Melanges and Their Bearing 

on Late Mesozoic and Tertiary 

Subduction and Interplate 

Translation at the West Edge 

of the North American Plate 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1198 
Melanges and Their Bearing 

on Late Mesozoic and Tertiary 

Subduction and Interplate 

Translation at the West Edge 

of the North American Plate 

By KENNETH F. FOX, JR. 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1198 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983 
Fox, Kenneth F., 1933-

UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Melanges and their bearing on late Mesozoic and Tertiary subduction and interplate translation at the western edge of 

the North American plate. 

(Geological Survey professional paper 1198) 

Bibliography: p. 36-40 

Supt. of Docs. no.: I 19.16:1198 

1. Melanges (petrology). 2. Geology, stratigraphic-Mesozoic. 3. Geology, stratigraphic-Tertiary. 4. Plate tectonics. 5. 

Geology-North America. I. Title. II. Series: United States. Geological Survey. Professional Paper 1198. 

QE471.15.M44F69 1983 551.1'36 8~00355 

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

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

Abstract ------------------------------------------------------------------------- 1 

Introduction --------------------------------------------------------------------- 2 

Late Cretaceous and Tertiary plate-tectonic setting ----- 2 

Definition of melange, broken formation, and petrotectonic assemblage ----------------------------------------------------- 3 

Origin of melange ------------------------------------------------------ 3 

Objectives of present investigation ------------------------------ 4 

Geometry of Humboldt- and Mendocino-type triple junctions ----------------------------------------------------------------------- 5 

Age and distribution of melanges and related rocks ----------- 10 

San Onofre Breccia ---------------------------------------------------- 10 

Olympic Peninsula and Oregon-Washington borderland- 10 

Franciscan Complex, Franciscan assemblage, Great Valley sequence, and correlative rocks -------------------------- 15 

Semantic distinction between Franciscan Complex 

and Franciscan assemblage and the relation of 

these rocks to the Great Valley sequence ----------- 15 

Hsu's (1969) concept of allochthonous, mesoallochthonous, 

autochthonous, and neoautochthonous rocks -------------- 18 

Evidence bearing on the age of formation of the Franciscan Complex ----------------------------------------------------- 18 

Morro Bay area --------------------------------------------------- 19 

Clear Lake-Covelo area ---------------------------------------- 20 

Bandon are a -------------------------------------------------------- 21 

Main elements of melanges and their bearing on plate-tectonic 

history -------------------------------------------------------------------- 23 

Page 

Franciscan Complex --------------------------------------------------- 23 

Age of deformation resulting in the Franciscan Complex ------------------------------------------------------------------ 23 

Cause of allochthonous deformation that created the 

Franciscan Complex ------------------------------------------ 24 

Comparison of Cretaceous through early Eocene tectonic regimes as predicted by the triple-junction 

hypothesis with concepts suggested by other evidence ---------------------------------------------------------------- 25 

Evidence from the Franciscan Complex contradictory 

to the triple-junction hypothesis ------------------------- · 27 

Oregon-Washington borderland ---------------------------------- 28 

Age, distribution, and plate-tectonic setting of the 

borderland -------------------------------------------------------- 28 

Origin of the borderland through plate collision ----- 29 

Reconciliation of the collision hypothesis with other 

tectonic features of the Pacific Northwest ---------- 32 

Melanges of the Olympic Peninsula ----------------------- 33 

The Aja Fracture Zone and the formation of a 

Humboldt-type triple junction ---------------------- 33 

Origin of melanges and broken formations ------- 33 

San Onofre Breccia ---------------------------------------------- 34 

Conclusions ---------------------------------------------------------------------- 36 

References cited -------------------------------------------------------------- 36 

ILLUSTRATIONS 

Page 

FIGURE 1. Sketch map showing geometry west of the North American plate from 60 to 80 Ma derived through extrapoliation of late 

Cenozoic plate motions ------------------------------------------------------------------------------------------------------------------------------------- 2 

2. Schematic model of early and middle Tertiary evolution of the plate geometry of the northestern Pacific -------------------------- 2 

3. Sketch map showing relation of oceanic features of the north eastern Pacific to melange and other features of the western part 

of the North American plate ----------------------------------------------------------------------------------------------------------------------------- 4 

4. Cross section showing hypothetical formation of melanges through tectonic churning and accretion of trench deposits to underside of a subduction zone as postulated in subduction-complex theory ------------------------~-------------------------------------------- 6 

5-8. Diagrams showing: 

5. Vector circuit representing the relative movement of three plates in mutual contact at a ridge-transform fault-subduction 

zone triple junction ----------------------------------------------------------------------------------------------------------------------------------- 7 

6. Evolution of plate geometry at a triple junction ----------------------------------------------------------------------------------------------- 8 

7. Progressive advance of Humboldt-type triple junction along a continental plate and the volume of material that must be 

disposed of during this advance ------------------------------------------------------------------------------------------------------------------- 9 

8. Formation of fold belt, thrusts, gravity thrusts, and melanges at Humboldt-type triple junction ---------------------------- 10 

9. Geologic map of western Oregon and Washington ------------------------------------------------------------------------------------------------------ 11 

10. Sketch map showing tectonic classification of the Franciscan Complex and associated rocks of western California and southwestern Oregon ---------------------------------------------------------------------------------------------------------------------------------------------------- 14 

11. Diagram showing correlation of boundary regimes and petrotectonic features along the west side of the North American plate - 17 

12. Schematic view to northwest showing transit of Early Cretaceous Humboldt-type triple junction with accompanying formation 

of melange and a proto-Coast Range thrust to east ---------------------------------------------------------------------------------------------- 26 

13. Paleomagnetic map of the floor of the northeastern Pacific Ocean ------------------------------------------------------------------------------- 30 

14. Diagram showing Late Cretaceous and early Tertiary plate geometry west of the North American plate ------------------------- 31 

15. Sketch map showing successive positions of the Aja fracture zone relative to the North American plate -------------------------- 34 

16. Schematic view to north showing deformation and generation of melanges and broken formations in early and middle Miocene 

time at the present site of the Olympic Peninsula ------------------------------------------------------------------------------------------------ 35 

III 
MELANGES AND THEIR BEARING ON LATE MESOZOIC AND 

TERTIARY SUBDUCTION AND INTERPLATE TRANSLATION 

AT THE WEST EDGE OF THE NORTH AMERICAN PLATE 

By KENNETH F. FOX, JR. 

ABSTRACT 

Melanges are commonly considered to be material scraped off an 

oceanic plate descending at a subduction zone, tectonically churned, 

and accreted to the underside of the overriding plate. Yet the correlation of Late Cretaceous and Tertiary melanges of western North 

America with subduction zones of that age is poor. During much of the 

middle and late Tertiary, this area was continuously or discontinuously bordered by a subduction zone within which the Farallon plate 

and much of its successor, the Juan de Fuca plate, were consumed. 

Yet known melanges of this age that can reasonably be linked to this 

process are rare and limited to those of the Olympic Peninsula of 

Washington. Melanges are also present within the Franciscan Complex of western California and within the Otter Point Formation of 

southwestern Oregon, mostly Eocene or older. 

An alternative to the subduction-complex theory is that melanges 

are material that was broken and sheared as it was plowed aside and 

either coasted or was rammed inland at a triple junction migrating 

along the edge of the continental plate. The required triple junction is 

of a singular dynamic type, referred to as a Humboldt-type, formed 

where an oceanic plate obliquely underthrusts a continental plate and 

advances laterally along the edge of that plate while ·following a retreating oceanic (or possibly continental) plate. The triple junction 

may be formed through the interection of either (1) a spreading ridge, 

transform fault, and subduction zone or (2) two transform faults and 

a subduction zone. 

The Franciscan Complex includes rocks that contain detritus 

eroded from preexisting melanges or detritus deposited by normal 

sedimentary processes on top of preexisting melange. These sequences were subsequently sheared, fragmented, and intermixed to 

form new melanges or broken formations, strata similar to melanges 

but containing no exotic blocks. The Franciscan in places contains a record of two or more distinct cycles of melange development. Evaluation of such constraints as are known on the ages of these cycles 

suggests three diachronous events, believed to represent the transit 

along the western margin of the continent of Humboldt-type triple 

junctions in Cretaceous and early Tertiary time. The youngest of 

these is fairly well bracketed by ages of nonpenetratively deformed 

rocks and penetratively deformed melange or broken formation near 

Morro Bay, Calif., and less satisfactorily in the Covelo-Clear Lake 

area of California. The ages suggest that the most recent period of formation of the Franciscan Complex and correlative rocks was during 

the Campanian at Morro Bay and early Eocene or perhaps later time 

near Covelo. Farther north, the age of the most recent overthrusting 

and imbrication of Franciscan-like rocks near Bandon, Oreg., also is 

bracketed within the early Eocene, but it is not certain that melange 

or broken formation formed contemporaneously with the thrusting. 

In California, the final episode of allochthonous deformation was 

probably a diachronous upheaval producing melange and broken formation that transited the continental margin at a rate of roughly 4 ern/ 

yr, reaching northern California by the early Eocene. This timing 

nearly coincides with the transit of the Kula-Farallon-North American 

triple junction, as inferred by Tanya Atwater in her constant-motion 

model of Late Cretaceous and Tertiary plate geometry. In early 

Eocene time, however, this transit apparently evolved into an event 

in which coastal areas of southwestern Oregon and northwestern 

California were contemporaneously deformed and the allochthonous 

oceanic crust now underlying northwestern Oregon and western 

Washington was formed and accreted to the craton. 

The basement rock of trus Oregon-Washington borderland consists of oceanic tholeiitic basalt of early and middle Eocene age, which, 

from published paleomagnetic data, is believed to have been rotated 

clockwise as much as about 70° by middle Tertiary time. The contact 

of the oceanic crust with the craton to the east is apparently defined 

by a zone of steep negative gravity gradients. The angular to jagged 

outline of this contact as inferred from published gravity maps 

suggests that the borderland is an aggregation of variably rotated 

blocks, rather than a single elongate and coherent crustal block. The 

reported attitude of source fissures of the tholeiite suggests derivation 

in part in a stress system with a tensional direction comparable to that 

of the Kula-Pacific ridge rather than of the supposedly nearby KulaFarallon ridge. 

Prior to 56 Ma (million years before A.D. 1950), the paths of 

Pacific and North American plates may have been convergent rather 

than parallel to the trend of the Queen Charlotte and San Andreas 

faults, as they have been for the past 25-30 million years. If they w,.ere 

convergent, the allochthonous crust of the borderland could have been 

accreted to the North American plate during a collision between that 

plate and the Pacific plate. 

It has been proposed that the Kula-Pacific spreading ridge vanished abruptly shortly after 56 Ma. The presence within the oceanic 

crust of the Oregon-Washington borderland of a tensional orientation 

comparable to that of this ridge suggests that the ridge, instead of 

vanishing, jumped northward, intersecting the North American plate 

northwest of the former Kula-Farallon-North American triple junction. 

The Pacific plate, enlarged by the addition of part of the Kula 

plate, then sideswiped the North American plate, driving a widening 

wedge of recently formed oceanic crust inland while crunching and 

stacking adjacent rocks of the craton in Oregon and Washington. By 

the impact of this collision, the Pacific and North American plates 

were deflected into their present paths parallel to their bounding 

transforms as the allochthonous wedge of oceanic crust was sheared 

off, fragmented, and rotated clockwise to form the basement of the 

Oregon-Washington borderland. 

The core rocks of the Olympic Peninsula consist of melange and 

broken formation, infaulted or imbricated with blocks of intact strata. 

Rocks peripheral to the core consist of the oceanic tholeiitic basement 

1 
2 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

of the Oregon-Washington borderland with interfingering clastic deposits, mainly overlain by shallow-water marine-shelf deposits. The 

core rocks are bathyal marine turbidite deposits. Melanges of the 

western core contain fossils whose reported ages are as young as early 

or middle Miocene. Published potassium-argon ages of the rocks of the 

eastern core suggest metamorphism after 29 Ma and cooling about 17 

Ma. 

Magnetic lineations of the northeastern Pacific step right laterally 

across the Aja fracture zone. From the age of these anomalies, it appears that north of the Aja, the spreading ridge system and coexisting 

subduction zone shrank, then vanished about 21 Vz Ma. The Aja would 

then intersect the Queen Charlotte fault and the subduction zone to 

the south and, with continued right-lateral movement of the Pacific 

plate, would form a Humboldt-type triple junction. That triple junction would then persist through about 51

/z m.y., finally dying 16 Ma as 

the ridge system south of the Aja stepped eastward and intersected 

the subduction zone. This timing nearly coincides with potassiumargon ages of cooling of the youngest melanges in the eastern core of 

the Olympic Peninsula. To account for the structural fabric and geographic extent of the Olympic melanges of Miocene age through the 

tectonism associated with this triple junction, the junction must have 

been situated immediately west of the Olympic Peninsula. If this spatial and temporal relation is valid, northwestward movement of the 

Pacific plate relative to the North American plate has averaged about 

6 crn/yr at least since middle Miocene time, a rate comparable to accepted estimates of the rate of movement of these plates averaged 

over the past 2 m.y. 

INTRODUCTION 

LATE CRETACEOUS AND TERTIARY 

PLATE-TECTONIC SETTING 

In Late Cretaceous and early Tertiary time, the 

North American plate was flanked to the west by the 

Kula plate (Grow and Atwater, 1970, p. 3717) and the 

Farallon plate (McKenzie and Morgan, 1969). Still 

farther west, the Kula and Farallon plates joined the 

Pacific plate (fig. 1) at spreading ridges from which all 

three plates grew. During this time, the oceanic plates, 

though moving in various directions with respect to each 

· . · . NORTH . ·s • ~~· .. AMERICAN .. ." · . PLATE.·\ 

Anc ora·g~· -~ . . ea .e:.: · ·_. .... S~~ · ·. :·." .· :- · .. · ·. · · ...... ·~. 

• · ~ • Fran.clsco · : · · · . Guaynli~s · : \\ • • • • 0 • • • • • 

\\ II~ r ~II 

II 

"' ~/~o !' ~~ Mall' (/ ~ 

\/'l-? 

'l-? 

FARALLON 

PLATE 

Mendocino 

fracture zone 

./·/·:·.:· ...... ·/''<· ... / ~;: ··.::. 

PLATE ''. 

FIGURE 1.-Plate geometry west of North American plate from 60-80 

Ma (million years before A.D.1950) (derived by Atwater, 1970, p. 

3531, through extrapolation of late Cenozoic plate motions). Arrows show directions of spreading and plate movements relative 

to the North American plate, arbitrarily held fixed. Pacific plate 

is assumed to be moving at a constant 6 crnlyr parallel to transform faults that later (in late Cenozoic) developed between it and 

the North American plate. Model explained by Atwater (1970, p. 

3531) as one of numerous alternatives permitted by her data. 

other, collectively moved right laterally past the North 

American plate (Atwater, 1970). Consequently, the subduction zone down which the Farallon plate plunged obliquely below the overriding North American plate 

gradually lengthened to the northwest. 

The spreading direction between Pacific and Farallon plates changed abruptly in early Tertiary time 

KULA 

PLATE . · ... · .NORTH · 

· .. · · · .· · · . ·AMERICAN ..... . . . PLATE · . 

JUAN 

DE FUCA 

PLATE 

A 

cocos 

PLATE 

· .. · ... · · .· ... ·. ·NORTH· .· · · . .'. 

. · · · . . · ... ·AMERICAN ... · .. . . · PLATE . . . 

8 

cocos 

PLATE 

Pioneer fracture ·. · 

zone · 

FIGURE 2.-Early to middle Tertiary evolution of plate geometry of 

northeastern Pacific (modified from Menard, 1978, p. 105). A, 

Early Tertiary (50 Ma). B, Middle Tertiary (36 Ma). 
INTRODUCTION 3 

(Menard and Atwater, 1968). Magnetic anomalies 

formed after this change radiate slightly fanwise about 

a distant pole (Menard, 1978, p. 104), suggesting that 

the Farallon plate gradually pivoted counterclockwise 

with respect to the Pacific plate. Menard (1978, p. 104) 

has postulated that about the time anomaly 21 was 

formed (50 Ma)1 the Farallon plate broke along the eastward projection of the Murray fracture zone into two 

parts, cutting the Juan de Fuca plate on the north away 

from the main part of the Farallon plate on the south, 

here referred to as the Cocos plate (fig. 2). 

At about the time of formation of anomaly 13 (36 

Ma), the Pioneer fracture zone also broke eastward to 

the edge of the North American plate, forming a small 

plate between the Pioneer and Murray fracture zones 

(Menard, 1978, p. 104). Shortly after about 30 Ma, the 

eastward projection of the Pacific plate between the 

Pioneer and Murray fracture zones contacted the North 

American plate. After 27 Ma, some 3 to 4 m.y. later, the 

segment of Pacific plate between the Mendocino and 

Pioneer fracture zones also contacted the North American plate. Subsequently, through northward advance of 

the Juan de Fuca- Pacific-North American triple junction (now at Cape Mendocino) and southward retreat of 

the Cocos-Pacific-North American triple junction, the 

plate geometry evolved into its present configuration 

(fig. 3)2• 

The evolution of plate geometry outlined here is 

based on the history of spreading of the northeastern 

Pacific, deduced chiefly from the magnetic lineations 

and morphology of its basaltic floor, and extrapolation of 

the present rate (roughly 6 crnlyr) and direction of relative movement between the Pacific plate and North 

American plate back to Late Cretaceous time. Although 

the gross geometry is probably correct, the model becomes progressively weaker proceeding back in time bec~use of the cumulative effect of errors in the assumptiOns and because much of the pre-Tertiary spreading 

record has been eradicated by consumption of oceanic 

crust at converging plate margins. 

Acknowledgments.-! appreciate the penetrating 

yet constructive criticisms of the manuscript by J. C. 

Matti, R. W. Tabor, M. C. Blake, Jr., D. L. Jones, and 

P. D. Snavely, Jr. Continuing dialogs with R. J. 

McLaughlin and M. C. Blake, Jr., on problems of the 

Franciscan Complex were very helpful, as were discussions with R. W. Kopf on the distinction between rockstratigraphic units and lithotectonic units, and distinc1 Ages assigned to magnetic anomalies in thiB report are baaed on the time scale or La 

Brecque and others (1977). 

~The Juan de Fuca plate was called the Vancouver plate by Menani (1978). At the riBk 

of some slight loaa in preciBion, thiB plate and other fragments or the Farallon plate, such 88 

the Cocos plate, are here referred to by the commonly accepted name or their major surviving 

remnant. 

tion between the terms "Franciscan Complex" and 

"Franciscan assemblage." 

DEFINITION OF MELANGE, BROKEN FORMATION, 

AND PETROTECTONIC ASSEMBLAGE 

Geologic features indicative of particular plate-tectonic regimes of the past (petrotectonic assemblages of 

Dickinson, 1971) include ophiolites, paired metamorphic 

belts, old volcanic arcs, thrust belts, and melanges. Following Hsu (1968, p. 1065), "melange" refers to those 

enigmatic, though "mappable bodies of deformed rocks 

characterized by the inclusion of tectonically mixed fragments or blocks, which may range up to several miles 

long, in a pervasively sheared, fine-grained, and commonly pelitic matrix." The melanges include both exotic 

and native blocks. Again quoting Hsu (1968, p. 1065), 

"Native blocks are disrupted brittle layers which were 

once interbedded with the ductilely deformed matrix. 

