... 

.., 

Tectonic Features of the 

Precambrian Belt Basin and 

Their Influence on 

Post-Belt Structures 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 866 
· Tectonic Features of the 

· Precambrian Belt Basin and 

Their Influence on 

Post-Belt Structures 

By JACK E. HARRISON, ALLAN B. GRIGGS, and JOHN D. WELLS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER X66 

U N IT ED STATES G 0 V ERN M EN T P R I NT I N G 0 F F I C E, \VAS H I N G T 0 N 19 7 4 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 74-600111 

) 

For sale by the Superintendent of Documents, U.S. GO\·ernment Printing Office 

'Vashington, D.C. 20402 - Price 65 cents (paper cO\·er) 

Stock Number 2401-02554 
CONTENTS 

Page 

Abstract................................................. 1 Phanerozoic events-Continued 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Late Mesozoic through early Tertiary-Continued 

Genesis and filling of the Belt basin . . . . . . . . . . . . . . . . . . . . . . . 1 Idaho batholith ................................. . 

Is the Belt basin an aulacogen? . . . . . . . . . . . . . . . . . . . . . . . 5 Boulder batholith ............................... . 

Precambrian Z events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Northern Montana disturbed belt ................. . 

Phanerozoic events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Tectonics along the Lewis and Clark line .............. . 

Paleozoic through early Mesozoic . . . . . . . . . . . . . . . . . . . . . . 6 Late Cenozoic block faults ........................... . 

Late Mesozoic through early Tertiary . . . . . . . . . . . . . . . . . . 6 Conclusions ............................................. . 

Kootenay arc and mobile belt . . . . . . . . . . . . . . . . . . . . . 6 References cited ......................................... . 

ILLUSTRATIONS 

FIGURES 1-4. Maps: 

1. Principal basins of sedimentation along the U.S.-Canadian Cordillera during Precambrian Y time (1,600-800 m.y. 

ago) ............................................................................................... . 

2. Principal tectonic elements of the Belt basin reentrant as inferred from the sedimentation record ............. . 

3. Principal folds and faults in Belt terrane ................................................................. . 

4. Principal folds and faults near the junction between the Hope, Ninemile, and St. Marys faults ............... . 

III 

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TECTONIC FEATURES OF THE PRECAMBRIAN BELT BASIN 

AND THEIR INFLUENCE ON POST-BELT STRUCTURES 

By JAcK E. HARRISON, ALLAN B. GRIGGs, and JoHN D. WELLS 

ABSTRACT 

The Belt basin represents a slowly sinking reentrant on the North 

American craton that began to form about 1,500 m.y. ago and persisted 

for more than 600 m.y. This sinking block somewhat resembles an 

aulacogen, but the basin is not a true grabenlike trough extending into 

the craton at a plate separation. The sinking block was at times almost 

triangular in shape; a central platform was bounded by the North 

American craton and associated narrow troughs on the south and 

northeast, and on the northwest by the Cordilleran miogeocline that 

extended past several reentrants along the North American craton. 

Following the end of Belt sedimentation and the onset of the East 

Kootenay orogeny about 850 m.y. ago, the Belt basin has been subjected to a variety of stresses. Within the basin the strain appears to 

reflect the inhomogeneities of the platform and troughs formed in Belt 

time. During and after the East Kootenay orogeny the central platform 

acted as a somewhat rigid block and contains gentle folds and vertical 

block faults; the southern trough contains a series of tear faults and 

tight folds of the Lewis and Clark line; the northeastern trough and old 

cratonic edge are the site of the Montana disturbed belt; the intersection of the two troughs forms an embayment that contains the Boulder 

batholith and related volcanics; and the old miogeocline on the 

northwest is the site of the Kootenay arc mobile belt, which contains 

gneiss domes and thrusts that rode eastward up over the platform 

block. 

Although this model aids in explaining many tectonic elements of 

the basin, some elements are still enigmatic. Unexplained are the· 

genesis and location of the Idaho batholith, the Coeur d'Alene mining 

district, and the Montana disturbed belt. 

INTRODUCTION 

The influence of Precambrian structures on 

Phanerozoic tectonic patterns of continents has become 

more apparent as information on the ·Precambrian accumulates. Within the past decade much of the terrane 

containing Belt rocks of Precambrian age (about 1,500-

850 m.y.) has been remapped or mapped in modest 

detail for the first time. Compilation of the major tectonic features (fig. 3) based on this new information, 

together with a significant increase in knowledge of Belt 

geologic history, provides an opportunity to update and 

reexamine the tectonics of the basin. Completion of a 

new geologic map of the basin at 1:250,000 is still 

perhaps a decade away, so this report should be considered to be interim and preliminary. 

Among the many tectonic events recorded in rocks of 

the basin, some can now be reinterpreted with confidence, some can be reinterpreted with trepidation in 

view of new data and geologic restraints, and others 

must still be considered as controversial, or at least unresolved. We will focus herein on (1) genesis of the basin; 

(2) Paleozoic events previously neglected in analyzing 

basin tectonics because of limited preservation in the 

geologic record; (3) some aspects concerning the origin of 

the Montana disturbed belt; and ( 4) tectonic events 

along the Lewis and Cla~k line-a zone first noticed by 

Calkins and Jones (1914), named and brought to geologic 

prominence by Billingsley and Locke (1939), and 

speculated about by many geologists. We, too, will 

speculate about the Lewis and Clark line, even though 

some structures within the line still~ in our opinion, do 

not have a satisfactory geologic explanation. 

We gratefully acknowledge helpful discussion with S. 

W. Hobbs, G. D. Robinson, M. R. Mudge, and R. L. 

Earhart on basin tectonics and problems. We especially 

thank E. T. Ruppel, who contributed his knowledge of 

and thoughts about Belt rocks and other rocks of Belt 

age in southwestern Montana and adjacent parts of 

Idaho, and whose help was fundamental to construction 

of a paleotectonic model of the Belt basin. The 

manuscript benefited from critical reviews by S. S. 

Oriel, J. L. Talbot, R. M. Weidman, and R. L. Earhart. 

W. G. Pierce not only made constructive suggestions but 

also called our attention to pertinent literature we had 

overlooked. R. A. Price patiently noted factual errors in 

the original manuscript and persuaded us to rethink or 

qualify certain interpretations. 

GENESIS AND FILLING OF THE BELT BASIN 

Our working hypothesis is that the Belt basin is one of 

several epicratonic reentrants formed along the eastern 

edge of the Cordilleran miogeocline during Precambrian 

Y time 1 (fig. 1). The most complete record of sedimentation in both the reentrants and the miogeocline appears 

'The letter refers to an interim time scale for the Precambrian adopted for use by the U.S. 

Geological Survey (James, 1972). Terms used in this manuscript are Precambrian Y (1,600 to 

800 m.y. ago) and Precambrian Z (800 to 570 m.y. ago). Time of deposition of the Belt 

Supergroup corresponds approximately to Precambrian Y, and that of the Windermere Group 

or the Windermere System of Canada, approximately to Precambrian Z. 

1 
2 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

to be that of the Belt basin, where 20 km of relatively undisturbed low-grade metasedimentary rock still remains. 

Correlations between the extensive Belt section and 

other sediments deposited during the 800-m.y. time span 

0 100 200 300 MILES 

I I 1 1 1

1 I 

0 100 200300 KILOMETERS 

of Precambrian Y time are not yet completely worked 

out, because of scattered limited exposures, insufficient 

mapping at some places, and difficulties in 

geochronologic dating of sedimentary rocks (Crittenden 

and others, 1972). However, nearly continuous deposition of sediments from the North American craton clearly filled the reentrants and at places dumped a clastic 

wedge into the miogeocline (fig. 1). 

