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) I 

Plate Motions Recorded in Tectonostratigraphic 

Terranes of the Franciscan Complex and 

Evolution of the Mendocino Triple Junction, 

Northwestern California 

By R.j. McLAUGHLIN, W.V. SLITER, N.O. FREDERIKSEN, 

W.P. HARBERT, and D.S. McCULLOCH 

U.S. GEOLOGICAL SURVEY BULLETIN 1997 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

Text edited by George A. Havach 

Illustrations edited by Taryn A. lindquist 

, I 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 

For sale by 

Book and Open-File Report Sales 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Plate motions recorded in tectonostratigraphic terranes of the Franciscan complex 

and evolution of the Mendocino triple junction, northwestern California/ by 

R.J. Mclaughlin ... [et al.]. 

p. em. - (U.S. Geological Survey bulletin ; 1997) 

Includes bibliographical references. 

1. Plate tectonics-California. Northern. 2. Geology, StructuralCalifornia, Northern. I. Mclaughlin, Robert E. II. Series. 

QE75.B9 no. 1997 

[QE511.4] 

557.3 s---dc20 

[551.1 '36'09794] 92-23140 

CIP 
CONTENTS 

Introduction 1 

Tectonostratigraphic· framework of the Cape Mendocino. region and adjacent northern 

Coast Ranges 2 

Franciscan Central belt and its contact relations with the Coastal Belt 2 

Yager terrane of the Coastal Belt 3 

Age and paleolatitudinal setting 1 0 

Deformation 12 

Paleomagnetic studies 13 

Coastal terrane of the Coastal Belt 13 

Sandstone, shale, and conglomerate 13 

Basaltic rocks 14 

Limestone 19 

Carbonate concretions and thin carbonate beds 19 

Pelagic limestone 19 

Blueschist 19 

Age of the Coastal terrane 22 

Age of the carbonate concretions associated with argillitic rocks 22 

Age and depositional setting of the pelagic limestone associated 

with basaltic rocks 25 

Paleomagnetic data 27 

Structure 29 

Implications of structure for hydrocarbon exploration 30 

Accretion of the Coastal terrane 30 

King Range terrane of the Coastal Belt 34 

Composition and structural framework 36 

Point Delgada subterrane 36 

King Peak subterrane 36 

King Range terrane thermal anomaly 38 

Vizcaino structural block 38 

Accretion of the Vizcaino structural block to the Pacific plate and initiation of the 

San Andreas transform system 43 

Models for accretion of the King Range terrane and their relation to propagation 

of the San Andreas transform 47 

Proximal-ridge and leaky-fracture-zone models 47 

San Andreas transform model 48 

Structural relations of the modern Mendocino triple junction 51 

Faults defining the configuration of the Triple junction 51 

Seismicity 54 

Quaternary uplift 54 

Conclusions 54 

References cited 56 

FIGURES 

1. Index map showing locations of geologic and geographic features mentioned in 

text 4 

2. Geologic map of Cape Mendocino region 7 

3. Geologic map of Central belt of the Franciscan Complex near Garberville 8 

4. Photographs showing sedimentary rocks of the Yager terrane 9 

5. Photographs showing folds in sedimentary rocks of the Yager terrane 12 

6. Photographs showing sedimentary rocks of the Coastal terrane 14 

Contents Ill 
7. Detailed geologic map of part of the Coastal terrane near Usal 16 

8. Photographs showing volcanic rocks and interbedded arkosic sandstone of the 

Coastal terrane in Dugans Opening area near Usal 17_ 

9. Plots of major-element chemistry of volcanic rocks from terranes of the Coastal 

belt, in comparison with other volcanic rocks 20 

10. Plots of minor- and trace-element chemistry of volcanic rocks of the Coastal 

terrane and from Deep Sea Drilling Project Site 173 24 

11. Photographs showing melange of the Coastal terrane south of Cape 

Mendocino 25 

12. Photomicrographs of pelagic limestone of the Coastal terrane from Dugans 

Opening near Usal 26 

13. Photomicrograph of pelagic foraminiferal limestone of the Coastal terrane near 

Parkhurst Ridge 26 

14. Detailed geologic map of Coastal terrane at Parkhurst Ridge near Honeydew 28 

15. Photograph showing pelagic foraminiferal limestone overlying basalt of Coastal 

terrane, located at Parkhurst Ridge 29 

16. Photograph showing boudined sandstone in penetratively sheared argillite in 

melange of the Coastal terrane 29 

17. Maps illustrating structure of the Cape Mendocino area 31 

18. Diagrams illustrating plate-tectonic model in a fixed-hotspot reference frame for 

accretion history of the Coastal terrane 32 

19. Diagrams illustrating accretion history of pelagic limestone and sedimentary rocks 

of the Coastal terrane 33 

20. Photographs showing rocks of the King Range terrane 34 

21. Photograph showing pillow basalt of the Point Delgada subterrane 36 

22. Photographs showing folded rocks of the King Peak subterrane 38 

23. Plot of contoured vitrinite-reflectance values and inferred paleotemperatures of 

metasedimentary rocks of the Franciscan Complex in the Cape Mendocino 

region 39 

24. Map of offshore area south of the Mendocino Fracture Zone 41 

25. Diagram illustrating plate-tectonic model for early Miocene initiation of the 

modem San Andreas transform 44 

26. Diagram illustrating model for northward migration of tpe Mendocino Fracture 

Zone and propagation of the San Andreas transform fault 46 

27. Diagram illustrating model for origin of the King Range terrane, assuming little or 

no latitudinal translation 48 

28. Diagram illustrating alternative model for origin of the ~ing Range terrane, 

assuming large-scale translation with the Pacific plate 49 

29. Maps of the Mendocino triple junction area 53 

TABLES 

1. Pollen in rock samples from the Coastal and Yager terranes 11 

2. Major-, minor-, and trace-element chemistry of basaltic :rocks of the Coastal belt 

and of other selected igneous rocks along the northern <California margin 22 

3. Radiolarians and planktonic foraminifers from the King: Range terrane and the 

lower part of the drill hole at Deep Sea Drilling Project' Site 173 42 

IV Contents 
Plate Motions Recorded in Tectonostratigraphic Terranes 

of the Franciscan Complex and Evolution of the 

Mendocino Triple Junction, Northwestern California 

By R.J. Mclaughlin, W.V. Sliter, N.O. Frederiksen, W.P. Harbert, and D.S. McCulloch 

Abstract 

The Mendocino triple junction area of northern California 

is underlain by the Coastal belt of the Franciscan Complex, 

flanked on the east by the Central and Eastern belts of the 

Franciscan Complex. The Coastal belt is further divided into 

three tectonostratigraphic terranes, from northeast to southwest, the Yager, Coastal, and King Range terranes. Upper 

Cretaceous through middle Miocene rocks included in these 

terranes were accreted to the North American plate margin 

partly during normal convergence with the Farallon plate between 49 and 25 Ma at poleward rates of 2 to 5 cm/yr (Coastal and Yager terranes), and partly during translation with the 

Pacific plate between 14 and 2 Ma at poleward rates of 3 to 

6 cm/yr (King Range terrane). Except for sparse blocks of Upper Cretaceous limestone and basalt scraped off the Farallon 

plate, as well as rare blocks of blueschist of uncertain origin, 

the Coastal belt terranes are composed principally of Paleocene to Eocene arc- and continental-margin-derived turbiditic 

detritus. Midlatitudinal affinities of foraminifers and palynomorphs in the Coastal and Yager terranes, and the presence of 

an Eocene conglomerate containing clasts of jadeitic and lawsonitic metasandstone, radiolarian chert, and serpentinite, 

suggest no more than about 600 km of right-lateral postdepositional displacement of the clastic rocks. Furthermore, the 

presence of arc-derived turbidites with interbedded Late Cretaceous ocean-ridge or seamount basalt near Usal, Calif., suggests access of sediment derived from the American plate to 

the Pacific-Farallon Ridge area sometime between 82 and 

69 Ma. Uplift of the California convergent margin during the 

middle Miocene (ca. 15 Ma) resulted in overlap of the Franciscan rocks already accreted to the margin by shelf and slope 

deposits. These overlap deposits, in turn, were downfaulted 

later in the Neogene to form the Humboldt forearc and related smaller basins. Until middle to late Miocene time, strata of 

the King Range terrane were deposited in a trench-slope or 

slope setting along the Farallon-North American plate boundary, possibly as far as 435 km south of Cape Mendocino. A 

Manuscript approved for publication, May 17, 1991. 

13.8-Ma metalliferous hydrothermal system, as well as evidence indicating that the accompanying thermal anomaly affected the entire King Range terrane, suggests that these rocks 

were rifted from the California margin about 14 Ma, over a 

slabless window (slab gap) at the north end of the San Andreas transform, and accreted to the northeast side of the 

Vizcaino structural block of the Pacific plate. The King Range 

terrane then was translated northward with the Mendocino 

triple junction and obductively reaccreted to North America 

during the early Pleistocene. Compressional tectonics at the 

northwestern California margin since about 3 Ma accounts for 

active uplift and thrusting near Cape Mendocino, compression 

of earlier-formed, Neogene pullapart basins, and the absence 

of Coast Range volcanism north of Clear Lake. Offshore and 

onshore structural relations suggest that the present-day Mendocino triple junction is situated near the hamlet of Petrolia, 

Calif., north of the King Range terrane, rather than off shore, 

where it has traditionally been located. The San Andreas fault 

zone thus may include the east and north boundaries of the 

King Range terrane. 

INTRODUCTION 

This report describes the Late Cretaceous to late Cenozoic evolution of the tectonostratigraphic terranes surrounding the Mendocino triple junction. We focus on 

determining the structure and boundary relations of the 

various tectonostratigraphic terranes of the Franciscan 

Complex, their ages, formational settings, amounts of displacement, and the timing and mechanisms of their accretion to the California margin. We attempt here to relate the 

evolution of terrane accretion to recent models of relative 

motions between plates in the adjacent Pacific Basin during the Mesozoic and Cenozoic (Engebretson and others, 

1985; Tarduno and others, 1986; Stock and Molnar, 1988). 

Our reconstructions indicate that although a convergent 

subduction setting dominated the northern California accretionary margin during early Cenozoic time, major dextral motion occurred during the Late Cretaceous and 

earliest Tertiary and, to a lesser extent, also contributed to 

Introduction 
the motion between rocks of the Franciscan Complex and 

North America during Paleogene time. 

Furthermore, our investigations suggest that the King 

Range terrane of the Franciscan Complex was accreted to 

the California margin during late Cenozoic time after undergoing dextral displacement of about 435 km within the 

San Andreas fault system. Structures associated with the 

presently active Mendocino triple junction reflect northeast-southwest-oriented compression between the North 

American and Pacific plates. 

Acknowledgments.- Our investigations in the Mendocino triple junction region have involved cooperation, exchanges of data and ideas, and discussions with many 

professional colleagues, over a timespan of at least 8 years. 

The senior author has greatly benefited from the work and 

insights of the following geologists: E. C. Beutner (Franklin 

and Marshall College, Lancaster, Pa.); K.R. Lajoie, S.D. 

Ellen, and S.H. Clarke (U.S. Geological Survey); G.A. 

Carver, K.R. Aalto, and S.D. Morrison (Humboldt State 

University, Arcata, Calif.); and M.B. Underwood (University of Missouri, Columbia, Md.). D.E. Champion and R.E. 

Wells (U.S. Geological Survey), D.C. Engebretson and 

S.G. Miller (University of Washington, Bellingham) and 

R. Sharps (Stanford University, Stanford, Calif.) generously 

·provided their time and expertise to paleomagnetic pilot 

studies in the region, and A. Griscom (U.S. Geological 

Survey) has been helpful in providing insights into the 

deep-crustal structure beneath the triple-junction area from 

aeromagnetic data. In addition to the paleontologic contributions of the coauthors of this report, we acknowledge 

additional support from the following paleontologists: J.A. 

Barron, C.D. Blome, B.L. Murchey, K. McDougall, and 

R.Z. Poore (U.S. Geological Survey); W.R. Evitt (Stanford 

University, Stanford, Calif.); K.D. Berry (Chevron Oil 

Co.); B. Roth (California Academy of Sciences, San Francisco, Calif.); R. Feldman (Kent State University, Kent, 

Ohio); S.A. Kling (Micropaleo Co., Encinitas, Calif.); and 

G. Keller (Princeton University, Princeton, N.J.). Numerous 

property owners have allowed us access to their land in the 

course of this project; without their support, this study 

would have been impossible. In particular, we thank the 

following individuals for their continued cooperation and 

interest in our work: Mary and Mike Etter, Jan Smith, and 

the Chambers family of the Mattole Valley and coastal area; 

and Thornton Smith of Harris, Calif. We also thank the 

Georgia Pacific Lumber Co. for permitting access to their 

land near U sal. 

TECTONOSTRATIGRAPHIC FRAMEWORK 

OF THE CAPE MENDOCINO REGION 

AND ADJACENT NORTHERN COAST RANGES 

The basement rocks of the Cape Mendocino region 

are assigned to the Franciscan Complex. In northern California, the Franciscan Complex is divided into three broad 

belts, the Eastern, Central, and Coastal (fig. 1 ), which generally become younger and less metamorphosed from east 

to west and have different structural relations to adjacent 

rocks (Irwin, 1960; Blake and others, 1985, 1988). Within 

each of these belts, numerous tectonostratigraphic terranes 

have, been mapped (Blake and others, 1982, 1985; 

McLaughlin and Ohlin, 1984). In this report, we are concerned primarily with the terranes and tectonostratigraphic 

framework of the Coastal belt. 

We here s~bdivide the Coastal belt into three faultbounded terranes that overlap in age and composition, 

from northeast to southwest: the Yager terrane of Paleocene(?) and Eocene age; the Coastal terrane of Late Cretaceous, Paleocene, and Eocene age; and the King Range 

terrane of Late ·Cretaceous, possibly Paleogene, and middle Miocene age (fig. 2). 

Franciscan Central Belt and its 

Contact Relations with the Coastal Belt 

Rocks of the Franciscan Central belt in the Cape Mendocino region compose a tectonic melange that consists of 

numerous tectonostratigraphic terranes enclosed in a penetratively sheareq argillite matrix metamorphosed to at least 

pumpellyite, possibly as high as low blueschist, grade. In 

addition to enclosing the tectonostratigraphic terranes, this 

melange matrix also encloses outcrop-size to larger exotic 

blocks of glaucophane schist, eclogite, garnet amphibolite, 

and other rocks of high-temperature and high-pressure 

metamorphic-mineral assemblages. The terranes enclosed 

by the melange matrix, though diverse, are generally separable into two types. The first type consists chiefly of sandstone and argillite, representing remnants of disrupted 

deep-sea-fan, sl9pe, and trench-slope depositional settings. 

The second typ~ consists of ultramafic and mafic intrusive 

rocks, basaltic ~ows, flow breccias and tuffs, radiolarian 

chert, and pelagic limestone, representing various oceanic 

settings. Some bf the terranes are composites of oceanic 

rocks overlain by terrigenous rocks. 

Some of th~se oceanic or composite oceanic and terrigenous assemblages are inferred to be fragments of the 

Coast Range ophiolite and Great Valley sequence displaced from the west side of the Great Valley into the 

Central belt of the Franciscan Complex during an episode 

of dextral translation in Late Cretaceous to Paleocene time 

(McLaughlin and others, 1988a). Paleomagnetic and paleontologic data strongly suggest that other oceanic assemblages associated with pelagic limestone and radiolarian 

chert, such as the Calera Limestone and Laytonville Limestone (informal usage of Alvarez and others, 1980), and 

rocks of the Nicasio Reservoir, Marin Headlands, and The 

Geysers basalt-chert localities, were deposited near the 

Equator and translated northward on one or more plates of 

2 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
the late Mesozoic to early Cenozoic Pacific Basin (Alvarez and others, 1980; Gromme, 1984; Tarduno and others, 

1985, 1986; Hagstrum and Murchey, 1991). 

The various sandstone-argillite terranes of the Central 

belt vary widely in age (from Late Jurassic to late Late 

Cretaceous) and composition (from lithic-volcanic and chert 

rich to arkosic). The sandstone terranes also vary considerably in their degree of metamorphism. Numerous large slabs 

of metasandstone entrained in the eastern part of the Central 

belt, which locally are interleaved with chert and mafic 

igneous rocks and contain blueschist metamorphic-mineral 

assemblages, are correlative with the Eastern belt of the 

Franciscan Complex (McLaughlin and others, 1988a). Other 

arkosic to lithic sandstone units, generally of mid-Cretaceous to Late Cretaceous age, containing incipient prehnitepumpellyite or pumpellyite-lawsonite metamorphic-mineral 

assemblages, are of uncertain relation to rocks in the Eastern 

belt. Some large sandstone-argillite terranes of the southwestern part of the Central belt are metamorphosed to laumontite-prehnite-bearing zeolite assemblages and consist of 

lithic to arkosic sandstone containing abundant detrital Kfeldspar. These K-feldspar-bearing, zeolite-grade sandstone 

terranes are generally mid-Cretaceous or Late Cretaceous in 

age and, except for their older ages, resemble the sandstone 

terranes in the Coastal belt to the west. 

The west boundary of the Central belt is a shallowly 

to steeply east dipping thrust fault. Movement on this fault 

has resulted in emplacement of the Central belt over rocks 

of the Coastal belt (fig. 3). This thrust fault is generally 

known as the Coastal belt thrust (Jones and others, 1978), 

although the names "Eel River fault" or "Garberville 

thrust" have been proposed for this feature in the vicinity 

of Garberville (Underwood, 1982; Bachman and others, 

1984). The Coastal belt thrust is a nearly flat contact that 

is warped into a broad southeast-plunging synform and antiform (figs. 2, 3). Overridden rocks of the Yager terrane 

of the Coastal belt contain fossils as young as late Eocene 

and are complexly folded, indicating that emplacement of 

the Central belt over the Coastal belt occurred after late 

Eocene time and either accompanied or postdated the folding. Mildly folded, middle Miocene to Pliocene marine 

strata overlap the Coastal belt thrust on the northeast side 

of the Eel River basin and in the vicinity of Garberville, 

and rocks of early Miocene age (Chetelat and Ingle, 1987) 

occur at the base of the postaccretionary Wildcat Group 

along the coast south of Centerville Beach. These constraints argue that displacement of the Central belt along 

the Coastal belt thrust must have occurred largely during 

the Oligocene, when a marine depositional hiatus occurred 

in the north coastal region. 

Northeast of Arcata, zircon from small trachytic alkalic intrusions have been dated (Meyer and Naeser, 1970) at 

about 35.3±2.0 Ma (Fickle Hill) and 36.0±0.9 Ma (intrusion north of Fickle Hill) by fission-track techniques 

(fig. 2). Somewhat northeast of these localities, at Coyote 

Peak, the Central belt is intruded by another Oligocene, 

diatreme-like alkalic intrusion (Morgan and others, 1985). 

These Oligocene volcanic rocks apparently intruded the 

Central belt concurrently with or after emplacement of the 

Central belt over the Yager terrane along the Coastal belt 

thrust. Broad, northwest-trending folds that deform the upper and lower plates of the Coastal belt thrust probably 

were initiated during or after the late Miocene. 

Yager Terrane of the Coastal Belt 

Rocks of the Yager terrane were initially named the 

Yager Formation by Ogle (1953), who believed them to be 

Late Cretaceous in age. More recent investigations (Evitt 

and Pierce, 1975; Underwood, 1983; McLaughlin and others, 1985a), however, have shown these rocks to be of early Tertiary age. The strata are highly deformed by folding, 

and the terrane is fault bounded. Because of its complex 

structure, we assign the Yager terrane to the Franciscan 

Complex (Underwood, 1983; Blake and others, 1985; 

McLaughlin and others, 1985a) as a tectonostratigraphic 

terrane of the Coastal belt. 

The Yager terrane is composed largely of silty mudstone, sandstone, and conglomerate. Mudstone and thinbedded sandstone generally dominate the northeastern part 

of this terrane (fig. 4A). Thick-bedded channelized sandstone is most abundant along the southwest and northeast 

sides of the terrane, although it also locally dominates the 

section in other areas. 

Conglomerates of the Yager terrane are polymict, containing generally subrounded and rounded clasts as large 

as boulders, composed chiefly of resistant rock types. 

These resistant clasts consist of silicic, tuffaceous, and 

porphyritic volcanic rocks and shallow-intrusive rocks; 

black, green, and red felsite and chert; and quartzite. The 

remaining clast component includes lesser amounts of 

mafic igneous rocks, metavolcanic rocks, reworked metasandstone, and autoclastic argillite. Locally, some of the 

chert and, possibly, some metasandstone clasts may be reworked from rocks of the Ce~tral belt of the Franciscan 

Complex. The conglomerates occur as lens-shaped channels, as much as several tens of meters thick and more 

than a kilometer long, along the west side of the Yager 

terrane between Bull Creek and Honeydew (fig .. 4B). 

Sandstone of the Yager terrane is predominantly arkosic 

to feldspathic; it is locally lithic, containing abundant clasts 

of chert or mudstone autoclasts. Chloritized detrital biotite 

commonly delineates ripple-drift laminations. After felsite 

and silicic volcanic-rock fragments, biotite and pink to 

white muscovite are the most common detrital accessories, 

followed by epidote and clinozoisite, zircon, and garnet. 

The sandstone beds range in thickness from thin-bedded 

flysch with abundant fine-scale sedimentary structures, to 

channeled units as much as 167 m thick near Cummings. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 3 
.,. 

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EXPLANATION FOR FIGURE lA 

.-~-----.1 Exposed Mesozoic and younger rocks that 

were accreted to continental margin 

before Eocene time 

__;;;;!::;_ Fault or fracture zone-Arrows indicate 

.......-- direction of relative movement 

....... 

I 

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45° 

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Figure 1. Index maps showing locations of geologic and geographic features mentioned in text. A, 

Tectonic features along coast of North America. FZ, fracture zone. 8, Major geologic features in 

California. Faults dotted where concealed by water. CL, Calera Limestone; CM, Cape Mendocino; 

CP, Coyote Peak; ERP, Eagle Rest Peak; HCF, Hayward-Calaveras fault; HSGF, Hosgri-San Gregorio 

fault; LL, Laytonville Limestone; LQ, Logan Quarry; NV, Neenach Volcanics; PF, Pilarcitos fault; 

PMB, Page Mill Basalt; PV, Pinnacles Volcanics; QSV, Quien Sabe Volcanics; TF, Tolay fault, TV, 

Tolay Volcanics. C, Cape Mendocino area. 
124°30' 

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Figure 1.-Conti nued 

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Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 5 
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PACIFIC PLATE ........: 

16.2-Ma basalt of Cooskie Creek 
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EXPLANATION 

i\:~· :· /: :/\'] Late Cenozoic overlap assemblage (Pleistocene 

to early Miocene) 

E3 

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Franciscan ComplexCoastal beltKing Range terrane40° 

00' 

King Peak subterrane (middle Miocene and early 

Tertiary?) 