Exotic blocks are tectonic inclusions detached from 

some rock-stratigraphic units foreign to the main body 

of the melange." A body of broken strata containing no 

exotic blocks but otherwise similar to a melange is defined as a "broken formation" (Hsu, 1968, p. 1065-1066). 

These and analogous features are major guides to 

past plate geometries. Even where correctly identified, 

the precision with which they can be applied is reduced 

because (1) the features are inherently difficult to date· 

or (2) their genetic correlation with a 'particular plate~ tectonic regime is tenuous; or (3) the spatial relation is 

diffused because the petrotectonic feature forms over a 

broad area at a considerable or indefinite distance away 

from the causal plate boundary regime; or ( 4) the spatial 

relation is confused by lateral translation of unknown 

magnitude before final accretion to the craton. Attempts 

to verify, calibrate, and rigorously extend Atwater's 

(1970) model by dating the petrotectonic features of the 

continental plate have generally been frustrated. 

ORIGIN OF MELANGE 

The recognition of the loose association of melanges 

with presumed consuming plate boundaries, the internal 

tectonic disruption of the melanges, and the incorporation of trench deposits and ophiolitic bodies within them 

suggested to many workers that the melanges formed 

through scraping off and tectonic churning of the upper 

surface of the oceanic plate as it was being subducted 

(Hamilton, 1969, p. 2415-2416). By this concept, 

melanges were considered to be imbricated slices of material scraped off the descending oceanic plate and accreted to the underside of the overriding plate during 

the subduction process (fig. 4). Indeed, the presence of 

melanges within a geologic terrane is now the single 

most important indicator of the subduction complex. 
4 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

One of the inadequacies of this subduction complex 

theory is that, in detail, the correlation between the location and age of melanges and the inferred location and 

duration of past subduction regimes is poor. For example, according to Atwater's (1970) model, during much 

of the middle and late Tertiary, the western side of the 

North American plate was continuously or discontinuously bordered by a subduction zone within which the 

Farallon plate and much of its successor, the Juan de 

Fuca plate, were consumed. Yet known melanges or 

melangelike rocks of this age that could possibly be related to this subduction process are rare, being limited 

to those of the Olympic Peninsula (Stewart, 1971; Tabor 

and Cady, 1978a). Perhaps the subduction process is inherently occult, its products concealed from observation 

except when unusual circumstances not now understood 

intervene. If it is, the subduction-complex hypothesis in 

its present form makes no verifiable predictions. 

An alternative hypothesis to explain the origin of 

the melanges of the Franciscan Complex was recently 

'Jo·w 100 200 300 400 500 KILOMETERS 

advanced by Fox (1976). According to this hypothesis, 

melanges form as a series of coalescing and imbricated 

gravity slides and thrust slices at a triple junction as it 

migrates along the edge of the continental plate. 

Further, the required triple junction is of a singular dynamic type referred to as a "Humboldt" triple junction. 

At a Humboldt-type triple junction, a transform fault 

bounding the continental plate is converted to a subduction zone through lateral transport of the two adjacent 

oceanic plates. Melanges, then, represent a chronological and spatial tie between the point on the continental 

plate where it joins with two adjacent oceanic plates and 

one at which there is a transition from a strike-slip tectonic regime to a subduction regime. 

OBJECTIVES OF PRESENT INVESTIGATION 

One of the objectives of this paper is to test the subduction-complex and triple.:junction hypotheses by comFIGURE 3.-Relation of oceanic features of northeastern Pacific to melange and other features of western part of North American plate. Oblique Mercator projection about late Cenozoic pole of rotation between Pacific and North American plates at lat 52° N., long 73°W. (Chase, 

1972). Melange terranes too small to show located in San Juan Islands (S.J.I.) and at Cedros Island (C. I.). Oceanic features after Atwater 

and Menard (1970), Naugler and Wageman (1973), Vine (1968), Raff and Mason (1961), Barr and Chase (1974), Moore (1973), and Larson 

(1972). 
INTRODUCTION 5 

paring the timing and locus of melange formation with 

that suggested by the known or inferred evolution of 

plate geometry along the western margin of the North 

American plate. Except for the Neogene, however, 

neither the evolutionary history of plate geometry nor 

the age of formation of the melanges is known with sufficient exactitude to provide a very sensitive test. Meaningful comparisons can indeed be made for the Neogene, 

but only very crude comparisons for the early Tertiary 

and Cretaceous. For that part of the geologic record, 

the recognition of an association of melanges with specific triple junctions could potentially improve both perception of past tectonic regimes and precision in their correlation with particular plate configurations. This leads to 

the second objective of this paper; to summarize evidence now available concerning the ages of the melanges 

and, by interpreting them as records of the existence 

and pa~sage of Humboldt-type triple junctions, to modify and amplify the Late Cretaceous and early Tertiary 

plate-tectonic history of western North America. 

!))))~ Melange 

- San Onofre Breccia 

• P Pinnacles Formation of Andrews ( 1936) 

• N Neenach Volcanics 

GEOMETRY OF HUMBOLDT- AND MENDOCINO-TYPE 

TRIPLE JUNCTIONS 

Triple junctions have been classified into 16 types 

according to the nature of the intersecting plate boundaries, that is ridge-ridge-ridge, ridge- trat;lsform-subduction zone, transform-transform-subductio'lt zone, and 

so on, and the conditions under which they are stable 

defined (McKenzie and Morgan, 1969). In this discussion, my interest is focused chiefly on understanding the 

tectonic processes at the margin of a continental plate. 

I therefore restrict my inquiry to the dynamics of those 

stable ridge- transform-subduction zone and transformtransform-subduction zone triple junctions in which the 

edge of a continental plate is a transform fault on one 

side of the triple junction and a subduction zone on the 

other side. 

The subduction zone is presumed to dip continentward at an angle· within the range of dip of modern Benioff zones, approximately 20°-65° (Turcotte and 

EXPLANATION 

. . . . 

F. Z. 

Subduction zone-Sawteeth on upper 

plate: sawteeth in direction of dip 

Transform fault-Dotted where concealed 

or inferred 

Fracture zone 

--Sc-- Magnetic anomaly-Queried where uncertain 

Boundary of Oregon-Washington bor· 

derland 

Spreading ridge-Dashed where inferred 

FIGURE 3.-Continued 
6 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

Schubert, 1973, p. 5880), whereas the transform fault is 

presumed to be nearly vertical, also in accordance with 

seismic studies of modern transform faults such as the 

San Andreas fault. The triple junction. marks, then, the 

point at the surface of the Earth where the inclined 

plane of the subduction zone intersects the nearly vertical plane of the transform. The third leg of the triple 

junction is either a spreading ridge or transform 

separating two oceanic plates. 

The rate and direction of lateral migration of triple 

junctions at which a continental plate contacts two adjacent oceanic plates depends on the relative motion of the 

three plates with respect to each other. Although that 

fact will seem self-evident, the details of the geometric 

relation between the movement of the triple junction 

and that of the three plates may seem obscure except to 

those who closely followed the development of plate-tectonic theory. The geometric details may be clarified by 

considering the following hypothetical example of relative movement between three plates, A, B, and C, as 

depicted in figures 5 and 6. 

I define the triple junction between plates A, B, 

and C to be stable in the sense of McKenzie and Morgan 

(1969); that is, the relative movement between these , 

plates sums to zero and can therefore be represented by 

the vector triangle shown in figure 5. In this example, 

spreading at the ridge between plates B and C is ortho-

. gonal to the ridge and is symmetrical; that is, equal increments of new oceanic crust are accreted to plates B 

and C during any given interval of time, and so plates B 

and C move away from the ridge at equal rates. That 

being the case, all points on the ridge and its imaginary 

prolongations at either end must lie on the perpendicular bisector of line B-C (fig. 5). 

The triple junction must lie both on the perpendicular bisector of line B-C and also on the line of which the 

vector between C and A is a segment, hence must be located at J in figure 5. The vector J A thus represents the 

direction and velocity of movement of the triple junction 

J with respect to plate A, that is, 1 cm/yr N. 40° W. if 

the spreading direction is N. g7o E. 

In this example, plate B moves N. 40° W. at 6 cm/yr 

relative to plate A, and plate C moves N. goo E. at 6 em/ 

yr relative to plate B. Were these rates the same, but 

the spreading direction N. goo E. rather than N. g7o E. 

(fig. 5), the triple junction would plot at J 2, coincident 

with the apex of the triangle at A. In this situation, the 

triple junction would not move relative to plate A. Were 

the spreading direction N. 75° E. (fig. 5), the triple junction would plot at J3 in figure 5, hence would move 1 em/ 

yr S. 40° E. relative to plate A. In this example, the direction and rate of movement of the triple junction relative to plate A is very sensitive to minor differences in 

spreading direction. Differences in spreading rate, angle 

of intersection of ridge and the margin of the continental 

plate, or in rate of offset of the continental plate with reEarly Cretaceous turbidites 

20 

40 

~ 60 w 

fw 

::2: 

~ 80 

:r:.' 

fa.. 

~ 100 

120 

140 

160 

Trench 

----;7' 

EXPLANATION 

Intrusive igneous rocks 

Sialic crust 

Early Cretaceous turbidites 

Late Cretaceous turbidites 

Early Cretaceous 

melange 

Fault-Showing relative movement 

Base of oceanic plate 

Moho discontinuity I ----------·---

0 20 KILOMETERS 

L___j 

CONTINENTAL PLATE 

Former location of 

the subduction zone 

FIGURE 4.-Cross section showing hypothetical formation of melanges through tectonic churning and accretion of trench deposits to 

underside of a subduction zone, as postulated in subduction-complex theory. 
INTRODUCTION 7 

spect to the oceanic plates could similarly affect the 

movement of the triple junction. 

~elative to the continental plate, then, the triple 

junction could (1) move in the same direction as the 

transform-fault-bounded oceanic plate but at the same 

or a lesser rate, thereby extending the length of the 

subduction zone; or (2) not move; or (3) move in the direction opposite (1), thereby extending the length of the 

transform fault. 

Returning to our example (fig. 6) and its vectorial 

representation (fig. 5), it is convenient to consider the 

spreading ridge as simply a crack along which upwelling 

magma is plastered to plates B and C. From the vector 

diagram (fig. 5), we see that a point on this crack (P) 

must move 4 crnlyr toward N. 3° W., that is, toward triple junction J 6 (fig. 6). Hence the crack, or spreading 

ridge, is being shortened at a rate of 4 ern/yr. 

The vector CJ, which represents the direction and 

rate that plate C moves with respect to the triple juneN.1~:~-----7 -----Oo E I ----- N. 8 _...:.---·-// 

---- I ---- 1 I 

----..:::.-=-==----~- N 87° E I I a~-~-~--------------------------------,~~-------------·--------·----~t~c 3 cm/yr 

4 cm/yr 

3 cm/yr f 

I 

I 

I 

I 

I 

I 

d 

/,1 ~ I o It ~ I/ <-. 

I 5.4 cm/yr 

FIGURE 5.-Vector circuit representing the relative movement of 

three plates, A, B, and C, at a ridge-transform fault-subduction 

zone triple junction (J) that is stable in the sense of McKenzie and 

Morgan (1969) and at which the three plates are in mutual contact. A point on the spreading ridge is represented by P. The direction of spreading is orthogonal to the ridge, and the rate of 

spreading symmetrical with respect to the ridge. Three cases are 

represented; in all, plate B moves N. 40° W. at 6 crnlyr with respect to plate A, and the spreading rate at the ridge is also 6 emf 

yr. Case 1 (solid lines): plate C moves N. B'r' E. with respect to 

plate B; case 2 (long-dashed lines): plate C moves N. 80° E. with 

respect to plate B; case 3 (short-dashed lines): plate C moves N. 

75° E. with respect to plate B. 

tionJ, trends N. 34° E. Hence the edge of the subducted 

slab (dotted line in fig. 6A) must also trend N. 34° E. 

(ignoring the effects that curvature, dip, partial melting, deformation, and other processes might have on the 

outline of the subducted slab as projected to the surface). To clarify this picture, we define three points that 

initially are superimposed on triple junction J 6 in figure 

6A. Point R is attached to plate B, point S to plate C, 

and point Q is fixed with respect to the crack (spreading 

ridge). At this instant in time, a brief magnetic polarity 

change is recorded in the cooling basalt along the ridge 

system, forming anomaly 5. 

After an interval of time, the plate geometry will 

have evolved to that portrayed in figure 6B. Relative to 

plate A, point R has moved 6 crnlyr toN. 40° W., point 

S has been subducted and moved 5.4 crnlyr toN. 24° E., 

where it lies on theN. 34° E.-trending edge of the subducted part of plate C. The triple junction has moved 

from J 6 to J 4 , and point Q now lies on an imaginary prolongation of the crack (spreading ridge), which has been 

shortened at the rate of 4 crnlyr. Anomaly 5 is visible 

alongside the spreading ridges. The length of that anomaly in plate C, including the part that has been subducted, equals its length in plate B. 

The evolution of this plate geometry is followed 

through four more time periods, assuming no changes in 

spreading rates, in figures 6C, 6D, 6E, and 6F. Note 

that the triple junction and the leading edge of the subducted slab move steadily northwest, until the ridgeridge transform fault forming part of the boundary between plates B and C intersects the margin of plate A. 

At that time, the velocity of the triple junction with respect to plate A abruptly accelerates from 1 crnlyr to 6 

cm/yr, and there is a concomitant acceleration and 

change in outline of the leading edge of the subducted 

slab. 

Consider now the space problems that arise if the 

triple junction and leading edge of the subducted slab 

are forced to migrate laterally along the margin of the 

continental plate. If, as in the example given, the subduction zone is extended at the expense of the transform 

fault bounding the continental plate, the descending 

plate must incrementally displace the wedge-shaped volume of lithospheric material partly bounded by the 

transform, the base of the continental plate, and the laterally projected surface of the subduction zone (fig. 7). 

As shown in figures 6 and 7, the space problem may be 

exacerbated while a ridge-ridge transform fault is subducted. 

Because of its buoyancy, the displacement of the 

volume of crustal material involved must be accomplished by the plowing up and ramming back of the lip 

of the continental plate above and ahead of the advancing prow of the subducting slab (fig. 8). Thus elevated, 
8 

A 

D 

---- .....--

MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

B 

. .. . . . . 

. .. . . · . . . , 

~\)f!~\~i,j;~~ 3. J' ••• {? m·~.:.:..;,;.:um· ......... ,.·.~·!.-.:,.:..._ 

A 

················· 

E 

EXPLANATION 

A 

:··:: .. 

············ c 

. . 

. ·~/ 

-~~:.',~.,.· 

@$8~,4 \ . J: •...•..•..••. ~:!:/:: 

F 

·········· ··-::···.·.······. 

A 

A 

Subduction zone-Sawteeth on upper plate; 

arrow shows relative movement ........... Projection to surface of northwestern edge of 

subducted extension of plate C 

Transform fault-Long dashed where inactive; 

short dashed where projected to surface. 

Arrows show relative movement 

Spreading ridge 

Projection to surface of former positions of edge 

of subducted extension of plate C 

FIGURE 6.-Evolution of plate geometry at a triple junction between three plates A, B, and C. Spreading rates, nomenclature, and assumptions 

are same as those in figure 5 (case 1). A, Initial configuration of the three plates. B, C, D, E, and F, Configuration after lapse of successive 

and equal increments of time. Movement and growth of plates B and C are shown with respect to plate A, arbitrarily held fixed. Dotted 

line, projection to surface of northwest edge of subducted extension of plate C. Former positions of this edge are shown in B through F 

by fine-dotted lines. Numbered lines 5, 4, 3, 2, and 1 represent magnetic anomalies formed at the time of frames A, B, C, D, and E, respectively, shown as solid lines at surface of plates Band C; dotted lines, where subducted beneath plate A. J 6

, J 4, J 3, J 2, J 1

, and J 0 represent 

location of triple junction at time of successive frames A, B, C, D, E, and F, respectively. R, Q, S, points mentioned in text. 
................ -,, _______ : 

.... /'--,..; 

.... 

L ading edgesf;J-------~ e f subducted - ·-;': 

0 '' slab in A i i 

B :: 

II ''

II c ': 1: I I II 

I I I I 

I J --------'-/ I, •, "--------· 

FIGURE 7.-Diagram showing progressive advance of Humboldt-type triple junction along a continental plate (tilted back) and volume of material that must be displaced during 

this advance. Although rates of spreading, subduction, and right-lateral movement are constant, intersection of ridge-ridge transform with transform fault between oceanic 

and continental plates exacerbates space problem. A, Tiny segment of spreading ridge north of ridge-ridge transform will vanish as spreading continues. Dashed lines on underside of continental plate show position of leading edge of subducted slab reached in B and C. Thin arrows show spreading directions, thick arrows direction of movement of 

oceanic plates relative to continental plate; B, Truncation of subsurface part of continental plate accelerates as triple junction evolves to transform-transform-subduction zone 

configuration. Magnetic anomalies generated at A shown by long-dashed lines in oceanic plates; C, Ridge again intersects transform boundary with continental plate. Dotted 

lines on underside of continental plate show successive positions of leading edge of subducted slab at future time intervals equivalent to those between A, B, and C. 

z 

1-:3 

~ 

0 

t':l 

~ 

0 

1-:3 

-

0 

z 

<.0 
10 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

the lip of crustal material will probably slough away as 

subhorizontal gravity slides, imbricated thrust fault 

slices, and intercalated melange and broken formation 

(Fox, 1976). 

A triple junction whose movement is associated 

with the lateral advance and impingement of the subducted slab on space occupied by the continental plate, 

resulting in the displacement of parts of the continental 

plate by the subducted slab, has been defined as a Humboldt-type triple junction (Fox, 1976). Two other situations can be visualized. In one, the triple junction does 

not perceptibly move relative to the continental plate. 

With time, the tectonic situation at the margin of that 

plate will stabilize, with a strike-slip regime to one side 

of the triple junction and a subduction regime to the 

other side. 

Alternatively, if the triple junction moves in such a 

way that the transform fault bounding the continental 

plate is extended at the expense of the subduction zone, 

then the wedge-shaped space formerly occupied by the 

obliquely descending lithospheric plate must be filled 

through upwelling of mantle or by accordion-folding of 

lithospheric material. This type of triple junction has 

been defined as a Mendocino-type (Fox, 1976). 