Broad tectonic elements within the Belt basin can now 

be identified with various degrees of certainty from 

recently accumulated data on stratal thicknesses, composition of detritus, directions of clast coarsening, and 

various primary structures. These elements are shown in 

figure 2, where we have also named some of them for convenience in discussion. The Flathead and Coeur d'Alene 

troughs, the Helena embayment, and a dome of late Belt 

time in the area of the Purcell platform were identified 

(but not named) by Harrison (1972) from isopach maps 

of various Belt formations. Isopach maps, facies, and 

fan-shaped directions of deltaic sedimentary transport 

radiating outward from south to north show clearly that 

sediments of many Belt formations had a cratonic source 

area from the south side of the Belt basin (Harrison, 

1972; Hrabar, 1971) and thus indicate that at least at 

times a large cratonic prong or island formed the south 

side of the Belt basin reentrant. The basin boundary 

fault and associated coarse facies (LaHood Formation) 

of old Belt age in the Helena embayment were studied in 

some detail by McMannis (1963). Recent investigations 

by Winston (1973) indicated that much of the youngest 

Belt rock had a source area from the south and that a 

coarse conglomeratic facies of young Belt age occurs 

along the west side of the Dillon. block. This leads to our 

inference that the older Precambrian crystalline rocks of 

the Dillon block are bounded by high-angle faults on the 

north and west, a structure similar to that bounding the 

southeastern part of the Beartooth block about 100 km to 

the east. Whether the Belt basin connected through to 

the south to form our inferred Belt seaway or whether 

these younger conglomerates accumulated in a southern 

arm of the Belt basin is a matter of conjecture, from 

currently available data. In figure 2 we have shown the 

Belt seaway in patterns of both craton and Belt basin to 

indicate that the craton may have connected with Belt 

island at various times during Precambrian Y. 

Ruppel (1973) has mapped and described a thick sequence of clastic rocks of Belt age in the area we call the 

Belt seaway. The uppermost part of the section is similar 

to Belt rocks, but the great bulk of the section is quartzFIGURE 1 (left).-Principal basins of sedimentation along the U.S.-

Canadian Cordillera during Precambrian Y time (1,600-800 m.y. 

ago). Compiled largely from Gabrielse (1972), King (1969), and 

Harrison (1972). Numbers show approximate thickness, in 

kilometers, of remnant Precambrian Y sedimentary rocks. Dashedline pattern in northwest part of craton indicates possible extension 

of craton. 

\ 
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GENESIS AND FILLING OF THE BELT BASIN 3 

ite and siltite containing negligible carbonate and red 

beds, and thus it only slightly resembles Belt. The lower 

and middle parts cannot be matched directly with the 

Belt stratigraphic section without calling upon what 

appear to be unreasonable facies changes and drastic 

changes in sedimentation environment over very short 

distances-a sedimentational characteristic contrary to 

the habit in the Belt basin itself. Ruppel suggested that 

the section has been telescoped by eastward transport on 

thrusts having 160 km or more of tectonic translation. 

The estimated amount of eastward translation (160 km) 

is approximately the same order of magnitude as that 

proposed by Price and Mountjoy (1970) for eastward 

translation in the southern Canadian Rockies (200 km) 

and by Peterson (1973) for that of the thrust and fold 

belt in northern Utah and southern Idaho (80-120 km). 

Ruppel inferred that the lower part of the rock stack is 

Precambrian Y age, but it is a clastic section of the Cordilleran miogeocline that has been brought subjacent to 

Belt basin rocks which were deposited in the Belt 

seaway. We accept Ruppel's tentative interpretation, 

and it requires that the Cordilleran miogeocline of Belt 

time be located relative to Belt island and the Belt 

seaway approximately as shown in figure 2. 

A final bit of evidence for Belt island and the Belt 

seaway comes from a study by Myers (1952) of Belt age 

rocks in thrust plates now in the northeastern part of the 

Belt seaway (fig. 2). There, westward coarsening in grain 

size and increase in feldspar content of highly 

feldspathic quartzites indicate a granitic source terrane 

to the west-our proposed Belt island. The amount of tectonic transport necessary to put these rocks in their present position far east· of the proposed source area is, 

again, about 160 km. 

Sedimentation within the Belt basin and along the 

Cordilleran miogeocline occurred over the interval from 

about 1,500 to 850 m.y. ago during Precambrian Y time. 

Within the Belt basin, early deposits, particularly along 

the join of the basin and the miogeocline, contain abundant turbidites (Edmunds, 1973). Later deposits were 

largely red-bed assemblages and minor carbonate both 

within the basin and in adjacent parts of the 

miogeocline. The thickest young Belt deposits accumulated in the Helena embayment. Except for the 

rare conglomerates deposited· along the faults separating 

the Belt basin from the Dillon block, the sediments are 

all fine grained; disconformities are difficult to identify, 

angular unconformities are rare, and facies changes are 

subtle. Thus, although troughs and a dome are identifiable tectonic elements of the basin, the total picture 

of basin filling is one of slow sinking and warping of an 

epicratonic reentrant in almost perfect balance with a 

slowly rising craton of probable low relief. Even the low 

dome of late Belt time received sediment during most of 

its existence, attesting to differential subsidence rather 

than to uplift as its primary cause. 

0 100 200 MILES 

0 100 200 KILOMETERS 

FIGURE 2.-Principal tectonic elements of the reentrant that formed the 

Belt basin, as inferred from the sedimentation record. Shading 

represents Belt depositional area. Paleotectonic reconstruction as interpreted by J. E. Harrison and E. T. Ruppel. Bar and ball on 

downthrown sides of faults. 

Magmatic activity in the Belt basin during Precambrian Y time appears to have been minimal. The first 

(about 1,300 m.y. old) of two groups of basic sills were intruded in particular abundance into sedimentary rocks 

along what we infer to have been the border between the 

reentrant and the miogeocline, as were the only known 

granitic plutons-the Hellroaring Creek stock and its 

associated bodies. A local deformation of Belt rocks in 

that area is the only known folding of any significance 

during Belt time. One volcanic event that resulted in the 

Purcell Lava occurred in late Belt time (about 1,100 

m.y.) in the northeastern part of the basin. 

By the end of Belt time several crustal inhomogeneities had developed in and near the Belt basin. 

These include not only the reentrant itself, the sharp 

bend .of the Cordilleran miogeocline around Belt island, 

and the intersection of the reentrant with the 

miogeocline, but also several sags and swells that must 

have been reflected in the basement of the reentrant. 

Each irregularity affected many subsequent geologic 

events. 
4 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

Figure 3 is a compilation of the major structures in the 

Belt basin. Many of these will be discussed in detail in 

later pages, but even a ·cursory comparison of figures 2 

. ' ( 

) Approximate limit of out· 

WASHINGT~ crop of rocks of Belt age -OREGoN \ (Prec;ambrian vi 

\ 

I 

' / 

I 

I 

( 

"' 

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\ Lf 

19""t' 

, ..... 

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and 3 shows some striking similarities between the broad 

tectonic eleptents of the Belt basin of sedimentation and 

major patterns of post-Belt deformation. 

EXPLANATION 

. . .• . .......... 

High-angle fault 

Showing apparent strike sli'p. Bar 

and ball on down thrown side 

Thrust fault 

Sawteeth on upper plate 

Anticline 1 Syncline 

A A 

Overturned anticline Overturned syncline 

Areas where some Middle Cambrian (and younger) rocks 

overlie Belt rocks 

MONTANA r-wY"oMiNG-

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--.I'--_____ l\j 

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0 50 100 MILES 

. 0 50 100 KILOMETERS 

FIGURE :3.-Principal folds and faults in Belt terrane. Compilation by J. E. Harrison; revised from Bayley and Muehlberger (1968), largely through 

application of as-yet-unpublished 1:25,000 and 1:250,000 geologic mapping by various members of the U.S. Geological Survey, including 

A. B. Griggs, J. D. Wells, M. R. Mudge, R. L. Earhart, J. E. Harrison, F. K. Miller, G. D. Robinson, and E. T. Ruppel. 

'

)
PHANEROZOIC EVENTS 5 

IS THE BELT BASIN AN AULACOGEN? 