Point Delgada subterrane (Late Cretaceous) 

Coastal terrane (Late Eocene to Late Cretaceous) 

Volcanic rocks (Late Cretaceous) 

Yager terrane (late Eocene to Paleocene?) 

Central belt (early Eocene? to Early Jurassic) 

Trachyte of Fickle Hill and Granite Butte (Oligocene) 

Contact-Dotted where concealed 

Western boundary of mapping of rock units offshore 

Fault, late Cenozoic activity-Dashed where inferred; 

queried where uncertain. Sense of slip unspecified 

except where arrows show direction of relative 

movement 

Thrust fault, Pliocene or younger activity-Dashed where 

inferred; queried where uncertain. Sawteeth on 

upper plate 

Thrust fault, inactive since early Miocene-Sawteeth on 

upper plate 

Basal thrust fault of Cascadia subduction zone-Dashed 

where inferred; queried where uncertain. Sawteeth 

on upper plate 

Fold axis in offshore area-Dashed where inferred 

Blueschist block 

....... ;:) , . . , 

\ 

0 10 20 KILOMETERS 

~ ~ Branscomb~~ 

EK 

Radiolarian 

chert-pebble 

' conglomerate 

at base of 

' ',vagerterrane 

Paleontologic localities-Age abbreviations: P, Pliocene; LM, late Miocene; 

MM, middle Miocene; E, late and middle Eocene; EK, Early Cretaceous; EP, early Eocene or late Paleocene; K, Late Cretaceous 

(Maastrichtian to Campanian) 

• LM Foraminifer • E Dinoflagellate * K Radiolarian 

p EP 

1111.: Mollusk ~ Spore; pollen • MM . . . fl Dwtom; s1hco agellate 

* E Crab fragment 

Figure 2. Geologic map of Cape Mendocino region, showing major tectonostratigraphic terranes, distribution of paleontologic sample localities, 

and locations of radiometrically dated rocks discussed in text. Offshore geology from McCulloch (1987a) and Clarke and Field (1989). FZ, fault 

zone. See figure 1 8 for location. 
124°00' 

40° 

30' 

40° 

15' 

\ 

Tpm 

COASTAL 

BELT, 

UNDIVIDED 

m 

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EXPLANATION 

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'-'"" lt... \\ '-~ feb m 

qf) ',, m 

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I Tpm I Marine overlap deposits (Pliocene and Miocene) ~ Metasandstone derived from Eastern belt 

of Franciscan Complex-Locally 

includes minor metabasalt and metachert Central belt of Franciscan Complex (early 

Eocene? to early Jurassic}-Consists of: 

Melange -Includes blocks of limestone, sandstone, 

chert, basaltic rocks, and high-grade blueschist, 

all mixed in argillite matrix. Locally, matrix 

includes large blocks, here divided into: 

Sandstone containing less than 2 percent 

K-feldspar 

Sandstone containing more than 2 percent 

K-feldspar-Locally laumontitic 

Basaltic rocks-Locally includes lenses of chert 

Rocks of Coast Range ophiolite (Jurassic) 

apd associated overlying strata of Great 

Valley sequence (Late Jurassic and Early 

Cretaceous) 

Contact-'--Dashed where inferred 

Fault-Dashed where inferred 

.......... Thrust fault-Sawteeth on upper plate 

Figure 3. Geologic map of Central belt of the Franciscan Complex near Garberville, illustrating contact relations 

between Central and Coastal belts. Geology mapped by R.j. Mclaughlin, S.D. Ellen, A.S. Jayko, and M.C. Blake, 

Jr., 1982-85. See figure 1 C for location. 

8 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
Figure 4. Sedimentary rocks of Yager terrane. A, Massive argillite intercalated with thinly laminated, fine-grained sandstones. Beds show fine-scale dislocation or slump features 

and ripple-drift structure. Beds are right side up. B, Rounded 

polymict conglomerate in stacked channels, located east of 

Honeydew. Note flat tops and irregular, tapered lower contacts of channels, which are marked by load features. C, 

Well-bedded, upside-down argillite and sandstone, showing 

conspicuous crossbedding and load features. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 9 
The channeled sequences commonly display upsectiop 

thickening (Piercy-Leggett area) and upsection thinning 

(Cummings area). The sandstone turbidites commonly exhibit fine-scale ripple-drift crosslaminations, as well as convolute or traction structures near the tops of beds indicative 

of downslope creep (fig. 4C). 

The Yager terrane is thought to have been deposited in 

a channeled-slope setting or in a series of slope basins (Underwood, 1983; Bachman and others, 1984). The presence 

of conglomerate, stacked sandstone channels, and overbankchannel-levee deposits in many areas suggests the p~oximity 

of a major sediment-distributary system. Underwood and 

Bachman (1986) proposed a mixed provenance for the Yager 

sandstones, including granitic-metamorphi~ sources in the 

Idaho batholith, a quartz-rich Sierra Nevada plutonic source, 

recycled detritus from an orogenic backarc area, volcanic 

detritus from an active Paleogene magmatic arc, and recycled Franciscan detritus. 

The Yager terrane is bounded by both low-angle and 

steep faults that separate it from the Coastal terrane on the 

west and the Central belt on the east. Differences in the 

age, lithology, metamorphism, and structural state of rocks 

across the Coastal belt thrust suggest that significant shortening has occurred between the Central belt and Yager terrane across this fault. Although sandstones of the Coastal 

terrane vary more widely in composition, compositional 

similarities to sandstones of the Yager terrane make it difficult to evaluate the magnitude of displacement along the 

Yager-Coastal terrane boundary; indeed, this displacement 

may be relatively minor. Underwood (1983) suggested that 

sandstones of the Coastal terrane that are similar in composition and appearance. to those of the Yager terrane may 

have been deposited in distributary channels which carried 

Yager sediment across the continental slope, into the lower-slope and trench-slope areas, where sandstones of the 

Coastal terrane were being deposited and deformed. 

Age and Paleolatitudinal Setting 

The ages of rocks of the Yager terrane are based almost 

entirely on fossil dinoflagellates and pollen and spores. 

Many of the dinoflagellate localities within the Yager terrane 

were reported on by Evitt and Pierce (1975) and Damassa 

(1979). We have since collected from additional localities 

for this and related studies (for example, McLaughlin and 

others, 1982; Sliter and others, 1986). We have, in addition, 

separated and dated radiolarians from green chert pebbles in 

a conglomerate near the easternmost exposed part of the 

Yager terrane west of Laytonville, and collected fragments 

of a fossilized crab from a carbonate concretion in thinbedded mudstone near Garberville. The distribution of fossil 

localities in the Yager terrane are shown in figure 2, and 

palynomorph ages are listed in table 1. 

Dinoflagellate assemblages from the Yager terrane range 

in age from Paleocene to late Eocene (Evitt and Pierce, 1975; 

Damassa, 1979). The spore and pollen floras indicate an age 

range from late Paleocene to middle or late Eocene, although 

probable reworked Cretaceous and Paleocene to early Eocene forms occur in many samples (table 1; Evitt and Pierce, 

1975). The presence of reworked palynomorphs introduces 

some uncertainty into the age of the Yager terrane, and so we 

assign a Paleocene(?) through early(?) to late Eocene age to 

the Yager terrane of the Coastal belt. 

South of the study area, Damassa (1979) studied dinoflagellate flora from the eastern part of the Coastal belt 

near Ukiah (which may, in fact, be correlative with a part 

of the Yager terrane) and assigned an early Paleocene 

(Danian) age to those rocks. She observed that many dinoflagellate taxa in. the Coastal belt of the Willits-Ukiah area 

are restricted in their distribution to the eastern part of the 

Coastal belt. We also note from our data (table 1) that numerous genera and species of dinoflagellates (including 

Areoligera sp., Danea impages, and Spiniferites spp.) are 

largely restricted to the Yager terrane. Damassa noted that 

terrestrially derived spores and pollen were sparse in her 

Danian sample from the Coastal belt, in contrast to the 

partly age equivalent, middle to outer neritic Moreno Formation of the San Joaquin Valley. She attributed this difference to closer proximity of the Moreno Formation 

depositional setting to a continental source of detritus. The 

depositional setting of the Yager terrane may thus have 

been closer to • the California margin than that of the 

Coastal terrane, a conclusion consistent with sedimentologic and structural data. 

Reworked spores and pollen of Cretaceous age from 

the Yager terrane include Aquilapollenites, Eucommiidites, 

Vitreisporites, a11d at least some Corollina and members of 

the Normapolle~ group (except the early Tertiary species 

Nudopollis terminalis), as well as Proteacidites and Siberalpollis (table 1). The presence of Aquilapollenites, a middle- to northern-latitude genus, suggests its derivation from 

a Cretaceous soJrce terrane in central to northern California 

(Frederiksen, 19:87). Reworked dinoflagellates observed in 

samples from t~e Yager terrane include Isabelidinium of 

Late Cretaceous age (W.R. Evitt, written commun., 1985) 

and Gonyaulacysta jurassica (Deflandre) (Evitt and Pierce, 

1975), which ocburs in both the Yager and Coastal terranes 

and requires reJorking of unmetamorphosed source rocks 

of Kimmeridgian or Oxfordian age. 

The crab fragments were identified by Rodney Feldmann (written commun., 1986) as belonging to Callianassa sp., a crab group of Cenozoic or younger age. Related 

Callianassa species are now found living in coarsely clastic, moderately nearshore habitats of the gulf coast. Thus, 

the crab fragments in the Yager terrane were probably displaced downslope from a shallower depositional environment into a slope or slope-basin setting. 

The Yager terrane is essentially devoid of any foraminifers or radiolarians. A likely explanation for the absence of 

marine microfossils is that the depositional site of the Yager 

1 0 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
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Table 1. Pollen in roc~ samples from the Coastal and Yager terranes 

[Samples arranged generally from southwest to northeast. All samples are from Humboldt County except R3425N, which is from Mendocino County. X, present; P, probably present;?, possibly present; 

-, absent. C, definite Cretaceous taxa; Pa, taxa probably not ranging above the Paleocene; P-LE, taxa mainly confined to the Paleocene and lower Eocene; LE, taxa probably confined to the lower 

Eocene; ME, taxa probably not ranging below the middle Eocene] 

Coastal terrane Yager terrane 

2 3 4 5 6 7 8 9 10 

-- Sample(s) ...... ---................................ ------............................ -------....... R3469 R342SN R3473A-O R3425B R3712A R3472A-D R3470A-C R3425D, R3712B R3712C, 

Field No(s) ............................................................................................................. S-85-133 MT-213-83 NFSSC 16-19 MT-4-83 MT-165-85 NF85C 12-15 NFSSC 1-3 MT-35-83 MT-174-85 MT-174-85, 

MT-175-85 

A/angiopollis cribellata (Srivastava) Frederiksen (PA)·----·-----------------------·- - - - - X 

sp. 1 of Frederilsen, 1983 (ME?)··············································· - - - - - - - X 

AlnpoDenites 1lefUS Potonie----------------------------·--------------------------- X X X X X X X X X X 

AnnutrjJorites sliJconwxus F rederillsen (ME)--------------------------------------- X - X - - X p - X p 

Aquilapollenites sp. (C) --------------------------------------·-------------------- - - - - - X - - - Betulaepo/lenites spp. (P-LE) ----------------------------------------------------- X - - - X X X X X 

Bombacacidites ferepadis Frederksen---·········································· X - - - - - - - X 

cf. B. fereparilis (rounded triangular)-·········································· - - - - - - - - - X 

nanabrochatus Frederksen + Bombacacidites atf. B. nanabrochatus --------------. - X - X X - X X X X 

cf. B. nanobrochatus (rounded triangular)---·-···-------··---------------------- X - - - - X 

reticu/atus Krutzsch-·························································· - X 

"Bombacacidites nacimientoensiS' (And81S0n) Elsil of Drugg (1967)·················· X X 

Carya spp.(<29 m~ ----··--··-··-···--••• •• •• ···-··--••• ··--··-----··-------·• • • · - X ? X ? p ? X X X 

Caryapo/lenites prodromus Nichols and at+ C. impara/is Nichols and Ott (PA) -------- X - - - X X - - X 

Ce~eidiphyDites sp---··-----··--··-··-••· --··-··---------··--··------------------- - - - - - - - - X 

Corollina spp. (PA)- ----··--------· --------··-··---·-----··--··--··-------------•• - - - - - - - X X 

Corsinipollenites sp. 1 of Frederiksen 1983 (ME)-·····-·····-····-·····-·····-· X - - X - X 

Cupanieidites sp. ----------- ·--·--------.----------------------· -----·----------·- - - - X C~.pu/iferoidaepollenites spp------------------------------------. --. --------------- - - - X 

Eucommiidites sp. (C)---··-··················--···---•••••• -------·- •.•••••••••••• - - - - - X 

lnsu/apo/Jenites spp. ·-• • • • ·-- -• ·---------·-• • • ·-• ·-------• • • • • • ·------·-----• • · • • · · X X - - X X 

lntratrporopollenites pseudinstructus Mai ---------·------.----·-· ·----·-----.-----. - - - X X - - - X X 

Mompites coryloides Wodehouse+M. strictus Frederiksen and Christopher·-----.. -.. -. - - - - X - X - X 

NonnapoOes group (Cretaceous forms) (C)-·········'··············· .•.............. - - - - - - X X X 

Nudopo/5s tennina5s (Pflug and Thomson) Pflug ·---------. ·--------. --.--- --•• ---.- - - - - X - - - - ParaalnpoOenites confusus (Zakfin9<aya) Hills & Wallace (P-LE) -.--·--•• --.--.-.---- - p - - X X - - X 

PistillipoOenites macgregorii Rouse·---------------.------.----.-•..• -•. -----.-----. X X X - - X X X X X 

P/atycaryapollenites trip/icarus (Eisk) Frederilsen and Christopher (LE) . -• ·-... -.. -. - - - - - - - X - - Proteacidites spp. + Siberiapollis spp-·•··•· ---··· .. -·--.. ··-.-... -... -.----.. -.... X X X X X X X X X X 

SubtriporopoOenites all. S. anu/atus Pflug and Thomson ---.-·---.•.••••. -.--.. --.--. - - - - - - - - X 

Triatriopo/lenites intennedius (Giadkova) Kedves (ME) ••••••.....•.•••.........••••. - - - - - - - - X 

/Lbomirovae (Giadkova) Kedves- ·-----··-------·------------·--• ·-.--.. --.---.-. - - - - - - - - p 

triangu/us Frede~sen---······································ ·· .•••.••.•••••. - - - - X - - - X 

Triporopollenites spp. (simple morphology) --------------·.--· ·---• ·---.-----.• ----- X X - - X X X X X 

UlmipoOenites krempii (Anderson) Frederksen ------------------•• .. --.---.-----.--. - X - - - - - - tricostatus (Anderson) Frederksen ----··--··-. ----------·-.---.-.----••.•••••• X - - - - - X X 

undulosus Wolff---····- ----··---·-------------------. ·-·-------------••••••••• - - - - - - - - X 

Vitreisporites spp. (C) • · • • · • • • • • • • • • • • ·-• • • • • ·------------------·-· ·------·------ - - - X X X X X X 

Sample locations: 1. Cape Mendocino 7.5' quadrangle, SE1/4 sec. 3, T. 1 5., R. 3 W. 6. Bridgeville 7.5' quadrangle, NW1/4 sec. 17, T. 1 N., R. 3 E. 

2. lincoln Range 7.5' quadrangle, NW1/4 sec. 19, T. 21 N., R. 16 W. 7. Bridgeville 7.5' quadrangle, NE1/4 sec. 16, T. 1 N., R. 3 E. 

3. Red Crest 7.5' quadrangle, NE1/4 sec. 11, T. 1 N., R. 2 E. 8. Myers Flat 7.5' quadrangle, NW1/4 sec. 32, T. 1 5., R. 3 E. 

4. Weott 7.5' quadrangle, SE1/4 sec. 27, T. 1 5., R. 2 E. 9. Harris 7.S quadrangle, W1/2 sec. 21, T. 55., R. 4 E. 

5. Garberville 7.5' quadrangle, E1/2 sec. 22, T. 55., R. 4 E. 10. Harris 7.5' quadrangle, E1/2 sec. 22, T. 55., R. 4 E . 
terrane received a thick and rapid influx of land-derived, 

organic-rich, fine-grained siliciclastic materials which greatly diluted the micro-organisms, or, alternatively, that the 

sparse foraminifers and radiolarians were removed through 

postdepositional diagenesis. A deep-marginal basin high on 

the continental slope or a marine deltaic setting might provide an appropriate depositional model for the Yager terrane. 

Local source areas in the Central belt probably provided detritus to the Yager terrane during Paleocene(?) to late 

Eocene time. West of Laytonville, along the east side of 

the Yager terrane, thick lenses of conglomerate occur. Although most of the conglomerate clasts are rounded, one 

lens at the faulted base of the Yager terrane is composed 

of angular green radiolarian chert similar in appearance to 

bedded chert occurrences in the adjacent Central belt of 

the Franciscan Complex. Radiolarians from these chert 

clasts are Early Cretaceous (Berriasian to Valanginian) in 

age (C.D. Blome, written commun., 1983) and are identical to assemblage MH-5 of the Marin Headlands and 

Geysers chert terranes of the Central belt (McLaughlin and 

Pessagno, 1978; Murchey, 1984; Murchey and Jones, 

1984). Furthermore, this radiolarian fauna is restricted to 

the Franciscan Complex and does not occur in the Great 

Valley sequence. Acanthocircus dicranocanthos, which occurs in the radiolarian assemblage from chert clasts of the 

Yager terrane, was noted by Murchey (1984 ), Pessagno 

and others (1984), and Pessagno and Blome (1986) as indicative of a low-paleolatitude oceanic setting. 

Deformation 

The Yager terrane underwent complex deformation 

characterized by multiple folding (fig. 5) within the broad 

time interval between the last deposition of Yager strata 

(ca. 41 Ma) and the earliest deposition of little-deformed, 

nonaccretionary rocks above the Yager and Coastal terranes (ca. 22 Ma). The folding was accompanied or followed by penetrative shearing, especially near faulted 

contacts with the Central belt to the east and the Coastal 

terrane to the west. 

We do not know whether this folding and shearing occurred during the entire 19-m.y.-long Oligocene to Miocene interval or, instead, was relatively brief. For example, 

the deformation may have lasted only 6 m.y. if, as local 

field relations suggest, it preceded the 36- to 35-Ma intrusion of trachytic rocks near Arcata into the upper plate of 

Figure 5. Folds in sedimentary rocks of Yager terrane. A, Upright fold, located along Van Duzen River. Note faulting of 

fold crest and truncation of thick sandstone bed by fault in 

apex of fold. Fold plunges north. Hammer (circled) shows 

scale. 8, Refolded rocks, located along Eel River near Garberville. Beds in fold are overturned, indicating an earlier 

fold event that predates fold exposed here. C, Recumbent 

fold, located east of Briceland. Fold axis dips southwest, and 

vergence is eastward. Note hammer (circled) for scale. 

12 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
the Coastal belt thrust. However, if these intrusions are not 

rooted below the thrust or are sheared off by it, then they 

have no bearing on the timing of Yager deformation. 

At least three major fold sets are evident in the Yager 

terrane. The earliest folds (/1) consist of map- and outcropscale recumbent folds with subhorizontal axes. In the few 

areas where overturnedf1 fold limbs can be observed, tectonic transport to the north-northeast is indicated. This direction 

may have been associated with early, landward-vergent tectonic transport of the Yager terrane during Oligocene time, 

which preceded displacement across the Coastal belt thrust. 

The / 1 folds are refolded by upright to asymmetric isoclinal 

folds (/2) that are overturned steeply to the southwest or 

northeast, indicating continued northeast-southwest-oriented compression. We speculate that those / 2 folds may have 

been associated with later Oligocene to early Miocene emplacement of the Central belt over the Yager terrane along 

the Coastal belt thrust. A much later, / 3 generation of open, 

map-scale folds has warped lower-plate Yager terrane, upper-plate Central belt, and overlying middle Miocene to Pliocene strata into anticlinal and synclinal warps that plunge 

southeast. This latest folding probably occurred in association with transpression during northward propagation of the 

San Andreas fault system in very late Cenozoic time. 

Penetrative deformation has also occurred within 

rocks of the Yager terrane, especially along its faulted east 

and west sides. This deformation is characterized by the 

development of anastomosing, undulatory shear surfaces, 

boudinage, and brittle fracturing. Southeast of Benbow, at 

least two shallowly dipping zones of penetratively sheared 

rocks, several hundreds of meters thick (melange zones), 

cut the Yager terrane. These melange zones apparently 

crosscut both / 1- and / 2-fold trends and are, in turn, folded 

by latest !J folds. Their low-angle trend, subparallel to the 

Coastal belt thrust south of Garberville, suggests that they 

may be cogenetic with emplacement of the Coastal belt 

thrust. However, detachment faulting may be as defensible 

as thrusting for the origin of the melange zones within the 

Yager terrane. Melange zones along the western margin of 

the Yager terrane locally exhibit extensional deformation 

and may be related to normal faulting along the margin of 

the Yager Basin. Extensional deformation of the· Yager terrane possibly occurred in conjunction with uplift of the 

adjacent Coastal terrane to the west. 

Rocks of the Yager terrane, together with those of 

other terranes of the Coastal belt, are metamorphosed to 

zeolite grade, though less so than those of the Coastal 

terrane. Where sandstone beds of the Yager terrane are 

broken, fractured, or shattered, the fractures are filled 

with laumontite+calcite+quartz. This zeolite assemblage 

is especially prevalent along the faulted west and east 

sides of the Yager terrane, where the rocks are most extensively fractured. This fracturing apparently increased 

rock permeability locally and enhanced the circulation of 

hydrothermal fluids from which the zeolites precipitated. 