AGE AND DISTRIBUTION 

OF MELANGES AND RELATED ROCKS 

Melanges of late Mesozoic and possible early Tertiary age recognized along the western margin of the 

North American plate (fig. 3) have been cataloged by 

Jones and others (1978). Exclusive of melanges in 

Alaska, which are outside the scope of the present 

study, the melanges include those of the Franciscan 

Complex in California and its presumed correlatives in 

Oregon and the Baja Peninsula of Mexico and melanges 

on both the San Juan Islands of Washington and the 

west coast of Vancouver Island, British Columbia. To 

FIGURE 8.-Diagram showing fonnation of fold belt, thrusts, gravity 

thrusts, and melanges at Humboldt-type triple junction. 

these must be added the melanges of the Olympic Peninsula of Washington, reported to be of Eocene (Snavely 

and Pearl, 1975; Snavely and others, 1977), and Miocene 

age (Rau, 1975). The melanges of the Olympic Peninsula, because of their relative youthfulness, permit comparison of the plate-tectonic situation that prevailed at 

the time of their formation, as deduced from the magnetic lineations of the northeastern Pacific, with that 

predicted by the subduction-complex and triple-junction theories. 

The Olympic melanges occupy part of the northern 

end of a 640-km-long coastal strip apparently underlain 

by oceanic crust composed chiefly of tholeiitic basalt and 

subordinate volcaniclastic sediments (Snavely and 

others, 1977, p. 9). The oceanic crust is early and middle 

Eocene and was probably accreted to the craton in middle Eocene or later time (MacLeod and others, 1977, p. 

226). Melanges are not found outside the Olympic Peninsula within this volcanogenic borderland, but because its 

history forms an important link in the story of interaction between oceanic and continental plates, its geology 

is briefly outlined below. 

The San Onofre Breccia (Woodford, 1925), though 

not a melange, is briefly mentioned because its origin 

may be related to the plate interactions that are the 

main concern of this paper. Correlation of these rocks 

with a particular tectonic setting is deferred to the section below entitled ~~Main Elements ofMelanges and***." 

SAN ONOFRE BRECCIA 

The San Onofre Breccia consists of sandstone, conglomerate, and breccia, together aggregating at least 

795 m in thickness (Woodford, 1925, p. 185). The deposit 

is unusual in two respects, first because it contains angular blocks of blueschist and other crystalline rocks as 

much as 4. 6 m long (Woodford, 1925, p. 186); second, because, though interlayered with marine deposits derived 

entirely from the craton to the east, the breccia itself 

was derived through erosion of a briefly emergent submarine source area to the west (Woodford, 1925 p. 236-

239). The deposit formed as a subaerial alluvial fan and 

bordering shallow-water marine-fan and delta complex 

in late Saucesian, Relizian, and Luisian(?) stages of the 

Miocene (Stuart, 1976). 

OLYMPIC PENINSULA 

AND OREGON-WASHINGTON BORDERLAND 

Between Bandon, Oreg., and Vancouver Island, 

British Columbia (fig. 9), the continent is bordered by a 

200- to 300-km-wide strip in which the apparent basement rock is chiefly tholeiitic basalt of early and middle 

Eocene age, overlain by younger volcanic and volcaniclastic strata (Snavely and others, 1966). This area, here 
45° 

43° 

PACIFIC 

OCEAN 

Tillamook 

Volcanics --ta..c • .. 

AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 

MARINE DEPOSITS 

Shallow to deep-water 

sedimentary rocks with 

minor intercalated vol· 

canic rocks (Eocene to 

middle Miocene) 

EXPLANATION 

D 

Surficial deposits 

(late Miocene to Quaternary) 

MIOCENE TO EOCENE BEDROCK 

CONTINENTAL DEPOSITS 

11 

ROCK TECTONIC UNITS 

- Deep-water sedimentary 

rocks (Eocene to 

Miocene) Volcanic and interlayered 

sedimentary rocks 

(Eocene to Quater· 

nary) 

Melanges and broken 

formations (Miocene 

and possibly Oligo· 

cene and Eocene) 

Alkalic basalt, partly 

subaerial (late Eocene) 

Tholeiitic 

~ 

~ 

basalt partly 

subaerial (early and 

middle Eocene) 

PRE-MIDDLE EOCENE BEDROCK 

Metamorphic, igneous, sedimentary, and 

tectonic rocks 

-·-·?-

Zone· of high gravity gradient (interpreted 

from gravity maps by Bromery and 

Snavely ( 1964), Berg and Thiruvathukal 

(1967). Dehlinger and others (1967), 

Macleod and others ( 1977), and Bonini 

and others (197 4) 

... ~ 

Thrust fault- Sawteeth in direction of dip; 

dotted where concealed 

Fault-Showing relative horizontal movement; dotted where concealed or engulfed 

by younger plutonic rock 

..... ~ 

-=7\~ 

Volcano 

FIGURE 9.-Geologic map of western Oregon and Washington. Chiefly from Huntting and others (1961), Wells and Peck (1961), Bromery and 

Snavely (1964), MacLeod and others (1977), and Tabor and Cady (1978b). 
12 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

referred to as the Oregon-Washington borderland, underlies only the western part of an early and middle Tertiary eugeosyncline (Snavely and Wagner, 1963) that 

also included the area to the east underlain by fringing 

shelf deposits of eugeosynclinal aspect. 

As pointed out by Bromery and Snavely (1964, p. 

N-1), the contrast between the high density of the basement rock and the lower density of the superjacent 

sedimentary rocks makes feasible the mapping of the 

basement surface through gravity surveys. Gravity 

maps by Bromery and Snavely (1964), Bonini and others 

(1974) and MacLeod and others (1977) confirm the coincidence of positive gravity anomalies with. areas in 

which the tholeiitic basement rocks crop out. These 

anomalies are bounded on the east by zones of steep 

negative gradients, here assumed to represent the contact in the subsurface of oceanic crust with less dense 

crustal material of the craton (fig. 9). The zones of high . 

gradient appear to be segmented, though linear within 

individual segments. Considering the smoothing effect 

of the gravity-measurement and map-contouring process, the actual contact is probably angular and in some 

areas jagged. 

The Eocene basalt includes, from south to north, 

the Siletz River Volcanics (Snavely and others, 1968), 

the Crescent Formation (Arnold, 1906), and the Metchosin Volcanics of Clapp (1910) (Snavely and others, 1966, 

p. 456). These volcanic rocks apparently are· chiefly 

Ulatisian (late early and early middle Eocene) in age 

(Rau, 1964, p. G-4; Snavely and others, 1968, p. 467). 

The Siletz River Volcanics has been dated at 49.3 to 54.7 

m.y. by potassium-argon methods (Duncan, 1977). 

The volcanic rocks generally include a thick lower 

member of marine pillow basalt and interbedded deepmarine sediments, locally overlain by an upper member 

of shallow-water to subaerial tholeiitic or alkalic basalt, 

the whole aggregating as much as 15 km in thickness 

(Cady, 1975, p. 575). According to Snavely and others 

(1968, p. 480), the lower part of the Siletz River Volcanics rose from the mantle along north-trending fissures. 

Sedimentary rocks interlayered with basalt of the 

Crescent Formation contain clasts of continental provenance including boulders of quartz diorite as much as 3 

m in diameter, suggesting proximity of the basalts to 

the North American continent at the time of their extrusion (Cady, 1975, p. 579). In Washington, the Crescent 

basalt is conformably overlain by, or intertongues with, 

a sequence of deep-marine clastic sediments mapped as 

the Aldwell and overlying Lyre Formations (Olympic 

Peninsula) or Mcintosh and overlying Northcraft Formations (southwestern Washington) of upper Ulatisian 

to middle Narizian (middle and early late Eocene) age 

(Rau, 1964, p. G-4; Snavely and others, 1958, p. 17-22). 

These units are unconformably overlain by shelf deposits, the unconformity marking a significant orogenic 

episode in the opinion of Snavely and others (1977, p. 7, 

20). 

In Oregon, the Siletz River Volcanics underlies the 

Tyee Formation, a 3,000-m-thick sequence of rhythmically interbedded sandstone and siltstone of continental 

provenance deposited by northward-flowing turbidity 

currents (Snavely and Wagner, 1963, p. 7). The Tyee is 

in turn overlain by the late Eocene Yachats Basalt 

(Snavely and McLeod, 1974). 

The pre-Pliocene bedrock of the Olympic Peninsula 

(fig. 9) is broadly composed (Tabor and Cady, 1978a) of 

peripheral rocks and core rocks. The peripheral rocks 

consist of a thick stratiform sequence of Eocene to 

Miocene age, which, though folded and faulted, is essentially stratigraphically intact (compare Glassley, 1974, 

p. 786). In contrast, the core rocks, though also of 

Eocene to Miocene age, consist of rocks that have been 

tectonically disrupted, forming a mass in which 

melanges and broken formations are imbricated and infolded with fault-bounded blocks and slivers of intact 

strata. 

The peripheral rocks consist of tholeiitic basalt of 

the Crescent Formation and interfingering deep-water 

clastic deposits. Except for the northwestern part of the 

peninsula, these rocks are overlain by a succession of 

shallow-water marine shelf deposits. In contrast, the 

core rocks consist of bathyal marine turbidite deposits. 

Melanges of the northwestern core are "composed 

of sheared middle Eocene basalt, large infolded blocks of 

turbidite sandstone, and broken formation" according to 

Snavely and Pearl (1975). These rocks are depositionally 

overlain by deep-water marine siltstone and sandstone 

of latest Eocene to middle Miocene age, and the sequence is itself strongly deformed and unconformably 

overlain by tilted strata of latest Miocene and Pliocene 

age (Snavely and Pearl, 1975). 

Melanges in the west-central part of the core zone 

are described by Rau (1973, p. 5) as part of the Hob rock 

assemblage, which, in addition to melange, contains 

much-deformed turbidite deposits, chiefly rhythmically 

interbedded and graded sequences of siltstone, 

sandstone, graywacke, and conglomerate. Most of the 

foraminiferal assemblages contained within these deposits indicate deposition at no less than upper bathyal 

depths (Rau, 1975). The melanges typically are composed of blocks or slabs of sandstone, graywacke, or 

conglomerate embedded in a matrix of much-sheared 

siltstone. The blocks commonly are as much as a meter 

or more in length; and Rau (1973, p. 8), in his excellent 

descriptions and photographs of the melanges, noted 

that the blocks include some as large as houses and some 

even larger ones that form sea stacks, promontories, 
AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 13 

and small islands. The slabs range to several kilometers 

in length and appear to be "floating" in melange 

(Snavely and others, 1~77, p. 21). 

At the seacliff exposures to which the I was directed by W. W. Rau, the blocks are angular to nearly 

equant and faceted. Outer surfaces are polished and 

commonly striated, and the blocks, though essentially 

intact, are crisscrossed by fractures. The melange locally contains zones composed predominantly of 

sandstone or graywacke that is sheared ubiquitously but 

not penetratively at hand-specimen scale. Exotic clasts 

of greenstone are present in the melange, but blueschist 

and eclogite have not been reported. 

The foraminifers of the Hoh rock assemblage have 

been extensively collected and studied by W. W. Rau. 

The following paragraph summarizes his findings (in 

Rau, 1975): 

Foraminifers, though rare, have been collected 

from 50 localities within the Hob rock assemblage. Most 

of these fossils suggest either an early or middle 

Miocene age (Saucesian or Relizian Stages of Kleinpell, 

1938) of deposition; a middle Eocene assemblage was 

found at one locality and a late Eocene at two. Some of 

the youngest assemblages (five localities) suggest a Relizian age, and one very poorly preserved assemblage 

tentatively suggests an age as young as late Miocene. 

Collections from the melanges themselves suggest 

upper Saucesian to possibly Relizian (middle Miocene) 

age. The single significant megafossil assemblage found 

indicates an early or middle Miocene age, according to 

W. 0. Addicott (cited by Rau, 1975). 

The Hob rock assemblage is beveled by an angular 

unconformity and on land is overlain by the late 

Miocene(?) and Pliocene Quinault Formation, flat-lying 

siltstone, sandstone, and conglomerate (Rau, 1975). 

Offshore, according to Rau (1975), late Miocene strata 

are at least in places present between the structurally 

complex rocks of early and middle Miocene age and the 

strata of Pliocene age. 

The eastern core rocks have recently been described by Tabor and Cady (1978a) as consisting of 

shale, siltstone, sandstone, and minor conglomerate, 

basalt, basaltic volcaniclastic rock, diabase, and gabbro, 

varyingly metamorphosed to slate, semischist, phyllite, 

greenstone, and greenschist. These rocks range from 

faulted or shear- zone bounded, but intact, bedded sequences to completely disrupted broken formations composed of sandstone or semischist clasts embedded in a 

matrix of slate or phyllite. Exotic clasts such as blueschist or eclogite have not been found. The rocks are multiply cleaved and lineated, and both bedding and cleavage 

have been folded and refolded. 

Fossils are very rare; those found range from early 

Eocene to early Oligocene. An intensive investigation of 

the potassium-argon geochronology of the northeastern 

core led Tabor (1972) to conclude that the age of regional 

metamorphism was about 29 Ma and the age of a later 

episode of faulting and quartz veining about 17 Ma. 

Measured ages (66 reported determinations) range from 

16.2 to 227.4 m.y. (Tabor, 1972, table 1), with an apparent inverse correlation between metamorphic grade and 

age. Potassium-argon ages of graywacke and semischist 

range downward to about 29 m.y. This lower limit was 

defined by three samples from which separates of matrix material yielded ages of 31.2±0.6, 29.1±0.9, and 

29.0±0.7 m.y.; separates of coexisting clast material 

yielded corresponding ages of 35.2±0.7, 36.2±0.9, and 

39.1 ± 1.3 m.y. Considering the possibility of imperfect 

separation and consequent cross-contamination of matrix and clast material, noted by Tabor (1972, p. 1810), it 

seems probable that the age of pure metamorphic matrix material is significantly less than 29 Ma. 

The potassium-argon age of slate and phyllite 

ranged down to 27 Ma. The 10 determinations of the age 

of phyllite breccia, however, clustered at about 17 Ma 

and ranged from 16.2 to 19.9 Ma. The samples of breccia 

are from widely scattered localities, some adjacent to 

outcrops of rocks giving much older ages (Tabor, 1972, 

p. 1811, fig. 10). 

Core rocks are separated from peripheral rocks by 

a curving system of observed and inferred faults and 

shear zones following or splaying away to the inside of 

the Olympic horseshoe (fig. 9). On the north, the system 

includes the Calawah fault, which diverges westward 

from the inferred shear system between peripheral and 

core rocks, cutting through the core rocks as a zone locally more than a kilometer and a half wide (Gower, 1960; 

Tabor and Cady, 1978a). Gower (1960) inferred that the 

zone was a left-lateral strike-slip fault. MacLeod and 

others (1977, p. 227) concurred in this opinion, noting 

that major differences in lithology and provenance of 

Eocene deep-water marine sandstone north and. south of 

the fault suggest strike-slip offset. They concluded that 

because of this lithologic contrast and the wide zone of 

shearing, offset was probably substantial. 

Tabor and Cady (1978a), after analyzing the structural fabric, concluded that the parental rocks of the 

core were isoclinally folded, then faulted, imbricated, 

overturned westward, and pressed by east-west compression into the basaltic horseshoe. The horseshoe 

could have formed by this deformation, or it could have 

been already in existence. Continued compression 

caused the core rocks to yield upward and outward by 

shear folding, forming a "mushroom-like dorrle" (Tabor 

and Cady, 1978a). 1 

The age, or ages, of the deformation of the core, 

zone rocks is critical to any understanding of their origin. In the eastern core, the age of some or all of the 
14 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

broken formation and of the most recent tectonism must 

be late Oligocene or younger, on the basis of Tabor's 

(1972) potassium-argon age study. Tabor's work establishes a strong presumption that these rocks were 

metamorphosed after 29 Ma and finally cooled through 

the blocking temperature of fine-grained mica about 17 

Ma. In Tabor's opinion, the potassium-argon age data 

suggest two events, the ages of which are closely approximated by the clusters of potassium-argon ages at 

29 Ma (late Oligocene) and 17 Ma (early Miocene). 

The melanges of the Hoh rock assemblage of the 

western core must be entirely or in part at least as 

young as the youngest fossils found within them, that is, 

possibly Relizian. This age would imply a maximum age 

of about 16 m.y. (time scale of Van Eysinga, 1975). The 

presence of little-deformed late Miocene sediments unconformably resting on the western core rocks similarly 

places a younger age limit of at least 5 m.y. on the age 

of the most recent deformation. That there was at least 

one previous episode of major deformation is indicated 

by the presence of latest Eocene sediments depositionally overlying melange containing middle and early 

Eocene rocks (Snavely and Pearl, 1975). 

In summary, the deformation of the core-zone rocks 

was apparently episodic, not continuous. Snavely and 

Pearl (1975) had suggested that the core rocks reflected 

two major orogenic events, the first of which was in late 

middle or early late Eocene, the last in middle Miocene. 

Their conclusion, insofar as it applies to the western 

core, seems amply sustained by the available evidence. 

Tabor's age data suggest that deformation of the eastern core during the last orogenic event ended in latest 

early Miocene time and further may point to the existence of a third major event of late Oligocene age. Confirmation of the late Oligocene event should be sought in 

the stratigraphic record. 

0. 50 100 150 KILOMETERS 

Allochthonous 

slabs and plates 

Hunters Cove Formation of 

Dott (1971) 

EXPLANATION 

FRANCISCAN COMPLEX 

~ 

L2.l 

Mesoallochthonous 

units 

Melanges and allochthonous 

and mesoallochthonous 

units, undifferentiated 

FIGURE 10.-Sketch map showing tectonic classification following Hsu (1969) (see also Hsu and Ohrbom, 1969) of the Franciscan Complex and 

associated rocks of western California and southwestern Oregon. Modified chiefly from Jennings (1977) with additions from Coleman (1972), 

Baldwin (1974), Dott (1971), and Beaulieu and Hughes (1976). 
AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 15 

FRANCISCAN COMPLEX, FRANCISCAN ASSEMBLAGE, 

GREAT VALLEY SEQUENCE, 

AND CORRELATIVE ROCKS 

SEMANTIC DISTINCTION 

BETWEEN FRANCISCAN COMPLEX 

AND FRANCISCAN ASSEMBLAGE 

AND THE RELATION OF THESE ROCKS 

TO THE GREAT VALLEY SEQUENCE 

The late Mesozoic and early Tertiary rocks underlying much of the western seaboard of central and northern California are collectively referred to as the Franciscan assemblage (Bailey and others, 1964, p. 11) or Franciscan Complex (Berkland and others, 1972). Although 

these names are applied to nearly the same rocks, the 

Franciscan assemblage and the Franciscan Complex embody different stratigraphic concepts and are therefore 

not synonymous. The Great Valley sequence borders 

~ ~ 

Ri R? '.?o·w 

PACIFIC 

m 

Salinian block 

the Franciscan Complex on the east, and similar rocks 

are found in isolated areas surrounded by the Franciscan Complex (fig. 10). 

The semantic distinction between the age of the assemblage and that of the complex is like that between 

the age of the trees in a forest or several forests and 

a house built from those trees. The trees might be of 

various ages within a discrete range; after cutting, sawing, planing, assembling, and nailing, the result is a new 

entity, a house, whose age is less than that of any of its 

components. Thus when we refer to the Franciscan 

Complex, we refer to the "house"; when we refer to the 

Franciscan assemblage, we refer to the "trees" from 

which the house was built. The problem is more complicated than this, in that like a house to which various 

wings have been added at various times, the Franciscan 

Complex has been built in various stages, places, and 

times. 