The subsiding block resulting in the Belt basin has 

some features in common with an aulacogen (Salop and 

Scheinmann, 1969) or a taphrogeosyncline. Aulacogens 

were described by Hoffman, Dewey, and Burke (1974) as 

"long lived deeply-subsiding transverse troughs, at times 

fault-bounded, that extend from orthogeosynclines far 

into adjacent foreland platforms." The Belt basin also 

meets some of the other requirements for an aulacogen, 

such as evidence of internal tectonic movement that was 

largely. vertical and absence of much high-grade 

metamorphism. It does lack, however, "periodic alkalic 

basalt and fanglomerate," although some of each is 

known in Belt time. Paleocurrents in Belt rocks are both 

parallel and transverse to the structural trend, perhaps 

because of the triangular shape of the reentrant, whereas 

those of the ideal aulacogen are parallel to the grabenshaped epicontinental reentrant. Hoffman, Dewey, and 

Burke noted that many aulacogens form at plate 

separations and that sediment sources from both sides 

provide infill to the orthogeosyncline. For Belt and 

Windermere sediments, however, no evidence has yet 

been found for a western sediment source, other than 

Belt island, either within the Belt basin or along the 

geosyncline (Price, 1964; Stewart, 1970; Gabrielse, 1972; 

Harrison, 1972; Edmunds, 1973). The concept, however, 

that the Belt basin is a modified aulacogen or 

taphrogeosyncline is certainly viable and should prove to 

be a useful working hypothesis. 

PRECAMBRIAN Z EVENTS 

Rocks of Precambrian Z age unconformably overlie 

rocks of Precambrian Y age in parts of Canada 

(Gabrielse, 1972) and conformably or unconformably 

overlie Precambrian Y rocks in some parts of the United 

States (Crittenden and others, 1971, 1972; Stewart, 

1972). In the northern Cordillera and in southeastern 

British Columbia, uplift and mild deformation followed 

by erosion reflect, respectively, the Racklan and East 

Kootenay orogenies of about 800 m.y. ago (Gabrielse, 

1972). During uplift the now-filled reentrants behaved as 

part of the craton, and Windermere and equivalent 

strata overlapped and were deposited seaward from Belt 

and equivalent strata of the older Cordilleran 

miogeocline. Windermere sediments and volcanic rocks 

unconformably overlie Belt strata only along the western 

edge of the Belt basin. Stewart (1970, 1972) considered 

Precambrian Z strata as the first deposits in the Cordilleran geosyncline and argues for a continental separation at that time. It seems to us, however, that the Cordilleran geosyncline was present all along the western 

edge of the North American continent in Precambrian Y 

time. We are not aware of evidence for a western source 

for either Precambrian Y or Z sediments. We, therefore, 

find it difficult to accept genesis of the Cordilleran 

geosyncline as a plate separation during either Precambrian Y or Z time. 

The Precambrian Z sedimentation continued uninterrupted through Lower Cambrian (Stewart, 1970), 

but there are no known sedimentary rocks belonging to 

that sedimentation cycle within the Belt basin. The uplift during the East Kootenay orogeny apparently was 

sufficient to expose Belt rocks, and they underwent their 

first major period of erosion. 

We assume that tectonic and magmatic events 

recorded beneath the Middle Cambrian quartzites 

(Flathead Quartzite and equivalents) in the United 

States part of the Belt basin represent effects of the East 

~ootenay orogeny. These events include gentle folding, 

h1gh-angle faulting, and intrusion of another group of 

basic sills. Middle Cambrian quartzites overlie the Belt 

rocks disconformably in many areas (fig. 3) and with low 

angular relation at some places. Steep block faults of 

pre-Flathead age with several hundred meters of throw 

have been mapped in the area northwest of Coeur 

d'Alene (King and others, 1970), in the area just west of 

Missoula (Campbell, 1960; Wells, 1974), and in the area 

just east of Missoula (Maxwell, 1959, 1965). Lead 

isotopes interpreted to be of Precambrian age have been 

.found in galena along high-angle faults of the Osburn 

and Hope fault zones as well as along many other faults 

scattered throughout the basin (Zartman and Stacey, 

1971). Basic sills folded in with broad open structures in 

many parts of the basin give ages in the range 750-850 

m.y. (J. D. Obradovich, unpub. data; Harrison and 

others, 1972; Mudge, 1972). Basic sills near the Purcell 

Trench thin toward or stop at the Hope fault, which 

suggests that the Hope fault predates the sills (Harrison 

and others, 1972). · 

'W_e interpret these data as indicating very gentle 

foldmg along north- to northwest-trending axes (fig. 3) 

that probably extended over most of the basin but now is 

most evident in the Purcell anticlinorium where it is well 

preserved on the Purcell platform. This folding was accompanied or followed by block faulting, mostly along 

the trends of the folds in many parts of the basin. The 

Osburn fault zone may have formed during the East 

Kootenay orogeny, a possibility that we discuss later in 

the report. 

PHANEROZOIC EVENTS 

Phanerozoic geologic history of the general area including the Belt basin is most conveniently discussed in 

units-Paleozoic through early Mesozoic, late Mesozoic 

through early Tertiary, and late Cenozoic to Holocene. 

We will depart from this format at one point to discuss 

the Lewis and Clark line-a zone of tear faults controversial in origin, formed essentially along the Co~ur d'Alene 

trough ,(figs. 2, 3) and intermittently active from 

Precambrian Z through Tertiary time. 
6 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST-BELT STRUCTURES 

PALEOZOIC THROUGH EARLY MESOZOIC 

Beginning. ·with the encroachment of Middle 

Cambrian seas and extending through Middle Jurassic 

time the Belt basin behaved as somewhat inhomogeneous 

pseudobasement. As noted by McMannis (1965), the 

Helena embayment continued to sink intermittently, 

and it consistently received thicker sediments than adjacent areas of the shelf. Wheeler (1966, p. 41) summarized 

part of the history for the southern Canadian Cordillera 

by stating: 

From Early Mississippian to mid-Jurassic time much of the eastern 

belt was, at intervals along its length, an intermittently emergent sill, 

or mobile geanticline which separated a deeply subsiding region on the 

west [the Cordilleran geosyncline], in which accumulated an abundance of volcanics and sediments, from a variably subsiding shallower 

basin to the east which received sediments almost entirely. 

The "mobile geanticline" projected into the United 

States includes the Purcell platform-the site of the late 

Belt dome-and perhaps Belt island. The "shallower 

basin to the east" corresponds. roughly to the Canadian 

part of the Rocky Mountain Trench and is outlined in 

part on isopach maps by McMannis (1965). McMannis 

showed that the shallow basin extended back onto the 

craton but clearly included thicker sediments deposited 

in the Flathead trough, the Helena embayment, and the 

Belt seaway. In the United States, the Rocky Mountain 

Trench follows along the eastern edge of the Purcell platform and thus swings away from the old craton edge, but 

the sag containing Paleozoic and Mesozoic sedimentary 

rocks follows along the old craton edge as it did in late 

Belt time. 

Identification of tectonic events in most of the basin 

during Paleozoic to early Mesozoic is difficult owing to 

limited preservation of sedimentary rocks (fig. 3) and 

lack of intrusive episodes. Middle Cambrian Flathead 

and equivalent quartzites, however, were deposited over 

the entire basin on very gently folded rocks or, at the 

eastern margin, on gently tilted rocks (Mudge, 1972, p. 

18-19). As described previously, pre-Flathead block 

faulting is known and may have been widespread. Unfortunately for the geologic historian, no Flathead is 

preserved on the tight folds of the Coeur d'Alene mining 

district along the Osburn fault zone, so the evidence to 

prove or disprove Precambrian tight folding is missing. 

