rocks; 

(2) metamorphic rocks, mainly eugeosynclinal (of 

Paleozoic age in east, of Mesozoic age in west) ; (3) intrusive rocks, the ages being indicated by different 

colors; (4) sedimentary rocks in postorogenic depressions (of Triassic age in east, of late Cenozoic age in 

west) ; (5) terrestrial volcanic rocks in the. western foldbelts, mainly postorogenic and of Cenozoic age.

Ages of deformation in the foldbelts are not indicated 

on the "Tectonic Map of the United States." The gross 

ages of the foldbelts in the United States have long been 

familiar to geologists the Paleozoic deformation in the 

Appalachian region on the east, and the Mewzoic and 

Cenozoic deformation in the Cordilleran regain on the
APPRAISAL OF EXISTING TECTONIC MAPS 9

west and to attempt further refinements requires an 

untangling of many superposed deformations, or of extrapolation beyond available data (see also Longwell, 

19Ma, p. 1772). The gross 'ages of deformation are implied by the representation of metamorphic and intrusive rocks of Paleozoic age in the east, and of Mesozoic 

and Cenozoic ages in the west. This method of representation, although satisfactory for showing the tectonics of a single country, is less well adapted to 

representation of continents or other large regions, 

where the ages of deformation in the foldbelts are much 

more diverse.

"TECTONIC MAP OF AUSTRALIA"

On the "Tectonic Map of Australia" (Geological Society of Australia, 1960) the whole continent is shown 

in color patterns, which represent stratigraphic units. 

The classification of the units is as follows:

Stratified rocks. Sedimentary and volcanic rocks, tbe latter 

shown by black overprinted patterns, where present. 

Cenozoic: Undifferentiated; Tertiary. 

Mesozoic: Undifferentiated.

Paleozoic: Upper (Permian to Upper Carboniferous) ; Middle (Upper Carboniferous to Middle Devonian) ; Lower 

(Middle Devonian to Cambrian).

Proterozoic: Undifferentiated; Upper (subdivided into upper

and lower) ; Lower (subdivided into upper and lower).

Archean: Undifferentiated; gneiss; sediments and metasediments.

Intrusive igneous rocks. (Each rock type shown in same color 

throughout, the ages of the different bodies being indicated 

by letter symbols: acid (granite and porphyry); alkaline 

(mainly syenite) ; intermediate to basic (hypabyssal) ; 

ultra basic.

Each stratified unit is shown by the same color pattern 

over the entire continent; the units are not themselves 

tectonic, as their tectonic significance varies from one 

region to another. However, the tectonic significance of 

each is indicated in an accompanying table, of which 

the entries for the lower Paleozoic unit are a sample:

Queensland, folded Tasman geosynclinal zone in east, gently 

folded sediments in west; New South Wales, Tasman geosynclinal zone; Victoria, Tasman geosynclinal zone; Tasmania, geosynclinal deposits, moderately to strongly folded; South Australia, folded Adelaide geosyncline, with gently warped strata 

to northwest; Western Australia, marine and continental deposits, gently folded; Northern Territory, intracratonic basins, 

gently folded.

The method of representation of the first-order components used on the "Tectonic Map of Australia," while 

differing from that on the "Tectonic Map of the United 

States," is equally objective. It is well adapted to a 

continent where detailed tectonic information is only 

partly available; thus, the existence of many basins, 

uplifts, and other features in the poorly known northwestern part of the continent is suggested mainly by 

the distribution of the color patterns, rather than by 

structural symbols.

"TECTONIC MAP OF TT.S.S.R. AND ADJACENT AREAS," 

AND "TECTONIC MAP OF EUROPE"

Representation of first-order components in the fold1 - 

belts on the "Tectonic Map of U.S.S.R. and Adjacert 

Areas" (Schatsky and others, 1956) has served as a prototype for many subsequent tectonic maps, and notably 

for the "Tectonic Map of Europe" (Sohatsky, 1962); 

the features of these two maps will be considered hero.

On both maps, foldbelts are colored according to th^ 

age of the assumed climatic orogeny, although the rock^ 

within the foldbelts themselves may be much older o^ 

much younger. On the "Tectonic Map of the U.S.S.B., 

and Adjacent Areas" eight ages (or regions) of folding 

are distinguished, three in the Precambrian and fiv*. 

in the Phanerozoic: namely, Archean, Proterozoic, Baikalian (Riphean), Caledonian, Hercynian, Mesozoic of 

Pacific border, Alpine, and Cenozoic of Pacific border. 

On the map of Europe eight ages (or regions) of folding are distinguished, five in the Precambrian and thre^. 

in the Phanerozoic: namely, Archean, Svecofennian 

(Karelian), Gothian (Daslandian), Jotnian, Baikalian 

(Cadomian, Assyntian), Caledonian, Variscan (Hercynian), and Alpine; there are additional categories for 

Precambrian folding undivided and Paleozoic folding 

undivided.

Each foldbelt is subdivided in turn into many map 

units, of which the units on each map for the Variscan 

or Hercynian regions are given below as samples:

"TECTONIC MAP OF U.S.S.R. AND ADJACENT AREAS" 

Regions of Hercynian folding

19. Precambrian of cores of anticlinoria.

20. Lower structural stage undivided (China and Taimyr, Cairbrian; Tien-Shan, Riphean-Ordovician).

21. Lower structural stage, lower substage (Kazakhstan, Rip] - 

ean-Cambrian; Altai, Cambrian-Ordovician; Ural, Ripl - 

ean-Cambrian; east slope of Ural, Riphean-Ordovician).

22. Lower structural stage, upper substage (Altai, Ordovician; 

Kazakhstan, Cambrian3-Ordovician; Ural, Ordovician an-l 

locally Cambrian).

23. Middle structural stage, lower substage (Tien-Shan an* 

Kazakhstan, Silurian-Devoniani-a; Ural, Silurian-Devorian and locally Ordovician; China, Silurian-Devonian; 

Altai and Taimyr, Ordovicians-Silurian).

24. Middle structural stage, upper substage (Tien-Shan, Kazakl - 

stan and Altai, Devonians-Carboniferousi; China, Devoniani-CarboniferouSi; Taimyr, Devonian-Permian; Ura* 

Devonians; elsewhere Devonian-rCarboniferouSi).

25. Upper structural stage, interior basins (China, Carboniferousi-Permian; Tien-Shan ana Kazakhstan, Carboniferous" 

Permian; Ural, Carboniferousi-TriaseiCi).
10 THE TECTONICS OF NORTH AMERICA

26. Upper structural stage, regional depressions ( Ural foreland, 

OarboniferouSj-Triassici; Kuznets, Carboniferous-Triassic 

Taimyr foreland, Permian-Triassic; Donetz, Carboniferous-Permian).

Intrusive igneous rocks: Ultramafic intrusives, Caledonian 

granitoids, granitoids undivided, late Hercynian granitoids, 

and alkalic intrusives.

"TECTONIC MAP OF EUROPE" 

Region* of Variacan or Hercynian folding

KV. Reworked massifs of Karelian and older folding (Sudetes,

Armorica, Bohemia).

BV. Reworked massifs of Baikalian folding. 

CV. Reworked massifs of Caledonian folding. 

VB, etc. Ancient cores reworked by Variscan folding (various

ages indicated by second index letter). 

Migmatized older Paleozoic and Precambrian of Ural*.

Bugeosynclinal (interior) zones

eV. Undivided.

eVl. Lower structural stage (western Europe, Cambrian-Silurian ; Great Caucasus, Riphean-Paleozoic i; Ural, Paleozoic i-Devonian a ) .

eV2. Middle structural stage (western Europe and Or eat Caucasus, Devonian-Carboniferousi; Balkans and Anatolia, 

Silurian-Carboniferousi; Ural, Devonian»-Carbonif erOUSl).

Miogeosynclinal (exterior) zones

mV. Undivided (south Ireland and Trans-Caspian region, Devonian-Carboniferous).

mVl. Lower structural stage (western Europe, CambrianDevonian; Armorica, Cambrian-Devonian; Morocco, 

Cambrian-Carboniferous; Or eat Britain, Silurian-Devoniani; Little Caucasus, PaleozoiCi; Ural, OrdovicianDevoniaih; Moravia, Devonian*.*).

mV2. Middle structural stage (western Europe, Devonian; 

Armorica, Carboniferous i; Little Caucasus and Swientokrzysky Mountains, Devonian-Carboniferous i; Ural, 

Devonian ^-Carboniferous 2 ; southern France and Moravia, Devonian 2-Carboniferous i; Morocco, CarboniferOUSi).

mV2-3. Middle and upper structural stages, undivided (mostly 

Devonian-Carboniferous t ; Great Britain, Devonian «- 

Carboniferous.).

All zones

V3. Upper structural stage (western Europe, CarboniferousPermiani; Balkans, Anatolia, and Bwientokrzysky 

Mountains, Carboniferous ^Permian; Morocco, Qaxfooniferoust-Penniani; Caucasus, Paleozoic*; Mangyshlak, 

Permian j-Triassic).

Ya. Foredeepe.

KZ -Foredeeps covered by Mesozoic and Oenozoic sediments. Ve

Intrusive igneous rocks

Varisoan granitoids: (a) Synorogenic, (b) postorogenic.

The type of representation used on these two maps is 

claimed by its proponents to be "an historical-morphological method," in that it shows not only the morphological (structural) features, but also how these features 

and the f oldbelt as a whole developed through geologic

time.8 The climactic orogeny of the f oldbelt is selected 

as the one during which a previous geosynclinal region 

was consolidated into a platform a time wh<*Ti formation of alpinotype structures gave place to formation 

of germanotype structures. Each orogenic era (Caledonian, Variscan, Alpine, and so forth) is conceived of 

in broadest terms. Each orogenic era affected extensive 

areas along linear belts and had a wide time range; thus, 

the climactic orogeny in the "regions of Vari^an folding" might have occurred several periods earlier or later 

at one place in the f oldbelt than in another.

Evolution of a f oldbelt is expressed by a sequence of 

"structural stages" ("etages structuraux"), vhich succeed each other in an invariable order, which are explained as follows (Schatsky and Bogdamff, 1957, 

p. 16 1959, p. 8).

The long history of geosynclinal regions is generally characterized by inherited development. Their cross-sections therefore lack such sharply distinguished structural streres as the 

basement and blanket of cratons. However, close analysis of 

the structure and history of development of geosynclinal regions 

also uncovers a series of clearly defined structural phases; each 

of these phases, corresponding to a given stage of development 

of the geosynclinal region, consists of a group of formations 

that is often separated from those above and below by regional 

unconformities. Deep-seated (lower) structural phases are usually dominated by volcanically derived and sedimentary formations (of the spilite-keratophyre type and others), corresponding to early stages of geosynclinal development. The middle 

structural phases often contain carbonates, shales and graywacke formations, pierced by granitoid intrusions. Upper structural phases contain flysch, molasses, coal-bearing basins and 

other formations.

Thus, the distinction between successive "structural 

stages" is two-fold; each "stage" is, first of all, a distinctive body of rocks whose facies is closely related to the 

tectonic evolution of the foldbelt, and second, each 

body of rocks is separated from those above and below 

by regional unconformities.

Use of the term "structural stage" for subdivisions 

of the sequence in foldbelts creates an unfortunate confusion with the "stratigraphic stage," which is defined 

as a time-stratigrrapMc unit next m rank below a series 

(American Committee on Stratigraphic Nomenclature, 

1961, article 31, p. 658-659; see also Gignoux, 1955, p. 

12-15), hence a unit which is essentially of th°> same age 

and scope at all places (for example, Oxford*an, Cenomanian, Maestrichtian). Study of the excerpts from the 

legends which are quoted above makes it abundantly 

clear that "structural stages" are rock-8tratig*«vpJtic and 

not time-stratigrrapMc units; the range o* a single 

"stage" may be from Upper Carboniferous to Lower

 This and the next paragraph are paraphrased from Schatsky and 

Bogdanoff (1967) and from several pamphlets that have been prepared 

by them for the Snboommiwion for the Tectonic Map of the World. 

See especially Schataky and Bogdanoff (1960) and Bogd-noff (1962).
APPRAISAL OF EXISTING TECTONIC MAPS 11

Triassic in one place, from Permian to Triassic in another, or from Carboniferous to Permian elsewhere, yet 

the rocks of the "stage" are presumably of much the 

same sedimentary facies from one locality to another. 

The legend for the "Tectonic Map of the U.S.S.R. 

and Adjacent Areas" does not seem to provide for longitudinal subdivisions of foldbelts, even though such subdivisions might have had different histories. This is 

rectified on the "Tectonic Map of Europe," in which the 

foldbelts are divided into eugeosynclinal (internal) and 

miogeosynclinal (external) zones, each with its own 

sequence of "structural stages" (see excerpt from legend 

quoted above).

The concept of "structural stages" in the sequence of 

strata of foldbelts is valid, although it is questionable 

whether unconformities make significant boundaries between them. "Stages" can be recognized in most of the 

foldbelts of North America, an ideal example being 

in the Paleozoic rocks of the Ridge and Valley province 

of the southern Appalachians:

4. Coarse clastic deposits, continental or brackish water, 

with coal measures; broadly "molasse." (Mainly 

Pennsylvanian, with Lower Permian locally at 

top; base varies downward from base of Pennsylvanian to as low as Upper Devonian). 

3. Fine-grained marine elastics, including both typical 

"flysch" and broad sheets of deposit. (Largely 

middle Paleozoic; base varies from base of Middle 

Ordovician to base of Upper Ordovician, or even 

higher.)

2. Carbonate sequence (largely Cambrian and Lower 

Ordovician, but with top at variable levels, as indicated above).

1. Basal clastic sequence; quartzite, arkose, shale, basal

conglomerate on Precambrian rocks. (Earliest fossiliferous Cambrian, but extending downward into

unfossiliferous strata classed as Cambrian?).

Sequences of "stages" with very similar lithologic

characters but with widely variable time ranges, can be

recognized in parts of the other foldbelts of North

America. In some foldbelts are incomplete or repeated

sequences of "stages," and in still others are sequences

of "stages" of very different lithologic character (as in

the California Coast Ranges).

Nevertheless, the present compiler has doubts as to 

the value of "structural stages" in tectonic mapping. 

Does representation of the outcrops of the "stages" on 

a tectonic map give a true picture of the historical development of the foldbelt? Is it possible to represent 

adequately both the morphology of a foldbelt and its 

history of development on a single sheet of paper? The 

"structural stages" in foldbelts characteristically crop 

out where the rocks are much deformed, hence their outcrops may be very narrow. These deformed areas are,

further, the ones where the structural features represented by symbols are most crowded, thus adding to th^ 

complexity of the map pattern (fig. 5). The surfac^ 

area occupied by a "stage" in a deformed region may 

have little relation to its relative lithologic or historica I 

significance; moreover, outcrops of the "stages" may b^ 

interrupted by later superposed structures, or by overlapping postorogenic deposits (fig. 6). The true significance of any "structural stage" can only be represented 

on a paleotectonic map which will show its original extent beyond its present outcrops, both where the "stage" 

has been removed by erosion, and where it has been 

buried beneath younger strata.

A final comment can be made on the method of coloring the foldbelts on the two maps. The colors used express the climactic times of orogeny in the foldbelts, different units of the sequence being shown by tints of th^ 

prevailing colors, whatever the actual ages of the units. 

The maps thus vividly emphasize the foldbelts, producing an eloquent picture on the wall, but the technique 

makes detailed study of the foldbelts difficult without 

constant reference to the legend. Precambrian basement 

rocks are shown in many different colors, from one foldbelt to another, and postorogenic deposits of the internal and external basins are give the same color as th<* 

climactic orogeny, although they may be many period 0 

younger. In some foldbelts the outcrops of the actusJ 

deformed strata are rather small, as they are nearly 

buried by extensive postorogenic deposits. Many cf 

these postorogenic deposits are a platform cover, which 

is coextensive with the platform cover of surrounding 

regions. Drawing a boundary between foldbelt color and 

platform color in postorogenic deposits becomes highly 

subjective, unless there are abundant subsurface date.

"TECTONIC MAP OF CANADA"

The specifications of the two maps just discussed coirbine several disparate items the nature of the rock?, 

the evolution of the rock sequence, and the ages of deformation. Many later tectonic maps use similar specifications, but with significant divergences in the relative 

emphasis given to the different items.

The "Tectonic Map of Canada" (Stockwell, 1969) 

thus places primary emphasis on the ages of deformrtion. On this map, 15 orogenies are differentiated, five in. 

the Precambrian and 10 in the Phanerozoic, namely: 

Kenoran (late Archean),Hudsonian (late Aphebianc1" 

Early Proterozoic), Elsonian (late Paleohelikian c* 

early Middle Proterozoic), Grenvillian (late Neohelilrian or late Middle Proterozoic), East Kootenay (middle 

Hadrynian or Late Proterozoic), Taconic (early Late 

Ordovician), Acadian (Middle to Late Devonian), E^- 

lesmerian (Early Mississippian), Appalachian (Lat"> 

Mississippian to Early Pennsylvanian), Melvillean
12

85°

36°

35°

KENTUCKY 

TENNESSEE

THE TECTONICS OF NORTH AMERICA

84° v ___ __ 83° VIRGINIA

82°

EXPLANATION

Sedimentary contac*.

852 _ _ KENTUCKY _ ___ 84 ' ' '.'.'.TENNESSEE' ' "^ "

36°

35'

83° VIRGINIA EXPLANATION

Sedimentary contact

B
85°

JCENTUCKY

TENNESSEE

APPRAISAL OF EXISTING TECTONIC MAPS

84° ^_ _____. 83" VIRGINIA

13

82°

EXPLANATION

Axis of anticline 

Axis of syncline

Precambrian 

crystalline rocks

35°

100 KILOMETERS

G

FIGURE 5 (left and above). Three maps of eastern Tennessee and western North Carolina showing different methods 

of representing the geology in a region of strongly deformed rocks. In figures .4. and 7> the representation of timestratigraphic and rock-stratigraphic units much obscures the representation of structural features, even though 

their scale is about 3 times that of the ''Tectonic Map of North America." Compiled from geologic maps of 

Tennessee and North Carolina, .i, Map showing time-stratigraphic units (geologic systems). /?, Map showing 

rock-stratigraphic units ("structural stages"). C, Structural map, showing folds and faults.

(Late Pennsylvania!!), Tahltaniaii (Early to Middle 

Triassic), Inklinian (Late Triassic to Early Jurassic), 

Nassian (Middle Juracsic), Columbian (Late Jurassic 

to Early Cretaceous), and Laramide (Late Cretaceous 

to Eocene).

Each orogeny is correlated over the whole country; 

for example, rocks aft'ected by the Appalachian orogeny 

are mapped not only in the Appalachians but also in 

the Arctic Islands. The areas aft'ected by the different 

orogenies are shown by separate colors, the color of the 

dominant orogeny being used also for areas reworked 

from earlier orogenies and for unconformably overlying deposits. The rocks affected by each orogeny are 

subdivided in turn according to their lithology and origin, their degree of metamorphism, and the. deformation to which they have been subjected; some have 

undergone two or more prior orogenies.

Two examples of the subdivisions of the rocks of the 

foldbelts which are made on the ''Tectonic Map of Canada" are quoted below, one for the Precambrian, one for 

the Phanerozoic:

Hudsonian orogeny (H) ; folding and granitic intrusions during late Aphebian.

Hg, Granitic intrusions emplaced during the Hudsonian,

including highly granitized gneisses. Hgd, discordant

intrusions. 

Hb, Basic intrusions. Hb', mainly gabbro. Ha, mainly

anorthosite. 

Hn, Aphebian sedimentary and volcanic gneiss and scl ist.

Hug, mixed with granitic material. Hr, granulite and

charnockite. 

Hm, Aphebian miogeosynclinal deposits. He, Apliel ian

eugeosynclinal deposits. 

Heu, Aphebian and (or) Archean slightly metamorphosed

eugeosyuclinal deposits. Him, gneissic equivalents. 

Hu, Aphebian and (or) Archean sedimentary and volcanic

gneisses, commonly mixed with granitic material. Fun,

metamorphic rocks in largely unmapped areas, with

undivided granitic intrusions. 

HKg, Granitic intrusions, emplaced during the Kenoran

and re\yorked during the Hudsonian. HKg, intrusions

emplaced during the pre-Kenoran and reworked during

the Kenoran and Hudsonian.

HKe, Slightly metamorphosed Archean eugeosynclinal deposits, folded during the Kenoran and reworked during

the Hudsonian.
39°30'

EXPLANAT

Tertiary volcanics an

Upper Mesozoic volcanic

NEVADAN OR

Lower Mesozoic 

shelf deposits

Permian and 

Triassic volcanics

SONOMA 

OROGENY

Eugeosynclinal T

Lower 

NEVADA

\

39°30'

\

20 KILOMETERS

20 MILES

FIGURE 6. Tectonic map of north-central Nevada illustrating the difficulty of 

making a meaningful representation of "structural stages" on small-scale 

tectonic maps in an area of complex superposed rocks and structures. The 

Paleozoic and Mesozoic rocks consist of three sequences, each divisible ini:o 

"structural stages." During Paleozoic and Mesozoic time the sequences were

telescoped by low-angle thrusting; later, their rocks 

partly covered by postorogenic deposits, and disrup

that they are now exposed in small, disconnecte

county geologic maps of Nevada, and from variou

publications.
APPRAISAL OF EXISTING TECTONIC MAPS 15

HKn, Gneissic equivalents, mixed with granitic material.

HKr, Granulite and charnockite.

Acadian orogeny (A) ; folding and granitic intrusions mainly 

during the late Middle to early Late Devonian. A', folding 

during middle Early Devonian. A", folding possibly during 

Late Silurian.

Ag, Granitic intrusions.

An, Upper Ordovician to Middle Devonian miogeosynclinal 

deposits. Am', Cambrian to Middle Devonian. Ae. Silurian 

to Middle Devonian eugeosynclinal deposits. Ae', Ordovician to Middle Devonian eugeosynclinal deposits. An', 

derived gneiss and schist.

A'm, Hadrynian to Early Devonian miogeosynclinal 

deposits.

A"g, Granitic intrusions. A"b, basic intrusions.

A"e, Lower Ordovician to Middle Silurian eugeosynclinal 

deposits. A"n, derived gneiss and schist.

ATg, Granitic intrusions emplaced during the Taconic and 

modified during the Acadian. ATb, basic intrusions emplaced during the Taconic and modified during the 

Acadian.

ATin, Cambrian to Ordovician miogeosynclinal deposits 

folded during the Taconic and refolded during the Acadian. ATM, metamorphic equivalents. ATe, Upper 

Hadrynian to Middle Ordovician eugeosynclinal deposits, 

folded during the Taconic and refolded during the Acadian. ATE, metamorphic equivalents. ATn, derived gneiss 

and schist

AK"M, Hadrynian or older metamorphosed miogeosynclinal 

deposits, folded during the middle or late Hadrynian and 

modified during the Acadian. AK"E, similar metamorphosed eugeosynclinal deposits. AK"n, derived gneiss, 

schist and migmatite.

AK'g, Granitic intrusions emplaced during the early 

Hadrynian or before and modified during the Acadian.

AK'M, Metamorphosed early Hadrynian or older miogeosynclinal deposits folded during the early Hadrynian or 

earlier and refolded during the Acadian. AK'ng, derived 

gneiss and schist, mixed with granitic material.

AK'e, Earlier Hadrynian or older eugeosynclinal deposits 

folded during the early Hadrynian or earlier and refolded during the Acadian.

The orogenies listed above are differentiated partly 

by geologic evidence stratigraphically dated unconformities, intrusive relations, and the like for which 

the record is unusually complete in parts of Canada. To 

a considerable extent, however, they are based on the 

extensive radiometric dating program of the Geological 

Survey of Canada, and especially potassium-argon dating. Most of the rocks that have been dated are either 

intrusive or metamorphic.

The five Precambrian orogenies are widely spaced, 

their climaxes being at about 2,400, 1,700, 1,300, 900, 

and 700? m.y. (million years) ago, or 700, 400, 400 and 

200 ? m.y. apart. The five Paleozoic orogenies lie within 

a span of 200 m.y. (Middle Ordovician to Late Pennsylvanian), and the five Mesozoic orogenies within a span 

of 155 m.y. (Early Triassic to end of Cretaceous). The 

figures cited might suggest an acceleration of orogeny 

through time, but the large number of Phanerozoic

orogenies recognized in Canada would be merely phases 

of the three or four orogenic eras recognized in Europe 

and the U.S.S.R., whose climaxes occurred at different 

times in different places. Very likely the widely spaced 

Precambrian orogenies recognized in Canada are actually groups of orogenies like those in the Phanerozoic, 

for which the. individual records have become blurred.

The classification of the Precambrian rocks and orogenies in the Canadian Shield on the "Tectonic Map of 

Canada" is useful and expressive. It blocks out larr^s 

regions or provinces which have been subjected to orogenic events of widely varying ages, showing within 

each province the intrusive and supracrustal rocks that 

formed during, or only a little before, the climactic 

orogeny, as well as the relics of older rocks affected by 

earlier orogenies that were reworked by the climactic 

orogeny. Representation of the Precambrian rocks of 

the Canadian Shield will be discussed at greater length 

later (p. 33-36).

The classification of the Phanerozoic rocks and ore 7- 

enios is less appealing to the present compiler. T^<* 

large number of orogenies differentiated, and the larre 

number of rock units mapped in each category obscrre 

the broader relations. The differentiation gives the impression, perhaps unintended, of many distinct foJdbelts built against each other, whereas the orogenic 

units are actually not sharply defined in either time or 

place; this compiler considers these units to be subdivisions or phases of gross foldbelts that evolved during 

lengthy tectonic cycles. Moveover, unless unusually complete age data are available, such a classification involves a large measure of inference and speculation. A 

comparable classification would be difficult to make ebewhere in North America, where the available age data 

are less complete.

"TECTONIC MAP OP MEXICO" AND "TECTONIC MAP 

OF U.S.S.R."

A divergence in another direction from the specifications of the "Tectonic Map of the U.S.S.R. and A djacent Areas," and the "Tectonic Map of Europe" is 

illustrated by two maps the "Tectonic Map of Mexico" 

(de Cserna, 1961) and the "Tectonic Map of the 

U.S.S.R." (Spizaharsky, 1966). On these maps, emphasis is not on the climactic orogenies in the foldbelts, 

nor on details of the orogenic episodes, but on the broad 

tectonic cycles during which the foldbelts were built.

On the "Tectonic Map of Mexico" the country is interpreted as being composed of "structural belts," erch 

of which became consolidated during a "geotectonic 

cycle," the cycle being the interval of time during which 

an orthogeosynclinal belt became a craton.4 The fol4 The theoretical basis for the map is summarized from de Cserna 

(1960) and from the brief text printed on the map.
16 THE TECTONICS OF NORTH AMERIO

lowing structural units, or belts, are recognized in Mexico: basement complex (Precambrian); Jaliscoan structural belt (early Paleozoic?); Huastecan structural 

belt (late Paleozoic to early Mesozoic); Sonoran belt, 

poorly defined (early Mesozoic); Mexican structural 

belt (Mesozoic and Cenozoic); Gulf Coast geosyncline 

(largely Cenozoic); Trans-Mexican volcanic belt (Quaternary) ; and Quaternary clastic deposits. On the map, 

rocks of the Jaliscoan belt are shown in tints of purple, 

those of the Huastecan in tints of blue, and those of the 

Sonoran and Mexican in tints of green, the colors suggesting the relative ages.

Within the "geotectonic cycles" several phases are 

recognized, namely: (1) an orthogeosynclinal phase, 

with development of eugeosynclines and miogeosynclines; (2) an anatexitic phase, with emplacement of 

batholiths and regional metamorphism in the eugeosynclinal area, and deposition of a flysch wedge over the 

miogeosyncline; (3) an erogenic phase during which the 

miogeosynclinal rocks and the flysch wedge were folded 

and thrust toward the foreland; (4) a taphrogenic 

phase, during which the whole terrane was blockfaulted, molasse was deposited, and subsequent magmatic activity produced both intrusions and extrusions.

The Jaliscoan and Huastecan structural belts are exposed only in small areas, and their original extent and 

subdivisions are hypothetical. The Mexican structural 

belt forms most of the surface of the country, and the

rocks produced during its "geotectonic cycle" s.re clearly 

displayed; they are classified as follows on tl ^ map:

Mexican geotectonic cycle (or structural 6eZ*)

Magmatic recks 

Final volcanic rock*.

Subsequent volcani? rocks and

subsequent intrur've rocks. 

Anatexitic intrusive rocks.

Sedimentary and metamorphic

rocks

Clastic volcanic debris. 

Postorogenic debris or molasse,

continental and marine. 

Preorogenic clastic or flysch

wedge. 

Eugeosynclinal and miogeosynclinal deposits.

A somewhat similar rendering appears on the "Tectonic Map of the U.S.S.R." (Spizaharsky, 1966), but in 

more elaborate form, partly because of the more varied 

tectonic features of the country, partly for theoretical 

reasons. The foldbelts of Phanerozoic age in the Soviet 

Union are divided into 13 systems, each colored differently, of these, 2 are called "geosynclinal" and are presumably still in process of deformation, the remainder 

are called "folded" because the cycle of deformation 

has been completed. These foldbelts are: ge^synclinaZ 

systems, Pacific and Alpine; and folded systems, Uralian, Kazakhstanian, Altay-Sayan, Baikal, MongoloAmur, Tien-Shan, Zaysan, Sikhote-Alin, Verkhoyansk, 

Chukotka, and Taimyr.

The rocks of each system are subdivided ii turn into 

tectonic units, of which the following is a sample :

Uralian system (or foldbelt)

Tectonic regime

"Orogenic" structures.

Superposed structures and 

intrusives.

Geosynclinal and parageosynclinal structures (second 

group).

Geosynclinal and parageosynclinal structures 

(first group).

Basement structures.

Supracrustal rocks

Undivided structural stages.

Intrusive rocks

None present in this system.

« 

Fourth structural stage.

Third structural stage.

Second structural stage.

First structural stage.

Third structural stage.

Second structural stage.

First structural stage.

Undivided.

Granites.

Alkaline rocks. 

Granites and granodiorites. 

Gabbro and diorite. 

Ultramafics.

Granites and granodiorites. 

Gabbros. 

Ultramafics.

Alkaline rocks. 

Granites and granodiorites. 

Plagiogranites. 

Gabbro. 

Ultramafics.

Granites. 

Gabbros.

Alkaline rocks.

Granites. 

Gabbros.

Age

Late Triassic to Ea rly Jurassic

Permian to Early Triassic.

Carboniferous

Middle Devonian to lower 

Carboniferous 

(Tournaisian)

Ordovician to Early Devonian

Late Proterozoic to Early 

Cambrian.

Middle Proterozoic.

late Early Proteroroic to 

early Middle Prcterozoic.

Early Proterozoic to Archean.
APPRAISAL OF EXISTING TECTONIC MAPS 17

The theoretical basis for the representation on this 

map and its legend has been presented at length by 

Spizaharsky and Borovikov (1966). In brief, the earth's 

crust is considered to be heterogeneous, and to be divided 

into gross blocks along abyssal fractures; if so, each 

block would undergo its own independent history, and 

universal times of orogeny would be impossible. The 

tectonic cycle in each folded system began with a geosynclinal period; the supracrustal rocks that were 

formed during the period are divisible into a sequence of 

"structural stages" (rock-stratigraphic units like those 

on previous Soviet maps), and there is a parallel sequence of intrusive rocks (shown in an adjoining column in the legend). Folding was largely completed by 

the end of the geosynclinal period, but orogenic climaxes are not mentioned. In the folded systems the geosynclinal period was followed by an "orogenic" and 

"koilogenic" period, during which the crust was epeirogenically unwarped and downwarped. These authors 

use "orogenic" in a geomorphological rather than a 

tectonic sense that is, for the formation of mountainous 

topography by uplift, rather than by deformation. In 

the geosynclinal systems, the "orogenic" and "koilogenic" period has not yet been attained.

According to the map legend, the geosynclinal period 

(and presumably also the periods of folding) ended in 

one folded system during the Cambrian, but in others 

during the Permian, the Triassic, the Early Cretaceous, 

and the Late Cretaceous; in the geosynclinal systems 

the geosynclinal period (and period of folding) has 

not yet ended. Were it not for the theoretical predilections of the compilers, the folding in these systems could 

be assigned, as on the "Tectonic Map of the U.S.S.R. 

and Adjacent Areas" and the "Tectonic Map of Europe," to the broadly defined Baikalian, Variscan, Mesozoic, and Alpine orogenies.

On the tectonic maps of Mexico and the U.S.S.R. the 

supracrustal rocks are thus subdivided in a manner like 

that on some of the tectonic maps hitherto considered, 

into a sequence of "structural stages" (although these 

are not so designated on the map of Mexico). Placing 

the intrusive and (or) magmatic units in a column in 

the legend adjacent to the supracrustal sequence is a 

worthy innovation, as it illustrates the parallel evolution of the two classes of rocks during the tectonic cycles in the foldbelts. The resulting legend is fairly 

simple on the "Tectonic Map of Mexico." On the Tectonic Map of the U.S.S.R." the great variety of features 

to be represented produces a legend vastly larger and 

more complex than on any tectonic map hitherto considered. This increases the difficulty of using the map 

and legend, but it certainly presents a truer picture of 

the local peculiarities and histories of each foldbelt;

these are obscured by the generalizations made on othei" 

maps, where tectonic features from widely separated 

regions are grouped together and correlated.

REPRESENTATION OF SUBSEA AREAS

On most of the tectonic maps just reviewed only the 

sea-bottom configuration is shown, by topographic cortours and bathmetric tints. Even by themselves, subsea 

topographic contours are more expressive of tectonics 

than topographic contours on the land, as the structures 

of the sea bottom have been less modified by erosion?.! 

and depositional processes. Hence, fault scarps, fault 

blocks, upwarps, and downwarps are evident merely 

from their topographic configuration.

Although interpretation of features beneath the sea 

on tectonic maps cannot, as yet, be made with as much 

assurance as features on the land, recent oceanographic 

advances have encouraged proposals for more specify 

tectonic representation of subsea features. Such representation, especially by Soviet geologists and oceanog-- 

raphers, has appeared on some of the more recent teetonic maps. On the "Tectonic Map of Eurasia" 

(Yanshin, 1966b) the ocean bottoms are subdivided into 

first-order units shown by color patterns, as follows:

Structure?, of the Sea and Ocean Floors

Regions of pre-Cenozoic folding; continental platforms [=Cortinental shelves] :

1. Regions of epi-Mesozoic and older platforms. 

Cenozoic folded and geosynclinal regions :

i?. Folded and geosynclinal systems.

3. Regions of pre-Neogene folding.

4. Deep basins without granitic layer.

5. Deep ocean trenches.

6. Deep trenches of inland seas. 

Regions of oceanic platform [= abyssal plains and ridges] :

7. Arched oceanic elevations of the basaltic crust; swell'.

8. Marginal swells of oceanic platforms and Philippine 

basin.

9. Oceanic ridges of block structure.

10. Midoceanic ridges, and graben structures of Bay of 

Aden and Red Sea.

11. Old oceanic plates between zones of elevation (parts 

thickly covered by sediments shown by overprinted 

pattern).

12. Oceanic plates without granitic layer, originating iu

Paleozoic and Mesozoic. 

Volcanicity:

Submarine volcanic ridges and plateaus (shown by overprinted patterns) : a, basaltic; b, mixed composition 

mostly andesitic.

Besides the first-order units, second-order symbols ar^ 

used on the map to indicate morphological or tectonic 

boundaries, faults, folds, flat-topped seamounts, atollr 

and the like.

Many of the first-order units used on the map am 

merely morphological or descriptive, such as deep
18 THE TECTONICS OF NORTH AMERICA

oceanic trenches, marginal swells, and mid-ocean ridges; 

these would probably be nearly as evident from detailed 

topographic contours alone. Other units imply rock 

composition, such as granitic or basaltic crust and thick 

sedimentary cover, and have been verified by geophysical measurements at least in places. Still others seem 

more speculative, such as units involving specific times 

of folding, as these times are not verifiable by present 

oceanographic methods.

Even more radical proposals for representing subsea 

tectonics have been made by Soviet geologists. These 

involve classification of areas by rock composition and 

ages of folding, placing heavy reliance on particular 

theories of oceanic origin and evolution.

Today, oceanographic and marine geological investigations are revealing not only subsea topography to 

considerable depths, but results are as yet available only 

in a few places. Accurate portayal of sea-bottom tectonics in the manner of the more radical proposals 

mentioned above must await a much wider extension of 

these investigations.

"THE TECTONIC MAP OF NORTH AMERICA" 

COMPILATION OF MAP

Compilation of the "Tectonic Map of North America" 

was facilitated by the existence of tectonic maps of many 

of the areas or countries involved some printed and 

available at the time the compilation began, others in 

progress at that time and printed subsequently, and 

others still in manuscript at the time of this writing. 

The maps in manuscript or in progress during the time 

of compilation were made available to the compiler 

through the courtesy of their authors.

A major source of information was the "Tectonic 

Map of the United States, exclusive of Alaska and Hawaii" (Longwell, 1944b; Cohee, 1962), but configuration 

of the surface of the basement rocks in this region was 

taken from the "Basement Map of North America Between 24° and 60° N.," compiled by a committee of the 

American Association of Petroleum Geologists (Flawn, 

1967). The tectonics of the State of Alaska is shown on 

a manuscript map assembled in 1958 by George Gryc 

and his associates in the U.S. Geological Survey, but for 

the North American map the data shown there were 

greatly amplified by information obtained subsequently 

by these same geologists.

Canada is covered by a published tectonic map 

(Deny, 1950), but this is superseded by a new tectonic 

map compiled by the Geological Survey of Canada and 

a committee of Canadian geologists, under the direction 

of Clifford H. Stockwell (Stockwell, 1969). The new 

map, which was made available to the compiler in manuscript, is the chief basis for representing nearl^ half of 

the land area of North America. The part of this map 

covering the Canadian Shield was published first 

(Stockwell, 1965), the whole map in 1969. As will be 

seen later, the tectonic classification of the Pre^ambrian 

used on this map was the principal basis for classification of the Precambrian used elsewhere on the North 

America map. Additional data on western Car ada were 

obtained from a tectonic map of the western Cordillera, 

in British Columbia and neighboring areas by William 

H. White (1966b, fig. 10-1).

All of Greenland is shown on a manuscript tectonic 

map by Asger Berthelsen prepared for the Geological 

Survey of Greenland. Eastern and northern Greenland are also shown on maps by John Haller of the 

Danish East Greenland Expeditions. Many of the latter 

maps have been published in various formr and on 

various scales (Haller, 1961b; Haller and Kulp, 1962, 

fig. 3 and pi. 4; Haller, 1968, maps 1-3), but others are 

still in press. The easternmost part of the U.S.S.R., 

lying within the area of the North America nap base, 

is shown on a tectonic map by S. M. Tillmar and his 

associates of the Siberian Section of the Acr demy of 

Sciences of the U.S.S.K. (Tillman and others, 1966), 

which was generously made available by itr authors 

prior to publication.

Mexico is shown on a tectonic map compiled by Zoltan de Cserna (1961), which was the chief basis for 

representing that country on the North America map. 

In addition, as indicated below, the scheme of classification adopted by de Cserna in Mexico greatly aided 

the present compiler in working out a general tectonic 

classification for North America. South of Mexico, the 

next published tectonic maps are of Venezuela (Bucher, 

1950; Smith, 1962). The tectonics of the intervening 

regions of Central America and the Antilles was filled 

in from geologic maps and miscellaneous data, both 

published and unpublished. For Central America, major assistance was obtained from Gabriel Dengo of the 

Organization for Economic Integration of Central 

America. A tectonic map of Cuba did not become available until after completion of the North America map 

but representation of the country on this map is much 

the same as on the present map (Puscharovsky and 

others, 1966).

The subsea topography surrounding North America 

was assembled from contour charts of the U.S. Navy 

Hydrographic Office, the U.S. Coast and Geodetic Survey, the U.S. Bureau of Commercial Fisheries, and the 

U.S. Geological Survey, extensively supplemented by 

publications of the staffs of Scripps Institution of 

Oceanography, Lament Geological Observatory, and
SPECIFICATIONS OF MAP ir

Woods Hole Oceanographic Institution. These compilations were corrected by Henry W. Menard and 

Robert L. Fisher of Scripps Institution of Oceanography and by Bruce C. Heezen of Lament Geological 

Observatory. The subsea topography of the Arctic 

Ocean and its surroundings was obtained from charts 

by the Marine Sciences Branch, Canadian Department 

of Mines, Energy, and Resources, which were furnished 

in manuscript through the courtesy of M. M. de Leeuw.

Preparation of the North America map was materially aided by the opportunity afforded to the compiler to view tectonic features in the field during many 

excursions, including excursions to such remote areas 

as Alaska, Newfoundland, southern Mexico, and Guatemala. The compiler also had several mutually helpful 

conferences with Clifford H. Stockwell and others at 

the offices of the Geological Survey of Canada, and with 

Zoltan de Cserna and others at the offices of the Instituto de Geologfa de Mexico. He* further benefited 

from attendance at conferences abroad which dealt with 

tectonic problems. Further, the compiler acknowledges 

with pleasure his rewarding association with Professor 

A. A. Bogdanoff of Moscow University, U.S.S.R., a 

world leader in the subject of tectonic mapping.

Finally, the "Tectonic Map of North America" could 

not have become a reality without the great and good 

help of Douglas M. Kinney, geologic map editor of the 

U.S. Geological Survey, first for his sage counsel and 

advice during the work of compilation, and second for 

his assistance in obtaining a printed product that faithfully reproduced the original specifications.

SPECIFICATIONS OF THE MAP

When planning the specifications of the "Tectonic 

Map of North America," the compiler considered the 

legends and the philosophical bases of the maps previously discussed, and adapted their best features to the 

new map. Rendering of second-order features on the 

North America map closely follows that on the "Tectonic Map of the United States" (Longwell, 1944b; 

Cohee, 1962), but many innovations are made in the 

rendering of the first-order features. All the land areas 

are subdivided into tectonic units, shown by color patterns, instead of being only partly colored. A practical 

reason for this change is that, whereas structural and 

tectonic data are complete enough within the conterminous United States for many features to be effectively 

represented by symbols alone, data are less complete 

elsewhere and many tectonic features in poorly known 

areas can only be blocked out by color patterns.

Clearly, the units to be shown by color patterns on the 

"Tectonic Map of North America" must differ from 

those on conventional areal geologic maps; they must be

tectonic units, rather than stratigraphic units. But what 

are tectonic units? The preceding review of existing 

tectonic maps indicates a wide diversity of opinion 

among makers of tectonic maps as to what a tectonic 

unit should be a diversity which reflects botl 

philosophical and practical considerations.