~ 

~ 

"> 

--fOCEAN 

Neo-autochthon 

~ 

k 

formation 

A 

~ 

L2:2J 

Autochthon 

Contact-Dotted where inferred Fault-Dotted where inferred 

FIGURE 10.-Continued 
16 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

The Franciscan Complex is a structural aggregation 

of rocks in which fault slices and blocks of relatively intact bedded deposits are imbricated and interwoven 

with shear zones, melanges, and broken formations. The 

structural fabric of the unit, dominated by subparallel to 

anastomosing fracture surfaces, closely spaced in the 

·melanges and broken formations and widely spaced in 

the tectonic slabs and blocks, along with accompanying 

fragmentation and granulation, is the· chief diagnostic 

feature of the complex. Berkland and others (1972, p. 

2297) stipulated that the Franciscan Complex was the 

basement terrane of the California Coast Ranges, and 

their assignment of some rock units to the complex was 

partly based on an interpretation of whether or not they 

constituted part of the local basement (1972, p. 2299). 

As noted in an earlier paper (Fox, 1976, p. 740), the 

Franciscan Complex is probably not basement in some, 

and perhaps much, of its area of outcrop. Except for 

that element requiring it to be a basement terrane, 

Berkland and others' (1972, p. 2297) name "Franciscan 

Complex," with their definition of it, is here adopted in 

place of the name "Franciscan Formation." 

The parental rocks were chiefly massive graywacke 

interbedded or interlayered with siltstone, conglomerate, bedded chert, greenstone, and locally limestone. 

During their structural disruption, slabs of serpentine, 

greenstone, and ophiolitic rocks were tectonically introduced into the complex. These rocks were zeolitized 

and locally metamorphosed to low-grade blueschist, 

greenschist, or amphibolite. In addition, high-grade 

blueschist and eclogite of enigmatic origin is present in 

the melanges as ubiquitous, though commonly widely 

scattered, exotic clasts. 

The term "Franciscan assemblage" (Bailey and 

others, 1964, p. 11) is an informal designation collectively applied to various lithologies that make up the 

Franciscan Complex as defined above. Bailey and others 

(1964, p. 148) stressed that their so-called Franciscan 

rocks are sheared, disrupted, and deformed, but did not 

imply that the resulting structural fabric is a diagnostic 

part of their concept of the lithologic entity they designated an assemblage. Though only sparsely fossiliferous, the assemblage does contain scattered fossils of 

Late Jurassic, Cretaceous, and early Tertiary age 

(Irwin, 1957; Bailey and others, 1964; Blake and Jones, 

1974). It is therefore correct to say that the Franciscan 

assemblage, as indicated by the available fossil evidence, ranges in age from Late Jurassic to early Ter-

. tiary, since by "age" we mean depositional age of the 

various parts of the assemblage. 

The Franciscan Complex is defined as a structural 

complex, a rock-stratigraphic unit according to Article 

6j of the "Code of Stratigraphic Nomenclature" (American Commission on Stratigraphic Nomenclature, 1974) 

though there seems to be a difference of opinion on this 

point (compare Berkland and others, 1972, p. 2299), and 

its age is that of its diagnostic structural fabric; its age 

at any given place is that of the deformation that produced the melanges and tectonic disruption characteristic of the complex. The older limit of that age is the age 

of the youngest fossils locally present (R. W. Kopf, written commun., 1976). Contrary to the opinion of Berkland and others (1972, p. 2300), the presence of fossils of 

Late Jurassic to early Tertiary age does not necessarily 

mean that the complex has a comparable range in age 

(melange rule three of Hsu, 1968, p. 1067). The deformation is penetrative with respect to the complex as a 

whole, though not necessarily at hand-specimen or even 

outcrop scale, a feature that could lead to difficulties in 

the practical application of this definition. More recent 

faulting and folding without this grossly penetrative aspect may displace the Franciscan Complex but is not 

considered part of its distinguishing structural fabric. 

As defined by Berkland and others (1972, p. 2299) 

the Franciscan Complex geographically occupies three 

belts, an eastern belt, a central belt, and a coastal belt. 

In the eastern belt, the rocks are dominantly metaclastic and have a strong to barely perceptible schistosity. 

In the central belt, the dominant lithology is melange. 

The coastal belt, originally recognized and roughly delineated by Bailey and others (1964), contains 

graywacke, shale, and conglomerate but only a little 

greenstone, chert, serpentine, and blueschist and is 

structurally deformed in a manner analogous to other 

parts of the Franciscan Complex (see Bailey and others, 

1964, p. 13). 

The ages given by Blake and Jones (1974, p. 351) 

for fossils and fossil assemblages from the Franciscan 

assemblage of northern California (fig. 11) range from 

Late Jurassic to Eocene. Except for those of Late Cretaceous and early Tertiary age, most of which were 

found in the coastal belt, the fossils were found either in 

the matrix of the melange or within allochthonous blocks 

and slabs. 

Rocks included within the Great Valley sequence 

range in age from Late Jurassic (Tithonian) to Late Cretaceous, making them equivalent in age to some of the 

lithologic elements incorporated within the Franciscan 

Complex (Irwin, 1957; Bailey and others, 1964, p. 123-

139; Berkland, 1973, p. 2396-2399). The Franciscan 

Complex and Great Valley sequence are not found in depositional contact with each other. Rather, the two units 

are faulted together, or are faulted against folded sheetlike bodies of serpentine and variably serpentinized ultramafic rocks that separate the two units. In places, 

the basal part of the Great Valley sequence depositionally overlies these ultramafic rocks (Bezore, 1969; Bailey 

and others, 1970). Curiously, the metamorphic grade of 
MIOCENE 

OLIGOCENE 

EOCENE 

PALEOCENE 

CAMPANIAN 

SANTONIAN I 

Age 

(m.y.) 

10 . 

VANCOUVER SEATTLE PORTLAND CAPE MENDOCINO 

::;::: . · \ Younger melange of 

\:: ::,·:::n~lympic Peninsula SUBDUCTION REGIME 

20 (Juan de Fuca plate) 

SAN FRANCISCO 

30 (Farallon plate separated into Juan de Fuca and Cocos plates about 30 Mal 

SUBDUCTION REGIME 40 

belt (in panl (Farallon plate) 

Granule conglomerate 

50 of Jasper Ridge 

60 1 ?•1et~~o~i~, :~eSCE 

70 

80 

t.f1[1[!i![' '';';''''''':::::::r::::::::::::::::::::::::::::tttit:::::::rrr:::::::::::::::::::::rrrr:m :[![!_li![i!i-!i![i[[i!!ill!iil!l!l[l~ .. :it0ttJL2::~:~::j;;;;he I Ill {-Atascadero Fm I 

90 

100 

110 

120 

130 

140 

150 

D Allochthonous slabs 

and plates 

Tectonic classification of rock-stratigraphic units (modified from Hs:~, 1969) 

EXPLANATION 

D .. .. Mesoallochthonous unit Protolith of melange (age 

marks maximum age of 

episode of allochthonous 

deformation) 

Autochthonous unit put in 

contact with melange during 

episode of allochthonous 

deformation 

[77l 

L.d Neo-autochthonous unit 

CEDRO$ ISLAND 

FIGURE 11.-Correlation of boundary regimes and petrotectonic features along west side of North American plate. All features are projected vertically to reference meridian of 

oblique Mercator projection shown as line A-A' in figure 3. Note that the 31&-km displacement postulated by Turner and others (1970) and Matthews (1973) between the Pinnacles Formation. of Andrews (1936) and the Neenach Volcanics (P and N in fig. 3) along San Andreas fault has been restored. Boundaries between tectonic regimes defined as 

follows: heavy line (queried where very speculative), represents transit of Humboldt-type triple junction; thin line (queried where very speculative), transit of Mendocino-type 

triple junction; short-dashed line, other types of transitions. Light vertical lines represent age range of fossils found in the Franciscan Complex, chiefly from Blake and Jones 

(1974). Crosses represent potassium-argon ages of exotic clasts composed of eclogite or high-grade blueschist and found in Franciscan Complex, as reported by Coleman and 

Lanphere (1971). Stratigraphic nomenclature is taken from several sources and does not necessarily follow that of U.S. Geological Survey. Age range of rock units and other 

features discussed individually in text is shown by vertical bars or blocks (boundaries short-dashed where established by potassium-argon or rubidium-strontium dating). Correlation of periods, epochs, and ages with geologic time follows Van Eysinga (1975). 

> 

0 

t_%j 

> 

z 

0 

0 

Ul 

-

~ 

::d 

-lXI 

c:: 

~ 

0 

-

z 

0 

"%j 

a:: 

t_%j 

t""' 

> z 

0 

t_%j 

Ul 

> z 

0 

::d 

t_%j 

t""' 

~ t_%j 

0 

::d 

0 

() 

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-.;J 
18 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

the Franciscan assemblage increases toward the contact 

with the ultramafic rocks, and in places so does the 

metamorphic grade of the Great Valley sequence (Blake 

and others, 1967, p. 3--6). These age and contact relations hypothetically result from underthrusting of the 

Franciscan Complex below the Great Valley sequence 

(Bailey and others, 1964, p. 16~165; Irwin, 1964; Blake 

and others, 1967, p. 6-7; Bailey and Blake, 1969, p. 148). 

HSU'S (1969) CONCEPT OF AllOCHTHONOUS, 

MESOAllOCHTHONOUS, AUTOCHTHONOUS, 

ANDNEOAUTOCHTHONOUSROCKS 

Rarely, the conglomerates within the Franciscan 

Complex contain scattered pebbles of blueschist similar 

to the blueschist found as exotic blocks within the 

melange, indicating that these deposits were derived 

through erosion of a preexisting part of the Franciscan 

Complex (Cowan and Page, 1975). Also rarely, shelf deposits that appear locally to unconformably overlie the 

deformed rocks of the complex elsewhere have been 

broken, disrupted, and imbricated with melange and 

broken formation, thereby assuming the same structural fabric as the complex as a whole (Hsu, 1969). The' 

Franciscan Complex, in some places, then, contains a 

record of at least two episode(s) of deformation and 

melange formation. The earlier episode(s) was in places 

followed by cannibalization of the structural complex 

and deposition of detritus from it on a surface eroded 

across it. This cycle was followed by another cycle of deformation and melange formation. The Franciscan Complex is in places the product of two and possibly more 

distinct cycles of sedimentation and deformation. 

The concept of tectonic cycles was first articulated 

by Hsu (1969), who classified the rocks in the Morro Bay 

area as allochthonous, mesoallochthonous, autochthonous, and neoautochthonous. According to Hsu (1969, p. 

12-13): 

(1) Allochthonous rocks are those that have been 

deformed by overthrusting or by gravity sliding and 

have been transported for a considerable distance from 

their original site of deposition. 

(2) Mesoallochthonous rocks are those rocks that 

were deposited upon an allochthonous basement and 

transported for a considerable distance from their original site of deposition during a later episode of allochthonous deformation. A mesoallochthonous slab may 

include only mesoallochthonous sediments, or it may include both the mesoallochthonous sediments and some of 

the basement that has been deformed by two or more 

episodes of allochthonous movements. 

(3) Autochthonous rocks are those rocks that have 

not been transported from their original site of deposition. 

(4) ~eoautochthonous rocks are those rocks that 

were deposited upon allochthonous rocks after all allochthonous deformations had taken place. 

EVIDENCE BEARING ON THE AGE OF FORMATION 

OF THE FRANCISCAN COMPLEX 

In applying Hsu's (1969) concepts of allochthonous, 

mesoallochthonous, autochthonous, and neoautochthonous rocks to the Franciscan Complex (fig. 10), it is stipulated that: (1) the craton to the east be regarded as a 

fixed reference frame; (2) tectonic dislocations, lateral 

translation, and accretion as tectonostratigraphic terranes (Beck and others, 1980) of the lithospheric crust 

underlying or including the Franciscan Complex and 

Great Valley sequence before the Cretaceous be ignored; and (3) the mode of deformation of the allochthonous rocks include overthrusting, gravity sliding, and (or) any other process capable of forming the 

penetrative fabric of the Franciscan Complex. 

The ages of the "allochthonous deformations" are 

bracketed by the ages of neoautochthonous and mesoallochthonous deposits and by the ages of nlesoallochthonous and the parental autochthonous deposits. 

Hsu's classification appears to be relevant to the 

lithologic contrast between rocks of the coastal belt and 

the blueschist-bearing melange of the central belt. The 

coastal belt contains fossils as young as late early 

Eocene (Orchard, 1978), yet is imbricated, penetratively 

deformed, and includes broken formations, evidence of 

one or more episodes of allochthonous deformation 

(Blake and Jones, 1974, p. 347). 

The nature of the contact of coastal-belt rocks with 

the central belt is controversial. Kleist (1974) concluded 

that melange near Laytonville in the central belt was 

deposited on the coastal belt, but Kramer (1976, p. 4) 

found that the contac~ in the Fort Bragg-Willits area 

was a high-angle reverse fault. Kramer (1976, p. 4, 5) 

also found isolated remnants of the coastal belt depositionally overlying central-belt rock and speculated that 

the reverse relations reported by Kleist (1974) might 

have resulted from Tertiary and Quaternary landsliding. 

Detailed mapping of the coastal belt is incomplete; 

it may be found that some of the Eocene rocks are neoautochthonous. ~ onetheless, exotic tectonic blocks of 

blueschist and eclogite are absent or very rare except at 

the margins of the belt. The coastal belt was probably 

deposited after conclusion of the tectonic event or 

events that stirred the exotic blocks into the melange of 

the central belt and, assuming that the coastal-belt 

rocks depositionally overlie central-belt rocks at least 

locally, must be chiefly mesoallochthonous. 

Blocks of foraminiferal limestone attributed to the 

coastal belt that contain Cenomanian fossils have been 
AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 19 

tectonically mixed with central-belt melange (Blake and 

Jones, 1974, p. 350--351). Cenomanian fossils have been 

reported from "Franciscan" sandstone and questionable 

Cenomanian fossils from a metagraywacke unit (Hull 

Mountain belt) enveloped by melange (Blake and Jones, 

1974). By these findings, fossils as young as Cenomanian 

are locally present along with blueschist blocks in the 

melanges of northern California. The episode of deformation during which these rocks were mixed together, 

then, occurred in Cenomanian or later time but probably 

before deposition of the bulk of the coastal belt. 

Farther south, near Jasper Ridge, 50 km southeast 

of San Francisco (fig. 10), rocks comparable in age to 

those in the coastal belt appear to be neoautochthonous. 

At this locality, greens~one and chert of the Franciscan 

Complex, along with serpentine that contains pods or 

blocks of blueschist, is overlain by a thick sequence of 

sandstone and siltstone, containing fossils of middle and 

late Eocene age (Pampeyan, 1970; Page and Tabor, 

1967). The Franciscan rocks form the core of an appressed west-northwest-trending anticline defined by 

steeply dipping and locally overturned Eocene rocks 

that form the limbs of the folq. The Eocene rocks of the 

south limb include a conglomerate as much as 60 m thick 

that depositionally overlies the Franciscan rocks (Pampeyan, 1970). A conglomerate is also present at the base 

of the middle and upper Eocene sandstone on the north 

side, but there it is much thinner, being only about 3 to 

9 m thick (Pampeyan, 1970), and measurably older, for 

it contains a fauna suggesting a late Paleocene or early 

Eocene age (Graham, 1967). This conglomerate depositionally overlies the nearby serpentine, and is composed 

of packed granule-sized Franciscan detritus, mainly 

greenstone, in a calcium carbonate cement (Page and 

Tabor, 1967; Graham, 1967). 

Page and Tabor (1967, p. 5-8) observed that the 

superjacent middle and upper Eocene beds contain unusual chaotic zones consisting of "disordered mudstone 

containing isolated sandstone bodies which have been 

detached from formerly continuous beds and have been 

more or less haphazardly distributed in the mudstone 

matrix." They concluded that the chaotic beds probably 

formed through submarine sliding in the late Eocene but 

that the steep folding of the Eocene beds and their close 

proximity to the San Andreas fault might be suggestive 

of a tectonic origin. Because neither the granule conglomerate nor the overlying Eocene beds are penetratively sheared, these rocks are here tentatively considered neoautochthonous (fig. 10). In this area, it appears 

that the Franciscan Complex had been tectonically 

formed, then exposed to erosion by early Eocene at the 

latest. 

Search of the literature reveals three areas where 

Hsu's (1969) classification either has been applied or 

could be applied on the basis of published descriptions: 

the Morro Bay area of central California, the Clear 

Lake-Covelo area of northern California, and the Bandon area of southern Oregon. 

MORRO BAY AREA 

In the Morro Bay area, Hsu (1969, p. 17-18) mapped a slab of broken formation ("broken formation A") 

composed in part of graywacke containing about 10-20 

percent K-feldspar, a few granite clasts, and abundant 

Franciscan debris. Blocks of similar graywacke were 

found in a melange below this slab, justifying Hsu's designation of the unit as mesoallochthonous (Hsu, 1969, 

fig. 2, p. 13; Smith and Ingeroll, 1978). Two 

palynomorphs from two samples of shale interbedded 

with the graywacke in this slab were identified by W. R. 

Evitt (in Hsu, 1969, p. 18) and considered by him to be 

Late Cretaceous, most likely Campanian or older. 

Approximately 15 km inland, Cowan and Page 

(1975, p. 1089) found recycled Franciscan detritus, including glaucophane-lawsonite schist, as clasts in a mass 

of sandstone, the Las Tablas unit, about 2.0 by 0.75 km 

in area, tectonically enveloped in Franciscan melange. 

Three palynomorphs (including the two species found in 

broken formation A) from three samples of shale intercalated with the sandstone were identified by W. R. 

Evitt (in Cowan and Page, 1975, p. 1093) and considered 

by him to be Late Cretaceous and, quite possibly but 

not necessarily, Campanian. 

Rocks about 25 km southeast of the Las Tablas 

unit, mapped as the Atascadero Formation by Hart 

(1976), are composed of massive sandstone and conglomerate, along with bedded sandstone, siltstone, and 

mudstone. In this unit, according to Hart (1976, p. 15): 

Internal deformation is widespread and locally intense. This includes: (1) pervas.ive shears in sandstone 

subparallel to the bedding; (2) pinched-off sandstone 

beds, including occasional boudins; and (3) pinching and 

swelling of mudstone beds. Some of the deformation 

may be of "soft rock" type (for example, slumping and 

sliding contemporaneous with deposition), but much of it 

is "hard rock" (as indicated by common microscopic 

shears and deformed grains in sandstone) and probably 

is the result of large-scale overthrusting or gravity sliding. 

It appears, then, that these. rocks have been affected by one or more episodes of allochthonous deformation. According to Hart (1976, p. 16), the Atascadero 

is probably Cenomanian or Turonian to late Campanian 

or Maestrichtian. 