Wheeler discussed mid-Paleozoic tectonism and 

plutonism north of the Belt basin as well as possible 

movement on faults in the trench during the early 

Paleozoic, but effects within the _Purcell platform are not 

known. The Antler orogeny of Nevada, another major 

tectonic disturbance of middle to late Paleozoic age, may 

have extended as far north as central Idaho (Roberts and 

Thomasson, 19Q.4), but there, too, effects if any within 

the Belt basin are unknown. The preservation of Cambrian strata in the Libby trough (fig. 3) seems to us to 

require subsidence of the graben in Paleozoic time, inasmuch as the Purcell platform was essentially positive 

and was being eroded from the mid-Paleozoic through 

the Mesozoic. 

In summary, the meager data suggest only slight vertical movement in the Belt basin during Paleozoic and 

early Mesozoic time. The Purcell platform was at times a 

positive mass, and the various troughs of Belt age again 

received marine sediments. How much, if any, sedimentary rock of Devonian and younger age was deposited on 

the Purcell platform, Belt island, and the intervening 

area will forever remain a conjecture, bec·ause none are 

now preserved in that area. We suggest that the vertical 

adjustments of the time were probably expressed ·in 

renewed movement on high-angle faults formed in 

Precambrian Z time, some of which bounded and others 

of which were in the Purcell platform. 

LATE MESOZOIC THROUGH EARLY TERTIARY 

The rather quiet, nonmagmatic, tectonic regime 

dominated by vertical displacements in and around the 

old Belt basin ended dramatically with the tectonic 

magmatic upheaval that began in the Jurassic and extended to the Eocene. The broad regional events were 

described succinctly by Robinson (1971), who wrote: 

The entire Cordilleran orogenic belt in Canada and U.S. may be 

viewed as a zone along which the upper Precambrian-Paleozoic 

geosynclines moved relatively eastward in late Mesozoic and early Tertiary time against the lower Precambrian crystalline craton, and over 

the Phanerozoic platform. 

During this time the many inhomogeneities of the old 

Belt basin, some of which had already affected post-Belt 

sedimentation, were reemphasized in the strain patterns 

that still dominate the geologic textures of the basin today (fig. 3). 

Most of the old Belt basin became a positive tectonic 

element. Significant amounts of sedimentation occurred 

only along the eastern edge, where sediments shifted 

from marine to dominantly nonmarine and 

volcaniclastic (McMannis, 1965; Mudge, 1972). A conglomerate of limited areal extent filled the Purcell 

Trench (fig. 3) in the western part of the basin (Harrison 

and others, 1972). 

Tectonic or tectonic-magmatic features that were 

formed or reactivated during that time include the 

Kootenay arc, the Idaho batholith, the Boulder 

batholith, the northern Montana disturbed belt, and the 

Lewis and Clark line. Of these, .all but the Idaho 

batholith appear to be readily relatable to Precambrian 

structures of the old Belt basin. 

KOOTENAY ARC AND MOBILE BELT 

The Kootenay arc (fig. 3), which has been defined in 

several ways, is identifiable on most geologic maps (see 

Wheeler, 1966, fig. 10-1, for example) as the arcuate contact between Belt rocks and Windermere rocks or the 

subparallel contact a few kilometers to the west between 

/ 
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PHANEROZOIC EVENTS 7 

Precambrian and Paleozoic rocks. The Kootenay arc is 

near, but not at, the eastern edge of a zone of intensive 

deformation, metamorphism, and intrusion-the 

Kootenay arc mobile belt. This mobile belt extends as 

far east in the old Belt basin as the Purcell Trench (fig. 

3), and it extends northward into Canada to include the 

fan-folded gneiss dome of the Shuswap terrane 

(Wheeler, 1966). The mobile belt encompasses the 

highly metamorphosed and thrust zones between the 

Kootenay arc and the Purcell Trench recently mapped 

by Clark (1973) and by F. K. Miller (written commun., 

1972) and may extend 160 km to the west where Fox and 

Rinehart (1973) recently identified another gneiss dome. 

The belt is the "mobile infrastructure" of Price and 

Mountjoy (1970), and the zone of gneiss domes appears 

to follow along the old Cordilleran miogeocline (fig. 1) at 

least from mid-Canada to .mid-United States (Price, 

1971). 

The Kootenay arc marks the limits of exposure of Belt 

rocks, and a pertinent question is whether the arc is a 

primary or a secondary feature. The Belt rocks are still 

very thick and show no evidence of depositional thinning 

where they pass westward under a cover of younger 

sediments in the zone of the Kootenay arc or under an 

extensive cover of Columbia River Basalt, which also 

masks the-southern end of the arc. Thus the actual configuration of the Belt sedimentary prism may always be 

conjectural, but it surely extended many kilometers 

westward from the westernmost exposures seen today. 

Several facts suggest, however, that the arc was a significant crustal feature during late Precambrian time: (1) 

conglomerates (or diamictites) are associated with 

volcanics in the Windermere along the arc, (2) the 

western part of the basin bordering the arc contains the 

most abundant and thickest basic sills of Precambrian 

age, and (3) in the zone with the basic sills are the only 

known granitic intrusives of Belt time as ~ell as the 

unique Sullivan lead-silver deposits. As noted by Yates 

(1973), the zone of the Kootenay arc was one of alternating eugeosynclinal and miogeosynclinal sedimentation beginning with Windermere time; he concludes, 

therefore, that the arc is not a plate junction and that 

the curvature is primary. Our paleotectonic reconstruction (fig. 2) requires hinge lines or fault boundaries completely around an epicratonic reentrant, and we suggest 

that the join between the Belt basin and the Cordilleran 

miogeocline represents a deep-seated break that 

provided access for the intrusives and extrusives of late 

Precambrian age along the arc. We concur with Yates 

(1973) and further suggest that the primary curvature of 

the arc represents the zone where the Cordilleran 

miogeocline swung out and around Belt island. Belt 

sedimentation covered the old hinge line, and some 

sediments may actually thicken westward where they 

filled the shallow basin and spilled over into the deeper 

miogeocline (see Harrison, 1972, figs. 7, 12, for example). 

The Kootenay arc was, in our opinion, established in 

Belt time, rejuvenated at the beginning of Windermere 

time, and reemphasized during Mesozoic-Cenozoic time 

when the mobile belt rode eastward and upward to shove 

the miogeoclinal sediments up the old hinge line onto the 

edges of Belt island and the Belt basin. 

IDAHO BATHOLITH 

The Idaho batholith is an irregular but generally 

hourglass-shaped compound body. No detailed study of 

the batholith and related tectonics has been published, 

but the batholith appears to contain both Cretaceous 

(100 m.y. or older) and Tertiary intrusives and related 

volcanic rocks. Thrusting on the eastern side and around 

the northeastern end shows tectonic transport away from 

the batholith (fig. 3) and involves rocks as young as Early Cretaceous (Calkins and Emmons, 1915; Gwinn, 1961; 

Nelson and Dobell, 1961;. Kauffman and Earll, 1963; 

McGill, 1965; Maxwell, 1965). 

The northern half of the batholith occupies the eastern 

end of the old Belt island, adjacent parts of the Belt 

seaway, and the Belt basin as far north as the Coeur 

d? Alene trough. At least part of the batholith has gneiss 

dome characteristics (Chase and Talbot, 1973) and may 

involve mobilization of pre-Belt rocks. Whether some 

high-grade metamorphic rocks in roof pendants of the 

batholith are Belt or pre-Belt is moot, in part because no 

means has been found to establish the kinds and ages of 

rocks that may have ridden up on or over Belt island during. formation of the. Cordilleran fold and thrust belt. 

Because Belt island was necessarily a source for some 

Belt sediments (Harrison, 1972) it seems likely that at 

least some of the high-grade metamorphic rocks isolated 

in the batholith represent the pre-Belt crystalline basement. We have no explanation for the location of the 

batholith. 