Paleomagnetic Studies 

Sampling of the Yager terrane for paleomagnetic study 

has been limited to unweathered river-bottom exposures of 

thin-bedded, carbonate-cemented, fine-grained siltstones 

showing common ripple-drift laminations. The areas that 

have been sampled include a section along Salmon Creek 

(D.E. Champion, oral commun., 1985), the Eel River at 

Scotia (R.E. Wells and others, unpub. data, 1985); and Sonoma Creek, a tributary to the Eel River near Whitlow 

(S.G. Miller, oral commun., 1985). Sample suites from 

these localities failed inclination-only fold tests and (or) 

showed evidence of major postdepositional remagnetization. The timing and mechanism of this remagnetization 

are unknown, and so, to date, all paleomagnetic data from 

the Yager terrane are inconclusive. 

Coastal Terrane of the Coastal Belt 

The Coastal terrane of the Coastal belt, like the Yager 

terrane, is composed of sandstone and argillite. Unlike the 

Yager, however, the Coastal terrane includes ( 1) minor but 

significant exotic blocks of basaltic intrusive rocks, flows, 

and breccias that in places are associated with pink to gray 

pelagic limestone; and (2) rare blocks of garnet-bearing 

glaucophane schist. In addition, sandstones of the Coastal 

terrane are compositionally more heterogeneous than those 

of the Yager terrane, a significantly larger area of the 

Coastal terrane is composed of tectonic melange and rocks 

deformed by penetrative shear, and parts of the Coastal 

terrane are significantly older than the Yager terrane. 

Sandstone, Shale, and Conglomerate 

Sandstones of both the Coastal and Yager terranes range 

petrographically from feldspathic to arkosic, typically containing more than 50 volume percent plagioclase in outcrops 

in the northern parts of the Yager and Coastal terranes, more 

than 50 volume percent monocrystalline quartz in the south, 

and as much as 20 volume percent K-feldspar along the 

entire length of both terranes (Underwood and Bachman, 

1986). Many sandstones of the Coastal terrane are more 

lithic than those of the Yager (Underwood and Bachman, 

1986), although there is essentially 100-percent overlap of 

the compositional field of the Yager terrane by that of the 

Coastal terrane. The most distinctive lithic components in 

sandstones of the Coastal terrane are igneous-rock fragments 

of shallow-intrusive and extrusive origin. These igneousrock fragments are dominantly silicic in composition and 

microfelsitic to coarsely porphyritic in texture, suggesting 

an island-arc derivation. The other fragments consist of lesser amounts of silicic tuff, chert, mafic igneous rocks, quartzand white mica-bearing schist, phyllite, slate, and shale. 

Nonopaque detrital heavy-mineral accessory assemblages 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 13 
are dominated by chloritized biotite, muscovite, epidoteclinozoisite, sphene, garnet, zircon, and schorlitic tourmaline. As is true of the Yager terrane, these data suggest a 

mixed provenance, including major deeply eroded plutonic 

sources that were plagioclase rich to the north and more 

quartzose to the south; a magmatic arc; and regionally metamorphosed orogenic terranes from which recycled detritus 

was derived (Underwood and Bachman, 1986). 

Conglomerates of the Coastal terrane consist of clastic 

suites identical to those of the Yager terrane. For example, 

conglomerates of the Coastal terrane, like those of the 

Yager terrane, contain recycled rounded clasts of hypabyssal intrusive and extrusive igneous rocks of intermediate to silicic composition, black and green chert and 

metachert, quartzite, hornfelsic meta-argillite, and metaigneous rocks. 

At one locality in the Coastal terrane, approximately 

5 km south of Cape Mendocino near Devils Gate, conglomerate is interbedded in an unusually intact sequence of arkosic sandstone and argillite of Eocene age (see subsection 

below entitled "Age of the Coastal Terrane"). The sandstone and argillite at this locality display very well preserved graded bedding, ripple-drift laminae, and traction 

features indicative of channelized turbidite deposition on a 

submarine slope and overbank deposition in interchannel 

areas. Abundant soft-sediment deformational features, including liquefaction and load structures and soft-sediment 

extensional structures, suggest deposition in an area of submarine slope failure (figs. 6A, 6B). The conglomerate at 

Devils Gate contains, in addition· to the clastic suite mentioned above, another clastic suite indicative of derivation 

from the Eastern and Central belts of the Franciscan Complex. This second clastic suite includes subangular to subrounded, pebble- to boulder-size clasts of red radiolarian 

chert and metachert, metabasalt, serpentinized peridotite, 

and rare clasts of lawsonite- and jadeite-bearing metasandstone reconstituted to textural zone 2 (of Blake and others, 

1967) (figs. 6C-6E). Blueschist-grade metasandstone of the 

Franciscan Complex that contains jadeite in the same abundance as found in the conglomerate clasts at Devils Gate 

(approx 5-7 volume percent) is known in California mainly 

from the central Diablo Range, particularly in the Mount 

Hamilton area (Maddock, 1955; Kerrick and Cotton, 1971). 

These data suggest that the Coastal terrane may have received detritus from uplifted rocks in the vicinity of the 

central Diablo Range during Eocene time. 

Basaltic Rocks 

Blocks of basaltic rocks have been mapped in several 

areas of the Coastal terrane. The most extensive exposures 

occur along Parkhurst Ridge, near Honeydew, and near 

Usal, southwest of Piercy. Minor but significant outcrops 

of basalt flows occur near Hacketville, Whitethorn, and 

south of Fort Bragg along the coast. Elsewhere in the 

Coast Ranges, Kramer ( 1976) mapped a large area of basaltic volcanic rocks in the Coastal terrane west of Willits. 

The basalts chiefly are pillow flows, flow breccias, and 

glassy aquagene tuff (hyaloclastite). However, diabase 

Figure 6. Sedimentary rocks of Coastal terrane from Devils 

Gate area. A, Well-bedded sandstone and argillite slope 

deposits. Section faces north (to right). 8, Right-side-up rippledrift crosslaminae and soft-sediment deformation. C, Metasandstone clast (to right of hammer) in conglomerate lens, 

displaying pronounced metamorphic fabric. Contains both 

lawsonite and jadeite, indicating derivation from Eastern belt 

of Franciscan Complex, probably from Diablo Range. 0, Photomicrograph of metasandstone clast shown in C; magnification, x1 00. J, jadeitic pyroxene; L, lawsonite. Crossed nicols. 

E, Same as figure 60, but with uncrossed nicols. 

14 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
Figure 6.-Conti nued 

0.1 mm 

0.1 mm 

dikes and sills occur with the extrusive rocks at Parkhurst 

Ridge and Usal, and isolated diabase intrusive bodies 

occur at Whitethorn. At all of these localities except Usal, 

the basaltic rocks are in fault contact with enclosing 

sheared sandstone and interbedded argillite of the Coastal 

terrane and thus are considered to be tectonic blocks. In 

the Usal area near Dugans Opening, basalt flows are interbedded with thick beds of arkosic sandstone, argillite, and 

altered glassy basaltic tuff (figs. 7, 8). The interbedded 

sandstone, argillite, tuff, and basalt form a composite 

block in this area, a point of some importance to our platetectonic reconstruction of the Coastal terrane. Basaltic 

rocks of the Coastal terrane are associated with pink to 

gray, manganiferous, pelagic, foraminiferal limestone at 

many localities. 

The basaltic rocks have glassy to intersertal ophitic textures but locally are vesicular, suggesting local extrusion in 

a shallow submarine setting (Usal). The intrusive rocks are 

subophitic to ophitic. Primary clinopyroxenes are predominantly augite (locally, titanaugite) and pigeonite; the plagioclase is largely albitized; accessory opaque minerals 

include magnetite, ilmenite, and pyrite; and nonopaque accessories include sphene and apatite. Glass in the flow rocks 

and tuffs commonly is altered to pumpellyite, celadonite, 

and smectite, and vesicles and vugs are filled with calcite+ 

quartz+chlorite or pumpellyite. An early, moderate-temperature (290-400°C) alteration assemblage, suggestive of 

subsea hydrothermal activity, includes chlorite+pumpellyite 

or chlorite+prehnite+pumpellyite. A later, lower-temperature, vein-filling zeolite assemblage indicative of temperatures from 225 to 230°C at about 3 kbars (Liou and others, 

1985) includes chlorite+laumontite+calcite+pumpellyite+ 

natrolite+datolite (see Blake and others, 1988). 

The formational setting of basaltic rocks of the Coastal 

terrane is suggested by various plots of their chemistry 

(figs. 9, 10; table 2). On a ternary discrimination plot of 

MnO versus P20 5 versus Ti02 contents (fig. 9A; Mullen, 

1983), the basalts lie dominantly within the compositional 

field of midoceanic ridge basalts. Three samples from the 

vicinity of Usal plot in the field of island-arc tholeiites, one 

very close to the calc-alkaline boundary; however, these 

rocks are from an area of widespread secondary Mn mineralization, probably due to submarine hydrothermal processes. One sample, from Parkhurst Ridge near Honeydew, 

plots in the field of seamount tholeiites. On a plot uf 

Na20+K20 versus Si02 contents (fig. 9B), the basalts are 

transitional from tholeiitic to alkalic in composition. On a 

discrimination plot of P20 5 versus Ti02 contents (fig. 9C; 

Ridley and others, 1974), the basalts clearly lie within the 

compositional fields of oceanic-ridge and seamount basalts. 

On a plot of Ti02 content versus FeO/MgO ratio (fig. 9D; 

Hekinian and Fouquet, 1985; Shervais and Kimbrough, 

1985), the basalts follow fractionation trends typical of East 

Pacific Rise basalts and midoceanic-ridge basalts in general, unlike arc-related basaltic rocks. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 15 
On figure 10, we present several scatter plots of minor 

and trace elements in an attempt to further discriminate 

the formational settings of basaltic rocks of the Coastal 

terrane. It is relatively clear from these plots that the basalts formed in an ocean-floor or ridge setting and not in 

an island arc, as shown in figures lOA and lOB by high Ti 

20 e-L-

~ 

71 

34 

e-ll-

-++-

--+-

Loc 3 &:)) 

Loc 2.@ 

Inclined 

Overturned 

Strike and dip of basalt flow or 

tufT-Ball indicates top 

direction known 

Inclined 

Vertical 

Strike and dip of shear fabric 

Inclined 

Vertical 

Foraminifer sample locality-Number 

refers to locality discussed in text 

Carbonate concretion (late early 

Eocene, Planktic Zone P9) 

Pelagic limestone (Late Cretaceous, 

Maastrichtian to Campanian) 

e Limestone lens 

'X' Manganese prospect 

Landslide-Arrow indicates direction 

of downslope movement 

0 

content (7,500-17,000 ppm) relative to Cr (80-230 ppm) 

and V (60-500 ppm), in figure lOC by high Ni content 

(60-200 ppm) and high Ti/Cr ratios, and in figure lOD by 

high Ti and low to moderate Zr contents (60-160 ppm). 

Although the plots of Ti versus Cr contents (fig. lOA), V 

versus Ti contents (fig. lOB), and Ti/Cr ratio versus Ni 

D 

D 

Dugans 

Opening 

EXPLANATION 

Sandstone and shale, laumontized, 

penetratively sheared and 

broken-Forms matrix of Coastal 

terrane. Generally contains fossils 

of Eocene age. May include older 

undivided sandstone and shale 

Sandstone and shale, laumontized, 

broken, displaying well-preserved, 

intact bedding locally-Locally 

interbedded with manganiferous 

basalt and limestone that contains 

fossils of Late Cretaceous age 

Basaltic pillow flows, tuffs, and 

breccias-Locally includes minor 

coarse-grained intrusive diabase 

and lenses of red to gray, manganiferous, foraminiferal limestone 

that contains fossils of Late 

Cretaceous age 

Contact-Dashed where inferred. 

Showing dip where known 

Fault-Dashed where inferred; 

dotted where concealed. 

Showing dip where known 

.®Lac 2 

2 KILOMETERS 

Julias 

Opening 

Figure 7. Detailed geologic map of part of Coastal terrane near Usal, showing location of foraminifer samples discussed in 

text. Geology mapped by R.j. Mclaughlin, 1983-85. See figure 2 for location. 

16 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
Figure 8. Volcanic rocks and interbedded arkosic sandstone 

of Coastal terrane, located in Dugans Opening area near 

Usal. A, Basalt flow interbedded with arkosic sandstone. Basalt (dark rocks, left, with hammer (circled) for scale) overlies 

tuffaceous shale and thinly interbedded laumontized sandstone (white streaked beds, right center), which, in turn, 

overlie thinning- and fining-upward sequence of channeled 

laumontized sandstone (light colored rocks, far right). Section faces east (to left). Basaltic rocks are associated with 

ferromanganese mineralization. 8, Thinning- and fining-upward sequence of arkosic sandstone depositionally below 

and west (to right) of basalt flow shown in A. Note undulatory loading along base of sandstone beds and local channeling into underlying shaly sequences. Section faces east (to 

left). Note hammer (circled) for scale. C, Attenuated and laumontized basalt flows intercalated with altered glassy tuff 

(hyaloclastite). 0, Photomicrograph of basaltic tuff, extensively altered to smectite mineral assemblages (filling conspicuous parting in upper center); magnification, x40. 

Plagioclase laths are locally altered to laumontite, and clinopyroxene is locally replaced by epidote-group minerals (epidote, pumpellyite, clinozoisite). 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 17 
0 0.01 mm 

L__j 

Figure 8.-Conti nued 

18 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
content (fig. 10C) suggest affinities to midoceanic-ridge 

basalts, the plot of Ti versus Zr contents (fig. 10D) suggests that an elevated offridge oceanic setting, probably a 

seamount or plateau, may best fit the data. 

Thus, the field relations, petrography, and geochemistry all indicate oceanic near-ridge and (or) somewhat-offridge formational settings for basaltic rocks of the Coastal 

terrane. Some of the vesicular flow rocks were likely extruded at or near sea level. This oceanic setting contrasts 

with the apparent arc-continental provenance of the sedimentary rocks within which the basaltic rocks are tectonically incorporated. In spite of this overall contrast, at Usa], 

sandstones are clearly interbedded with and overlie basaltic flow rocks (figs. 8A, 8B). These sandstones are also 

arkosic, containing angular quartz and feldspar, as well as 

14 to 21 vofume percent K-feldspar. Furthermore, the interbedded sandstones contain minor detrital quartz-mica 

schist, biotite, muscovite, garnet, and epidote-group minerals, consistent with their derivation from deeply eroded 

granitic-plutonic and regionally metamorphosed orogenic 

sources along the American plate margin. Together, these 

data argue that basaltic rocks of the Coastal terrane 

formed in a near-ridge oceanic setting which was at least 

within reach of sediment derived from the American plate 

margin. 

Basaltic rocks of the Coastal terrane are Late Cretaceous in age (Campanian to Maastrichtian), on the basis of 

the ages of planktic foraminifers in pink to gray pelagic 

limestone locally interleaved with the flow rocks (Sliter 

and others, 1986) at several localities (fig. 2). This relation 

requires that the arkosic sedimentary rocks interbedded 

with the basalts at Usal are coeval, although the sheared 

arkosic sandstone and argillite which tectonically enclose 

the basalts (and interbedded sandstone) at Usal and elsewhere in the Coastal terrane are early to late Eocene in 

age (see subsection below entitled "Age of the Coastal 

Terrane"). 

Limestone 

Limestone of the Coastal terrane occurs in two distinctly different lithologic settings of varying depositional 

origin. The first occurrence is as carbonate concretions 

within thin-bedded sequences of black argillite and minor 

interbedded sandstone, which commonly are penetratively 

deformed into zones of scaly argillite mapped as melange. 

The second occurrence is as reddish-pink, greenish-gray, 

or white pelagic limestone lenses, from less than 7 to 

more than 10 m thick and as much as 500 m long, intercalated with or overlying basaltic rocks. 

Carbonate Concretions and Thin Carbonate Beds 

The carbonate concretions in the Coastal terrane are 

similar in their associations to the carbonate concretions in 

the Yager terrane. However, the associated argillitic rocks 

of the Coastal terrane commonly are significantly more 

deformed than the argillite of the Yager terrane, as reflected by a penetrative shear fabric (fig. 11A). These argillitic 

rocks of the Coastal terrane probably represent hemipelagic and interchannel-overbank deposition in a lower-slope 

or trench-slope setting. 

Locally, thin carbonate-bearing beds occur within areas of mildly deformed, thin-bedded flysch, interbedded 

with channelized sandstone and conglomerate. These intact carbonate-bearing sequences have sedimentary structures (ripple-drift laminations, load structures, current 

features, and synsedimentary deformation structures) and 

sandstone compositions similar to those in the Yager terrane. Carbonate-bearing beds in these intact blocks commonly are laterally more extensive than in more deformed, 

argillitic parts of the Coastal terrane. 

The carbonate concretions associated with argillitic rocks 

of the Coastal terrane, particularly in the area surrounding 

Petrolia, have a high organic content, as indicated by an abundance of carbonized plant matter, palynomorphs, and rare 

occurrences ofliquid hydrocarbons. Unlike those in the Yager 

terrane, however, the carbonate concretions in the Coastal 

terrane locally contain planktic foraminifers (fig. 11B) and 

commonly show abundant bioturbation features. 

Pelagic Limestone 

The pelagic limestone associated with basaltic rocks 

of the Coastal terrane, unlike the carbonate concretions associated with argillitic rocks, is relatively pure. It locally 

contains minor admixed mafic, glassy volcanic detritus 

(fig. 12A), laminar horizons of chemically precipitated red 

to green clay, manganese oxide, or cherty tuff (Parkhurst 

Ridge) that possibly represents devitrified ash. Bioturbation and dissolution partings are common to abundant. At 

Usal, where arkosic terrigenous sandstone is interbedded 

with basalt, pink pelagic limestone from within the basalt 

sequence is invaded along microfractures by microscopic 

clastic dikes of arkosic silty sandstone (figs. 12B, 12C). 

Blueschist 

Blocks of garnet-free and garnet-bearing glaucophane 

schist (types III and IV blueschist of Coleman and Lee, 

1962) occur locally within melange of the Coastal terrane 

along its contact with the King Range terrane. These blueschist blocks, which probably were originally amphibolite 

and garnet amphibolite (Moore and Blake, 1986), are 

identical to those in the Central belt of the Franciscan 

Complex (McLaughlin and others, 1982). The melange 

zones in which these blueschist blocks occur have been 

viewed as sutures between the King Range terrane on the 

west and other terranes of the Coastal belt to the east 

(McLaughlin and others, 1982). The restricted occurrence 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 19 
EXPLANATION 

• Usal 

EB Whitethorn 

+ Parkhurst * Hacketville 

0 

Mn0(x10) 

EXPLANATION 

0 Cooskie Creek 

+ Point Delgada 

~ Queen Peak 

Mn0(x10) 

EXPLANATION 

• Black Point 

X Iverson 

-$- Mindego Hill 

[;iii Fickle Hill 

-$- DSDP 

Mn0(x10) 

A 

CAB 

CAB 

CAB 

Figure 9. Major-element chemistry of volcanic rocks 

from terranes of Coastal belt, in comparison with selected other volcanic rocks. Analytical data given in table 2. 

A, Ternary plot of Ti02, MnO, and P20 5 contents, showing fields for island-arc tholeiites (IAT), oceanic-island 

tholeiites (OIT), midoceanic-ridge basalts (MORB), 

calc-alkaline basalts (CAB), and oceanic-island alkali 

E-t 

z 

~ 12.--.--~.---.---.---~--~~ 

u 

~ 

~ 10 

E-t 

::r: 

d 8 

~ z 6 

-E-t~ 

z 4 ~ 

~ 0 2 

u 

0 0 ~ 4~2--~4~6--~5~0---5~4---5L8---6L2---6~6--~70 

+ 

0 

C'l 

(1j 

z 

0~~--~~--~--~--L---L-~ 

42 46 50 54 58 62 66 70 

Si02 CONTENT, IN WEIGHT PERCENT 

B 

basalts (OIA) from Mullen (1983). Dashed line, approximate field boundary. B, Na20+K20 versus Si02 contents, showing boundary between fields of alkalic (A) 

and tholeiitic (TH) compositions from Macdonald and 

Katsura (1964). C, P20 5 versus Ti02 contents, showing 

fields of alkali basalts (AB), oceanic-island tholeiites 

(OIT), and oceanic-ridge basalts (ORB) from Ridley and 

20 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
~ J:il 

C) 

0.8 ,.--.------.-.---.------.-.---, 

0.6 

0.4 

0.2 

1 2 3 4 5 

~ 0.8 .--.------.-.---...-------.-.---, 

fl: 

d ~ 0.6 

~ z 0.4 

-

1 2 3 4 5 

0.8 .--.------.-.---...-------.-.---, 

0.6 AB 

0.4 

0.2 

0~-~-~--~-~-~ 

0 1 2 3 4 5 

Ti02 CONTENT, IN WEIGHT PERCENT 

c 

3.5 .------r-----,---.------.---..-----. 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

EPR 

9N. 

SCRO 

(ARC FIELD) 

OL--~--J--~-~--L-~ 

0.5 1.0 1.5 2.0 2.5 3.0 3.5 

3.5 .------r-----,---.-----.----..------, 

~ 

z 

J:il 3.0 

C) 

~ 

fl: 2.5 ~ 

::X:: 

d 

~ 2.0 

z 

"'""" 1.5 ~A 

~ 1.0 

0 

C) 

C'l 0.5 

0 

E:: 

EPR 

9N. 

SCRO 

(ARC FIELD) 

0~--~-~--~--~-~-~ 

0.5 1.0 1.5 2.0 2.5 3.0 3.5 

3.5 ,.-----.----.--.-----.----r------, 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

SCRO 

(ARC FIELD) 

Fickle Hill 

D 0 L..-_-L _ ____L __ ~__....~..._ _ __.__ _ __J (FeO*/ MgO 

0.5 1.0 1.5 2.0 2.5 3.0 3.5 =21.0) 

TOTAL Fe(as FeO*)/MgO RATIO 

others (1974). 0, Ti02 content versus total Fe (recalculated as FeO*)/MgO ratio, showing trends and fields 

for rocks from East Pacific Rise (EPR) from Hekinian 

and Fouquet (1985), as well as fields for rocks from 

northern Coast Range ophiolite (NCRO), southern 

Coast Range ophiolite (SCRO), and trend for opensystem fractionation of midoceanic-ridge basalts 

(open arrow) from Shervais and Kimbrough (1985). 