In the judgment of the compiler, the ages of deformation of the rocks should not be the major criterion for 

distinguishing tectonic units. Many contrasting kinds of 

deformation can occur nearly simultaneously, in different rocks, and in different environments; these rockr 

and their environments are as significant as the timer 

when they were deformed. Moreover, although specific 

ages of deformation can be determined in some local 

areas, regional ages of deformation are seldom clearcut. Regional ages of deformation are seemingly clearest 

in the Precambrian rocks, which are readily divisible, 

into gross provinces or foldbelts, but this is largely because of the great length of Precambrian time and tho 

incompleteness of the record in these ancient rocks 

Within the Phanerozoic rocks, many deformational 

events can be recognized, which not only can be more 

specifically dated than those in the Precambrian but 

which are more closely spaced in time. Differentiation of 

all such events on a tectonic map is of dubious significance, at least on a map of continental scope; the extent 

of many of the individual events is very local, or can be 

specifically proved only in local areas. Much more significant is the gross time (that is, the tectonic cycle) 

during which the structures in each foldbelt were 

created.

The units shown by color patterns on the "Tectonic 

Map of North America" should, then, combine several 

significant tectonic items. Within the deformed regions 

the gross units should be the f oldbelts, which evolved by 

continuing deformation during lengthy periods of geologic time and each of which had its own distinctive 

history and time span; these foldbelts should be distinguished by different colors. Subdivisions of each foldbelt should represent the kinds of rocks involved, which 

are, to a greater or lesser degree, products of this evolution such as eugeosynclinal, miogeosynclinal, and other 

sedimentary rocks, and various metamorphic, plutonic, 

and volcanic rocks. These kinds of rocks can be differentiated by patterns of the prevailing colors. Finally, 

deformations of specific ages should be indicated only 

where they imply significant pulses in the evolution of 

the foldbelt as a whole; they can be shown by various 

tints of the prevailing color. These concepts correspond 

closely to the concepts that were the basis for tho 

"Tectonic Map of Mexico" (de Cserna, 1961), and the 

present compiler acknowledges his indebtedness to thoso
20 THE TECTONICS OF NORTH AMERICA

concepts in evolving specifications for the "Tectonic 

Map of North America."

These philosophical considerations are all very well, 

but they require modification in order to be represented 

on a map of North America on a 1:5,000,000 scale. Many 

tectonic items of theoretical significance are difficult or 

impossible to map at all, and others would require representation on a scale of 1:2,500,000, or even larger. The 

"structural stages" which adorn many recent tectonic 

maps are a case in point. The "stage" concept is valid in 

expressing the evolution of the f oldbelts through time, 

and "stages" can be recognized in the rock sequences of 

the f oldbelts of North America; to map them is another 

matter. Sequences of "stages" are generally best marked 

in strongly deformed areas, thus, both crowded outcrop 

patterns and crowded structural symbols would coincide 

on the map (figs. 5 and 6). The present compiler believes 

that the structures in such areas are more significant 

than the rock sequence, hence that representation of the 

latter must be sacrificed. Nevertheless, as will become 

apparent later, many of the subdivisions of the f oldbelts 

used on the "Tectonic Map of North America" are gross 

"structural stages" in the sense of existing maps, but 

these are broad enough in scope to be represented clearly 

on the 1:5,000,000 scale.

A final disclaimer is in order: Tectonic classification 

will always be much more subjective than stratigraphic 

classification and will always be greatly influenced by 

the judgment and predilections of the map compiler. 

No two compilers will make identical tectonic classifications of the rocks of the same area for example, as to 

what rocks are eugeosynclinal or miogeosynclinal, or as 

to what rocks are synorogenic or postorogenic. For the 

"Tectonic Map or North America," the present compiler 

has appraised the source data and the opinions of geologists that relate to each region, but judgment as to the 

final representation on the map was made by him alone. 

What he has shown on the map will differ more or less 

from what would be shown by another compiler, and 

may be erroneous in places; every feature shown on 

the map is controversial in some degree. Nevertheless, 

in making this map a single compiler has viewed North 

America in all its parts, and has compared the tectonic 

features of each part with each other part, thus hopefully arriving at a balanced judgment of the relative 

significance of each.

On the tectonic map, North America and its surroundings are divided into the following tectonically significant major units:

Platform areas:

(A) Little deformed Precambrian deposits overlying more 

deformed earlier Precambrian rocks (Canadian 

Shield and elsewhere).

(B) Platform deposits on Precambrian basement (cratonic 

area of central United States, western Crnada, and 

the Arctic Islands).

(C) Platform deposits on Paleozoic basement (Atlantic and 

Gulf Coastal Plains).

(D) Platform deposits on Mesozoic basement (ArcticCoastal Plain of Alaska and northern Onada). 

(E) Volcanic rocks and associated sediments of North Atlantic province, on Precambrian and younger basement (Iceland, Greenland, and part of Baffin 

Island).

(F) Icecaps of Quaternary age, on Precambrian and younger basement (Greenland and parts of Arctic 

Islands). 

Foldbalts of Precambrian age:

(G) Kenoran foldbelt (Canadian Shield and elsewhere). 

(H) Hudsoniaa foldbelt (Canadian Shield and elsewhere). 

(I) Greenville foldbelt (Canadian Shield and elsowhere). 

Foldbelts mainly of Paleozoic age :

(J) East Greenland foldbelt (northern east Greenland). 

(K) Innuitian foldbelt (Arctic Islands and no*th Greenland).

(L) Appalachian foldbelt (southeastern North America). 

(M) Ouachita foldbelt (southern North Americr). 

Foldbelts mainly of Mesozoic age :

(N) Andean foldbelt (South America, in southeastern corner of map).

(O) Cordilleran foldbelt (western North America). 

Foldbelts mainly of Cenozoic age:

(P) Pacific foldbelt (west coast of North Amer'ca).

(Q) Antillean foldbelt (Antilles, southern Central America,

and Caribbean coast of South America). 

Subsea areas.

On the map and its legend, these tectonic units and 

their subdivisions are indicated by colors that follow 

a prismatic scale, according to the ages of tl Q, geotectonic cycles during which they were formed brown 

and red for Precambrian, purple for older Paleozoic, 

blue for younger Paleozoic, green for Mesozoic, and yellow and orange for Cenozoic. The units and their subdivisions are also indicated by symbols. Prefires of the 

symbols are the capital letters in the preceding list. 

These letters are followed by numerals or letters which 

designate the subdivisions thereof numerals for the 

sedimentary and metasedimentary subdivisions, Greek 

letters for the plutonic and volcanic subdivisions.5

However, some of these subdivisions occur in more 

than one of the foldbelts. For example, "thiclr deposits 

in structurally negative areas" are characteristic not 

only of the Cordilleran foldbelt, but also of tt^ adjoining Pacific and Antillean foldbelts. Moreover, rocks of 

various Precambrian foldbelts come to the surface in

8 Nine Greek letters are used in the legend, which are placed in conventional order (alpha, beta, gamma, delta, and so forth), reading from 

bottom to top of each column; however, certain letters in the sequence 

are omitted that would not reproduce clearly on the map (s"ch as zeta, 

eta, kappa, and nu). The letters are used indiscriminately IT whatever 

rock type appears in the sequence, so that granite, for example, is 

designated in one place as alpha, in others as beta, delta, and theta. 

This usage differs from that on some European maps where all granites 

are designated as gamma, all basalts as beta, and so forth, regardless of 

their place in the sequence.
PLATFORM AREAS 21

the foldbelts of later ages. On many of the maps previously reviewed, such subdivisions are colored, patterned, and symbolized in an entirely different manner 

from one foldbelt to another, and it is difficult to observe 

their obvious kinships. In order to express such kinship 

on the "Tectonic Map of North America," the same 

colors and patterns are used for the subdivisions in all 

places but they are repeated in the legends of each foldbelt, only their symbols differing from one foldbelt to 

another.

PLATFORM AREAS

Platform areas are those parts of the continents in 

which flat-lying or gently tilted deposits, mainly sedimentary, are underlain at varying depths by a basement 

of rocks that had been consolidated, not only by earlier 

deformation, but in part by metamorphism and plutonism. This definition, while seemingly clear, is not everywhere easy to apply. In North America there is much 

debate among geologists and geophysicists as to which 

rocks should be called basement and which should not; 

in compiling the "Tectonic Map of North America" 

some arbitrary decisions have been made in places. The 

most extensive platform areas in North America are 

those with a Precamorian basement in the central craton 

and those with a Paleozoic basement in the Atlantic 

and Gulf Coastal Plains. The other platform areas are 

less extensive or otherwise less typical, but they can 

more appropriately be placed in this category than in 

any other.

The tectonic features of the platform areas are most 

effectively portrayed by means of contours on the upper surfaces of their underlying basements. These surfaces were produced by erosional truncation before the 

platform deposits were laid over them, but erosion ordinarily had advanced far enough so that any residual 

topographic features are scarcely apparent in the contours on a small-scale map. Most of the configuration 

of the surfaces is therefore the sum of all the deformations that were imposed on them after they were truncated. Contours on the surfaces of the underlying 

basements are available for all of the central cratonic 

area, for a large part of the Atlantic and Gulf Coastal 

Plains, and for the Greenland icecap; these are represented on the "Tectonic Map of North America." Foi 

the remaining platform areas configuration of the underlying basement is either unknown or is known only 

in small areas. These platform areas are relatively inconsequential and are not contoured.

(A) PLATFOBM ABEAS WITHIN THE PBECAMBBIAN

In parts of the Canadian Shield are areas of flatlying or gently tilted sedimentary or volcanic rocks oi

Precambrian age, which lie on truncated surfaces of 

rocks-that were strongly deformed during earlier Precambrian orogenies. These orogenies are of diverse agee. 

and the ages of the deposits that overlie the rocks deformed by them are probably equally diverse; on the 

map, these deposits are classed as of Early, Middle, and 

Late Proterozoic ages (Al, A2, and A3).e Most of these 

flat-lying or gently tilted rocks are now preserved in 

relatively small areas, but such rocks are very likely the 

last remnants of what were originally much more extensive platform deposits.

One well-known Precambrian platform deposit is the 

Keweenawan Series of the Lake Superior region a 

very thick sequence of mafic lavas interbedded with and 

succeeded by red continental sandstones (A2). The series has been downwarped into a broad syncline, faulted, 

and invaded by thick sheets of gabbro (Aa) but it r 

unmetamorphosed and is tilted rather than folded, so 

that its structure contrasts with the underlying Precambrian which has undergone thorough erogenic deformation and moderate to strong regional metamorphism (James, 1955). Radiometric dating of the gabbro 

sheets indicates an age of about 1,100 m.y. (Goldich and 

others, 1961, p. 96), so that the part of the Keweenawar 

Series intruded by them is Middle Proterozoic; thehigher parts of the series might extend into the Upper 

Proterozoic (Stockwell, 1964, p. 12). Other Precambrian continental and volcanic rocks which occur ir. 

more remote parts of the shield, such as the Athabaska, 

Dubawnt, and Coppermine River Groups, are even lesr 

disturbed than the Keweenawan Series, and like it f onr 

a platform cover over more disturbed earlier Precambrian rocks. Toward the northwest, on the shores of the 

mainland and in the southern parts of the Arctic Islands, 

the Precambrian platform deposits include marine carbonates and a few evaporite units (Blackadar and 

Fraser, 1961, p. 363-368). Many of these groups, liko 

the Keweenawan Series, have been radiometrically 

dated as Middle Proterozoic.

Besides the extensive Middle Proterozoic platform 

deposits there are smaller areas of Precambrian platform deposits of earlier and later ages. No platform deposits of Archean age remain, if such ever existed. However, Lower Proterozoic supracrustal rocks extend in a 

few places as platform deposits (Al) over the deformed1 

Archean, as in the region between Lake Huron anc^ 

Lake Temiskaming. More commonly, they are eroded 

back to the edges of their foldbelts, where they form 

tilted sequences that lie against the deformed Archean 

(the "marginal homoclines" of Stockwell, 1965; here included in unit H5). The deformed rocks of the Middle

 Classification of the Precambrian used on the 

North America" is discussed on pages 30-32.

"Tectonic Map o*
22 THE TECTONICS OF NORTH AMERICA

Proterozoic Grenville foldbelt are overlain by a few 

small patches of flat-lying Upper Proterozoic deposits 

(A3), such as the Double Mer Sandstone near Hamilton 

Inlet; the Upper Proterozoic attains much greater bulk 

in parts of North America outside the shield, as will be 

seen later.

Included with the Upper Proterozoic platform deposits on the "Tectonic Map of North America" are the 

pre-Upper Cambrian rocks of the Wichita Mountains 

of Oklahoma. Most of the pre-Upper Cambrian rocks 

that are exposed are intrusive granites and gabbros, but 

subsurface data in surrounding areas demonstrate that 

these are not basement in the usual sense, but are sheets 

that intrude a thick and extensive terrane of lavas and 

graywackes (Ham and others, 1964, p. 21-37). Isotopic 

dating of the intrusives indicates ages of about 500 m.y. 

suggesting an Early Cambrian rather than a Precambrian age; the host rocks are of about the same age or 

slightly older. Regardless of an eventual decision as to 

the age classification of these pre-Upper Cambrian 

rocks, they are more closely related tectonically to the 

later Precambrian (A3) than to the Paleozoic, and are 

so treated here (see p. 64).

Shown on the "Tectonic Map of North America" by 

the same color patterns as the Middle and Upper Proterozoic platform deposits are strata of similar ages in 

parts of the Appalachian and Cordilleran foldbelts. 

These strata are properly geosynclinal rather than eratonic, yet they resemble the platform deposits in that 

they overlie deformed basements of earlier Precambrian rocks, and were themselves not materially deformed except by the Phanerozoic orogenies. They will 

be discused more specifically later.

(B) PLATFORM DEPOSITS ON PRECAMBRIAN 

BASEMENT

In the central craton of North America the exposed 

Precambrian rocks of the Canadian Shield are surrounded by areas in which they are covered by varying 

thicknesses of platform deposits of Paleozoic and 

younger ages. These platform deposits cover the interior lowlands south of the shield in the United States, 

from whence they extend northwestward through the 

plains of western Canada into the Arctic Islands and 

northern Greenland. A broad outlier of similar platform deposits covers the center of the shield southwest 

of Hudson Bay. Between the shield and the Appalachian foldbelt on the southeast, the platform deposits 

make only a discontinuous, narrow strip along the St. 

Lawrence River.

On the "Tectonic Map of North America" the upper 

surface of the Precambrian in the central craton is contoured on a 500-m (1,640-ft) interval, and its configuration is indicated by layer tints. During the last few 

decades the top of the basement in the cratoi south of 

the 60th parallel has been penetrated at numerous places 

by wells drilled for water, oil, or gas, so thrvt control 

for contouring is excellent except near the centers of 

the deeper basins (Flawn, 1967); here, the basement 

configuration must be extrapolated from the structure 

of overlying strata. North of the 60th parallel, in Yukon 

Territory and the District of Mackenzie, very few deep 

wells have been drilled, but to complete the map, the 

sparse data from these wells have been used to extend 

the basement contours hypothetically to tl ?. Arctic 

Ocean.

The paleogeology of the buried Precambrian surface 

is of much tectonic interest, as it indicates the extensions 

of the Precambrian foldbelts away from their surface 

exposures, in the Canadian Shield and in outlying areas. 

The paleogeology can be deduced with much confidence 

in many places by means of well data. For the most part, 

however, it is not feasible to show the paleog?«logy on 

the "Tectonic Map of North America," except where 

exposed boundaries are extended for short distances 

beneath the cover as dotted lines. The paleog<M>logy of 

the cratonic area in the United States is indicated on 

another map, so far as it can be deduced (Bayley and 

Muehlberger, 1968); data from that map, and from 

other sources, are summarized in figure 10.

The paleogeologic data show that in most places the 

Precambrian surface is a true basement of met amorphic 

and plutonic rocks. Nevertheless, in some areas the Paleozic cover is separated from the basement by m oderately 

deformed younger Precambrian sediments an^ lavas 

themselves ancient platform deposits like those in the 

Canadian Shield. Buried extensions of the Ker^eenawan 

Series and the related Sioux Quartzite occ^r in the 

states immediately south and southwest of Lake Superior, and there are areas of rhyolitic lava in Oklahoma, north Texas, and adjacent states (Mu^hlberger 

and others, 1966, p. 6422). At present, it is impracticable 

to represent the configuration of the lower srrfaces of 

these Precambrian platform deposits, and the basement 

contours are perforce carried over their tops.

The basal deposits of the platform cover a TO Paleozoic, the earliest being mostly Late Cumbrian in age, 

but the Cambrian is overstepped in many areas by 

younger strata. As indicated on the various geologic 

maps of North America, Paleozoic rocks also lie at the 

surface over extensive parts of the craton, especially 

toward the southeast, but they are covered r^estward, 

in the plains east of the Cordillera of the United States 

and Canada, by thick Mesozoic deposits and thinner 

Tertiary deposits.
PLATFORM AREAS 23

The structure of the strata in the platform cover is 

of much tectonic interest and is known in detail, especially as a result of drilling for oil, gas, and water. 

Within each area the sequence of strata form a "layer 

cake." "Each layer is separated from the other by an 

unconformity; each layer of geology is completely independent of other layers above and below; there is no 

clue in the upper layer of either the existence or character of the next layer below; and each layer has its own 

oil and gas geology, completely independent from each 

of the other layers" (Levorsen, 1943, p. 912). Some of 

the unconformities between the layers are local; others 

extend over much or all of the craton, and the strata 

between them can be integrated into gross "sequences" 

(Sloss, 1963, p. 95). Four gross sequences have been 

recognized in the Paleozoic, the 'boundaries between 

them lying in the lower part of the Ordovician, the 

middle of the Devonian, and near the base of the Pennsylvanian; other gross sequences have been distinguished 

in the Mesozoic and Oenozoic (Sloss, fig. 6, p. 110).

On the tectonic maps of the United States (Longwell, 1944b; Cohee, 1962) and the "Tectonic Map of 

Canada" (Deny, 1950) the outcropping edges of some 

of these layers or sequences have been represented on 

the former, the bases of the Pennsylvanian, the Cretaceous, the lower Tertiary, and the upper Tertiary. 

Similar usages have been followed on some tectonic 

maps of other areas or continents. Both the United 

States and Canadian maps also show structure contours 

on various strata within the platform cover. Contours 

on some of these strata can be extended over vast areas, 

as on the top of the Trenton Limestone (Ordovician) 

in the eastern United States, and on the top of the Dakota Sandstone (Cretaceous) east of the Cordilleran 

front from Kansas northward through Alberta. These 

contours usefully supplement the contours on the surface of the underlying basement and bring out details 

of the tectonics of the different layers or sequences in 

the cover.

Nevertheless, representation of structure within the 

platform cover has not been attempted on the "Tectonic 

Map of North America." To represent the extent and 

nature of the different layers or sequences in the cover 

would require many superposed lines or patterns, unsuitable on a map on this scale; their place is on maps 

of larger scales, or on a series of maps. Also, the differences between contours on the strata in the cover 

and those on the basement are of such a low order of 

magnitude that they are scarcely apparent on a map of 

small scale with a large contour interval; hence, only 

confusion would result from superposing them. Such 

contours would be much more meaningful on maps of 

larger scales and with smaller contour intervals.

Faults occur at different levels in the platform cover 

and its basement in the central craton and pose problens 

of three-dimensional representation. The faults are of 

three kinds: (1) those which displace units in the baroment, but not the overlying strata; (2) those which displace the basement surface, but which may or may not 

extend upward to the ground surface; (3) those which 

displace the surface strata, but which may or may not 

extend downward to the basement surface. On the "Te < > 

tonic Map of North America," faults which display 

units in the buried parts of the basement are mostly 

not shown; they are more appropriate as components of 

a paleogeologic map. Faults which displace the surfa^ 

of the basement but which do not extend to the ground 

surface are shown by dotted lines (as concealed faults). 

Faults which displace the surface strata are shown by 

solid lines; some of these extend downward to the bar <v 

ment, some do not. Those which do not extend downward to the basement may be thought of as "floating" 

above the deeper lying structures that dominate the 

platform areas on the map.

The central craton as it existed during early Paleozoic time extended southwestward, well into the Cordilleran region, into what is now the Central and Southern 

Rocky Mountains, the Colorado Plateau, and parts of 

the Basin and Range province in Arizona and New 

Mexico. This part of the early craton has been reactivated or disrupted during later tectonic events.

In the Southern Rocky Mountains reactivation begrn 

during later Paleozoic time, but it was extended ov^r 

much greater areas during the orogenies of Mesozc^c 

and early Cenozoic time. During each of these times, tJ- * 

basement was raised into elongate uplifts or geanticlines, between which it was depressed into troughs of 

varying depth; during the later reactivations the baa^ 

ment and its cover were folded and faulted, and the 

weakened crust was penetrated by many intrusives all 

of these events producing the modern structure of this 

part of the Rocky Mountains.

This record is reflected in the superincumbent Pha*ierozoic strata. During the early Paleozoic the region 

received thin cratonic deposits. During times of reactivation, as in the later Paleozoic and in the Paleocene ar i 

Eocene, thick marine or nonmarine deposits, largely 

elastics, accumulated in troughs or basins between til ^ 

geanticlines. During pauses in reactivation, as duririr 

the later Mesozoic, sheets of sediments, some of geosynclinal proportions, were spread over the region from 

the mobile parts of the Cordillera to the west. Thero 

groups of strata are structural layers, or "stages," eacK 

of which has its own tectonic features that would d<*- 

serve representation on large-scale maps. On a smallscale map of the whole continent, however, ft*
24 THE TECTONICS OF NORTH AMERICA

dominant tectonic feature is the configuration of the 

deformed basement. Consequently, on the "Tectonic 

Map of North America, 7 ' these areas are colored with the 

same relief tints as on the remainder of the platform, 

and contours on the Precambrian basement are extended 

to the western and southwestern edges of the Colorado 

Plateau that is, up to the edge of the folds and thrusts 

of the Cordilleran miogeosyncline.

In that part of the Basin and Range province south 

and southwest of the Rocky Mountains, the former eraton was mildly reactivated between late Paleozoic and 

early Cenozoic time, but it was strongly disrupted by 

block faulting later during the Cenozoic, producing the 

present pattern of ranges of older rocks and intervening 

basins deeply filled by late Tertiary and Quaternary 

nonmarine deposits (O12). Precambrian rocks form 

the surface of many of the ranges, especially in Arizona, 

and their cover rocks occupy correspondingly smaller 

areas. Configuration of the basement can be indicated 

by contours only in parts of this disrupted region.

(C) PLATFOBM DEPOSITS ON PALEOZOIC BASEMENT

The Atlantic and Gulf Coastal Plains of southeastern 

and southern North America are formed of Mesozoic 

and younger platform deposits that were laid over the 

deformed Paleozoic and older rocks of the Appalachian 

and Ouachita foldbelts. The platform deposits thicken 

and slope seaward from the exposed parts of these foldbelts, the basement descending beneath them. The continental shelves which border these coastal plans are 

their submerged extensions. The Atlantic Coastal Plain 

is prolonged far southward in the Florida Peninsula, 

and extends thence, mainly submerged, through the 

Bahama Islands and up to the front of the Antillean 

foldbelt in Cuba. The Gulf Coastal Plain projects well 

into the central United States in the Mississippi Embayment, but it narrows southward into Mexico where it is 

partly interrupted by the outer folds of the Cordillera; 

it widens again farther south in the Yucatan Peninsula.

From New Jersey to the Llano uplift in central Texas 

the landward border of the "platform deposits on Paleozoic basement" is drawn on the tectonic map at the edge 

of the Cretaceous and (or) Tertiary deposits of the 

coastal plains, where they overlap on their basement. 

This border is used for practical reasons; alternatively, 

there is logic for placing the border farther coastward 

west of Alabama, at the fronts of the Appalachian and 

Ouachita foldbelts. Except in the Ouachita Mountains, 

this front is largely buried from. Alabama to west Texas 

and is shown as a dotted line (or a concealed fault) on 

the tectonic map. Westward and southward from central Texas, the edge of the Cretaceous extends far inland 

from the Gulf Coastal Plain; here, the border of the

platform is arbitrarily placed at the fronts of the Ouachita and Cordilleran foldbelts.

The platform as thus defined has a heterogeneous 

basement. The most characteristic parts am the deformed rocks of the Appalachian and OuacHta foldbelts of both the external and internal (miogeosynclinal and eugeosyiiclinal) zones west of Ala-bama, of 

the internal zone alone to the northeast (the Piedmont 

province and its buried extensions). The internal zones 

are variably metamorphosed and that of the Appalachian foldbelt is invaded by plutonic rocks. Most of 

the deformed rocks are Paleozoic, but the intemal zone 

of the Appalachian foldbelt includes probable Precambrian that has been reworked by the Paleozoic 

orogenies.

As the platform is defined, it must perforc0. also include less characteristic basement rocks. In the upper 

part of the Mississippi Embayment and in nor*h-central 

Texas the Cretaceous and Tertiary deposits spread over 

the little disturbed Paleozoic rocks of the central craton, 

so that one layer of platform deposits overstepr another; 

contours on the Precambrian basement beneath the 

lower layer are mapped under the upper layer as dotted 

lines. Well to the southeast of the. Appalachiar foldbelt, 

in the Smvaiiee basin of northern Florida and southern 

Georgia, is another area of little disturbed Paleozoic 

strata, known only from drill data; the basin is- bordered 

on the north and south by crystalline rocks, but its basal 

configuration is unknown.

Beneath the platform cover from the Nortl Atlantic 

States south westward to Texas are also Triassic rocks 

 or at least continental sedimentary rocks and mafic 

igneous rocks that are lithically identical with the 

Upper Triassic Newark Group of the inner zone of the 

Appalachians (L8). Some geologists have proposed that 

these form a "broad terrane" toward the coast, hence are 

the initial platform deposits (Spangler and Peterson, 

1950, fig. 18, p. 87; McKee and others, 1959, p?. 10), but 

more plausibly these rocks are preserved in fault troughs 

like the exposed Newark Group (Durham and Murray, 

1967, p. 432). The Eagle Mills Formation, from which 

a few Triassic plants have been recovered, has Hen penetrated in a trough-like belt that extends acrosr southern 

Arkansas into Texas (Scott, Hayes, and Fietz, 1961). 

The Triassic rocks probably predate the platform 

sequence.

On the "Tectonic Map of North America," configuration of the Paleozoic basement (C) is indicated, by 500-m 

contours and by layer tints in a manner similar to that 

of the Precambrian basement (B), but in a different 

color. These contours are drawn over the tops of all the 

heterogeneous rocks just described, whether they are 

characteristic basement or not. From Alabama eastward
PLATFORM AREAS 25

configuration of the basement is fairly well documented 

by drill penetrations out to the coast; near the coast, in 

the Bahama Islands, and on the continental shelves 

many additional data are afforded by seismic refraction 

surveys. From Mississippi westward through Texas the 

basement surface descends more rapidly seaward, so 

that all drill penetrations are within 150 km (100 miles) 

of the inner border of the coastal plain. Toward the 

coast the basement is far beyond reach of the drill, or 

even of very meaningful results from geophysical surveys. Nevertheless, hypothetical contours 'on the basement are shown as far as the coast and down to the 

13,000-m contour on the tectonic map; these are derived 

from the "Basement Map of the United States" (Bayley 

and Muehlberger, 1968), whose authors compiled them 

from available geophysical data and by extrapolation 

from the structures of higher horizons.

The stratigraphy and tectonics of the segment of the 

coastal plain from Mississippi through Texas has been 

thoroughly documented by more than half a century of 

petroleum exploration. Many of the results have been 

presented on the two versions of the "Tectonic map of 

the United States" (Longwell, 1944b; Cohee, 1962), 

including not only faults and salt domes, but structure 

contours at many levels on the strata of the coastal plain 

sequence. Contours on the strata within the sequence are 

omitted from'the "Tectonic Map of North America" 

because they obscure the regional picture on a 1:5,000,- 

000 scale; admittedly, contours on these strata have a 

firmer factual basis than hypothetical contours on the 

basement, but contours on any single horizon can be extended over only a small part of the whole coastal 

plain over long belts parallel to the strike where the 

strata are persistent, over short belts where they are not 

persistent (fig. 7). On the "Tectonic Map of the United 

States" of 1962, it was necessary to contour 24 different 

horizons in this part of the coastal plain to produce 

reliable results, 15 of which are in the later Tertiary of 

southern Louisiana alone.

In Mexico, south of this segment of the coastal plain, 

the Paleozoic basement again lies at relatively shallow 

depths, but drill penetrations are widely spaced and no 

regional contour maps on the basement were available to 

the compiler. Here, the "Tectonic Map of North America" shows contours on the top of the Lower Cretaceous, 

copied from the "Tectonic Map of Mexico" (de Cserna, 

1961).

The oldest emerging strata of the platform cover are 

Lower Cretaceous from Mississippi westward, and 

Upper Cretaceous from Alabama eastward (except for 

the thin Lower Cretaceous Potomac Group north of the 

Potomac River). Still older components of the platform 

cover wedge in downdip beneath the surface Jurassic

from Mississippi westward, Lower Cretaceous from 

Alabama eastward. Probably the oldest components of 

the cover are the Werner Formation and Louann Salt, 

approximately earliest Jurassic, the latter being the 

source of most of the salt domes that penetrate the 

higher coastal plain strata from Mississippi westward.

The platform sequence extends upward through the 

Cretaceous and Tertiary into the Quaternary. In the 

Atlantic Coastal Plain, regional unconformities cav^e 

the Eocene to overstep the Cretaceous in places, and the 

Miocene to overstep both Eocene and Cretaceous in 

others. The sequence is relatively thin even near the 

coasts, but seismic refraction surveys indicate thicVnesses as great as 4,500 m (15,000 ft) toward the edge 

of the continental shelf (Drake, Ewing, and Sutton, 

1959, p. 176-184). The sequence is also thin in the northern part of the Florida Peninsula, but thickens rapidly 

toward its southern end to more than 7,000 m (23,000 

ft) (Sheridan and others, 1966, p. 1983-1986). From 

Mississippi westward through Texas the sequence is 

more nearly complete and vastly thicker, especially ne.<vr 

the coast. Along the coast of southern Louisiana and 

eastern Texas, the maximum thickness of sedimerts 

above the Qligocene is as much as 19,000 m (62,000 ft), 

the Pliocene as much as 1,800 m (6,000 ft), and the 

Quaternary as much as 2,400 m (8,000 ft) (Crouch, 

1959), but these maxima are not all preserved in a single 

sequence. The thickness of the older part of the platform sequence near the coast is unknown but is undoubtedly also great.

The great thickness of sediments near the Louisiana 

and Texas coast, and the great depths of basement I ?.- 

neath, implies an extraordinary crustal subsidence. 

Some geologists attribute this subsidence to isostatic 

adjustment caused by loading of the crust by the voluminous sediments brought down by the Mississippi 

and other rivers, but it may have an ultimate tectoric 

cause in a postorogenic collapse of the interior of the 

Ouachita foldbelt in its great arc between Alabama 

and central Texas. Related to this collapse are the 

synthetic and antithetic faults in the coastal plain strata 

(shown on the map), which are concentric to the arc 

of the foldbelt.

Also shown as an area of "platform deposits on Paleo 

zoic basement" (C) is a small enclave within the Ccrdilleran foldbelt in southern Coahuila immediately 

north of the Parras basin, where Cretaceous strata are 

more broadly folded and warped than in the surrounding ranges and where the basement lies so near the surface that it emerges in many places (for example, at 

Las Delicias; R. E. King and others, 1944, p. 25-3J). 

Classification of this enclave as a platform rather than 

as part of the Cordilleran foldbelt must be made with
26 THE TECTONICS OF NORTH AMERICA

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PLATFORM AREAS 27

reservations; de Cserna (oral commun., 1964) has 

pointed out to the compiler that other ranges in the 

eastern part of the Mexican Cordillera have similar 

features, although perhaps to a lesser degree, so that 

distinctions are not absolute.

(D) PLATFORM DEPOSITS ON MESOZOIC BASEMENT

Classed as an area of "platform deposits on Mesozoic 

basement" (D) is the Arctic Coastal Plain a wellmarked but somewhat discontinuous ^eomorphic feature that extends across northern Alaska, past the 

mouth of the Mackenzie River in Canada, and along the 

northwestern edge of the Arctic Islands nearly to Axel 

Heiberg Island. This coastal plain differs technically 

from those just discussed, and is more heterogeneous, 

so that its validity as a true platform is somewhat 

doubtful.

In northern Alaska the coastal plain lies between 

the front of the Cordillera (Brooks Range) and the 

Arctic Ocean, and extends from west of Point Barrow 

nearly to Yukon Territory, where the Cordillera impinges on the coast (Gryc, 1959, p. 107-110). Much of 

it is masked by Quaternary deposits, which lie in part 

on Upper Cretaceous, in part on marine Tertiary. Unlike the strata of the Atlantic and Gulf Coastal Plains, 

those of this coastal plain slope landward from the 

coast, and thicken into a foredeep along the front of the 

Brooks Range; basement is within 720 m (2,500 ft) of 

the surface near Point Barrow, and stratigraphic evidence suggests at least the ephemeral existence of a basement massif offshore beneath the present continental 

shelf. The structure of the base of the coastal plain 

deposits in northern Alaska is indicated on the tectonic 

map by partly hypothetical contours on a basement 

which may be equivalent to the metamorphosed lower 

Paleozoic rocks of the core of the Brooks Range.

On the Canadian mainland, in Yukon Territory and 

the District of Mackenzie, another segment of the Arctic 

Coastal Plain includes the delta of the Mackenzie River 

and some areas on either side, all mantled by Quaternary deposits. Here, the underlying bedrock and its 

basement slope steeply seaward, as attested by a single 

deep oil test well in the delta which failed to pass 

through the Lower Cretaceous at a total depth of 3,841 

m (12,668 ft) (British-American, Shell, and Imperial 

Oil Companies, Reindeer D27 well, completed 1966).

In the northwest part of the Arctic Islands the Arctic 

Coastal Plain is formed by the Beaufort Formation 

(Craig and Fyles, 1961, p. 406-408) of preglacial Pliocene or earliest Pleistocene age a deposit several hundred feet thick that was laid down by streams draining 

northwestward toward the Arctic Ocean. On Banks 

Island and part of Prince Patrick Island the Beaufort

lies on Paleozoic platform deposits, but farther northeast on disturbed Mesozoic strata of the Sverdrup basin 

(a part of the Innuitian foldbelt).

(E) VOLCANIC BOCKS AND ASSOCIATED SEDIMENTS 

OF THE NORTH ATLANTIC PROVINCE

The plateau basalts and associated volcanics and sediments of the northeastern part of the area of the "Tectonic Map of North America" are part of an extensive 

petrographic province variously termed the North J tlantic, Brito-Arctic, or Thulean province, which includes some of the British Isles, the Faeroe Islands, and 

Spitzbergen, beyond the map area (Wenk, 1961, p. 27f). 

Within the map area these rocks form Iceland, Jan 

Mayen Island, parts of the east and west coasts of 

Greenland, and a small part of the east coast of Bafn 

Island (fig. 8).

The basalts and associated rocks are not entirely comparable to the platform deposits previously discussed, 

but they do lie with little deformation on basements 

with more complex histories. The basalts and associated 

rocks of Greenland and Baffin Island were spread ov°,r 

continental crust mainly metamorphic and plutonic 

rocks of the Hudsonian and earlier Precambrian foldbelts, but northward along the east coast of Greenland 

upon the diverse rocks of the Paleozoic East Greenland 

foldbelt. The basalts in Iceland and the other islards 

may have been erupted on oceanic crust.

Whether the basalts and associated rocks in the widely 

separated parts of the North Atlantic province were 

ever originally connected is a question which has implications as to the origin of the North Atlantic Ocean 

whether by continental foundering, continental drift, 

or some other mechanism but this question is beyond 

the scope of the present discussion. The occurrence of 

these rocks near the 70th parallel on both the east and 

west coasts of Greenland, as well as in Baffin Islard, 

suggests former connections, even though they are now 

separated by the Greenland icecap and the waters of 

Davis Strait. The peculiar configuration of the base of 

the Greenland icecap near the 70th parallel may have 

some relation to the volcanic structures. Nevertheless a 

connection across Greenland beneath the icecap has be°-n 

questioned, because the sediments and lavas overlap inland from the coasts (Wager, 1947, p. 29; Wenk, 19^1, 

p. 279).

The basal sediments in Greenland (El) include bcth 

Lower and Upper Cretaceous on the west coast, and 

high Upper Cretaceous on the east coast; these are followed by Paleocene sediments with volcanic components 

that foreshadow the succeeding volcanic episode. Similar 

basal sediments occur on Baffin Island (Wilson and 

Clarke, 1965), but are not shown separately on the mrp.
THE TECTONICS OF NORTH AMERICA

Probable areas 

of plateau basalt 

beneath icecap

Tertiary plateau basalt Quaternary volcanics Midocean ridge

FIGURE 8. Map of Xorth Atlantic Ocean and adjoining lands, including areas beyond the limits of the "Tectonic Map of North 

America," showing plateau basalts of North Atlantic province, and related features.
PLATFORM ABBAS 29

The overlying plateau basalts (Eft) have a thickness 

of nearly 8,000 m (26,000 ft) in east Greenland (Wager, 

1947, p. 21) and are as thick or thicker in west Greenland (Bosenkrantz and others, 1942, p. 55). In east 

Greenland they are capped by remnants of late Eocene 

to Miocene marine sediments, a feature indicating that 

the eruptions were completed during Eocene time. Associated with the plateau basalts in east Greenland are 

plutons of various sizes and compositions (Ea), including the famous Skaergaard layered mafic intrusive 

(Wager and Deer, 1939), as well as alkali syenites and 

profuse dike swarms. Intrusives are inconsequential in 

the other areas and are not mapped.

In Iceland, sequences of plateau basalts as thick as 

7,000 m (26,000 ft) are exposed, with thin sedimentary 

intercalations not shown on the map). The oldest 

radiometric dates are late Tertiary (about 16 m.y.), but 

plant remains in the sedimentary intercalations indicate 

various earlier ages back to the Eocene (Askelsson and 

others, 1960, p. 11-12). The basement on which the basalts were erupted is not exposed. On the "Tectonic 

Map of Europe" (Schatsky, 1962) it was assigned to 

"Precambrian of the marginal part of Laurentia", but 

this is very implausible. Seismic surveys indicate crustal 

layers beneath Iceland much like those elsewhere on the 

Mid-Atlantic Ridge beneath the surface basalts an 

intermediate layer 16 km thick with a velocity of 6.7 

km/sec, and a deep layer 10 km thick with a velocity of 

7.4 km/sec. The velocity of the intermediate layer is 

between that of oceanic and continental crust and may 

be hydrated oceanic crust (L. C. Pakiser, oral commun., November 1967).

Extending across the center of Iceland is a broad belt 

of Quaternary volcanics (E?) of more varied composition than their predecessors, erupted along the trace of 

the Mid-Atlantic rift zone, and including several volcanoes still in vigorous eruption (Askelsson and others, 

1960, p. 21-28, 32-45). Within the belt, special symbols 

are used on the map to express Quaternary volcanic 

features: (1) '*mountains of volcanic rubble" ("moberg" 

or "palagonite mountains"), produced by eruptions under the Pleistocene icecap, and (2) "fractures", which 

are either rifts in very young lavas or alined clusters 

of cinder cones.

<P) ICECAPS

Glaciers, ice fields, and icecaps cover extensive land 

surfaces in the northern part of the area of the "Tectonic Map of North America," some being relics of 

Pleistocene time. Parts of this ice are truly platform 

deposits (although not composed of conventional sedimentary rocks) for they conceal, in places to great thickness, a deformed bedrock or basement whose varied 

structure and age is largely undetermined. The great

icecap that covers the bedrock in all but the coastal 

parts of Greenland, and the smaller icecaps in the northeastern part of the Arctic Islands are therefore represented as platform deposits on the tectonic map. Most 

geographic maps overemphasize the ice-covered areas at 

the expense of the ice-free areas, but while the ice is 

geographically interesting not all of it is a platform 

cover in a tectonic sense. On the tectonic map, minor ice 

patches, valley glaciers, and ice areas with many r<r.k 

ridges and nunataks are omitted; hence, the rendering 

of platforms covered by ice differs materially from the 

ice-covered areas shown on geographic maps.

Geographic maps show many ice fields and glaciers 

in the northern Cordillera, mostly near the Pacific Cor»«t 

and north of the 56th parallel, but none of these can be 

classed as platforms in a tectonic sense. The most likr'y 

candidate is the great ice field of the St. Elias Mountains, near the common corners of Alaska, Yukon T^rritory, and British Columbia; to consider it a platform 

area is the more tempting because several major tectonic 

belts in the bedrock with uncertain mutual relations converge toward it. Nevertheless, large-scale geograplic 

maps indicate that the ice field is by no means continuous and is interspersed throughout with rock ridges, 

many of which are still unexplored geologically.

On the tectonic map, the configuration of the baw*- 

ment beneath the Greenland icecap is represented by 

500-m contours and by layer tints, in the same manner as the configuration of the basement in other platform areas. Data on the configuration of the basement 

of the other icecaps are not available, but this configuration is probably of less consequence. The configuration 

in Greenland is derived from variably spaced geophy^- 

cal traverses across the ice. Many versions of the results 

of these traverses have been prepared; the latest is by 

John Haller of the Danish East Greenland Expeditions 

who generously made available his manuscript map for 

use in compiling the tectonic map.

These contours show vividly the extensive area in 

the center of Greenland where the basement beneath 

the ice descends below sea level obviously in isostatic 

response to the load of the icecap. The ice above the 

basement is as much as 3,410-m (11,150 ft) thick, and 

its upper surface rises to a maximum altitude of 3,300- 

m (10,800 ft). Exceptional features of the basement configuration are the transverse ridges and troughs ne^r 

the 70th parallel, which may have been shaped by local 

tectonic causes. The bedrock surface rises toward the 

coasts, where it emerges in bare plateaus and mountains a few kilometers to as much as 300 km (160 mi) 

wide. These attain imposing heights along the east coa<< 

culminating in the plateau basalts south of Scoreshy 

Sound at 3,700-m (12,150 ft). The eastern coasNl
30 THE TECTONICS OF NORTH AMERICA

mountains were raised, relative to the bordering sea, by 

upflexing and upfaulting during Tertiary time (Wager, 

1947, p. 51-62); their western flank was depressed beneath the ice during Pleistocene time.

FOLDBELTS OF PRECAMBRIAN AGE

All the Precambrian rocks of North America (with 

the exception of the platform deposits already discussed) are parts of foldbelts that were formed during 

tectonic cycles embracing various parts of Precambrian 

time. Special problems are associated with the foldbelts 

of Precambrian age; hence, they are discussed separately from those of Phanerozoic age.

Precambrian rocks form the surface of vast areas in 

the northern and northeastern parts of North America, 

in the Canadian Shield and its extensions into Greenland, the Arctic Islands, the Adirondack uplift, the 

Lake Superior region. Precambrian rocks also form the 

basement beneath the platform deposits in the surrounding parts of the craton, where they emerge in small 

inliers on the crests of some of the uplifts, and they extend beneath at least the edges of the younger foldbelts 

where they have been reworked by Phanerozoic orogenies and are brought to the surface in the cores of 

the ranges.

Because of the large area of Precambrian exposure 

in the Canadian Shield, this region offers the best opportunity in North America for determining a sequence 

of Precambrian rocks, the orogenies of Precambrian 

time, and the foldbelts produced thereby. Great progress 

in such determinations has been made in recent decades 

by the Canadian geologists; as a result of accelerated 

programs of geologic mapping and radiometric dating. 