The. southern half of the batholithic hourglass occupies a part of the old Belt miogeocline, and the wasp 

waist of the hourglass coincides with our reconstruction 

of the join (hinge zone) between the old miogeocline and 

Belt island. Regional gravity studies by Don R. Mabey 

(oral commun., 1973) show a line bounding a regional 

gravity differential that cuts through the Idaho batholith 

at this same point and extends southeast across the 

Snake River Plain to the Wasatch Front. This line of 

gravity change was discovered independently of our 

geologic reconstruction, but the line is identical with our 

inferred hinge zone along and southeast of Belt island 

(fig. 2). The southern half of the batholith, in our opinion, is in the old miogeocline south of Belt island as is 

the Kootenay arc mobile belt on the north; it thus has 

the potential for a causative relation to the thrust zone 

now in the Belt seaway (fig. 3). This causative relation 

has been investigated by Scholten (1971) and by Ryder 
8 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

and Scholten (1973), who believe that the eastwarddirected thrusts on to the old craton are "regional gravity 

gliding features, with the energy provided by a crustal 

· undation in the area of the Idaho Batholith ***" 

(Scholten, 1971). Whatever the cause of the thrusting, at 

least 160 km of eastward translation would have been 

required to bring Precambrian Y rocks of the 

miogeocline against Belt rocks of the Belt seaway 

(Ruppel, 1973). 

BOULDER BATHOLITH 

The vast amount of magma represented by the 

Boulder batholith and its associated volcanics welled up 

from the anomalously deep depression of the Helena embayment. Overlapping plutonism, volcanism, and tectonism of the Boulder batholith has been well 

documented by Robinson, Klepper, and Obradovich 

(1968), who noted that all these events took place within 

20 m.y., between 85 and 65 m.y. ago. Folding and 

thrusting near the batholith began somewhat earlier 

and ended sooner than folding and thrusting in the Montana disturbed belt which ended in late Eocene about 40 

m.y. ago; however, the main period of deformation in the 

two areas was approximately at the same time (Mudge, 

1970). These disturbances are younger than some of the 

tectonic events associated with the 100+-m.y.-old Idaho 

batholith. Thrusting on the eastern side of the Idaho 

batholith involves rocks as young as Early Cretaceous. In 

at least one area midway between the Idaho and Boulder 

batholiths, thrusts and overturned folds indicate 

transport to both east and west (Gwinn, 1961); there, the 

west-directed stress from the Boulder batholith has 

overridden, tilted, or refolded older east-directed thrusts 

and folds associated with the Idaho batholith (McGill, 

1965). 

NORTHERN MONTANA DISTURBED BELT 

Our analysis of the basin and its influence on tectonic 

patterns can be applied to the controversy regarding 

genesis of the Montana disturbed belt. Two opposing 

views on the genesis of the imbricate thrust structures in 

the disturbed belt have been advocated: gravity gliding 

off a vertical uplift (Mudge, 1970, 1971), and lateral 

gravitational spreading from a deeply rooted z~ne of upwelling in a mobile infrastructure (Price and Mountjoy, 

1970; Price, 1971). 

In our opinion both sides of the argument have merit 

for the original areas for which they were conceived, and 

the peculiarities of the Belt basin reentrant can be used 

to explain the differences. The mobile infrastructure 

referred to by Price is exposed in the Shuswap terrane, 

whose oldest recognizable sedimentary rocks are highly 

metamorphosed formations of Windermere age that are 

subjacent to the foreland fold and thrust belt. These 

Windermere rocks are inferred to rest on a Hudsonian 

(pre-Belt) basement having a remarkably uniform gentle 

slope away from the old craton. This inference requires 

that at least 12 km of Belt rocks has disappeared along 

strike in the old miogeocline within a few tens of 

kilometers, which seems unlikely to us. But this is actually a minor point to Price's interpretation, for the 

relatively smooth surface he requires may be a combination of Hudsonian crystalline rocks of the old craton and 

a wedge of low-grade metamorphic Belt rocks smoothed 

by erosion to a nearly flat surface during the East 

Kootenay orogeny before or during deposition of the 

Windermere System. What is of consequence is that the 

zone of mobile infrastructure, which lies close to the old 

hinge line and downdip from the old craton in the 

geosynclinal structure of southeastern British Columbia, 

follows along the Kootenay arc and the wider zone of the 

Kootenay arc mobile belt as it approaches the U.S. 

border. In our interpretation, mobile infrastructure 

formed at geosynclinal depths and, consequently, follows 

the main geosyncline and not the Belt reentrant. 

Although Mudge (1971) is correct in stating that no 

mobile infrastructure exists in western Montana, complex gneiss domes and fold and thrust structures do occur in eastern Washington and western Idaho in the 

Kootenay arc mobile belt. There, the "mobile infrastructure" is separated from the Montana disturbed belt by 

240 km of gently folded Belt rocks of the Purcell platform; the Belt rocks are cut by some high-angle faults of 

several thousand meters of throw that surely involve 

basement and that are both pre-Middle Cambrian and 

possibly Paleozoic in age. Therefore, no smooth surface 

is now evident along which to translate the Jurassic 

through Cretaceous updip movement from the Kootenay 

arc mobile belt through the Purcell platform to the Montana disturbed belt. Neither can we find evidence for a 

now-missing Phanerozoic prism of sediments thick 

enough to transmit the stress. Thus, Price's hypothesis 

may be acceptable for observed structures in southern 

British Columbia and other parts of the Cordilleran fold 

and thrust belt where no rigid block exists between 

mobile infrastructure and old cratonic edge. It may apply to genesis of the thrust structures in Belt (and preBelt?) rocks at the eastern edge of the Kootenay arc 

mobile belt in the United States where no Phanerozoic 

rocks exist to take up thin-skinned deformation, but the 

Montana disturbed belt seems related to the southern 

Canadian thrust and fold belt only insofar· as both occur 

at the edge of the old Belt craton. In other words, the 

zone of mobile infrastructure goes around, not through, 

the old Belt basin. One might suspect that the Cordilleran thrust belt, like the Rocky Mountain Trench 

(Leech, 1964, 1972), shows some topographic or geologic 

continuity related to the hinge zone of the old craton but 

may not everywhere have the same structural genesis. 

When we consider what we believe are serious objections to Price's proposed mechanism operating within 

... 
PHANEROZOIC EVENTS 9 

the Belt basin, Mudge's hypothesis of gravity sliding 

seems more enticing. Nevertheless, it leaves unexplained 

some critical points. Mudge (1970) showed his main 

glide surface (decollement) cutting gently and smoothly 

up section from lower Belt at the Rocky Mountain 

Trench into Paleozoic rocks in the thrust belt. Evidence 

is abundant for block faulting of pre-Middle Cambrian 

age in many parts of the Belt basin and of prethrusting 

age in the Rocky Mountain Trench (Leech, 1964); such 

evidence conflicts with Mudge's smooth decollement as 

it did with Price's. Mudge also calls on the now-classic 

abnormal fluid pressures to buoy and lubricate the slide 

surface. Most of the inferred decollement, however, is in 

rocks of lower Belt which were prograded into the biotite 

zone of the greenschist facies in Precambrian time and 

whose "bed" permeability is about zero owing to extensive recrystallization of the rock. This metamorphism 

presents serious problems to application of the fluid 

pressure hypothesis in metamorphic rocks as noted 

originally by Davis (1965). And finally, new mapping in 

the Rocky Mountain Trench and adjacent areas subsequent to Mudge's compilation and analysis has failed to 

reveal any trace of either a breakaway zone or a glide 

plane, but, more significantly, it has revealed an open 

fold structure with opposing dips of 20° or more, which 

contrasts sharply with the monoclinal structure of the 

Mission Mountains on the east (fig. 3). Mudge's 

hypothesis, which suggests extension in the heel zone of 

47 km or slightly more, then requires that the simple 

monoclinal dip of about 20° found in the Mission Mountains be restored over the folds to the west in the Mission 

Valley. Such restoration does not match structures even 

if we allow significant eastward tilting of the Mission 

Mountains block during the formation of the frontal 

fault zone on the west face of the Mission Mountains. 

Thus, we do not believe that either hypothesis fully explains all geologic data now available. Nor do we, at present, have data to support an alternative hypothesis; 

this places us in agreement with Robinson (1971), who, 

on the basis of a related but somewhat different approach, reached essentially the same conclusions. 