Two different fractionation trends for rocks from EPR at 

lat 12°50' N. are shown (dashed lines). 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 21 
Table 2. Major-, minor-, and trace-element chemistry of basaltic rocks of the Coastal belt and of other selected igneous rocks 

[Major-element analyses in weight percent; minor- and trace-element analyses in parts per million. Analysts: 1-4, 7, D. Vivit and J. Kent (X-rayHamlin ("single solution" method of Shapiro, 1975); 15, 16, P. Elmore, S. Botts, G. Chloe, L. Artis, and H. Smith (methods of Shapiro and Brannock, 

Data sources: 15, 16, Wentworth (1966); 17, 18, Haehl and Arnold (1904); 19, MacLeod and Pratt (1973); all others, this report] 

Analysis······· 1 

Terrane • •• • Coastal 

Location-· • • • • Usal Sample··· MT-38-85 

Coastal 

Usa I 

MT-47-85 

3 

Coastal 

Usa I 

MT-58-850 

4 

Coastal 

Usal 

MT-50-85 

Coastal 

Usa I 

MT-4~ 

6 

Coastal 

Usa I 

MT-43.1-82 

Coastal 

Whitethorn 

MT-1818-85 

8 

Coastal 

Honeydew 

MTB-15C-82 

9 

Coastal 

Partthurst Ridge 

MT-74-82 

10 

Coastal 

Hac:ketvllle 

MT-30-82 

11 

King Range 

Point Delgada 

MK-14 

Major elements 

5102 ....... 45.9 50.1 47.5 44.5 49.8 46.6 46.1 47.2 50.2 46.6 47.4 

A1203 ...... 12.6 15.5 14.7 14.0 14.8 13.9 12.6 14.3 16.0 12.6 16.2 

Fe203------ 14.4 10.3 11.8 17.0 10.8 15.2 15.2 10.5 8.69 13.5 9.6 

Feo ........ 1.8 

MgO-------- 7.44 5.97 5.56 5.52 6.00 5.83 6.12 7.62 5.49 7.11 7.2 

eao ........ 9.80 6.74 8.22 4.10 8.75 8.06 9.29 10.5 9.31 10.1 8.2 

Na20------- 2.32 2.48 2.98 2.42 3.22 2.69 2.87 3.64 3.49 2.49 2.9 

K2o ........ .56 2.73 .76 .83 1.74 2.10 .32 .04 1.51 .21 1.6 

H20+ ....... 2.9 

H2o· ....... .26 

Tl02 ....... 1.71 1.18 1.75 2.56 2.03 2.00 1.87 1.55 1.83 1.86 1.6 

P205· •• ••• • .18 .10 .19 .33 .22 .22 .26 .15 .19 .18 .18 

MnQ ........ .20 .28 .29 .88 .19 .19 .24 .18 .12 .20 .16 

C02········ .03 

Minor elements 

1\b ......... 13 <10 <10 <10 13 

Fb ......... <10 53 10 27 <10 

Sr •••••·••• 260 140 270 190 270 

Zr---------- 110 60 110 160 160 y .......... 28 22 30 56 40 

Trace elements 

aa ......... 200 270 130 380 

ee ......... <30 <30 30 30 

La ......... 30 <30 <30 <30 Qj ......... 200 290 120 120 

Nj .......... 90 72 87 110 

Zn ......... 90 180 97 180 

Cr ••••••••• 150 150 230 80 y .......... 362 241 312 451 

1 Reported as CaO 

of blueschist blocks along this fault boundary suggests 

that these rocks were tectonically incorporated into the 

Coastal terrane during emplacement of the King Range 

terrane. It is unclear, however, whether the blueschist 

blocks (whose protoliths were basaltic rocks) were somehow tectonically recycled from the Central belt (which 

may underlie the Coastal belt) or whether they are derived 

directly from sources deep in the crust or upper mantle. 

Age of the Coastal Terrane 

The ages of rocks of the Coastal terrane are based on 

palynomorphic and foraminiferal assemblages in limestone. These fossils indicate that the carbonate concretions 

associated with argillitic rocks contain faunas and floras 

distinctly different in age and depositional setting from 

those in the pelagic limestone associated with basaltic 

rocks. 

140 

40 

<30 

160 

60 

90 

100 

74 

Age of the Carbonate Concretions Associated with Argillitic Rock~ 

The carbonate concretions associated with argillitic 

rocks of the Ceastal terrane have yielded planktic foraminifers, dinoflagellates, and spores and pollen. Planktic foraminifers identified in samples from numerous localities 

between Hacketville to the north and the Mattole River to 

the south (fig. 2) are of middle Eocene, possibly late middle 

Eocene age, on the basis of their assignment by Sliter and 

others (1986) to the Globigerapsis subconglobatus and 

Morozovella lehneri Zones (P 11 to 12, 49-43 Ma) and the 

Truncorotaloides rohri Zone (P 14, ca. 42.5-41 Ma). On 

the basis of ti,e absence of keeled subtropical to tropical 

forms and the !abundance of Globigerina officina/is, a temperate form, these early Tertiary faunas are believed to indicate depositfon at temperate,. middle latitudes between lat 

30° and 40° N. (fig. llB). Berry (1982) reported the presence of an early Eocene planktic fauna from the Coastal 

22 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
along the northern California margin 

fhio'rescence and inductively coupled plasma spectroscopy); 11-13, J. Reid and P. Hearn (X-ray spectroscopy and methods of Shapiro, 1975); 14, Z. 

1962.); 17, 18, E.T. Allen; 19, unknown (methods of Shapiro and Brannock, 1962, supplemented by atomic-absorption spectroscopy and spectrography). 

Analysis·---- t2 t3 t4 t5 t6 t7 t6 t9 20 

Terrane • • • King Range 

Location· • • Point Oelgada 

Sample··· MK668 

King Range 

Queen Peak 

G·20 

King Range 

Cooskle Creek 

SK·t78 

Black Point 

Gualala 

6240 

Iverson Basalt 

Gualala 

6243 

Santa Cruz Mountains 

Mlndego Hill Basalt 

Diabase I 

Santa Cruz Mountains 

Mindego Hill Basalt 

Diabase II 

Vizcaino Block 

DSDP Site t73 

Core 36 

Fickle Hill 

Trachyte 

FHT4A 

SI02 • •• • • ••· 

AI203 •• • •• •• 

Fe203 • • • •• • • 

FeO ••••••••• 

Mg0········· 

CaO········· 

N820········ 

K20 ••• ••• ••• H2Q+ •••••••• 

H20· • •• •• ••• 

TI02 • •• •• ••• 

P205 • · · •• ••• 

MnO········· 

C02 •• • ••• ••• 

fib •••••••••• 

Pb •••••••••• 

Sr ••·••••••• Zr··········· 

y •.•.•••..•. 

Ba •••••••••• 

Ce •••••••••• La •••••••••• 

cu .•...••••• 

Nl··········· 

Zn •••••••••• 

Cr •••••••••• v .......... . 

46.2 

t4.6 

3.t 

7.7 

6.5 

tt.t 

3.t 

.84 

2.6 

.28 

t.9 

.t4 

.27 

.04 

48.5 

t5.0 

t2.8 

1.7 

4.6 

5.0 

4.7 

.tt 

2.t 

.67 

3.2 

.30 

.23 

.04 

Major 

49.9 50.6 46.0 

t2.0 t3.9 t6.2 

3.7 1.5 3.2 

t1.4 6.6 6.7 

4.2 9.6 7.4 

8.0 9.6 8.5 

2.6 3.7 3.2 

1.3 .t4 .85 

t.3 3.t 2.4 

.48 .25 1.2 

2.9 .7t 2.3 

.75 .tt .4 

.27 .t7 .t2 

.7t .3 .05 

Minor 

Trace 

terrane northwest of Branscomb, characterized by Morozovella (M. aragonensis, M. Caucasica, M. subbotinae) and 

Acarinina pseudotopilensis. We also found late early Eocene faunas assignable to the Acarinina pentacamerata 

Zone (P 9, 53.4-52 Ma) in the Usal area. The Usal faunas 

are meager, but they also include rare fish debris and large 

nodosariid and agglutinated benthic foraminifers that appear to have been transported from a shallow-neritic to upper-bathyal setting and redeposited. Farther south, near 

Ukiah, the matrix of a melange unit assigned to the Coastal 

belt was found by Orchard (1979) to contain foraminifers 

of late early Eocene age. This fauna is characterized by 

agglutinated taxa in addition to Epistomina helicella and 

Quinqueloculina triangularis, which Orchard interpreted to 

indicate an outer-shelf setting. Thus, the highly deformed 

rocks of the Coastal belt may be older to the south and may 

contain strata that were originally deposited closer to the 

California margin, although the fauna referred to by Orelements 

50.t2 49.60 

t8.52 t6.56 

2.47 4.28 

4.tt 4.44 

2.68 5.38 

8.99 9.22 

5.22 3.3t 

t.46 t.25 

1.64 t.44 

3.09 2.58 

t.33 1.66 

.t8 .30 

trace .08 

elements 

elements 

51.3 

t6.6 

5.0 

2.7 

5.0 

6.6 

3.3 

t.t 

2.t 

4.2 

t.2 

.26 

.06 

1 .05 

<t5 

420 

200 

36 

560 

<30 

20 

t5 

t90 

t60 

47.2 

t4.3 

t0.5 

7.62 

t0.5 

3.64 

.04 

t.55 

.t5 

.t8 

chard also occurs in slope settings at bathyal depths. In 

contrast, the rare benthic foraminifers that occur within 

some matrix concretions in the Coastal terrane to the north 

include Cyclammina, Frondicularia, Gyroidina, and Haplaphragmoides, consistent with a slope setting at bathyal 

depths. 

Dinoflagellate assemblages indicate a probable late 

Eocene age for matrix concretions in the western part of 

the Coastal terrane between Cape Mendocino on the north 

and the Mattole River and Honeydew to the south (W.R. 

Evitt, written commun., 1983). Several of the species listed in table 1 are al::5sent in the one sample of this age from 

the Yager terrane that was examined, and so some of these 

forms may be restricted to the Coastal terrane. As in rocks 

of the Yager terrane, reworked dinoflagellates of Mesozoic 

age have been noted in rocks of the Coastal terrane. These 

reworked taxa include Gonyaulacysta jurassica (Deflandre) of Kimmeridgian or Oxfordian age (Evitt and Pierce, 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 23 
z 25,000 

0 

~ ....:l 

~ 20,000 

~ 

~ 

UJ. 

E-t 

~ 15,000 ~ 

~ 

E-t~ 

z 10,000 

~ 

E-; 

z 

0 

u 5,000 ~ 

A 

Coastal 

terrane 

~ .................... ! I ' I '...._ \ .................. 

\~ ............... 

\ . /. \ / \ /t/ 

VAB 

OFB 

OL-------~------~------~----~ 

0 1 00 200 300 400 

Cr CONTENT, IN PARTS PER MILLION 

1000 

c 

Coastal 

terrane 

/-\--. I MORB 100 I 

I 

I 

0 I 

........ e ,' ~ I 

p:: I 

I l-< 

u 

~ 

E-; 

10 

IAT 

1~----~------~------~~~~ 

10 100 1000 

Ni CONTENT, IN PARTS PER MILLION 

24 

0 22 

~ UJ. 20 ~ 

0 

::r:: 18 

E-; 

~ 16 

0.. 

g3 14 

~ 12 

z 

-~ 10 

E-t 

z 

~ 8 

E-t 

z 

0 6 

u 

~ 4 

2 

00 

MORB 

AB 

2 4 6 8 1 0 12 14 16 18 20 

Ti CONTENT, IN PARTS PER THOUSAND 

D 

( 

I 

50 

/ 

/ 

/ 

/ 

100 

I 

I 

.... 

150 

t 

200 

Zr CONTENT, IN PARTS PER MILLION 

EXPLANATION 

e Usal t DSDP Site 173 

ffi Whitethorn * Most primitive 

FAMOUS glass 

Figure 10. Minor- and trace-element chemistry of volcanic rocks of Coastal terrane and from Deep Sea Dri IIi ng 

Project (DSDP) Site 173. Analytical data given in table 2. 

Ti content converted from Ti02• A, Ti versus Cr contents, 

showing boundary between fields of ocean-floor basalts 

(OFB) and volcanic-arc basalts (VAB) from Garcia (1976). 

B, V versus Ti contents showing boundary between fields 

of island-arc tholeiites (IAT), midoceanic-ridge basalts 

(MORB), and alkali basalts (AB) from Shervais (1982). C, 

Ti/Cr ratio versus Ni content, showing boundary between 

fields of island-arc tholeiites (IAT) and midoceanic-ridge 

basalts (MORB) from Beccaluva and others (1979). 0, Ti 

versus Zr contents, showing field of oceanic tholeiites 

(OTB) from Floyd and Winchester (1975), fields of midoceanic-ridge basalts (MORB) and island-arc basalts (lA) 

from Pearce and Cann (1973), and plot of most primitive 

glass from French-American Mid-Ocean Undersea Survey (FAMOUS) from Shervais and Kimbrough (1985). 

24 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple junction, Northwestern California 
1975) and' Paleocystodinium of Coniacian or younger age 

(J.L. Clark, written commun., 1986). 

Spores and pollen examined from two widely separated localities in the Coastal terrane (table 1) contain middle 

Eocene and late Paleocene or Eocene floras, respectively. 

The middle Eocene flora comes from the Devils Gate locality near Cape Mendocino; in addition, these rocks contain probable reworked Cretaceous forms (Proteacidites 

and Siberiapollis) and probable reworked Paleocene to 

early Eocene taxa (Betulaepollenites, Caryapollenites prodromus, and C. imparalis). About 90 krn southeast, west 

of Branscomb, terrigenous rocks of the Coastal terrane 

contain a late Paleocene or Eocene flora and little, if any, 

reworked pollen. 

The collective fossil evidence indicates that the terrigenous strata which compose the matrix of the Coastal terrane in the Cape Mendocino region are predominantly 

middle to late Eocene in age. The presence of strata of 

Paleocene and early Eocene age from about Usa] southward suggests that the Coastal terrane becomes older to 

the south. Dinoflagellates, spores and pollen, and foraminifers in the Cape Mendocino region indicate that the terrigenous rocks of the Coastal terrane are predominantly 

younger (middle to late Eocene) than those of the Yager 

terrane (Paleocene? to middle Eocene), which is dated 

chiefly on the basis of spores and pollen and some dinoflagellates. The palynomorphic and foraminiferal data also 

suggest that most terrigenous matrix rocks of the Coastal 

terrane (excluding sandstones interbedded with basalts at 

Usa!) were deposited at middle latitudes north of about lat 

30° N., probably in an outer-slope or inner-trench-slope 

setting, at bathyal depths. In the southeastern part of the 

Coastal terrane, the depositional setting may have been 

shallower and somewhat closer to the continental margin. 

Age and Depositional Setting of the Pelagic Limestone 

Associated with Basaltic Rocks 

The pelagic limestone, which is interbedded with exotic blocks of oceanic basalt, contains planktic foraminifers (fig. 12B) ranging in age from early Campanian (ca. 

82 Ma) to middle Maastrichtian (ca. 71-69 Ma) (Sliter, 

1984; Sliter and others, 1986). The pelagic fauna in these 

rocks is characterized by the presence of keeled forms 

(fig. 13) typical of low-latitude (lat 20° N. to 20° S.) Tethyan environments of the Pacific Ocean (for example, Gansserina gansseri s.l. and Rosita wal.fischensis s.l.; Harbert 

and others, 1984; Sliter, 1984; Sliter and other~. 1986). 

Several biofacies recognizable in the pelagic limestone 

suggest its deposition near active volcanic edifices at bathyal 

....... Figure 11. Melange of Coastal terrane, located south of 

Cape Mendocino. A, Melange matrix of scaly, sheared argillite. Armored, rounded to lenticular fragments below and to 

right and left of knife (circled) are carbonate concretions (C); 

other fragments are attenuated sandstone beds. 8, Photomicrograph of carbonate concretion, showing foraminifers of 

midlatitudinal affinity; magnification, x1 00. Located at mouth 

of Peter B. Gulch near Petrolia. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 25 
depths, in a slope or basinal setting. At Parkhurst Ridge, at 

least two manganiferous biofacies occur: a mildly bioturbated facies and a highly bioturbated dissolution facies. The 

A.. ' 

0 

I 

0.5mm 

0.1 mm 

I 

mildly bioturbated biofacies contains rare, small planktic 

foraminifers and benthic foraminifers that include Aragonia, 

large, coiled, calcareous Gyroidinoides spp. and Ellipsoglandulina? sp., agglutinated Bathysiphon, and rare fish 

debris. This fauna indicates deposition in a deep, lowerbathyal setting at or just below the carbonate-compensation 

depth (approx 3,000 m) that probably received rare debris 

displaced downslope. The highly bioturbated facies consists 

of stylolitic micrite containing distinct layers of fragmented 

and distorted planktic and benthic foraminifers that include 

small agglutinated genera, broken nodosariids, and small 

gyroidinids. These fossils have undergone partial dissolution 

as a result of either their downslope transport to well below 

the carbonate-compensation depth or, alternatively, their 

proximity to areas of hydrothermal venting. Manganiferous 

pelagic limestone near Usal contains a meager foraminiferal 

fauna assignable to the Campanian and to the early Maastrichtian Globotruncanella havanensis Zone (ca. 74 Ma). 

0.1 mm 

Figure 13. Photomicrograph of pelagic foraminiferal limestone of Coastal terrane, located near Parkhurst Ridge, 

showing keeled, low-latitude Campanian foraminifers; magnification, xl 00. 

..... Figure 12. Photomicrographs of pelagic limestone of 

Coastal terrane, from Dugans Opening near Usal. A, Volcanic-glass fragments cemented by foraminiferal limestone; magnification, x20. 8, Clastic dikelet of K-feldspar-bearing 

arkosic sandstone cutting foraminiferal limestone; magnification, x20. Note discordance of dikelet to stylolitic partings in 

limestone. C, Same clastic dikelet as in B, but at x40 magnification and with crossed nicols. K, detrital K-feldspar; P, plagioclase; Q, quartz. 

26 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple junction, Northwestern California 
Rare, reworked, calcareous benthic foraminifers, bivalve 

fragments, and fish debris present in this fauna probably 

were transported downslope, from outer-neritic and upperbathyal (200-500 m) depths, to a predominantly bathyal 

setting. Other transported debris a~sociated with these limestones include abundant volcaniclastic rocks (some vesicular) and angular clasts of radiolarian ooze, further suggesting 

proximity to a volcanic edifice. Both the detrital and foraminiferal components of the pelagic limestone at Usal display 

graded bedding, suggesting that turbidity currents may have 

been involved in their transport to bathyal depths. 

The local interbedded relation of arkosic sandstone 

with the pelagic limestone associated with basaltic rocks 

at Usal (figs. SA, 8B) indicates that at least some terrigenous rocks of the Coastal terrane also are of Late Cretaceous age and were depo~ited at a low-latitude site. The 

complex relation between the composite terrane of Upper 

Cretaceous basalt, limestone, and interbedded sandstone 

and the enclosing Eocene argillite and sandstone matrix of 

the Coastal terrane near Usal is illustrated in figure 7. The 

following Late Cretaceous planktic foraminifers have been 

identified from the Coastal terrane near Usal (locs. 1, 2, 

fig. 7): 

Aragonia sp. 

Archaeoglobigerina cretacea 

Globotruncana area (Cushman) 

bulloides 

linneiana (d'Orbigny) 

rosetta 

ventriocosa White 

Globotruncanita stuarti 

stuartiformis (Dalbiez) 

Planoglobulina sp. 

Pseudotextularia sp. 

Rosita fornicata (Plummer) 

plummerae (Gandolfi) 

The following Early Tertiary planktic foraminifers have 

been identified from the same area (loc. 3, fig. 7): 

Acarinina Broedermanni (Cushman and Bermudex) 

spinuloinjiata (Bandy) 

Planorotalites pseudoscitula 

Pseudohastigerina micra (Cole) 

Subbotina linaperta (Finlay) group 

senni (Beckmann) 

Truncorotaloides topilensis (Cushman) 

Pink to red pelagic limestones intercalated with basaltic flows and aquagene tuffs at Dugans Opening near Usal 

(Joe. I, fig. 7) have yielded a low-latitude Campanian to 

Maastrichtian fauna. Nearby, a carbonate concretion within a lenticular body of sandstone enclosed by basalt (loc. 

1, fig. 7) contains a similar low-latitude Campanian to 

Maastrichtian fauna. However, approximately 2 km northwest of Dugans Opening (loc. 3, fig. 7), several limestone 

concretions collected from sheared argillitic matrix of the 

Coastal terrane contain late early Eocene (planktic zone 

P 9) foraminifers, indicative of deposition at temperate, 

middle latitudes. Approximately 2 km north of this locality, basaltic flow rocks are interbedded with thick beds of 

laumontized arkosic sandstone, presumably of Late Cretaceous age, similar to the sandstone lens within basalt at 

Dugans Opening to the southeast (figs. 7, 8). 

Paleomagnetic Data 

Paleomagnetic studies have been conducted on the terrigenous and low-latitude oceanic components of the 

Coastal terrane. Paleomagnetic data from the Devils Gate 

area yield no eyidence for the latitude of deposition of 

these rocks. Cores collected from well-bedded unweathered terrigenous rocks at Devils Gate were found to have a 

characteristic remanent magnetization that was acquired 

after folding, indicating that the rocks are remagnetized. 

The timing of remagnetization is constrained only by the 

middle Eocene age of the Devils Gate section, and so it 

could be any time between the middle Eocene (possibly 

during early interaction of the California margin with the 

oceanic component of the Coastal terrane) and the 

Pleistocene. 