The oldest rocks in this area that are neither dis- . 

rupted by penetrative shearing nor found as tectonic inclusions in .the melanges make up the arkosic Asuncion 
20 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

Formation of Taliaferro (1944), according to Hsu (1969, 

p. 25). Campanian and Maestrichtian fossils found in the 

upper part of this unit indicate the age of deposition 

(Taliaferro, 1944; Popenoe and others, 1960; Hsu, 1969, 

p. 25). 

The most recent period of melange formation in this 

part of California appears to have been in Campanian 

time. The presence of mesoallochthonous deposits implies that there was at least one earlier period of 

melange formation. 

The rocks of central California classified by Hsu 

(1969) as mesoallochthonous include rock units of Early 

Cretaceous age, mapped as part of the Marmolejo Formation by Taliaferro (1944). Fossils reported by 

Taliaferro (1944, p. 469) from the Marmolejo were assigned to the late Valanginian by Popenoe and others 

(1960, chart 10e, annotation no. 12, p. 1520), but according to Hsu (1969, p. 18), specimens of Buchia from the 

formation include both Late Jurassic and Early Cretaceous species. Hsu (1969) described these rocks as being 

evenly bedded siltstone and shale characterized by the 

preservation of stratal continuity, occurring both as 

large mappable slabs and tectonic inclusions too small to 

map. Hsu (1969, p. 16) correlated broken formation B,, 

composed in part of graywacke commonly containing 2-5 

percent detrital K-feldspar and conglomerate and 

sedimentary breccia containing clasts of glaucophane 

schist, with the rocks mapped as the Marmolejo by 

Taliaferro (1944) on the basis of their K-feldspar content. The designation of the fossiliferous Marmolejo as 

mesoallochthonous, rather than simply allochthonous, 

hinges on this correlation. Fossils were not found in broken formation B. If truly mesoallochthonous, an episode 

of tectonism in which blueschist exotics were incorporated in melange occurred prior to the deposition of the 

Marmolejo. 

About 100 km southeast, the latest Jurassic and 

early Cretaceous (Valanginian) Toro Formation, as described by Hart (1976), seems to grade into melange. 

While evidence for a pre-Marmolejo and pre-Toro 

episode of melange formation is somewhat tenuous, 

there can be no doubt of one after deposition of these 

rocks. 

CLEAR LAKE-COVELO AREA 

Near Clear Lake, Swe and Dickinson (1970) described a succession of clastic sedimentary deposits, 

ranging in age from Late Jurassic to Late Cretaceous, 

that they correlated with the Great Valley sequence to 

the east. They postulated that these deposits, aggregating 10,700 m (35,000 ft) in thickness, along with lower 

Tertiary beds, represented outliers of imbricated sheets 

that were overthrust to the west onto the Franciscan 

assemblage, or that the Franciscan was underthrust to 

the east below Great Valley rocks, and that these imbricated sheets were subsequently infolded with the Franciscan. The rocks correlated with the Great Valley sequence fortrl four fault-bounded segments within which 

the strata appear to be conformable. 

The lower Tertiary strata consist of a lower unit of 

sandstone and shale aggregating 1,300 m (4,230 ft) in 

thickness and an upper unit of conglomeratic sandstone 

aggregating 335--365 m (1,100-1,200 ft) in thickness 

(Brice, 1953, p. 28--30). These two units had originally 

been assigned to the Martinez (early Paleocene) and 

Tejon (late Eocene) Stages, respectively, by Dickerson 

(1914, 1916), but as Berkland (1973, p. 2391) reminds us, 

the "Tejon" beds were subsequently reassigned to the 

Meganos (late Paleocene) Stage by Clark and Vokes 

(1936, p. 856, fig. 2). The basal contact of the Paleocene 

beds was given careful attention by Swe and Dickinson 

(1970, p. 183). They found that the Paleocene strata are 

in thrust contact with underlying rocks correlative with 

the lower part of the Great Valley sequence. On this 

basis, they inferred that the Great Valley and Tertiary 

rocks were tectonically emplaced against Franciscan 

rocks after deposition of the lower Tertiary beds. 

The thrusting may have produced a certain degree 

of mixing of Franciscan and Great Valley rocks, for Swe 

and Dickinson (1970, p. 171) observed such mixing in 

several areas. In one area, small fossiliferous thrust 

slices of the Great Valley sequence were caught up in 

the underlying Franciscan tectonic breccia. In another 

(1970, p. 169) the Great Valley is extensively sheared 

and intricately mingled with serpentinite breccia. In a 

third, highly deformed massive graywacke and pebble 

conglomerate mapped as the Franciscan assemblage lie 

on the strike of a 61/2-km ( 4 mi)-long belt of graywacke 

and conglomerate in the adjacent Great Valley sequence 

(1970, p. 168-169); these Franciscan and Great Valley 

rocks had previously been mapped as a continuous belt 

of the Knoxville Formation by Brice (1953, p. 13-14, pl. 

1). 

The preservation of the Cretaceous and Paleocene 

depositional sequence suggests that the Great Valley sequence found within the Clear Lake outlier has not been 

grossly dismembered and therefore probably has not 

been transported great distances from its original depositional site. These rocks are probably autochthonous. 

However, their deformation and local incorporation in 

Franciscan tectonic breccia imply that here the Franciscan Complex includes rocks that were tectonically detached from the Great. valley sequence. The episode of 

allochthonous deformation during which these rocks 

were mixed necessarily occurred in Paleocene or later 

time. 

At Rice Valley, Cretaceous strata correlated with 

the Great Valley sequence and overlying Paleocene and 

Eocene(?) strata form an elongate slab 0. 7 km by 3 km 
AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 21 

in plan that is tectonically enclosed by ultramafic rock 

and chaotic rocks of the Franciscan Complex (Berkland, 

1973). The Cretaceous rocks consist of greenish-gray 

shale with thin-bedded K-feldspar-bearing sandstone interlayers aggregating 915 m (3,000 ft) in thickness, 

overlain by biotitic arkosic K-feldspar-rich sandstone 

about 150 m (500 ft) thick. The Paleocene strata overlie 

the Cretaceous strata with slight angular discordance 

(Berkland, 1973, p. 2396) and in ascending order consist 

of an unfossiliferous massive sandstone at the base, 

overlain by polished pebble conglomerate, also unfossiliferous but containing abundant reworked Franciscan· 

detritus; quartz grit; fossiliferous concretionary 

sandstone containing a rich fauna of Meganos (upper 

Paleocene) age; and an uppermost unit of glauconitic 

sandstone containing a sparse fauna that could represent a horizon "possibly as young as Eocene" according 

toW. 0. Addicott (in Berkland, 1973, p. 2398). 

According to Berkland (1973), the Cretaceous part 

of the Rice Valley outlier is a conformable sequence, 965 

m (3,500 ft) thick, that ranges in age from Hauterivian 

to Cenomanian on the basis of fossils from two horizons. 

As Berkland (1973) suggested, the difference in thickness between this section and the 12,000-m (40,000 ft) 

section of the Great Valley sequence at the western 

edge of the Sacramento Valley is in part due to erosion 

prior to deposition of the overlying Paleocene rocks. The 

Hauterivian to Albian part of the autochthonous Great 

Valley sequence in the Wilber Springs area (fig. 10), 50 

km southeast of the Rice Valley outlier, is itself 5,100 m 

(17,000 ft) thick (Bailey and others, 1964, p. 132). Hence 

the thinness of the Rice Valley sequence implies extreme primary depositional thinning of the Great Valley 

sequence toward the west. 

Berkland's (1973, fig. 2) map shows that the slab is 

folded into an appressed syncline with the dip of both 

limbs approaching the vertical near the bounding faults. 

Berkland presumed that the Rice Valley sequence is a 

remnant of an upper-plate underthrust by an oceanic 

plate of Franciscan materials (Berkland, 1973, p. 2400-

2402). He suggested that the melange characteristic of 

the Franciscan Complex in this area may represent a 

tectonic mixture of adjacent thrust plates. In support of 

this concept, Berkland mentioned the presence along 

the east side of Rice Valley of a slab of unmetamorphosed K-feldspar-bearing graywacke and minor chertpebble conglomerate similar to the Lower Cretaceous 

part of the Great Valley sequence. 

The structural relations of the Rice Valley outlier to 

surrounding ultramafic rock and melange of the Franciscan Complex are equivocal. The actual contacts are 

chiefly high-angle faults, according to Berkland's (1973) 

map and cross sections. There is little direct evidence to 

support the conventional view that the slab is a downfaulted block or an erosional remnant of a once-continuous sheet of Great Valley sequence thrust over the 

Franciscan. On the basis of structural relations described by Berkland (1973), the possibility cannot be 

ruled out that the slab is (1) an upfaulted block of the autochthonous basement on which the Franciscan rests, or 

(2) a large rip-up of that basement engulfed in melange, 

or (3) a mesoallochthonous deposit. 

In the Covelo area, irregular "islands" formed by 

intact sequences of Upper Cretaceous, Paleocene, and 

Eocene strata are surrounded by a "sea" of broken and 

sheared rocks of the Franciscan Complex (Clark, 1940). 

The Franciscan Complex here consists of layers of 

melange and. coherent sequences of mudstone and 

feldspathic arenite or graywacke; the melange contains 

fossils as young as Late Cretaceous (Albian through 

Cenomanian) according to Gucwa (1975, p. 107). Gucwa 

(1975) noted that one of the coherent units-the White 

Rock unit of Late Cretaceous age-is depositionally 

overlain by melange. He attributes this contact relationship to emplacement of the melange as a gravity mass 

flow during Late Cretaceous time or later. 

The Upper Cretaceous strata forming the islands 

consist of sandstone, shale, and minor conglomerate that 

is in places conformably or disconformably overlain by 

Martinez (lower Paleocene), Meganos (upper 

Paleocene), and Capay (lower Eocene) sandstone 

(Clark, 1940). Locally this sequence of sediments depositionally overlies the Franciscan Complex (Gucwa, 1974, 

p. 39, 66). 

We ask, then, are these Cretaceous and early Tertiary sequences mesoallochthonous or neoautochthonous? The field evidence bearing on this question seems 

to be somewhat equivocal. Gucwa (1974, p. 39) noted 

that one small outlier consisting of strata similar to the 

fossiliferous Upper Cretaceous rocks is overturned. He 

suggested that this section was deposited on melange 

and then partly incorporated into the melange. However, other sections of the Upper Cretaceous and Tertiary rocks do not give any indication of substantial 

postdepositional displacement, hence in Gucwa's opinion 

were probably deposited after formation of the subjacent melange. Probably additional field evidence will be 

needed to resolve this question. However, on the basis 

of the map and contact relations depicted and described 

by Clark (1940) and Gucwa (1974, 1975), it seems likely 

that the Late Cretaceous strata were deposited on 

melange, then dismembered and engulfed within 

melange in early Tertiary, hence are probably mesoallochthonous. 

BANDON AREA 

Ih southern Oregon, the closest lithologic correlative of the Franciscan Complex is the Otter Point For-
22 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

mation. The Otter Point consists of interstratified 

mudstone, graded sandstone (arkosic to lithic wacke), 

pebbly mudstone, volcanic breccia, bedded chert, conglomerate, and pillowed lava flows (Koch, 1966, p. 36-

43). The presence of numerous detached garnetiferous 

blueschist blocks, universal shearing, and the absence of 

preserved bedding (Beaulieu, 1971, p. 30) imply that the 

Otter Point is in fact a melange (Dott, 1971, p. 27; 

Baldwin and Beaulieu, 1973, p. 13), hence should be considered a northern continuation of the Franciscan Complex. 

The Otter Point Formation is regarded as Late 

Jurassic by the presence within it of a sparse fauna that 

includes Buchia piochii (Gabb) (Koch, 1966, p. 42-43). 

Cretaceous fossils have been found in other formations 

in the area, for example, the Upper Cretaceous Cape 

Sebastian Sandstone and Hunters Cove Formation of 

Dott (1971, p. 31-42) and the Lower Cretaceous Rocky 

Point Formation and underlying Humbug Mountain 

Conglomerate (Koch, 1966, p. 43-48). The Cape Sebastian Sandstone unconformably overlies the sheared 

Otter Point Formation (M. C. Blake, Jr., written commun., 1979). But as the contact between the Humbug 

Mountain and Rocky Point Formations and Otter Point 

melange is apparently tectonic (Koch, 1966, p. 43; Dott, 

1971, p. 57), the episode or episodes of deformation that 

produced the Otter Point melange must be pre-Cape 

Sebastian San<:fstone and could be post-Rocky Point 

Formation in age. 

Along the coast, the type Humbug Mountain Conglomerate is gradationally overlain by the Rocky Point 

Formation; here the two formations are composed of interbedded conglomerate, mudstone, and sandstone, and 

aggregate about 2, 750 m (9,000 ft) in thickness (Koch, 

1966, p. 43-50). The two formations appear to be Valanginian in age (Koch, 1966, p. 44-45). The Humbug 

Mountain unconformably overlies the Upper Jurassic 

Galice Formation, composed of black argillite, slaty or 

phyllitic mudstone, and gray sandstone (Dott, 1971, p. 

11, 21-23). Overthrusting of the Humbug Mountain and 

Rocky Point Formations by the Colebrooke Schist (Coleman, 1972, pl. 1) shows that their deposition was followed by one and possibly more than one episode of allochthonous deformation. 

The youngest rocks in the Bandon area that have 

been penetratively deformed (at least locally) and tectonically juxtaposed against the Otter Point melange are 

those of the Roseburg Formation of Baldwin and 

Beaulieu (1973) and Baldwin (1974, p. 60), of early Tertiary age. Their Roseburg includes strata classified by 

Turner (1938, p. 5) as the lower member of Diller's 

(1898) Umpqua Formation~ As defined, their Roseburg 

consists of a lower member of basaltic flows, pillow 

lavas, volcanic breccias and interflow sediments, chiefly 

conglomerate and tuffaceous sandstone~ and an upper 

member of tuffaceous siltstone and rhythmically bedded 

sandstone. The formation could aggregate 3, 700 to 4,600 

m (12,000 to 15,000 ft) in thickness (Baldwin, 1974, p. 6-

8). 

According to Baldwin (1974, p. 8-10), the Roseburg 

includes strata that at various localities have yielded a 

fauna of Paleocene, early Eocene, and rarely, Late Cretaceous age. In his opinion, rocks containing the Late 

Cretaceous fossils may have been tectonically introduced into the Roseburg. 

The Roseburg is isoclinally folded and thrust over 

severely deformed strata in part considered to be the 

sedimentary upper member of the formation (Baldwin 

and Beaulieu, 1973, p. 20), and in places, as at Coos 

Bay, the Roseburg (nee lower Umpqua) "displays several melange features" (Beaulieu, 1971, p. 30). Steeply 

folded Roseburg strata also appear to be overthrust by 

the Colebrooke Schist (Baldwin and Lent, 1972, p. 125; 

Beaulieu and Hughes, 1976, p. 13). The Roseburg is herein considered to be autochthonous (fig. 9), although it 

is apparent from the descriptions given that parts of the 

formation might be allochthonous. 

The Colebrooke Schist is composed of thin-bedded 

shale, sandstone, and associated minor pillow lavas, 

tuff, and chert metamorphosed at temperatures and 

pressures intermediate between those of the blueschist 

facies and greenschist facies (Coleman, 1972, p. 29, 43). 

This formation occupies an area of about 260 km2 (100 

mi2) in southwestern Oregon, much of which, according 

to Coleman (1972, p. 56), represents an allochthonous 

sheet thrust eastward over the Otter Point, Dothan, 

and other pre-Tertiary formations in the area. 

The folded and beveled Roseburg and pre-Tertiary 

rocks are unconformably overlain by a mildly deformed 

conglomerate, sandstone, and siltstone, the 

Lookingglass Formation of Baldwin (1974), which contains a Penutian (late early Eocene) fauna (Baldwin, 

1974, p. 12-16). Strata assigned to the Lookingglass by 

Baldwin (1974, p. 16, arid geologic map) were considered 

by Turner (1938, p. 5) to be the upper member of the 

Umpqua.· They also include the beds at dne locality 

(Boulder Creek) mapped as Umpqua by Coleman (1972, 

p. 19, 28, pl. 1) that Baldwin (1974, geologic map) shows 

depositionally overlying the Colebrooke Schist, its basal 

decollement, and part of the subjacent imbricated serpentinite sheet. 

The structural fabric of the Colebrooke Schist 

suggests multiple episodes of penetrative deformation, 

the most recent of which Coleman believed to be Late 

Cretaceous (1972, p. 19, 29, 56). However, the field relations of the Colebrooke to the Roseburg and 

Lookingglass appear to bracket the age of final tectonic 

emplacement of the Colebrooke Schist as early Eocene, 

·as suggested by Baldwin and Lent (1972, p. 125) and 

Beaulieu and Hughes (1976, p. 13). 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 23 

MAIN ELEMENTS OF MELANGES AND THEIR 

BEARING ON PLATE-TECTONIC HISTORY 

The origin of the melanges of the western part of 

the North American plate and their bearing on platetectonic history from Cretaceous through Tertiary time 

involves the formation of three main elements: (1) Cretaceous through early Tertiary melanges of the Franciscan Complex, (2) the Eocene Oregon-Washington borderland, and (3) middle(?) Tertiary melanges of the 

Olympic Peninsula of Washington. Here, attention is focused on whether the age of the most recent episode of 

allochthonous deformation during which part of the 

Franciscan Complex was formed coincides with the conversion of a transform boundary to a subduction zone at 

a northwestward-migrating triple junction, or spans the 

entire time during which the subduction zone was active. 

The topic of the Oregon-Washington borderland 

represents a departure from the main subject of this 

paper, that is, the tectonic implications of the melanges. 

Yet an understanding of the origin of the borderland is 

necessary in order to link the Cretaceous through early 

Tertiary history of the Franciscan Complex with the 

middle(?) Tertiary history of the Olympic Peninsula. 

In the third section of the discussion, attention is 

focused on whether the age of deformation of the Olympic Peninsula coincides with the age or ages of momentary formation and transit of a Humboldt-type triple 

junction between the Pacific, Juan de Fuca, and North 

American plates, as deduced from the magnetic record 

of the ocean floor. 

FRANCISCAN COMPLEX 

AGE OF DEFORMATION 

RESULTING IN THE FRANCISCAN COMPLEX 

The internal disruption and deformation characteristic of the Franciscan Complex has been attributed to 

(1) processes occurring at or above an active subduction 

zone (Hamilton, 1969) and, alternatively, to (2) processes occurring at or near a triple junction whose migration is associated with conversion of a transform plate 

boundary to a subduction zone (Fox, 1976). According to 

the subduction-zone hypothesis, deformation continues 

along the entire extent of the subduction zone for its 

life. According to the triple-junction hypothesis, deformation is episodic, occurring only in the immediate vicinity of the triple junction. Further, because of the migration of this triple junction, the age of formation is 

progressively younger along its course. We ask, then, 

was the formation of the Franciscan Complex a continuous process or was it an episodic process? If episodic, 

was all or most of the complex synchronously formed, or 

was the deformation that produced the complex diachronous, becoming younger northwestward? 