The results obtained in the shield can, moreover, be 

extended with much success into the Precambrian rocks 

and foldbelts elsewhere in North America, so that these 

results are used as a basis for the stratigraphic and 

tectonic classifications on the 'Tectonic Map of North 

America."

STBATIGBAPHIC CLASSIFICATION

Classification of rocks on a tectonic map is based ultimately on a stratigraphic classification, but the difficulties of making a meaningful stratigraphic classification 

of the Precambrian rocks are well known difficulties 

resulting from the impossibility until recently of making any reliable correlations of rock sequences beyond 

local areas. Fossils of value for correlation are virtually 

lacking in Precambrian rocks, but the methods of radiometric dating that now exist have proved to be at least 

a partial substitute. These datings have not only opened 

the way for correlating Precambrian rocks over extensive areas, but have also shown the immense spans of 

Precambrian time during which these rocks weir formed 

(James, 1960, p. 104-107).

Radiometric methods of dating are neverthelQes still 

imperfect; the geochemical and geological limitations 

have been discussed at length in many publications. 

Radiometric methods, used in conjunction with geologic 

methods, can indicate broad correlations, but precise 

correlations are often elusive. A case in point i* the relation between the original Huronian Series of Ontario 

and the Animikie Series of northern Michigan, nearby 

to the west (James, 1958, p. 33-34). Although trolitianally the two have been correlated, and although they 

obviously lie within the same general age span, there 

is little evidence that they are actually conteirDoraneous and much geological evidence that they are not; 

conversely, if the two are not contemporaneors there 

is little evidence as to which is the older and which the 

younger.

Many proposals for a general Precambrian classification have been made during the last century, but the 

earlier proposals are now obsolete and have only historical interest; they have been summarized in many 

publications (see, for example, Holmes, 1963). More to 

the present purpose are recent proposals, some of which 

are shown in table 1. Any classification that can be 

generally accepted must, however, await an international 

agreement among geologists. Such an international 

agreement cannot be made until several fundamental 

questions have been resolved, namely: Are Precambrian 

rocks and Precambrian time capable of subdivision into 

named systems and periods comparable to thoro of the 

Phanerozoic ? Should these subdivisions be basec* on type 

sequences of rocks in one part of the world or mother? 

Or should they be defined by means of clusters of radiometric dates which presumably express the times of 

orogeny that terminated such sequences? Could the existing terminologies of Precambrian rocks be retained 

and adapted to the radiometric dates that became available later, or should a new terminology be croated?

As the "Tectonic Map of Korth America" has been 

completed before an international agreement on Precambrian classification has been reached, an interim 

classification must be used, but none of the existing classifications of the contributing geological surveys in 

North America are well adapted to this purpose.

The U.S. Geological Survey originally divided the 

Precambrian (or "Proterozoic Era") into the "j^.rchean" 

and "AlgonMan" Systems, presumably with time-rock 

connotations, but in actual practice in Geologi^l Survey reports the systems were used empirically for 

example, "Archean" for dominant plutonic rocks and 

"Algonkian" for dominant supracrustal rocks. The re-
FOLDBELTS OF PRECAMBRIAN AGE 31

TABLE 1. Comparison of various classifications of the Precambrian

[In the table the rock terms'' Lower," "Middle," and" Upper" are used, although some of the sources quoted use the time terms "early" ("earlier"), "middle," and "late"

riater")]

U.S. Geological Survey

Before 1933 (1)

Cambrian

PROTEROZOIC ERA

Algonkian 

System

Boundary 

not 

clearly 

denned

Archean 

System

After 1933

Cambrian

| PRECAMBRIAN

Divided 

informally 

into: 

Upper, 

lower

or

Upper, 

Middle, 

Lower

Minnesota 

Geological 

Survey, 1961 (2)

Cambrian

PRECAMBRIAN

Grenville 

orogeny 

1,100 m.y.

fc

a&

Penokean 

orogeny 

1,700 m.y.

_CJ 

T3-a

3

Algoman 

orogeny 

2,500 m.y.

<5 Laurentian 

| orogeny 

3 Age??

Geological Survey of Canada

1957, 1962 (3)

Cambrian

PROTEROZOIC

Divided 

into:

Upper, 

Middle, 

Lower

or

Upper, 

Lower

ARCHEAN

After 1964 (4)

Cambrian

PROTEROZOIC

Hadrynian

Grenville 

orogeny 

ending 880 

m.y.

Neohelikian

Elsonian 

orogeny 

ending 

1,280 m.y.

Paleohelikian

Hudsonian 

orogeny 

ending 

1,640 m.y.

Aphebian

Kenoran orogeny 

ending 2,390 

m.y.

ARCHEAN

Baltic Shield (5)

Cambrian

Sparagmite Series 

("Eoeambrian")

Daslandian 850-1,200 

m.y.

Gothian platform 

1,200- deposits 

1,500 1,300 

m.y. m.y.

Rapikivi 

granites

 1,600 

m.y. 

Karelian and Svecofennian 1,500-1,900 

m.y.

Belomorian 1,900- 

2,100 m.y.

fc 

< 

W 

W Saamian 2,150- 

0 2,900 m.y.

PH

< Katarchean 2,770- 

3,950 m.y.

Tectoric 

Map of 

NortI 

America 

(presert 

report.)

Cambrian

PROTEROZOIC

Upper

Middle

Lovar

ARCHEAN

1. Wilmarth (1926. p. 42,103,127. and pi. 1).

2. Qoldich and others (1961, p. 5).

3. Harrison (1957, p. 27); Stocfrwell (1962, p. 126 and fig. 3).

4. Stockwell (1964, p. 7-9; 1965; 1966. p. 33-34).

5. Compiled from Holtedahl and others (1964), Magnusson (1960), Simonen (I960). Polkanov and Qerling (196(0, and other sources. The names used seem to apply interchangeably to rock units, to the tectonic cycle during which they were formed, and to the terminal orogenies; for the foldbelts created by the orogenies the suffix 

"-ides" is commonly substituted for "-ian." The Katarchean occurs only as relics in the Saamian gneisses of the Kola Peninsula. The Gothian and Daslandian infracrustal rocks occur only in the southwest part of the shield, and the Jotnian supracrustal rocks only in the central part.

suits had become so incongruous by 1933 that formal 

subdivisions of the Precambrian were abandoned, after 

which only informal subdivisions were used, described 

as "lower" and "upper" ("older" or "younger") or 

"lower," "middle," and "upper"; these have been applied locally, seldom with any implications of general 

correlation. The English language provides only three 

such relative descriptive terms, whereas modern knowledge demonstrates that the Precambrian contains four 

or more major subdivisions. Additional categories might 

be provided by expressions such as "lower upper" or

"early late," but these are so offensive and confusing 

that they do not merit serious consideration.

In 1964 the Geological Survey of Canada adopted five 

named subdivisions of the Precambrian "Areher.n," 

"Aphebian," "Paleohelikian," "NeoheliMan," and 

"Hadrynian" (see table 1). The last four names, wlich 

are used for parts of an inclusive "Proterozoic," are 

derived from Greek words which indicate relative degrees of maturity (aphebos, heUhis, and hadrynes^ or 

past maturity, maturity, prematurity). The meritr of 

these names and the acceptance which they will recftive
32 THE TECTONICS OF NORTH AMERICA

remain to be determined. The novelty of the names 

militates against comprehending either their meaning or 

the sequential relations of the units.

On the "Tectonic Map of North America" the interim 

classification of the Precambrian therefore reverts to the 

traditional terms "Archean" and "Proterozoic," which 

have been hallowed by long usage in geological textbooks and among geologists. The subdivisions thereof 

can be expressed by descriptive terms, and the Proterozoic is so divided into "Lower," "Middle," and "Upper" 

("Early," "Middle," and "Late"); a similar subdivision 

of the Archean might be possible at a later time, but is 

not attempted on the map. These traditional terms are 

open to many objections it is true ambiguities in their 

earlier definitions and how they were applied, and later 

questions as to how they could be redefined more precisely on the basis of radiometric dates. As suggested by 

table 1, there seems to be little agreement between North 

American and European geologists as to the age of the 

termination of the Archean. This interim classification, 

whatever its deficiencies, at least makes possible a subdivision of the Precambrian into more than the three 

descriptive categories available in the English language, 

and provides the subdivisions with names whose ages 

and sequential relations can be inferred by the geologist 

who uses the map.

Besides the general time-stratigraphic units just discussed, the captions in the legend of the "Tectonic Map 

of North America" mention the names of a few local 

Precambrian rock units (the Keewatin, Temiskaming, 

Huronian, Animikie, Grenville, and Keweenawan Series, the Sudbury Norite and Duluth Gabbro). These 

are included merely for mnemonic purposes and as examples ; all are from the southern part of the shield in 

Canada and the United States, where the Precambrian 

is familiar to geologists. Their use does not imply that 

the names can be extended to rocks outside their typical 

areas, although such extensions have been proposed in 

the past. Elsewhere in the shield the rock units have 

properly been given other local names, but those in 

remote places and those recently proposed are less familiar to geologists. Many of the names for other rock units, 

and the probable ages of the units, have been tabulated 

in publications by Stockwell (1962, fig. 3; 1964, table 3) 

and by Goldich and others (1961, table 2).

TECTONIC CLASSIFICATION

Tectonic classification of the Precambrian rocks of 

North America on the basis of radiometric dating and 

other evidence is clearer than the stratigraphic classification. As more radiometric dates of Precambrian rocks 

become available, it is increasingly evident that they 

cluster during periods several hundred million years

long, separated by periods as long or longer with few or 

no dates (for example, see Gastil, 1960, figs. 1 and 2; 

Stockwell, 1964, fig. 2). Most of the datings h*ve been 

made on plutonic and metamorphic rocks, and the clusters of dates express times of plutonism and metamorphism. Inferentially, the clusters also express times of 

orogeny, of which the plutonism and metamorpHism are 

partial manifestations. This is confirmed in places by 

structural unconformities between the rocks tl *.t yield 

the dates and overlying less deformed rocks. TTie times 

which yield clusters of radiometric dates are thus commonly interpreted by geologists as erogenic times, and 

they are so treated on the "Tectonic Map cf North 

America."

Nevertheless, a qualification is needed. The clusters of 

dates have definite climaxes, but the whole spread of a 

cluster may be as great as 400 m.y. or as long as all 

geologic time since the beginning of the Devonian. During Devonian and later time, geologists hav?i distinguished many separately named orogenies, the number 

depending on the predilections of different geologists. 

Thus, the Precambrian "orogenies" are probably comparable, not so much to the orogenies within the foldbelts of later times, as to the tectonic cycles which 

created the f oldbelts as a whole; probably the Precambrian "orogenies" themselves consisted of imny such 

lesser orogenies, now blended together by imperfections 

of the record.

The nature of the Precambrian orogenies and tectonic 

cycles has been debated, opinions being colorec1 by conflicting theories as to the origin and evolution of the 

earth's crust. Many geologists in the past have assumed 

that crustal behavior during much of Precambrian time 

differed substantially from that of Phanerozoic time. 

An ancient fallacy that has caused much mischief is an 

assumption of the universality of Precambrian orogenies; remnants of this fallacy persisted even into later 

times. Bucher (1933, p. 419) thus summarized the 

thinking of his day:

The best evidence of the diminution in the course cf geologic 

time of the areas occupied by the orogenic belts ir the progressive decrease in the width of the zones of crystalline 

schists and gneisses produced in successive eras. * * * For 

Archeozoic time, crystalline schists and gneisses occupy apparently 100 percent of the folded belts so far as they are 

accessible to view today. For the Proterozoic, the percentage 

is still large, but much less than 100 percent. The contrast between the Proterozoic and the Paleozoic is still greater. * * *

A modern variant of these views is expressed by 

Stockwell (1966, p. 34-35):

The orogens of the shield characteristically cover very broad 

regions quite unlike the mountain chains of later times. Another difference is the long time interval between deposition 

of sediments and their involvement in orogeny (about 400
POLDBELTS OF PRECAMBRIAN AGE 33

m.y. in two instances), as contrasted with the close time sequence of geosynclinal deposition, sinking, and mountain 

building in younger rocks. Still another difference is the very 

extensive overlapping of a younger orogen on an older one and 

this occurs in regions not likely ever to have been the site of 

intervening geosynclinal deposition. On the whole, the thesis 

that the orogenic development of the shield differed from that 

of the younger rocks seems worth considering and, it may be 

suggested, resulted from deep crustal or subcrustal movements 

unrelated to the sinking of geosynclinal belts.

At least some of the alleged contrasts between Precambrian and Phanerozoic orogenies may be more apparent than real. The depth of denudation of a foldbelt 

is progressively greater the older the rocks; hence the 

exposed areas of crystalline rocks are also greater with 

age. Moreover, the universality of crystalline rocks in 

the Archean is probably not a product of any single 

world-wide orogeny, but results from the great length 

of Archean time during which many orogenies built 

foldbelts in different places. Also, if the foldbelts mentioned by Stockwell are actually products of gross 

tectonic cycles rather than single orogenies they are 

more comparable with those of Phanerozoic time; the 

Cordilleran foldbelt of western North America which 

was built during the last 150 m.y. of geologic time compares favorably in breadth and length to the Precambrian foldbelts, and likewise contains relics of 

earlier foldbelts that were reworked during the Cordilleran orogeny. Even allowing for probable changes 

in the nature and behavior of the earth's crust with 

time, it is reasonable to assume (as Sederholm did more 

than half a century ago) that the resemblances between 

the Precambrian and the Phanerozoic foldbelts are 

greater than the differences (see Holmes, 1963, p. xxixxii).

FOLDBELTS OP CANADIAN SHIELD

In the Canadian Shield, three great clusters of radiometric dates have been obtained in the Precambrian 

rocks, whose times probably express major orogenies; 

these times are so designated on the "Tectonic Map of 

North America," following previous usage. As explained earlier, the so-called orogenies of Precambrian 

time are more properly gross orogenic times or tectonic 

cycles in terms of Phanerozoic events, and this will be 

implicit in the discussion henceforth. The orogenies 

have been classified as shown in table 2.

These dates, and the orogenies which they express, 

occur in well-defined areas the "provinces" of Canadian geologists which are here considered to be the 

foldbelts produced by the orogenies (fig. 9). The provinces (or foldbelts) of each age are listed below, in order 

of increasing age: 

Grenville orogeny

Grenville province

Hudsonian orogeny

Churchill province

Southern province

Bear province

Nain province 

Kenoran orogeny

Superior province

Slave province

Nain province (minor)

The restriction of orogenic effects during Precambrian time to specific foldbelts (provinces) is illustrated 

by the passage of deformed supracrustal rocks in the 

foldbelts into the platform deposits that extend o^er 

earlier foldbelts, as described under an earlier heading 

(p. 21-22). Platform deposits of Lower Proterozoic age 

(Al) thus define Kenoran foldbelts, those of Middle 

Proterozoic age (A2) Hudsonian foldbelts, and those 

of Upper Proterozoic age (A3) Grenville foldbe'ts. 

Such relations are most dramatically expressed during 

Middle Proterozoic time, when the rocks of the Gr?nville foldbelt in the southeastern part of the shield w*»re 

subjected to deep-seated deformation and accompanying metamorphism and plutonism, whereas in the gre«ter part of the shield to the northwest, supracrustal pb.tform deposits (A2) were being laid down over rocks 

already consolidated by earlier orogenies the deformed rocks and the platform deposits both yielding 

nearly contemporaneous radiometric dates.

TABLE 2. Classification of orogenies in Canadian Shield

Rock sequence

Upper Proterozoic 

(minor iu shield).

Middle Proterozoic.

Lower Proterozoic.

Archean.

Orogenies

Canada '

(I) Grenville 

orogeny.4

(«)

(II) Hudsoninn 

orogeny.

(Q) Kenoran 

orogeny.

Minnesota '

Keweenawan 

igneous 

activity.

Penokean 

orogeny.

Algoman 

orogeny.

Climax of 

orogeny '

Rang" of 

orogeny *

Million years

945

1, 735

2.490

880-1,000

1.640--1.820

2.390-2.600

1 Modified from Slock we ^1 (1964, tables 1 and 2) and other papers.

: Modified from Goldich and others (1961, table 2; 1966, table 5).

'The climaxes and ranges of the orogenies are inferred from statistical summaries 

of recorded radiometric dates in Canada (Stockwell, 1964, p. 2-7); the clitnaier and 

ranges of the orogenies are based on the abundance of the radiometric dates', the 

ends being assumed to be the mean minus the standard deviation.

4 Pertinent objections have been raised by Qilluly (1966, p. 104-108) to such terms 

as" Grenville orogeuic belt" and "Grenville orogeny." Rightly or wrongly ,-the terms 

are now so firmly entrenched in usage that it is undesirable to substitute some new 

and unfamiliar nomenclature.

« Stockwell distinguishes an additional Elsonian orogeny between the Hudsonian 

and Grenville; this is appraised on p. 56.
34

130

THE TECTONICS OF NORTH AMERICA 

120° 110° 100° 90° 80° 70° 60° 50° 40°

pPf^X] & V^^\,

50

1000 KILOMETERS

0 1000 MILES

::::::::::::::::

^\\^

* * ' " * * ^ *

Province containing Province containing Province containing Province containing Little deformed Edge of Phanerozoic

Kenoran foldbelts Hudsonian foldbelts Kenoran and Hudsonian Grenville foldbelt supracrustal strata platform co^er

elements, overprinted mainly middle

by Elsonian event Proterozoic

FIGURE IX Map of Canadian Shield showing provinces that contain different assemblages of Precambrian rocks. Modified from

Stockwell (19&4, fig. 1).
FOLDBELTS OF PRECAMBRIAN AGE 35

Besides the major orogenies, there are indications of 

other events of possible erogenic significance during 

Precambrian time in the Canadian Shield, two of which 

merit discussion:

Archean time was prolonged, and it is presumed that 

some or many orogenies occurred before the terminal 

Kenoran orogeny. One of these, the "Laurentian" orogeny is suggested by geological evidence in the Lake 

Superior region. As early as 1885, A. C. Lawson recognized that the Keewatin greenstones of the region were 

intruded by the so-called "Laurentian" granite (a misnomer, as the original "Laurentian" is in the Grenville 

province, and very much younger), and that the eroded 

surfaces of both were overlain by younger strata, now 

classed as later Archean (the Knife Lake); he ascribed 

to this event a major role in Precambrian history. Subsequent investigations indicate, however, that the 

younger Kenoran orogeny and the accompanying Algoman granites are much more prominent in the region, 

and the "Laurentian" has been relegated to a lesser rank 

(Goldich and others, 1961, p. 73-74). The age of the 

"Laurentian" event is still indeterminate, as no radiometric dates from it have survived; very likely they 

were overwhelmed by the Kenoran orogeny. Similar 

(but not necessarily contemporaneous) events may have 

occurred elsewhere in the Kenoran foldbelts during 

Archean time, as suggested by the occurrence of granite 

clasts in many of the sediments. Radiometric data indicate relics of a 3,550 m.y. event in the gneisses of the 

Minnesota River valley (Goldich, Lidiak, and others, 

1966, p. 5395-5396), but their relation, if any, to the 

"Laurentian" event is unknown.

The record of pre-Kenoran tectonic events during the 

Archean is so imperfectly preserved that Stockwell 

(1965) was unable to map their effects in the Canadian 

Shield, and these effects are accordingly not indicated 

on the "Tectonic Map of North America."

Stockwell (1964, p. 2-3) proposed an "Elsonian orogeny" on the basis of a scattering of radiometric dates 

in the Nain province in the northeastern part of the 

Labrador Peninsula with a mean age of about 1,370 m.y. 

The area from which the dates were obtained is a terrane of gneisses and embedded granitic plutons, apparently not differing from the adjoining Hudsonian foldbelt except for the occurrence of bodies of anorthosite 

and gabbro like those in the Grenville foldbelt to the 

south (I£); Stockwell suggests that all the anorthosite 

and gabbro bodies are of Elsonian age, those in the 

Grenville foldbelt having been reworked during the subsequent Grenville orogeny. Credence in the existence of 

an "Elsonian orogeny" is enhanced by the occurrence of 

radiometric dates within the same age range elsewhere 

in North America outside the shield.

Nevertheless, the significance of the Elsonian eveit 

remains dubious. Even in the eastern part of the shield, 

radiometric dates within this age range are few, and 

have no prominent peak of abundance like those of the 

other Precambrian orogenies; radiometric dates within 

this age range are virtually lacking elsewhere in the 

shield, except in the Southern province (Michigan and 

Wisconsin). Besides the anorthosites and gabbros, there 

are no obvious geological manifestations of an orogeny 

and the event can hardly have produced a foldbelt in the 

usual sense. The compiler infers that the Elsonian is a 

minor event in the Canadian Shield that produced no 

more than an overprint on rocks already consolidated by 

the Hudsonian orogeny. On the "Tectonic Map of Nor+h 

America" the rocks of the Nain province are assign**! 

to the Hudsonian foldbelt, the overprint of the Ekonian event being indicated, where appropriate, by a 

superposed diagonal ruling (H2).

Although the Laurentian, Elsonian, and other problematical Precambrian events have only minor significance in North America, they may reflect orogenies of 

greater significance in other continents. Thus, the Gothian deformation in the southwestern part of the Baltic 

Shield seems to be nearly contemporaneous with the 

Elsonian event in North America (table 1).

Subdivision of the rocks in the Precambrian foldbelts of the Canadian Shield on the "Tectonic Map of 

North America" largely follows the classification of 

Stockwell (1965). Within the Kenoran foldbelts little 

subdivision can be made, except to separate the supracrustal sedimentary and volcanic rocks (G2) from the 

granitic rocks (Gy); the migmatites (Gl) probably 

represent partial conversions of the former into the 

latter. More complex classifications are possible in the 

Hudsonian and Grenville foldbelts, although even here 

it is necessary to show areas of undifferentiated rocks 

(HI and II), where the terranes are little mapped or 

poorly understood. It has been proposd that the closely 

accordant radiometric dates in each of these foldbeJts 

indicates not only the ages of their climactic orogenies 

but also the general age ranges of all the rocks of ea^-h 

foldbelt (J. T. Wilson, 1950, p. 107-108). Stockwell's 

more detailed review indicates greater complexities. 

Each foldbelt includes not only geosynclinal supracrrstal rocks that formed during the tectonic cycle that gave 

rise to the foldbelt, but also plutonic and supracrustal 

rocks which are relics of earlier cycles reworked by t\e 

later ones; these reworked earlier rocks are indicated 

on the tectonic map by superposed diagonal rulings. TT- e 

Hudsonian foldbelts thus contain reworked relics of 

the Kenoran foldbelt (H3, Ha); the Grenville foHbelt contains reworked relics of both the Kenoran and 

Hudsonian foldbelts (12,13, la).
36 THE TECTONICS OF NORTH AMERICA

The famous "Grenville front," or prominent northwestern border of the Grenville foldbelt, truncates at 

high angles all the structures of the adjoining Kenoran 

foldbelt, and there has been much speculation as to its 

meaning. Certainly throughout its length it marks a 

significant contrast in style of deformation and metamorphism, and certainly through parts of its length it 

is a zone of major faulting and northwestward thrusting. Its actuality as a structural feature is confirmed by 

the occurrence through part of its length of a pronounced linear negative gravity anomaly (Canada 

Dominion Observatories, 1957). Nevertheless the 

"front" does not, as has sometimes been assumed, juxtapose a terrane of younger rocks in the Grenville foldbelt on the southeast against terranes of older rocks in 

the foldbelts to the northwest. Stockwell (1964, p. 18- 

21) has been able to trace relics of Archean and Lower 

Proterozoie rocks involved in the Kenoran and Hudsonian orogenies through the Grenville foldbelt (12, 

13), and he proposes that even the Grenville Series (14) 

of the southern part of the foldbelt (from which the 

names of the foldbelt and the orogeny are derived) is 

not younger than the Early Proterozoic, and hence is not 

simply a geosynclinal precursor of the much later 

Grenville orogeny.

FOLDBELTS OF GREENLAND

The island of Greenland is obviously a detached 

northeastern extension of the Canadian Shield, with 

analogous Precambrian rocks and structures. Incomplete geophysical surveys suggest that the intervening 

Baffin Bay and Davis Strait are floored by oceanic 

crust, so that the detachment was more likely by drift 

than by foundering of shield rocks. Nevertheless, the 

geometry of the Precambrian rocks and structures does 

not match convincingly from one coast to another so 

that the original fit of the two shield areas is still 

uncertain.

Precambrian shield rocks underlie much of Greenland, but they are covered in small part by the plateau 

basalts and associated rocks (E/J), and in much larger 

part by the central icecap (F), so that they only emerge 

near the coasts. Along the east and north coasts, besides, 

the shield is impinged by the East Greenland and Innuitian foldbelts (J and K), whose Precambrian components have been reworked by Paleozoic orogenies. 

Berthelsen and Noe-Nygaard (1965) have made an excellent summary of the Precambrian rocks of Greenland. 

Parts of these rocks are known in much detail, especially in the shield area on the west coast as far north as 

the 73d parallel (Berthelsen, 1961) and in the East 

Greenland foldbelt between the 70th and 80th parallels 

(Haller, 1961a, b; Haller and Kulp, 1962); some other

parts are virtually unexplored, especially on the east 

coast, south of the 68th parallel. Less radiometric dating has been done on the Precambrian rocks of Greenland, compared with that of the Canadian Shield, but 

the available results are consistent with those of the 

shield.

The oldest clearly recognizable foldbelt in Gr-^nland 

is the Ketilidian, which forms all the west coas* as far 

north as the 63d parallel. Most of it consists ci deepseated metamorphic and plutonic rocks (Gl) tlat have 

undergone long and complex histories. These have been 

divided into several named complexes of varying composition and metamorphic grade, which are separated in 

part by strike-slip faults. A few radiometric dates between 2,100 and 2,700 m.y. are available which indicate 

a correlation of the foldbelt with the Kenoran foldbelts of the mainland (fig. 10). Supracrustal rod's (G2) 

occur in a few places, notably in the Ivigtut ar^ near 

the 61st parallel, where a sequence of sediments and 

volcanics as thick as 4,000 m (13,000 ft) is preserved. 

All the metamorphic and plutonic rocks of the east 

coast south of the plateau basalts at the 68th parallel 

are also probably part of the Ketilidian foldbelt (GlH3a) but they are little known. Farther north, similar 

rocks form the autochthone of the East Greenland foldbelt at the head of Scoresby Sound and have yielded 

maximum radiometric dates of 2,300 m.y.

On the west coast at S0ndre Str0mf jord near the 67th 

parallel, rocks of the Ketilidian foldbelt are su "-ceeded 

northward by other metamorphic and plutonic rocks of 

the Nagssugtoqidian foldbelt (H4), which have yielded 

radiometric dates of about 1,500 to 1,650 m.y., hence are 

correlative with the Hudsonian foldbelts of th*. mainland. The Nagssugtoqidian foldbelt, like the Ketilidian, 

is divided into named complexes of varied nature. Supracrustal rocks (H5), called the Agpatides, are reported around Umnak Fjord north of Disko Islr.nd, but 

they have been little studied.

In southernmost west Greenland the Ketilidirn foldbelt has been overprinted by the Sanerutian e^ent, so 

that all radiometric dates here are 1,600 m.y. or less, 

hence (like the Nagssugtoqidian) being correlative with 

the Hudsonian orogeny of the mainland. The area includes the extensive Julianehaab Granite (Ha), pctually 

a heterogeneous complex of various granites with, relics 

of sedimentary and volcanic suprastrata; although 

originally formed during the Ketilidian it was immobilized during the Sanerutian. The area also includes discordant bodies of postorogenie granite (H8) which yield 

Sanerutian dates. On the east coast the Ketilidip n foldbelt is overprinted by the Sanerutian event as fa r north 

as Angmagssalik at the 66th parallel.

Southernmost west Greenland also contains the still
FOLDBELTS OF PRECAMBRIAN AGE 37

younger Gardar Group continental sandstones and 

volcanics (A2), block-faulted but otherwise little disturbed, and contemporaneous alkalic intrusives (Aa). 

These are platform rocks like the Keweenawan Series 

of the continental interior, and like them yield radiometric dates of about 1,000 m.y. They are not comparable to the infracrustal rocks of the Grenville foldbelt 

which strike out to sea on the opposing coast in Labrador, even though they yield similar dates, nor are they 

correlative with the remnants of Upper Proterozoic 

platform deposits (A3) that overlie the Grenville rocks. 

as was once proposed (Kranck, 1939, p. 30-32).

The age of the basement rocks of northern and northeastern Greenland is undetermined, but it is probably 

similar to that of the basement in Baffin and Ellesmere 

Islands, where a few Hudsonian dates have been obtained by the Geological Survey of Canada. Between 

Inglefield Land and Cape York, opposite Ellesmere 

Island near the 78th parallel, the basement is succeeded 

by the little disturbed platform deposits of the Thule 

Group (A2), which are overlain by a few remnants of 

the Cambrian and Ordovician (B). Although tihe Thule 

has not been dated, it is interpreted on the tectonic map 

to be of Middle Proterozoic age, like the comparable 

Precambrian platform deposits in the northern part of 

the Canadian Shield.

In northeast Greenland the Thule reappears from 

under the icecap around the head of Independence 

Fjord, where it is broadly folded and is overlain unconformably by the Upper Proterozoic Hagen Fjord Group 

(included with B on the map). Eastward in the East 

Greenland foldbelt, according to Haller (1961a), relations are even more discordant; the Thule is metamorphosed and plastically deformed (Jl) by the Carolinidian orogeny, a precursor of the early Paleozoic orogeny that affects the succeeding rocks of the foldbelt 

(fig. 11). The Carolinidian orogeny has not been closely 

dated, but Berthelsen and Noe-Nygaard (1965) suggest its possible correlation with the Grenville orogeny, 

an interpretation adopted in the legend of the tectonic 

map. Within the East Greenland foldbelt this Carolinidian basement is followed by a pile of Upper Proterozoic geosynclinal deposits (included in J2) which are 

discussed on page 54.

FOLDBELTS SOUTH OF THE CANADIAN SHIELD

South of the Canadian Shield in the United States 

and Mexico, outcrops of the Precambrian are smaller, 

and are closely to widely separated by the cover of 

younger strata. In the central craton they emerge in a 

few of the higher uplifts the Black Hills, the Sioux 

uplift, the St. Francois Mountains (in the Ozark uplift), the Arbuckle Mountains, and the Llano uplift. 

In the Appalachian foldbelt, metamorphic and plutonic 

rocks form some of the ranges near the central ayis, 

but the remainder of the Precambrian here consists of 

younger supracrustal strata. In the northern part of 

the Oordilleran foldbelt only younger Precambrian 

supracrustal strata come to the surface, but in the Central and Southern Rocky Mountains metamorphic and 

plutonic rocks are extensive in the cores of the rang<\s. 

They reappear in the Basin and Range province w<?st 

and southwest of the Colorado Plateau, in and near the 

edge of the Cordilleran miogeosyncline. They form t he 

surface of large parts of the ranges of the province in 

Arizona and the desert region of southeastern California, whence they extend westward into the Transverse Ranges to within a few miles of the Pacific Coa st. 

In most of Mexico, Precambrian metamorphic and pJutonic rocks emerge only in small inliers on the crests of 

higher folds and fault blocks, but there is a larger area 

of uncertain dimensions in Oaxaca in the far south, as 

shown by radiometric dates in the metamorphic ttmplex (Ol). The occurrences in the Transverse Ranfres 

and in Oaxaca are the only authentic Precambrian in 

the Pacific border region of North America.

These scattered outcrops of Precambrian metamorphic and plutonic rocks could not have been assembled 

into any kind of a coherent picture, were it not tH.t 

similar rocks have been penetrated by many drill holes 

in the central cratonic area, and were it not tl at 

many radiometric dates have been obtained, not only 

from the rocks of the outcrops, but from the rocks of 

the drill holes. These have made it possible to construct 

paleogeologic maps of the surface of the buried part of 

the Precambrian, to connect rocks and structures in the 

outcrops, and to assign the Precambrian metamorphic 

and plutonic rocks over large parts of North America 

to provinces or foldbelts like those in the Canadian 

Shield.

For practical reasons it is not possible on the "TT ctonic Map of North America" to represent the connections of these foldbelts where they are buried between 

the outcrops. For these, the reader should consult paleogeological maps, such as the map by Bayley and 

Muehlberger (1968). The general relations of the P?ecambrian foldbelts of North America, exposed or 

buried, are represented in the accompanying text figure 

(fig. 10). Radiometric datings south of the Canadian 

Shield are summarized in table 3, based in part on an 

extensive project of Goldich, Muehlberger, and others 

(Goldich, Lidiak, and others, 1966; Goldich, Muehlberger, and others, 1966; Muehlberger and others, 19C*>; 

Lidiak and others, 1966; Muehlberger and others, 196?).
38 THE TECTONICS OF NORTH AMERICA
130

t v:. 1 /..-?.'^,,*;' V::;U/i/, "^C-:-, /7 >

L

Cam

M

(a

Outcrops of 

Precambrian rocks

Ro

Contacts between 

Precambrian units 

Ro

or

Fronts of Phanerozoic 

foldbelts

Roc

Roc

or

R

or

120°

110

FIGUKE 10. Map of North America showing inferred extent of exposed and buried provinces of Precambrian rocks. Compiled from 

Stockwell (1964, fig. 1), and Bayley and Muehlberger (1908).
40 THE TECTONICS OF NORTH AMERICA

TABLE 3. Summary of Precambrian rocks and events south of Canadian Shield

Canadian

Shield (Modified from

Stockwell,

1964)

PALEOZOIC

 600 m.y. 

UPPER

PROTEROZOIC

Grenville 

orogeny 

~ 880-1,000

m.y.

OH- 1

O

(S3

O

tf

W

O

tf

OH

Elsonian

event

- 1,280-

1,460 m.y.

W

Q

p i i

Hudsonian

orogeny 

1,640-1,820

m.y.

LOWER

PROTEROZOIC

Kenoran

orogeny

2,400-2,600

m.y.

ARCHEAN

Appalachian

foldbelt 

(Rodgers,

1967, and

other sources)

Avalonian

event

(coastal

region from

Newfoundland southward)

Holy rood

granite

580 m.y.

Grenville 

orogeny 

(in basement of 

interior 

Appalachians) 800-

1,000 m.y.

(No earlier

dates re-

- corded) -

-

Northeastern

United States 

(Lidiak and

others, 1966)

Metamorphism

east of 

"Grenville 

- front," Ohio - 

800-1,000

m.y.

St. Francois igneous activity, 1,200-

- 1,350 m.y. -

Plutonic complex of Iowa,

1,3007-1,500?

m.y.

-Relics, 1,600- -

1,800 m.y.

North-central

United States 

(Goldich, Lidiak

and others,

1966)

Keweenawan 

igneous 

- activity, - 

1,000-1,200

m.y.

Folding of 

Sioux Quartzite, 1,200

m.y.

Apparent ages,

1,200-1,400

m.y.

Event, 1,490

m.y.

Penokean and

Black 

Hills orogenies 1,600-

1,800 m.y.

Algoman

orogeny

~ 2,400-2,750

m.y.

Minnesota

Valley relics 

3,550 m.y.

South-central

United States 

(Muehlberger

and others,

1966)

Wichita igneous activity

500 m.y.

Llano orogeny 

- 1,000-1,150 - 

m.y.

Panhandle and 

Spavinaw 

igneous

activities,

1,100-1,300

m.y.

Nemaha igneous activity

1,350-1,450

m.y.

-Granites, -

Arbuckle

Mountains,

1,320-1,400

m.y.

(No earlier

dates recorded)

-

Western United States 

(Greatly generalized

from many sources)

Pikes Peak

Granite,

Intrusives Coloin Apache rado; 

Group, Southern 

- 1,100 m.y. CaliDates on fornia

Belt Se- plutonries 900- ics, 1,000 

1,300 m.y. m.y.

Sherman Granite, Colorado; younger

granites of Arizona,

- 1,300-1,500 in.y.

Mazatzal orogeny, Arizona; final meta-

_ morphism _ 

in Front Range, Colorado, and prfr-Belt

basement, Montana,

1,650-1,750 m.y.

Granites and metamor- _

phics, NW. Wyoming,

2,400-2,600 m.y.

Relics, NW. Wyoming 

3,100 m.y.

Mexico (Fries,

196^; Fries 

and Riac6nOrta, 1965)

Eastern Mexico (Tamaulr>as)

710 m.y.

Southern 

Mexico 

- (Oaxaca) - 

770-980

m.y.

Eastern Mexico

- (Hidalgo) -

1,210 m.y.

Nortl -"western

Mexico

(Soiora) 

1,320-1,710

m.y.
FOLDBELTS OF PRE CAMBRIAN AGE 41

The oldest radiometric dates outside the Canadian 

Shield in North America have been obtained from Precambrian rocks in the mountains of northwestern 

Wyoming and adjacent Montana (Beartooth Mountains, Bighorn Mountains, Wind River Mountains, and 

other mountain ranges); they are consistently more 

than 2,400 m.y. old, with a few relics as great as 3,100 

m.y. The dates obtained from the outcrops, and from 

drill samples in surrounding areas, define a province 

comparable in age to those produced by the Kenoran 

orogeny in the Canadian Shield; nevertheless, these 

provinces are not connected, as younger dates have been 

found in the intervening area. The rocks of the province 

are mainly granites and granite gneisses (Gy), but they 

include the mafic Stillwater Complex of the Beartooth 

Mountains (Ga), and some schist and iron-formation 

in central Wyoming and elsewhere (G2). In the Black 

Hills, where most of the Precambrian is Lower Proterozoic, 2,500-m.y. dates have been obtained in the small 

Nemo district, from rocks probably representing an 

earlier basement.

The remaining Precambrian metamorphic and plutonic rocks of the western Cordillera all yield younger 

radiometric dates. Dominant dates from Montana to 

Arizona are between 1,500 and 1,800 m.y.; the rocks 

which yield them are classed on the tectonic map as 

Hudsonian (H5 or O2; Hy or Oa), but this is a gross 

generalization of varied rock assemblages, all of which 

have had complex histories. In the north, the basement 

rocks below the Belt Series of Montana and comparable 

supracrustal strata in the Uinta and Wasatch Mountains 

of Utah have yielded dates within this age range (but 

relic earlier dates occur in the Little Belt Mountains, 

Mont.); the Belt Series itself has been dated by a variety 

of methods as between 1,300 and 900 m.y. old (Obradovich and Peterman, 1967), hence is Middle Proterozoic 

or somewhat younger.

In the south, the basement beneath the Grand Canyon Series and Apache Group, and beneath the Precambrian supracrustal strata near Caborca, northwestern 

Sonora, was deformed by the Mazatzal orogeny, dated as 

1,650-1,750 m.y. ago (Wasserburg and Lanphere, 1965, 

p. 755), although it was once ascribed a younger ("Elsonian") age. Actually, two closely spaced events can be 

discriminated in the orogeny, one 1,650-1,715 m.y. ago, 

another 1,715-1,750 m.y. ago, the younger dominating 

toward the south (Silver, 1967). In several different 

parts of Arizona the rocks deformed and metamorphosed by the Mazatzal orogeny include sequences of sediments and volcanics as thick as 6,000 m (20,000 ft) 

(Anderson, 1966, p. 6-8). A basal unconformity has been 

observed beneath one of these sequences (Blacet, 1966), 

but its regional significance remains to be established.

The supracrustal rocks of this region are probably 

Middle Proterozoic like the Belt Series in the nor*h; 

diabase sills in the Apache Group have been dated at 

1,100 m.y. (Shride, 1967, p. 76-77).

Special problems attend the Precambrian metamorphic and plutonic rocks of the Southern Rocky Mountains in Colorado and nearby Wyoming; clearly, tHy 

have had a complex history. The older paraschists r.nd 

paragneisses (Idaho Springs Formation and other 

units) were originally laid down as geosynclinal sfiiments and volcanics and were afterwards deformed r.nd 

metamorphosed. Still later, they were plastically folded, 

and another regional metamorphism was superposed; 

this event is dated at 1,750 m.y. ago (Pearson and others, 

1966; Hedge and others, 1967, p. 554), or at about the 

time of the Hudsonian orogeny. These rocks, and arsociated plutonic rocks, are classed on the tectonic map 

as Kenoran reworked by Hudsonian orogeny (H3, Ha). 

Sequences of Lower Proterozoic supracrustal strata 

(H5) are preserved in places, as in the Medicine low 

Mountains of southern Wyoming and the Needle Mo'\ntains of southwestern Colorado, and there are several 

sets of younger plutonic rocks. Anorthosite (Ift) like 

that in the eastern part of the Canadian Shield forms 

the central part of the Laramie Range, Wyo. (Newhouse and Hagner, 1957); extensive granites such as 

the Sherman and Silver Plume were emplaced 1,200- 

1,300 m.y. ago, and the less extensive Pikes Peak Granite 

was emplaced 1,000 m.y. ago.

Plutonic activity in the Southern Rocky Mountains 

thus extended well past the time of the Hudsonian orogeny, into the times of the Elsonian event and the Gr-mville orogeny. A similar situation obtains farther southwest, in Arizona, where granites emplaced at the c^ose 

of the Mazatzal orogeny are dated at 1,650 m.y. r.^o, 

and are followed by later granites dated at 1,375 ruy. 

ago (Silver, 1966); various stray younger Precambr^an 

dates reported in Arizona are probably an overprint related to these younger granites. The history implied by 

radiometric dates is also complex in the Transverse 

Ranges of southern California (Silver and others, If ^3; 

Silver, 1968); gneisses and plutonics in the San Gabriel 

Mountains are dated at 1,650-1,700 m.y. ago (hence 

Mazatzal or Hudsonian); they were followed by a metamorphic event 1,420-1,450 m.y. ago, and later by intrusion of an anorthosite-syenite complex (Oft) 1,220 r^y. 

ago. The Precambrian rocks were long afterwards invaded by Permian to Triassic and by later Mesorcoic 

plutonic rocks.

Radiometric dates intermediate between the Hudsonian and Grenville orogenies have been determined in 

many of the Precambrian basement rocks penetrated by 

drilling east of the Cordillera, in the central CK ton
42 THE TECTONICS OF NORTH AMERICA

(Goldich, Lidiak, and others, 1966, p. 5384-5386). Some 

are merely apparent dates or an overprint, but others 

pertain to actual plutonic or volcanic events, as shown 

on table 3. Thus, rhyolitic volcanic rocks were spread 

widely in Missouri, Oklahoma, and northwestern Texas, 

and have been dated from place to place as 1,200-1,350 

m.y., 1,150-1,300 m.y., 1^100-1,200 m.y., and 500 m.y. 

ago (Muehlberger and others, 1966, p. 5421-5422); there 

was also granite emplacement during some of these 

times. In the compiler's judgement, these events do not 

justify classing the basement rocks of the southeastern 

part of the craton as an extension of the proposed Elsonian foldbelt of the shield.7

The youngest Precambrian metamorphic and plutonic 

rocks south of the Canadian Shield are in the southeastern part of the United States, most of which are 

broadly correlative with those of the Grenville foldbelt. 