TECTONICS ALONG THE LEWIS AND CLARK LINE 

Originally the Belt basin part of the Lewis and Clark 

line was defined as the zone of tear faults between 

Helena and Missoula on the east and Spokane on the 

west (Billingsley and Locke, 1939, p. 36). For convenience in our discussion we will consider the Hope 

fault zone also to be a part of the Lewis and Clark line, 

although for esthetic (fig. 3) as well as some geologic 

reasons, such an addition is not totally warranted. We 

will refer to this zone simply as "the line" in the rest of 

this report. 

New geologic data have been collected along virtually 

the entire length of the line between Missoula and Sandpoint (fig. 4) in the past few years. Among the principal 

new discoveries are continuity of the Hope and Ninemile 

faults, recognition and extent of the St. Marys fault, extension of the zone of tight west-northwest folds south of 

the Osburn fault into the area south of the St. Marys 

fault and north of Missoula, recognition of major thrusts 

from the southwest that ride out into the line, and discovery of a series of small thrusts hinged at the north 

showing maximum displacement where they abut the 

Hope or St. Marys faults. It has also become obvious 

that the Rocky Mountain Trench, the Libby trough, and 

probably the Purcell Trench terminate at the line as 

defined by the Osburn and St. Marys faults; those faults 

must, then, be as old as or older than the trenches. 

Tectonics along the line appear deceptively simple. 

Many geologists, using a standard technique of interpreting tectonic events from gross structural "patterns," 

have concluded that the angle of tight folds within the 

line (fig. 3) or a supposed continuity between batholithic 

terrane south of the line and that in the Kootenay arc 

mobile belt demonstrates left-lateral movement of many 

kilometers along the line. In our opinion, such assumptions of apparent continuity of "patterns" are not only 

unwarranted but also unsupported by details of the 

geology. For example, first, our studies of facies and formations of Belt rocks across the line indicate that the apparent offset in units not affected by major thrusts can 

be only a few kilometers. Two illustrations of this lack of 

major offset are (1) exceptionally thick sections of the 

Burke Formation (Wells, 197 4) match almost directly 

north across the line; and (2) the Empire Formation, 

which thins rapidly both east and west from its maximum 

thickness of 450 m in the Mission Mountains north of the 

line, shows approximately the same thicknesses in contiguous parts of the formation within the line. Second, if 

the fold pattern were formed by left-lateral movement, 

then the movement must be as young as the Upper 

Cretaceous rocks involved in the pattern. However, (1) 

Coeur d'Alene veins older than Late Cretaceous cut folds 

of the "pattern," and (2) geologic offset of other folds of 

the "pattern" was several kilometers right lateral in 

Cretaceous-Tertiary time, which would require a large 

left-lateral movement followed immediately by an even 

larger right-lateral movement-a strange if not unreasonable geologic event. We concur with Weidman 

(1965), who cautioned against attributing the fold and 

fault patterns along the line to a single event. In the 

following paragraphs we will analyze data now available 

and attempt to establish a credible sequence of events. 

The tectonic record for Belt time shows an almost 

stable but slowly sinking platform accumulating sediment that was slightly thicker in a shallow trough 

parallel to the line. The sedimentation record along the 

line shows_ no evidence of any significant folding or 

faulting during Belt sedimentation. 
10 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

I 

I Zone of Eocene 1 

porphyry dikes I< oz 

::r:::< <E-< 

a,~ I~ 

I 

... 

.J. 

:::> 

WEST~ 

A Fold 

c 

No ver~ical exaggeration 

EAST 

A' 

Approximate 

level of present 

topographic surface 

SKETCH SECTION THROUGH THRUSTS ADJACENT 

TO HOPE-ST MARYS FAULT ZONES 

0 

I 

10 

10 

I I 

20 

20· 

I I 

30 

I 

30 

I 

40 MILES 

I 

40 KILOMETERS 

EXPLANATION 

p 

b 

Contacts 

Shown between selected formations where needed 

to emphasize structural trends. p, Prichard 

Formation; b, Burke Formation; r, Revett Formation; s, ·st Regis Formation. ·or its stratigraphic correlative; the Spokane Formation. In section only: 

A, Ravalli Group; W, Wallace Formation 

Anticline 

High-angle fault 

Showing direction of 

lateral movement. Bar 

and ball on downthro'wn 

side 

A 

. . Thrust fault 

Sawteeth on upper plate 

Syncline Overturned 

Folds showing direction of plunge 

FIGURE 4.-Principal folds and faults near the junction between the Hope, Ninemile, and St. Marys faults. Fold axes labeled A, B, C, and D are 

probable correlatives across major faults. Principal sources of data: Sandpoint area, Harrison, Kleinkopf, and Obradovich (1972); KelloggCoeur d'Alene area, Griggs (1974); Alberton area, Wells (1974); remainder of area, 1:250,000 unpublished mapping by Griggs, Harrison, and 

Wells. Fault movement in section: A, away from viewer; T, toward viewer. 

). 
PHANEROZOIC EVENTS 11 

Precambrian Z (Windermere time) and Early Cambrian events along the line can be inferred only by identification of events occurring pre-Middle Cambrian 

(Flathead and equivalent rocks) or events occurring 

prior to or during intrusion of dated basic sills at about 

850 to 750 m.y. ago. Some steep block faults of several 

thousand feet throw formed in pre-Flathead time, as 

shown by overlap of offset Belt strata by Flathead Quartzite in the area northwest of Coeur d'Alene (King and 

others, 1970), in the area just west of Missoula 

(Campbell, 1960; Wells, 1974), and in the area just east 

of Missoula (Maxwell, 1959, 1965). Basic sills folded in 

with the broad open structures at the north edge of the 

Lewis and Clark line-one just east of the Purcell Trench 

(Harrison and others, 1972) and one just west of the 

Rocky .Mountain Trench (J. D. Obradovich, unpub. 

data)-are al;Jout 850 m.y. old, or East Kootenay orogeny 

in age. Some thick basic sills that persist to the north for 

tens of kilometers near the Purcell Trench thin toward or 

end at the Hope fault, which suggests that the Hope 

fault existed in some form at the time of sill intrusion 

(Harrison and others, 1972). Whether any other major 

faults of the line formed in latest· Precambrian time is 

moot. Thus, the regional patterns previously described 

and the local patterns along the line combine to indicate 

that broad open folds, some steep block faults, and a 

precursor of the Hope fault formed at about 850-750 m.y. 

ago. 

At this point in the discussion we must face squarely 

the controversial issue of genesis and age of the tight 

west-northwest folds within the line south of the Osburn 

and St. Marys faults (figs. 3, 4). We become involved immediately in long-standing controversies over age and 

genesis of the Coeur d'Alene lead-silver ores on the west 

end of the line as well as relations of tectonic events 

along the line to tectonism accompanying intrusion of 

the Idaho and Boulder batholiths and formation of the 

thrust belt on the east end of the line. Pertinent geologic 

relations are described below. 

A zone about 40 km wide of west-northwest-trending 

folds lies south of the Osburn and St. Marys faults along 

their entire length. The folds tend to be tight or slightly 

overturned in the western two-thirds of the zone and 

somewhat more open in the eastern third. A pervasive 

cleavage accompanies the tight to overturned folds. In 

the Coeur d'Alene district steep veins cut the folds or are 

along their flanks; although some veins are faulted off or 

sheared, they are not folded (Hobbs and others, 1965). 