The fact that units within the Franciscan Complex 

can be classified as allochthonous, mesoallochthonous, 

autochthonous, and neoautochthonous strata, a division 

made by Hsu (1969), implies that deformation was an 

episodic process, interrupted at least locally by periods 

of erosion and sedimentation. 

The age of the terminal episode of deformation is 

closely bracketed within Late Cretaceous (Campanian) 

in the Morro Bay area and early Eocene in the Bandon 

area. The youngest deposits presumed to be mesoallochthonous in the Covelo-Clear Lake area are late 

Paleocene, and possibly early Eocene at Rice Valley, 

and late Paleocene and early Eocene at Covelo. The 

coastal-belt rocks immediately to the west also are at 

least locally as young as Early Eocene. 

The age of this terminal episode of allochthonous 

deformation varies from place to place and apparently 

becomes progressively younger to the northwest (fig. 

11). The simplest history consistent with the data (fig. 

11) is as follows: beginning in Late Cretaceous, a diachronous upheaval producing melange or broken formation and the younger part of the Franciscan Complex 

progressed northwest along the margin of the continental plate at about 4 crnlyr, reaching central California in 

early Eocene time. 

The age of the deformation of the Eocene coastal 

belt and rocks in the Covelo area and Eocene(?) rocks in 

Rice Valley is not narrowly limited by the ages of 

younger neoautochthonous deposits. Conceivably, in 

these areas this event could be younger than early 

Eocene, implying that its rate of northwestward migration was less. On the other hand, the velocity could be 

greater if the Franciscan Complex in the Morro Bay 

area has been telescoped by right-lateral movement exceeding the 315 km of Neogene and later offset recognized by Matthews (1973) on the San Andreas fault. 

The Campanian and early Tertiary event probably 

did not extend southeastward to the Los Angeles lowland. Crystalline rocks forming the basement of the Los 

Angeles lowland were beveled in middle Cretaceous 

time, forming the "Los Angeles erosion surface" (fig. 

11), and were unconformably overlain, beginning in the· 

Cenomanian, by a sequence of strata that, though folded 

and faulted, have apparently not been affected by a regional episode of penetrative allochthonous deformation 

(Woodford and Gander, 1977). 

The continental borderland west of the Los Angeles 

lowland is underlain in places by the Catalina Schist 

(Bailey and others, 1964, p. 93), in other places by Cretaceous (Maestrichtian?, Campanian, Coniacian, and 

Cenomanian) and younger strata (Paul and others, 1976, 

p. 16). Neither the contact relation of the schist to the 

Cretaceous sediments nor the relation of the basement 
24 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

rocks of the borderland to those of the Los Angeles lowland has been established with certainty (Howell and 

Vedder, 1981). 

According to Howell and Vedder (1981), the borderland is composed of four terranes, probably representing large structural blocks. By northwest-directed lateral movement, the blocks have been individually dislocated with respect to each other and collectively dislocated with respect to the Los Angeles lowland. Hence, 

deformational episodes affecting the borderland may not 

be synchronous with those affecting the Los Angeles 

lowland. Howell and Vedder (1979, 1982) noted that 

"middle Cretaceous and late Paleocene hiatuses in 

sedimentation and concurrent regional lapses in magmatism may represent times of transform faulting that 

interrupted subduction." Until the movement history of 

the borderland is accurately reconstructed, the transitions from transform faulting to subduction implied by 

this statement cannot be placed in context with episodes 

of allochthonous deformation sensed in the Morro Bay 

area to the north. 

That there were episodes of allochthonous deformation preceding the Campanian and early Tertiary event 

is implicit in the characterization of the Las Tablas unit · 

of Cowan and Page (1975), the "broken formation A" of 

Hsu (1969), the Atascadero Formation of Hart (1976), 

and similar units as mesoallochthonous. Although accurate definition of thes~ earlier deformational episodes is 

not yet feasible, available information is sufficient to at 

least suggest their existence and place rough limits on 

their age and extent. 

In northern California, the latest Cretaceous and 

early Tertiary coastal belt is presumed to be chiefly mesoallochthonous and deposited after an episode of penetrative deformation that occurred in Cenomanian and 

Maestrichtian time. This Late Cretaceous episode (fig. 

11) probably postdates the Hauterivian to Cenomanian 

sequence at Rice Valley and the Late Jurassic to Early 

Cretaceous (Valanginian) Marmolejo Formation of 

Taliaferro (1944) and the Toro Formation of the Morro 

Bay area. It must predate the early Tertiary Roseburg 

Formation and Covelo outlier, the Paleocene rocks at 

Rice Valley, the "broken formation A" of Hsu (1969), 

the Las Tablas unit, and the Atascadero Formation. At 

Rice Valley, this episode of allochthonous deformation 

probably occurred during the time represented by the 

unconformity between Cenomanian and Paleocene 

rocks. 

A middle Early Cretaceous episode of melange formation in northern California also seems required, for 

most of the melanges there contain only a comparatively 

abundant Tithonian through Valanginian fauna without 

admixture of younger fossils (Blake and Jones, 1974, p. 

350). 

The middle Early Cretaceous event could also account for pre-Marmolejo development of melange in central California and perhaps at Baja, Calif. (fig. 11). The 

age of melange at Baja is not known with any certainty, 

however. At one locality on Cedros Island in Baja, 

Calif., the latest Jurassic through Early Cretaceous 

Eugenia Formation reportedly includes blueschist detritus believed to have been derived from the nearby 

melangelike Cedros Formation of probable Late Jurassic age (Kilmer, 1977). Potassium-argon ages of white 

mica and blue amphibole from blueschist at Cedros Island, however, range from 94.4±4 to 110±2 m.y. 

(Suppe and Armstrong, 1972). If not thermally retrograded, these ages indicate metamorphism and possibly 

formation of melange in or after Turonian time, that is, 

after deposition of the Eugenia Formation. · 

This middle Early Cretaceous episode could also be 

responsible for some of the structural disruption of the 

allochthonous terranes of the San Juan Islands. The age 

or ages of these rocks has not been firmly established. 

Some parts contain fossils as young as Cenomanian and 

Valanginian age (J. T. Whetten, written commun., 

1977). 

In summary, the development of the Franciscan 

Complex and allied rocks was apparently an episodic 

process. In some areas, such as the Morro Bay area, the 

evidence suggests that there were several episodes of 

deformation. The final episode was apparently diachronous; earlier events may have also been diachronous, but 

the data for them is less compelling. The age of much of 

the complex probably decreases systematically to the 

northwest, ranging from Late Cretaceous (Campanian) 

at Morro Bay, California, to early Tertiary (Eocene) in 

northern California. Within this geographic span, however, parts of the complex formed prior to Campanian 

time and were unaffected by more recent episodes of 

penetrative deformation may represent discrete terranes. 

CAUSE OF ALLOCHTHONOUS DEFORMATION 

THAT CREATED THE FRANCISCAN COMPLEX 

The structural fabric of the Franciscan Complex may 

have been produced through gravity thrusting and sliding away from one or more triple junctions that migrated northwestward along the interface between the 

North American plate and the oceanic plates to the 

west. The chief bases for this hypothesis, as originally 

proposed (Fox, 1976), were (1) the existence of triple 

junctions of the required dynamic type is implicit in the 

plate-tectonic theory; and (2) the structural fabric of the 

Franciscan Complex is more compatible with shearing 

at low confining pressure inherent in this process than 

the high confining pressures seemingly required by the 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 25 

subduction theory; and (3) this process provides a means 

of recovering and stirring exotic clasts of eclogite and 

blueschist into the· melange. To these reasons a fourth 

may now be added, namely, that the deformation that 

produced the Franciscan Complex was apparently 

episodic and, where best dated, was diachronous. 

If the premise of episodic deformation is accepted, 

the age data schematically projected in figure 11 provide 

a means of identifying and tracking the causal triple 

junctions. The Late Cretaceous and early Eocene deformational event is the track of a Humboldt-type triple 

junction that migrated northwestward along the plate 

margin at a rate of roughly 4 em/yr. At any given time, 

the margin of the North American plate was bordered 

by a transform fault to the northwest and by a subduction zone southeast of this triple junction. Hence the 

northwest transit of the triple junction marks the end of 

a strike-slip-dominated tectonic regime and the beginning of a subduction-dominated tectonic regime. 

Both the middle Early Cretaceous and middle Late 

Cretaceous events are presumed to represent northwestward transits of Humboldt-type triple junctions 

(fig. 11), as yet too obscure to justify any firm conclusion 

as to their precise age or rate of northwestwardly migration. Tentatively, they appear to have moved somewhat faster than the Late Cretaceous (Campanian) and 

early Tertiary event of central and northern California 

(fig. 11). 

The Campanian and early Tertiary event marks a 

transition from what was previously strike-slip-dominated to subduction-dominated tectonism along the continental plate margin. Presumably, each of the preceding episodes of allochthonous deformation do also. Intervening episodes in which the subduction zone was converted to a strike-slip transform fault could be episodes 

during which a Mendocino-type triple junction (Fox, 

1976) transited the margin of the continental plate, or 

episodes during which a change in plate motion was 

coupled with a ridge jump. 

Cooper and others (1976) showed that the lithosphere underlying the eastern Bering Sea basin is a fragment of oceanic plate that broke away and was trapped 

as the subduction zone at the northern edge of the Kula 

plate jumped south to its present location at the Aleutian trench. They estimated the date of this event to be 

approximately 70 Ma (Cooper and others, 1976, p. 1125). 

Presumably this event was part of a general reorganization of the boundaries of the Kula plate that roughly 

coincided with the middle Late Cretaceous (approximately 75 Ma) transition along the California coast from 

subduction to strike-slip faulting. If so, that transition 

was probably precipitated by a ridge jump (fig. 11), 

rather than by transit of a Mendocino-type triple junction. 

On the basis of the spreading history of the Pacific 

plate, Larson and Chase (1972) postulated a 2,000-km 

southeastward jump of the Kula-Farallon ridge about 

100 Ma. This jump probably entailed a jump of the KulaFarallon-North American triple junction, hence coincides with a transition from subduction to strike-slip 

faulting along a segment of the western edge of the 

North American plate (fig. 11). . 

The following model (fig. 12) is proposed: Beginning 

in latest Jurassic or Early Cretaceous time, the spreading ridge between two oceanic plates first intersected 

the margin of the North American plate, forming a triple junction with the transform fault to the northwest 

and the subduction zone to the southeast. Subsequently, 

this triple junction migrated northwest along the margin 

of the continental plate at a rate comparable to that 

suggested in figure 11, that is, about 5% cm/yr (or more, 

if, as is likely, pre-Late Cretaceous elements of the 

Franciscan Complex have been telescoped by faulting an 

amount greater than the 315 km restored in fig. 11). 

The space problem at the triple junction caused by 

the encroachment of the subducting oceanic slab on the 

transform-bounded edge of the continental plate was relieved by the plowing up of the lip of the continental 

plate and the consequent detachment of slabs and sheets 

of the Late Jurassic and Early Cretaceous turbidite deposits underlying the continental slope. These slabs cascaded or were rammed inland, stacking melanges, broken formations, and allochthonous imbricated masses of 

rocks on the floor of the forearc basin to the east, which 

was locally underlain by Late Jurassic (Knoxville Formation) and Early Cretaceous rocks deposited over 

oceanic basement. Rip-ups from this autochthonous 

floor were locally incorporated in the melanges. The 

subjacent autochthon was depressed beneath the weight 

of the melange and, being under compression from the 

west, broke along the crustal flexure that formed near 

the eastern extremity of the allochthonous mass, and 

was then partially overridden by the Knoxville Formation and the Great Valley sequence of the craton on a 

zone of thrust and high-angle reverse faults (the first 

phase of thrusting on the Coast Range thrust of Bailey 

and others, 1970). This scenario was apparently repeated with varying degrees of severity during middle 

Late Cretaceous and again in Late Cretaceous (Campanian) and early Tertiary time. In these later episodes, 

strata laid down on the melange were themselves disrupted and imbricated. 

COMPARISON OF CRETAC.EOUS THROUGH EARLY 

EOCENE TECTONIC REGIMES AS PREDICTED 

BY THE TRIPLE-JUNCTION HYPOTHESIS 

WITH CONCEPTS SUGGESTED BY OTHER EVIDENCE 

The plate configuration from which the Early Cretaceous Humboldt-type triple junction evolved is un-
26 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

known. Ernst (1965, p. 905) suggested that the Late 

Jurassic elements of the Franciscan "Group" represent 

trench deposits, a suggestion adopted as a working 

hypothesis by Hamilton (1969). This implies that in Late 

Jurassic and Early Cretaceous time, central western 

North America was bounded by a subduction zone or 

zones. Plutonic rocks (fig. 11) that could be related to 

this subduction zone include both Late Jurassic and 

Early Cretaceous plutonic rocks with representatives in 

northern California (Lanphere and others, 1968) and 

central California (Evernden and Kistler, 1970, p. 623). 

Further definition of the Jurassic setting is required before the nature of the transition, which could involve as 

yet undefined plates, from Late Jurassic subduction to 

the Cretaceous melange-forming transit of the KulaFarallon spreading ridge can be specified. 

The timing and geographic extent of the Late Cre-. 

taceous subduction zone required by the scenario given 

in figure 11 correlates roughly with the plutonic episode 

Evernden and Kistler named the Cathedral Range intrusive epoch (1970, p. 623). The Early Cretaceous subduction zone correlates less satisfactorily with the Huntington Lake intrusive epoch. The Yosemite intrusive . 

epoch and the age of plutonic rocks of the Trinity Mountains (Lanphere and others, 1968) do not correlate with 

any subduction episode represented in the boundary regimes (fig. 11). This may imply that marginal parts of 

the North American plate have been dislocated by 

Wrangellia 

strike-slip faulting from the Early Cretaceous volcanic 

arcs of the craton and subsequently rejoined elsewhere 

to the craton as tectonostratigraphic terranes. 

The relations depicted in the schematic chart (fig. 

11) imply that a strike-slip regime prevailed along the 

western margin of the North American plate for about 

20 m.y. in Late Cretaceous and early Tertiary time. 

This conclusion is vital to the interpretation of plate 

geometry developed in later sections, hence corroboration independent of the triple-junction hypothesis is particularly desirable. Snyder and others (1976) postulated 

development of a strike-slip regime in latest Cretaceous 

and early Tertiary time, on the basis of timing of offset 

on the proto-San Andreas fault, which seemingly requires a period of strike-slip faulting roughly 50-70 Ma 

(1976, p. 103). Development of the transform through a 

rise-trench encounter was followed by a cessation of arc 

magmatism to the east. Their compilation of ages of 

igneous rocks ~n~icates th~t there was a null period in 

magmatism about 65--40 Ma. They attributed the apparent 5- to 10-m.y. timelag in cessation and renewal of 

magmatism to the time required for the trailing edge of 

the subducted slab to pass and the leading edge of the 

succeeding slab to reach appropriate d~pth for melting 

to begin (Snyder and others, 1976, p. 92, 103). 

The Late Cretaceous to early Eocene transit of a 

Humboldt-type triple junction inferred from the ages of 

melanges is roughly synchronous with the transit of the 

,;·:···. 

······.·····, .. 

FIGURE 12.-View to northwest showing transit of Early Cretaceous Humboldt-type triple junction with accompanying formation of melange 

and a proto-Coast Range thrust (CRT) to east. Western source of detritus in the Franciscan Complex is a tectonstratigraphic terrane (Beck 

and others, 1980), such as Wrangellia (Jones and others, 1977), locked in an oceanic plate and drifting with it to northwest relative to craton 

and source area of the Great Valley sequence. 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 27 

Kula-Farallon-North American triple junction originally 

proposed by Atwater (1970, p. 3531) through extrapolation of present rates of plate movement back to that 

time. The rate of right-lateral offset between the Pacific 

and North American plates of 6 cm/yr adopted by Atwater (1970) in her constant-motion model was based principally on Larson and others' (1968) estimate of the average spreading rate for the past 2 m.y. across the extension of the East Pacific Rise into the Gulf of California. The slip rate was amended slightly by Minster and 

others (1974) to 5¥2 cm/yr on the basis of simultaneous 

solution of rotation vectors between 11 major plates. 

Atwater and Molnar (1973) deduced greatly different slip rates between the North American and Pacific 

plates throughout most of Tertiary time by chaining 

spreading rates between the North American, African, 

Indian, East and West Antarctic, and Pacific plates. On 

this basis, they suggested that the rate of motion between the Pacific and North American plates averaged 

about 5 cm/yr between 29 and 21 Ma, 1.3 cm/yr between 

21 and 10 Ma, 4 cm/yr between 10 and 4.5 Ma, and 5.5 

cm/yr since 4.5 Ma. While cautioning that the details 

should not be taken literally, in particular noting weaknesses in the 21-m.y. and 29-m.y. reconstructions, they 

concluded that "an average relative rate of about 2 em/ 

yr between 38 and 10 Ma is almost certainly realistic" 

(Atwater and Molnar, 1973, p. 142). 

If the rates postulated by Atwater and Molnar 

(1973) are approximately correct, the transition from 

strike-slip faulting to subduction at any given place 

along the plate margin would be expected to have occurred much more recently than is implied by the latest 

Cretaceous through middle Eocene age of the younger 

melanges of the Franciscan Complex. If the triple-junction hypothesis for the origin of the melanges is correct, 

then the deformational record of those melanges 

suggests that the average rate of movement from Cretaceous to the present approaches the present rates of 

plate movement and in general validates Atwater's 

(1970) direct extrapolation of those rates back to the 

Cretaceous ("constant-motion model"). Conversely, if 

the rates of offset proposed by' Atwater and Molnar 

(1973) are approximately correct, the triple-junction 

hypothesis must be wrong. 

EVIDENCE FROM THE FRANCISCAN COMPLEX 

CONTRADICTORY TO THE 

TRIPLE-JUNCTION HYPOTHESIS 

Resistance to the hypothesis that melanges of the 

Franciscan Complex originated through tectonism at 

migrating triple junctions as advanced by the author 

(Fox, 1976) commonly focuses on two questions: (1) Do 

any of the ophiolite-Knoxville Formation-Cretaceous sequences found as numerous discrete terranes structurally enclosed by Franciscan Complex actually represent 

exposed elements of the autochthonous pre-melange 

basement, or are they all part of an allochthonous upper 

plate? (2) Does the triple-junction hypothesis provide a 

structural setting in which the exotic blocks of highgrade blueschist and the more extensive terranes of 

low-grade blueschist could form? 