In the Appalachian foldbelt, rocks yielding dates of 

about 1,000 m.y. (LI) form the basement of the Paleozoic miogeosynclinal and eugeosynclinal sequences and, 

where present, that of the Upper Proterozoic supracrustal strata (L2) (King, 1969, p. 61-62); they emerge 

along the axis of the foldbelt in the Long Range of Newfoundland, the Green Mountains and other uplifts in 

New England, and in the Blue Ridge as far south as 

North Carolina. They can be extended westward by 

drill data to a well-defined boundary (probably a prolongation of the "Grenville front" of the shield) that 

is traceable southward from Michigan, through Ohio, 

into Kentucky (Bass, 1960).

Rocks affected by this event are recognizable again 

in Texas, where the Llano orogeny of the Llano uplift 

has been dated at 1,000 to 1,150 m.y. (Muehlberger and 

others, 1966, p. 5422-5423); comparable dates have been 

obtained from nearby drill samples and from west 

Texas outcrops (Wasserburg and others, 1962). Apparently the belt continues southward with much the 

same trend into eastern Mexico, although details are 

uncertain because of sparse outcrops (which are labeled 

O2 on the map, a symbol used elsewhere for rocks affected by the Hudsonian orogeny). Precambrian inliers in Tamaulipas have yielded a minimum date of 

770 m.y., and those in Hidalgo a date of 1,210 m.y. The 

metamorphic complex in Oaxaca in the far south has 

yielded nine or more dates with a scatter between 700 

and 1,100 m.y. (Fries and others, 1962, p. 45-52; Fries 

and Rincon-Orta, 1965, p. 81-87). The Precambrian 

age of the metamorphic rocks in Oaxaca is further

7 W. R. Muehlberger has contributed a dissenting comment (written 

commun., April 1968) : "All the events yon cite are on volcanic rocks 

and their associated shallow granitic intrnsives which are clearly 

younger than the basement on which they rest or into which they were 

Intruded. This basement almost everywhere in the region from New 

Mexico eastward to Ohio has Isotopic ages of 1,350-1,450 m.y. This 

basement I would include within the Eteonlan of the exposed Canadian 

Shield. This is the event which we named the Nemaha to distinguish it 

from the Elsonian inasmuch as there is no clear connection between the 

two because of the later Grenville events."

attested by the recent discovery of unconformaHy overlying fossiliferous strata of Late Cambrian and younger 

Paleozoic ages (Pantoja-Alor and Robison, 1967).

There was thus a wide and lengthy belt along much 

of the southeastern edge of North America that was 

subjected to metamorphism, plutonism, and prcbably to 

orogeny about 1,000 m.y. ago, or at a time comparable 

to the Grenville orogeny and the formation of the Grenville foldbelt in the southeastern part of the Canadian 

Shield (fig. 10). This was during a part of Precambrian 

time when the remainder of North America to tl <». northwest had been largely stabilized, and was receiving only 

supracrustal cratonic or geosynclinal deposits.

Some final Precambrian events deserve notice, although they are inadequately represented on the "Tectonic Map of North America" (data regarding them 

were received too late for inclusion). An Avalonian 

event (or orogeny) is now well documented in southeastern Newfoundland (Rodgers, 1967, p. 409-410; 

Poole, 1967, p. 14-17), where the Holyrood Granite 

(La) intrudes the Harbour Main Volcanics and these 

are succeeded by as much as 6,000 m (20,000 ft) of Upper Proterozoic strata that are topped by foseiliferous 

Lower Cambrian; the granite has yielded a radiometric date of 575 m.y. (McCartney and others, 1966). 

Comparable very late Precambrian events may have 

occurred in the basement rocks below the Lover Cambrian in the Maritime Provinces and possibly in southeastern New England (Isachsen, 1964, p. 812-816), although on the tectonic map these rocks are shown either 

as Grenville metamorphics (LI) or as Paleozoic plutonics (L8). Dates comparable to those of the Holyrood 

Granite have been obtained from drill samples near 

the Atlantic Coast in North Carolina (Denison and 

others, 1967), and perhaps in Florida (fig. 10). Farther 

northwest in North America, the only significant very 

late Precambrian to early Paleozoic events were the 

Wichita igneous activity (A3), referred to earlier (p. 

22, 40) and the unconformity between the Belt and 

Windermere Series and their equivalents (O3 and O4) 

in the northern part of the Cordilleran fokHMt.

The Avalonian event, although scantily represented 

in North America, is of interest because it is nearly 

contemporaneous with the Pan-African or Damaran 

orogeny, dated at 450-550 m.y. ago, during which many 

foldbelts throughout Africa were deformed and metamorphosed (Kennedy, 1964; Clifford, 1967). An orogeny of about the same age is also reported in the Precambrian rocks of the Atlantic coastal area in Brazil.

PRECAMBRIAN OF NORTHERN SOUTH AJTERICA

Brief mention should be made of the Pre".ambrian 

of the Guyana Shield in Venezuela and adjacent countries of northern South America, whose edge projects
PHANEROZOIC FOLDBELTS 43

into the southeastern corner of the "Tectonic Map 

of North America." The part of the shield shown in the 

map consists of metamorphic and plutonic rocks, from 

which radiometric dates of 2,000-2,500 m.y. have been 

obtained (McConnell and others, 1964), hence are 

classed as of Archean age and part of the Kenoran foldbelt on the tectonic map (G2 and Gy). Other dates, both 

older and younger, have been reported from the 

crystalline rocks of various parts of the Guyana Shield, 

outside the area shown on the tectonic map. To the 

southeast, also outside the area shown on the map, the 

crystalline rocks are overlain unconformafoly by the 

nearly undef ormed sandstones of the Roraima Formation, once thought to be of Paleozoic or Mesozoic age. 

However, mafic sills in this formation have yielded 

radiometric dates of 1,700 m.y. (McConnell and others, 

1964), showing that it is likewise Precambrian and is 

of Lower Proterozoic age. The prolonged stability of 

the Guyana Shield, as indicated by these data, is 

noteworthy.

Northwest of the shield, in the Venezuelan Llanos, 

nearer the front of the Andes, small areas of crystalline 

rocks (Nl and N/3) project near El Baul, but are probably all of Paleozoic rather than of Precambrian age; 

Cambrian fossils have been found in some of the 

phyllites, and the intrusive granites have yielded 

radiometric dates of 270 m.y.

PHANEROZOIC FOLDBELTS

In North America, the foldbelts of Phanerozoic age 

lie farther away from the center of the continent than 

the Precambrian shield and its bordering platforms, and 

nearer the surrounding oceans. The continent is thus 

almost ideally symmetrical; however, this apparent 

symmetry may not be a valid generalization, as it is 

much less perfectly expressed, if at all, in other 

continents.

The Phanerozoic foldbelts evolved through time from 

early in the Paleozoic to the present, the evolution of 

some of them having been completed long ago, of others 

more recently, whereas in a few the evolution still continues. Their sedimentary and tectonic histories become 

plainer the younger their age, but the histories of all 

of them are better preserved than those of the Precambrian foldbelts. This makes it possible to classify their 

components in more detail than those of the Precambrian, but it creates correspondingly greater problems 

as to how to represent them on the "Tectonic Map of 

North America."

THE TECTONIC CYCLE

All foldbelts evolved through time, passing through 

what is here termed a tectonic cycle, consisting of preorogenic, orogenic, and postorogenic phases. These cycles consolidated originally mobile parts of the crust into 

cratons, the structures formed during successive phases 

changing from alpinotype to germanotype in the s^iase 

of Stille. The orogenic phase was the climax of mobUity 

in the foldbelt, when the folds, faults, and plutonic 

structures that were produced exceeded in magnitude 

and complexity any that were produced before or after.

The rocks that formed in the foldbelts during the 

tectonic cycles also changed with time. During the errly, 

or preorogenic phase, the sites of most foldbelts v^ere 

geosynclines, or tectonically unstable linear trougl s in 

which sediments and volcanics accumulated, commonly 

to a greater thickness than in the adjoining cratons. 

Preorogenic and orogenic phases of the foldbelts overlapped; parts of the geosynclines were being oro^-^nically deformed while other parts were still receiving: deposits. The net effect of the orogenic phase was to reduce 

the areas of accumulation of supracrustal rocks and to 

break the earlier broad geosynclinal tracts into irore 

localized basins, many of which were surrounded by 

areas of much topographic relief; localized deposition 

continued into the postorogenic phase, partly in faultblock depressions. In most foldbelts, marine deposition 

decreased progressively from the orogenic into the postorogenic phase, with a corresponding increase in 

nonmarine deposition. A still later phase occurs in the 

Phanerozoic foldbelts that formed earliest; they became 

so stable that their truncated surfaces were overspread 

by platform deposits.

During the tectonic cycle the foldbelts also underwent 

a magmatic evolution. During the preorogenic phase 

volcanics were erupted in submarine environment^ in 

parts of the geosynclines; during the orogenic and p ostorogenic phases the environment of volcanism became 

progressively more terrestrial; and during the Hter 

phases in many of the foldbelts the deformed bedrock 

was nearly or wholly concealed by broad sheets of volcanic products surmounted by chains of volcanic cones. 

Deep-seated crustal activity, of which the volcanism 

may be a surface manifestation, reached its climax 

during the orogenic phase, when various suites of 

plutonic rocks were emplaced, some in great volume. 

During the postorogenic phase plutonic rocks v^ere 

emplaced in lesser volume, but they intruded both the 

original geosynclinal tracts and the adjacent disrupted 

parts of the original cratonic areas.

The foregoing paragraphs summarize the tectonic 

cycles that have been observed in many of the foldbelts* 

of the world, details of the history and terminology 

being purposely suppressed so as to give the sumrrary 

its widest possible application. More elaborate vers: ons 

of the cycles have been proposed, but these obviously 

apply only in places; even the present version may be
44 THE TECTONICS OF NORTH AMERICA

too particular. There are no "invariable laws" governing the evolution of foldbelts, as has sometimes been 

claimed; the histories of many of the foldbelts fail to 

conform, in a few or in many respects, to the classical 

concepts.

Nevertheless, tectonic cycles of the kind outlined occurred widely enough hi North America, and are sufficiently similar from one foldbelt to another to serve 

as a frame on which to build the classifications adopted 

on the "Tectonic Map of North America." Although the 

cycles in each of the foldbelts are thus broadly similar, 

they and the rocks that formed during the cycles are 

sufficiently different from one to another to warrant 

listing them separately in the legend rather than 

grouping map units together in foldbelts of the same 

general age, as has been done on some other tectonic 

maps.

TERMINOLOGY

Many tectonic terms are afflicted with confusing 

usage, and the terms "geosyncline" and "orogeny" used 

in the preceding summary are no exception, hence they 

require further explanation.

GEOSYNCLINE

The term "geosyncline" was first used in slightly different form by J. D, Dana in 1875, but the concept itself 

originated with James Hall in 1859. Since then, the 

term "geosyncline," the concept which it expresses, and 

the varieties of both, have undergone vast proliferation 

and mutation, as ably set forth by Glaessner and 

Teichert (1947). Of particular interest are the terms 

and concepts proposed by Stille, in several publications 

(1936b, 1940, and others), which were applied in expanded form to North America by Kay (1951); some 

of this terminology, especially the names "eugeosyncline" and "miogeosynoline" are discussed hi the following section (p. 47-49).

The compiler regards geosynclines as tectonically unstable linear troughs in which sediments and volcanics 

accumulated, commonly to a greater thickness than in 

the adjoining cratonic areas, and the term is so used 

throughout this account. It is therefore applied to the 

"orthogeosynclines" of Stille and Kay, and not to the 

"parageosynclines" of those authors, which are basins 

of sedimentary accumulation that formed in the cratons 

under conditions of greater tectonic stability.

Even the tectonically unstable linear troughs that are 

here considered to be "geosynclines" have a wide variety 

of characters and forms, some of which are mutually 

contradictory because they apply to different kinds or 

to different parts of geosynclines. Thus, a more precise 

definition of them would not only be difficult, but misleading and unduly restrictive. Some years ago the compiler (King, 1959, p. 56-57), rather than undertake a 

precise definition, listed ten "geosynclinal attributes," 

which were intended to encompass these variations.

In the preceding section (p. 43), it was stated that 

the preorogenic phase of the tectonic cycle in a foldbelt 

was also commonly a geosynclinal phase, and tH inevitability of a geosynclinal phase as a prelude to orogeny 

has been proclaimed by many geologists, beginning with 

James Hall in his first statement of the concept. According to de Sitter (1956, p. 351), geosynclines ar^ "those 

accumulations of sediments of great thieknes^ which 

have beien severely folded." Other geologists 1 <vve attemped to set a critical limit to the thickness of sediments that could accumulate before the inevitable 

orogeny began for example, about 12,000 m (40,000 ft) 

(Knopf, 1960, p. 132-133).

No such inevitability was intended by the compiler, in 

his outline of the geosynclinal concept presorted above, 

and it appears to be unlikely; again, there are no 

"inevitable laws" governing the evolution of foldbelts. 

In the northern Cordillera the Middle and Upper 

Proterozoic Belt and Windermere Series accumulated 

to a thickness that far exceeds the figure cited, yet they 

remained virtually undeformed until the orogenies of 

Mesozoic time. The Mesozoic and Cenozqic sedimentary 

accumulation along the Gulf Coast likewise far exceeds 

this figure, yet the region remains one of very low mobility. Where accumulation of geosynclinal sediments 

did closely precede the orogeny in a foldbelt, neither the 

formation of the geosyncline nor the succeeding orogeny 

were a result of the sedimentary loading itself; instead, 

the creation of a trough capable of receiving a thick 

accumulation of sediments was an early phas^ of the 

mobility of the foldbelt.

OROGENY

Kegarding the meaning of the term "orogeny," it is 

instructive to quote the views of several earlier geologists (see Dennis, 1967, p. 112-113, for summary of early 

literature). According to Gilbert (1890, p. 340), who 

was one of the earliest to use "orogeny" in a technical 

sense:

The displacements of the earth's crust which produce mountain ridges are called orogenic. For the broader displacements 

causing continents and plateaus, ocean beds and continental 

basins, our language affords no term of equal corvenience. 

Having occasion to contrast the phenomena of the narrower 

geographic waves with those of the broader swells, I shall take 

the liberty to apply to the broader movements the adjective 

epeirogenic * * *. The process of mountain formation is orogeny, the process of continent formation is epeirogeny, and the 

two collectively are diastrophism. It may be that orogenic and 

epeirogenic forces and processes are one, but so lonp at least 

as both are unknown it is convenient to consider them 

separately.
PHANEROZOIC FOLDBELTS 45

Soon after, the concept of "orogeny" and "epeirogeny" became encrusted with further connotations. 

According to Upham (1894, p. 385) :

Gilbert has recently supplied to our science the terms epeirogeny and epeirogenic, to designate the broad movements of uplift 

and subsidence which affect the whole or large parts of continents and the oceanic basins. Previously the terms orogeny and 

orogenic had come into use, denoting the process of formation 

of mountain ranges by folds, faults, upthrusts and overthrusts, 

affecting comparatively narrow belts and lifting them in great 

ridges, while epeirogenic movements of the earth's crust produce 

and maintain the continental plateaus and the broad depressions 

which are covered by the sea.

These early statements illustrate a fundamental 

dichotomy that has plagued the term "orogeny" to this 

day between the processes which produced the rock 

structures within the mountain chains and the processes 

which produced the mountainous landscapes. It was not 

fully realized until later that most of the present mountainous landscapes were not caused directly by the processes that formed the rock structures, but were caused 

instead by the differential erosion of broadly uplifted 

masses of deformed rocks. Stille ( 1936a, p. 850-851) has 

well expressed the resulting dilemma:

As a matter of fact, orogeny in the tectonic sense generally 

fails as an explanation for the existence of the topographically 

great mountains of the earth, such as the Alps of Europe or 

the Cordilleras of North America. These mountains exist or 

still exist as a result of post-orogenic en bloc movements, for 

the most part still going on, and, belonging to the category of 

epeirogenic processes. Thus arises the terminologic contradiction, that the mountains as we see them today owe their origin 

not to what is called orogeny, but to an entirely different type 

of movement that is to be strongly contrasted with the orogenic 

process. Orogeny and topographic mountains are indirectly connected in the sense that the orogenic units, at least in the beginning, coincide with the units of epeirogenic movement. Thus the 

Alps are a unit not only morphologically but also in relation to 

orogenic history.

Stille's remarks reflect the conception of the term 

"orogeny" prevalent among geologists today, and in 

particular among the makers of tectonic maps. The futility of attempting to revert to Gilbert's sparse and 

uninformative definition is amply illustrated by the results which modern geologists have obtained when they 

applied his concept (Gilluly, 1966, p, 98). In this account, and on the legend of the "Tectonic Map of North 

America," "orogeny" is therefore used for the processes 

by which the rock structures within the mountain 

chains or foldbelts are created.

NAMES OF OROGENIES

The orogenic times in the foldbelts that is, the 

times of formation of the rock structures have been 

individually named, but here again there has been confusion of usage whether these names should be applied 

to rather brief episodes, of deformation, or to times of

prolonged orogenic activity that group together m?ny 

-episodes of shorter duration, or whether orogeny has 

been so nearly continuous through time in the foldbelts 

that specific names are unjustified.

According to common European usage (expresred, 

for example, on the "Tectonic Map of Europe," Scratsky, 1962), Phanerozoic time in that continent vas 

marked by four orogenic times, or "eras" the Assyrian (Baikalian), Caledonian, Variscan (Hercynian), 

and Alpine, which are, respectively, of late PrecsMnbrian to earliest Paleozoic, of early Paleozoic, of late 

Paleozoic, and of Mesozoic and Cenozoic ages. As a 

result of more detailed analyses, Stille (see for example, 

Stille, 1936b, p. 851-854; Knopf, 1948, p. 652-657) has 

distinguished 40 orogenies during Phanerozoic time, 

most of them being classed .as phases of the broader 

orogenic times. While most of Stille's orogenies were 

based on European field examples, he supposed that each 

was world-wide in its effects.

In North America, various major orogenies have b°.en 

commonly recognized for example, the Taconian, A cadian, Appalachian (more properly Allegheny), Nevadan, and Laramide, which are, respectively, of early 

Paleozoic, of middle Paleozoic, of late Paleozoic, of 

middle Mesozoic, and of late Mesozoic to early Tertiary 

ages. However, opinions have varied as to their meaning whether they represent broad orogenic times like 

those in Europe, or specific episodes of short duration. 

Based on the latter concept, European geologists have 

regarded the North American orogenies as phases of 

their own orogenic eras. On the same basis, North 

American geologists have added other named orogeries 

not of the same age as the five mentioned, or named 

orogenies of the same age which occur far from the 

type areas of those mentioned; a formidable list of 

named orogenies could be assembled from the North 

American literature. Other North American geologists 

regard orogeny in the foldbelts as nearly continuous 

through large parts of Phanerozoic time, although probably episodic at any single locality (Gilluly, 1965, p. 

21).

The compiler (1955b, p. 737-738) regards specific 

times of orogeny as episodes within the orogenic 

phases of the tectonic cycles which created the foldbelts. 

He doubts the correctness of the proposition that orogeny has been nearly continuous through the life of the 

foldbelts; instead, orogeny that produced actual rock 

structures (rather than ephemeral mountainous topography) was concentrated in a succession of episodes 

during each orogenic phase. Individual episodes were 

certainly not world-wide, and may not have been extensive even within the limits of a single foldbelt; names 

for such episodes are appropriate only for local p fir-
46 THE TECTONICS OF NORTH AMERICA

poses. Of broader interest are the orogenic times which 

are natural groupings of these episodes. The compiler 

believes that many of the traditional names for North 

American orogenies (like the Taconian and others, 

previously mentioned) apply most appropriately to 

such orogenic times, and has so used them on the legend 

for the tectonic map; they are thus comparable in scope, 

but not in age span, to the orogenic times in Europe 

(the Caledonian and others, previously mentioned, p. 

45). Although such orogenic times, real or fancied, are 

no more than gross generalizations of many complex 

events, the names used for them are useful for expressing the general geological ages of the tectonic features.

SEDIMENTARY UNITS 

GEO8YNCLXNAX, DEPOSITS

The older sedimentary units of the Phanerozoic foldbelts formed during the preorogenic phase of the tectonic cycle in geosynclines or troughs of accumulation 

that were much more extensive than those of the succeeding phases; various geosynclinal units are thus 

represented on the "Tectonic Map of North America." 

Nevertheless, the deposits and the troughs in which they 

formed are complex features; it is thus misleading to 

speak of a single Appalachian geosyncline or a single 

Cordilleran geosyncline. The nature of the deposits in 

one part of a geosynclinal trough varied with time, different segments of a single trough had contrasting histories, and quite different troughs developed through 

time in different parts of the longer foldbelts. Because 

of the small scale of the tectonic map, not all these complexities can be represented in the units that have been 

adopted.

In most of the foldbelts, a longitudinal subdivision 

can be made on the map between contrasting suites of deposits in the eugeosynclines and miogeosynclines. These 

terms, proposed by Stille" (1940), are now widely used 

for the features in question in Europe, North America, 

and elsewhere. They supplant the same author's earlier 

terms "pliomagmatic" and "miomagmatic" geosynclines 

(1936b), and correspond broadly to the terms "internides" and "externides" of some other authors internal 

and external referring in this case to the parts of the 

foldbelts nearer to their central axes or farther away 

from them, rather than nearer to or farther away from 

the bordering cratons. (For a useful discussion of terms, 

see Knopf, 1960, p. 127-129; Kay, 1967).

As conceived by Stille", the pliomagmatic or eugeosynclines were marked by strong volcanism during the 

early phases and by synorogenic plutonism during the 

later phases. Miomagmatic or miogeosynclines were 

more peripheral, with lesser magmatism and mobility, 

and were commonly little deformed until the orogeny

in the adjoining eugeosynclines were nearly completed. 

The differences in magmatism and mobility have produced contrasting deposits those of the eugeosynclines 

being principally submarine volcanics, cherts, slates, and 

gray wackes; those of the miogeosynclines being principally carbonate rocks, quartzites, and shales.

Use of the terms "eugeosyncline" and "mio^eosyncline" for these features creates the largely unwarranted 

implication that they were, in fact, separate geosynclinal troughs. This impression is enhanced by the fact 

that through lengthy segments of the foldbelts the two 

sets of deposits are separated by structural boundaries 

In many parts of the Appalachian foldbelt the two lie 

on opposite sides of a welt of basement rocks (tH Long 

Range in Newfoundland, the Green Mountains and 

others in New England, the Blue Ridge farther south). 

In other places one set of deposits lies agains*., or is 

carried over the other set along major low-range thrusts 

(for example, the Taconic thrust in New York State, the 

Roberts thrust in Nevada), both sets consisting of sequences of nearly identical age span, but with radically 

different facies. In a few other places, however, transitional deposits can be traced from one into the other 

through the complex structures that were superposed on 

them later, relations which suggest that in general the 

two kinds of "geosynclines" were actually different 

parts of a single trough.

Many geologists have attempted to use the lithologic 

features mentioned as absolute criteria for distinguishing eugeosynclinal from miogeosynclinal deposits. In 

some areas the two kinds of deposits can be separated 

clearly by the lithologic criteria alone; nevertheless, the 

distinctions are blurred elsewhere, or deposits of the 

two kinds occur in different parts of the same sequence. 

Triimpy (1960, p. 899) describes the problems of nomenclature in the central and western Alps:

Distinction between eugeosynclines and miogeosynclines * * * 

may be controversial. Volcanism cannot be used as the only 

criterion; the Triassic of the Dolomite Mountains is a good example of miogeosynclinal deposition with volcanic mate Hal, the 

Bundnerschief er of the Pratigau exemplifies eugeosynclinal sedimentation without volcanism. Should Flysch formations be 

classed as miogeosynclinal or eugeosynclinal? They develop over 

mio- and eugeosynclines alike; they have no volcanic materials, 

but conditions of relief and of contemporaneous mobility approach those of true eugeosynclines.

Clearly, the lithologic criteria are simply manifestations of the respective tectonic environments. Thus, distinctions between eugeosynclines and miogeosynclines, 

and between their respective deposits, must be r^ade in 

broadest terms, after considering the gross history and 

pattern of the particular foldbelt.

These considerations were used in classifying the 

eugeosynclinal and miogeosynclinal deposits on the
PHANEROZOIC FOLDBELTS 47

"Tectonic Map of North America"; hence, the classifications made rely heavily on the judgment of the 

compiler, not only as to the history and nature of the 

different sequences, hut as to what extent these require 

generalization in order to he shown on a map of this 

scale. Even so, a few sequences seem not to be clearly 

assignable to either category, and are labeled simply 

as "geosynclinal deposits." Where differentiation is 

made on the map, the two kinds of deposits are shown 

in different shades of color, the stronger shades being 

used for the eugeosynclinal deposits.

During the orogenic phases the eugeosynclinal and 

miogeosynclinal rocks were subjected to contrasting 

styles of deformation, resulting from their differing 

environments and rock compositions; these are reflected 

by /the different kinds of symbols that are needed to 

represent their structures on the tectonic map. The 

eugeosynclinal rocks were deformed in a deep-seated 

environment to the accompaniment of much metamorphism and plutonism, and generally as a mass because their monotonous sequences contain few units 

of contrasting competence. The miogeosynclinal rocks 

were deformed in shallower environments, where their 

stratified units of contrasting competence were deformed disharmonically. The deformation was accomplished mainly by lateral thrust from the internal or 

eugeosynclinal parts of the foldbelts, assisted in places 

by gravitational forces. Deformation terminated rather 

abruptly along a structural front, where the thick miogeosynclinal sequences gave place to the thinner sequences in the craton or foreland.

It has thus come about that large parts of the eugeosynclinal rocks were plastically folded, with little or 

no breakage and few contemporaneous faults; most of 

their breakage was along high-angle faults that were 

superposed later. On the tectonic map the pattern of 

the folding in the eugeosynclinal areas is indicated by 

trend lines, although their culminations into broader 

anticlinoria and synclinoria are indicated by axial symbols. The miogeosynclinal rocks, by contrast, were deformed piecemeal, according to the competence of their 

stratified units. They were thrown into long, nearly 

parallel anticlines and synclines, commonly asymmetrical away from the internal part of the foldbelt, and 

were broken by low-angle to high-angle faults that were 

thrust in the same direction, all the structures at a given 

stratigraphic level being based on a surface of decollement in one of the incompetent strata. On the tectonic 

map the structures of the miogeosynclinal areas are thus 

represented by variously crowded folds and thrust 

faults, the anticlines being shown by spindle symbols 

whose width and spacing expresses the intensity of the 

deformation.

EUGEOSYNCLINAL DEPOSITS

Eugeosynclinal deposits occupy large areas in both 

the Appalachian and Cordilleran foldbelts ( L3, L4, 

L5; O6, O7), and they also occur in the Innuitian, Pacific, and Antillean foldbelts (Kl, PI, P2, Q2, Q4a). In 

the Ouachita foldbelt the early Paleozoic (pre-Mississippian) deposits have many eugeosynclinal characters, 

but these are overlain by a thick body of late Paleozoic 

flysch; because of the small outcrop areas of each, they 

are not divided on the map (M). The East Greenland 

foldbelt seemingly lacks eugeosynclinal depositr at 

least within the present limits of exposure.

As already implied, deformation began in the eugeosynclinal areas during the geosynclinal phase itself, 

and a succession of pulses led up to the orogenic climax. Some of these pulses are sufficiently prominent to 

warrant indication on the tectonic map. In the northern 

part of the Appalachian foldbelt the Cambrian and 

Ordovician eugeosynclinal deposits that were deformed 

by the Taconian orogeny (L3) are separated fronr the 

Silurian and Devonian eugeosynclinal deposits that 

were deformed by the succeeding Acadian orojyeny 

(L4). In the Cordilleran foldbelt a separation is made 

between older eugeosynclinal rocks that were deformed 

by several Paleozoic orogenies (O6) and the younger 

eugeosynclinal rocks that were deformed only by the 

middle Mesozoic (Nevadan) orogeny (O7). However, 

in parts of the eugeosynclinal areas regional rr°itamorphism has been so great that the stratigraphic record is lost; here, the only indication of multiple orogenic pulses is afforded by a scatter of radiometric d^.tes, 

and the rocks and areas affected by these pulses cannot 

be separately mapped.

MIOGEOSYNCLINAL DEPOSITS

The miogeosynclinal areas are those parts of the geosynclines nearest the cratons. The deposits of the miogeosynclinal and cratonic areas are much alike, except 

that the sequences in the former are thicker and more 

complete. In many places, the boundaries between the 

two sequences were sufficiently fixed through time as 

to suggest that a hinge line of structural origin had 

been established there early in the tectonic cycle, and 

persisted later. Thus, the edges of the miogeosynclinal 

areas in many parts of the Appalachian and Cordilhran 

foldbelts are marked by a wedging out of the Lower 

and Middle Cambrian deposits, so that Upper Ombrian or even younger deposits were the first laid down 

on the adjacent craton; moreover the position of this 

wedge-out corresponds closely to that, of the wedging 

out or thinning of many of the succeeding units. Ne^ertheless, the boundaries between the miogeosynclinal 

and cratonic areas fluctuated somewhat with time, r*aking it difficult to draw a firm line between them or the
48 THE TECTONICS OF NORTH AMERICA

tectonic map. On the map, the contact is arbitrarily 

shown in most places at the outer limit of strongly 

folded and faulted rocks, on the assumption that this 

change in style of deformation was caused by a change 

from a thick to a thin column of sediments. Boundaries 

drawn on such structural criteria do not necessarily 

correspond to boundaries indicated by stratigraphic 

criteria; very likely some miogeosynclinal rocks that 

escaped deformation are shown as cratonic on the map, 

and some cratonic rocks that were deformed are showp 

as miogeosynclinal.

The sequences in the miogeosynclinal areas are generally conformable. Most gaps in the record are along 

surfaces of disconformity; in only a few places are there 

angular discordances produced by significant deformation prior to the final orogeny. Nevertheless, miogeosynclinal deposition was much influenced by orogenic 

events that were taking place in the adjoining eugeosynclinal areas. In many miogeosynclines, initial quartzose 

elastics are followed by thick masses of carbonate rocks, 

indicative of general crustal quiescence. These are succeeded, however, by wedges of clastic rocks that taper 

away from the eugeosyncline, across the miogeosyncline 

and onto the craton, the first elastics being fine-grained 

and marine, the latter coarse-grained, largely continental, and partly coal bearing. The fine-grained elastics 

are probably related to the beginnings of orogeny in the 

eugeosynelinal area, the coarse-grained elastics to the 

orogenic climax or to postorogenic phases.

The successive units in the miogeosynclinal sequences 

are "structural stages" in the sense of European geologists, the upper clastic units corresponding to their 

"flysch" and "molasse." It has even been claimed that 

"flysch" and "molasse" are invariable sedimentary products of orogenies, hence that where they are absent there 

could have been no orogeny. However, the meanings of 

"flysch" and "molasse" in this broad sense are so ambiguous that in the present account the deposits which 

have been so called are mostly referred to in other terms. 

It would seem desirable to restrict "flysch" and "molasse" as far as possible to their original meanings.

Where first used in the Alps of central Europe, 

"flysch" and "molasse" apply to distinctive rock-stratigraphic units that are related to the tectonic evolution 

of tiie area. The original "flysch" is a sequence of distinctively interlay ered sandstones, mudstones, and marls 

that seemingly accumulated in relatively narrow, deep, 

rapidly subsiding troughs. Nearly identical deposits occur elsewhere in the world (including the Appalachian 

and Ouachita foldbelts of North America), but large 

parts of the fine-grained clastic wedges of the miogeosynclinal sequences have few of the characteristics of 

"flysch." The original "molasse" is a less distinctive

younger sequence of marine and brackish water sandstones, with some layers of gravelly or boulder^ conglomerate ("nagelfluh"). Elsewhere, "molasse" ha" been 

used indiscriminately for almost any kind of pc^torogenic deposit, such as shallow-water marine sandstones 

and limestones, coal measures, red beds, or the terrestrial gravel filling of intennontane basins thus deriving the term of any specific meaning.8

Many of the miogeosynclinal areas in North Anerica 

have sequences of the sort described, although their age 

spans vary widely from one foldbelt to another. In the 

Appalachian foldbelt miogeosynclinal deposition (L6) 

began in the Cambrian or even earlier; carbonate deposits were succeeded by fine-grained elastics as early 

as the Middle Ordovician in some places and f s late 

as the Silurian and Devonian in others. In the Eocky 

Mountains of Canada and in parts of the Great Basin 

of the United States, miogeosynclinal depositior (O9) 

began as early as in the Appalachians, but carbonate 

deposits were not succeeded by fine-grained elastic^ until 

Triassic tune or later. In eastern Mexico the span of 

miogeosynclinal deposition (O9) was entirely in the 

Mesozoic; the carbonate deposits are Jurassic and Lower 

Cretaceous, and the fine-grained elastics are Upper 

Cretaceous.

Not all the miogeosynclinal deposits in North America 

have so ideal a sequence of units. In some, carbonate deposits and clastic wedges are repeated, in others clastic 

wedges overlie cratonic deposits with the first Hlf of 

the geosynclinal sequence unrepresented, in still others 

the lithologies of all the subdivisions differ from any of 

those described. In the Great Basin of eastern ITevada, 

for example, Cambrian to Devonian carbonates r.re followed by a middle Paleozoic clastic wedge derive! from 

the Antler orogenic belt to the west, which is top-ied in 

turn by upper Paleozoic carbonates.

Comment has already been made (p. 11,20) on the difficulty of mapping "structural stages" or other rock 

units in strongly deformed areas, and this applies with 

special force to the miogeosynclinal belts. On the "Tectonic Map of Mexico" (de Cserna, 1961) on a wale of 

1:2,500,000, units of miogeosynclinal deposits lil ° those 

in the ideal sequence described above are separately 

mapped, but this is much, less feasible of the "T^tonic 

Map of North America" on half the scale, either for 

Mexico or elsewhere (figs. 5 and 6). These difl^ulties 

are multiplied in the more complex sequences th^t have 

been cited. Consequently, on the "Tectonic Map of 

North America" the miogeosynclinal deposits in each 

foldbelt are shown as single units, thus grossly ^neral-

 Some European geologists have attempted to provide for these 

alleged varieties of "molasse" by using the word in a plural form, bat this has ridiculous connotations when transferred to American English. 

Thus, "intennontane basins filled with molasses" would mem, to filled with th< "' ' ----- 

or pancakes 1
DEPOSITS OF SUCCESSOR BASINS 49

izing their many details of composition and age; however, some explanation of these details is given in the 

captions of the legend.

METAMOBPHISM OF GEOSYNCLINAL DEPOSITS

In many of the Phanerozoic f oldbelts of North America the geosynclinal rocks have been metamorphosed to 

varying degrees, especially in the eugeosynclinal areas; 

the rocks that are strongly metamorphosed are distinguished on the tectonic map by an overprinted pattern in red from those that are weakly metamorphosed 

or umnetamorphosed. In places, the strongly metamorphosed rocks pass along the strike into little metamorphosed rocks. Metamorphism thus increases 

southwestward in the eugeosynclinal rocks of the 

Appalachian foldbelt, and a boundary is indicated in 

northern New England that is determined by the garnet 

isograd; this is not a stratigraphic boundary, because 

units of equivalent ages extend across it from the little 

metamorphosed into the strongly metamorphosed rocks. 

Outside of New England, regional data on the metamorphic grades of the geosynclinal rocks are so incomplete that portrayal of the strongly metamorphosed 

rocks has had to be done on a relative basis, and is 

probably inconsistent from one area and from one 

foldbelt to another.

The regional extent of metamorphic rocks of different 

grades in North America is now being studied by geologists of the U.S. Geological Survey and Geological 

Survey of Canada, as part of a project for the "Map of 

the Metamorphic Rocks of the World," so that great 

improvements in representation of metamorphic rocks 

on tectonic maps can be anticipated at a later time.

In parts of the Cordilleran foldbelt additional units 

are distinguished as metamorphic complexes (Ol). In 

the northern Cordillera of Canada and Alaska they 

generally occur in the median part of the foldbelt. In the 

far south they form much of the Sierra Madre del Sur 

along the Pacific coast of Mexico and extend eastward 

into Guatemala and Honduras. Earlier geologists interpreted these complexes as an Archean protaxis separating geosynclinal deposits on the two sides, but the 

ages of both the original rocks and their metamorphism 

have since been variously interpreted. Stratigraphic 

evidence indicates that some of the complexes are older 

than less metamorphosed Mesozoic or Paleozoic rocks, 

hence probably include Precambrian, yet they have 

yielded many radiometric ages as young as 150 m.y. or 

less, indicating a long-persistent plutonic and metamorphic activity, a late uplift and unroofing, or both. 

Whatever the ages of the original rocks in the metamorphic complexes, and their subsequent histories, they 

probably were formed by prolonged burial at great 

depths in the crust.

Metamorphic complexes are also indicated on the map 

in the Antillean foldbelt (Ql), especially in the islands 

of Cuba, Jamaica, and Hispaniola. These complexes are 

older than the oldest dated units of the Mesozoic st~atified sequence, but they very likely include frly 

Mesozoic eugeosynclinal deposits.

DEPOSITS OF SUCCESSOR BASINS

In the internal parts of many of the Phanerozoic foldbelts the eugeosynclinal deposits are succeeded by deposits that were laid down in more restricted basins, 

during the orogenic phase and the early part of the 

postorogenic phase; they generally overlie rocks that 

were much deformed by the first strong orogenies of the 

foldbelt. The extent and preservation of such deposits 

vary greatly from one foldbelt to another and from one 

segment to another. Originally, they were probably laid 

down widely in most of the f oldbelts, but they have rmce 

been lost in many places by deep and prolonged erofion.

For the areas in which these deposits accumulated the 

term "successor basins" is used here and on the legend 

of the "Tectonic Map of North America." Some of them, 

notably the area of late Paleozoic deposits in the Maritime Provinces of southeastern Canada, were crlled 

"epieugeosynclines" by Kay (1951, p. 5$-57), but this 

term seems overly complex, and elsewhere Kay has used 

it for features somewhat different from those here discussed. The "successor basins" of Alaska have been 

called "geosynclines" (Gates and Gryc, 1963, p. 269- 

272), but they are much less extensive and much more 

interrupted by nondepositional areas than the true 

geosynclines that preceded them.

In the Appalachian foldbelt the deposits of successor 

basins are late Paleozoic (chiefly Mississippian and 

Pennsylvanian) (L7), and are extensively preserved in 

the Canadian Maritime Provinces. Smaller outliers occur 

in Newfoundland and New England but they are unknown farther southwest. They were laid down rfter 

the climactic middle Paleozoic (Apadian) orogeny, but 

they were in part much deformed by later orogenies 

(L7a). Triassic deposits (L8) extend for much of the 

length of the foldbelt; they are postorogenic and were 

tilted and blocks-faulted rather than folded. In the Cordilleran foldbelt successor basins form nearly half of the 

interior of Alaska; they are large but more separated 

in Yukon Territory and British Columbia. Their deposits are late Mesozoic (latest Jurassic and Cretaceous) 

(O10), and although much deformed are younger than 

the climactic middle Mesozoic orogenies. In the wertern 

United States such deposits are preserved in the interior 

of the foldbelt only in patches too small to map, but 

extensive contemporaneous deposits occur to the east 

and west and are differently classified. In southern
60 THE TECTONICS OF NORTH AMERICA

Mexico the early Tertiary "red conglomerates" have 

tectonic affinities to the late Mesozoic deposits of successor basins farther north, but they are mapped with 

the younger basinal deposits (Oil). The Innuitian 

f oldbelt of the Arctic Islands is partly covered by upper 

Paleozoic, Mesozoic, and Tertiary deposits of the extensive Sverdrup basin (K3), and small basins of Devonian 

deposits occur in the East Greenland f oldbelt (J3).

The deposits in the successor basins in many places 

attain thicknesses of 4,500-9,000 m (15,000-30,000 ft), 

but mostly represent localized, rapid sedimentation. All 

of them include fine to coarse debris derived from previously deformed older rocks of the surrounding highlands, and some of them include granite clasts from 

nearby plutons that were emplaced only a little earlier. 

Marine deposits are more restricted or intermittent than 

earlier, and brackish water or continental deposits are 

correspondingly more extensive. Some areas, such as 

the New Brunswick basin of the Appalachians and the 

Sverdrup basin of the Arctic, contain evaporite beds 

that have been deformed into domes and other diapir 

structures. Most of the sequence in the successor basins 

in Alaska is poorly sorted graywacke, with much volcanic debris and a few thin to thick interbedded lavas. 

The marine sequence is longest and least interrupted 

in the Sverdrup basin, but its deposits have many resemblances to those of a coastal plain; the basin was 

probably open on the northwest toward the Arctic Ocean 

(Tozer,1961,p.400).

YOUNGER BASINAL DEPOSITS

Several classes of younger sedimentary rocks are 

shown in some of the f oldbelts on the "Tectonic Map of 

North America." Basinal deposits of Tertiary age, 

mainly marine and in part very thick, characterize the 

Pacific and Antillean f oldbelts (P4, Q4). Most of them 

were moderately to strongly deformed by orogenies during the Cenozoic. The well-known Tertiary sequence of 

the California Coast Ranges presents a record of shifting basins and uplifts, and of many local episodes of 

deformation that culminated during a few times of 

more general orogeny, the last one in the Pleistocene. 

The records in some of the other sequences, especially 

in the Antilles, are equally complex. On large-scale 

maps it would be desirable to divide these sequences 

into "structural stages," according to their variations 

in lithologic facies and their times of deformation, but 

on the small scale of the "Tectonic Map of North America" few subdivisions are possible.

Eugeosynclinal deposits that are distinctly different 

in nature and origin from the other early Tertiary deposits form areas large enough to map separately along 

the south coast of Alaska, in the Coast Ranges of Oregon

and Washington (P2), and in parts of the Greater Antilles (Q4a), In the Greater Antilles, where orogeny 

was largely completed during the early part of tH Tertiary, postorogenic deposits (Q5) are showr in a 

separate category.

THICK DEPOSITS IK STRUCTURALLY NEGATIVE ABTA8

Another class of younger sedimentary rocks includes 

the terrestrial deposits of Pliocene and Pleistocene age 

(O12) which attain great thicknesses in areas of late 

down warping and downfaulting in the Cordillera n foldbelt. The class does not include river alluvium, terrace 

deposits, or glacial drift, which cover the bedrodh thinly 

in parts of the f oldbelt; such deposits have no tectonic significance and are not shown on the map.

In the Basin and Range province of western United 

States and northern Mexico the older rocks of the foldbelt and the adjacent craton were disrupted by blockfaulting during the latter part of Cenozoic tine, and 

the basins between the ranges were filled with erosional 

debris. These deposits interrupt the continuity of the 

earlier structures in the adjacent ranges, and t\e bedrock beneath them is generally unknown and unmappable. Representation of the deposits on the map serves 

further to indicate the pattern of the very late ("neotectonic") deformation that was superposed on previous 

Cordilleran structures.

In central Alaska, much broader plains ar? interspersed between the mountain ranges, one of the largest 

being Yukon Flats on the upper course of the Yukon 

River. These are thinly to thickly mantled by Pleistocene and perhaps older Cenozoic deposits. The plains 

are partly of erosional origin, as indicated by bedrock 

hills that project through the cover here and tl °;re, but 

to some extent they are products of late erustal downwarping, hence deserve representation as tectonic 

features.