Thus the veins are definitely post-folding. Lead isotope 

studies (Zartman and Stacey, 1971) suggest a model lead 

age of about 1,200 m.y ., but a different set of geologically 

valid assumptions to determine the youngest reasonable 

age for remobilized lead yields an age of mineralization 

of 825 ( +395, -500) m.y. The age of pitchblende from the 

Sunshine mine in the Coeur d'Alene district has been 

calculated as about 1,100-1,200 m.y. by Eckelmann and 

Kulp (1957, p. 1130); those authors also noted that the 

high content of fine-grained galena and the low content 

of thorium prompted certain assumptions concerning 

best values to use for corrections, and these caused a 

change from an earlier determination that gave an age of 

about 885 m.y. for the pitchblende. Because the 

stratigraphic column lacks evidence of a tectonic disturbance at 1,200 m.y. (about mid-Belt or Wallace Formation time), we prefer to accept a young(post-Belt) age as 

more realistic geologically than the model lead or 

"corrected" uranium-lead age. Our interpretation supports that of other investigators who have suggested that 

the Coeur d'Alene ores were remobilized from 

depth-perhaps from Sullivan-like deposits in lower 

parts of the Prichard Formation-sometime in post-Belt 

but pre-Cretaceous time. 

Toward Missoula the zone of tight folds becomes complicated. In the Missoula-Alberton area additional structural elements are present; westerly trending thrusts 

showing transport to the northeast occur, and Cambrian 

strata are folded along with the Precambrian rocks. Imbricate thrusts with tear faults bounding them are now 

known 60 km west of Alberton (fig. 4) where the thrust 

plates override broad open folds south of the line. 

Thrusts of similar trend and direction of tectonic 

transport are also found within the line. At least one 

such thrust south of the Ninemile fault has sheared 

along the south flank of an overturned fold but when 

traced to the west terminates abruptly into a northwesttrending fold (Wells, 1974), indicating that the thrust 

formed from continuation of the same stress that caused 

the overturned folds. Several thrusts are present in both 

the southern and the northern parts of the line, an indication that stresses extended across it. The Ninemile fault 

appears to offset one of the thrusts where the two intersect southeast of Missoula (fig. 4); however, the actual amount of offset, if any, is conjectural, inasmuch as 

the intersection is buried beneath Tertiary and younger 

valley fill. 

Between the Ninemile and St. Marys faults is a zone 

where the more competent rocks-Helena and Wallace 

Formations and younger-form relatively open 

northwest-trending folds, whereas along the axis, down 

section, toward the center of the parallel fold, the less 

competent rocks of the Prichard Formation show tight 

overturned folds having prominent axial-plane cleavage. 

The relief of the stress created by compression of the 

strata in the center of the fold was by movement upward 

and to the northeast, as shown by the inclination of the 

cleavage and the asymmetry of the folds. Here, too, the 

stress that caused the folding was also relieved eventually by thrusting up the south-facing flanks of anticlines. 

The abrupt change in tectonic style at the St. Marys 

. fault to open uncleaved north-trending folds north of the 
12 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

fault (fig. 3) shows that the St. Marys fault acted as a 

tectonic boundary at the time of the tight folding and 

thrusting south of it. 

Here then, the northwest-trending thrusts and related 

folds are younger than Cambrian but are older than 

movement on the Ninemile fault that tilted Tertiary 

strata as well as cut off a tear fault associated with one of 

the thrusts north of the fault (fig. 4). 

This zone continues eastward from Missoula, where it 

contains west-northwest-trending folds and thrusts 

north of the Ninemile fault but near to it (fig. 3). Some of 

the folds that parallel the thrusts involve Cambrian 

rocks (Nelson and Do bell, 1961; Kauffman and Earll, 

1963). Still farther east, folds of the zone not only involve 

rocks as young as Cretaceous but also turn south, and 

although the inflection point may be at or near the projected position of the Ninemile fault or a branch from it 

(fig. 3), the folds are not offset along a fault (Gwinn, 

1961). Thrusts definitely related to the tight to overturned north-trending structures and that show 

translations both eastward and westward are abundant 

in the area between the Idaho and Boulder batholiths 

(fig. 3). 

Farther toward Helena only the St. Marys fault is still 

traceable. The various splays of the line pointing toward 

the Boulder batholith are not identifiable in or beneath 

either younger (Eocene) volcanics or older (Late 

Cretaceous) volcanics associated with the batholith and 

do not cut through either the volcanics or the batholiths. 

Our analysis of the events leading to the geologic picture described above should be classified somewhere 

between a speculation and an interpretation. In our 

opinion, the data suggest the following: (1) The westnorthwest folds are of several ages and origins, (2) no 

fault of the line is through going, (3) various faults have 

taken up the stress at various times, and ( 4) the apparent continuity of folds and faults of the zone is-misleading in terms of origin and timing just as in the examples previously discussed of the Rocky Mountain 

Trench and the Cordilleran thrust belt. 

Weidman's (1965) concise summary of the interpretations of tectonic events along the line presents the 

variety of thoughts by many different geologists and the 

enigmas inherent in many of these interpretations. Not 

all these explanations are repeated here, but by careful 

selection of various parts of previous interpretations one 

could arrive at ours, which hardly makes ours original. 

Nonetheless, ours does have an input of more data from 

additional mapping that puts some restraints on sequence and timing and thus on interpretations. We 

suggest that the tight folds within the line in the Coeur 

d'Alene district are post-Belt and pre-Flathead. 

Although their echelon arrangement and angle suggest 

to some a left-lateral movement on the Osburn fault, all 

geologic evidence of offset on the Osburn, Hope, 

Ninemile, and St. Marys shows right-lateral displacement. Direction of movement on the St. Joe fault is 

equivocal from all published geologic data; only a sketch 

map published by Reid and Greenwood (1968) suggests 

the direction of the relative slip, which would be left 

lateral if thrusts at the east end (figs. 3, 4) actually connect with the St. Joe fault. If the thrusts override the St. 

Joe fault or are cut off by it, then data are still not adequate for determining relative movement. As all other 

faults in the line are right lateral, we infer that the St. 

Joe also is right lateral. We, therefore, accept the concept of a wedge caught between converging· right-lateral 

structures-essentially the Osburn and St. Joe 

faults-as sustaining drag and compression, similar to 

the proposal by Hobbs, Griggs, Wallace, and Campbell 

(1965, fig. 34). A wedge was also proposed by Smedes 

(1958), but its southern boundary was considered as the 

Idaho batholith; this explanation no longer seems possible, because broad open folds are now known south of the 

St. Joe (fig. 4). The wedges and consequent tight folding, 

which we infer to be pre-Flathead, could not have extended past the junction of the Osburn and Ninemile 

faults, because Cambrian rocks are in only slight angular 

unconformity with Belt rocks east of that junction. 

In our opinion, the Coeur d'Alene ores fit best into this 

episode of Precambrian Z tectonism. The "minimum" 

ages previously cited (p. 11) calculated by both the leadlead and the uranium-lead methods fall in the 800- to 

900-m.y. bracket, the time of the East Kootenay 

orogeny. 

We attribute the tight folds in the Missoula-Alberton 

area (fig. 4) to accommodation of stress accompanyingintrusion of the Idaho batholith, stress that was resolved 

into upward and northeast-directed strain as far north as 

the St. Marys fault. Some stress probably was 

transmitted through (about at right angles) some faults 

of the line, but thrusts also eventually rode across the 

line. West from Missoula we suggest that the southward 

curving of thrusts and tight folds (fig. 3) reflects the 

stresses related to emplacement of the Idaho batholith of 

Early Cretaceous age. Opposed overturned folds and 

thrusts of westward transport reflect the Late 

Cretaceous and early Tertiary tectonics associated with 

the Boulder batholith, one of several alternatives offered 

by McGill (1965). 

Apparent right-lateral movement determined from 

offset of major contacts is about 26 km along the Osburn 

fault (Hobbs and others, 1965), 26 km along the Hope 

fault (Harrison and Jobin, 1963), 29 km along the 

Ninemile fault (Wells, 1974), and 13 km along the St. 

Marys fault (J.E. Harrison, unpub. data). The Osburn, 

Hope, and Ninemile faults also all have an apparently 

large component of dip slip (all down to the south in a 

down-to-basin configuration to the Coeur d'Alene 

trough), which complicates determination of true slip. 
CONCLUSIONS 13 

Offset of fold axes (fig. 4) suggests that the true strikeslip component on the Osburn is about 26 km, whereas 

the true strike-slip component on the Hope is 10-13 km. 