Another locality in the Coastal terrane that was studied 

in some detail is on Parkhurst Ridge north of Honeydew 

(fig. 14). At this locality, a large lens of low-latitude, pink 

to red limestone of Campanian to Maastrichtian age is folded into a steeply east plunging syncline (Harbert and others, 

1984; Sliter and others, 1986). The limestone depositionally overlies a large composite body of diabase dikes and 

pillow basalt (fig. 15). Harbert and others (1984) found that 

the observed mean inclination of this limestone (/=-47°) is 

about 23° shallower than the expected mean cratonal inclination for North America (/=-62.8°) during Campanian to 

Maastrichtian time. This result was interpreted to approximate the primary remanent inclination of the limestone, 

although application of a fold test to the data was inconclusive at the 95-percent-confidence level, owing to the steep 

plunge of the folded limestone. Harbert and others concluded from their paleomagnetic and paleontologic data that the 

limestone on Parkhurst Ridge was deposited between 80 

and 70 Ma at latitudes near or at the Pacific-Farallon Ridge 

between about lat 20° and 28° N.; the paleomagnetically 

determined mean is lat 28° N. Recently, Hagstrum (1990) 

has suggested that the remanent magnetization in the limestone of Parkhurst Ridge could reflect a remagnetization 

acquired before folding, but at the time that the limestone 

reached the subduction margin. By his interpretation, the 

limestone would have formed south of lat 28° N. Also, he 

has made tenuous assumptions ( 1) that the limestone was 

flat and undeformed until reaching the subduction margin 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 27 
40° 

17' 

30" 

40° 

16' 

30" 

1957 

X 

~ ~ ~ 

I ~ v > L-> " v > L- " v > L-> v > L-> v > ( 

. /v /... > v '- > " '- > v '- ~> v '- '> v 

II -1 "' --1 " "1 A -1 1'\ -1 " ..,.) 

·~v -~ v > v > v > v > v 

. / " ..Y,_ " > L- " > L- " > L- " > L- " 1 ./1 1- > v '- > v '- > v I- > v /... > " '- > 

/ ~ "~ "~ "~ "~ "~ "/ ~ / v > v > v > v > v > v > 

_ ___... I , > '- ' > '- ' > t- ' > '- ' > '- ' > '- i ~ / >" '->V '->V '->V t->V '->V '-

2368 

X Long /. " -1 " -1 2275 ~ " ~ " -1 " -1 

( >v>>" X v>>v >v >v > 

t-, 1-, \ '- ~.-,>t-, 

\

'- > v '- > v '- > v /... 

A -1 /1 -1 1\ -1 

\

v > v > v > v 

'>t-, >t-, >t-, > 

'-...., ~ > v L- > v L- 83-4 · '\_ A -1 1\ -1 1\ -1 

/ > v > v . 

creeJt _...,../ t " > L- " > L- 8:1-3 L- 0 

\ 

rd ~· > v ,_ > v ,_ 84-2 c . v ,_ ~ itch~·---· ~ ~ ~ 83-5 _, pr I v >v v >v 

'"'""-" , ......-' . ~ '- \ > 1- ' > (lJ ' > '-

,y.. > v L- > \ ~ > v 

__..--/.<{-- · 

.~X ~ 1485 

~ ( 

fi.. 

. .-/ ' " _, " VI " 

, . > "> " • .P " > ____> r 

-~ 1~ ~~ 

\ .... r·· ..... ··\··~ 

\ ····~ Q._ \ 

\ \ ( ·~· .. ...... ,...._.. __ I ~( ~ 

J 4~\ ( 1: \ (fJ ~--4 (( "" ~ (( ""' ---.. . ( ~. 

2059 

0 0.5 1.0 KILOMETERS 

EXPLANATION 

\ 

I 

~ 

) 

.G. 

""'\. /' \ ' ~ ).~ 

r ~ 

J>.:~Q; ~ / ~~ 

CJ Sandstone 

CJ Limestone 

D Basalt 

--- Contact-Dashed where inferred; dotted where 

concealed 

--- Fault-Dashed where inferred; dotted where 

concealed 

@ Quaternary landslide complex- Dotted line shows 

smaller landslide unit within composite slide. 

Arrow indicates direction of downslope 

movement. Hachured line shows landslide 

scarp; solid line indicates base of scarp, and 

hachures point uphill. Locally, includes: 