Other thick late Cenozoic deposits are showr on the 

map by the same pattern in the Pacific, Antillean, and 

Andean f oldbelts (P5, Q6, N5). Some of these were 

laid down in intermontane basins like those in the Cordillera, others accumulated on narrow coastal plains. 

Parts of the deposits are terrestrial and reseml le those 

in Cordillera, but they include marine units, especially 

near the coasts. In Central America and sor^e other 

places the deposits include large volumes of pumice derived from surrounding volcanic areas, anc? in the 

coastal areas of the Antilles they include uplifl vl limestone reefs and banks.

VOLCANIC UNITS

Volcanic rocks form large and complex suites in all 

the foldbelts. During the early phases of the tectonic
GRANITIC BOCKS 51

cycle thin to thick masses of submarine volcanics were 

erupted in the eugeosynclinal areas. Their presence here 

is implicit hi the definition of eugeosynclines and they 

are not separated from the other eugeosynclinal deposits on the tectonic map, except for those of early Tertiary age in Oregon and Washington (P2a), which are 

areally extensive. Volcanic rocks that formed during 

the late phases of the tectonic cycle are mostly not preserved in the older Phanerozoic foldbelts, if they ever 

existed there, but they are areally extensive in the 

younger ones. During these late phases, terrestrial volcanic rocks were spread widely in the Cordilleran, Pacific, and Antillean foldbelts, over rocks that had been 

deformed during the climactic orogenies. Most of the 

volcanism in these foldbelts occurred during Tertiary 

time; it began during the Cretaceous in places and has 

continued through the Quaternary in others.

The later terrestrial volcanic rocks pose various practical problems of representation on the "Tectonic Map 

of North America." On the one hand, they are merely 

an inconvenient cover which interrupts the continuity of 

the older dominant structures of the foldbelts, and in 

the more extensive volcanic fields masks them entirely. 

In these more extensive volcanic fields, as in the northwestern conterminous United States and in western 

Mexico, they must be shown if only to indicate that the 

underlying rocks and structures are unknown. On the 

other hand, many small volcanic patches have been 

omitted. Between these extremes are numerous areas 

about which arbitrary decisions regarding representation have been made, as in medium-sized volcanic fields, 

and in the innumerable small remnants of once extensive fields that are now broken up by faulting, folding, 

and erosion.

These later terrestrial volcanic rocks also pose many 

problems of tectonic interpretation and subdivision. 

Certainly all of them have tectonic significance, but 

this significance is not always known or understood. A 

tectonic significance is most apparent for plateau basalts 

that were products of fissure eruptions, for ignimbrites 

(ash-flow tuffs) that were spread widely as clouds of 

incandescent glass, and for andesites that were built into 

eruptive mountain ranges and chains of volcanic cones, 

mainly near the continental borders. A tectonic significance is less obvious for the heterogeneous volcanic sequences that are extensive in many parts of the continent 

and are unclassifiable at present. Even when the theoretical significance of one of the classes of volcanic 

rocks is known, only fragmentary information on its 

areal extent is usually available.

However, the extent of the plateau basalts is well 

known in the Cordilleran foldbelt of Oregon, Washington, and British Columbia (P?), as well as in Greenland, Iceland, and Baffin Island (Ejff). The last three 

occurrences are here classed with the platform deposits 

and have been described under an earlier heading (p. 

27-29). Aside from the plateau basalts, lack of data precludes differentiating the volcanic rocks as to kini or 

origin. Lack of differentiation is partly compensated by 

the use of symbols on the map to represent volcanic 

cones, calderas, and other volcanic structures.

The principal separation among the volcanics that is 

made on the Tectonic Map of North America is between the extensive areas that are mainly of Ternary 

age (O/t, P8, Q8) and the more restricted ones that are 

mainly of Quaternary age (O*-, Pe, Qe). The latter seemingly deserve differentiation on the map because they indicate volcanic activity along lines of very late crustal 

rupture. Eocks of Quaternary age form the volc^mc 

fields of the Snake Eiver Plain in Idaho and of the 

Transverse Belt of southern Mexico, both of which extend across the earlier Cordilleran structures. T^ey 

also form volcanic chains parallel to the prevailing 

structure in the Cascade Range of Oregon and Washington, along the Pacific coast of Central America from 

Guatemala to Costa Rica, and in the island chains of 

the Aleutians and Lesser Antilles. As previously noted 

(p. 29), another belt of Quaternary volcanics extends 

across the center of Iceland (E?).

PLUTONIC UNITS

Embedded in the geosynclinal deposits of the Phanerozoic foldbelts are various kinds of plutonic rocks especially in the internal or eugeosynclinal zones that vere 

deformed in deep-seated environments. They were emplaced by various combinations of magmatic, metosomatic, and tectonic activity as an integral part of the 

tectonic cycles that produced the foldbelts.

GBAKITIC ROCKS

Granites and related felsic plutonic rocks occur in 

large volume in the internal zones of all the foldbelts, 

and are extensively exposed in most of them. They exhibit the widest variety of form, structure, and relation 

to the country rocks in which they lie from migmatitic permeations, through discrete but still concordant 

bodies, to massive, thoroughly discordant bodies. Hie 

compiler regards all these as parts of a granite senes 

in the sense of Read (1957, p. 374-375), or parts of a 

single evolutionary sequence resulting from progressive mobilization of crustal material during the preorogenic, orogenic, and postorogenic phases of the 

tectonic cycle. However, the subject of the granitic rocks 

has many more ramifications and problems then can be 

covered in this brief outline of the concept, and r^ost 

of these are beyond the scope of this account. Moreover,
52 THE TECTONICS OF NORTH AMERICA

some geologists have challenged the concept itself in 

many particulars (for example, Gilluly, 1963, p. 164- 

167; 1965, p. 2&-2S; Hamilton and Meyers, 1967, p. 

17-23). On the "Tectonic Map of North America" it 

has not been possible to distinguish the different structural varieties of the granite series, partly because of 

the scale of the map, partly because of lack of adequate 

regional data.

Granitic rocks are extensively exposed in the internal 

zones of the Appalachian foldbelt (L8, Le), and to a 

smaller extent in the East Greenland, Innuitian, and 

Antillean foldbelts (Jo, Ka, Q£); none are exposed in 

the Ouachita foldbelt, whose internal zone is concealed, 

but there is some suggestion of their existence from subsurface data. By far the most extensive exposures (and 

probably also the greatest volume) of granitic rocks 

occur in the internal zone of the Cordilleran foldbelt 

throughout its length from Alaska to Central America 

(O ,O(9).

Most of the granitic rocks were emplaced during or 

shortly after the main orogenies of the foldbelts, but 

radiometric dating and some stratigraphic evidence indicates that the time of emplacement was more prolonged than previously supposed. In the Appalachian 

foldbelt the main emplacement was during the Devonian (L8), but some granitic bodies are as old as Ordovician and others as young as Carboniferous; the White 

Mountain alkalic rocks in northern New England are 

early Mesozoic (Le), and the small hypabyssal intrusives 

of the Monteregian Hills in southern Quebec are Cretaceous (L0). In the Cordilleran foldbelt some granitic 

bodies in Alaska, Canada, and Mexico are of proved 

Paleozoic age, but the greater part is Mesozoic. The 

Mesozoic batholiths in the Cordillera have been shown 

by detailed study to be internally complex in both composition and age, and to consist of many discrete plutons 

that were emplaced over a long period. The rocks of the 

Sierra Nevada batholith in California vary from west 

to east from quartz diorite to granite and from Middle 

Jurassic to middle Cretaceous, but also include some late 

Triassic plutons on the east (Kistler and others, 1965). 

The Coast batholith of British Columbia has similarly 

been found to be a complex of plutons ranging from 

mafic to felsic and from early Mesozoic to Tertiary in 

age, with many included shreds and ghosts of the original country rocks (Roddick, 1966).

It is questionable whether representation of details of 

the age and composition of the granitic rocks would 

serve a useful purpose on the "Tectonic Map of North 

America," even were complete data available. These are 

facts of historical geology which are more appropriate 

on areal geologic maps, or on tectonic maps of larger 

scale. On the tectonic map, the granitic rocks shown

separately from the rest are those of markedly different age, such as those of early Mesozoic age in ncHhern 

New England (Le), and those of Paleozoic age in the 

Cordillera (O«).

However, in the Cordilleran, Pacific, and Antillean 

foldbelts intrusive and plutonic rocks of Tertiary age 

are separately shown (OA, P£, Qy). They have a wide 

range of structures, composition, and age, but are generally of such small dimensions that a more detailed 

representation is not possible on the map. Many of them 

are plugs, laccoliths, and other hypabyssal bodies, but 

a few are deep-seated plutons that approach the dimensions of those of Mesozoic time. Most of the larger 

bodies were emplaced in the early part of Tertiary time, 

hence have been referred to as "Laramide," but a few 

of them in northern Washington, southern Pritish 

Columbia, and elsewhere are as young as middle 

Tertiary.

The Tertiary intrusives of the Cordilleran foldbelt 

extend well to the east of the plutonic bodies of Mesozoic time, across the miogeosynclinal area, and into the 

reactivated parts of the craton beyond. Large granitic 

plutons of early Teritiary age occur, for example, in 

some of the uplifts of the Southern Rocky Mountains, 

where they have risen to their present position through 

the Precambrian basement. Plutons of this kind have 

had no apparent direct relation to any generation of 

magma in the eugeosynclinal area. Stille has arcribed 

them to "allochthonous synorogenic plutonism fed by 

lateral migration of sialic magma" (Knopf, 1960, p. 

131), but this seems inherently unlikely. They ar^ more 

likely autochthonous, and were derived from deep levels 

of a crust that had been weakened and broken by many 

erogenic events.

MAFIC PLUTONIC BOCKS

Many of the granitic rocks of the foldbelts have more 

mafic phases, and there are occasional discrete plutons 

of gabbro, diorite, and other rocks which are either 

contemporaries of the dominant granitic plutons or of 

slightly earlier ages. On the tectonic map most of these 

are not separated from the granitic rocks, but a few in 

the Appalachian and Cordilleran foldbelts occupy sufficiently large areas to warrant separate representation

(Ly,0«).

UX.TRAMAFIC BOCKS

A very different class of plutonic rocks from any of 

the others comprise the ultramafics peridotites, dunites, and others, now widely serpentinized. They form 

small but numerous bodies in the Appalachian foldbelt 

(L£) and much larger masses in the Cordilleran, Pacific, and Antillean foldbelts (Oy, Pa, Qa). The ultramafic bodies in the Appalachians are small pods and
DESCRIPTION OF FOLDBBLTS 53

lenses that form clusters and chains in the metamorphic 

bedrock of the internal zones. Many of the large bodies 

in California and Cuba are thick, gently dipping sheets 

at the bases of thrust plates. Most of those in Guatemala 

and some in California are nearly vertical tabular bodies lying between high-angle transcurrent faults.

Most geologists have ascribed a significant role to 

the ultramafic rocks in the tectonic processes, but opinions as to what this role might be have fluctuated from 

year to year and from geologist to geologist. A formerly appealing interpretation was that they were intruded into the geosynclinal rocks near the central axis 

of the foldbelt during the first downwarping of the 

tectogene beneath it (Hess, 1939, p. 270-271), but this 

interpretation has lost plausibility along with the tectogene hypothesis itself. Many Mediterranean geologists 

are convinced that ultramafic rocks were extruded as 

magma on the sea floor of the eugeosynclines during 

their early phases, but no North American examples of 

such magmatic extrusions have been proved. It now 

seems clear that few, if any, of the ultramafic rocks in 

the North American foldbelts arrived as magma in the 

positions which they now occupy, rather that they were 

emplaced tectonically as cold, nearly solid bodies, their 

movement being facilitated by slippage along their 

serpentinized parts. Such bodies may originally have 

been intruded as magma into rocks at lower levels of the 

crust, or they may be wedges or slabs of the underlying 

mantle itself, carried tectonically high above their place 

of origin (Hess, 1966, p. 5-6).

DESCRIPTION OF FOLDBELTS

The tectonic features of the different foldbelts of 

Phanerozoic age in North America are reviewed below, 

in clockwise order around the continent, and in general 

from oldest to youngest. The descriptions deal solely 

with surface and near-surface rocks and structures of 

the kinds which can be shown appropriately on a tectonic map, and emphasis is given to the means of so 

representing them. The surface and near-surface rocks 

and structures of the foldbelts involve many other facts 

and problems which are not treated, and for which 

there is a voluminous literature. Further, no mention is 

made of the structure of the deeper crustal layers and 

their attendant geophysical problems, even though 

these would contribute greatly to a full understanding 

of the tectonics of the foldbelts; the place for these 

subjects is on other maps and in other publications.

(K) INNUITIAN FOLDBELT

The Innuitian foldbelt extends across the northern 

part of the Arctic Islands of Canada, and into Peary 

Land of northern Greenland (Thorsteinsson and Tozer, 1961, p. 346-349). Its eastern and western ends 

plunge beneath the Arctic Ocean, and its western end 

(in Prince Patrick Island) is also overlapped by Mesozoic and Tertiary strata. How much farther westvard 

it continues is uncertain; it may be related to the e^rly 

Paleozoic structures of the core of the Brooks Rs^.nge 

in northern Alaska.

The Innuitian foldbelt developed from the Franklinian geosyncline (Thorsteinsson and Tozer, 1961). In 

the Arctic Islands, a miogeosynclinal belt borders the 

Canadian Shield and its platform cover, forming the 

Parry Islands (Melville and Bathurst Islands) and extending most of the length of Ellesmere Island. It contains a sequence as much as 6,000 m (20,000 ft) thick 

of Ordovician to Upper Devonian strata, mainly carbonates, but with minor elastics and evaporites (I"2). 

In the Parry Islands these are thrown into long symmetrical east-west folds beautifully displayed on aerial 

photographs, the deformation being later than the 

Devonian and earlier than unconformably overlying 

Middle Pennsylvanian rocks (K3a).

The continuity of the miogeosynclinal structures is 

interrupted in Cornwallis and nearby islands, where 

the older miogeosynclinal strata (K2a) were folded 

between Silurian and Middle Devonian time, mainly 

along north-south axes that were probably draped over 

an extension of the Boothia uplift, which project? toward the area from the Canadian Shield (Kerr and 

Christie, 1965, p. 919).

A eugeosynclinal belt extends across northern Ellesmere Island and into the northern tip of Axel HeiHrg 

Island. Its boundary with the miogeosynclinal beH is 

displayed only in eastern Ellesmere Island, where carbonates give place northwestward to graptolite shales 

and other elastics; elsewhere the boundary is concealed 

by the younger strata of the Sverdrup basin (see below). The eugeosynclinal rocks are as thick or thicker 

than the miogeosynclinal rocks, and in northern Ellesmere Island they are graywackes, shales, volcanics, 

with minor amounts of carbonates, all variably nr^tamorphosed (Kl and Kla); they have yielded a few 

Ordovician and Silurian fossils. Schists and gneisses 

at the base may extend the sequence downward into the 

Proterozoic. The time of deformation of the eu<*eosynclinal rocks is uncertain; the deformation was earlier 

than the overlying Pennsylvanian, and it may 1 "we 

produced the elastics which occur in the Middle and 

Upper Devonian of the miogeosynclinal sequence. T>ere 

are a few embedded granitic plutons (Ka), probably 

of Paleozoic age.

Less is known about the extension of the foldbelt 

into Peary Land, as it has been crossed by only a few 

ground traverses. Its rocks are elastics, with some car-
54 THE TECTONICS OF NORTH AMERICA

bonates and possible volcanics, that become increasingly 

metamorphosed northward; all are shown as eugeosynclinal (Kl) on the tectonic map, but with doubt. Open 

folds at the edge of the craton are succeeded northward 

by more complex structures, including low-angle thrusts 

that are strangely directed northward, away from the 

craton. Some southward-directed thrusts occur in the 

western segment, which are thought to result from 

superposed Tertiary deformation (Haller and Kulp, 

1962, p. 33). The time of deformation in Peary Land 

is uncertain, except that it is older than the overlying 

Middle Pennsylvanian. Long after the deformation, the 

foldbelt was split by longitudinal faults which raised 

the northern part as a horst.

In the northwestern Arctic Islands the core of the 

Innuitian foldbelt is concealed by upper Paleozoic, 

Mesozoic, and lower Tertiary (Paleocene or Eocene) 

strata in the extensive Sverdrup basin (K3a,-b-c) 

one of the "successor basins" discussed above. These 

strata attain a thickness of 9,000 m (30,000 ft) in the 

eastern part of the basin, without any interruption by 

structural unconformities. The Pennsylvanian and 

Permian part of the sequence includes carbonates and 

evaporites, but the much thicker Mesozoic strata are 

mainly sandstones and shales. Sedimentation during 

Mesozoic time may have resembled that during Cenozoic time along the Gulf Coast, vast amounts of detritus 

being delivered by streams draining the continental interior streams which were prograded toward the 

bordering sea, in this case the Arctic Ocean (Tozer, 

1961, p. 400; Tozer and Thorsteinsson, 1964, p. 216- 

218). To complete the analogy, the evaporites in the 

lower part of the sequence have risen through the higher 

strata as domes and diapirs like those on the Gulf Coast 

(Thorsteinsson and Tozer, 1961, p. 355-356). The strata 

of the Sverdrup basin are little deformed in its western 

part, but are sharply folded and thrust in the eastern 

part, in Axel Heiberg and Ellesmere Islands; this deformation is a late feature, as it involves Tertiary beds 

at the top of the sequence. Many of the folds in this 

part of the basin trend north-south, and are perhaps 

on a posthumous extension of the older structures of 

the Boothia Uplift and Cornwallis Island.

Between Ellesmere Island and Greenland the Innuitian foldbelt is interrupted by the Kennedy and Kobeson Channels whose straight-sided shores suggest a rift, 

probably produced by left-lateral faulting. The amount 

of lateral shift, if any, is difficult to prove, as the foldbelt crosses the channels at an oblique angle; nevertheless, the front of the foldbelt in Greenland is 225 km 

(140 miles) northeast of the front in Ellesmere Island.

(J) EAST GREENLAND FOLDBELT

The East Greenland foldbelt nearly converge with 

the Innuitian foldbelt at the northeastern corner of 

Greenland, and the two probably meet at an acut> angle 

off the coast. From there the East Greenland foldbelt 

extends southward to Scoresby Sound at the 70th parallel, where it plunges under the Tertiary plateau basalts 

(E0); it does not reappear farther south, and p^^esumably is cut off at the edge of the continental she1 ^ The 

exposed part of the foldbelt is 1,400 km (750 miles) 

long and as much as 300 km (160 miles) wide. TH complex rocks and structures were deciphered by tl ^ staff 

of the Danish East Greenland Expeditions, under the 

direction of Lauge Koch, and have been ably S^mmarized and interpreted by Haller in several publications 

(Haller, 1961a, b; Haller and Kulp, 1962, p. 31-61; 

Haller, 1968).

The East Greenland foldbelt, like the Innuitian foldbelt, is mainly an early Paleozoic feature, and both 

have been called "Caledonian" in a broad sense (Haller 

and Kulp, 1962, p. 68-73); however, details of their 

character and history differ. The main bulk of its geosynclinal accumulation is a mass of Upper Prot^rozoic 

sediments as much as 16,000 m (50,000 ft) thick (the 

Eleanore Bay and Tillite Groups in the sou*h, the 

Hagen Fjord Group in the north), which are followed 

nearly conformably by much thinner Paleozoic strata, 

mainly Cambrian and Ordovician, but topped by Silurian in the north. On the tectonic map, the whole geosynclinal assemblage is shown as a single uni* (J2). 

In the legend the accumulation is simply referred to 

as "geosynclinal," but all of it is broadly of miogeosynclinal type; no deposits of truly eugeosyncliral type 

occur within the width of exposure of the foldbelt. The 

geosynclinal strata overlie a crystalline basement (Jl), 

which emerges widely between the 76th and 80tl parallels and forms smaller inliers to the south. In the north, 

this is composed of Middle Proterozoic rocks deformed 

by the Carolinidian (Grenville?) orogeny (fiv. H); 

farther south, it is earlier Precambrian.

Both the'geosynclinal rocks and their crystalline basement were strongly deformed; the Silurian is involved 

in the north, hence the orogeny was Caledoniar in the 

classical sense. During deformation, the suprrcrustal 

strata were thrust westward over the Greenland Shield 

and its platform cover. Thrusting is prominently displayed in Kronprins Christian Land north of the 80th 

parallel, where westward movement was as much as 40 

km (24 miles). Farther south, the frontal thrusts are 

largely concealed by the icecap, but they emerge for 

short distances, as in Dronning Louise Land (76th to 

77th parallels) and in Gaaseland west of Scoresby 

Sound (70th parallel). Except along its western edge,
DESCRIPTION OF FOLDBELTS 55
56 THE TECTONICS OF NORTH AMERICA

most of the f oldbelt between the 70th and 76th parallels 

exposes deeper levels than farther north, and displays 

a highly mobile infrastructure (J2a), in which the Upper Proterozoic geosynclinal strata and their underlying basement have been migmatized and have flowed 

plastically into domes, recumbent folds, and mushroomshaped bodies all magnificently displayed on the steep 

walls of the fjords. It is surprising that such complex, 

deep-seated infrastructures should occur so near the 

foreland of the f oldbelt.

Between the 72d and 74th parallels the Caledonian 

folded structures are overlain by Middle and Upper 

Devonian clastic deposits (J3), mostly red and continental and as much as 7,000 m (23,000 ft) thick; they 

have been called "molasse" and were deposited in "successor basins" as defined in this report. These strata were 

involved in a second orogeny at about the same time as 

the main deformation of the Innuitian foldbelt to the 

west; the orogeny has been called "young Caledonian," 

but it is contemporaneous with the Acadian orogeny 

farther south in North America. The surrounding preDevonian rocks, already thoroughly deformed by the 

Caledonian orogeny, were refolded and in part remobilized, along axes transverse to the earlier ones. On the 

tectonic map these younger superposed structures are 

shown as thin lines that extend across the earlier folds. 

Both the Devonian and the surrounding older rocks are 

invaded by late erogenic or postorogenic granite plutons 

(Ja), which have yielded radiometric dates of 395 m.y.

Overlying all the earlier rocks and structures, especially near the coast, is a sequence of postorogenic 

Pennsylvanian, Permian, and Mesozoic formations 

(J4), capped in places by outliers of the Tertiary 

plateau basalts. The postorogenic formations have been 

little deformed, except by tilting and blockfaulting.

(L) APPALACHIAN FOLDBELT

The Appalachian foldbelt, which also formed mainly 

during Paleozoic time, is exposed for 3,700 km (2,000 

miles) along the southeastern side of North America, 

from Newfoundland to Alabama. In the Canadian 

Maritime Provinces it is exposed to a width of 650 km 

(400 miles) and in Tennessee and North Carolina to a 

width of 400 km (250 miles); however, large parts of 

its original extent are now concealed by younger deposits, or submerged beneath the sea. Because of such 

concealment, it is nearly bisected near New York City. 

Northeastward, the Appalachian foldbelt runs out to 

sea along the coast of Newfoundland with no dimunition in strength of deformation, and must surely continue to the edge of the continental shelf 100 km (60 

miles) beyond. Its nearest counterparts in this direction 

are the Caledonian and Variscan f oldbelts of the British

Isles on the opposite shore of the Atlantic Ocean, from 

which it is separated by a wide expanse of oceanic crust. 

Southwestward, it plunges beneath the Mesozoic and 

Tertiary strata of the Gulf Coastal Plain in Alabama, 

again with no dimunition in strength of deformation. In 

the Atlantic Coastal Plain the southeastern part of the 

internal zone of the foldbelt is similarly covered t y these 

younger strata.

The Appalachian foldbelt is notably more repilar in 

plan than most of the others in North America, with 

fewer differences in structure from one part to another, 

and with fewer interruptions by superposed younger 

structures. Its longitudinal subdivisions maintain their 

identity for long distances. It pursues a sinuous course, 

and is divided transversely into a succession of salients 

and recesses, each salient being about 650 km (400 miles) 

long.

Nevertheless, as mentioned above, the expos?d part 

of the foldbelt is nearly bisected at about mid -length 

near New York City; there are various contrasts between the two halves, some real, and some merely apparent. Geographically, the term "Appalachian Mountains" is thus commonly used only for the southern half, 

the highlands to the north being given other names. 

There is also a traditional belief that the northern half 

of the foldbelt is primarily an early Paleozoic feature 

and the southern half primarily a late Paleozoic feature. This has given rise to a concept, common in the 

European literature, of a "Variscan foldbelt" crossing 

westward over a "Caledonian foldbelt" near th°< place 

of bisection. The fallacy of this concept is now demonstrated by the nearly identical radiometric dates in the 

internal zones of both halves of the foldbelt (see p. 61). 

It has also been proposed (Drake and Wood war!, 1963) 

that a fundamental zone of transcurrcnt faulting crosses 

the foldbelt near the place of bisection, extending inland 

from the line of Kelvin Seamounts in the Atlantic 

Ocean basin to the southeast. While there are j?30physical indications of faulting along this trend in the ocean 

basin, neither geological nor geophysical data provide 

convincing evidence for through-going faults in the 

continent either at the surface or at depth even though 

structural belts on the south are bent westward from 

corresponding belts on the north.

Whether superficial or fundamental, there are sufficient .differences between the northern and southern 

halves of the Appalachian foldbelt to warrant separate 

treatment in the remainder of this account.

NORTHERN APPALACHIANS

In the northern Appalachians the front of the foldbelt nearly impinges on the Precambrian rocl-? of the 

Canadian Shield (or its outlier in the Adirondack up-
NORTHERN APPALACHIANS 57

lift), with only a narrow foreland of Palezoic platform 

deposits (B) between. In southern Quebec the front of 

the foldbelt is the major low-angle thrust fault of Logan's Line, but frontal thrusts are discontinuous southward, and if present to the northeast are beneath the St. 

Lawrence Estuary.

Precambrian basement rocks (LI, La) emerge in the 

higher uplifts within the foldbelt itself, where they have 

been reworked by the Paleozoic orogenies. They are exposed in a chain of uplifts a short distance southeast of 

the front of the foldbelt, in the Long Range of Newfoundland, the Green Mountains and others of New 

England, and the Hudson River Highlands of New 

York State. They are exposed again in another chain 

of uplifts well to the southeast, in the Avalon Peninsula 

of Newfoundland, Cape Breton Island, southern New 

Brunswick, and perhaps in southeastern New England 

(although not there separated on the map from the 

Paleozoic plutonics, L8). The basement rocks to the 

northwest are extensions of the Grenville foldbelt and 

yield characteristic radiometric dates of about 1,000 m.y. 

Those to the southeast were all seemingly involved in 

the younger Avalonian event, and in Newfoundland at 

least yield radiometric dates of 600-550 m.y. The significance of the Avalonian rocks in the history of the 

northern Appalachians remains to be appraised, but in 

Newfoundland they may form the southeastern border 

of the Paleozoic foldbelt (Williams, 1964, p. 1155-1156; 

Poole, 1967, p. 19). In Newfoundland, the Holy rood 

Granite of the Avalonian basement is overlapped by 

fossiliferous Lower Cambrian, but in nearby areas a sequence of Upper Proterozoic strata (L2) as much as 

12,000 m (40,000 ft) thick intervenes (Weeks, 1957, p. 

145-147).

Miogeosynclinal deposits of Cambrian and Ordovician age (L6) are well displayed in western Newfoundland and from Vermont southward into Pennsylvania, 

but are absent, at least at the surface, through the central segment in Quebec. Although no more than 2,500 m 

(8,000 ft) thick, their sequence is thicker and more complete than that in the adjacent foreland; they are topped 

by younger Ordovician shales that are part of a clastic 

wedge which extends westward into the foreland, related to the late Ordovician Taconian orogeny. This 

orogeny is further expressed south of the Adirondack 

uplift by an angular unconformity between the Ordovician and the Silurian. Above the Silurian in the foreland to the west, from the Catskill Mountains southward, is a much thicker Devonian clastic wedge, coarse 

and continental toward the top, which reflects the Late 

Devonian Acadian orogeny within the foldbelt. Miogeosynclinal deposits of the same age as those on the 

northwest also occur on the southeastern side of the foldbelt in Newfoundland, where they are preserved in snail 

patches on top of the Avalonian basement and the Up r»er 

Proterozoic strata, but both facies and faunas are more 

closely related to those in Europe than to those in the 

northwestern part of the Appalachian foldbelt.

Eugeosynclinal deposits of Cambrian to Devonian age 

form a large part of the remainder of the northern Appalachians. On the tectonic map they are divided in*o a 

Cambrian and Ordovician part (L3) and a Silurian and 

Devonian part (L4). They are further divided into an 

unmetamorphosed or weakly metamorphosed part wlich 

dominates to the northeast, and a strongly metamorphosed part (L3a, L4a) which dominates to the southwest, the boundary being placed at the garnet isograd.

Throughout the central segment in Quebec, Cambrian 

and Ordovician eugeosynclinal rocks extend to the front 

of the foldbelt, next to the shield and its narrow border 

of platform deposits. In the Taconic Mountains of Few 

York State and in western Newfoundland, where C"mbrian and Ordovician miogeosynclinal rocks form the 

front d-f the foldbelt, they are overlain with thrust contact by broad sheets of eugeosynclinal rocks of the same 

ages that were emplaced during early phases of the Taconian orogeny, probably by gravity sliding from the 

internal zone (Zen, 1967; Rodgers and Neale, 19*3). 

Eugeosynclinal rocks of Cambrian to Devonian age 

form the central and greater part of Newfoundland, 

and are bordered not only on the northwest but on the 

southeast by Precambrian massifs with a thin miofyeosynclinal cover (see above); (Williams, 1964). On the 

mainland of Nova Scotia, the eugeosynclinal rocks extend to the Atlantic coast, but probably formed in another depositional trough southeast of the belt of J.valonian basement (Poole, 1967, p. 18-19).

The eugeosynclinal sequences are generally thicker 

than the miogeosynclinal. In Vermont, they are dramatically so, the Cambrian and Ordovician changing f~om 

2,500 m (8,000 ft) of miogeosynclinal deposits wee4 of 

the Green Mountain uplift to 9,000-15,000 m (30,000- 

50,000 ft) of eugeosynclinal deposits east of it, where 

they are followed by 4,500-6,000 m (15,000-20,000 ft) 

of Silurian and Devonian (Cady, 1960, p. 541-544). 

Elsewhere thicknesses are variable; there may have Hen 

zones of uplift or volcanic chains along the siter of 

some of the present anticlinoria, although these are difficult to unravel because of subsequent def ormatior.

Parts of the Cambrian and Ordovician eugeosyncFnal 

rocks were deformed during the Taconian orogeny before the succeeding strata were laid over them. Taconian 

folding seems to have been greatest along the northwestern border in Quebec, parts of which were not greatly 

deformed afterwards. These folded rocks are overlap r»ed 

from the southeast by Silurian and Devonian shelf de-
58 THE TECTONICS OF NORTH AMERICA

posits (shown as miogeosynclinal, L6 on the map) which 

pass within a short distance into a thick eugeosynclinal 

sequence; here, the eugeosyncline had a two-phase history. A two-phase history is also evident in Newfoundland, where the Silurian, although containing volcanic 

components like the Ordovician, includes shallow-water 

arkoses, red ,beds, and conglomerates that contain clasts 

derived from the older rocks. Elsewhere in the eugeosynclinal part of the foldbelt, Taconian deformation is 

less obvious, and if present has been much obscured by 

later deformation; unconformities occur in places, and 

in New Hampshire the Silurian overlaps Ordovician 

plutons, but in places the sequence appears to be conformable from Ordovician into Silurian.

The greatest orogeny in the northern Appalachians 

was the Acadian, whose climax was during later Devonian time (Poole, 1967, p. 33-37). (There is no convincing evidence of a Caledonian orogeny in the classical 

sense, between the Silurian and Devonian.) From Maine 

northeastward, deeper downfolds preserve late Early 

Devonian continental plant-bearing beds lying conformably at the top of the eugeosynclinal sequence beds 

which foreshadow the climactic orogeny. Deposits that 

succeed the orogeny are mostly Carboniferous, although 

in places they include the latest Devonian.

In Quebec and New England, Silurian and Devonian 

eugeosynclinal rocks are preserved in two deeply depressed synclinoria, separated by a median belt of 

anticlinoria that exposes the older Paleozoic the 

Connecticut Valley-Gaspe synclinorium on the northwest and the Merrimack synclinorium on the southeast 

(not labeled on the map). South westward, the synclinoria become tightly compressed and the grade of 

metamorphism increases; in Connecticut schists and 

gneisses of the synclinoria and the intervening anticlinoria have been plastically folded into nappes of 

Pennine type. A third synclinorium forms most of the 

peninsula of Nova Scotia, southeast of the Precambrian 

massifs of Cape Breton Island and southern New Brunswick, but most of its rocks are Cambrian and Ordovician 

and the overlying Silurian and Devonian are minor.

Carboniferous deposits (with latest Devonian at the 

base in places, and some Permian at the top) (L7) 

accumulated in successor basins in many parts of the 

northern Appalachians, but most extensively in the 

Maritime Provinces. The largest, or New Brunswick 

basin, occupies more than half of that province, as well 

as Prince Edward Island and a large part of the floor 

of the Gulf of St. Lawrence. Farther southeast the 

basins are separated by massifs of Precambrian and 

older Paleozoic rocks, some of which stood as highlands 

during Carboniferous time. The first deposits of the

basins (Mississippian) are partly marine, but the 

younger (Pennsylvanian) are largely continental, with 

workable coal measures. Degree of deformation varies. 

In the New Brunswick basin most of the surface rocks 

are nearly flat-lying, but there and elsewhere the sequence contains structural unconformities, ard the 

degree of deformation increases downward; the rocks 

of some of the smaller basins are more folded, as shown 

by a separate symbol on the map (L7a). Configuration 

of the surface of the underlying basement that wns consolidated by the Acadian orogeny is indicated on the 

map by 500-m contours.

Carboniferous deposits occupy smaller successor 

basins in western Newfoundland and southeastern New 

England, and while they overlie much deformed earlier 

rocks they are themselves more deformed than most of 

their counterparts in the Maritime Provinces. In the 

Narragansett basin of Rhode Island coal beds have been 

converted to graphite, and the southern end of th? basin 

is intruded by late Paleozoic granite (Le).

Upper Triassic rocks of the Newark Group (L8) form 

a downfaulted strip in the Connecticut Valley of New 

England, and another in the Bay of Fundy whose southeastern edge projects onto the shore of Nova Scotia; 

the latter strip extends southwestward into the Gulf of 

Maine, where its extent has been determined I y geophysical means (the edge being shown by a dotted line 

on the map). The Triassic rocks are wholly postongenic, 

unaltered, and merely tilted and block faulted; they are 

red beds and arkoses, with interbedded mafic lavas and 

related intrusives.

The Devonian and earlier eugeosynclinal rocl*?1 were 

little broken during the Acadian deformation, bnt they 

and the succeeding Carboniferous and Triassic rocks 

are traversed by many high-angle faults, most of which 

are grossly parallel to the grain of the foldbelt, These 

include the so-called Cabot fault of J. T. Wilson (1962), 

who proposed that this was a through-going fracture 

with left-lateral displacement, formed during Carboniferous time, that extended from Newfoundland 

through the Maritime Provinces into southeastern New 

England. It is true that there is an unusual con".entration of high-angle faults of about this age in the places 

mentioned, but they form a zone as much as 100 km 

(60 miles) wide. None of the faults in the zone clearly 

persist for its whole length; some have proved leftlateral displacement, others right-lateral, whereas still 

others have only proved dip-slip displacemert. The 

Triassic normal faults of the Bay of Fundy seem to be 

reactivations within the zone. The Triassic fault? of the 

Connecticut Valley are a separate system farthe^ west;
SOUTHERN APPALACHIANS 59

they include the Ammonoosuc normal fault of New 

Hampshire, once thought to be a west-dipping thrust.

Ultramafic rocks (L/3) form pods and lenses in the 

Cambrian and Ordovician eugeosynclinal rocks on both 

the mainland and Newfoundland. In Newfoundland 

they lie in two belts, symmetrically placed on each side 

of a presumed central axis, but on the mainland they are 

concentrated in a belt on the northwest that extends, 

with interruptions, from Gaspe to southern New England. All of them are probably of Late Ordovician age; 

the thrust sheets of western Newfoundland that were 

emplaced as gravity slides during the Taconian orogeny 

contain ultramafic bodies that are rootless like the strata 

enclosing them.

Granitic rocks occupy parts of the site of the eugeosynclinal area, forming dispersed plutons to the northeast, but more extensive and more crowded plutons to 

the southwest. Both stratigraphic relations and radiometric dates indicate that they were emplaced during a 

prolonged period, from early Paleozoic into Mesozoic 

time. The earliest granitic rocks are pre-Silurian and 

probably mainly Late Ordovician. Early plutons have 

been identified in New Hampshire and Maine and others 

probably exist, but their known extent is too small to 

warrant separation from the younger Paleozoic granitic 

rocks on the map. The main assemblage of granitic rocks 

(L8) is of middle Paleozoic age, and yields radiometric 

dates of 350-400 m.y.; they are broadly contemporaneous with the Acadian orogeny, ranging from synorogenic to early postorogenic. A representative suite forms 

the New Hampshire Plutonic Series, but other series 

occur both in that state and elsewhere. Mafic plutonic 

rocks (Ly), in part related to the dominant granitic 

rocks, form a few areas large enough to indicate 

separately on the tectonic map.

Younger granitic rocks are wholly postorogenic and 

are generally more alkalic than the earlier ones (Lc). 

They are exemplified by the White Mountain Plutonic 

Series of New Hampshire, which includes a spectacular 

array of ring dikes, some of which enclose remnants of 

supracrustal volcanics. The White Moutain Series is 

early Mesozoic, with radiometric dates of about 185 

m.y., but the alkalic rocks farther south in New England are late Paleozoic. The oldest precede the Carboniferous deposits of the successor basins, but these deposits are themselves invaded by nonalkalic granite at 

the south end of the Narragansett basin.

The youngest intrusives of all, the Monteregian (L0), 

form a chain of stocks that extends eastward from 

Montreal (for which they are named) in the platform 

area into the outer edge of the foldbelt. They yield 

radiometric dates of about 110 m.y. and are of 

Cretaceous age.

SOUTHERN APPALACHIANS

South of its bisection near New York City, the Appalachian foldbelt is continuous on the surface to 

Alabama. Throughout this distance it borders the 

Paleozoic platform deposits of the craton to the ncrthwest with a well-defined topographic and structural 

front, along which steep folds and prominent thrusts 

end abruptly, giving place to more open structure in 

the platform rocks beyond. On the tectonic map, this 

is shown as the edge of the miogeosynclinal deposits 

(L6), although stratigraphic data indicate that this 

edge is actually a broader, less well-defined zone.

Throughout the length of the southern Appalachians 

the miogeosynclinal rocks form a belt 65-130 km 

(40-80 miles) wide, expressed topographically by the 

Valley and Ridge province. The miogeosynclina] sequence, about 9,000 m (30,000 ft) thick, extends from 

Lower Cambrian to Pennsylvania!!, the upper half (beginning as low as the Middle Ordovician in pla?«s) 

being largely clastic deposits derived from the southeast. 

Parts of the elastics (notably the Ordovician Martinsburg Formation) are typcial flysch laid down in deep 

longitudinal troughs (McBride, 1962, p. 87-88), but 

other parts are more varied and form wedges that trper 

across the miogeosyncline into the craton. Notable 

among the wedges is the great mass of Middle and 

Upper Devonian elastics in the northeastern half of the 

southern Appalachians, which are as much as 3,00^ m 

(10,000 ft) thick at the apex, but which taper not only 

northwestward toward the craton, but southwestward 

along the trend of the foldbelt. Clastic deposits nearly 

as thick occur on the Middle and Upper OrdovHan 

farther southwest, and in the Pennsylvanian of Alabama in the far southwest. Large parts of the elastics 

are marine, but they become continental higher up, 

especially in the Pennsylvanian.

The clastic deposits were derived from the erosion of 

uplifted and probably deformed parts of the interio^ of 

the foldbelt, and their maxima of different ages in different places suggest orogenic climaxes in adjacent segments of the internal zone; the Ordovician and Devonian maxima may be related to orogenies correlative 

with the Taconian and Acadian of the northern Appalachians. However, a large part of the thick Penrsylvanian elastics in the far southwest was probably related 

to orogeny in the impinging Ouachita foldbelt, now 

buried beneath the Gulf Coastal Plain to the south.

There are few breaks of any structural consequence 

within the miogeosynclinal sequence itself, and no strata 

lie unconformably above, hence the times of its deformation are inferential. The Pennsylvanian is preserved 

in places in the deeper downfolds in the miogeosyncl^al
60 THE TECTONICS OF NORTH AMERICA

belt, and the lowest Permian is conformable above the 

Pennsylvanian in the mildly folded part of the adjacent 

foreland, so that much of the deformation was late in 

Paleozoic time. This has been called the "Appalachian 

orogeny" (or "Appalachian Revolution"), but it is 

hardly Appalachian hi a broad sense, and the more 

specific term "Allegheny orogeny" is more appropriate.

The structures of the miogeosynclinal belt the classical "Appalachian structure" of American geology includes long, parallel folds generally asymmetrical 

toward the northwest, which are broken by low- to highangle faults that are thrust in the same direction (fig. 

5(7). Some of the thrust faults persist for great 

distances; the Saltville fault extends from Virginia to 

Alabama, and others are nearly as lengthy. Much of the 

structure is disharmonic so that the folds and faults 

overlie surfaces of decollement in various incompetent 

units low in the sequence; the underlying strata and the 

basement beneath are probably little deformed, or deformed in a different manner (Rodgers, 1953).

At the surface, the northeastern half of the miogeosynclinal belt contrasts with the southwestern, as the 

first is dominated by folds and the second by thrust 

faults the faults so crowded in places that no intervening folds are preserved. Structures at depth are more 

alike in the two halves, surfaces of decollement being 

equally prominent in each. Wells drilled on the Nittany 

arch, the great culmination of miogeosynclinal folds hi 

central Pennsylvania, passed through two or more 

thrusts within 1,500-3,000 m (5,000-10,000 ft) of the 

surface, and some of them ended in strata higher in the 

sequence than those in which they started.

Surfaces of decollement also extend out from the 

foldbelt into the adjoining platform deposits, and have 

determined much of the structure in this part of the 

craton or foreland. Near the common corners of Virginia, Kentucky, and Tennessee the Pine Mountain fault 

has moved a block of Carboniferous strata 200 km (124 

miles) long and 40 km (25 miles) wide at the edge of 

the foreland for about 7 km (4 miles) northwestward 

along an incompetent shaly layer (Rich, 1934; and 

many later references). Less prominent but even more 

pervasive decollement surfaces of the same kind are 

now known from subsurface and outcrop data in the 

foreland of Pennsylvania, West Virginia, and Tennessee (Gwinn, 1964; Wilson and Stearns, 1958).

In Alabama, the miogeosynclinal rocks are separated 

from the crystalline rocks of the internal zone by the 

Talladega belt, a thrust slice about 200 km (125 miles) 

long made up of Paleozoic strata of a different facies, 

largely slaty below with f ossilif erous Devonian cherts 

in the middle and plant-bearing Carboniferous above.