The contact relations between internally deformed 

rocks of the Franciscan Complex and the stratally intact 

ophiolite-Knoxville-Cretaceous sequences that are commonly referred to the Great Valley sequence have been 

and remain a vexing problem. In part because of inherent stratigraphic and structural complexity and in part 

because of poor exposure, geologic opinion on this problem has been dominated by successive reigns of various 

ruling theories which may or may not have a sound observational basis. Three such theories can be readily distinguished: (1) the Knoxville Formation is younger than, 

and grades downward into, the Franciscan "Formation" 

of former usage (Taliaferro, 1943); (2) the Great Valley 

sequence is broadly coeval with the Franciscan assemblage, and the two groups of rocks have been juxtaposed by faulting (Bailey and others, 1964; Irwin, 

1957); (3) the various isolated remnants of the Great 

Valley sequence are parts of a formerly continuous 

sheet underthrust by the Franciscan assemblage (Bailey 

and others, 1970). 

Contributing to the credibility of the third 

hypothesis is the fact that in broad areas of central 

California, Upper Cretaceous rocks contiguous with, 

and commonly correlated with, the Great Valley sequence do indeed structurally overlie Franciscan rocks. 

This relation is perhaps best illustrated in the area of 

the Diablo antiform (fig. 10) (Bailey and others, 1964, p. 

154). Detailed maps (Dibblee, 1973, 1975) show that a 

conformable sequence of stratally intact Upper Cretaceous to Eocene rocks wrap around Franciscan rocks exposed in the core of the southeast-plunging nose of this 

fold. And as previously noted, north of the Diablo antiform, both Lower and Upper Cretaceous rocks of the 

Great Valley sequence, sensu stricto (that is, as exposed 

along the western flank of the Great Valley), have structurally overridden the Franciscan Complex along a reverse fault or thrust fault. 

Proponents of the subduction-complex theory, extrapolating from these relations, argue not unreasonably that all the isolated patches of Great Valley-like 

rocks to the west structurally overlie the Franciscan 

Complex; if the local contact is in fact a high-angle fault 

or if it can be demonstrated that the Franciscan structurally overlies the Great Valley-like rocks, then the 

contact is interpreted as a folded or overturned thrust 

fault with Franciscan in the lower plate. In some recent 

compilations, all patches of Great Valley-like rocks are 
28 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

shown as klippen, regardless of the present attitude of 

the contact and of whether the original mapper believed 

the contact to be depositional or structural. 

This dogma is contradicted by both the conception 

of the Franciscan Complex as being part mesoallochthonous and partly allochthonous (Hsu, 1969) and by 

the implication of the triple-junction hypothesis that the 

Franciscan could overlie a basement composed in part of 

Great Valley-like rocks. What is in question, then, is the 

structural and depositional relations of the isolated 

areas of Great Valley-like rocks to the surrounding 

Franciscan Complex. Field evidence is commonly ambiguous because the contacts between these rocks are 

typically poorly exposed and either are, or have been 

modified by, high-angle faults. The presence of Franciscan structurally overlying Great Valley rocks locally has 

been reported, as at the St. John Mountains thrust 

fault, on which Franciscan rocks have been thrust over 

the Knoxville Formation a distance of at least 8 km (5 

mi) (Weaver, 1949, p. 137; Bailey and others, 1964, p. 

157). 

Stratigraphic differences between the rocks in 

these "outliers" and the Great Valley sequence, sensu 

stricto, have been noted, leading some workers to post-. 

ulate that the outliers were deposited in local basins of 

deposition separate from the main body of the Great 

Valley sequence (Maxwell, 1974). Moore and Karig 

(1976) postulated that the outliers might represent sediments accumulated in local basins on the trench slope 

adjacent to, or within the surface trace of, the subduction zone. In their view, these sediments may have been 

deposited on a part of the accretionary wedge previously converted to melange, and subsequently or concurrently deformed by distributed thrusting as subduction continued. This concept, which was echoed by Howell and others (1977) and by Underwood (1977), has 

merit in that it does recognize and, to a degree, accounts 

for the contact and structural relations peculiar to the 

mesoallochthonous deposits. 

In summarY, observational and theoretical basis is 

now sufficient to reopen the question of the relations, 

both structural and stratigraphic, of the outliers of 

Great Valley-like rocks to the Franciscan Complex. 

These outliers could include: (1) klippen of the Coast 

Ranges thrust, (2) an autochthonous basement that is 

structurally overlain by the Franciscan Complex, (3) 

mesoallochthonous slabs that depositionally overlie 

older melange, and are structurally overlain by younger 

melange; and (4) neoautochthonous strata that depositionally overlie the Franciscan Complex. 

The origins of (1) the regionally metamorphosed 

generally low grade blueschist formations, such as the 

South Fork Mountain Schist (Blake and others, 1967) 

and the Colebrooke Schist (Coleman, 1972), and (2) the 

high-grade blueschist and eclogitic exotic blocks are two 

of the very puzzling riddles of the Franciscan Complex. 

It may be significant that the potassium-argon ages of 

the exotic blocks reported by Coleman and Lanphere 

(1971) are older at any given place than the admittedly 

speculative middle Early Cretaceous event (fig. 11). 

This could imply that these blocks formed during a 

period of strike-slip-dominated tectonism preceding that 

event. Possibly they originated as blocks of material 

that were scraped off the wall of the oceanic plate at its 

transform boundary, metamorphosed at local sites of 

high contact pressure, and left as horses in the fault 

zone, then elevated and incorporated in melange during 

the ensuing passage of the Humboldt-type triple junction. 

The rubidium-strontium isochron age of 125 ± 18 

m.y. for the Cole brooke Schist (reported as 128 ± 18 

m.y. by Coleman, 1972, p. 54, and recalculated by Lanphere and others, 1978, using refined decay constants) 

compares with the probable metamorphic age of 115-120 

m.y. of the South Fork Mountain Schist (Lanphere and 

others, 1978). These ages straddle the time of passage of 

the middle Early Cretaceous triple junction as conjectured in figure 11, suggesting that metamorphism of 

these rocks was related to this episode of allochthonous 

deformation. 

The questions raised here bear directly on the validity of the triple-junction concept. Even assuming that 

this concept is correct in principle, the tectonic scenario 

given in figure 11 for the Early and early Late Cretaceous must be regarded as tentative because of the extreme fuzziness of the dating of the melanges of those 

ages, and because of the possibility of undetected lateral 

translation of the data points in part, perhaps, as discrete tectonostratigraphic terranes. 

The speculative nature of that scenario can perhaps 

be accentuated by framing some of the more troublesome questions that occur to the author: (1) Were there 

in fact two diachronous episodes of allochthonous deformation in Cretaceous time prior to the Campanian, or 

only one, possibly with a more complex path? (2) What 

is the significance of the fact that exotic clasts of 

blueschist and eclogite were mainly introduced into the 

melange dUring the deformational events prior to the 

Campanian? 

OREGON-WASHINGTON BORDERLAND 

AGE, DISTRIBUTION, AND PLATE-TECTONIC SETTING OF 

THE BORDERLAND 

The early and middle Eocene age of the allochthonous oceanic crust represented by basalts of the Metchosin, Crescent, and Siletz River Volcanics, the basement 

of the Oregon-Washington borderland, roughly equates 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 29 

with the expected time of transit past northwestern 

California and western Oregon of the Kula-Farallon 

spreading ridge (fig. 11). Presuming that the basalts are 

indeed oceanic crust, their restricted age indicates that 

they represent a block sliced away from the near-flank 

and perhaps the axial zone of a spreading ridge. That 

the spreading ridge was near the continental plate is 

confirmed by the 3-m-diameter quartz diorite boulders 

incorporated in sedimentary beds interlayered with 

Crescent Formation as reported by Cady (1975). 

Paleomagnetic data from the western Cordillera of 

North America suggest that the region consists of large 

crustal blocks which were rotated clockwise and translated northwestward relative to the North American 

craton, as if caught and rotated like ball bearings in a 

wide zone of right-lateral shear (Beck, 1976). In this 

context, rocks of the Oregon-Washington borderland 

are remarkable and to a degree atypical, in that some, 

though relatively youthful, show very substantial rotations (Cox, 1957; Simpson and Cox, 1977) but no detectable northward translation (Plumely and Beck, 1977; 

Beck and Burr, 1979, p. 178). 

Paleomagnetic data reported by Simpson and Cox 

(1977) indicate that after deposition the Siletz River 

Volcanics was rotated clockwise en bloc about 68° ± 12°, 

the overlying middle Eocene Tyee Formation and related rocks about 64°± 16°, the late Eocene Yachats 

Basalt about 51 o ± 33°, an Oligocene sill about 28°, and 

nearby Miocene basalt 0 ± 44 o. Simpson and Cox propose 

two alternative tectonic models: (1) a 225-km segment 

of coastal Oregon originated as a small plate related to 

northward migration of the Kula-Farallon-North American triple junction, then rotated clockwise about a pivot 

near its southern end; (2) a much longer block encompassing what is now the entire Oregon-California borderland rotated clockwise about a pivot near its northern end. 

A variation of the first model better accords with 

the plate geometry required if the Campanian and early 

Tertiary episode of allochthonous deformation also represents the transit of this triple junction. To document 

this opinion requires a slight digression from the main 

thread of this discussion but one necessary to put the 

melange-forming processes that occurred before and 

after formation . of the borderland into better perspective. 

The angular and, in places, even jagged outline of 

basement as inferred in figure 9 suggests that the borderland is an aggregation of numerous crustal fragments 

wedged into a reentrant in the continental plate, rather 

than a single coherent crustal block. If it is, the blocks 

must have been individually rotated clockwise after 

their formation, some as much as about 70°, to account 

for the paleomagnetic data. The north-trending fissures 

that were the source of part of the Siletz River Volcanics (Snavely and others, 1968, p. 480) therefore must 

have trended about N. 70° W. when formed. These fissures thus reflect a tensional axis oriented parallel to 

that of the Kula-Pacific spreading ridge rather than the 

supposedly nearby but northeast-trending Kula-Farallon ridge. 

On the basis of Duncan's (1977) potassium-argon 

ages, which show that the Siletz River Volcanics ranges 

in age from 49.3 to 54.7 Ma, it appears that anomaly 22 

(53 Ma) and possibly anomaly 23 (55 Ma) are represented within the rotated blocks of oceanic crust making up the basement of the borderland. But study of the 

paleomagnetic map of the northeastern Pacific (fig. 13) 

shows that at the time of anomaly 24 (56 Ma), the easternmost part of the Kula-Pacific spreading ridge terminated at a ridge-ridge-ridge triple junction some 1,300 

km west of the North American plate. 

Yet it also appears from this map (fig. 13) that the 

Kula-Pacific spreading ridge either vanished (Byrne, 

1978; Geotimes, 1978, p. 24) or jumped some undefined 

but considerable distance northward shortly after formation of the Y -shaped anomaly labeled "24 ?" located at 

about 54° N., 158° W. The ridge jump is the preferred 

hypothesis because it better rationalizes the presence of 

the Kula-Pacific spreading orientation with the Siletz 

River Volcanics and, as will be discussed later, extensional faulting and rift-related volcanism in British Columbia and northern Washington. If there was such a 

jump, the Y -shaped anomaly 24? could be the fossilized 

triple junction between the Kula, Pacific, and Farallon 

plates formed immediately before the jump. 

ORIGIN OF THE BORDERLAND 

THROUGH PLATE COLLISION 

The fragmentation and differential rotation of the 

allochthonous blocks forming the borderland indicates 

that their emplacement was forcible. Could the borderland have formed through a collision of oceanic and continental plates? Atwater (1970, p. 3531), in her constantmotion model (fig. 1), assumed that in the Late Cretaceous and early Cenozoic, the motion of the Pacific plate 

was parallel to the edge of the North American plate 

and that the motion of the Kula and North American 

plates had a significant component of strike slip. Indeed, 

the Kula-North American motion may have been essentially strike-slip, as judged by the evidence summarized 

of a time-transgressive change in tectonism associated 

with northwestward advance of the subducting Farallon 

plate. But the paths of the Pacific and North American 

plates, not then in mutual contact, could have been 

slightly or even markedly convergent. 
30 MELANGE~UBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

If the paths of the North American and Pacific 

plates were convergent, a collision would inevitably follow, and the borderland could be a product of that collision. This concept accommodates the paleomagnetic evidence of a ridge jump, the evidence of tectonic fragmentation and rotation of the borderland, and the deformational features east of the borderland. The following 

scenario is suggested (fig. 14). As the Pacific plate approached the North American plate through plate 

growth and path convergence, the shrinking Kula plate 

broke apart, causing the Kula-Pacific spreading ridge to 

jump northward (fig. 14C) and intersect the continental 

margin. Enlarged by the addition of part of the Kula 

plate, the Pacific plate sideswiped the North American 

plate, driving a widening wedge of recently formed 

oceanic crust inland (fig. 14D) and crunching adjacent 

rocks of the craton in northern California, Oregon, and 

Washington. The Pacific and North American plates 

were deflected by the impact of this collision, ultimately 

adopting paths parallel to their bounding transform 

160'W 150"W 

55' N 

50' N 

45' N 

faults. As they did, the allochthonous wedge was 

sheared off and fragmented. Caught between two major 

plates moving right laterally past each other, the fragments rotated clockwise and lodged against the North 

American plate. 

The change in spreading directions between the 

Pacific and Farallon plates recorded by the difference in 

trend of anomalies 22 and 21 (53-50 Ma) (fig. 13) may 

represent adaptation of the movement of the Farallon 

plate to the changing paths of Pacific and North American plates. As a consequence of this change in spreading 

direction, several right laterally stepping fracture 

zones, including the Aja, formed along .the new segment 

of Pacific-Farallon spreading ridge (the northeast-trending segment that had been the Kula-Farallon spreading 

ridge). 

Relative movement between the Pacific and North 

American plates has assuredly been parallel to the 

transform between them since contact of the two plates 

in the area between the Pioneer and Murray fracture 

140'W 130'W 

100 200 300 KILOMETERS 

I I I 

FIGURE 13.-Paleomagnetic map of floor of northeastern Pacific Ocean (after Hilde and others, 1976, p. 223, except detailed inset, which is after 

Atwater and Menard, 1970, p. 448). Heavy lines indicate fracture zones, dashed where uncertain; light numbered lines indicate magnetic 

anomalies, dashed where uncertain. 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 31 

zones after about 30 Ma. The adjustment of the 

mechanism which drives the plates that was required to 

bring the direction of their relative movement into con-

·. · ~ · · · · • : .s~att.le · · · ·NORTH. AMERICAN ·PLATE· 

. · · ·. . · · · · · . anc~uv~r · • Portland · · · · · · · · · . • • . . . • 0. . . . . . . . . . . · . ·. · · · · . · Cape . San. . . . 

. · .M~nd,oclno · t;=ra ~~·-lr-~~~~~--~ 

A 75Ma 

• 0 •••• • 0 • • • • 

· Vancouver •Seattl~ · . · · . · · · •Portland 

. . . 

NORTH AMERICAN PLATE. 

Cape · 

.. ~e 1-+-· -(=7"-"--...:...,_....:.._---,.,.;.,., :::::::.: ... :.:::::::::~ .. :: ...... :. 

8 56 Ma 

C 55 Ma 

................................. ······················· ·········· 

::::::::::::::::::::::::::·:::::::: 

·;2,;;;; :FARALLON::; ;; ·::::::::::;PLATE:;:::::: ........................... 

·························~······ ':: ?.~:::: :~~~ti~~~::::: ··················· 

.· .·. ·· .' ·· · ·.· ·. · · ·v· · · • ·.Seattle . · : · NO.RTH AMERI.CAN PLATE·. 

...... : . · .· ... ·. a:~~.uve~ ... ~=;:~.a~~ ·.: S~n .· .· ·: ... .' ·: .. ·. :· . : 

D 51 Ma 

E 42Ma 

. · Francisco · · · · · 

....... ........... ....... 

.... :::::::::::FARALLON: 

:A:.FI.:::::n~T~:::::::: ························ 

'·:~.F,.,z:.:::::::: .. ........... 

: .·. ·. v~nc~~~·e~·. ·.se~tt!~ · . .' ·NoR.IH ~~E~ICAN ~LA'r( . · . . · · • Portland · · · . · · · . · 

·Cape. ·. · · San· · . · ·. · ·. · · . 

Me~adoc;:ino. ·. ~ra11<:isc;:o · . G 

FARALLON 

:::PLATE::: 

................ 

formity with the bounding transforms by about 30 Ma 

probably began contemporaneously with the jump of the 

Kula-Pacific ridge about 55 Ma, continued through the 

change in spreading directions between the Pacific and 

Farallon plates at 53 to 50 Ma (anomalies 22 and 21), and 

was essentially complete before 30 Ma. 

The Earth's hot spots appear to constitute a fixed 

frame of reference by which absolute motion of the 

lithospheric plates may be defined (Minster and others, 

1974). The only significant shift in direction of movement of the Pacific plate relative to the Hawaiian hot 

spot during early and middle Tertiary time occurred 

about 42 Ma, when the Pacific plate veered from a 

northward to a northwestward path (Dalrymple and 

Clague, 1976). During the earlier part of the Tertiary, 

the Pacific plate was apparently holding steady on 

course. The initial contact between Pacific and North 

American plates, if, as suspected, it occurred simultaneously with the jump of the Kula-Pacific ridge about 55 

Ma, is detectable in the magnetic record of the ocean 

floor some 13 m.y. before the Pacific plate finally veered 

away to the northwest. The path change at 42 Ma probaEXPLANATION 

Subduction zone-Sawteeth on upper plate; 

dashed where approximately located 

Transform fault-Queried where uncertain. 

Arrows show relative movement 

=?= = Spreading ridge-Queried where approximately located 

FIGURE 14.-Late Cretaceous and early Tertiary plate geometry west 

of North American plate. Spreading directions shown by thin arrows, plate movements relative to a fixed North American plate 

by broad arrows. Projection is oblique Mercator as in figure 3. A, 

About 75 Ma, Kula plate was moving right laterally past North 

American plate, Farallon plate was being slowly subducted, and 

Pacific plate was drifting toward North American plate. B, By 56 

Ma, with movement continuing as in A, triple junction between 

Kula, Farallon, and North American plates had moved northwestward past "San Francisco." C, About 55 Ma, ridge between Kula 

and Pacific plates jumped northward, adding a large block of Kula 

plate to Pacific plate. D, About 51 Ma, spreading directions between Pacific and Farallon plates changed, causing reorientation 

of ridge system. Aja (A.F.Z.) and Sila (S.F.Z.) Fracture Zones 

originated as ridge-ridge transform faults at this time. Motion of 

Pacific and North American plates was convergent, causing a salient of more rigid oceanic crust to impinge on North American 

plate. E, As direction of movement between North American and 

Pacific plates shifted to right-lateral strike-slip about 42 Ma, 

wedge of Pacific plate that had impaled North American plate 

sheared off, and Pacific-Farallon ridge jumped northwestward to 

position shown by dashed lines. 
3.2 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

bly marks the end of the period of convergence and collision between the North American and Pacific plates. 