~ Closed depression within landslide complex 

Spring within landslide complex 

-+-Syncline- Dashed where inferred. Arrow 

indicates direction of plunge 

~~~~ Gouge zone 

~ Strike and dip of bedding- Ball indicates top 

84_

3 direction known 

-----. Paleomagnetic coring site- Locality number refers 

1485 

X 

to table 2 in Harbert and others (1984) 

Spot elevation 

..---. ..__.p Drainage pattern-Arrow indicates downstream 

direction 

Figure 14. Detailed geologic map of Coastal terrane at Parkhurst Ridge near Honeydew, showing location of samples 

cored for paleomagnetic studies. Interpretation of data from selected coring sites given in Harbert and others (1984). 

Geology mapped by R.J. Mclaughlin, 1982. See figure 2 for location. 

28 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
and (2) that the limestone did not reach the margin until the 

Oligocene. 

Structure 

The overall structure of the Coastal terrane has been 

characterized by earlier writers as "broken formation" (usage of Hsu, 1968). The western part of the Coastal terrane, 

however, including most of the Cape Mendocino region, is 

deformed by highly penetrative shearing (fig. 16) and contains exotic blocks of blueschist and far-traveled oceanic 

rocks, and so it is more appropriately termed a melange 

(Hsu, 1968). The eastern part of the Coastal terrane is less 

penetratively deformed, and large areas of intact folded 

strata are interrupted by closely to widely spaced fractures 

and faults commonly lined with calcite+laumontite. In 

some areas, sandstone-rich parts of the section have the 

appearance of being shattered as a result of pervasive fracturing. We regard the pervasively sheared argillitic zones 

that are interspersed with broken rocks as a melange matrix (fig. 11A). The broken rocks are largely Paleocene to 

late Eocene in age but are, at least in part, Late Cretaceous 

(Campanian to Maastrichtian) locally (for example, in the 

Usal area). The argillitic matrix rocks (excluding the exotic and far-traveled blocks) are early to late Eocene in age. 

Deformational structures composing the tectonic fabric 

of melange in the Coastal terrane of the Cape Mendocino 

region record a Cenozoic transition from obliquely convergent subduction to transpressional right slip. Investigations 

of these deformational structures (Beutner and Hansen, 

1975) indicate that the earliest structure in these rocks is a 

Figure 15. Pelagic foraminiferal limestone overlying basalt of 

Coastal terrane, located at Parkhurst Ridge. Note drill holes 

in limestone where cores used in paleomagnetic study were 

taken. Limestone contains foraminifers of Late Cretaceous 

(Campanian) age. Section faces east (to left). 

widespread penetrative shear fabric (S1) characterized by 

north- to northeast-directed thrusting. Beutner and Hansen 

considered the date of this early deformation to be 55-45 

Ma, but the youngest affected rocks of the Coastal terrane 

are late Eocene in age. Thus, the S1 fabric and all later 

deformation in these rocks formed after about 41 Ma. We 

suggest that these early, north-northeast-directed thrusts are 

related to emplacement of the Coastal terrane along the 

California accretionary margin, possibly over or against a 

"backstop" composed of the Central belt. The direction of 

tectonic transport and the timing of this S1 deformation appear to correspond to f 1 recumbent folding in the Yager 

terrane, suggesting that eastward obductive emplacement of 

both terranes occurred during the Oligocene (possibly with 

the Yager terrane already overlying the Coastal terrane). 

The S1 fabric of the Coastal terrane is folded into upright 

compressional folds (f2) with orientations suggesting that 

east-northeast- to west-southwest-oriented compression followed S, deformation. The clockwise- rotated compression 

exhibited by / 2 structures (Beutner and Hansen, 1975) may 

Figure 16. Boudined sandstone in penetratively sheared argillite in melange of Coastal terrane, located on beach below 

lighthouse at Cape Mendocino. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 29 
reflect somewhat more head-on convergence along the California subduction margin after initial emplacement of the 

Coastal terrane. These f 2 folds, which may also correspond 

to .h folds in the Yager terrane, are overlapped by the less 

severely deformed Miocene and younger Bear River beds 

of Haller (1980) and the Wildcat Group of Ogle (1953). 

Beutner and Hansen (1975) also found that several 

steeply dipping shear zones, trending east-west to northwest, developed sequentially from south to north, that they 

exhibit left-lateral slip, and that all of these shear zones 

postdated S1 and / 2 structures. Our mapping suggests that 

this shearing is not confined to discrete zones except 

where incompetent argillitic rocks are present (for example, near Petrolia) and that it is discontinuous over relatively short distances. 

Rocks of the Neogene Wildcat Group were reported by 

Ogle (1953) to' be offset along east-west shears of the False 

Cape shear zone (fig. 17A). Related strata of middle Miocene and Pliocene age, exposed along the Bear River to the 

south of False Cape, are also juxtaposed with the Coastal 

terrane along faults of the same orientation as the shearing 

within the Coastal terrane and Ogle's False Cape shear 

zone. Harbert and Cox (1986) proposed that a major clockwise rotation by about 20° of the Pacific-Farallon Ridge 

(now the Pacific-Gorda and Pacific-Juan de Fuca Ridges) 

occurred between 3.86 and 3.4 Ma, revising the earlier 

work of Cox and Engebretson ( 1985). This rotation is 

thought to have initiated transpression along the San Andreas transform margin of California, as well as deformation of the Gorda plate due to a slowing of spreading at the 

south end of the Gorda Ridge (Wilson, 1986). Structural 

relations seemingly resulting from this deformation of the 

Gorda plate include ( 1) steepening and reorientation of subduction-related thrusts in the offshore accretionary wedge, 

as indicated by their present alignment with east-west- to 

northwest-southeast-oriented structures in the onshore 

Coastal terrane south of Cape Mendocino; and (2) overprinting of these subduction-related faults by east-west- to 

northwest-southeast-oriented, left-lateral shearing where 

the faults bend onshore. We tentatively attribute the northward-younging, left-lateral shearing described by Beutner 

and Hansen (1975) to sequential coupling of the Gorda 

plate to North America since 3.86-3.4 Ma (fig. 17B). 

South of Cape Mendocino, between Petrolia and the 

north boundary of the King Range, the foliation of eastwest- to northwest-trending shearing in the Coastal terrane 

dips at moderate to low angles south-southwest into and 

beneath the King Range terrane. Near Petrolia, a southdipping wedge of silty Pliocene mudstone containing a 

lower-bathyal (2,00~,000 m) foraminiferal fauna is imbricated with the Coastal terrane along thrust faults associated with this shearing (fig. 2). Nearby, Upper Miocene 

rocks assigned to the Wildcat Group that contain a middleto lower-bathyal foraminiferal fauna also are faulted against 

the Coastal terrane (Miller and others 1983). Adjacent to 

the King Range, rare blocks of high-grade blueschist occur 

in melange formed by this youthful Neogene shearing. We 

consider the late Cenozoic shearing to be associated with 

obductive north-northeast-directed emplacement of the 

King Range terrane over the Coastal terrane during Pliocene to Pleistocene time. 

Implications of Structure for Hydrocarbon Exploration 

South-dipping, late Cenozoic thrust faults associated 

with emplacement of the King Range terrane in the Petrolia area have important implications for hydrocarbon exploration in the onshore and offshore area. Historical oil 

production near Petrolia (MacGinitie, 1943) and numerous 

oil seeps that issue from argillitic rocks of the Coastal terrane along the coast between Cape Mendocino and the 

mouth of the Mattole River are evidence that the Coastal 

terrane is a source of hydrocarbons. Although high-paraffin-base hydrocarbon occurrences in this area probably are 

derived from organic-rich argillic rocks of the Coastal terrane, economically exploitable hydrocarbons probably are 

present only in the more permeable rocks. Permeability 

potentially provided by the highly fractured sandstone lithofacies of the Coastal terrane is compromised by the extensive laumontization of these rocks. Potential reservoir 

rocks with good permeability may, instead, be provided by 

the sandy lithofacies of tectonically buried remnants of 

Neogene strata thrust beneath rocks of the Coastal terrane. 

Such buried thrust remnants may be present in the offshore area west of Petrolia, north of the Mendocino Fracture Zone, and in the area immediately north and east of 

the King Range terrane boundary on shore. However, the 

youthful deformation and structural setting at the junction 

of the seismically active Mendocino Fracture Zone and the 

San Andreas fault may pose a considerable hazard, especially, to offshore exploration. 

Accretion of the Coastal Terrane 

Using the techniques of M.G. Debiche of Stanford 

University, Stanford, Calif., and Engebretson and others 

( 1985), Harbert and others ( 1984) formulated a terrane-trajectory model constraining the depositional latitude of the 

limestone on Parkhurst Ridge, the timing of its incorporation into the Coastal terrane along the North American 

plate margin, and its final accretion to northern California. 

In their reconstruction, the date of initial interaction of the 

pelagic limestone with North America is considered to be 

approximately 40 Ma because the limestone is enveloped 

by middle to upper Eocene terrigenous rocks. The field 

relations near Usal described in this report require a refinement of this model (figs. 18, 19). 

The age and paleolatitudinal setting of the pelagic 

limestone and the composition of associated basaltic rocks 

of the Coastal terrane suggest that the limestone and basalt 

30 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, North.western California 
40° 30' 

A 

PACIFIC 

PLATE VIZCAINO BLOCK OF 

PACIFIC PLATE ---, \ 

0 5 10 15 20 25 KILOMETERS 

EXPLANATION FOR FIGURE 17A 

c:J Late Cenozoic overlap assemblage 

00 

(Pleistocene to early Miocene) 

King Range terrane (middle Miocene and 

Late Cretaceous) 

~ Coastal terrane (late Eocene to Late 

Cretaceous) 

(z;;t:: Resistant map-scale block, largely of 

broken sandstone and argilliteEnclosed in melange matrix of 

Coastal terrane 

~ Yager terrane (late Eocene to Paleocene?) 

---Contact 

__.__._.__ 

~ 

~ 

A;- ----- ·........_ _-·, 

-- 500 

-eA A' 

Thrust fault-Dashed where inferred; queried where uncertain. 

Sawteeth on upper plate 

Thrust fault within offshore accretionary wedge-Dashed where 

inferred. Sawteeth on upper plate 

Basal thrust of Cascadia subduction zone-Dashed where inferred; 

queried where uncertain. Sawteeth on upper plate 

Fold axis within offshore accretionary wedge 

Onshore course ofMattole River and offshore submarine canyon 

Bathymetric contour-Shown in meters 

Strike of planar element (bedding, shear fabric, axial plane of fold) 

associated with onshore projection of Cascadia subduction 

complex 

Strike of planar element (bedding, shear fabric, axial plane of fold) 

within area that is compressed between Pacific and North 

American plates and broken by southwest-dipping thrust faults 

- · - ··- ·- Boundary of mapping of offshore units 

~ Fault-Dashed where inferred; queried 

where uncertain. Arrows indicate 

direction of relative movement L____j Location of cross section shown in figure 29C 

Figure 17. Structure of Cape Mendocino area. A, Offshore and onshore structural elements. Offshore geology modified from Clarke and Field (1989). Franciscan 

rocks south of Russ fault zone were originally designated as False Cape shear zone by Ogle (1953). 8, Hypothetical relation of sinistral shearing in Coastal terrane 

to coupling between North American plate and deforming Gorda plate. Numbers 1 through 3 indicate relative 

ages of northward-younging sinistral shearing of North 

American plate (from Beutner and Hansen, 1975). Solid 

sawteeth, basal thrust of Cascadia subduction zone; 

open sawteeth, thrust fault within offshore accretionary 

wedge; small arrows, sinistral shearing; open arrow, 

plate motion with respect to North American plate. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 31 
A. -80 Ma (Campanian) B. -70 Ma (Maastrichtian) 

C. -56 Ma (Paleocene) D. -40 Ma (Late Eocene) 

EXPLANATION 

Accretionary prism-Contains terrigenous sediment of Coastal 

and Yager terranes and blocks of pelagic limestone offscraped 

from oceanic plate 

Terrigenous sediment of Coastal and Yager terranes 

(largely Eocene)-Arrow indicates direction of transport 

Older terrigenous sediment of Coastal terrane (Cretaceous)-

Arrow indicates direction of transport 

Pelagic limestone of Coastal terrane (Cretaceous)-Area 

enclosed by dashed line in 18D indicates projected 

position oflimestone after subduction 

Lithosphere that could be either Kula or Farallon plate-Arrows 

indicate alternative plate-motion vectors 

Figure 18. Plate-tectonic model for accretion history of Coastal terrane, modified from reconstructions in fixed-hotspot reference frame by Engebretson and others (1985). FAR, Farallon 

plate (N, north; S, south); KUL, Kula plate; MFZ, Mendocino 

Fracture Zone; NAM, North American plate; PAC, Pacific 

plate. Longitudes in degrees east of Greenwich; present-day 

North American coastline shown for reference. A, From 82 to 

80 Ma, pelagic limestone is deposited on Farallon plate northeast of Kula-Farallon-Pacific Ridge triple junction, and sediment derived from continental margin at approximate latitude 

of present-day northern Mexico is sporadically transported by 

submarine fans or canyons and deposited in near-ridge area. 

B, From 70 to 69 Ma, pelagic limestone continues to be deposited on Farallon plate north of Mendocino Fracture Zone 

between lat 20° and 28° N., and terrigenous sediment contin-

+ + + Active subduction margin-Sawteeth on upper plate 

Active spreading ridge and transform fault-Ridge-transform 

geometry known from magnetic lineation data 

/l {1 Hypothetical position of now-subducted Kula-Farallon g· ,.._":! ridge-transform system (modified from Engebretson and 

""-? /1 · >...{/ others, 1985)-Queried segments show alternative positions 

l/ · 'lJ ofridge-transfonn system if lined area is either Farallon 

or Kula plate 

Abandoned Pacific-Kula ridge-transform system-Shown in 

18D only 

General direction of plate motion with respect to fixed-hotspot 

reference frame 

Hotspot-H. Hawaiian; Y, Yellowstone 

ues to be sporadically deposited near ridge. C, By about 56 

Ma, new supply of sediment, derived from interior of presentday California and Oregon, is deposited in trench and on 

trench slope above subduction zone of Farallon plate as nowcomposite terrane of pelagic limestone, basalt, and terrigenous clastic deposits nears continental margin. 0, Between 56 

and 40 Ma, central part of Farallon plate is nearly completely 

subducted, carrying most of pelagic limestone and terrigenous 

clastic deposits beneath North American plate, and accretionary prism is formed above trench of subduction margin by 

tectonic mixing of offscraped blocks, which include basalt, 

limestone, and interbedded low-latitude sandstone, with continually supplied, largely Eocene sedimentary deposits. Between 40 and 24 Ma, Coastal terrane is completely accreted 

to North American plate. 

32 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
formed between lat 20° and 28° N., in a near-ridge setting, 

between early Campanian (ca. 82 Ma) and middle Maastrichtian (ca. 69 Ma) time (fig. 19). The present position 

and interstratification of terrigenous sedimentary rocks 

with basalt and limestone near Usal further require that the 

oceanic-ridge system at the time of deposition was both 

within reach of sediment derived from the American plate 

margin and north of the Mendocino Fracture Zone (figs. 

18A, 18B). To satisfy these conditions, using the plate reconstructions of Engebretson and others (1985), the depositional setting of the pelagic limestone at 70 Ma is 

uniquely located at or just east of the Pacific-Farallon 

Ridge, near the Pacific-Kula-Farallon plate triple junction. 

Speculatively, sediment derived from the North American 

A 

46 or 42 

~ 41 

~ 

;::J 

~ ~ 

~ s s j ~ ~ 

r-4 cv:l 0 cv:l ~ 

~ cv:l ~ 

~ ~ c ~ 

28 

20 

Cape Cape Mendocino 40 Mendocino 

Diablo 

Range 

Great 

Valley Rp = 

1.8-4.9 

cm/yr 

36 

NORTH 

AMERICA 

Rp =7.1-7.2 crnlyr 

1 

t

···:·:: :.v.·l ~-::·::· :_ 

••• 

EXPLANATION 

Terrigenous sediment of Coastal and Yager terranes 

(Tertiary)-Arrow indicates direction of transport 

Older terrigenous sediment of Coastal terrane 

(Cretaceous)-Arrow indicates direction oftransport 

General orientation of subduction margin- Sawteeth 

on upper plate 

Figure 19. Summary of accretion history of pelagic limestone and associated sedimentary rocks of 

Coastal terrane near Cape Mendocino, showing change in rate of poleward component of motion (RP) 

before and after encounter with continental margin at 49 Ma. Paleolatitudes derived from reconstructions of Harbert and others (1984) and Engebretson and others (1985); poleward distances calculated by 

assuming that 1 o of latitude equals 111 km. Present-day coastline of North America shown for reference. A, Deposition of pelagic limestone of Coastal terrane (diagonal lines) occurred between 82 and 

69 Ma at paleolatitudes between 20° and 28° N. During this time, detritus from continental margin 

near northern Mexico was deposited in vicinity of Kula-Farallon-Pacific plate triple junction and transported northward with limestone on Farallon plate. Time and position of encounter with North American plate margin (star) is based on presence of jadeitized metasandstone detritus of inferred Diablo 

Range (paleolatitude 41 o N.) provenance in middle Eocene (ca. 49 Ma) conglomerate of Coastal terrane. Final accretion at Cape Mendocino (vertical lines) must have occurred sometime between about 

40 and 24 Ma on the basis of late Eocene age (ca. 40 Ma) of youngest fossils in Coastal terrane and 

early Miocene age (ca. 22 Ma) of oldest strata that overlap rocks of Coastal terrane. Position of Cape 

Mendocino on North American plate moved southward relative to Farallon plate between 40 Ma (paleolatitude 46° N.) and 24 Ma (paleolatitude 42° N.). Range of RP values before 49 Ma is based on 

poleward distances of either 13° (1 ,443 km) in 20 m.y. or 21 o (2,331 km) in 33 m.y. 8, Range of RP 

values after 49 Ma is based on present-day poleward distance between Diablo Range (lat 36° N.) and 

Cape Mendocino (lat 40° N.) of 4° (444 km), which is distance traveled in either 9 m.y. (final accretion 

at 40 Ma) or 25 m.y. (final accretion at 24 Ma); distance traveled parallel to present-day continental 

margin may have been as far as 600 km. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 33 
margin at the 70-Ma latitude (present position of northern 

Mexico, fig. 18B) may have been transported to the vicinity of the Pacific-Farallon Ridge in a major submarine-canyon or fan system, possibly analogous to the Zodiac Fan 

system of southern Alaska (Scholl and others, 1977; 

Byrne, 1979; Harbert, 1987). 

Cretaceous oceanic and terrigenous strata deposited on 

the Farallon plate moved obliquely northeastward toward 

the American plate margin at a poleward rate of about 7.1 

to 7.2 crn/yr (fig. 19). These rocks next interacted with 

younger sediment derived from the American plate margin 

between late Paleocene and late Eocene time (ca. 55-41 

Ma). A significant component of terrigenous detritus in the 

Coastal terrane near Cape Mendocino probably was derived during middle Eocene time (ca. 49 Ma) from the vicinity of the southern Diablo Range (figs. 18C, 18D) on 

the basis of the occurrence of conglomerate clasts of metasandstone containing abundant jadeitic pyroxene with a 

probable source in the present-day Diablo Range (figs. 

6C-6E). From about 49 to 41 Ma, as the Farallon plate 

continued its oblique convergence with North America at 

poleward rates of 1.8 to 4.9 crn/yr (fig. 19), terrigenous 

sediment, supplied to the slope and trench-slope area from 

the interiors of California, Oregon, Washington, and Idaho, 

contributed to formation of the accretionary margin of 

California (Underwood and Bachman, 1986). Latest late 

Paleocene and early Eocene (ca. 60-52 Ma) channeled 

shelf and slope deposits (not part of the Franciscan) unconformably overlie the Central belt of the Franciscan 

Complex and Great Valley sequence in the Coast Ranges 

locally (Short and Cummings, 1987). These channeled sequences may have transported the sediment that fed distributary-channel systems of the Coastal and Yager 

terranes. 

Accretion of the near-ridge basalt and pelagic limestone to the California margin and its incorporation into 

the Coastal terrane occurred after about 40 Ma, on the basis of the youngest terrigenous strata of the Coastal and 

Yager terranes (fig. 18D). Between latest Eocene (ca. 

40 Ma) and latest Oligocene (ca. 24 Ma) time, poleward 

translation of the limestone-basalt slowed from about 5 to 

less than 2 crn/yr (fig. 19), as it was scraped off the subducting Farallon plate and imbricated with slope and 

trench-slope sediment above the California subduction 

margin. This imbrication was accompanied by landwarddirected thrusting onto the Central belt of the Franciscan 

Complex and relative westward emplacement of the Central belt over the Coastal and Yager terranes along the 

Coastal belt thrust. 

Accretion was apparently completed by the early Miocene (fig. 19) because lower-bathyal strata of this age (ca. 

22 Ma) occur in lowermost Neogene strata of the Bear 

River area and in the Eel River basin (Chetelat and Ingle, 

1987). These strata locally overlie rocks of the Coastal and 

Central belts unconformably. 

King Range Terrane of the Coastal Belt 

The King Range terrane, which is the westernmost terrane of the Coastal belt, contains the youngest accreted 

rocks of the Franciscan Complex. The King Range terrane 

is subdivided into two subterranes on the basis of the 

structural discontinuity, contrasting lithology, and age differences of the rocks. These subterranes are considered to 

form the composite King Range terrane because both subterranes were affected by a major thermal event during 

mid-Miocene time that did not affect adjacent rocks of the 

Coastal terrane. The north boundary (Cooskie shear zone, 

which intersects the coast north of Cooskie Creek) and 

east boundary (which approximately aligns with Bear 

Creek) of the King Range terrane are marked by highly 

sheared and crush.ed rocks considered to be discrete shear 

zones by some workers (Nason, 1968; Beutner and Hansen, 1975). We interpret this boundary to be a major terrane suture but consider most shear deformation to be 

within the lower plate of the suture. On the basis of structural and aeromagnetic data, this suture dips about 30° 

SW., indicating that the King Range terrane structurally 

overlies the Coas'tal terrane (Griscom, 1980b; McLaughlin 

and others, 1982). 

The San Andreas fault zone may mark the southwest 

boundary of the King Range terrane, although this fault 

has never been located in the offshore area between the 

Mendocino Fracture Zone and Point Delgada (fig. 17A). 

South of Point Qelgada and opposite the mouth of Whale 

Gulch, the northernmost mapped segment of the offshore 

San Andreas fault zone is a zone of faulting as much as 

10 km wide (fig. 2; McCulloch, 1987a, b). The east side of 

this zone of faulting projects northward, parallel to the 

Bear Harbor fault along the coast (fig. 20A), and aligns 

with youthful faulting in Whale Gulch (Beutner and others, 1980). Anoth.er steep, conspicuous, north-south-trending fault at this latitude projects toward Point Delgada and 

is traceable southward to Point Arena (Curray and Nason, 

1967; McCulloch, 1987a). This segment, which is generally regarded as the trace of the San Andreas fault that was 

active during the 1906 San Francisco earthquake, may step 

westward and continue northward from Shelter Cove in 

the offshore area. Therefore, we regard the King Range 

terrane as bounded on the east, west, and north by faults 

..... Figure 20. South end of King Range, showing rocks of 

King Range terrane thrust northeastward over Coastal terrane. A, Southeast boundary of King Range terrane along 

Bear Harbor and 1Whale Gulch fault zones. Faults are dashed 

where inferred, dotted where concealed, and queried where 

uncertain; sawtee~h on upper plate. Arrows indicate direction 

of relative movement. View northward. Photograph by D.G. 

Herd, U.S; Geolpgical Survey. 8, Fault zone at mouth of 

Whale Gulch (see A for location). View northeastward. 

34 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
A Cape Mendocino 

King Peak 

/ 

Chamisal Mountain 

/ 

/ 

B 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 35 
of the San Andreas system and Mendocino Fracture Zone 

(fig. 2). The rocks on the southwest side of the presumed 

offshore San Andreas fault zone adjacent to the King 

Range terrane are assigned to the Vizcaino structural block 

(McCulloch, 1987a). Because the King Range terrane is 

adjacent to, and may have been displaced along, the San 

Andreas fault zone, its accretionary history and its relation 

to the presently adjacent Coastal terrane are controversial 

(McLaughlin and others, 1982; Miller and others, 1983; 

Bachman and others, 1984; Underwood, 1987). 

Composition and Structural Framework 

The King Range terrane is a composite terrane, consisting of rocks of diverse lithology that range widely in 

age and are structurally complex. It is divided into the 

Point Delgada and King Peak subterranes (fig. 2). 

Point Delgada Subterrane 

The Point Delgada subterrane is exposed only at Point 

Delgada in the intertidal area beneath Quaternary marine 

and nonmarine deposits (McLaughlin and others, 1982). 

The subterrane consists chiefly of basaltic pillow flows, 

flow breccias, and tuffs that locally are cut by diabase 

dikes (fig. 21). Red and gray calcareous mudstone intercalated with the volcanic flows contains a Late Cretaceous 

(Coniacian to Campanian) radiolarian fauna. 

Overlying and intercalated with the upper volcanic rocks 

is a melange consisting of sheared pebbly to bouldery mudstone. The clastic assemblage of this olistostromal melange 

is polyrnict, consisting of a mixture of rounded porphyritic 

hypabyssal rocks of intermediate composition, dark chert, 

mafic metaigneous (greenschist facies) rocks, metasandstone, and fine-grained grayish-brown limestone derived 

Figure 21. Pillow basalt of Point Delgada subterrane. View 

northward. Note hammer (circled) for scale. 

from calcareous concretions. Included within the melange 

unit is a large, subrounded, slickensided block of glaucophane schist (type IV of Coleman and Lee, 1962), approximately 3 m long. The melange unit is overlain by arkosic 

sandstone and shale turbidites derived from an arc-related 

terrigenous source. 

Sandstone and volcanic rocks of the Point Delgada subterrane are mildly metamorphosed to prehnite-pumpellyite 

grade, and both the sandstone and volcanic rocks are hydrothermally altered and mineralized locally along north-southto northeast-trending normal faults that are conjugate with 

northwest-trending faults of the San Andreas fault system. 

Unlike the Coastal terrane, sandstone of the Point Delgada 

subterrane generally lacks detrital K-feldspar, although this 

mineral may have been present before metamorphism and 

hydrothermal alteration. Potassic alteration, including hydrothermal K-feldspar, is locally conspicuous along mineralized faults. The Point Delgada sandstones are arkosic to 

feldspathic (24-42 volume percent plagioclase, 26-48 volume percent quartz) and moderately lithic (27-36 volume 

percent nonchert rock fragments), containing sparse volcanic detritus (less than 20 volume percent) and moderately 

abundant chert detritus (0.6-1.0 times as abundant as 

nonchert lithic materials). The volcanic rocks, melange, and 

sandstone are folded into tight southeast-plunging folds and 

separated from the King Peak subterrane to the northeast by 

a nearly vertical, northwest-trending fault thought to be a 

segment of the San Andreas fault system (McLaughlin and 

others, 1982, 1985b ). 

Late Cretaceous igneous rocks of the Point Delgada 

subterrane are somewhat enriched in alkalis (K, Na) and 

Mn, largely owing to hydrothermal alteration (figs. 9A, 9B). 

However, their P20sffi02 ratios (fig. 9C) and TiO/(FeO/ 

MgO) ratios (fig. 9D) are typical of oceanic-ridge basalt. 

The occurrences of calcareous red pelagic mudstone with 

pillowed volcanic flows and of subrounded basalt clasts in 

flow breccias suggest their deposition on the slopes of an 

oceanic volcanic edifice that reached wave-base elevations. 

We interpret the volcanic rocks of Point Delgada to have 

formed along the flanks of a ridge or near-ridge volcanic 

edifice. The setting of these rocks is, therefore, about the 

same as that of basaltic rocks of similar age in the Coastal 

terrane. Olistostromal melange and arc-derived terrigenous 

rocks intercalated in the upper part of the Point Delgada 

subterrane probably represent sediment deposited in a 

trench-slope setting, where the Late Cretaceous trench at 

the Farallon-American plate margin was close to the Pacific-Farallon Ridge. Alternatively, the terrigenous rocks may 

represent sediment that bypassed the filled trench and was, 

instead, deposited in an outer-trench or near-ridge setting. 

King Peak Subterrane 

The King Peak subterrane composes the main body of 

rocks referred to as the King Range terrane (McLaughlin 

36 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
and others, 1982). This subterrane is in contact on the 

west with the Point Delgada subterrane along a steeply 

northwest trending fault. It is also faulted to the north 

against the Coastal terrane along an east-west-trending 

shear zone (Cooskie shear zone of McLaughlin and others, 

1982). To the east, the King Peak subterrane is in fault 

contact with the Coastal terrane along a northwest-trending shear zone aligned with Bear Creek and Whale Gulch. 

These shear zones, which separate the King Range and 

Coastal terranes, correspond to the low-angle, southwestdipping suture that underlies the King Peak subterrane 

(fig. 2). 

The King Peak subterrane is composed dominantly of 

argillite, sandstone, and minor pelagic chert, limestone, 

and basalt. As is true of other terranes of the Franciscan 

Complex, the pelagic chert and limestone and associated 

basaltic rocks are considered to be oceanic rocks scraped 

off at the American plate margin (McLaughlin and others, 

1982). 

Sandstone and minor interbedded argillite in the King 

Peak subterrane, much like those in the Coastal terrane, are 

characterized by graded bedding, load and current traction 

features, and ripple-drift laminations, suggestive of deposition in a channeled-slope or trench-slope setting. The sandstone of this subterrane is arkosic to feldspathic (20-62 

volume percent feldspar, 23-57 volume percent quartz) and 

moderately to highly lithic (9-46 volume percent nonchert 

fragments). The most common lithic constituents include 

rounded to subangular mafic rocks, cherty felsite, and arcderived, silicic to intermediate volcanic and plutonic rocks. 

We estimate that mafic detritus may be 0.3 to 1.3 times as 

abundant as silicic to intermediate volcanic-rock fragments; 

chert detritus may be as much as 3 times as abundant as 

nonchert fragments and as much as 5 times more abundant 

than volcanic detritus. Predominant detrital biotite and 

muscovite and minor epidote and clinozoisite compose the 

principal sandstone accessory minerals. K-feldspar makes 

up 7 to 42 volume percent of the rock. 

Many coastal and eastern exposures of the King Peak 

subterrane consist of black, thin-bedded to massive, cherty 

and calcareous argillite and fine-grained sandstone. These 

sedimentary rocks probably represent hemipelagic and 

overbank-slope deposition. Soft-sediment slump folds and 

deformed burrows commonly are present. The argillitic 

rocks commonly are overprinted by a penetrative shear 

fabric, especially along the northern, eastern, and coastal 

margins of the King Peak subterrane. Locally, these rocks 

form a true melange. 

Basaltic rocks occur as rare blocks in melange of the 

King Peak subterrane; they range in length from a few meters along the coast to many hundreds of meters in the 

Queen Peak area northeast of Point Delgada. The basalts 

occur as pillowed flows, tuffs, and flow breccias, associated 

with pink to red pelagic limestone or red to green tuffaceous radiolarian chert. Two major-oxide analyses of these 

basaltic rocks, one of basalt from Queen Peak, another of 

basalt from Cooskie Creek, indicate that the rocks are alkalic and oceanic in derivation (fig. 9). The high P20 5ffi02 

ratios (fig. 9C), FeO:MnO:P20 5 proportions (fig. 9A), and 

TiOi(FeO/MgO) (fig. 9D), ratios all suggest that the basalts may have formed in a seamount setting. 

Rocks of the King Peak subterrane are of middle Miocene age, on the basis of radiolarians and planktic and benthic foraminifers. Early Tertiary foraminifers also are present 

locally, but at least some occur in rocks containing both 

radiolarians and foraminifers of middle Miocene age. The 

early Tertiary forms, therefore, are reworked (McLaughlin 

and others, 1982). At Queen Peak, a radiolarian fauna characterized by Cyrtocapsella tetrapera Haeckel and Stichocorys spp. occurs along with nondiagnostic diatoms in a tightly 

folded composite block of manganiferous and tuffaceous 

chert overlying pillow basalt. The same radiolarian fauna 

occurs in hemipelagic thin-bedded chert and interbedded 

calcareous argillite along the coast, and is ~ssociated with 

carbonate concretions in thin-bedded argillite and finegrained terrigenous sandstone along the east side of the King 

Range. Planktic foraminifers of middle Miocene age occur 

in carbonate concretions with this radiolarian fauna on Horse 

Mountain along the east side of the King Range, and in 

argillitic sections along the coast north of Point Delgada 

(McLaughlin and others, 1982). The age of basalt in the 

Cooskie Creek area (fig. 2), based on whole-rock K-Ar analysis, is 16.2±0.5 Ma (McLaughlin and others, 1985b ). 

Rocks of the King Peak subterrane are complexly folded, penetratively deformed, and pervasively veined with 

laumontite. Penetrative shearing predated or accompanied 

early folds, which vary in orientation from the northern to 

the southern part of the King Peak subterrane. Asymmetric 

early folds in the north half of the King Peak subterrane 

exhibit a consistent northward to northeastward vergence 

(fig. 22A), whereas folds in the south half exhibit largely 

westward to northwestward vergence (fig. 22B). The relation of these early folds to each other is unclear from our 

data, although Beutner and Hansen (1975) suggested two 

discrete fold generations: a north- to northeast-vergent generation (f;) overprinted by a later generation of chevron and 

complex folds (j2) whose axial surfaces are generally upright and trend northwest. Later broad, open warps (j3) with 

east-west-trending axes may be relatable to north-south 

compression and tectonic transport associated with the San 

Andreas fault system. Late deformation of rocks in the 

King Peak subterrane is exhibited in highly sheared and 

crushed rocks along the terrane margin north of Cooskie 

Creek and along its east boundary. Earlier penetrative 

shearing of rocks of the King Peak sub terrane, as well as f 1 

and / 2 folding, probably resulted from east-west convergence along the Farallon-North American plate boundary 

before about 14 Ma. We speculate thatf3 warping was initiated much later in the Cenozoic, possibly during an episode 

of northward translation between about 14 Ma and the 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 37 
present. Shearing associated with the King Range terrane 

suture, which includes coparallel shearing in the adjacent 

Coastal terrane to the north, occurred during final tectonic 

transport and accretion of the King Peak subterrane to 

northern California. This shearing is considered to have 

occurred between the Miocene and Pleistocene, on the basis of the earlier-described imbrication of Pliocene marine 

rocks into the Coastal terrane near Petrolia. 

King Range Terrane Thermal Anomaly 

The King Range terrane was affected by a high-temperature thermal event during middle Miocene time. At Point 

Delgada, this thermal event is indicated by hydrothermal 

alteration and mineralization that includes an unusual asFigure 22. Folded rocks of King Peak subterrane. A, Overturned, northward-vergent synclinal fold, located near north 

end of King Peak subterrane between Cooskie and Spanish 

Creeks. B, Complex westward-vergent folds in thin-bedded 

siliceous argillite and calcareous sandstone, located south of 

Big Flat. View northward. · 

semblage of epithermal Pb-, Zn-, and high-Ag-bearing sulfides (McLaughlin and others, 1985b ). The hydrothermal 

mineralization at Point Delgada occurs along north-northeast-trending tension faults conjugate to the north-south- to 

northwest-oriented faults of the San Andreas system. The 

base-metal sulfide mineralization crosses a presumed trace 

of the San Andreas fault system that separates the Point 

Delgada and King Peak subterranes (McLaughlin and others, 1985b ). Adularia associated with the potassic hydrothermal alteration and sulfide mineralization yielded a K-Ar 

age of 13.8±0.4 Ma (fig. 2). Thus, the Point Delgada and 

King Peak subterranes were amalgamated before 13.8 Ma. 

Vitrinite-reflectance studies in the Coastal belt of the 

Franciscan Complex (Underwood, 1987; Blake and others, 

1988) clearly show that the 13.8-Ma thermal event affected the entire King Range terrane (fig. 23). These data support the proposition that the Point Delgada and King Peak 

subterranes have been a composite terrane since the middle Miocene and, as such, have undergone negligible 

right-lateral slip relative to each other subsequently 

(McLaughlin and others, 1985b). Therefore, any presently 

or recently active segments of the San Andreas fault system along which major right-lateral offset has occurred 

(such as faults forming the present boundary between the 

North American and Pacific plates) must lie either inland, 

along the suture between the King Range and Coastal terranes, or offshore, west of or at the contact with the 

Vizcaino structural block. 

Vizcaino Structural Block 

The Vizcaino structural block (McCulloch, 1987a, b) is 

a large, structurally complex terrane on the Pacific plate in 

the offshore area south of the Mendocino Fracture Zone, 

immediately west of the King Range terrane (figs. 17 A, 24 ). 

Lithologic data for the Vizcaino block are scant (fig. 

24 ), and so, although complex structures are mapped from 

numerous seismic profiles (McCulloch, 1987a, b), a detailed comparison of these rocks with those of the adjacent 

King Range terrane on shore is not presently possible. 

Quartz-mica schist and slate recovered from the bottom of 

an exploratory drill hole at the southwest corner of the 

Vizcaino block just north of Point Arena (loc. L-1, fig. 14) 

have been provisionally identified as Salinia terrane basement (Hoskins and Griffiths, 1971; Bachman and Crouch, 

1987; McCulloch, 1987a, b). Because the location of this 

drill hole is at the faulted southwest boundary of the 

Vizcaino block, it is unclear whether the sampled basement was actually in the block or southwest of it, where 

Salinia rocks are known to occur. 

A conspicuous undated, northwest-trending, offshore 

magnetic anomaly that is truncated at the San Andreas 

fault just north of Point Arena strongly suggests that the 

southeastern part of the Vizcaino block is underlain by 

38 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple junction, Northwestern California 
shallow ophiolitic and (or) oceanic basement rocks at 0.5 

to 2.5 km below sea level (Griscom, 1980a; Griscom and 

Jachens, 1989). Onshore, south of Point Arena, basaltic 

basement rocks underlie Upper Cretaceous (Campanian) 

turbidites at Black Point near Gualala (Wentworth, 1968, 

1972). The undated basalt of Black Point is tholeiitic (fig. 

9B) and oceanic in composition with respect to Ti02 and 

P20 5 contents and FeO/MgO ratio (figs. 9C, 9D). Owing 

to its elevated MnO content, however, it shows affinities 

to island-arc tholeiites in a ternary plot of MnO, Ti02, and 

P20 5 contents (fig. 9A). These Campanian or older basaltic 

rocks may correspond to the upper part of the "ophiolitic" 

basement associated with the magnetic anomaly offshore 

and thus may compose part of the composite basement of 

the Vizcaino block, along with the overlying Upper Cretaceous and Paleogene to lower Miocene sedimentary section at Point Arena. Paleomagnetic data (Kanter and 

Debiche, 1985) suggested 37° of northward translation of 

the basaltic rocks of Black Point since the Early Cretaceous: 16° of translation of the overlying lower Tertiary 

section (German Rancho Formation of Wentworth, 1968) 

since the Paleocene, and 10° of translation of the overlying Tertiary volcanic section (Iverson Basalt of Weaver, 

1944) since the early Miocene (25 Ma). The chemistry of 

the lower Miocene Iverson Basalt (Wentworth, 1966) indicates that these rocks are alkalic, with ocean-floor (seamount) affinities (figs. 9A, 9D). The Iverson Basalt is 

compositionally similar to diabase and basalt of the approximately coeval Mindego Basalt in the Santa Cruz 

Mountains of the San Francisco Bay region. The Mindego 

Basalt presumably was erupted through Salinian basement. 

The Iverson Basalt also is compositionally similar to the 

Oligocene or early Miocene "andesitic" basement reported 

from the bottom of the drill hole at Deep Sea Drilling 

Project (DSDP) Site 173 (MacLeod and Pratt, 1973) at the 

northwest corner of the Vizcaino block. 

124°00' 

Figure 23. Contoured vitrinite-reflectance 

(R0) values and inferred paleotemperatures 

of metasedimentary rocks of Franciscan 

Complex in Cape Mendocino region. Modified from Underwood (1987) and Blake 

and others (1988). 

....<J 

'"70: 

~ :..0 Point Delgada 

40° 

00' 00: 

~ 

m 

c:J 

c:J 

v~· .. :.-::-:.<J 

~ EXPLANATION 

Percent ~ 

Paleotemperature, 

in degrees Celsius 

~2.0 ~275 

1.2- 2.0 225-275 

1.0- 1.2 175- 225 

0.7-1.0 125- 175 

~0.7 $.125 

Contour of vitrinite-reflectance 

value (Ro)--Dashed where 

inferred;-queried where uncertain 

Terrane boundary 

Vitrinite data point 

0 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 39 
~ 

0 

~ t"l 

0 

:::s 

0 

~ 

a 

~-

~ 

"C 

::r 

;::;· 

~ ~ 

:::s 

~ 

0 

:r -

~ 

:::r 

~ 

:::s 

t"l 

(ii" 

t"l 

~ 

:::s 

~ 

0 

3 

"C 

;:;-

>< 

~ 

:::s 

c. 

m 

< 0 

s 

o· 

:::s 

0 

:r -

~ 

~ ~ 

:::s 

c. 

0 

t"l 

:r 

0 

:;I 

~· 

;:;-

c 

:::s 

!l 

o· 

2 

z 

0 

::l 

::r 

~ ~ 

"' 

~ :::s 

~ 

~ 

~ :::s 

iii" 

127° .----

40° 

GORDA BASIN 

® 

PACIFIC 

PLATE 

126° 125° 

·VIZCAINO '--

, . BLOCK 

\\ ~ ~ ' ' 'b 12 ·- ~ \ ~ L-~ 2_,. '--, ', voo ' .?aa , __ _ 

®----.. , 

\ 

,' 

:.?sa 

:-a 

Vo_:, 

~ 

'3 v._ 

...:. ~-----~ ' ' -- - @A? . ' ', ' ' \ ' ' -- -~,' ' .,......, · '- ----, 

' ....... :___ _ PACIFIC , / --- _,_-

PLATE 

', 

0 25 50 KILOMETERS 

124° 

KING RANGE 

TERRANE rn=J 

EXPLANATION 

Positive magnetic anomaly-Circled number is 

assigned anomaly number 

~ Location of seismic profile 

- Uninterpreted reflection-Probably largely 

sedimentary contacts (shown in seismic 

profiles only) 

Unconformable contact on deformed basement 

(shown in seismic profiles only) 

~ Fault or fracture zone-Dashed where inferred; 

queried where uncertain. Ball and bar on 

downthrown block. Arrows indicate 

direction of relative movement 

~ 

~ 

Thrust fault--Sawteeth on upper plate. Dashed 

where inferred; queried where uncertain 

Inactive(?) convergent margin--Sawteeth on 

upper plate 

'-- 2500 - -"_ Bathymetric contour-In meters 

L-40 

L-1 

L-2 

L-3 

L-4 

L-~ 

Locality oflithologic data from dredge haul or 

drill hole on Vizcaino block. Lithologic 

descriptions: 

Quartz-mica schist and slate, possibly Salinia 

terrane(?) basement-From bottom of 

exploratory drill hole (Hoskins and 

Griffiths, 1971 ) 

Fine-grained sandstone containing Eocene to 

Oligocene nannofossils-Dredged from 

Noyo Submarine Canyon 

Volcanic basement overlain by uppermost 

Oligocene and younger siliceous pelagic 

strata containing reworked Cretaceous 

radiolarians-From drill hole DSDP 173 

at depth of 306.5 to 320 m 

Basalt, basaltic conglomerate, and serpentinite-Dredged from Mendocino Ridge 

Drill hole 
;;I 

I") 

0 

::I 

0 

~ 

~ 

~-

llol 

"C 

::r 

r;· 

., 

til 

3 

I'D 

~ 

0 

*' 0 

-:T 

I'D 

~ 

llol 

"C 

I'D 

~ I'D 

::I 

Q. 

0 

I") 

:r 

0 

;:11:1 

I'D 

C1Q 

s· 

::I 

llol 

::I 

Q. 

> Q. 

Qj" 

~ 

a 

z 

0 

;:::s. 

::r 

I'D 

... 

::I 

n 

g 

~ 

;:11:1 

llol 

::I 

C1Q 

I'D 

en 

~ 

TF~~~==~In=a=ct=iv=e=(:?.)~co~n-ve_r_g_e~nt_m_a_r_g_·n----~M~EENnNDro~C;IN~O~F~RA~C~T~U~R~E~~ZO~N=E~---------------o 

4 

· Overlap deposits ~~Basatt, ~~ l ~--,.. -=--~-=-~- --~~=-~--;..:~ ~/"" t-1 tfi'w"S=::±::= l-4 - ~~---2~~--VIZCAINOBLOCK ~---==1 --- -...... ~ -----~ ......... ____ .;-~ ,;;--~--- - -- ------ ~---- - ---- -- ~ OCEANIC CRUST 

2 2 

6 

8 

6 

GORDA Pl.A TE 

8 

0 

Inactive(?) convergent margin Oconostota 0 

2 I Ridge Overlap deposits ~~---::::=-~~-=- __ _ 4 --========--=- ----- ----==--- ~ =~-~~-;.-~-~~~-::: ......... , --t4 

6 -=---= --~~-~~~----VIZCA~O BLOCK .. 6 -- ----------- --- ~ 

--

(/) 

0 

z 

0 

(.) 

w 

(/) 

z 

w 

~ 

~ 

2 

-~ 8 OCEANIC CRUST L-8 

w 

> 0 <( 

a: 

..... 2 > <( 

~ 4 

0 

" -------T 0 Overlap depos1ts _ ....--

- ----~-=~· ~. - ~~~~- ~~ -t-- F ~~~~:!-~ a-=-:i------ ~~----= Ophiolitic(?) rocks at 

shallow depth 

2 

4 VIZCAINO BLOCK 

~ 6 6 

0 ~~-------------------------------------------------------------------------------rO ·P-4 

8 

2 -1 !Inactive(?) convergent margin 

I ::::==-~---~ .. ---- 4 -~.;;-, -- -------- __:::_""::::-=-_-----~;;;:._~ ........ ..::::::_....__~-........ --.......... ..... -~- --==--- - --- ..:::::..=::-_...=.=-..:::::;:...,::::::.:-~-~ ••• ~-::-....... ......._ ......... " 

6~=---- ~ --=-=---:::--~_:~:Z-_--._, .. ......_ VIZCAINO BLOCK ----:-- ......... _~ --- ,, --~ -.......... ----...__.....______-- .:::::;;:..----- ....... ------~ ........ -- OCEANIC CRUST 

2 

4 

6 

8 

Figure 24. Offshore area south of Mendocino Fracture Zone, showing locations of positive magnetic anomalies of Pacific plate and Vizcaino structural block, lithologic 

data and structural features of Vizcaino block and adjacent offshore area, and interpreted seismic profiles across major structures bounding Vizcaino block and Pacific 

plate (modified from McCulloch, 1987a, b). Ages of units delineated on seismic profiles: overlap deposits, middle Miocene(?) and younger; Gorda basin fill, Miocene 

and younger; oceanic crust, Oligocene and older; Vizcaino block, Oligocene to Cretaceous or Jurassic(?). 
Table 3. Radiolarians and planktonic foraminifers from the King Range terrane and the 

lower part of the drill hole at Deep Sea Drilling Project (DSDP) Site 173 

[Ages: K, Cretaceous; M, Miocene; T; Tertiary. R, reworked. Data from Ingle (1973), Kling (1973), von 

Huene and others (1973), and McLaughlin and others (1982)] 

Radiolarians 

King Range 

terrane 

Amphipyndax? sp. ------------------------------------------------------------------------- K 

Archaeodictyomitra sp. -------------------------------------------------------------------- K 

Cryptamphorella sphaerica (White) of Foreman (1973) ------------------------------- K 

DSDP 

173 

Cyrtocapsella japonica Nakaseko -------------------------------------------------------- M M 

tetrapera Haeckel ----------------------------------------------------------------------- M M 

Cyrtocapsella ? sp. ------------------------------------------------------------------------- M 

Dictyocephalus sp. of Pessagno ( 1963)? ------------------------------------------------ K 

Dictyomitra duodecimocostata Foreman ------------------------------------------------- K 

formosa Squinabo1 of Pessagno (1976) -------------------------------------------- K 

koslovae Foreman --------------------------------------------------------------------- K 

multicostata Zittel of Pessagno (1976)? --------------------------------------------- K K(R) 

Lychnocanoma grande (Campbell and Clark) ------------------------------------------ M 

Ommatartus? sp. --------------------------------------------------------------------------- M M 

Sphaeropyle robusta Kling --------------------------------------------------------------- M M 

Stichocorys delmontensis (Campbell and Clark) --------------------------------------- M M 

s p. ---------------------------------------------------------------------------------------- M 

Theocorys redondoensis (Campbell and Clark) ---------------------------------------- M 

Planktonic foraminifers 

Acarinina sp. -----------------------------------------------------------------------:.________ T 

Globigerina juvenilis Bolli ---------------------------------------------------------------- M M 

M 

M 

sp. --------------------------------------------------------------------------------------- M 

Globigerinita glutinata (Egger) ----------------------------------------------------------- M 

Morozovella sp. ---------------------------------------------------------------------------- T 

Orbulina sp. --------------------------------------------------------------------------------- M 

Planorotaloides sp. ------------------------------------------------------------------------- T 

Subbotina sp. ------------------------------------------------------------------------------- T 

Northwest of the drill holes offshore from Point Arena, fine-grained sandstone containing a nannofossil assemblage ranging in age from middle Eocene to Oligocene 

(McCulloch, 1987a, b) was dredged from the southwest 

wall of Noyo Submarine Canyon from a depth of about 

2,500 m (loc. L-2, fig. 24). These rocks are interpreted 

(McCulloch, 1987a, b) to be in the lower part of an accretionary prism, a depositional setting consist~nt with that 

for rocks of similar age in the Coastal terrane. Alternatively, these rocks could be part of a somewhat more distal or 

downslope lithofacies of the German Rancho Formation 

near Point Arena. 

Presumed basement rocks were sampled from two other localities (locs. L-3, L-4, fig. 24) in the northwestern 

part of the Vizcaino block, both west of long 125° W. 

Submarine volcanic breccia was recovered from the bottom of the drill hole at DSDP Site 173 (loc. L-3, fig. 24) 

at 3,233.5 to 3,248 m below sea level. On the basis of 

texture and a silica content similar to those of arc-related 

volcanic rocks of the Cascades, MacLeod and Pratt (1973) 

considered these rocks to be andesitic. However, the major-oxide and minor- and trace-element compositions are 

more compatible with ocean-floor or alkalic seamount volcanism in all the discrimination plots of figures 9 and 10 

except for (Ti/Cr)/Ni ratio (fig. 10C). Thus, these basement rocks may form part of a remnant of Oligocene or 

older ocean floor. 

A thick, relatively undeformed section of siliceous, 

pelagic mud containing a lower bathyal radiolarian and foraminiferal fauna. of latest Oligocene to late Miocene age 

overlies the volcanic unit at DSDP Site 173 (Ingle, 1973; 

Kling, 1973). Identical radiolarian and foraminiferal faunas are found in the middle Miocene part of the King 

Range terrane (table 3). Radiolarians of the Dictyomitra 

multicostata group, interpreted to be reworked from a Cretaceous Franciscan source, were also identified at the bottom of the sedimentary section at DSDP Site 173 (Kling, 

1973). This same radiolarian group occurs in a Late Cretaceous (Campanian or Coniacian) fauna at Point Delgada in 

the King Range t~rrane. A dredge haul along the crest of 

42 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
the Mendocino Ridge immediately northwest of DSDP 

Site 173 and east of long 126° W. (loc. L-4, fig. 24) consisted of basaltic flows and conglomerate, and serpentinite 

(Krause and others, 1964). The Miocene section at DSDP 

Site 173 thus appears to be faunally equivalent to that of 

the King Range terrane (in part), and the reworked Cretaceous radiolarians reported from DSDP Site 173 could 

have been eroded and transported from Point Delgada. 

Unlike the DSDP Site 173 rocks, however, the King 

Range terrane includes a major component of coarse terrigenous detritus, and rocks of the terrane are severely disrupted by folding and penetrative shearing. In addition, rocks of 

the King Range are veined with calcite and laumontite. The 

depositional setting of DSDP Site 173, therefore, differed 

significantly from that of the King Range terrane. The setting of DSDP Site 173 during the middle Miocene appears 

to have been distal and oceanic, and that of the King Range 

terrane more closely associated with a near-trench environment, within reach of terrigenous sedimentation. 

Seismic profiles across the northwest corner and west 

central part of the Vizcaino block, coupled with magneticlineation data (fig. 24; McCulloch, 1987a, b) show that 

oceanic crust within the Pacific plate, dated as Oligocene 

in age, is overlain by rocks of the Vizcaino b~ock along a 

northeast-dipping low-angle fault. This low-angle fault 

bounds the southwest side of the Vizcaino block and extends along the entire California margin west of the San 

Andreas transform system (profile P-1, fig. 24). This 

southwest boundary of the Vizcaino block and its extension southward of Point Arena have been interpreted as a 

late Oligocene or early Miocene "paleosubduction" boundary between the Pacific and North American plates (McCulloch, 1987a, b). According to this interpretation, the 

southwest boundary of the Vizcaino block represents part 

of the Oligocene to early Miocene subduction margin of 

the North American plate. This interpretation, however, 

conflicts with earlier work, because Oligocene to early Miocene motion of the Pacific plate relative to the North 

American plate is inferred to have been transtensional 

south of the Mendocino Fracture Zone, according to the 

plate reconstructions of Atwater ( 1970), Engebretson and 

others (1985), and Stock and Molnar (1988), and so subduction between the Pacific and North American plates 

would be difficult to achieve. 

Accretion of the Vizcaino Structural Block 

to the Pacific Plate and Initiation 

of the San Andreas Transform System 

A reconstruction of the accretion of the Vizcaino structural block to the Pacific plate constrains the tectonic mechanism(s) involved in initiation of the San Andreas transform 

and its subsequent propagation northward (fig. 25). Any 

such reconstruction must address the apparent enigma posed 

by offshore evidence for convergence between the Vizcaino 

block and the underthrusted oceanic (Pacific) plate, during 

a time when transtensional stress is inferred to have been 

dominant along the California margin. 

Although most plate models for the late Oligocene to 

early Miocene California margin south of the Mendocino 

Fracture Zone suggest that a transform setting had developed 

between the Pacific and North American plates, unequivocal 

geologic evidence is absent for San Andreas-related transform motion (that is, transform motion tied to northward 

propagation of the Mendocino Fracture Zone) before about 

16 Ma. Although restoration of pre-Tertiary basement rocks 

of the Coast Ranges to their original configurations apparently requires movements along faults that predate the modern 

San Andreas transform, restoration of middle Miocene or 

later right slip on the modern San Andreas fault system (here 

considered to include the San Andreas-Pilarcitos, HosgriSan Gregorio, and Hayward-Calaveras faults) apparently establishes continuity between some older units offset across 

the San Andreas fault, such as the Late Jurassic gabbros of 

Logan and Eagle Rest Peak (fig. 26). 

Northward-younging patterns of volcanism, together 

with strike-slip-fault-bounded structural basins that young 

northward, have been linked to northward propagation of 

the San Andreas transform system (Dickinson and Snyder, 

1979; McLaughlin and Nilson, 1982; Fox and others, 

1985). The ages of the volcanic rocks, as well as of basin 

fillings that exhibit this northward-younging pattern, appear 

to be younger than about 15 to 16 Ma (fig. 26). Widespread 

volcanism and basin formation also occurred earlier, especially about 24-22 Ma, east and west of the San Andreas 

fault. This earlier volcanism, which shows no obvious 

space-time relation to present or past positions of the Mendocino triple junction, is thought to be related to extensional tectonism rather than strike slip (Stanley, 1987). These 

data suggest that although a transtensional setting existed at 

least from 24 to 16 Ma, the modern San Andreas transform 

was not well developed, and so significant right slip did not 

occur until after about 16 Ma (fig. 25B). 

Structural relations along the north and southwest 

boundaries of the Vizcaino block indicate that its accretion 

to the Pacific plate resulted from eastward extension of the 

Mendocino Fracture Zone across the convergent boundary 

between the Vizcaino block and oceanic rocks of the Pacific 

plate, connecting the Mendocino Fracture Zone to the newly 

initiated San Andreas transform. Offshore data show that 

this jump of the Mendocino Fracture Zone occurred sometime between formation of magnetic anomaly 8 on the Pacific plate about 28-27 Ma and overlap of the convergent 

margin of the Vizcaino block by undeformed pre-Mohnian 

(probably middle Miocene, 16-12 Ma) sediment (fig. 24; 

see also, McCulloch, 1987a, b). The absence of any known 

strike-slip fault west of the Vizcaino block along which 

Pacific-North American plate transform motion could have 

been taken up before this eastward jump of the fracture zone 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 43 
suggests that all transform movement of the San Andreas 

system has been taken up along faults that broke to the east 

of the convergent west boundary of the Vizcaino block and 

its southward extension. The present positions of the Mendocino Fracture Zone and Vizcaino block are north of the 

r& " rt " ~ " 0 

..._<o 

" 

NAM 'll''ll'o 

New ldria diapir 

19- to 24-Ma Pinnacles 

and Neenach Volcanics 

J " ~ ..;>6'o 

" 

<?ao PAC 

~ <::;:, <!!, 

" 

..;><?o / I 

200 KILOMETERS 

0)~ 

,ct A. 24-20 Ma 

" 

~ <?'ll'o! & 0)~ <?ao 

" " 

Figure 25. Plate-tectonic model for early Miocene initiation of 

modern San Andreas transform, offering two hypotheses for 

convergent deformation along Pacific-North American plate 

margin after 16 Ma. Plate reconstructions, relative motions, 

and projections modified from fixed-hotspot reference-frame 

model of Engebretson and others (1985). ERB, deposits of Eel 

River basin; FAR, Farallon plate; FAR*, old Farallon plate that 

is now part of Pacific plate; MFZ, Mendocino Fracture Zone; 

NAM, North American plate; NAM*, old North American 

plate that is now part of Pacific plate; PAC, Pacific plate; VIZ, 

Vizcaino structural block. A, Pacific-Farallon Ridge initially 

came in contact with North American plate about 30 Ma, but 

we assume here that Farallon-North American plate margin 

Salinia terrane, and the position of the northern San Andreas 

fault system is east of the Salinia terrane (fig. 24). In comparison with reconstructions of the change in position of the 

Mendocino Fracture Zone over time (Atwater, 1970; Engebretson and others, 1985; Stock and Molnar, 1988) and with 

~ NAM 

..;>6' 

Diablo Range 

r& " 

8- to 12-Ma volcanic rocks 

(Tolay Volcanics, Quien Sabe 

Volcanics, and volcanic rocks 

in Berkeley Hills) 

14.8-Ma Page Mill Basalt 

.J<?o 

rt " 

and Neenach Volcanics 

~ " 

<?ao 

PAC 

~ " 

<?'II' / 

I 

200 KILOMETERS 

..._ct B.16-8 Ma 

" 

~ " <::;:, <!!, 

" 

north of present-day Gulf of California remained convergent 

until sometime after 24 to 20 Ma. During that period, PacificFarallon Ridge and increasingly segmented Farallon plate 

were compressed between North American and Pacific plates. 

8, First model suggests that convergent deformation along Pacific-North American plate margin was driven by active subduction of Farallon plate beneath North American plate and 

that west-convergent boundary of Vizcaino block represents 

now-abandoned subduction zone; model assumes existence 

of pieces of old Farallon plate and segments of failed PacificFarallon Ridge west of old subduction margin. By this model, 

Pacific-Farallon Ridge collided with North American plate 

about 16 Ma, and subduction stalled, resulting in reorientation 

44 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple junction, Northwestern California 
restored offsets of the San Andreas fault system (fig. 26), 

this geometry indicates that eastward extension of the fracture zone, resulting in accretion of the Vizcaino block to the 

Pacific plate and formation of the modem San Andreas fault 

system, could not have occurred earlier than about 16 Ma. 

Although the Miocene and older west convergent 

boundary of the Vizcaino block and its southward extension 

may be inconsistent with transtensional motion between the 

Pacific and North American plates for that time, a subduction 

margin is compatible with Oligocene to Miocene motion of 

~ " rt- " ~ " 

FAR 

/ 

NAM 

8- to 12-Ma volcanic rocks 

(Tolay Volcanics, Ouien Sabe 

Volcanics, and volcanic rocks 

in Berkeley Hills) 

New ldria diapir 

14.8-Ma Page Mill Basalt 

19- to 24-Ma Pinnacles 

and Neenach Volcanics 

~ " 

~6'o 

~ PAC 

,rl, 

" / ~'SI'o I 

0 

~ C.l6-8 Ma 

" 

~co <:;)<:;) 

" 

of active ridge, extinction of older ridge segments, and 

preservation of remnants of Farallon plate between abandoned subduction margin and extinct ridge. Reorientation 

of active ridge was accompanied by extension and rifting 

of North American plate near present-day Gulf of California between 16 and 12 Ma. About 14 Ma, Mendocino 

Fracture Zone broke eastward, connecting with newly oriented active ridge and initiating modern San Andreas 

transform. Reorientation of ridge and transform fluctuated 

until at least 8 Ma, resulting in transfer of various structural 

blocks, including Vizcaino block, to North American plate 

and back to Pacific plate near propagating north end of 

San Andreas transform. C, Alternatively, convergent defor-

,rl, 

" 

EXPLANATION 

~ Eocene overlap deposits 

~ King Range terrane 

~L>>~L j Coastal and Yager terranes 

l-:=:=:=:=:=:=J Central and Eastern belts 

f-: :-.-_: : :1 Salinia terrane 

'S1''S1'o 

,ct 

" ~ Volcanic rocks 

--- Contact 

--.... Terrane boundary- Dotted where 

concealed 

¢ San Andreas transform system-Arrows 

indicate direction of relative movement 

'>"oo .............., Thrust fault-Sawteeth on upper plate 

<:;)cb (shown inC only) 

" ...A.....A.... Active subduction zone-Sawteeth on 

upper plate 

~ Abandoned subduction zone--Sawteeth 

on upper plate (shown in B only) 

Active spreading ridge and transform 

fault-Queried where uncertain 

Abandoned spreading ridge and 

transform fault- Queried where 

uncertain; dotted where inferred 

to be concealed beneath North 

American plate 

c:::::::;> Plate motion with respect to North 

American plate 

mation was driven by extension and rifting in hinterland 

of North American plate, which caused thrusting along 

Pacific-North American plate margin; this model assumes 

that Pacific-Farallon Ridge had been subducted beneath 

or, possibly, overridden by North American plate and was 

no longer present west of subduction zone. Crustal thinning, extension, and rifting of North American plate above 

subducted ridge caused west-northwest-directed thrusting 

along Pacific-North American plate boundary. About 14 

Ma, as in 8, Mendocino Fracture Zone connected with 

newly oriented Pacific-Farallon Ridge, and San Andreas 

transform was initiated, transferring large piece of North 

American plate, including Vizcaino block, to Pacific plate. 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 45 
the Farallon plate relative to North America (fig. 25A). This 

compatibility leads us to suggest the presence of failed (or 

~~ ~ J " ~ ..... 0 

01ao 

FAR 

extinct) segments of the Pacific-Farallon Ridge west of the 

Vizcaino block (fig. 25B). 

~ " 

EXPLANATION 

NAM 