This sequence is more like that in the Ouachita foldbelt 

(M) than any elsewhere in the Appalachians, and it is 

so represented on the tectonic map.

Through the remainder of the length of the miogeosynclinal belt northeast of Alabama, it is bordered on 

the southeast by the Blue Ridge uplift, which brings 

to the surface rocks beneath the Paleozoic miogeosynclinal deposits. The basement of the uplift is crystalline 

rocks that are outliers of the Grenville foldbelt (Ll). 

Separating the basement from the Paleozoic are generally Upper Proterozoic supracrustal rocks (L2), mafic 

lavas to the northeast, nonvolcanic elastics to the southwest; the latter (Ocoee Series) are as thick or thicker 

than the adjoining Paleozoic miogeosynclinal sequence. 

The Upper Proterozoic rocks were little deforced before the orogenies of Paleozoic time.

At the boundary between the miogeosynclinal belt 

and the Blue Ridge uplift the structural style changes; 

the basement of the Blue Ridge is deformed ith its 

cover and regional metamorphism is apparent. Toward 

the northeast the uplift is overfolded against the miogeosynclinal belt, but thrust faults develop southwestward along the border, carrying the basement and other 

rocks of the uplift northwestward over the mic*yeosynclinal rocks. In the Tennessee-North Carolina segment, 

much of the Blue Ridge uplift is allochthonous, as windows of the thrusts emerge as much as 65 km (40 miles) 

behind their leading edges. Southwest of Virginia the 

Precambrian basement rocks of the Blue Ridg«> uplift 

and their supracrustal cover are juxtaposed agfinst the 

contrasting metamorphic and plutonic rocks of the 

Piedmont province to the southeast along the remarkably straight, narrow Brevard zone. Its principal 

structure is a high-angle fault, along which there are 

clear indications of a large component of righMateral 

strike-slip displacement including a nearly he Hizontal 

linear fabric, and inclusions of exotic rocks in the zone, 

far from any obvious sources (Reed and Bryant, 1964, 

p. 1192).

Besides the interpretation of the Brevard zor** as one 

of strike-slip displacement, others are possible which 

are not necessarily incompatible. The compiler Hs compared the zone (and some similar ones in the same region) with the root zone of the nappes on tl i south 

flank of the Alps (King, 1950, p. 653), and has termed 

it a "dejective zone," thus implying a structure more 

fundamental than the mere surficial syncline peculated 

by earlier geologists (King, 1955a, p. 356). This interpretation has been further exploited by Burcl fiel and 

Lavingston (1967) who point out that the alloclthonous 

Blue Ridge rocks northwest of it, like the nr.ppes of 

the Alps, have no large, obvious source area; hence that 

this source area must have been pulled down and oblit-
SOUTHERN APPALACHIANS 61

erated along the root zone. Credence is lent to this suggestion by the fraying out and disappearance of the 

Brevard zone to the northeast, at about the place where 

the Blue Ridge rocks cease to be allochthonous, and become part of an autochthonous uplift.

In the Piedmont province the southeastern, eugeosynclinal part of the foldbelt are supracrustal rocks 

younger than the Grenville basement, but their sequences and ages are enigmatic because of metamorphism, interruption by plutons, and a general absence 

of fossils. On the tectonic map they are indicated as undivided eugeosynclinal deposits, weakly to strongly 

metamorphosed (L5, L5a). In the Piedmont, the Grenville basement itself (LI) only comes to the surface in 

a cluster of mantled gneiss domes near Baltimore, Md.

So far as now known, the lower part of the sequence 

dominates in the northwestern part of the Piedmont 

province. It consists of very thick, nonvolcanic, clastic 

units, such as the Glenarm Series of Maryland and the 

Lynchburg Formation of Virginia; these much resemble the Ocoee Series of the southwestern part of the 

Blue Ridge uplift, and like it may be of Upper Proterozoic age (Hopson, 1964, p. 203-207). Higher units, 

many with large volcanic components, are infolded in 

the northwestern part of the Piedmont but dominate 

its southeastern part, especially in the Carolina Slate 

Belt; they contain a few Cambrian and Ordovician fossils at one place or another, and have yielded Ordovician radiometric dates elsewhere; whether other 

Paleozoic ages are represented is undetermined.

Most of the supracrustal rocks of the Piedmont province are regionally metamorphosed (L5a), and a metamorphic climax is attained (at least in North and South 

Carolina) in a sillimanite-almandine zone northwest of 

the slate belt (Overstreet and Bell, 1965, p. 54-57); this 

zone may have been the central axis of the foldbelt 

during some of the Paleozoic orogenies. Rocks of the 

Carolina Slate Belt itself are only weakly metamorphosed (Li5) except near plutons, and similar weakly 

metamorphosed rocks may form a large part of the 

basement beneath the Atlantic Coastal Plain.

The supracrustal rocks of the Piedmont province 

have been disrupted by large volumes of plutonic rocks. 

Gabbros and diorites (I/y) occur in Maryland and North 

Carolina, and are seemingly of early Paleozoic or latest 

Precambrian age. A more extensive younger array of 

granitic plutonic rocks (LS) includes foliated concordant varieties, and more massive cross-cutting bodies.

Radiometric dates on plutonic and metamorphic rocks 

in the Piedmont province cluster near 450-500 m.y., 350 

m.y., and 250 m.y., the first being more abundant to 

the northwest, the last to the southeast; they express 

times of plutonism, metamorphism, and presumably also

of orogeny that extended through a long span of the 

Paleozoic (Hadley, 1964). These times invite comparison with the known events in the northern Appalachians 

(the Taconian, Acadian, and later Paleozoic orogenies), 

but the extent of the rocks involved in each is too vaguely 

known to make it possible to represent it on the tectonic 

map. Various considerations, including the record of 

clastic wedges in the miogeosynclinal belt, suggest that 

the earlier radiometric dates express a greater erogenic 

event than the later dates.

Extending through the Piedmont province from 

Pennsylvania to North Carolina are faulted strips and 

patches of the Upper Triassic postorogenic rocks of the 

Newark Group (L8), nearly identical in lithology and 

structure to the rocks of the same group in the north ern 

Appalachians. They lie unconformably on the crys+alline rocks of the Piedmont province, and in Penn<*ylvania overlap the edge of the miogeosynclinal belt. As 

shown on the tectonic map, mafic dikes related to the 

intrusives in the Newark Group extend long distances 

into the surrounding older rocks and have a systematic 

regional pattern (King, 1961). As already indicated 

(p. 24), faulted strips of Triassic rocks in the internal 

zones of the Appalachian and Ouachita foldbelts continue beneath the coastal plain cover at least as far 

southwest as southern Arkansas.

(M) OUACHITA FOLDBELT

The Ouachita foldbelt is, in many respects, a western 

continuation of the Appalachian foldbelt; nevertheless, 

its rock sequence and structure differ significantly, hence 

it evolved during a different tectonic cycle. The foldbelt extends more than 1,600 km (1,000 miles) across 

the southern United States, from near the last exposure 

of the Appalachians in Alabama to western Texas, wl ^re 

it passes into Mexico. However, it is only exposed for 

510 km (275 miles) of this distance in a large r.rea 

in the Ouachita Mountains (Miser, 1929; and many 

later papers), and in the Marathon region and even 

smaller areas in western Texas (King, 1937). Through 

the remainder of its length it is covered by deposits of 

the Gulf Coastal Plain (C), but its position is indicated 

with varying degrees of precision by drill dat% (Flr.wn 

and others, 1961).

Because of the small extent of the exposed parts of the 

Ouachita foldbelt, its rocks are shown as a single unit 

(M) on the "Tectonic Map of North America" (fig. 12). 

The same symbol is used for inliers of deformed Paleozoic rocks in the eastern Cordillera of Mexico that w^re 

classed as parts of the Huastecan structural belt on the 

"Tectonic Map of Mexico" (de Cserna, 1961). In the 

Gulf Coastal Plain the front of the concealed part of the 

foldbelt is shown as a dotted line (or buried fault).
103

Leptogeosynclinal deposits Older flysch 

Devonian and older Paleozoic deposits 

la lowest exposed units, Mississippian 

separately shown in part

EXPLANATION

Younger flysch Synorogenic and Platform depos

deposits postorogenic deposits Cretaceous an

Pennsylvanian Younger Pennsylvanian Tertiary 

in A; Permian in B

Thrust fault Normal fault

__i__ 

Synclinal axis

30°

FIGURE 12. Tectonic maps of the two larger areas of exposure of the Ouachita folclbelt showing the tectonic subdivisions of its rocks

the "Tectonic Map of North America," where the subdivisions are not shown because of the small scale. A, Ouachita Mountains of 

B Marathon region in western Texas. Compiled from geologic maps of T*»xa«, Oklahoma, and Arkansas, with additions from othe
OUACHTTA FOLDBELT 63

The course of the Ouachita f oldbelt, like that of the 

Appalachian f oldbelt, is marked by salients and recesses, 

but of even greater amplitude. The Ouachita Mountains 

are at the apex of one salient, the Marathon region at 

another; a deep recess lies between the Ouachita and 

Appalachian foldbelts in Mississippi and Alabama, and 

another adjoins the Llano uplift in central Texas. 

Throughout its course, the foldbelt lies against the central craton, whose Paleozoic rocks have a different character and structure, commonly with thrust contact; in 

part it is bordered by deep basins containing upper 

Paleozoic synorogenic clastic deposits the Black Warrior basin of Alabama and Mississippi, the Arkoma 

basin of Arkansas and Oklahoma, and the Val Verde 

basin of western Texas.

The relation of the Ouachita foldbelt to the Appalachian foldbelt is incompletely determined. Trends of the 

two structures converge nearly at right angles, and drill 

data show that the frontal structures curve sharply from 

one trend to the other, but the manner of junction of 

the internal zones is unknown. The Ouachita foldbelt 

also meets the rocks and structures of the Wichita system nearly at right angles in southern Oklahoma; the 

junction is complexly faulted, but the Ouachita rocks 

are thrust over the Wichita rocks. In western Texas the 

Ouachita foldbelt again meets at right angles the 

younger Cordilleran foldbelt, which largely overwhelms 

it in Mexico.

The Paleozoic rock sequences of the exposed parts of 

the Ouachita foldbelt in the Ouachita Mountains and 

Marathon region are remarkably alike, especially considering their wide geographic separation and the very 

different nature of the contemporaneous rocks exposed 

in intervening and adjoining areas. The lowest exposed 

strata in both are high in the Cambrian. The remainder 

of the lower Paleozoic is dominantly slaty and cherty 

and is topped by a middle Paleozoic white chert or 

novaculite. These strata have many, but not all, of the 

characters of eugeosynclinal deposits. Their thickness 

is modest, generally no more than 900 m (3,000 ft), 

and they were probably laid down in a starved basin 

comparable to the leptogeosynclines of the Alps 

(Trumpy, 1960, p. 865-866). The only exception is the 

Silurian Blaylock Sandstone of the Ouachita Mountains, which wedges out rapidly northward and may be 

related to an early pulse of orogeny in the interior of 

the foldbelt.

Higher Paleozoic strata in both areas of exposure are 

dramatically different a great mass of flysch that 

marks the orogenic phase in the foldbelt (Cline, 1960; 

Goldstein and Hendricks, 1962; McBride, 1966). Within the Ouachita Mountains the middle and upper Mississippian part (Stanley and Jackfork Formations) is

5,200 m (17,000 ft) or more thick, but both here jvnd 

in the Marathon region this part wedges out alirost 

entirely at the front of the foldbelt. The early Pennsylvanian part (Atoka Formation) is 6,000 m (19,000 ft) 

thick at the front of the Ouachita Mountains, and 

wedges out almost entirely near the Ozark uplift in the 

craton to the north. In west Texas a similarly great but 

younger flysch sequence occurs along the front, mainly 

in the Val Verde basin, and includes earliest Permian. 

Successive pulses of orogeny in the Ouachita foldHlt 

are thus suggested, progressing outward from the internal zones and westward along its trend Mississippian and early Pennsylvanian pulses in both areas, a 

late Pennsylvanian pulse in the western area. Related 

to the younger pulses are structural unconformitier in 

the higher Paleozoic foreland strata.

As a result of these orogenies, the exposed rocks of 

the Ouachita foldbelt were deformed in much the sr.me 

manner as the rocks of the miogeosynclinal belt of the 

southern Appalachians, into asymmetrical folds overturned toward the north, with many low-angle to 

high-angle thrust faults. Thrusts are especially numerous and crowded in the western half of the Ouachita Mountains, near the impingement of the foldbelt 

on the Wichita system. The eastern half was formerly 

believed to be rather simply folded, even though the 

deepest parts of the structure and those parts fartHst 

south were known to be conspicuously metamorphosed; 

it is now known that frontal thrusts extend across all 

this part to the Mississippi Embayment, and that the 

structures behind the front include major recumbent 

folds or nappes (Viele, 1966).

In the buried parts of the foldbelt, drill holes h<we 

penetrated deformed older and younger Paleozoic 

rocks that are recognizably like the formations in the 

exposed areas, but these formations, in both the outcrops and the buried areas, are only in the frontal zo^es. 

In Texas, drill holes have penetrated an internal zone 

of highly sheared, medium-grade metamorphic rock? of 

undetermined age (Flawn, in Flawn and others, 1961, 

p. 79-81); these nowhere come to the surface, except 

in a tiny outcrop immediately south of the Rio Grande 

in Coahuila, Mexico. Granitic and other plutonic recks 

were penetrated in a few of the drill holes, but no large 

plutons have been found so far. East of Texas, the 

internal zones have not been reached by the drill, and 

are extensively blanketed by postorogenic deposits. 

Triassic redbeds (Eagle Mills Formation) occupy a 

long belt in Arkansas immediately south of the Criachita Mountains. South of them a single well in northeastern Louisiana (Union Producing Co., No. T-A 

Tensas Delta) penetrated undeformed Pennsylvar ian 

strata, which perhaps were deposited in a successor
64 THE TECTONICS OF NORTH AMERICA

basin, like basins in the Canadian Maritime Provinces, 

after the major orogeny in the interior of this part of 

the foldbelt had been completed.

8TBTJCTUBAL SYSTEMS BELATED TO THE OTJA.CHITA FOLDBELT

Before leaving the Ouachita foldbelt, discussion is 

desirable of other structural systems to which it is 

related in one way or another systems which have been 

mentioned only incidentally hitherto.

Structures of Paleozic age in the Wichita system extend west-northwest from their impingement with the 

Ouachita foldbelt for about 650 km (400 miles) across 

southwestern Oklahoma into the Texas Panhandle; they 

are vaguely connected thence northwestward through 

lesser structures with those of Paleozoic age in the 

Southern Rocky Mountains. Only a small part of the 

Wichita system projects to the surface, to form the 

Arbuckle and Wichita Mountains; the remainder is 

concealed by late Paleozoic postorogenic deposits, including extensive Permian redbeds to the west. On the 

"Tectonic Map of North America" the system is represented by its faults (mostly buried) and by contours 

on the surface of the Precambrian; its rocks are mapped 

as platform deposits on a Precambrian basement.

In the Wichita system the lower part of the Paleozoic sequence, through the Mississippian, is 3,700 m 

(12,000 ft) thick and mainly carbonate rocks; it much 

resembles that in the southern Appalachian miogeosyncline, except that it lacks most of the Cambrian components. The lower Paleozoic strata thin north, south, 

and west across the craton, where they lie on crystalline 

rocks consolidated during Middle Proterozoic orogenies. 

Within the system itself the Paleozoic strata lie on 

another body of rocks (Ham and others, 1964, p. 149- 

160). Where exposed in the Wichita Mountains these are 

primarily plutonic gabbros, granites, and rhyolites 

(shown as A3 on the tectonic map) but a few outcrops 

and extensive drill data demonstrate that the plutonic 

rocks are floored intrusives in the upper part of a supracrustal sequence 6,000 m (20,000 ft) thick, largely graywackes and volcanics. Radiometric datings on both the 

plutonics and volcanics yield surprisingly young ages 

of 500 m.y., thus placing them and at least part of the 

supracrustal sequence in the Cambrian rather than in 

the late Precambrian.

All these rocks and their basement were deformed 

during several stages of Pennsylvanian time, or broadly 

at the time of the orogenies in the nearby Ouachita foldbelt. Blocks of basement and supracrustal rocks were 

raised along high-angle faults, in part with lateral displacement, producing the massifs of the present 

Wiohita and Arbuckle Mountains, and others still concealed. Their erosional debris was shed into deeply subsiding adjoining or intervening troughs notably the

Anadarko basin on the north flank of the Wichita Mountains where another 4,500 m (15,000 ft) of later 

Paleozoic sediments accumulated, parts of which were 

severely folded and faulted during the final st^qres °f 

the deformation.

Details of the deformation of the Wichita system, 

the related sedimentation, and the interaction of both 

with the growing Ouachita foldbelt have been dc'vsribed 

at length in other publications. Here, it is sufficient to 

observe that the history of the Wichita system has been 

germanotype, rather than alpinotype like the Ouachita 

foldbelt, yet that this history was characterized by a 

much greater mobility than in most of the cratonic areas 

of North America.

The Huastecan structural belt of eastern Mexico (de 

Cserna, 1960, p. 598-601; 1961) is virtually a continuation of the Ouachita foldbelt, but it is represented at the 

surface by only a few inliers of deformed Prleozoic 

rocks that emerge in the higher folds of the easte rn part 

of the Cordilleran foldbelt (Flawn, in Flawn and others, 

1961, p. 90-103), so that its structural pattern is largely 

conjectural. It is commonly assumed that the Huastecan 

belt (that is, the Ouachita foldbelt) extends southward 

along the trend of the present Cordillera, but tl is is by 

no means assured. Some of the most significant Paleozoic inliers occur near Las Delicias in southwestern 

Coahuila, 480 km (300 miles) south of the Marathon 

region, where a thick sequence of marine Permian with 

volcanic components has been steply folded along northnortheast axes and intruded by early Mesozoic granite 

(R. E. King and others, 1944, p. 25-27), suggesting that 

orogenic movements in the Ouachita foldbelt persisted 

later in Paleozoic time here than they did farther 

northeast.

At Las Delicias and elsewhere in western Coahuila, 

the Mesozoic cover is thinner than in adjacent parts of 

the Cordillera, over a largely buried massif termed the 

"Coahuila peninsula," around which the Coriilleran 

folds are strongly deflected on the south, part of this 

massif is shown as platform area (C) on the tectonic 

map (see p. 25-27). This buried massif probably is a 

consolidated part of the internal zones of the Ouachita 

foldbelt.

(0) CORDILLEBAN FOLDBELT

The Cordilleran foldbelt extends for 8,000 km 5,500 

miles) along the western side of North America, from 

Alaska to northern Central America, with a vidth of 

650-1,600 km (400-1,00 miles). The foldbelt is sigmoid 

in plan, trending westward in the extreme north, meridionally in the main part, then eastward in the extreme south. Westward from Alaska it is separated by 

only a short stretch of shallow water from comparable 

f oldbelts in Asia. Southeast of Guatemala and Honduras 

in Central America the rocks and structures differ from
NORTHERN CORDILLERA 65

those to the north, and are described later as a part of 

the Antillean foldbelt (Q).

On its eastern side the Cordilleran foldbelt is bordered 

through most of its length by the central craton of 

North America, but in Alaska it fronts northward on 

the Arctic Coastal Plain, and in Mexico it fronts eastward on the Gulf Coastal Plain. On its inner side the 

Cordilleran foldbelt and the related Pacific foldbelt 

(P) are bordered for their entire length by the Pacific 

Ocean, without coastal plains and with only narrow 

continental shelves. In the northwest, the Pacific Coast 

is closely adjoined by the Aleutian Trench and in the 

south by the Middle America Trench, both with depths 

greater than 6,000 m (20,000 ft). In the more lengthy 

intervening area the ocean bottom next to the coast 

lacks trenches and lies at depths of 3,000-4,000 m 

(10,000-15,000 ft).

In the Cordilleran foldbelt, the climactic orogenies 

took place during Mesozoic time, but it had a long antecedent history of sedimentation and deformation. It 

arose from several geosynclines, some of which took 

form early in Paleozoic time, as in the Appalachian 

foldbelt, others as late as Mesozoic time. Early def ormational features have been largely overwhelmed by the 

later ones, so that their patterns are seldom clear; some 

were probably precursors of the later structures, others 

seem to have been quite discordant. The postorogenic 

history of the foldbelt was much more eventful and 

complex than that in the Appalachians, with orogenic 

and epeirogenic events extending through most of the 

Cenozoic. In many places the imprint of these later 

events is so great as to disrupt or obscure the more fundamental Mesozoic structures.

These later events are also largely responsible for 

the present system of Cordilleran Mountains. Through 

most of the length of the Cordillera these events have 

produced two major mountain chains, one near the Pacific Coast on the west, the other fronting the continental interior on the east, with lower plateaus and less 

continuous ranges between. These are only indirectly 

related to the original Cordilleran foldbelt, and in much 

of the western United States the eastern mountain chain 

(Central and Southern Rocky Mountains) is actually a 

reactivated part of the original craton (see p. 23-24).

As might be expected from the great length of the 

Cordilleran foldbelt, its geology, tectonics, and history 

differ from one part to another, which makes generalization difficult, both on the tectonic map and in the present 

text; this is reflected by the large number of units into 

which the foldbelt is divided on the map legend, some 

of which occur only in one part or another. Nevertheless, 

a certain degree of unity is evident for long distances in 

the foldbelt, with rather abrupt contrasts in tectonic

style across narrow transverse zones, so that the foldbelt is segmented (King, 1966, p. 2-3). How fundamental this segmentation may be is problematical, br t it 

is a convenient basis for the ensuing descriptions, wl^re 

the foldbelt is treated in terms of a northern Cordillera 

(Alaska and Canada), a central Cordillera (western 

United States), and a southern Cordillera (Mexico and 

Central America).

NORTHERN CORDILLERA

The Cordilleran foldbelt trends northwestward in 

Canada with a nearly constant width of about 800 km 

(500 miles). A little east of the Alaska-Yukon boundary 

the belt is constricted by a deep recess along its northeastern side. In Alaska the belt turns westward, widening and branching, one branch extending out to sea 

southwestward to form the Aleutian island arc. Despite 

the constriction and some tectonic differences on its t wo 

sides, the cross section of the foldbelt in both Canrda 

and Alaska is much the same, and longitudinal strips- of 

like rocks and structures extend through the whole segment, including a belt of miogeosynclinal rocks on the 

northeast and a belt of eugeosynclinal rocks on the 

southwest. In addition, a belt of younger rocks snd 

structures fringing the Gulf of Alaska is distinguisl <*! 

as a part of the Pacific foldbelt (P) and described under 

a later heading. The segment extends a short distance 

southward across the 49th parallel into Washington, 

Idaho, and Montana, where it terminates against overlapping volcanic rocks and the transverse faults of the 

Lewis and Clark zone.

Easternmost Siberia, opposite Alaska, contains foldbelts that are the obvious extensions of the Cordilleran 

and Pacific foldbelts of North America into the cortinent of Asia. Indeed, the rocks and structures of the two 

continents are separated only by the very shallow nor+hern part of the Bering Sea on the south and the Chukchi 

Sea on the north, underlain by continental crust. It is 

evident from published maps (notably Tilman and 

others, 1966) that rocks, structures, and orogenic tines 

occur in Siberia which are analogous to those in Alas!-a; 

nevertheless, their positions and trends do not clos^y 

match. There has been much speculation as to precise 

connections by Soviet and American geologists (among 

the latter, Ostenso, 1968; Michael Churkin, Jr., unpub. 

data), but more precise comparisons are desirable. Further information will no doubt become available as a result of oceanographic investigations now in 

progress on the sea-bottom geology of the intervening 

areas.

Between the 46th and 60th parallels the outer, or miogeosynclinal, part of the northern Cordillera forms the 

Rocky Mountains, a bundle of parallel ranges with little
66 THE TECTONICS OF NORTH AMERICA

curvature, about 120 km (75 miles) broad, which rise 

abruptly west of the plains of the continental interior, 

generally along a zone of thrusting (Bally and others, 

1966, p. 340-345). The Lewis thrust near the 49th parallel carries Middle Proterozoic strata (O3) eastward at 

least 30 km (20 miles) over the rocks of the plains (Ross, 

1959, p. 76-78); the similar but not identical McConnell 

thrust farther north carries Paleozoic strata eastward in 

the same manner. Behind the frontal thrust the rocks 

of the mountains are closely crowded into folds and 

thrust blocks, probably accompanied by decollement 

over the basement. East of the frontal thrusts the rocks 

of the plains are themselves intensely disturbed in a 

foothills belt 30 km (20 miles) wide.

North of the 60th parallel, the Rocky Mountains give 

place to the Mackenzie Mountains, also a bundle of 

ranges of folded and faulted rocks, but broader, more 

strongly arcuate, and offset a little to the east. An outer 

strand, the Franklin Mountains, deforms the rocks of 

the plains still farther east, nearly to Great Bear Lake 

and the edge of the Canadian Shield. Much farther 

northwest is the Brooks Range of northern Alaska, with 

a structure like that of the Rocky Mountains and Mackenzie Mountains, but trending nearly westward. Between the Mackenzie Mountains and the Brooks Range, 

the deep recess referred to above extends nearly to the 

Yukon River, into which structures curve from the 

ranges on either side. Within the recess are blocklike 

uplifts, such as the Richardson Mountains, separated 

by deep structural basins.

It should be evident from the "Tectonic Map of North 

America" that the Brooks Range is an integral part of 

the Cordilleran foldbelt (Gates and Gryc, 1963, p. 265- 

266), and is not an independent tectonic element, separated from the true Cordilleran foldbelt farther south 

by a "Yukon stable block," as has been claimed (Jeletzky, 1962, p. 62-66). The recess between the arcs of the 

Brooks Range and Mackenzie Mountains has features 

of a stable block, but this block does not extend indefinitely westward across Alaska. Large parts of the preMesozoic rocks south of the Brooks Range are Paleozoic 

geosynclinal deposits, partly metamorphosed, which 

have participated in two or more of the Cordilleran 

orogenies.

No Precambrian crystalline rocks emerge anywhere 

within the ranges of the miogeosynclinal belt, nor apparently elsewhere in the northern Cordillera. However, 

younger Precambrian supracrustal rocks are extensively 

exposed in the higher uplifts in the miogeosynclinal 

belt and the adjoining part of the eugeosynclinal belt 

(Reesor, 1957, p. 152-162; Gabrielse, 1967, p. 271-275). 

They form significant "structural stages" in the lower

part of the stratified sequence, hence are shown separately on the tectonic map (O3,04).

The Middle Proterozoic part of this supracrustal sequence (O3) is exemplified by the Belt (Purcell) Series 

that is widely exposed near the 49th parallel, but whose 

equivalents also occur in the outer ranges as far north 

as Yukon Territory. The Belt is formed of shallowwater, fine-grained clastic rocks and interbecMed carbonates; toward the east it overlies a Hudsonian (Lower 

Proterozoic) crystalline basement (H5, O2), but within 

the foldbelt its base is not visible even though it attains 

a thickness of 15,000 m (45,000 ft). It is everywhere unconformable beneath younger rocks, and in the interior 

of the foldbelt is succeeded by Upper Proterozoic supracrustal rocks (O4), exemplified by the Windermere 

Series near the 49th parallel, a sequence nearly as thick, 

but coarser and with some volcanic components, that 

is conformable or nearly so with the succeeding Paleozoic. The Upper Proterozoic includes the "grit unit" of 

the internal zones in Yukon Territory, a poorly sorted 

arkose, apparently derived from crystalline rocks farther southwest.

The Middle Proterozoic supracrustal rock? are geosynclinal deposits that formed in a trough antecedent 

to the main Cordilleran geosyncline, yet one which nearly corresponded in position to its successor. Most of its 

sediments were seemingly derived from the continental 

interior. This early geosynclinal phase was terminated 

by crustal movements, most of which were ep^irogenic, 

but which attained erogenic proportions, at least in 

Yukon Territory. The succeeding Upper P-oterozoic 

supracrustal rocks are more closely related to the main 

Cordilleran geosynclinal phase, and form its basal deposits where present. Their coarse sediments were derived from mixed sources, and part of them from lands 

farther in the interior of the foldbelt. Neithe^ of these 

Proterozoic sequences was materially disturbed until 

the much later Cordilleran orogenies, and t]i?ir metamorphic phases (O3a, O4a) date from that period.

On the tectonic map, strata shown as iniogeosynclinal 

(O9) in the northern Cordillera extend from Cambrian 

to Jurassic in Canada, and from Upper Devonian to 

Jurassic in Alaska where the lower Paleozoic is shown 

separately (O5). The Cretaceous is generally excluded, 

but is mostly preserved in the adjoining platform area 

(B) rather than in the foldbelt. On the map, the boundary between the miogeosynclinal and platform area is 

thus commonly drawn at the contact between the 

Cretaceous and older strata, which in manr places is 

also the outer limit of strong deformation. The deformed 

Paleozoic rocks in the outer strand of the foldbelt in the 

Franklin Mountains are therefore shown FS miogeosynclinal, although they might, with much propriety, be
NORTHERN CORDILLERA 67

considered as deformed platform cover. Also shown as 

miogeosynclinal are the pre-Cretaceous rocks in the 

recess between the Mackenzie Mountains and the Brooks 

Range, although not all of their complex sequence is 

truly geosynclinal (Jeletzky, 1962, p. 62-66). In places, 

miogeosynclinal rocks contrast greatly with adjacent 

eugeosynclinal rocks of the same ages (O6, O7), mostly 

where they are juxtaposed along faults. Elsewhere, the 

boundary is indefinite, different parts of the sequence 

partaking of one f acies or the other, so that the contact 

is arbitrarily drawn.

In general, the Paleozoic miogeosynclinal rocks are 

dominantly carbonates,, and the Mesozoic dominantly 

clastic wedge deposits, but the sequence varies greatly 

from one part of the belt to another (Bally and others, 

1966, p. 361-369; Gabrielse, 1967, p. 275-283; Brosge 

and others, 1962). Unconformities occur, especially in 

the upper part of the Devonian, but they were mostly 

produced by epeirogenic rather than erogenic movements, and angular discordances are local. The array of 

unconformities and the epeirogenic movements which 

they express is especially complex in the recess between 

the Mackenzie Mountains and the Brooks Range (Jeletzky, 1962, p. 62-71). However, true orogenic deformation seems to have occurred in the core of the eastern 

Brooks Range, where rocks below the Upper Devonian 

have been much metamorphosed (O5a); granites in the 

vicinity have been dated radiometrically between 370 

and 220 m.y., hence are middle to late Paleozoic (Oe). A 

possible relation between these deformed rocks and the 

Innuitian foldbelt to the northeast has already been 

suggested (p. 53).

Most of the Mesozoic clastic wedge deposits of the 

miogeosynclinal sequence are related to orogenic activity in the eugeosynclinal part of the foldbelt, rather 

than in the miogeosynclinal area itself. Nearly all the 

deformation in the miogeosynclinal part of the foldbelt 

was late in the Cretaceous or still later, hence is Laramide in the broad sense; in the southern part of the 

segment Paleocene strata are deformed equally with 

the Cretaceous in the foothill belt east of the Rocky 

Mountains. In the Brooks Range, however, Laramide 

deformation was preceded by an Early Cretaceous 

(Aptian) time of orogeny and thrusting.

Much of the remainder of the northern Cordillera in 

both Canada and Alaska was a eugeosyncline during 

large parts of Paleozoic and Mesozoic time. Its supracrustal rocks and deformational events were as varied 

as those in other eugeosynclines, and at least the younger 

of these are well documented in many places. Volcanic 

rocks are common, especially in certain parts of the 

sequence for example, from Devonian into Mississippian and from Permian into Triassic in some areas 

but thick limestone bodies also accur, although they are 

mostly of local extent. Coarser clastic rocks, including 

conglomerate, attest the uplift and probable deformation of adjacent areas.

On the "Tectonic Map of North America," the eug'°x>- 

synclinal rocks that were involved only in the Mesozoic orogenies, and which are primarily of Mesozoic age 

(O7) are separated from an earlier part, primarily of 

Paleozoic age (O6) which was involved in one or more 

previous orogenies and reworked later. Besides these, a 

still older unit of early Paleozoic age (O5) is showr in 

Alaska a unit which was involved in middle Paleozoic 

or earlier orogenies, and not all of whose rocks are of 

eugeosynclinal character.

Rocks of the last-named category (O5) are extensive 

in the interior of Alaska, where they include phyllites 

and schists (O5a) that underlie little metamorphosed 

rocks of known Mesozoic age. On the flank of the Brooks 

Range the metarnorphic rocks are traceable northward 

into unmetamorphosed formations beneath Upr»er 

Devonian conglomerates; elsewhere, their ages are 

known only from fossils in scattered limestone units. 

Clearly, these rocks have been involved in one or more 

orogenies during Paleozoic time, and perhaps early in 

that time. In the Seward Peninsula the metamorpMc 

and partly fossiliferous rocks were folded along northsouth axes during the Paleozoic, and were refolded 

along east-west axes during the Mesozoic or later (D. M. 

Hopkins, oral commun., 1966).

Southeast of Alaska older Paleozoic rocks are exposed only along the northeastern and southwestern 

edges of the eugeosynclinal area; those on the northeast 

are gradational into the miogeosynclinal deposits, those 

on the southwest (in the islands of the Alaska Panhandle) are more truly eugeosynclinal (Brew and others, 

1966, p. 157-166). In the northeastern belt, where th^se 

rocks are not separated from the younger Paleozoic, 

significant orogeny near the close of the Devonian has 

been recorded in the Cassiar area and elsewhere 

(Wheeler, 1966, p. 34). Plutonic rocks of early Paleozoic age (Oc) occur in the southwestern area.

Later Paleozoic and early Mesozoic eugeosynclinal 

deposits (O6, O7) extend the length of the northern 

Cordillera, and many sequences attain thicknesses of 

more than 7,600 m (25,000 ft). Proportions of the different components volcanics, chert, argillite, grr»ywacke, and limestone vary greatly from one place and 

from one part of the sequence to another, but the Me^ozoic generally contains coarser elastics than the Pal?.ozoic. Deposition was widely interrupted during early 

Triassic time, mostly by epeirogenic movements, hut 

locally by moderate deformation (White, 1966a, p. 187).

Mesozoic time was one of nearly continuous orogeny
68 THE TECTONICS OF NORTH AMERICA

from place to place in the eugeosynclinal area, and it is 

difficult to make any meaningful generalizations as to 

erogenic climaxes; the statement on the legend of the 

tectonic map that the eugeosynclinal deposits were deformed mainly during a middle Mesozoic (Nevadan) 

orogeny is a gross oversimplification (Misch, 1966, p. 

119). In many areas all the Mesozoic, and even part of 

the Tertiary, is steeply folded. Nevertheless, a significant part of the deformation had been accomplished 

well before the terminal Mesozoic (Laramide) orogeny 

in the miogeosynclinal belt to the northeast. The 

younger Mesozoic and the Tertiary rocks (OlO, Oil), 

even where steeply folded, did not participate in many 

of the deformations that affected the older rocks; they 

are less metamorphosed and plutonized, and they contain larger volumes of terrestrial deposits and of coarse 

immature elastics.

During Mesozoic time granitic rocks were emplaced 

in the eugeosynclinal area on a vast scale (O0). The 

great Coast batholith of western Canada is nearly continuous for 1,800 km (1,100 miles), from the 49th past 

the 60th parallel, and is 80-200 km (50-120 miles) wide, 

but' it is inhomogeneous internally and of several ages 

(Roddick, 1966, p. 76-79). Other sizeable but less continuous batholiths occur in Alaska to the northwest and 

in Canada to the east, especially in a belt along the 

northeastern side of the eugeosynclinal area (Cassiar 

Mountains southward to Monashee Mountains). Geologic relations and radiometric datings both indicate a 

prolonged time of emplacement from 250 m.y. to less 

than 70 m.y., or through all of Mesozoic and into Tertiary time. By far the most extensive granitic emplacement, especially in the Coast batholith, occurred during 

Cretaceous. The batholiths of early Tertiary age are 

late phases of the Mesozoic plutonism, but a few near 

the 49th parallel are as young as middle Tertiary (OA). 

Radiometric dating suggests pulses of granitic emplacement during the Mesozoic at intervals of 30 m.y., but 

while the pulses lie within the general period of orogeny, they have little obvious relation to the known 

times of deformation in the supracrustal rocks (Gabrielse and Reesor, 1964, p. 127-128).

Parts of the eugeosynclinal rocks have been strongly 

metamorphosed, mainly in areas near granitic plutons, 

which were also areas of long-persistent heating and 

recrystallization. Metamorphic rocks thus lie within 

and around the Coast batholith, as well as in the plutonic belt along the northeastern side of the eugeosynclinal area, where they are more continuous than the 

granitic rocks themselves. Many of the metamorphic 

rocks in these belts can be identified as equivalent to less 

altered Proterozoic, Paleozoic, and Mesozoic formations, and have been so mapped (O3a, O4a, O6a, O7a).

Other metamorphic rocks have been transformed by 

recrystallization and plastic flowage to such an extent 

that most indications of their original nature r.nd relations have been lost, and they are therefore mapped as 

metamorphic complexes (Ol). The complexes include 

the Birch Creek Schist of interior Alaska, the equivalent Yukon Group of Yukon Territory, the Shuswap 

complex of southern British Columbia, and various 

intervening bodies all in the northeastern plutonic 

and metamorphic belt.

Earlier geologists interpreted the metamorphic complexes as an Archean protaxis that separated younger 

geosynclines on the two sides, but subsequent investigations reveal a much more eventful history, involving heating and recrystallization in place, penetrative 

flowage, and superposed deformations through prolonged periods of time. Most radiometric dates in the 

complexes are surprisingly young 150 m.y. or less 

but those record only the latest thermal events and subsequent cooling, after burial at great depths in the 

crust. Parts of the complexes may be highly altered 

phases of Paleozoic or even Mesozoic supracrustal 

rocks, but other parts may once have been Proterozoic 

supracrustal rocks or an even earlier basement, now 

remobilized.

During Late Jurassic and through most of the Cretaceous time, the deformed eugeosynclinal rocVs of the 

northern Cordillera were partly covered by deposits of 

successor basins (OlO), which were themselves variably deformed later. The deposits are coarse elastics 

and interbedded volcanics, partly marine, partly continental, most of the clasts having been derived from 

surrounding uplifted areas; the deposits also include 

fragments from granite plutons that had Hen emplaced only a little earlier (Grantz and otheT-s, 1963, 

p. B58-B59). The successor basins are most extensive 

in Alaska (Gates and Gryc, 1963, p. 269-272), but the 

Bowser basin in central British Columbia is 200 by 320 

km (120 by 200 miles) across (Souther and Armstrong, 

1966, p. 178-180). Smaller basins of continental Tertiary deposits (Oil) occur here and there in the interior 

of the northern Cordillera.

Parts of the eugeosynclinal rocks are broken 17 faults, 

some of which originated during the geopynclinal 

period itself, but extensive low-angle thrusts h*.ve been 

described only in the northern Cascade Range of Washington (Misch, 1966, p. 120-136). Of greater interest 

are the very lengthy longitudinal faults, which are 

commonly expressed as topographic trenches o^* valleys 

on the land and as fiords between the offshore islands. 

The fault zone of the Rocky Mountain Trend and its 

continuation into the Tintina Trench extends with a 

few offsets from south of the 49th parallel across Can-
CENTRAL CORDILLERA 69

ada into Alaska, and the Denali fault zone extends 

through the high mountains of southern Alaska into 

the southeastern panhandle. The straight, or gently 

curved courses of these and other fault zones, the interchange of apparent upthrown and downthrown sides 

along their courses, and the frequent absolute differences between rocks on the opposite sides all suggest 

major strike-slip displacements; strike-slip displacements have been proved in a few segments, but the actual displacement in much longer segments remains 

enigmatic. The Fairweather fault near the coast of 

Alaska is active today and has given rise to major 

earthquakes, but most of the remainder have long been 

dormant.

CENTRAL CORDILLERA

The Cordilleran Mountains occupy most of the 

United States west of the 104th Meridan, but the central segment of the Cordilleran foldbelt (O), as here 

defined, forms only part of them. The foldbelt is limited northward by the Lewis and Clark transverse zone 

near the 46th parallel, and southward by another transverse zone near the 34th parallel the Transverse 

Ranges and the Texas Lineament of southern Calilyrnia and Arizona. The eastern part of the mountain system (Central and Southern Rocky Mountains and Colorado Plateau) is classed as a reactivated 

craton (B), and the western part (Cascade Range and 

Coast Ranges) is placed in the Pacific foldbelt (P). As 

here defined, the Cordilleran foldbelt is 1,100 km (700 

miles) wide near the 42nd parallel, but it is much narrower to the north and south.

Even within the central segment of the foldbelt as 

thus restricted, the fundamental Mesozoic and older 

orogenic structures are greatly obscured by younger 

rocks and structures, so that their gross pattern is by 

no means as evident on the tectonic map as it is in the 

northern Cordillera. Superposed on the Mesozonic and 

older structures are Cenozoic volcanic fields and a system of block faults which trend in other directions, and 

on the tectonic map these varied features play against 

each other in counterpoint. In the northern part of the 

segment, the continuity of the Mesozonic and older 

structures is interrupted entirely by a transverse belt 

of volcanic rocks that extends from the Snake River 

Plain of Idaho into the Absaroka Mountains of 

Wyoming.

The eastern, or miogeosynclinal part of the foldbelt 

is poorly defined north of the transverse volcanic belt, 

in Idaho and Montana, where it is closely crowded between cratonic and internal elements. It widens farther 

south in the Great Basin, where it extends westward 

400 km (250 miles) from the edge of the Colorado 

Plateau in Utah into central Nevada. In eastern California it extends as far west as the front of the Sierra 

Nevada, but in the Mojave Desert to the south its continuity is lost in the confused ranges of the southern 

transverse zone.

In western Wyoming and southeastern Idaho the 

frontal part of the miogeosynclinal belt consists of closely crowded folds and thrust slices, very much like those 

in the Canadian Rocky Mountains (Armstrong and 

Oriel, 1965). Southward in Utah, the frontal part is deflected into the eastern edge of the Great Basin, aromd 

the western end of the Uinta Mountains axis. The ranges 

of the eastern Great Basin contain low-angle thrust 

faults of great lateral displacement, many of which superpose strata near the base of the Paleozoic over Mesozoic strata; the thrusts can be correlated from one range 

to the next, through Utah into southern Nevada, in such 

a manner as to indicate that they are continuous features (Longwell, 1960; Crittenden, 1961; Armstrong, 

1968, pi. 1). Behind the frontal belt in western Utah and 

eastern Nevada, the thrusts within the ranges difl'er; 

higher parts of the miogeosynclinal sequence have 

moved over the lower parts, and the whole has moved 

over a major decollement near the base of the Paleozoic 

(Misch, 1960). Correlation from one range to the next 

indicates that the decollement is a continuous feoture 

like the frontal thrusts to the east. It is tempting to infer 

that the decollement is the root zone of the frcutal 

thrusts, although there is some evidence that the 

former originated somewhat before the latter (Armstrong, 1968, p. 444). Frontal thrusts and decollement are exposed across nearly the whole width of the 

miogeosynclinal belt in this latitude, and if they are 

parts of the same feature there was an eastward transport of very broad sheets of miogeosynclinal strata over 

their basement although for distances by no means as 

great as the observed breadth of exposure.