RECONCILIATION OF THE COLLISION HYPOTHESIS 

WITH OTHER TECTONIC FEATURES 

OF THE PACIFIC NORTHWEST 

The collision hypothesized here provides an explanation for several otherwise enigmatic structural features of probable Eocene age in Washington and adjacent areas. The Devils Mountain fault (fig. 9), for which 

Whetten (1978) has postulated 60 km of left-lateral displacement, may represent a tear fault formed during the 

early phase of the collision when plate paths were most 

convergent. The Straight Creek fault (fig. 9) and its 

probable continuation in Canada, the Hope fault, appear 

to be right-lateral strike-slip faults with displacement 

estimated at 200 km (120 mi) by Misch (1977). The trace 

of the fault is interrupted by younger plutonic rocks but 

appears to truncate the Devils Mountain fault zone and 

to truncate or merge with the Olympic-Wallowa lineament. The Straight Creek fault may have been an active 

strike-slip fault during the later phases of the collision 

as the paths of the North American and Pacific plates 

approached parallelism and the salient of oceanic crust 

forming the borderland was torn away from the Pacific 

plate. 

The eastern continuation of the Devils Mountain 

fault could be marked by the southeastern part of the 

Olympic-Wallowa lineament of Raisz (1945) (fig. 9), a 

zone of structural weakness marked chiefly by en echelon anticlines and associated reverse faults. According 

to Bentley (1977), these features formed by draping of 

Miocene and younger rocks over rotated basement 

blocks. Alternatively, the lineament may represent a 

pre-Miocene fault zone separating basement rocks of 

somewhat contrasting rheomorphic properties. By 

focusing regional stresses along this discontinuity, the 

fault has printed up through the cover of Miocene and 

younger rocks that are present along most of its extent. 

In British Columbia, the long period of volcanic 

quiescence that had begun in the Late J tirassic ended 

abruptly in the early Tertiary with widespread explosive eruption of lavas, mainly acidic to intermediate 

(Souther, 1970, p. 559). According to Souther, their 

eruption was accompanied by block foundering, northsouth block faulting, formation of large cauldron subsidence features, and emplacement of north-south dike 

swarms. Correlative volcanic rocks were erupted in 

northern Washington, particularly thick sequences accumulating in north-northeast-trending volcanotectonic 

depressions such as the Republic graben (Muessig, 1967, 

p. 95-96) and, Toroda Creek graben. (Pearson and Obradovich, 1977, pl. 1). Potassium-argon ages show that 

this episode of volcanism began about 53 Ma, climaxed 

about. 51-50 Ma, and continued with interruptions 

through 45 Ma (Mathews, 1964; Pearson and Obradovich, 1977). These volcanic rocks and the tensional 

features ass~ciated with them formed contemporaneously with the collision of the Pacific and North American plates postulated above. Because the formation of 

north-south tensional features is incompatible with eastwest compression, the impact area was probably entirely south of this area of volcanism. If it was, the 

oceanic plate immediately west of the volcanic field was 

probably decoupled from the Pacific plate during the collision. This suggests that rather than vanishing, the 

Kula-Pacific ridge jumped northward 5&-55 Ma, reestablishing itself southwest of the area of volcanism. Tensional features and the related volcanic rocks probably 

formed in response to the tearing effect of the right-lateral component of motion of the impinging Pacific plate. 

In Washington, the grabens and allied tensional features along with the associated volcanic and hypabyssal 

intrusive rocks have a north to north-northeast trend. 

The en echelon alinement of these volcanic features defines a diffuse, east-trending volcanic field that forms 

the western section of the so-called "Challis arc." The 

association of these volcanic rocks, at least those in the 

segment of the arc in Washington, with extensional features suggests that the arc originated through rifting, 

not through melting of a subducted slab. This suggestion is supported by the fact that in southern British 

Columbia and northeastern Washington, lavas and associated intrusive rocks near the base of the middle 

Eocene sequences are alkalic, including analcitic lava, 

extrusive rhomb prophyry, trachyte, phonolite, and 

trachyandesite (Daly, 1912, p. 98; Monger, 1968; 

Church, 1971; Fox, 1973). Though overlain by andesite, 

rhyodacite, and rhyolite, the alkalic rocks at the base 

give the sequence a compositional flavor more commonly 

associated with tension or rift-related volcanism than 

with the calc-alkaline andesitic volcanism of conventional island arcs. 

In southern Oregon, the early Eocene deformation 

of the Roseburg Formation associated with, or perhaps 

culminating in, the emplacement of an allochthon of the 

Colebrooke Schist cannot be readily explained by deformation related to the postulated transit in Campanian to 

Eocene time of the Humboldt-triple junction along the 

continental margin to the south. But the age of thrusting and related deformation (fig. 11) does roughly coincide with the change in spreading direction of the 

Pacific-Farallon ridge (53-50 Ma), and the rift-related 

volcanism to the north (53-45 Ma), may thus date extreme compression that occurred at the initial site of 

Pacific-North American plate contact and convergence. 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 33 

MELANGES OF THE OLYMPIC PENINSULA 

THE AJA FRACTURE ZONE AND THE FORMATION OF A 

HUMBOLDT-TYPE TRIPLE JUNCTION 

The magnetic lineations in the northeastern Pacific, 

as mapped by Naugler and Wageman (1973) and incorporated in figure 13, appear to step right laterally 

across the Aja fracture zone. The fracture zone and 

nearshore anomalies trend eastward into a disturbed 

zone within which the magnetic anomalies have apparently been obliterated by later heating (N augler and 

Wageman, 1973). Judged by the mapping, it is likely 

that north of the Aja fracture zone, the spreading ridge 

system and coexisting subduction zone shrank, then 

vanished about 21% Ma (fig. 13). 

A right-lateral transform, the ancestor of the Queen 

Charlotte fault, would at that time have intersected the 

Aja fracture zone and the still-active subduction zone 

along the continental shelf southeast of the Aja, forming 

a transform-transform-subduction zone triple junction. 

With northwest movement of the Pacific plate relative 

to the North American plate, this triple junction would 

necessarily be of the Humboldt-type. That triple junction would then persist through part of early Miocene 

time, finally dying as the ridge system south of the Aja 

fracture zone stepped eastward, intersecting the subduction zone. 

Judged by spreading rates and amount of right-lateral offset of the spreading ridge at the Aja fracture 

zone indicated by offset of the magnetic lineations, the 

Humboldt type triple junction would have persisted 

through about 5% m.y. (until about 16 Ma) before being 

succeeded by the stationary to slow-moving (Riddihough, 1977, p. 392-393) transform-spreading ridgesubduction zone triple junction that now forms the 

northern terminus of the Juan de Fuca ridge. The allochthonous deformation related to transit of the triple 

junction might then diminish gradually as the position of 

the leading edge of the subducted plate stabilized (fig. 

7C). 

ORIGIN OF MELANGES AND BROKEN FORMATIONS 

Melanges and other deformational features of the 

Olympic Peninsula were formed during at least two, and 

possibly three, episodes of deformation, the earliest in 

middle or late Eocene time (Snavely and Pearl, 1975) 

and the most recent in latest early and middle Miocene 

time. Potassium-argon ages from the rocks of the eastern core may record a third event at about 29 Ma 

(Tabor, 1972), somewhat later if those ages represent 

older material incompletely retrograded during 

metamorphism. 

The middle or late Eocene event evidently coincided with the initial impact of the Pacific and North 

American plates, which, as postulated in the preceding 

section, culminated in suturing of the block of oceanic 

crust now forming the basement of the OregonWashington borderland to the North American plate. 

As allochthonous deformation and formation of 

melanges or broken formation within and near the suture zone, particularly at the leading edge of the block, 

might be expected, no further explanation of the middle 

or late Eocene melanges seems required. 

The timing of the Humboldt-type triple junction 

formed by the intersection of the Aja transform with the 

Queen Charlotte fault and the subduction zone to the 

south coincides at least approximately with the time of 

formation of the latest early and middle Miocene 

melanges. In order to account for the structural fabric 

and geographic extent of these melanges through tectonism associated with this Humboldt-type triple junction, that triple junction at its inception about 211

/2 Ma 

would probably have been near the southern part of the 

Olympic melanges (fig. 15) and would have existed until 

it reached the approximate latitude of the northern part 

about 16 Ma. 

If this spatial relation is valid, the Olympic 

melanges and broken formations of Miocene age probably originated as follows. At about 21% Ma, the Aja 

fracture zone intersected the edge of the continental 

plate, forming a northwestward-migrating Humboldttype triple junction (figs. 15, 16). At this time the underthrusting part of the Juan de Fuca plate began incrementally extending itself northwestward along the continental plate margin at a rate commensurate with the 

northwestward slip of the Pacific plate. As it did, the 

underthrusting slab rammed, then plowed aside, the 

Queen Charlotte transform-fault-bounded lip of the continental plate, sending the contents of one or more deep 

marginal basins cascading toward the east in a catastrophic series of gravity slides and thrusts. These displaced masses bent the Crescent Volcanics and superjacent beds into the Olympic horseshoe, where, arrested 

by the restraining bulk of this barrier, they piled up at 

the present site of the Olympic Mountains. At about 16 

Ma, the spreading Juan de Fuca ridge again intersected 

the subduction zone (figs. 15, 16), and shortly thereafter 

the creation of melanges and broken formations of the 

Olympic Peninsula ceased. 

In examining the map of the northeastern Pacific 

(fig. 13), one observes other right-stepping fracture 

zones north of Aja; the northernmost of these persisted 

at least through the time of anomaly 10(?). Other such 

zones may once have existed farther north, but if they 

did, they have now been subducted beneath Alaska. As 

each of the active ridge-ridge transform faults whose 
34 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

existence the fracture zones record in turn intersected 

the Queen Charlotte fault, a Humboldt-type triple junction would form. The Olympic Peninsula or other areas 

immediately to the south could have been affected by 

tectonism associated with these triple junctions. As reconstructed by me, a Humboldt-type triple junction 

formed at the eastern end of the northernmost fracture 

zone shown on the map (fig. 13) about 29 Ma and was in 

existence for about 2 m.y. (fig. 11). 

Correlation of the Miocene allochthonous deformation of the Olympic Peninsula with transit of the Aja 

fracture zone, as postulated above, requires an average 

slip rate of about 6 crn/yr for the past 20 m.y. or so, comparable to that measured by Larson and others (1968) 

for the past 2 m.y. Were the average rate substantially 

1 00 200 300 400 

'Jo·w 

less, as postulated for example by Atwater and Molnar 

(1973), tectonism associated with transit of the Aja fracture zone between 21¥2 Ma and 16 Ma would be focused 

off the coast of British Columbia rather than the Olympic Peninsula. 

SAN ONOFRE BRECCIA 

The San Onofre Breccia of southern California may 

have originated through tectonism associated with the 

transit of an identifiable feature of the Pacific plate. 

Magnetic anomalies of the northeastern Pacific floor appear to be offset left laterally about 275 km on the 

Pioneer fracture zone (fig. 3). This relation implies that 

the segment of spreading ridge north of the Pioneer 

~ 16 Ma I.J.."'~·!. 

/ 

/ , 

, 

, ' ,/ 

I , 

I ' 

, 

1 -23112 Ma -i.l 

'<:("_.' 

//,' ~ , " 

,/,,,'' ~ 

EXPLANATION 

• • Subduction zone-Sawteeth on 

upper plate 

--- ::::::::; Fracture zone-Showing relative 

horizontal movement; dashed 

where inactive 

=== Spreading ridge 

-· 6 -- Magnetic anomaly 

--~ Transform fault-Dotted where submerged. Arrows show relative 

horizontal movement 

FIGURE 15.--Successive positions of Aja fracture zone relative to North American plate (arbitrarily considered fixed). About 23Vz Ma, a short 

segment of Pacific-Juan de Fuca ridge is still present north of Aja Fracture Zone but will vanish about 21 Vz Ma through continued spreading. About 16 Ma, ridge south of the Aja Fracture Zone first intersects plate margin. Humboldt-type triple junction that existed between 

21 Vz and 16 Ma probably created the Olympic melanges of early and middle Miocene age. Assuming that to be true, spatial relations between oceanic and continental features at 23Vz and 16 Ma shown above were adjusted to associate melanges with Humboldt-type triple junction. · 
MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 35 

fracture zone intersected the North American plate significantly later than did the segment of ridge to the 

south. 

Assuming that the half-spreading rate of about 4% 

cm/yr measured between anomalies 13 and 8 was maintained, the northern end of the spreading ridge between 

the Mendocino and Pioneer fracture zones probably intersected the North American plate after 27 Ma and was 

active at its southern end until after 25 Ma, some 5 to 6 

m.y. after initial contact of the Pacific and North American plates to the south. The spatial and temporal association of the San Onofre Breccia with the jump of the triple junction from the Pioneer to the Mendocino fracture 

zone (fig. 11) suggests that this jump might have initiated the catastrophic uplift and erosion of the source 

area of the San Onofre Breccia. That spatial and temporal association is admittedly less perfect than expected, however. In figure 11, the position of the San 

Onofre has been adjusted to compensate for the 315-km 

offset on the San Andreas fault, and the northwestern 

part of the deposit has been moved 160 km southeast to 

restore movement postulated by Stuart (1976) on the 

East Santa Cruz Basin fault system. With these adjustments, and given a rate of offset between the Pacific and 

North American plates of 6 cm/yr (as in fig. 11), the San 

Onofre appears to be displaced about 200 km farther 

northwest than expected. Were the rate only 5% crnlyr 

as calculated by Minster and others (1974), the discrepancy would be about 100 km. This amount can probably 

be accounted for by the displacement between the 

A 

FIGURE 16.-Views to north showing defonnation and generation of melanges and broken fonnations at present site of Olympic Peninsula in 

early and middle Miocene time. A, In early Miocene, Aja transfonn fault has intersected margin of North American plate, fonning a Humboldt-type triple junction. North side of the Juan de Fuca plate, defined by Aja transfonn at surface and by its subducted extension in subsurface, follows northwestward movement of Pacific plate, causing north side of subducted part of Juan de Fuca plate to impinge on North 

American plate at their interface in subsurface (toward direction indicated by broad arrow). North American plate is compressed and buckles upward near triple junction and downward along a curving axis to north and east. Speculatively, a curving reverse fault fonns at inflection in North American plate north and east of this curving downfold. B, In latest early Miocene, continuing impingement of Juan de Fuca 

plate drives a segment of North American plate landward and beneath main part of this plate. At triple junction, lip of North American 

plate buckles upward, and upper part of this uplifted mass sloughs away to north and east in a series of catastrophic gravity slides and 

thrust plates, fonning melange and broken fonnation. These sheets of displaced material pile up within curving downfold where they are 

further defonned through compression against overriding and upfaulted edge of North American plate to north and east (Olympic "horseshoe" of Cady, 1975). Lower parts of this pile are sufficiently heated through reestablishment of nonnal geothennal gradient to be weakly 

metamorphosed. 
36 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 

Pacific and North American plates that was distributed 

across faults outside the San Andreas fault system. For 

example, Thompson and Burke (1973) estimated that N. 

55° W. extension across the entire Basin and Range province is about 100 km. Hence it is likely that the total 

displacement between the San Onofre Breccia and the 

North American plate east of the Basin and Range province substantially exceeds the 315 km allowed for offset 

on the San Andreas fault, and that the outcrop of the 

San Onofre correlates more perfectly with the PioneerMendocino triple-junction jump than is shown on figure 

11. 

CONCLUSIONS 

Latest Cretaceous and Tertiary melanges of the 

North American plate apparently can be correlated in 

both time and location with triple junctions that migrated along the western edge of this plate. The existence of 

triple junctions of the required dynamic type, the Humboldt-type triple junctions of Fox (1976), is implied by 

the magnetic record of crustal formation preserved in 

the ocean floor. The temporal and also the spatial correlation is required if the rate of movement of the North 

American plate with respect to the Pacific plate has averaged roughly 6 cm/yr for the past 21% m. y. and approached that figure for the entire Tertiary. 

Atwater and Molnar (1973) have calculated that this 

rate averaged only 2 cm/yr between 38 Ma and 10 Ma. 

The discrepancy between this rate and the 6- cm/yr rate 

invoked to correlate the Miocene melanges of the Olympic Peninsula with the transit of the causal triple junction must be resolved before a genetic link between 

melanges and triple junctions can be accepted with complete confidence. Direct correlation of the melanges of 

the Olympic Peninsula with what are presumed to be 

the causal plate-tectonic features suggests that plate 

movements have been continuous and the rate of rotation roughly constant from at least earliest Miocene time 

to the present. 

If the melanges of the Franciscan Complex and 

Olympic Peninsula did form at migrating triple junctions, the ages of these melanges, together with the 

paleomagnetic record preserved in the ocean floor, 

suggest the following scenario: 

(1) In latest Cretaceous to early Eocene time, a triple junction between the Kula, Farallon, and North 

American plates migrated northwest:ward along the 

edge of the North American plate at about 4 cm/yr. 

(2) In early Eocene time, the southern part of the 

Kula plate broke away and was added to the Pacific 

plate through a northward jump of the Kula-Pacific 

spreading ridge. 

(3) During middle through late Eocene time, the 

convergent motion of the obliquely colliding Pacific and 

North American plates was arrested as fragments of the 

Pacific plate were sheared off, rotated clockwise, and 

sutured to the North American plate, forming the allochthonous Oregon-Washington borderland. 

(4) In latest Oligocene to middle Miocene time, 

right-stepping ridge-ridge transform faults, including 

the Aja, intersected the Queen Charlotte fault, forming 

Humboldt-type triple junctions west of the present site 

of the Olympic Peninsula. At these triple junctions, the 

convergence of the subducted part of the oceanic plate 

on the south and the transform-bounded part of the 

North American plate on the north culminated in the 

formation of the melanges and broken formations of the 

Olympic Peninsula. 

Latest Cretaceous to early Tertiary transit of the 

Kula-Farallon-North American triple junction apparently introduced a subduction-dominated tectonic regime that was to persist for at least 30 m.y. along the 

coast of California and to the present time along the 

coast of Oregon and Washington. Were melanges produced as part of the subduction process, melanges representing that age and age span should be widespread. 

Their general absence, except for the Olympic Peninsula, and the prevalence instead of melanges dating 

from the time of the postulated transition from strikeslip to subduction-dominated tectonism suggest that 

these melanges originated near the migrating KulaFarallon-North American triple junction. Corroboration 

or refutation of this hypothesis should be sought 

through refinement of the age(s) of the melanges of the 

Franciscan Complex, through reevaluation of the contact relations of the Franciscan to the patches of Great 

Valley-like rocks, and through further consideration of 

the origin of blueschist. 

The allochthonous Oregon-Washington borderland 

with its varyingly rotated blocks of oceanic crust seems 

to have originated through a glancing collision of the 

Pacific and North American plates. Many otherwise 

obscure tectonic features of the Pacific Northwest, including the allochthon of Colebrooke Schist, the Olympic-Wallowa lineament, the extensional volcanogenic 

grabens and related features of northern Washington, 

and, lastly, the allochthoneity of the borderland itself 

can be rationalized by this hypothesis. Such a collision, 

however, implies that the paths of the North American 

and Pacific plates were at least slightly convergent prior 

to late Eocene. Confirmation of this corollary should be 

sought through mapping of the tracks of hotspots and 

through determination of paleomagnetic poles for the 

Paleocene, Eocene, and Oligocene. 

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