~~~~ Eocene overlap deposits 

!;:;:(": ::::~{·/] King Range terrane 

~~ ~ 

" " 

~L~L~L~L Coastal and Yager terranes 

E=-=-=-=-=-=-=] Central and Eastern belts 

I ,-~ ', ·' : J Salinia terrane 

8- to 12-Ma volcanic 0 ~ Volcanic rocks 

rocks (Tolay Volcanics, 

Quien Sabe Volcanics, ---- Contact 

Ul"i7o and volcanic rocks in 

PAC Berkeley Hills) 

New ldria diapir 

---.... Terrane boundary- Dotted 

where concealed 

~ " 14.8-Ma Page Mill Basalt ~ " 

--=¢=~ San Andreas transform 

system- Arrows indicate 

direction of relative 

movement 

- -Inferred fracture zone19- to 24-Ma Pinnacles 

and Neenach Volcanics 

Queried where uncertain 

4 4 Active subduction zoneSawteeth on upper plate 

PAC 

/ 

I 

0 

NAM* 

200 KILOMETERS 

Figure 26. Model for northward migration of Mendocino 

Fracture Zone and propagation of San Andreas transform 

fault, showing reconstructed positions from approximately 16 

to 8 Ma of major terranes, structural blocks, and Eocene or 

younger rocks that overlap accreted terranes along North 

American plate margin. Model suggests that, before about 

16 Ma, San Andreas transform had not been initiated, or lay 

either entirely within offshore Pacific plate or along northeastdipping Pacific-North American plate boundary. After 16 Ma, 

modern San Andreas transform was initiated when Mendocino Fracture Zone broke eastward into North American plate 

margin, connecting newly reoriented Pacific-Farallon Ridge to 

NAM A A Abandoned subduction zoneSawteeth on upper plate 

I Active spreading ridge and 

transform fault 

Mendocino Fracture Zone. Increasing convergence of northwardly propagating triple junction with North American plate 

resulted in numerous northeastward jumps of Mendocino 

Fracture Zone and San Andreas transform fault, accompanied 

by dextral displacement and temporary to permanent transfer 

to Pacific plate of several fault blocks that were caught west 

of newly broken transform segments; two of these blocks are 

Salinia terrane and Vizcaino structural block. ERB, deposits of 

Eel River basin; FAR, Farallon plate; MFZ, Mendocino Fracture Zone; NAM, North American plate; NAM*, part of North 

American plate that has been transferred to Pacific plate; PAC, 

Pacific plate; VIZ, Vizcaino structural block. 