The crystalline basement (O2), probably part of the 

Hudsonian foldbelt, emerges near the eastern edg«. of 

the miogeosynclinal area in Utah, but it extends ae far 

west as Death Valley in southeastern California. It is 

succeeded in places by Middle Proterozoic strata (O3), 

comparable to the Belt Series farther north and the 

Grand Canyon Series farther east, and more widely by 

Upper Proterozoic strata. The latter attain imprersive 

thicknesses in the south (Noonday, Johnnie, Wyman, 

and other units), and like the Upper Proterozoic of the 

northern Cordillera are nearly conformable with the 

Paleozoic above; in this segment of the Cordillera their 

areal extent is not great enough to separate them from, 

the higher miogeosynclinal strata on the tectonic map.

The miogeosynclinal sequence of the central Cordillera (O9) is largely Paleozoic, but includes remnants of 

lower Mesozoic (Triassic and Jurassic) at the top, as
70 THE TECTONICS OF NORTH AMERICA

well as some infolded Cretaceous along the eastern edge. 

Farther back in the foldbelt, as at Eureka, Nev., the Cretaceous is postorogenic and continental, and was laid 

down in successor basins whose present remnants are 

too small to map. Throughout the Great Basin, the 

volume of the Paleozoic carbonate rocks is impressive. 

To the south, near Las Vegas, Nev., 5,500 m (18,000 ft) 

of dominantly carbonate rocks, extend from Cambrian 

to Permian. Farther north, near Eureka, Nev., carbonate rocks from Cambrian to Devonian alone are 4,500 m 

(15,000 ft) thick, but in this latitude they are succeeded 

by a Mississippian clastic wedge that expands westward 

toward the middle Paleozoic Antler orogenic belt in the 

eugeosynclinal area (Nolan and others, 1956); the thinner, higher Paleozoic, again contains carbonate strata. 

Further complications develop in the eastern part of 

the miogeosynclinal area in the same latitude, where the 

upper Paleozoic thickens to 9,000 m (30,000 ft) in the 

Oquirrh basin nearly 10 times its value in nearby 

areas in an exceptional area of extreme but transitory 

subsidence. On the other hand, in Idaho and Montana, 

ephemeral positive areas have caused large gaps in the 

Paleozoic sequence, and the Lower Cambrian is missing 

at the base.

The eugeosynclinal belt of the central Cordillera extends westward from the miogeosynclinal belt to the 

Pacific f oldbelt (P) near the coast, which had a different tectonic history. In California the boundary with 

the Pacific foldbelt is along the western edge of the 

Sierra Nevada and Klamath Mountains, which in the 

latter area is a zone of east-dipping thrusts (Irwin, 

1966, p. 33-36; Davis, 1968). In Oregon and Washington the boundary curves eastward in the Columbia arc 

(not labeled on the map), as shown by deflection of both 

structural trends and the quartz diorite petrographic 

boundary (Moore, 1959, p. 199). Whether the Columbia 

arc was indigenous, or whether it was created by oroclinal bending during the Cenozoic (Hamilton and 

Myers, 1966), is beyond the scope of this discussion.

As in the northern Cordillera, the eugeosynclinal 

supracrustal rocks of the central Cordillera are divided 

on the map into a unit of Triassic and Jurassic age (O7) 

deformed only during the Mesozoic orogenies, and a 

unit of Paleozoic age (O6) that was deformed during 

several earlier orogenies.

In Nevada, the eastern part of the eugeosynclinal belt 

is dominated by a lower to middle Paleozoic (Ordovician to Devonian) sequence of cherts and volcanics as 

much as 15,000 m (50,000 ft) thick that was deformed 

during the Antler orogeny of middle Paleozoic (Early 

Mississippian) time (Roberts and others, 1958, p. 2816- 

2821). The orogenic belt can be traced through the later 

structures that are superposed on it for a distance of

800 km (500 miles); its principal manifestation is an 

eastward transport along the low-angle Roberts thrust 9 

of the eugeosynclinal sequence over a miogeosynclinal 

sequence of the same age (fig. 6 and 13). Windows and 

klippen of the thrust occur from one range to tl ^ next 

across a breadth of 95 km (60 miles), which is the. minimum distance of transport. The known manifestation 

of the Antler orogenic belt involved only shallow parts 

of the crust, but its deep-seated origin is sugges+ed by 

its influence on subsequent tectonic history. The o**ogenic belt was the site of lesser disturbances durirg succeeding Paleozoic time, and the upper Paleozoic and 

lower Mesozoic eugeosynclinal deposits were laic1 down 

only farther west (Silberling and Roberts, 1962, p. 7- 

25). The lower Mesozoic eugeosynclinal rocks (O7) 

pass eastward into near-shore shelf deposits toward the 

site of the orogenic belt.

Elsewhere in the western part of the Cordilleran foldbelt eugeosynclinal conditions were long persistent. 

Volcanic rocks as old as Silurian and Devonian have 

been identified in parts of the Klamath Mountains and 

the Sierra Nevada, and volcanic rocks are prominent in 

the remaining Paleozoic and the older Mesozoic, but the 

Klamath Mountains sequence contains several prominent 

Paleozoic carbonate formations (Irwin, 1966, p. 23; 

Clark and others, 1962). The Triassic and Jurassic contain great volumes of andesitic basaltic submarine lavas 

and pyroclastics (Dickinson, 1962). Partial sequences 

of eugeosynclinal rocks in various areas are 9,000 m 

(30,000 ft) or more thick; however, the total of the 

whole eugeosynclinal column is undetermined. Structural unconformities at various levels indicate pulses of 

orogeny from one place to another at various timos during the Paleozoic and early Mesozoic, but their extent 

and magnitude is not as clearly defined as it is in the 

Antler orogenic belt.

The climactic middle Mesozoic orogeny in the Sierra 

Nevada has been dated as late in the Jurassic, and1 is the 

classic Nevadan orogeny; the climax may havo been 

earlier or later in other parts of the eugeosynclinal belt 

(Gilluly, 1963, p. 146-150). Deformation was progressively younger eastward across the foldbelt, and in the 

miogeosynclinal area there is no clear separation between a middle Mesozoic orogeny and a terminal Mesozoic orogeny; most of the events were at intermediate 

times. At Eureka, Nev., deformed miogeosynclinal rocks 

are overlain by remnants of Lower Cretaceous successor 

basin deposits, but at the eastern edge in the same 

latitude the coarse synorogenic deposits are Upper

 Termed the "Roberts Mountains thrust" In many publications, but 

the shorter form proposed by Gilluly (1963, p. 141-142) to preferred by 

the compiler.
EXPLANATION

120

CENTRAL CORDILLERA

118°

71

116° 114°

 3 

§ fc

Outcrop of eastern 

(miogeosynclinal) sequence

Outcrop of 

Roberts thrust

Inferred trace of Roberts 

thrust, where covered 

by younger deposits

Boundaries between 

tectonic regions

OREGON [lDAHO__ _ £_ _ _

"" NEVADA" '/

-38°

200 KILOMETERS

200 MILES

FIGURE 13. Map of northern Nevada showing extent of Antler orogenic belt of middle Paleozoic time. The Paleozoic and 

Mesozoic rocks of northern Nevada are divisible into three tectonic regions with the following characters: Region A, Lower 

and upper Paleozoic eugeosynclinal deposits not affected by Antler orogeny, deformed by Sonoma orogeny during Peririan 

time, overlapped from west by Triassic shelf deposits, and deformed by middle Mesozoic orogenies. Region B, Antler 

orogenic belt and area involved in Roberts thrust; lower Paleozoic eugeosynclinal deposits deformed by Antler orogeny 

in middle Paleozoic time and overlapped from east by upper Paleozoic shelf deposits. Region C, Lower Paleozoic miOTeosynclinal deposits, upper Paleozoic clastic wedges derived from Antler orogenic belt, and minor Mesozoic deposits; deforned 

during late Mesozoic time, with development of thrusts and decollements. Compiled from many sources, including Roberts 

and others (1958).

Cretaceous (Spieker, 1946). Major thrusting in the 

miogeosynclinal belt in Utah was completed by the middle of Late Cretaceous time (Armstrong, 1968, p. 451). 

Farther north, in southeastern Idaho and western Wyoming, successive thrust blocks formed from west to east 

between Late Jurassic and Paleocene time (Armstrong 

and Oriel, 1965, p. 1857-1861). The classic Laramide 

orogeny of Paleocene time occurred primarily in the 

reactivated craton, east of the Cordilleran foldbelt as 

here defined.

Ultramafic rocks (Oy) are prominent in the western 

part of the eugeosynclinal belt, especially in the Sierra 

Nevada and Klamath Mountains, where they form

gently to steeply dipping tabular bodies, some of great 

thickness and extent. Commonly these bodies are ? long 

fault planes, and whatever their original history they 

clearly have been emplaced into their present positions 

by tectonic processes.

Granitic rocks (O0) are as extensive in the eugeosynclinal belt of the central Cordillera as they are in the 

northern Cordillera. They coalesce into two major 

batholiths the Sierra Nevada batholith in the routh 

and the Idaho batholith in the north, the latter offset 

eastward from the former in accordance with the curve 

of the Columbia arc (p. 70). Elsewhere, granitic rocks 

form smaller plutons in the eugeosynclinal terrare, or
72 THE TECTONICS OF NORTH AMERICA

inliers in areas of younger rocks. Both of the larger 

batholiths are composite, being composed of many discrete plutons, and in the Idaho batholith including migmatized host rocks. The granitic rocks become more 

silicic eastward, changing from quartz diorite to quartz 

monzonite and granodiorite (Moore, 1959, p. 205-206). 

With some exceptions, they are also younger from to 

west to east; all are Mesozoic, but they yield radiometric 

dates between 180 and 80 m.y., or from late Triassic 

through the Cretaceous (Kistler and others, 1965, p. 

162-164). Most of those west of the Sierra Nevada batholith are Jurassic or older, whereas most of the dated 

plutons in the Sierra Nevada and Idaho batholiths are 

of Cretaceous age. The Boulder batholith near the front 

of the foldbelt east of the Idaho batholith invades Late 

Cretaceous rocks. In the Great Basin Mesozoic plutons 

are mingled with smaller Tertiary intrusives (OA); the 

latter extend thence across the miogeosynclinal belt and 

into the reactivated craton east of it. The age range of 

the Mesozoic granitic rocks broadly coincides with the 

time of erogenic deformation of the eugeosynclinal 

rocks, but those of Cretaceous age are later than the 

erogenic climax; the Sierra Nevada batholith breaks 

into and shoulders aside eugeosynclinal rocks that 

had already been steeply upended and regionally 

metamorphosed.

During Cenozoic time, wide areas of the deformed 

geosynclinal rocks were covered by terrestrial volcanic 

rocks (Oft, Oir). The cover is largely intact through 

Oregon into southern Idaho, but farther south it is 

dismembered by block faulting and erosion. Volcanic 

history was prolonged; the older Tertiary volcanics are 

much deformed and mineralized, the more extensive 

middle Tertiary volcanics are block faulted, the Quaternary volcanics are merely tilted in the transverse Snake 

River downwarp. The varied compositions and origins 

of the volcanic rocks cannot readily be indicated on the 

tectonic map, and are therefore not differentiated in the 

generalized units adopted. Basaltic and andesitic volcanics are prominent to the north and west, but silicic 

volcanics cover large parts of the southern Great Basin, 

and include widely spread sheets of ignimbrite; where 

known, the calderas from which they were erupted are 

shown by smbols on the map.

During Cenozoic time also, large parts of the central 

Cordillera were disrupted by block faulting, caused by 

crustal distention. Block faulting has occurred in other 

foldbelts of the world late in the tectonic cycle, but its 

magnitude here is without parallel. Block faulting and 

volcanism partly coincide in time, but the faulting continued later; major faulting occurred during the early 

Pleistocene, and minor faulting continues today in 

places. In the Great Basin, the previously deformed terrane was split longitudinally into broad to narrow 

blocks, whose displacement produced the present topography of disconnected ranges and intervening basins; 

larger, longer fault blocks along the edge of tho Colorado Plateau to the east and the Sierra Nevada to the 

west are more coherent manifestations of the same structure. The block faulting is a mountain-creating process, 

hence one of the few deformationa] events in the Cordillera to which the term "orogeny" can be applied in the 

narrow sense of Gilbert; in the broader sense of the term 

the block faulting is more properly postorogenic.

The faults at the edges of the mountain blocks are 

shown by normal fault symbols on the tectonic map, so 

far as they are known. Some of the known faults can be 

observed directly, the existence of others is proved by 

the even baselines of the mountains and the steep escarpments behind them. Other mountains may also have been 

fault blocks, but they were uplifted so long ago that the 

fault scarps have been destroyed by erosion, and the 

positions of the faults themselves are lost. The downfaulted blocks, or basins between the ranges, have been 

filled by debris eroded from the upfaulted areas, shown 

on the map as "thick deposits in structurally negative 

areas" (O12). Coarse fanglomerates were deposited near 

the edges of the basins and fine sands and silts near 

the centers, the whole accumulation of deposits attaining thicknesses of 3,000 m (10,000 ft) or more. On the 

tectonic map the distribution of these debris-filled basins 

suggests the pattern of latest deformation in tl v<^ central Cordillera, even where faulting is not apparent.

SOUTHERN CORDILLERA

The southern Cordillera is separated from tH central Cordillera by the so-called Texas Lineament 

(Albritton and Smith, 1957; King, 1965, p. 118), a transverse zone that extends east-southeast from the Transverse Ranges of coastal California, south of the Colorado 

Plateau and a little north of the United States-Mexican 

boundary, into western Texas. The zone is a strip oi 

country as much as 160 km (100 miles) wide that separates two parts of the Cordillera with different topographies, geologic histories, and styles of deformation. 

South of the zone the Cordilleran foidbelt exterds 800 

km (500 miles) farther east than on the north side, and 

for long distances its deformed rocks closely adjoin 

little deformed rocks in the Colorado Plateau and the 

block mountains of New Mexico, which are reactivated 

or disrupted parts of the former craton. These contrasts 

have not been produced by transverse faulting, a nd the 

Texas Lineament is not a through-going fault zone, as 

has sometimes been assumed. It is true that the Transverse Ranges on the west contain eastward-trending 

high-angle faults, some with left-lateral displacement,
SOUTHERN CORDILLERA 73

but there are few faults parallel to the zone farther east, 

although it contains many faults trending in other directions (as elsewhere in the Cordillera).

The southern Cordillera includes all of Mexico except 

the Gulf Coastal Plain on the east, an area of platform 

deposits (C), and the parts of Baja California on the 

west which belong to the Pacific foldbelt (P). From 

southeastern Mexico it extends eastward into northern 

Central America, where it forms much of Guatemala, 

Belize, and Honduras. Except in the extreme south (see 

p. 74), most of the surface of the southern Cordillera 

is formed of Mesozoic and Cenozoic rocks, and older 

rocks emerge only as small inliers in the higher folds 

and fault blocks, so that the pre-Mesozoic structural 

pattern is largely undetermined or speculative. Very 

likely the Mesozoic and Cenozoic rocks and structures 

are superposed on a very different set of Paleozoic and 

older geosynclines and foldbelts, along with their forelands and backlands.

In the far northwest, in the Altar district of Sonora, 

are incomplete exposures of a geosynclinal sequence, 

very much like that in the Great Basin of the central 

Cordillera (O9), consisting of carbonate rocks that include Lower Cambrian and the higher Paleozoic systems, topped by Triassic and Jurassic marine shales; 

beneath are Upper Proterozoic supracrustal rocks like 

those in the Great Basin (O4), which rest on a crystalline (Hudsonian) basement (O2) (Cooper and Arellano, 1946; Damon and others, 1962). The relations of 

this geosynclinal sequence to rocks of surrounding 

areas in Mexico is unknown, and it is separated from 

its analogue in the Great Basin to the northwest by 

650 km (400 miles), in which no trace of it is preserved.

Farther east, the front of the foldbelt of the southern 

Cordillera overwhelms the Ouachita foldbelt (M), 

whose characteristic rocks and structures are exposed in 

the Marathon dome and some other areas in western 

Texas. In northeastern Mexico the Coahuila peninsula 

notably distorts the Cordilleran folds, and is a massif 

that was evidently consolidated during the Ouachita 

orogeny. Inliers of Paleozoic and Precambrian rocks 

occur, both near the Coahuila peninsula and farther 

south along the Sierra Madre Oriental, but are too frag' 

mentary to afford much of an idea of the true pre-Mesozoic structural patterns. The continuation of the Ouachita foldbelt (or Huastecan belt) within the southern 

Cordillera remains speculative. Some geologists (for 

example de Cserna, 1960, p. 598-601) have proposed that 

the late Paleozoic foldbelt follows the younger Cordilleran structures southeastward through Mexico into 

Central America.

Much of the southern Cordillera is a Mesozoic and 

younger feature. Its miogeosynclinal belt includes the

present Sierra Madre Oriental, the Mesa Central on the 

west, part of the Sierra Madre del Sur on the south, and 

its extension into Central America. The miogeosynclinal 

rocks are dominantly later Mesozoic (mapped as O9, 

like the more heterogeneous miogeosynclinal deposits 

in Sonora). Jurassic and Lower Cretaceous carbonate 

rocks form most of the ranges, and are succeeded by 

Upper Cretaceous marine clastic wedge deposits. P«vneath the carbonates are earlier Jurassic and Triarsic 

red elastics, which include evaporites in some places r.nd 

volcanics in others. In the interior ranges of central md 

southern Mexico the deformed Mesozoic miogeosynclinal rocks are overlain by thick accumulations of synorogenic and postorogenic coarse continental deposits of 

Eocene and younger age, the so-called red conglonr^rates (O10) (Fries, 1960, p. 91-101). In other ranges, 

fronting the Gulf Coastal Plain, marine lower Tertiary 

deposits, including Oligocene and even younger a#es, 

are folded with the earlier rocks. Deformation of the 

miogeosynclinal belt of the southern Cordillera was thus 

primarily Late Cretaceous and early Tertiary, or 

broadly contemporaneous with the Laramide orogeny 

of the central Cordillera.

Deformation of the miogeosynclinal rocks produ?-ed 

a fine display of folds (Alvarez, 1949, p. 1330-13? 3), 

some long and closely crowded, others shorter and discontinuous; these are accompanied by few low-an^le 

thrusts. The closely crowded folds formed over a decollement in the weak layers of the lower part of the 

Mesozoic sequence, especially where evaporites are interbedded ; the more open folds involve the basement, wHch 

emerges in the cores of the uplifts. South of the 22d 

parallel the deformed miogeosynclinal rocks front 

the little disturbed rocks of the Gulf Coastal Plain ir an 

abrupt mountain face, but farther north domical uplifts 

stand in front (shown by structure contours in the 

coastal plain rocks on the tectonic map) the Sienr. de 

Tamaulipas, the Sierra de San Carlos, the Sierranias 

del Burro, and the Marathon dome in Texas. The rrain 

bundle of folds is strongly deflected westward near the 

25th parallel around the south end of the Coahuila peninsula, producing a prominent salient near Monterrey, 

Nuevo Leon. A lesser branch of the foldbelt, with or>en 

folds, extends northwest from Monterrey to the Big 

Bend of the Rio Grande (Weidie and Murray, 1967, p. 

689). South of the 20th parallel the frontal folds b^.nd 

eastward from Chipas into Guatamala and Belize, vdth 

a south-projecting recess in midcourse.

The Mesozoic miogeosynclinal rocks of the Mesa Central in Mexico pass westward beneath little deformed 

Tertiary volcanics (Oft), including extensive sheets of 

ignimbrite, which form the broad dissected plateau0 of 

the Sierra Madre Occidental. Relations between the prer
74 THE TECTONICS OF NORTH AMERICA

volcanic external and internal zones of the foldbelt are 

thus largely concealed, and eugeosynclinal rocks are 

poorly known on the mainland. Metamorphic rocks of 

uncertain age (mapped as O6a) are exposed west of the 

volcanic plateau in Sinaloa. The prevolcanic rocks on 

the south coast of Mexico west of the 102d meridian 

(mapped as Ol and O9) are now known to include Mesozoic eugeosynclinal slates and volcanics and embedded 

ultramafic rocks (Zoltan de Cserna, written commun., 

1968). The main occurrence of eugeosynclinal rocks is 

in Baja California, where they and their granitic plutons (O7, O7a, O0) form the backbone of the northern 

half of the peninsula and its southern tip. In the north 

they are submarine lavas, pyroclastics, and volcanic 

sediments, steeply upended and partly metamorphosed; 

they are known to include strata of Jurassic and Lower 

Cretaceous (Aptian) age, and may include older components (Allison, 1964, p. 12-15). Both the eugeosynclinal rocks and the plutons are overlain by nearly 

undeformed latest Cretaceous (Campanian) sediments 

(OlO), indicating that the erogenic climax in the southern Cordillera was later than the classic Nevadan orogeny of analogous parts of the central Cordillera.

In the Sierra Madre del Sur of southern Mexico, and 

eastward into northern Central America, pre-Mesozoic 

rocks and structures are more widely exposed than elsewhere in the southern Cordillera. A sequence of Pennsylvanian and Permian carbonate rocks and their basal 

elastics extends from Chiapas across Guatemala into 

Belize; it was deformed by variable but undetermined 

amounts before the Mesozoic miogeosynclinal deposits 

were laid over it, hence is shown on the tectonic map as 

a lower "structural stage" (older miogeosynclinal deposits, O8). A much greater part of the pre-Mesozoic of 

this region is a metamorphic complex (Ol), varying 

from paraschist with interbedded marble and amphibolite to highly migmatized gneisses (McBirney, 1963, p. 

187-197; de Cserna, 1965, p. 15-19). It contains embedded granitic plutons that have yielded radiometric 

dates from 300-90 m.y., or from middle Paleozoic to late 

Mesozoic age (Oe, O0), thus indicating (as in the metamorphic complexes of the northern Cordillera) prolonged heating and plutonic activity. The complex itself 

is probably of heterogeneous ages. Much of it in Oaxaca 

is Precambrian, as it has yielded radiometric dates between 700-1,100 m.y. and is overlain unconformably by 

Cambrian strata (Fries and others, 1962, p. 45-52; Fries 

and Rincon-Orta, 1965, p. 81-87; Pantoja-Alor and 

Robison, 1967). In Guatemala and Belize the complex is 

unconformable below the upper Paleozoic strata, but 

may include earlier Paleozoic (Dixon, 1956, p. 13-17), 

whereas in southern Guatemala and Honduras it may

include equivalents of the upper Paleozoic itself 

(Williams and others, 1964, p. 2-5).

The topographic Sierra Madre del Sur of southern 

Mexico trends east-southeast parallel with the Pacific 

Coast and the Middle America Trench immediately offshore, but the structures of both its miogeosyi^.linal 

Mesozoic rocks and its metamorphic complex extend 

southeastward across it to the coast, where they are 

remarkably truncated. In Guatemala and Belize the 

structures are alined more toward the east, ard are 

traversed by a bundle of closely spaced high-angle faults 

with the same trend. The high-angle faults extend eastward into the faults that bound the Cayman Trench on 

the floor of the Caribbean Sea; their westward extension 

toward the Pacific is concealed. They enclose slices of 

diverse and contrasting rocks Paleozoic, Mesozoic, and 

Tertiary sediments, metamorphic complex, granitic 

rocks, and ultramafic rocks (the latter possibly up^hrust 

mantle material). To the compiler, this set of faults has 

every indication of being a zone of major strike-slip displacement, yet little tangible field evidence for such 

displacement has been reported (Walper, 1960, p. 1312- 

1313; McBirney, 1963, p. 212-213).

Immediately north of the Sierra Madre del Sur the 

transverse Mexican volcanic belt (On ) extends from the 

Pacific to the Gulf of Mexico, and contains a dozen lofty 

volcanic cones (including Pico de Orizaba or Citlaltepetl, 5,698 m or 18,696 ft) and a host of lesser cones 

and craters, some still vigorously active. It originated 

late in Cenozoic time and no doubt follows a deep-seated 

line of weakness, but it is accompanied by little surface 

rupture and it does not perceptibly deflect the structures 

of the miogeosynclinal rocks that plunge beneath it. from 

either side.

(F) PACIFIC FOLDBELT

Cenozoic erogenic activity is especially pronounced in 

that part of the Cordilleran Mountain System ne^.r the 

Pacific Coast, and on the tectonic map it is appropriate 

to separate this part from the main Cordilleran foldbelt 

(O) as a rather vaguely defined Pacific foldbelt (P). A 

similar distinction has been made in the Asiatic ectuivalent of the North American Cordillera, where Soviet 

geologists separate a "region of Cenozoic foHing" 

(sometimes called "Kamchatkan") from a "region of 

Mesozoic folding" farther inland. The Pacific foMbelt 

is characterized by late erogenic, volcanic, and seismic 

activity, by Cenozoic rocks and structures, and by a basement of eugeosynclinal rocks that are younger than 

those of the main Cordillera. The Pacific foldbelt was a 

late accretion to the edge of the North American continent, and part or all of it may have formed on what 

was originally oceanic crust.
WASHINGTON AND OREGON 75

On the "Tectonic Map of North America" the Pacific 

foldbelt is shown as including the Aleutian Islands, the 

coastal mountains of southern Alaska, the Coast Ranges 

of Washington, Oregon, and California, part of the 

volcanic province of Washington and Oregon, and part 

of Baja California. Its boundary with the Cordilleran 

foldbelt is well defined in places, but indefinite in others, 

where it has been necessary to overlap the units distinctive of the two foldbelts. Thus, granitic rocks of the 

Cordilleran foldbelt (O0) are shown in the California 

Coast Ranges, and plateau basalt of the Pacific foldbelt 

(Py) is shown in the interior of British Columbia.

ALASKA

The characteristic rocks and structures of the internal 

part of the Cordilleran foldbelt extend westward across 

southern Alaska, south of the Alaska Range and into the 

Alaska Peninsula Mesozoic eugeosynclinal rocks (O7) 

with their embedded granitic plutons (O0), and younger 

Mesozoic and Tertiary successor basin deposits (OlO, 

Oil). Between them and the coast, in the Chugach 

Mountains, Kenai Peninsula, and Kodiak Island, are 

younger eugeosynclinal deposits a vast sequence of 

steeply upended and partly metamorphosed graywackes, containing granitic debris from the middle 

Mesozoic plutons farther inland, but themselves intruded by small plutons of Tertiary age (P/3) (Miller, 

1959, p. 20-21). Sparse fossils indicate that the greater 

part of this sequence is of Cretaceous age (PI), but it is 

overlain near Prince William Sound by similar deposits 

of Paleocene age (P2). The same sequence extends 

southwestward beneath the continental shelf off the 

Alaska Peninsula, where it reappears in the Shumagin 

Islands (Burk, 1965, p. 63-71). The base of the Cretaceous eugeosynclinal deposits is undetermined, and 

they may have been laid down off the edge of the original Cordilleran foldbelt, at oceanic depths at the foot 

of the continental slope. Overlying the eugeosynclinal 

deposits in their eastern segment is 7,600 m (25,000 ft) 

of younger Tertiary shallow water marine deposits 

(P4).

Northwest of the Cretaceous-Pa leocene eugeosynclinal belt is the younger Aleutian volcanic arc (Coats, 

1962), which extends 960 km (600 miles) through the 

Alaska Peninsula, ending northeastward at Mount 

Spurr in the Alaska Range. West of the peninsula, the 

arc runs out to sea for another 1,600 km (1,000 miles) 

through the Aleutian Islands; the islands surmount a 

submarine ridge between the deep Aleutian Trench on 

the south and the shallower Bering Sea on the north; the 

ridge continues westward through the Kommandorsky 

Islands to the coast of Kamchatka. The basement of the 

Alaska Peninsula is formed of Mesozoic and older rocks

of the Cordilleran foldbelt, and is succeeded by Tertiary 

volcanics and volcanic-derived sediments (PS, P4), 

which are surmounted by young andesitic volcanic cor0® 

(Pe), some still vigorously active (Burk, 1965, p. 119- 

122). Very little of the pre-Tertiary basement (PI) 

emerges in the Aleutian Islands, and they are mostly 

formed of Tertiary and younger volcanics, the young 

andesitic cones lying on the northern side of the arc, 

away from the Aleutian Trench.

WASHINGTON AND OREGON

The northwestern States of Oregon and Washington 

differ both in geology and tectonic style from nearby 

parts of the Cordilleran region, for here Cenozoic volcanic rocks and structures dominate the scene nearly 

to the exclusion of all others (Waters, 1955, p. V04-71S; 

Snavely and Wagner, 1963) lava plateaus, volcanic 

cones, and eruptive ranges that were produced by upbuilding, and folded or block-faulted ranges that were 

produced by deformation of the volcanic materials. Cenozoic volcanic rocks, and the sediments associated with 

them, extend along the Pacific Coast for 480 km ("% 

miles) and extend inland for as much as 650 km (4-00 

miles). Within the volcanic areas pre-Cenozoic rocks 

emerge only as occasional inliers, although they form 

most of its periphery.

The pre-Cenozoic rocks of the Inliers and of the periphery of the volcanic area belong to the eugeosynclinal 

part of the Cordilleran foldbelt, whose structures are 

recessed eastward in the Columbia arc (see p. 70); 

the crust within the recess probably differs from that 

of the Cordilleran foldbelt, and may have been oceanic 

during its early history. On the site of the present Co^-st 

Ranges within the recess, eugeosynclinal deposits (P2) 

were laid down during Eocene time, and are interbedded 

with large volumes of submarine pillow basalt (P£a). 

During Miocene time the eastern part of the recess v^as 

covered by great floods of plateau basalt of the Columbia 

River Group (Py), and at about the same time andesitic 

lavas and pyroclastics (PS) accumulated along the site 

of the Cascade Range to the west. Building of the Cascade Range continued into late Tertiary and Quaternary time, when a chain of volcanoes (Pe) grew along 

its north-south axis. During Tertiary time, small mafic 

to silicic plutons (P/?) were intruded in the volcanic 

rocks of the Cascade Range.

The structure of the volcanic area was produced by 

a combination of volcanic construction and of moderate 

deformation during Cenozoic time. The Coast Ranges 

on the west are an anticlinorium, openly folded except 

in the steeply upthrust Olympic Mountains at the 

north end. Between them and the Cascade Range to the 

east is a belt of downwarping expressed as a chain of
76 THE TECTONICS OF NORTH AMERICA

lowlands extending from Puget Sound southward to 

the Willamette Valley, whose deeper parts are thickly 

covered by late Tertiary and Quaternary deposits (P5). 

The plateau basalts within the recess of the Columbia 

arc form a broad structural basin, little deformed in 

places, but in others crossed by anticlinal folds.

CALIFORNIA AND BAJA CALIFORNIA

The Coast Ranges of California extend south-southeastward along the Pacific Coast for 925 km (575 miles) 

from Cape Mendocino past San Francisco Bay to Point 

Conception, and are separated by the Great Valley 

from the Cordilleran foldbelt in the Sierra Nevada to 

the east. South of Los Angeles, the Peninsular Ranges 

extend south-southeastward into Baja California. In 

the intervening area are the eastward-trending Transverse Ranges. Cenozoic strata (P4), largely of marine 

origin, are thick and extensive in the Coast Ranges 

south of San Francisco Bay, as well as in parts of the 

Transverse Ranges and adjacent basins, and have undergone several periods of erogenic deformation.

In the coastal belt of California great thicknesses of 

clastic rocks accumulated from latest Jurassic through 

Cretaceous time, or mainly after the climax of the Nevadan orogeny in the Cordilleran foldbelt. To the east 

these rocks are fossiliferous shelf deposits (P3), but 

to the west they are a. contemporaneous sequence of 

poorly fossiliferous eugeosynclinal deposits (Pi) the 

graywackes, shales, cherts, and pillow basalts of the 

Franciscan Formation and its kindred (Bailey and 

others, 1964). The Jurassic and Cretaceous shelf deposits are thrust to the west over the Franciscan deposits, 

and no transitional phases between them are known 

(Page, 1966, p. 268-272). The Franciscan deposits were 

probably laid down at oceanic depths at the foot of the 

continental slope, in a manner similar to the Cretaceous 

and Paleocene eugeosynclinal deposits along the south 

coast of Alaska.

Rocks similar to the Franciscan Formation probably 

extend southward along the continental shelf west of 

the Peninsular Ranges as far as southern Baja California, as they emerge in many of the offshore islands 

and coastal promontories of the peninsula, such as Catalina Island in the north and Sebastian Vizcaino Peninsula farther south (Allison, 1964, p. 14-15). Here, in 

contrast to farther north, they appear to be of about 

the same age as the eugeosynclinal rocks of the Cordilleran foldbelt to the east (O7), rather than younger as 

in California. Their sedimentary facies is decidedly 

different, and they contain no granitic plutons.

In other parts of the coastal mountains of California 

a basement of crystalline rocks emerges, not only in the 

Peninsular and Transverse Ranges, but also in a long

strip of the Coast Ranges that extends northward past 

San Francisco Bay. The basement of the Transverse 

Ranges includes Precambrian plutonic rocks ancf Paleozoic metamorphic rocks (Oa, O/?, O6a). In the Coast 

Ranges to the north are extensive granitic rocks (O0) 

and metamorphic rocks of probable early Paleozoic age 

(O6a); the granitic rocks have yielded radiometric 

dates of 80-90 m.y., hence are Cretaceous and are as 

young or younger than the Franciscan Formation, but 

they and the metamorphic rocks are everywhere faulted 

against the Franciscan, so that the original relations 

between them are unknown.

The Cenozoic strata of the Coast Ranges and Transverse Ranges have been described in many publications 

(Reed and Hollister, 1936, p. 1559-1596; Taliaferro, 

1943, p. 135-149; Page, 1966, p. 263-268). On the tectonic map they are shown as a single unit (P4), although their record is so complex that they could be 

divided into a number of "structural stages" or a map 

of larger scale. Deposits of Miocene and earlier ages 

are extensive and mainly marine, but most cf them 

grade eastward into continental equivalents. Pliocene 

marine deposits occur in more restricted coastal areas, 

such as the Los Angeles and Ventura basins, and in 

the southern part of the Great Valley. Marine Pliocene 

strata attain thickness of 4,500 m (15,000 ft) in some of 

the coastal areas, and nonmarine Pliocene in the interior 

basins is even thicker. In the coastal areas the marine 

Pliocene is succeeded conformably by thick lower Pleistocene marine and brackish water deposits, which are 

strongly unconformable beneath upper Pleistocene 

marine terrace deposits and alluvium.

The Cenozoic sequence of the Coast Ranges and 

Transverse Ranges records an eventful tectonic history, 

with times of deformation indicated at so many levels 

and with such varying magnitude from place to place 

that they are difficult to generalize into any erogenic 

climaxes. Nevertheless, a group of deformations during 

middle and late Miocene time appears to have been important, to have deformed the earlier Cenozoic strata 

widely, and to have restricted the areas of subsequent 

sedimentation. A final climax of deformation occurred 

during Quaternary time, mainly during the gap between the lower and upper Pleistocene deposits (Stille, 

1936, p. 867-868; Bailey, 1943, p. 1562-1564).

The sequence of Tertiary marine deposits along the 

western side of Baja California is nearly as complete as 

in California (Allison, 1964, p. 16-19), but neitHr they 

nor the late Cretaceous deposits beneath them hr.ve been 

as much deformed as farther north. In the southern 

half of the peninsula the upper Tertiary marir^ deposits pass eastward into a thick mass of terrestrial volcanics, pyroclastics, and volcanic sediments (Comondu
CALIFORNIA AND BAJA CALIFORNIA 77

Formation, P8) a mass which is broken off on the east 

by escarpments that face the Gulf of California.

The structure of the Coast Kanges, Tranverse 

Ranges, and northern ( Peninsular Ranges is confused 

by a multitude of faults. The widespread westward 

thrusting of Jurassic and Cretaceous shelf deposits over 

the Franciscan Formation has already been mentioned. 

Much more prominent and generally younger are the 

high-angle faults, many of which are of great length 

and have dominant components of strike-slip displacement (Crowell, 1962). The high-angle faults cross the 

grain of the ranges at an acute angle, mostly in a northwestward direction, but with a conspicuous east-trending set in the Tran§verse Ranges. Strike-slip displacement on the northwest-trending set is dominantly 

right lateral and on the east-trending set dominantly 

left lateral. The most lengthy high-angle fault is the 

San Andreas, which crosses all the coastal ranges from 

northern California to the United States-Mexican border with a nearly straight southeastward course, except 

for an eastward deflection in the Transverse Ranges.

Nearly every aspect of the high-angle faults of the 

coastal ranges has been diversely interpreted the time 

of their inception, the magnitude and kind of their 

movements, and their role in the geologic history of the 

region (Hill and Dibblee, 1953; Oakeshott, 1966; 

Dibblee, 1966). The San Andreas fault has been variously estimated to have originated in early Cenozoic 

time or before and to have been displaced laterally as 

much as 560 km (350 miles), or to have originated during the Quaternary and to have been displaced less than 

2 km (1 mile). In view of the marked contrasts between 

the rocks on the opposite sides of the San Andreas and 

most of the other high-angle faults, larger rather than 

smaller displacements seem likely; such movements 

probably account for the anomalous juxtaposition of the 

Franciscan basement against the crystalline basement 

in the Coast Ranges.

Along the San Andreas fault and many of the others 

are overwhelming indications of marked displacement 

during Quaternary time, and of continuing activity today. Modern activity is attested by the clustering of 

earthquake epicenters along the faults and by ground 

breakage along the faults at the times of earthquakes. 

Fault movements during the last few thousand years 

are indicated by characteristic landforms along the 

fault traces fault trenches or rifts, pressure ridges, 

and sag ponds. Even more striking is the distortion of 

modern topography along the faults by offset of ridges 

and stream channels; stream channels crossing the San 

Andreas fault have been offset right laterally 100-1,500 

m (325^,500 ft). The record of displacement along the 

San Andreas and other high-angle faults indicates that

movements were progressive, so that each topographic 

feature and each bedrock formation is more offset the 

greater its age.

The peninsula of Baja California, with eugeosynclinal rocks and granitic plutons in its basement, is .a 

long sliver of continental crust, separated from the 

Mexican mainland by the deep water of the Gulf of 

California. The northern half of the gulf is thickly 

fille.d with sediments, but the southern half is floored 

by oceanic crust; the gulf is evidently a rift, across 

which the peninsula drifted northwestward from the 

continent during Cenozoic time (Rusnak and others, 

1964, pp. 68-74). The faults which were formed during 

the drifting do not, however, extend directly down the 

gulf, and are not direct extensions of the high-angle 

faults of California. The latter trend southeastward 

into Mexico, each dying out on the Sonoran mainland. 

They are succeeded farther south by other faults, 

plainly indicated by the bathymetry of the gulf floc^, 

which also trend southeastward, with an en echelon 

pattern. The northwestward drift of Baja California 

and the strike-slip displacement of the high-angle 

faults of California are not independent phenomena, 

but are parts of a single grand movement of the western 

coastal urea of North America (Hamilton, 1961).

(Q) ANTILLEAN FOLDBELT

Another region of pronounced Cenozoic erogenic 

activity, here called the Antillean foldbelt (Q), lies between North and South America in the lands and islands 

surrounding the Caribbean Sea. On the tectonic m?.p 

the foldbelt is shown as including the islands of the 

Greater and Lesser Antilles, and the mainland are^s 

of coastal Venezuela and southern Central America. 

Southern Central America is included for convenience, 

but with equal propriety it could be classed as a southern 

prolongation of the Pacific foldbelt.

The Antillean foldbelt differs from those of NorMi 

and South America, as it is an oceanic rather than a 

continental feature. Much of it is far from any larjre 

continental areas, and most of the emerged parts ri^e 

steeply from oceanic depths, making it one of the mcst 

rugged mountain systems of the Americas. The Puerto 

Rico Trench is 8,000 m (26,000 ft) deep 130 km (80 

miles) north of the coast of Puerto Rico; the Caymr.n 

Trench is 7,000 m (23,000 ft) deep 40 km (25 miles) 

south of the coast of Cuba, and the Sierra Maestra ris^s 

another 2,000 m (6,500 ft) behind the coast. Peaks in 

the central cordilleras of Hispaniola and Costa Ri?.a 

project above 3,000 m (10,000 ft), but the other lard 

areas in the foldbelt are lower. As shown by the bathymetry on the tectonic map, the Caribbean sea floor is 

itself diversified by basins and trenches, with ridges ard 

swells between, many of which do not project above
78 THE TECTONICS OF NORTH AMERICA

sea level, and whose structure is largely conjectural. 

The land areas thus constitute only a fragment of the 

tectonics of the whole foldbelt, the remainder of which 

must be deduced from data obtained during oceanographic surveys.

Because of the fragmentary evidence, the origin and 

the tectonic history of the Antillean foldbelt are less 

certain than those of any of the foldbelts so far considered, and have been diversely interpreted. Many of 

the early hypotheses required extensive continental 

areas in the Caribbean region, which have now vanished by foundering or other causes. It is true that the 

crust of the Caribbean sea floor is somewhat thicker 

and less dense than normal oceanic crust, but it is by 

no means continental (Donnelly, 1964, p. 683-684); it 

was probably produced by hydration of normal oceanic 

crust, rather than by "oceanization" of continental 

crust. The tectogene hypothesis, which was at one time 

applied to the Antillean foldbelt (Hess, 1938) is discredited by newer geophysical data; as originally proposed, 

it involved downbuckling of a thicker and more sialic 

crust than is now known to exist.

Perhaps the Caribbean Sea was produced by the drifting apart of the North and South American continents, 

but most of the proposed reconstructions of drifting in 

the region involve implausible shifting, and even complete rotation of bits of continental crust to produce the 

present pattern of lands and islands. Other, more 

plausible reconstructions of the Caribbean by the drift 

hypothesis might be possible, but they are still in the 

formative stage. Hess writes (1966, p. 5), "It can be 

stated without question that the Caribbean existed during the whole of the Tertiary more or less with its present outlines, but how long before that was there a Caribbean?" Present evidence seems to indicate that, whatever may have been the origin and early nature of the 

Caribbean, the present lands and islands of the Antillean foldbelt were built up during Mesozoic and Cenozoic time on an original oceanic crust, by the addition of 

magmas from the mantle, followed by a complex tectonic evolution into the modern foldbelt (Dengo, 1962, 

p. 156-157; Donnelly, 1964, p. 689-695; Bowin, 1966, p. 

15-17).

This tectonic evolution is most nearly completed in 

the large, massive islands of the Greater Antilles on the 

north and in the coastal area of Venezuela and Trinidad 

on the south, both of which underwent erogenic deformation during late Mesozoic and early Cenozoic time, 

and have now attained relative stability. It is less complete in Central America on the west, which is bordered 

on the Pacific side by a chain of active volcanoes and by 

the Middle America Trench (Dengo, 1967). Even less 

mature are the small islands of the Lesser Antilles on the

east, which are mainly volcanic and rise individually out 

of deep water, with a trench extending at least part of 

the way around their Atlantic side.