At least for the Hope fault a Tertiary movement of 5-7 

km right lateral can be demonstrated from offset of the 

Purcell Trench and Selkirk Mountain frontal fault of 

Cretaceous age (fig. 3) and from dating of 50-m.y.-old 

porphyry dikes (fig. 4) that fill tension fractures of the 

pull-away zone north of the Hope fault at the Purcell 

Trench (Harrison and others, 1972). Because the Tertiary movement does not account for the total displacement, the faults evidently existed before the Tertiary. 

Branches from the Ninemile-Osburn system (fig. 3) 

appear to be identifiable in upper plates of thrusts (Maxwell, 1965) associated with the Idaho batholith southeast 

from Missoula, which suggests that some faults of the 

line are Early Cretaceous or older. Evidence for a 

Windermere age for inception of the Hope and possibly 

the Osburn and St. Joe has been presented previously. 

The line, therefore, seems to have begun forming late in 

Precambrian time and has had movement intermittently along it ever since. 

The 5-7 km of right-lateral movement identifiable at 

the intersection between the Hope fault and the Purcell 

Trench appears to have been taken up mainly along the 

Hope and St. Marys faults and to a lesser extent along 

the Ninemile. The apparent offset of the Cretaceous 

thrusts along the Ninemile fault near Missoula (fig. 4) is 

1.2 km or less. The reason for this selective Tertiary 

movement largely on the Hope and St. Marys becomes 

evident when we realize that by early Tertiary time all 

eastern segments of the line except the St. Marys fault 

were blocked by the deep-seated (Klepper and others, 

1971) intrusion of the Boulder batholith. 

Stress in early Tertiary time caused the block north of 

the line to move east. Such movement was proposed by 

Harrison, Kleinkopf, and Obradovich (1972) on the basis 

of data from the Hope fault-Purcell Trench area, and our 

new mapping adds further conclusive evidence to that 

interpretation. Hinged, or flap, thrusts north of the 

Hope-St. Marys faults are shown in plan and cross section in figure 4. None of the thrusts extend northward for 

more than a few kilometers, each ends in a slight warp, 

all abut and do not cross the Hope or St. Marys faults, 

and each has greatest displacement along those faults. 

The strain pattern results from an eastward-directed 

stress that compressed, then sheared, along the westfacing limb of an anticline in rocks previously folded 

moderately (B fold and C fold) or sheared along a gentle 

anticline with consequent accordion pleating in the upper plate of an old open fold (D fold). The total 

measurable crustal shortening on all three thrusts and 

associated folds is on the order of a few kilometers, which 

is consistent with the total crustal lengthening in the 

pulled-apart zone at the trench (Harrison and others, 

1972). This tectonic event, therefore, appears to require 

some eastward movement and counterclockwise rotation 

of the block north of the Hope and St. Marys faults to 

generate the unusual strain pattern that increases adjacent to the faults. If the St. Marys fault extends into the 

disturbed belt, the relatively small amount of movement 

along the St. Marys fault could not have contributed 

significantly to the thin-skinned deformation of the disturbed belt and may be unimportant in the major cause 

of the thrusting. 

LATE CENOZOIC BLOCK FAULTS 

Cenozoic block faulting (Pardee, 1950) that formed 

Basin and Range structures is the final basinwide tectonic event, although some fault movement is recorded 

by tilting or faulting of some Tertiary basin-fill 

sediments. Details of the fault pattern far exceed in 

number of faults and in complexity those shown in figure 

3, and an appreciation of them must await completion of 

mapping and publication of 1:250,000 geologic maps now 

being prepared. The pattern is remarkably similar to, 

but much better exposed than, the pattern described by 

Stewart (1971) for the Basin and Range structure in 

Nevada. The horst-and-graben structure of. the Belt 

basin terrane clearly is an expression of tensional stress, 

but as is obvious from prevfous discussions, we believe 

that such stress has affected the basin many times since 

the ·Belt rocks were deposited. Surely, extensive block 

faulting occurred during the early Cenozoic, and the 

U.S. part of the Rocky Mountain Trench was formed at 

that time, but we hasten to point out that much of the 

Cenozoic faulting may represent renewed movement on 

much older faults. The 12-15 km of tectonic relief 

between the Montana disturbed belt and the crest of the 

Purcell anticlinorium horst, for example, is assumed by 

Mudge (1970) to have formed entirely during 

"Laramide" time. This assumption not only seems to 

require what to us is excessive vertical movement in a 

short time but also ignores all evidence of pre-Tertiary 

vertical movements. If, however, part (perhaps half) of 

the apparent tectonic relief was formed (and eroded) 

during latest Precambrian to Jurassic, then the 

measurable tectonic relief is more readily understood 

even though a smaller amount of post-Jurassic relief is 

somewhat detrimental to Mudge's thesis. 

CONCLUSIONS 

Tectonic patterns apparent at first glance on geologic 

and tectonic maps that include the Belt basin appear to 

be Mesozoic to Cenozoic ("Laramide") in age. The accumulating data indicate that the puzzling 600-m.y. gap 

in tectonic events (end of Belt to Jurassic) is more apparent than real. Also, with increased knowledge of the 

geometry, sedimentation, and genesis of the Belt basin, 

the influence of that basin on subsequent tectonic events 
14 TECTONIC FEATURES OF PRECAMBRIAN BELT BASIN, POST -BELT STRUCTURES 

can be better understood. Thus, we can now demonstrate a probable nearly continuous succession of the 

events and explain the reasons for location of some of 

them. 

Four structural elements prominent in Belt time have 

been key factors that influenced subsequent reaction of 

the basin to various stresses. These .main elements are: 

1. The Cordilleran miogeocline, which extended along 

the North American craton during Belt and 

Windermere time and which apparently formed 

the western boundary of a shallow miogeocline or 

modified aulacogen, the Belt basin. Later features 

related to the miogeocline or its junction with the 

Belt basin in our opinion are the Kootenay arc, the 

Kootenay arc mobile belt, and perhaps the 

Sullivan lead-silver ores and the Purcell Trench. 

2. The west-northwest sedimentation trough of early to 

mid-Belt time, referred to in this report as the 

Coeur d'Alene trough. This zone of crustal 

weakness is now occupied by the right-lateral 

shear zone known as the Lewis and Clark line and 

contains on its western end the Coeur d'Alene 

lead-silver ores. 

3. The hinge of the Canadian Shield, which defined the 

northeast edge of the Belt basin. Major broad 

folds, block faults including the grabens of the 

Libby trough and the Rocky Mountain Trench, 

and the northern Montana disturbed belt all 

parallel this old feature. In addition, continued 

sagging along the hinge zone provided a sedimentation trough along the craton in both Belt and 

Phanerozoic time. A deep basin where that sag 

joins the sag along the Lewis and Clark line now 

contains not only Phanerozoic sediments but also 

the Boulder batholith and associated volcanics. 

4. The Purcell platform, a triangular-shaped block 

formed by the intersection of the three structural 

elements just listed. The platform is approximately in the position of a low dome formed in late Belt 

time, and it seems to have acted as a somewhat 

rigid block ever since. Although it is gently folded 

internally, the major tectonic relief in the block is 

vertical along high-angle faults. The Kootenay arc 

mobile belt rides ·up on its west edge, and the 

Rocky Mountain Trench follows along its east 

side. 

Several important elements in the Belt basin are still 

not well understood. Among these are the geometry and 

sedimentation of the southernmost part of Belt terrane, 

the Idaho batholith, and the reason for and genesis of the 

Montana disturbed belt and the ores of the Coeur 

d'Alene district. Our lack of understanding for the first 

two is in part due to lack of data in areas where little 

geologic work has been done. By contrast, abundant information on the Montana disturbed belt and the Coeur 

d'Alene district still lends itself not to definitive analysis 

but only to various speculations within broad geologic 

constraints. 

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