Another item must be added to complete the tectonic 

picture the great fault zones on each side of tH Caribbean Sea, extending through Venezuela and rrinidad 

on the south and the Greater Antilles on the ncrth. The 

Cordillera de la Costa in Venezuela and the North 

Eange in Trinidad are a line of narrow ridges a long the 

sea, composed of metamorphosed Mesozoic roclrs (Q2a) 

unlike the unmetamorphosed Mesozoic rocks south of 

them. They have reached their present position Vj rightlateral displacement along a fault zone on the south 

side of the ridges (Rod, 1956, p. 474-476).

On the northern side of the Caribbean Sea the Cayman Trench extends diagonally through the Greater 

Antilles between Cuba and Jamaica, with steep inf acing 

scarps on each side (Meyerhoff, 1962, p. 1-3). Farther 

east, and not in direct alinement, the Anegada Passage 

forms another, smaller trench between the Greater and 

Lesser Antilles. Both these features are certainly fault 

bounded, and at least some left-lateral displacement has 

occurred along them (or displacements opposite to those 

in Venezuela and Trinidad), but there are complications. The Cayman Trench is floored by an oceanic crust, 

much thinner than that beneath the islands and submarine rises on either side, suggesting that th? trench 

is a rift, perhaps comparable to the rift in the Gulf of 

California, mentioned above (Ewing and others, 1960, 

p. 4095-4096). Whatever the nature of the fault movements north and south of the Caribbean Sea, they detached the more mature parts of the foldbelt from the 

unstable parts between, where tectonic evolution is still 

in progress.

No pre-Mesozoic tectonic units have been proved in 

the Antillean foldbelt, and their existence is doubtful 

in most of it. The last exposure of the pre-Mesozoic is 

in northern Central America where its metr.morphic 

complex (Ol) plunges southward beneath Tertiary volcanics (Q8) near the Honduras-Nicaragua border, and 

it is cut off eastward by the Caribbean coast. Farther 

south in Central America the basement is th? Nicoya 

Complex (Q2), resembling the Franciscan of the Pacific foldbelt, and like it probably no older than Mesozoic (Dengo, 1962, p. 138-142). The pre-Mesozoic metamorphic complex seems to extend east of the coast beneath the Nicaraguan Rise and Cayman Ridge, but 

whether it emerges again in the Greater Antilles is undetermined. The basement of the Greater Antilles is also 

termed a metamorphic complex (Ql), but this is known 

only to be older than Late Jurassic. Most of if", is probably older Mesozoic eugeosynclinal material, although
CALIFORNIA AND BAJA CALIFORNIA

pre-Mesozoic components have been proposed in Cuba 

(Khudoley, 1967, p. 672-674; Hatten, 1967, p. 780-782).

Mesozoic eugeosynclinal deposits (Q2), with large 

volumes of volcanics and volcanic-derived sediments, occur in all areas of the Antillean foldbelt. In southern 

Central America they are overlain by nonvolcanic Upper Cretaceous deposits (Q3), but in the Greater Antilles the eugeosynclinal sequence extends in most places 

to the top of the Mesozoic, and in southeastern Cuba 

into the Eocene (Q4a). In the Cordillera de la Costa of 

Venezuela, where all the eugeosynclinal rocks are regionally metamorphtosed (Q2a), the largely nonvolcanic Caracas Group is bordered on the south by the structurally 

separated but probably older volcanics of the Villa de 

Cura Group. Mesozoic miogeosynclinal deposits, including thick carbonates, border the eugeosynclinal rocks on 

the north in Cuba (Q4), and on the south in Venezuela 

(N3), and are continuous with the platform deposits of 

the Bahama Islands and the Venezuelan interior. Succeeding older Tertiary deposits (Q4) are varied, largely 

clastic in many areas, but including volcanic components 

in southern Central America and thick carbonate units 

in parts of the Greater Antilles.

A significant Middle Cretaceous orogeny occurred in 

Cuba, but throughout much of the foldbelt the main 

orogenies occurred from Late Cretaceous through Eocene time. They were accompanied by emplacement of 

granitic plutons (Q/?) of middle Cretaceous to Eocene 

age, largely more mafic than those of the other f oldbelts 

(diorites and quartz diorites). In Cuba the eugeosynclinal rocks were carried northward over the miogeosynclinal rocks in great thrust sheets or nappes of Alpine type (Hatten, 1967, p. 785-788), many of which 

have tabular masses of ultramafic rocks (Qa) along 

their soles.

In the more mature parts of the foldbelt, deformation 

was largely completed by Eocene or Oligocene time, so 

that the later Tertiary is postorogenic and little deformed (Q5). Deformation continued later in Hispaniola, and there was significant Miocene orogeny in Costa 

Kica, accompanied by emplacement of small granitic 

plutons in the central Cordillera (Qy) (Dengo, 1962, 

p. 143-147).

Young volcanic rocks are lacking in the more mature 

parts of the foldbelt, but they are important in the less 

stable intervening parts. Large areas in southern Central America are covered by late Tertiary lavas and 

pyroclastics, probably derived from fissure eruptions 

(Q8). On these, on the Pacific side, younger volcanics 

(Q«) have been built, surmounted by a chain of volcanic cones that extends from the eastern border of 

Mexico to Costa Kica (Williams, 1952,1960). An arcuate 

chain of volcanic cones also forms most of the islands

of the Lesser Antilles to the east, but in the northern 

half of the arc the cones are bordered on the Atlantic 

side by half a dozen lower islands with a foundation of 

older Tertiary volcanics and diorite plutons, capped l^y 

later Tertiary limestones and other sediments (Q4).

(N) ANDEAN FOLDBELT

Many features like those in the North American Cc iwdillera are repeated in the South American Cordillera, 

or Andean foldbelt (N). Most of the Andean foldbelt is 

outside the area of the "Tectonic Map of North Amc * - 

ica," but its northern prongs extend into it, in Colombia 

and Venezuela. The tectonic features of the Andean 

foldbelt are shown on the map down to the 6th parallel, 

mainly to provide a southern frame for the Caribbean 

Sea and its Antillean foldbelt (Q). The structures represented are copied from compiled maps, such as the 

"Geologic-Tectonic Map of Northern Venezueb" 

(Smith, 1962), and publications on the foldbelt have 

been studied only sufficiently to establish usable map 

units (see, for example, Jacobs and others, 196^; 

Mencher, 1963). The tectonics of the foldbelt and its 

map units will be shown more authoritatively on the 

"Tectonic Map of South America," which is now in 

preparation by geologists of the South American 

countries.

In Colombia, the Andes Mountains split northward 

into a Cordillera Occidental, a Cordillera Central, and a 

Cordillera Oriental. The first two plunge northward 

beneath the lowlands of the Rio Magdalena and its 

tributaries, but the third extends northeastward into 

Venezuela, bifurcating in turn into the Sierra de Perija 

and the Cordillera de Merida, which enclose the Mar*- 

caibo Basin. The Sierra Nevada de Santa Marta along 

the Caribbean coast in Colombia is an independent 

massif. The various cordilleras and sierras are composed 

of Mesozoic and older rocks, and the lowlands between 

them are underlain by Cenozoic rocks.

Pre-Mesozoic rocks, largely metamorphosed (Fl, 

Na), form much of the Cordillera Central and Sierra 

Nevada de Santa Marta, and emerge in smaller inliers in 

the Cordillera Oriental and Cordillera de Merida. Th^.y 

include fossiliferous rocks of various Paleozoic agos, 

and probably also Precambrian rocks; the pre-Mesozc^c 

basement of the Cordillera Oriental includes reworked 

parts of the edge of the Precambrian Guyana Shield. 

Orogenies at various times during the Paleozoic are 

suggested in places by unconformities or gaps in tV 

sequence, but they are too imperfectly known to gi^e 

much of an idea of the resulting structures. Embedded 

in the metamorphic rocks are granitic plutons (N/P), 

primarily Mesozoic but possibly including Paleozo;<*.. 

Kecent radiometric determinations in Colombia shew
80 THE TECTONICS OF NORTH AMERICA

that they are dominantly Mesozoic, the large pluton in 

the Sierra Nevada de Santa Marta is dated at 160-170 

m.y. (Jurassic) and the large pluton near Medellin in 

the Cordillera Central is dated at 70 m.y. (Late 

Cretaceous).

Tectonic history becomes plainer in the Mesozoic. The 

Cordillera Occidental and the lower ranges along the 

Pacific coast are formed of Mesozoic eugeosynclinal deposits (N2) submarine lavas and associated sediments 

that are largely of Cretaceous age, but which may also 

include Jurassic. The Cordillera Oriental and its extensions into Venezuela is formed of Mesozoic miogeosynclinal deposits (N3), including thick sequences of 

Cretaceous carbonates. Beneath them, however, are 

continental red elastics (Giron Group), which are 

largely of Jurassic age.

The Andean Cordilleras were deformed and shaped 

into much their present outlines by several orogenies 

between Late Cretaceous and Eocene time. The miogeosynclinal rocks of the Cordillera Oriental and Sierra 

de Perija were thrown into closely crowded, east-directed folds and thrust slices, but the cordilleras of 

metamorphic and plutonic rocks were raised mainly as 

block uplifts bordered by high-angle faults, some with 

strike-slip displacement. The Bocono fault, which extends the length of the Cordillera de Merida, has a large 

component of right-lateral displacement, and may be 

still active (Rod, 1956, p. 463-468).

Basinal deposits of Eocene and younger Tertiary 

ages (N4), partly marine and partly continental, and 

both synorogenic and postorogenic, fill the depressions 

between the cordilleras, and have been extensively explored for petroleum. In deeper depressions along the 

Magdalena River in Colombia, and in the Maracaibo 

Basin in Venezuela, these deposits are covered in turn 

by late Tertiary and Quaternary deposits (N5). The 

configuration of the pre-Tertiary basement beneath the 

Maracaibo Basin is shown on the map by 500-m contour lines.

SUBSEA AREAS

On the "Tectonic Map of North America" the terrain 

beneath the seas and oceans surrounding North America 

is represented by 500-m (1,640-ft) topographic contours, with the first 200-m (656-ft) contour added, and 

by bathymetric tints for each 1,000-m (3,280-ft) interval. The many sources from which the contours have 

been compiled were summarized earlier (p. 18-19). In 

addition, a special symbol is used to represent prominent faults or fracture zones on the ocean floor, whose 

existence is indicated by topographic scarps, and in 

part by geophysical evidence. The treatment of the subsea areas adopted on the North America map corresponds to that on most tectonic maps previously 

published.

The topography of the sea bottom is clearly more 

expressive of tectonics than the topography on the land, 

because the original structural surfaces have bien much 

less modified by subsequent erosion and deposition. 

Hence, fault scarps, fault blocks, upwarps, and downwarps are evident merely from their surface configuration. Moreover, the internal structure of the topographic 

features on the sea bottom cannot, as yet, be deciphered 

with as much assurance as on the land. The features 

are not available for field inspection, but must lv> probed 

indirectly by various kinds of oeeanographic surveys. 

These surveys are still incomplete in the areas surrounding North America, and some of the methods of 

survey are still being perfected.

On the North America map, as on many others, the 

subsea areas are not classified into tectonic units like 

those on the land. However, subsea tectonic units are 

shown on some of the newer tectonic maps, such as the 

"Tectonic Map of Eurasia" (Yanshin, 1966) discussed 

earlier (p. 17-18). Many of the tectonic units shown on 

such maps are merely morphological or descriptive, and 

correspond to the physiographic provinces which have 

been mapped on the sea bottoms around North America 

(for example, Hcozen and others, 1959, p. 20), and 

include such features as continental slopes, continental 

rises, and abyssal plains. Most of these provinces are 

evident in the subsea topography itself. Other tectonic 

units shown on such maps indicate the surface or subsurface composition of the sea bottom, such as areas 

of oceanic crust or continental crust, and areas with 

thick sedimentary cover. Many data on the composition 

of the sea bottom have been obtained around North 

America, but no maps have been published of large 

arear which show instriunentally verified compositions. 

Other tectonic units on such newer maps ar^ of more 

questionable value, as these imply ages of th° units or 

ages of the deformation, which cannot be verified by 

existing oceanographic methods, and whicl imply a 

choice between several possible theories of oceanic 

tectonics.

Some of the existing survey methods offer more possibilities than others for refining the tectonic classification of the subsea areas. Continuous seismic reflection 

profiling provides information on the structure of reflecting surfaces more than 1,000 m (3,300 ft) below 

the bottom surface, revealing features that can be interpreted as flat-lying, tilted, or folded sedimen tary beds, 

as well as faults and unconformities. Many profiles have 

now been made on the continental shelves and slopes 

around North America for example, off th«, Atlantic 

Coast of the United States (Uchupi and Emery, 1967).
FAULT ZONES 81

These surveys will aid materially in an eventual tectonic mapping of the shelves and slopes around the continent, areas which are shown without symbols on the 

present map.

Magnetic surveys also have tectonic implications. A 

total intensity magnetic survey has been made of a large 

area off the Pacific Coast of the United States (Vacquier 

and others, 1961; Raff and Mason, 1961) which, although incomplete, has revealed a remarkable pattern 

of narrow, parallel belts of positive and negative 

anomalies, offset along the subsea fracture zones. Similar 

parallel belts of magnetic anomalies have been found on 

each flank of the Mid-Atlantic and other mid-ocean 

ridges (Vine and Matthews, 1963), and it has been proposed that all these anomaly belts are products of sea 

floor spreading (Wilson, 1965a; Vine and Wilson, 1965). 

Parallel anomaly belts, apparently of a different kind, 

are known on the continental shelf and slope off the 

Atlantic Coast (Drake and others, 1963), but the western Atlantic Ocean basin between Bermuda and the 

continental slope is reported to be magnetically featureless (E. R. King and others, 1961; Heirtzler and 

Hayes, 1967). Magnetic surveys, when more complete, 

will have an important influence on the tectonic mapping of the subsea areas around North America.

FAULT ZONES

Further discussion is desirable on the only subsea 

structural features specifically shown on the tectonic 

map the faults or fracture zones. The most notable of 

these are off the Pacific Coast of North America; the 

zones have been described and interpreted by Menard 

and others in various publications (for summary, see 

Menard, 1964, p. 41-53). From north to south, those 

shown on the map are the Mendocino, Pioneer (minor), 

Murray, Molokai (incompletely represented by the 

Shirley Trough), Clarion, and Clipperton. Still other 

fracture zones extend westward from the coast of South 

America, beyond the map area (fig. 14).

The fault zones on the Pacific Ocean floor are remarkably evenly spaced, about 1,000 km (600 miles) 

apart, and trend nearly westward from the coast along 

great circle courses for distances of many thousands 

of kilometers. All of them are expressed topographically 

by escarpments or lines of ridges and troughs, and at 

least some of them form the borders of ocean bottoms of 

different depths; thus, the ocean bottom south of the 

Mendocino zone is at least 1,000 m (3,300 ft) lower than 

north of it. The areas between the zones also differ in 

tectonic behavior, as shown on the map by the nearly 

featureless surface between the Mendocino and Murray 

zones, contrasted with the abundant seamounts north 

and south of them. Seismic surveys show that these express differences in crustal thickness in the areas between 

the zones. The age of the faulting is undetermined. In 

view of the fact that few ages older than Cretaceous 

have been verified in the oceanic areas, they cannot be 

very ancient, yet no earthquake epicenters occur along 

them except near their intersections with other features. 

Despite the strong expression of the faults on the 

Pacific Ocean floor and their close approach to the 

coast of North America, remarkably little indication of 

them can be discovered on the land. The only zone directly traceable to the land is the Mendocino, which 

offsets the edge of the continental shelf right laterally 

about 80 km (50 miles), but even it is not identifiable 

in the surface rocks farther inland. It has been proposed 

on geological or geophysical evidence that the rone extends long distances eastward across North Amer; ",a 

(Gilliland, 1962; Fuller, 1964); however, the proposals 

are as yet unconvincing. The Murray zone disappears 

near the foot of the continental slope, perhaps bener.th 

a sedimentary cover. It may be related to the faults w;th 

left-lateral displacement in the Transverse Ranges of 

southern California; these, in turn, are at the western 

end of the Texas Lineament, which is not a fault zone. 

The Clarion zone disappears far west of the coast, bit 

is alined with the Trans-Mexican volcanic belt, which 

again is not a fault zone. The Clipperton zone disappears far west of the coast, but is alined with miror 

transverse structures in Costa Rica, and these in turn 

with the lengthy faults on the north coast of South 

America. A broken, zig-zag connection between the 

Clipperton zone and the transverse faults of Guatemala can also be imagined through the northeast-tren ding Tehuantepec fracture zone. The vagueness of all 

these relations strengthens the belief that the faults of 

the Pacific Ocean floor are structures in the ocearic 

crust, and that if they extend eastward into the North 

American continent, it is beneath the upper crust, whose 

surface has been affected indirectly at most.

Where the Mendocino fracture zone leaves the corst 

it is expressed by a northward-facing scarp, but 120 km 

(75 miles) farther west the topography reverses to a 

southward-facing scarp as much as 3,000 m (10,000 ft) 

high, which continues to the termination of the zone, 

showing that displacements on the zone have been complex. This complexity is more strikingly demonstrated 

by matching magnetic anomalies across it. The anoiralies indicate that west of the coast there has been an r,stonishing left-lateral offset of 1,170 km (785 miles), as 

compared with an 80 km (50 miles) right-lateral offret 

of the continental shelf along the same zone. Similar'y, 

matching the anomalies across the Murray zone indicates as much as 640 km (395 miles) of right-lateral 

offset at sea, decreasing eastward, whereas the faults in
40e

30

10

Faults and fracture zones Nonfaulted transverse 

zones on the land

Oceanic trenches Oceanic ridges and rises Seam

FIGUKE 14. Map showing transverse structural features on the lands and beneath the seas in the southern part of the area of th

America," and in adjacent regions. Compiled from "Tectonic Map of North America," and from publications of H. W. Menard, B. 
GLOSSARY

the Transverse Ranges on the land are offset left 

laterally.

These paradoxes seemed inexplicable until J. T. Wilson (1966) proposed the concept of "transform faults," 

along which lateral displacement has occurred in opposing senses on the two sides of a zone of oceanic 

spreading. The zone of spreading in this case is the 

East Pacific Rise, which is supposed to extend northward from the ocean beneath the continent in the Gulf 

of California and to emerge again in the ocean floor 

north of the Mendocino fracture zone. The oceanic 

spreading along the East Pacific Rise, and the accompanying transform faulting, if these exist, had a major 

influence on the creation of the younger tectonic features 

of the Cordilleran and the Pacific f oldbelts of the North 

American continent.

The existence of lengthy fracture zones on the Pacific 

Ocean floor has implications on the origin of the fault 

zones on the north and south sides of the Caribbean Sea 

(p. 78), especially since east-west fracture zones, also 

transform faults, are now known to cross the MidAtlantic Ridge between the equator and lat 15° N. 

(Heezen and Tharp, 1%1). The faults in the Caribbean Sea may therefore not be local features produced 

by a movement pattern in the Antillean foldbelt alone, 

but may be parts of a circumglobal fracture system in 

the oceanic crust (fig. 14).

Aside from the subsea faults in the Caribbean Sea, 

only one other is shown on the tectonic map on the 

eastern side of North America. This is supposed to follow the arc of the Kelvin Seamounts ("Bermuda-New 

England seamount arc" of Northrop and others, 1962) 

onto the continental shelf and slope near the 40th parallel. The existence of the fault on the shelf and slope is 

suggested by a 145-km (94-miles) right-lateral distortion or actual offset of sedimentary basins and belts 

of magnetic anomalies that have been determined by 

oceanographic surveys (Drake and Woodward, 1963, p. 

53-55). A landward prolongation of the fault across 

the Appalachian foldbelt has been proposed on seemingly tenuous evidence, although there is admittedly a 

discontinuity at about this latitude between the northern 

and southern Appalachians, so that the structures of 

the former lie eastward of those of the latter (p. 56).

GLOSSARY

This glossary defines the tectonic terms used in the 

text, not all of which are familiar to the non-specialized 

reader. The initial sentences under many of the items 

are abstracted from the "International Tectonic Dictionary" (Dennis, 1967). Succeeding sentences are explanations or qualifications by the compiler. Some of the 

terms are discussed at greater length at appropriate

places in the text. French equivalents of some of tH 

terms are included to facilitate comparison with tH 

legend of the "Tectonic Map of Europe" (Schatsky, 

1962). For brevity, sources and authorships of the terms 

are seldom included; for literature references and for 

history of usage the reader should consult the "International Tectonic Dictionary."

Allochthone. A mass of rocks which has been mowi 

by tectonic forces from its original site of origin, as in 

a thrust sheet or nappe. Opposite, autochthone.

Alpinotype tectonics. The tectonics of alpine mountain belts, which are produced from orthogeosyncline^. 

Characterized in the internal parts by deep-seated pla^- 

tic folding and plutonism, and in the external parts 17 

lateral thrusting, which has produced nappes, thru^ 

sheets, and closely crowded folds.

Antithetic structures. Structures produced by minor 

movements that oppose the prevailing larger movements 

in the area; for example, faults which are downthrown 

in a sense opposite to the general uplifting or arching. 

Opposite, synthetic structures.

Autochthone. A mass of rocks which, though variably deformed, has remained at its original site of origin, 

where it is still rooted to its basement. Opposite, 

allochthone.

Basement rocks. Rocks beneath the platform are^s 

and the f oldbelts that were consolidated during earlier 

deformations, which are overlain by a less deformed 

younger structural layer or cover. Ideally, basement 

rocks are metamorphic and plutonic, and contra^ 

strongly with their cover, but parts of the basement are 

composed of intermediate rock types so that distinctions 

are blurred in places. In the central part of the continent the basement rocks are Precambrian, but they are 

of younger ages elsewhere. French, socle. (See p. 21.)

Batholith. Traditionally used for large, subjacent 

and discordant plutonic bodies, supposedly floorless in 

contrast to other intrusive bodies; most batholiths are 

formed of granitic rocks but presumably could be of 

other compositions. In the western Cordillera of tJ ^ 

United States and Canada the term is used specifically 

for regionally extensive aggregates of granitic rocks, 

complex internally, and consisting of many individual 

plutons of varied compositions and ages; the "Tectonic 

Map of North America" and the present text defer to tl -> 

latter usage. Examples, Sierra Nevada batholith, Idaho 

batholith, Coast batholith. (See p. 51-52.)

Block faulting. The breakup of a previous more 

homogeneous terrane into blocks as a result of faultily 

that is mainly normal or tensional. The resulting structure is an array of uplifted mountains (preserving the;r 

block forms in the early stages), separated by downfaulted depressions. Block-faulting is primarily a pos4--
THE TECTONICS OF NORTH AMERICA

orogenic feature and disturbs terranes with varied prior 

histories, ranging from those which have undergone 

little earlier deformation to those of thoroughly deformed rocks. Example, the Basin and Range province 

of the southwestern United States, (See p. 24,72.)

Craton. A part of the earth's crust which has attained stability, and which has been little defonried for 

a prolonged period. As originally defined, the cratons 

included parts of both the continents and the ocean bottoms, but the proposed examples of the latter are unproved or dubious. In the present text the term, is used 

only in the continental areas, where it includes both 

shields and platforms.

Decollement. Detachment of rocks along a stratigraphic surface, resulting in independent styles of deformation in the rocks above and below. The term is 

merely descriptive, and the detachment could have been 

produced by any one of a number of tectonic forces, including lateral thrusting and gravity sliding.

Diapir. A fold or dome which contains a core of more 

mobile rock that has pierced and intruded the overlying 

less mobile rock. Ordinarily the more mobile rock has a 

lower density than its cover, but it may be of many 

kinds, including salt (as in salt domes), other evaporites,orclay.

Disharmonic folding. Folding in which there are 

notable differences in kinds of structures between adjacent rock bodies, resulting from bedding-plane slip 

along sedimentary layers. This type of deformation is 

characteristic of stratified sequences and is, in fact, demanded by the geometry of their folds. In extreme form, 

bedding-plane slip produces decollement, and the two 

structures are commonly associated.

Disrupted areas. Used specifically in the text for 

previously stabilized or cratonic areas that have been 

broken up by block-faulting and related processes. Example, the part of the Basin and Range province in 

southern Arizona and New Mexico. (See p. 24,72.)

Epeirogeny. Broad uplift and subsidence of the 

crust, primarily by vertical movements, which has affected large parts of the continents. Epeirogeny occurs 

both in the cratons and in foldbelts during the final 

stages of their tectonic cycles. It has produced the present mountainous topography in many of the foldbelts, 

and thus would be "orogeny" as some geologists have 

defined the term (but not the present compiler). Even 

as here defined, epeirogenic and orogenie structures 

grade into each other in detail, although most of them 

differ distinctly from each other. Counterpart, orogeny. 

(See p. 44-45.)

Epieugeosyncline. A sedimentary trough or basin on 

the site of a former eugeosyncline. Nearly synonymous

with successor basin, the term preferred in tb* present 

text.

Eugeosyncline. The internal part of a geosyncline 

(orthogeosyncline), away from the craton, characterized by strong volcanism during its early phases and by 

synorogenic plutonism during its later phases. Deposits 

of eugeosynclines include submarine volcanics, cherts, 

slates, and graywackes. Counterpart, miogeosyncline. 

(See p. 46-47.)

Event. A noncommittal term used for any incident 

of probable tectonic significance that is suggested by 

geological, radiometric, or other evidence, whose full 

implications are unknown. Used especially for minor 

clusters of radiometric dates whose relations to geologic 

structures or processes have not been precise1 y determined.

Extemides. The external or outer part of a foldbelt, 

nearest the craton or foreland, commonly the site of a 

miogeosyncline during its early phases and sul'jected to 

marginal deformation later. Opposite, internides. The 

terms externides and internides express relative positions of units within the foldbelts, and not their relation to adjacent parts of the continents. Ir. relation 

to the central craton of North America, the extftrnides of 

the Phanerozoic foldbelts would be internal, and the 

internides external. (See p. 46).

Flysch. A sequence of distinctively interlayered 

sandstones, mudstones, and marls believed to have accumulated in relatively narrow, deep, rapidly subsiding 

troughs. The term originated in the European Alps, but 

nearly identical deposits occur in many other foldbelts 

of the world. Flysch is commonly interpreted as a synorogenic deposit, and hence the term has been used improperly by many geologists for any clastic sediment 

that was laid down during the time of the orogeny. (See 

p. 48).

Foldbelt. A linear belt that has been subjected to 

folding and other deformation during the orogenie 

phase of a tectonic cycle. Each foldbelt evolved during 

a time span different from that of any of the others, 

and details of their histories differ, although they can 

be grouped broadly according to generalized worldwide times of orogeny. Individual foldbelts and the 

tectonic cycles which created them, are used as fundamental units on the "Tectonic Map of North America." 

Synonyms, mobile belt, orogenic belt, and (less properly) mountain belt. French, region de ptissement.

Foredeep. The part of the foreland or craton next to 

a foldbelt that was deeply downfolded and thhkly filled 

with sediments during the climactic orogeny.

Foreland. A stable area marginal to a foMbelt, toward which its rocks were thrust or overf olded. The area
GLOSSARY 85

is commonly continental, and is a craton or platform 

area.

G&rmanotype tectonics. Tectonics of the cratons and 

the stabilized foldbelts, derived from the structures in 

Germany north of the Alps. The milder phases of this 

type of tectonics are epeirogenic, but it also includes 

broad folds dominated by vertical uplift and high-angle 

faults, block-faulted terranes, and sedimentary basins 

deformed within a frame of surrounding massifs.

Infrastructure. Structure produced at a deep crustal 

level, in a plutonic environment, under conditions of 

elevated temperature and pressure,, which is characterized by plastic folding and by granitic and other migmatitic and magmatic rocks. This environment occurs in 

the internal parts of most of the foldbelts, but the term 

is used especially where the infrastructure is clearly contrasted with a less disturbed or altered higher layer, or 

suprastructure.

Intemides. The internal part of a foldbelt, farthest 

away from the craton, commonly the site of a eugeosyncline during its early phases, and subjected later to 

plastic folding and plutonism. Opposite, extemides.

KoUogenic. A term, used by Spizaharsky and Borovikov (1966) for broad downwarping or subsidence of 

the crust, of a sort that would be classed as a negative 

epeirogenic movement in the present text.

Leptogeosyncline. A part of a geosyncline (orthogeosyncline) in which the water was of great depth, but 

which received only small thicknesses of sediments over 

prolonged periods; a form of starved sedimentary basin.

Massif. A massive topographic and structural feature in a f oldbelt, commonly formed of rocks more rigid 

than those of its surroundings. These rocks may be protruding bodies of basement rocks, rocks consolidated 

during early deformations in the foldbelt, or younger 

plutonic bodies.

Miogeosyncline. The external part of a geosyncline, 

where there was little or no magmatic activity during its 

depositional phase, and which was little deformed except during the closing phases of the orogeny. Deposits 

of miogeosynclines include carbonate rocks, quartzites, 

and shales; these are identical with those laid down on 

the cratons, but they were laid down to a much greater 

thickness and in a more complete sequence. Counterpart, 

eugeosyncline. (See p. 47-49.)

Miomagmatic geosyncline. A term proposed by 

Stille in 1936, but superseded by his later term miogeosyncline. (Seep.46.)

Molasse. A term used in the European Alps for a 

Tertiary sequence of marine and brackish water sandstones, with some layers of gravelly to bouldery conglomerate. As the molasse was laid down after the 

climax of the Alpine orogeny the term has been used

widely throughout the world for any kind of postorv 

genic deposit. Because of the great variety of alleged 

molasse deposits the term is not considered useful outside its original area in the present text. (See p. 48).

Neotectonics. The youngest tectonics of the earth, 

which was produced during the later part of the Tc?- 

tiary, during the Quaternary, as well as the tectonics 

in process of formation at the present time.

Oroclinal structure. An arcuate structure supposed 

to have been formed by bending of the crust in a horizontal direction, or by "deformation in plan". Some 

geologists propose that deformation of this kind produced many or most of the arcuate patterns of the 

foldbelts.

Orogenie phase. The median part of the tectoric 

cycle in a foldbelt, characterized by the climax of crustal 

mobility and erogenic activity, and by the formation of 

alpinotype structures. The erogenic phase is commonly 

shorter than the preorogenic and postorogenic phaees 

which precede and follow it, and may be less than a 

geologic period in length, although it is usually prolonged by a succession of erogenic pulsations. RocH 

formed during the erogenic phase are term**! 

synorogenic.

Orogeny. The processes which create the rock stn^?^ 

tures within the mountain chains or foldbelts. The wo^d 

orogeny actually means "the formation of mountains," 

which implies not only the formation of the rock structures in the mountains but also of the mountainous lardscapes. When the term first began to be used a century 

ago, these were supposed to be parts of the same process 

and the distinction between them was only recogniz^xi 

later. Most geologists today use the term orogeny as he^ 

defined, and consider the formation of the mountainous 

landscapes to be a postorogenic or epeirogenic process. 

(See p. 44-45.)

Orthogeosyncline. A major linear sedimentary 

trough lying outside the craton, generally divisible into 

eugeosynclinal and miogeosynclinal parts. The term is 

synonymous with geosyncline as originally defined and 

as used here; it was proposed in order to distinguish it 

from sedimentary troughs and basins in the crate ns 

which were called parageosynclines^ but these are rot 

accepted as geosynclines in the present text.

Overprint. The superposition of a younger evmt 

on a dominant earlier event, as indicated by a mil or 

scatter of young radiometric dates in a terrane doimnated by earlier dates. Where valid, the younger dates 

can sometimes be ascribed to renewed orogeny or to recurring plutonic activity, but the reasons for many of 

them are obscure.

Paleogeographic map. A map which purports to 

show the extent and outlines of lands, seas, and otl <** 
86 THE TECTONICS OF NORTH AMERICA

geographic features at a given time in the geologic past. 

(See p. 2.)

Paleogeologic map. A map which shows the areal 

geology of the surface beneath an unconformity, which 

has been covered by younger strata. (See p. 2.)

Paleotectonic map. A map which represents the 

tectonic features as they existed at a given time in the 

geologic past. (See p. 1.)

Parageosyncline. A sedimentary trough or basin in 

the craton. Although different varieties of parageosynclines have been recognized and separately named, none 

of them are considered to be true geosynclines in the 

present text. Counterpart, orthogeosynoline.

Pennine-type structure. Structure like that in the 

Pennine Alps of central Europe, characterized by plastic deformation of the crystalline basement and its cover, 

and by the formation of large recumbent folds or 

nappes.

Platform. That part of a continent which is covered 

by flat-lying or gently tilted strata, mainly sedimentary, 

which are underlain at varying depths by basement 

rocks that were consolidated during earlier deformations. A part of the craton of the continent. French, 

plateforme. (See p. 21).

Pliomagmatic geosyncline. A term proposed by 

Stille in 1936, but superceded by his later term eugeosyncline. (See p. 46.)

Pluton. Originally proposed as a noncommittal term 

for a body of intrusive igneous rock of any shape or 

size, hence including batholiths and many other varieties. However, in the western Cordillera of the United 

States and Canada the term is commonly used for the 

individual, relatively restricted bodies of particular 

composition and age which are components of the larger 

aggregates, or batholiths.

Plutonism. Processes which take place at depth in 

the earth's crust under conditions of elevated temperature and pressure, which produce plutonic rocks that 

is, metamorphic, migmatitic, and magmatic rocks. In 

the present text the term plutonic rocks is used specifically for the migmatitic and magmatic varieties, especially the granitic types. Plutonic processes are prominent in the internal parts of foldbelts, and the plutonic 

environment can be termed infracrustal.

Postorogenic phase. The final phase of the tectonic 

cycle in a foldbelt, following the climatic orogeny. In 

some foldbelts the postorogenic rocks and structures are 

minor and do not obscure the erogenic structures, in 

others they nearly overwhelm them; in all foldbelts, 

the postorogenic events have produced the present 

mountainous landscapes. Postorogenic structures are 

germanotype and epeirogenic. Postorogenic sediments

are varied; by common European usage they are called 

molasse^ but this term has little utility. Postorogenic 

plutonic bodies include discordant granitic plutons and 

a wide variety of hypabyssal intrusives. In mary of the 

foldbelts, terrestrial volcanic rocks were spread widely 

over the deformed terrane during this phase.

Preorogenic phase. The initial phase of the tectonic 

cycle of a foldbelt, prior to the climactic orogeny. The 

phase is the time of formation of geosynclines, most of 

which are clearly divisible into eugeosynclinal (internal) and miogeosynclinal (external) parts, the first 

characterized by abundant submarine volcanism, the 

second by little magnatism and by carbonate-cuartzite 

sedimentation. The plutonic rocks of the preorogenic 

phase include ultramafic bodies and rare early granitic 

plutons.

Protaxis. An antique term for the central rxis of a 

mountain chain, supposedly consisting of the oldest 

rocks and structures., (See p. 49.)

Pulsation. A minor time of deformation which is 

one part or phase of a more prolonged e^xxih of 

orogeny.

Reactivated areas. Used specifically in the present 

text for the parts of a previously stabilized craton that 

have been involved in one or more renewed deformations. Example, Southern Rocky Mountains. (See p. 

23-24.)

Reworked structures. Structures that were formed 

during an earlier time of deformation or orogeny, which 

have been involved in later deformation that has refolded or otherwise modified them.

Shield. A large area of exposed basement rocks in a 

craton, commonly surrounded by sediment-covered platforms; the term is virtually restricted to expored areas 

of Precambrian basement rocks. French, boucli°r.

Structural stage. A rock-stratigraphic unit or layer 

in a foldbelt, characterized throughout by a common 

facies related to the tectonic evolution of the foldbelt. 

As originally conceived, successive structural stages are 

supposed to be separated by unconformities, but this 

is dubious. Rocks of a single stage may vary considerably in age span from place to place, hence a structural 

stage is not a stratigraphic stage, or time-stratigraphic 

unit. French, etage structuraux. (See p. 10-11.)

Successor basin. A sedimentary basin of relatively 

restricted extent in the internal part of a foldbelt, 

formed during the erogenic phase or early part of the 

postorogenic phase. Its deposits overlie geosynclinal 

rocks that were deformed by the climatic oro,?enies of 

the foldbelt, but they themselves have been var ably deformed during the later phases of the orogeny. Includes 

epieugeosynclines. (See p. 49-60.)
REFERENCES CITED

Supracrustal rocks. Sedimentary and volcanic rocks 

of the suprastmcture in a foldbelt, which have been 

subject only to near-surface deformational processes, 

and not to deep burial, plutonism, or metamorphism.

Suprastrueture. The upper structural layer in a 

foldbelt, subjected to relatively near-surface deformational processes, in contrast to an underlying and differently deformed infrastructure.

Synorogenic rocks. Rocks formed during the 

erogenic phase of the tectonic cycle of a foldbelt. Synorogenic sediments are preserved mainly in the external 

parts of the foldbelt; they include flysch as strictly 

denned, as well as broad sheets of deposit spread far 

from their erogenic sources. Synorogenic plutonic rocks 

include concordant granites which have disrupted or 

replaced the rocks of the internal parts of the foldbelt.

Synthetic structures. Structures produced by minor 

movements that are in harmony with the major movements of the area; for example, faults which are downthrown in the same sense as the general uplifting OT 

arching. Opposite, antithetic structures.

Tectonic cycle. The interval of time during which an 

original mobile area has evolved into a stabilized foldbelt, passing through preorogenic, orogenic, and postorogenic phases, each characterized by distinctive structures and by distinctive suites of sedimentary, volcanic, 

and plutonic rocks. Called a geotectonic cycle by some 

geologists. (See p. 43-44.)

Tectonic map. A map which portrays the architecture of the upper part of the earth's crust, or the 

features produced by deformation and other earth 

forces, and represents them by means of symbols, patterns, and colors. (See p. 1-2.)

Transform, fault. A term applied to a class of faults 

on the ocean floor; no valid equivalents have been 

demonstrated on the continents. They are transverse to 

mid-ocean ridges (and rises), which are commonly offset geographically across them (see fig. 14). The axes 

of the ridges are inferred to be loci of the addition of new 

crustal material, which results in spreading of the ocean 

floor and of strike-slip displacement of the transform 

faults on the two flanks. However, the geographic offsets of the ridges themselves are only indirectly related 

to such displacements. As a result of spreading of the 

ocean floor from the ridge the strike-slip displacement 

on the faults is in opposite senses on the two flanks. The 

displacements can be inferred in places from offsets of 

sea-bottom topography, but they can be demonstrated 

even more conclusively by offsets of narrow parallel 

belts of magnetic anomaly. At each end, the transform 

faults lose displacement and die out on the ocean floor 

at variable distances from the ridge axes.

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iNDEX OF UNITS ON "TECTONIC MAP OF NORTH AMERICA" WHICH ARE REFERRED TO IN 

TEXT

Page

A1-. ___.__.______ 21, 23 

Ao._ _.___________ 21, 27 

A2._ ______.___ 21, 33, 37 

A3._ .____ 21, 22, 33, 37, 42, 64 

B... _____ 22,24,37,57,66,69 

C______________ 24, 61, 64, 73 

D________________-_..__ 27

El__._______________ 27 

Ea___-______.______ 29 

E/S___._______.__ 29, 36, 51, 54 

E7...................._..___ 29, 51

F_______________.___ 29, 36

Gl________________._ 35, 36 

G2______________ 35, 36, 41, 43

Ga.___-___._...._. -_._. 41

Gy__ ___________ 35, 41, 43 

HI______.__________ 35 

H2_________..__.__. 35 

H3__.-_-________- __...... 35, 41

H3a______..______ _. 36

H4_____....._.__....... 36

H5_. ________. 21, 36, 41, 66 

Ha________________ 35, 36, 41

H-y___.....____......__-.__ 41

H«__________________ 36 

II___.___.....__.......__ 35

12______________...... 35, 36

13__________________ 35, 36 

14__....____._...._.. 36

la___-- _...-.__ ._- 35

I-/S_____________ .. ___ 35, 41 

Jl_______________.___ 37, 54 

J2_______________. __ . 37, 54 

J2a_________________ 56 

J3______________.___ 50, 56 

J4_____.__.__.___._ 56 

Ja__________________.. 52, 56 

Kl_____..__......_.... 47, 53, 54

Kla_____________-___ 53 

K2_____________.___ 53 

K2«. __...._.....__ 53

Page

K3a.______________ 50, 53, 54 

K3b__________________ 50, 54 

K3c..._______________ 50, 54 

Ka_________________ 52, 53 

LI__ .___________ 42, 57, 60, 61 

L2________________ 42, 57, 60 

L3________________ .. 47, 57 

L3a_________________ 57 

L4________________ __ 47-57 

L4a_________________ 57 

L5____________ _____ 47-61 

L5a_________________. 61 

L6_____________ 48, 57, 58, 59 

L7_______.________- 49, 58

L7a_________________. 49, 58 

L8______________ 24, 49, 58, 61 

La__________________ ___ 42, 57 

L/S__________________ . 52, 59 

Ly,....___________ .. 52, 59, 61

L«____________ 42, 52, 57, 59, 61 

L«__________________ 52, 58, 59 

LO........................... 52, 59

M____________ __ 47, 60, 61, 73 

Nl__________________. 43, 79 

N2______..._______... 80 

N3_________________ 79, 80 

N4_____________._..__. 80 

N5__ . _____ . ____ _ __ .. 50, 80 

Na_______...__.__... 79 

N/S_ _ __ .____________ _ 43, 79 

Ol____________ 37, 49, 68, 74, 78 

O2__ . _____ ___ 41, 42, 66, 69, 73 

O3________________ 42, 66, 69 

O3a______.________ ._ 66, 68 

O4________________ 42, 66, 73 

O4a_________________ 66, 68 

O5__________________ 66, 67 

O5a__.......___._.____- 67

O6_________.____-__--_- 47, 67, 70

O6a_____________._. 68, 74, 76 

O7___. _____ 47, 67, 70, 74, 75, 76

O7a________________ 68 74 

O8_______________..... 74

O9____________ 48, 66, 69, 73, 74 

O10________._ 49, 68, 73, 74, 75 

Oil________________. 50, 68, 75

O12________________ 24, 50, 72 

Oa___________ ____ . _ 41, 76 

O/S__________________ 41, 76 

Oy.......................... 52, 71

O«_____......._.________ 52

O«_________________ 52, 67, 74

OO............... 52, 68, 71, 74, 75 76

OX__ . _____________ 52, 68 72 

Qn........................ 51, 72. 73

Ow__.___..__________ 51, 72, 74

PI________________ 47, 75 76

P2___.___,___._._______ 47, 75

P2a__.____...._____ 51, 50 75

P3_________________. 76

P4______________________ 50, 75 76

P5___________________ 50, 76 

Pa______.__________. 52

P/S___________________ 52, 75 

Py.......................... 51 75

P*________________ 51,75 77 

Pe______________ . ____ 51, 75 

Ql___________________ 49, 78 

Q2._.._._.__. ______. 47, 78, 79

Q2a_________________ 79 

Q3____________________ 79

Q4__________________ 50, 79 

Q4a_..._.____...__........ 47, 50, 79

Q5____ ____....__..... 79

Q6_._._.-.___..._..__.... 50

Qa__ ________________ 52, 79 

Q/S_______.._..._____.. 52, 79 

Q-K . - _--__. 52, 79 

Q$-__.._ ___..._.... 51,78, 79

Q«_______ . _______ __ 51, 79

95

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