46 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
By this model, remnants of the Farallon plate would 

be caught between segments of the failed Pacific-Farallon 

Ridge and a preserved subduction margin between the Farallon and North American plates, represented by the southwest boundary of the Vizcaino block. We speculate that 

subduction of this trapped remnant of the Farallon plate 

south of the Mendocino Fracture Zone may have persisted 

until 16-12 Ma after initial contact of the Farallon-Pacific 

Ridge with the North American plate. Between 28 and 

16 Ma, the subducting remnant of the Farallon plate would 

have become increasingly fragmented as the segmented 

Pacific:-Farallon Ridge approached the subduction margin 

and transtension developt(d along an increasingly larger 

area of contact between the North American and Pacific 

plates (fig. 25A). Simultaneously, motion of the Pacific 

plate would have propagated the Mendocino Fracture 

Zone north~ard to a position along the North American 

plate margin north of the Salinia terrane. Between about 

16 and 14 Ma, dying segments of the Pacific-Farallon 

Ridge would have been accreted to the Pacific plate with 

the Vizcaino block, as the Mendocino Fracture Zone simultaneously extended eastward and the modern San Andreas transform was initiated, connecting the fracture zone 

to the active Pacific-Farallon Ridge system to the south 

(fig. 25B). 

If this model is valid, evidence for extinct ridge segments, in the form of west-to-east reversals in the age sequence of magnetic anomalies on the Pacific plate, should 

be present among undated anomalies between anomaly 8 

(28-27 Ma) and anomaly 5 (10-9 Ma). However, confirmation of such age reversals would be difficult, owing to 

attenuation of the magnetic anomalies and complex faulting of the sea floor (D.C. Engebretson, oral commun., 

1988). Progressive fragmentation and incorporation of 

parts of the Farallon plate into the Pacific and North 

American plates have been proposed previously (Menard, 

1978). In addition, failed segments of spreading ridges and 

(or) rift-initiation sites have been recognized elsewhere in 

some microplate fragments that formerly were part of the 

Farallon plate (Mammerickx and others, 1988). 

Alternatively, westward-directed thrusting could have 

occurred in response to extension and rifting above segments of the Pacific-Farallon Ridge that were subducted a 

considerable distance south of the Mendocino Fracture 

Zone (fig. 25C). This extension-related thrusting would 

have occurred after Oligocene ridge subduction and before 

establishment of a transform boundary between the Mendocino Fracture Zone and the ridge system at 16--14 Ma. 

This model is attractive because it eliminates the necessity 

for hypothetical extinct segments of the Pacific-Farallon 

Ridge west of the Vizcaino block. 

Both of these models suggest that at least some of the 

so-called "transtensional volcanism" in the Coast Ranges 

before 16 Ma (Stanley, 1987) may actually have been generated above an active convergent margin. 

Models for Accretion of the King Range Terrane 

and Their Relation to Propagation 

of the San Andreas Transform 

Two different models have been proposed to explain the 

accretionary setting and heat source for the thermal anomaly of the King Range terrane. One model implies significant 

right-lateral displacement of the King Range terrane; the 

other does not. 

Proximal-Ridge and Leaky-Fracture-Zone Models 

Between 16 and 12 Ma, the Pacific-Farallon Ridge sys-. 

tern was very close to the North American plate subduction 

margin (Engebretson and others, 1985; Underwood, 1987, 

1989). The presence of youthful, hot, buoyant oceanic crust 

along the subduction margin would have promoted hydrothermal circulation of connate fluids in marine sediment 

deposited in the trench and on the inner trench-slope and 

slope. A variation of this model appeals to a shallow magma 

point source leaked from along "unstable" precursors of the 

Blanco, Sovanco, or Surveyor Fracture Zones that may have 

intersected the Cape Mendocino margin during middle Miocene time (Underwood, 1987, 1989). These models assume that little, if any, northward translation of the King 

Range had occurred, that hydrothermal circulation was initiated at about the present latitude, and that the King Range 

terrane was merely "troweled" aside as the Mendocino triple junction propagated to its present latitude (fig. 27; Underwood, 1987, 1989). 

Although the above models offer a reasonable explanation for the present setting of the King Range terrane and 

for the formation of its thermal anomaly, several details 

remain unclear. If proximity of the Pacific-Farallon Ridge to 

North America promoted hydrothermal activity, we would 

expect a significant thermal anomaly to be expressed in 

near-trench middle Miocene deposits along the Washington 

to Oregon margin. The presence of such an anomaly needs 

confirmation to validate this model. Moreover, at present we 

know of no areas of hydrothermal sulfide mineralization of 

middle Miocene age (16-13 Ma) along the subduction margin north of the Mendocino Fracture Zone with a paragenesis comparable to that of Point Delgada (McLaughlin and 

others, 1985b ). A subduction-related structural setting that 

includes Upper Cretaceous oceanic rocks (Point Delgada 

subterrane) juxtaposed against middle Miocene slope and 

trench-slope deposits (King Peak subterrane) has no known 

analogs along the Cascadia subduction margin. Thus, areas 

that include rocks of these ages involved in subductionrelated structural settings still need to be documented. 

The close association of the mineralization at Point 

Delgada with northeast-trending tensional faults conjugate 

to northwest-trending strike-slip faults implies an origin for 

the hydrothermal system that is closely linked to right-lateral faulting. Because of this relation, right-lateral motion, 

possibly associated with a leaky transform, seems to be an 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 47 
unavoidable element in any model for the King Range thermal anomaly, whether or not this anomaly formed at or 

south of its present position. 

San Andreas Transform Model 

Our preferred reconstruction for deposition, thermal 

metamorphism, and translation of the King Range terrane 

infers transport along the San Andreas transform margin 

with the offshore Vizcaino block of the Pacific plate 

(fig. 28). Two important considerations constrain the latitudinal origin of the King Range terrane by this model: (1) the 

location and nature of igneous heat sources of appropriate 

age (ca. 14-13 Ma) initiated during propagation of the San 

Andreas transform (Dickinson and Snyder, 1979; Johnson 

NAM 

8- to 12- Ma volcanic 

rocks (Tolay Volcanics , 

Quien Sabe Volcanics, 

and volcanic rocks in 

Berkeley Hills) 

EXPLANATION 

1:{,'::: .. ··~·/··.:>>.'j King Range terrane 

~~~~~~~~ ~ Coastal and Yager terranes 

r============-j Central and Eastern belts 

:::'·':''-' ~ Salinia terrane 

~ Volcanic rocks 

Contact 

--.... Terrane boundary- Dotted where 

concealed 

~ San Andreas transform system-

.....- Arrows indicate direction of 

relative movement 

'~~'O o ---- Inferred fracture zone-Dark line 

indicates position at approximately 13 Ma; lighter lines 

indicate positions at other 

times; dotted where subducted; 

queried where uncertain 

~Active subduction zoneSawteeth on upper plate PAC New ldria diapir ~ Abandoned subduction zoneSawteeth on upper plate 

14.8- Ma Page Mill Basalt I Active spreading ridge and 

transform fault 

0 100 KILOMETERS 

19- to 24-Ma Pinnacles 

Volcanics Hydrothermal circulation above 

magmatic heat source along 

leaky fracture zone 19- to 24-Ma Neenach 

Volcanics 

~ ~--~~~~~------------~----~----~~~~L-~~------------~~6---~ 

~ 

c:::::::;> Plate motion with respect to 

North American plate 

Figure 27. Model for origin of King Range terrane that assumes little or no latitudinal translation, as suggested by Underwood (1989), showing reconstructed North American plate 

margin at approximately 13 Ma. According to this interpretation, King Range terrane was deposited near its present-day 

latitude, near Farallon-North American plate subduction margin, and subsequently was "troweled" aside as Mendocino 

triple junction propagated northward. Thermal anomaly associated with King Range terrane is attributed to hydrothermal 

circulation caused by shallow magma generated in Farallon 

plate along partially subducted leaky fracture zone, which has 

since migrated northward. High geothermal gradient was 

present in Farallon plate, owing to proximity of spreading 

ridge to subduction margin. ERB, deposits of Eel River basin; 

FAR, Farallon plate; KRT, King Range terrane; MFZ, Mendocino Fracture Zone; NAM, North American plate; PAC, Pacific 

plate; VIZ, Vizcaino structural block. 

48 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
.Jeo 

0 

PAC 

FAR 

Point Delgada 

hydrothermal 

system 

100 KILOMETERS 

I 

NAM 

8- to 12-Ma volcanic 

rocks (Tolay Volcanics, 

Quien Sabe Volcanics, 

and volcanic rocks in 

Berkeley Hills) 

New ldria diapir 

14.8- Ma Page Mill Basalt 

19- to 24- Ma Pinnacles 

Volcanics 

19- to 24- Ma Neenach 

Volcanics 

EXPLANATION 

!':' :) =:'.::_ /)] King Range terrane 

~ Coastal and Yager terranes 

=============~ Central and Eastern belts 

,, . '~: -, ~ :I Salinia terrane 

~ Volcanic rocks 

----- Contact 

--.... Terrane boundary- Dotted where 

concealed 

~ San Andreas transform system-

......- Arrows indicate direction of 

relative movment 

---- Inferred fractu re zone 

~ Active subduction zoneSawteeth on upper plate 

~ Abandoned subduction zoneSawteeth on upper plate 

I 

Slabless window (slab gap) filled 

by hot a sthenosphere at 

approximately 13 Ma 

Active spreading ridge and 

transform fa ult 

~Magma 

~ 

intrusion above newly 

opened slabless window 

(slab gap) at a pproximately 

13.8 Ma- Shown in'B only 

' ~--~ A ..;>7~------------~-~~---- ~~~ ~~ ~,~~------------~~~6'---~ 

' ' 

Hydrothermal circulation above 

magma intrusion that was 

caused by newly opened 

slabless window (slab gap) at 

approximately 13.8 Ma 

VIZ 

B 

segment 

NAM 

Subducting 

slab boundary 

Plate motion with respect to 

North American plate 

Figure 28. Alternative model for origin of King Range terrane 

that assumes large-scale translation with Pacific plate, showing 

reconstructed North American plate margin at approximately 

13 Ma. ERB, deposits of Eel River basin; FAR, Farallon plate; 

KRT, King Range terrane; MFZ, Mendocino Fracture Zone; 

NAM, North American plate; PAC, Pacific plate; VIZ, Vizcaino 

structural block. A, According to this interpretation, King 

Range terrane was assembled considerably south of its presentday latitude, along Farallon-North American plate subduction 

margin, and was accreted to Pacific plate as Mendocino Fracture Zone extended eastward into subduction margin. Point 

Delgada hydrothermal system was initiated when slabless window (s lab gap) opened beneath King Range terrane about 13.8 

Ma. King Range terrane was then translated northward with 

Pacific plate to its present-day latitude, where it was accreted 

back to North American plate margin as motion between Pacific and North American plates became increasingly transpressive at propagating north end of San Andreas transform. 8, 

Enlargement of area of Point Delgada hydrothermal system, 

simplified from figure 28A, showing eastward jump of Mendocino Fracture Zone, realignment of subduction zone, and 

magma intrusion above newly opened slabless window (slab 

gap). 

Tectonostratigraphic Framework of the Cape Mendocino Region and Adjacent Northern Coast Ranges 49 
and O'Neil, 1984; Fox and others, 1985; McLaughlin and 

others, 1985b; Stanley, 1987), and (2) the position of the 

Mendocino Fracture Zone when the King Range terrane 

was transferred from the North American plate margin to 

the Pacific plate (Engebretson and others, 1985). 

Dickinson and Snyder (1975, 1979) pointed out that 

the geometric instability created by propagation of the 

Mendocino triple junction requires a triangular "slabless 

window" adjacent to the San Andreas transform and east 

of the Mendocino Fracture Zone. This window beneath the 

crust is defined on the north by eastward projection of the 

subducted south boundary of the Gorda plate, on the south 

by the point along the trailing Pacific-North American 

plate transform where the East Pacific Rise intersects the 

North American plate margin, and on the east by a line of 

intersection at the base of the North American plate connecting the subducted easternmost part of the Gorda plate 

with the East Pacific Rise-North American plate intersection (fig. 28A). Theoretically, the slabless window has 

opened continuously with northward lengthening of the 

San Andreas transform and eastward subduction of the 

Gorda plate, allowing hot asthenosphere to continuously 

rise upward into the window and heat the overlying crust. 

Dickinson and Snyder (1979) used this model to explain 

the relation of magma emplacement to propagation of the 

San Andreas transform (fig. 28B). The same model also 

has been invoked to explain heat-flow distribution along 

the San Andreas fault (Lachenbruch and Sass, 1980) and 

the velocity structure of the crust beneath the Coast Ranges (Zandt and Furlong, 1982). Recently, the concept of a 

triangular slabless window of asthenosphere has been 

challenged by Severinghaus and Atwater (in press), who 

argue that subducted slabs heat up and become incorporated into the asthenosphere over time. The effect of this 

heating is to obliterate the triangular geometry of the 

"slabless window" and, instead, produce a large "slab gap" 

southeast of the triple junction. 

Underwood (1987) argued that the appearance of thermal anomalies in the crust of the Coast Ranges lagged behind passage of the Mendocino triple junction by as much 

as 3 m.y., owing to the time necessary to conductively 

transfer heat into the crust overlying the asthenospherefilled slabless window. However, this argument does not 

seem to apply to the Coast Ranges because initiation of 

volcanism at 2.6-2.0 Ma in the Clear Lake area lagged 

behind passage of the Mendocino triple junction by only 

0.7 to 1.3 m.y. (Donnelly-Nolan and others, 1981; 

McLaughlin, 1981; McLaughlin and others, 1985b ). This 

shorter lag may be attributable to the likelihood that thermal anomalies in the upper 10 km of crust above the 

slabless window (slab gap) are more directly tied to extensional strain adjacent to newly lengthened segments of the 

transform than to conductive heat transfer. Extensional 

faulting over the asthenosphere-filled slab gap would establish conduits for upward movement of magma and allow convective hydrothermal circulation to occur above 

and adjacent to the magma bodies. We have therefore assumed an approximate lag time of 1 m.y. between passage 

of the Mendocino triple junction and initiation of volcanic 

activity. 

The latitude of origin of the King Range terrane was 

reconstructed by restoring slip on major faults of the San 

Andreas system that postdate 16 Ma, and by assuming that 

the King Range thermal anomaly formed in close proximity to 15-13-Ma ~olcanism in the Coast Ranges. We consider volcanism of this age and the thermal anomaly to 

have been initiated over faulted crust above a slabless 

window (slab gap) shortly after passage of the Mendocino 

triple junction. In reconstructing the 16-13-Ma California 

margin, 115 km of Neogene slip are restored on the Hosgri-San Gregorio fault, 28 km on the peninsula segment of 

the San Andreas fault (Cummings, 1968; Fox and others, 

1985), and 102 km on the Pilarcitos fault, amounting to 

245 km of combined slip. Restoration of 45 km of slip 

across the Tolay~Hayward fault and 145 km across the 

Calaveras fault (for a combined slip of 190 km) are necessary to restore the 12-8-Ma Tolay Volcanics of Morse and 

Bailey ( 1935), v0lcanic rocks in the Berkeley Hills, and 

the Quien Sabe Volcanics of Leith (1949) to the same pre12-Ma position (McLaughlin and others, 1990). This reconstruction further restores the 14.8-Ma Page Mill Basalt 

on the San Francisco peninsula to a position south .of the 

Quien Sabe Volcanics (Johnson and O'Neil, 1984; Fox and 

others, 1985). The resulting minimum post-16-Ma dextral 

displacement across the San Andreas system is about 

435 km (figs. 26, 28). 

Previously discussed structural and stratigraphic relations indicate that between about 17 and 14 Ma, strata in 

the King Peak subterrane of the King Range terrane were 

deposited in a lower bathyal trench setting along the 

Farallon-North American plate subduction margin, north of 

the Mendocino FFacture Zone. We speculate that this trench 

or trench-slope depositional setting may have overlapped 

the Central belt, as well as the Coastal and Yager terranes 

of the Coastal belt (fig. 28). Deposition of strata in the King 

Peak subterrane continued until shortly after 15 Ma, when 

the northward-propagating Mendocino triple junction approached about lat 36.5° N. By that time, the composite 

Late Cretaceous rto middle Miocene Vizcaino block and 

Point Delgada subterrane had already been accreted to the 

northwestward-moving Pacific plate. Upper Cretaceous 

oceanic rocks of the Point Delgada subterrane then were 

juxtaposed with the King Peak subterrane along an early 

transtensional fault of the San Andreas regime at about 

14 Ma, and hydrothermal circulation was initiated at about 

13.8 Ma along normal faults conjugate to the strike-slip 

system. This hydrothermal event, which affected the entire 

King Range terrahe, most likely resulted from intrusion of I 

magma into the spallow crust above a newly opened partition of the slaoless window (McLaughlin and others, 

50 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendoci..,o Triple Junction, Northwestern California 
1985b). We postulate that the 14.8-Ma Page Mill Basalt and 

younger Quien Sabe Volcanics, Tolay Volcanics, and volcanic rocks in the Berkeley Hills are surface manifestations 

of the magmatic source of the King Range thermal anomaly 

and that these volcanic rocks were erupted over the slabless 

window considerably east of the transform margin (fig. 28). 

We also postulate that the Mendocino Fracture Zone again 

extended eastward during the episode of crustal extension 

which accompanied the King Range thermal event, from 

the northeast corner of the Vizcaino block, into the trenchslope of the North American plate. This eastward propagation of the Mendocino Fracture Zone was accommodated 

along the trailing transform margin by another eastward 

jump and slight clockwise rotation of the transform boundary, thus accreting the King Peak subterrane to the east side 

of the Vizcaino block at 13.8 Ma. Subsequently, the King 

Range terrane was translated northward with the Pacific 

plate (fig. 28). , 

Owing to about 20° of clockwise rotation of the PacificFarallon Ridge between 3. 7 and 3.4 Ma (Harbert and Cox, 

1986), the northern section of the San Andreas fault zone 

changed direction to a more north-trending orientation north 

of Point Arena. Since the time of this major reorientation, 

the transform margin has been dominantly transpressive. 

This change from dominantly transtension to transpression 

appears to have prevented magma in the underlying slab 

window from rising into shallow levels of the crust. Thus, 

volcanism in the Coast Ranges has not extended north of the 

Clear Lake area since the Mendocino Fracture Zone propagated past that latitude about 3 Ma. Other tectonic implications of the change from transtension to transpression 

include (1) major uplift of the northern Coast Ranges; (2) 

late Cenozoic compressional structures parallel to the San 

Andreas fault system, which are superposed on earlier transtensional basins; and (3) dominantly north-northeast oriented compression presently occurring at the northeast corner 

of the Pacific plate south of the Mendocino Fracture Zone. 

STRUCTURAL RELATIONS OF THE MODERN 

MENDOCINO TRIPLE JUNCTION 

We now turn to the modern tectonic setting of the 

Cape Mendocino region and discuss the geologic evidence 

for the present position of the Mendocino triple junction. 

Faults Defining the Configuration of the 

Triple Junction 

The modern Mendocino triple junction is defined by 

several major faults in the onshore and offshore areas, 

with different orientations and senses of slip (figs. 24A, 

29). Offshore, the modern Pacific plate is bounded to the 

north by the active Mendocino fault zone. West of long 

126° W., the Mendocino fault zone separates oceanic crust 

of the Gorda and Pacific plates along a prominent, southfacing fault scarp. East of long 126° W., the Mendocino 

fault zone follows the base of the steep, north-facing Gorda Escarpment, separating the Gorda plate on the north 

from rocks of the Vizcaino structural block to the south 

(figs. 24, 29). East of long 125° W., the Mendocino fault 

zone, if present at all offshore, is difficult to delineate. Onshore, however, northwest of Petrolia, a N. 47° W.-trending fault that truncates the south side of a large Quaternary 

alluvial terrace projects offshore and aligns with Mendocino Canyon. 

Another fault defined in seismic profiles (S.H. Clarke, 

Jr., oral commun., 1988; Clarke, 1992) branches southeastward from the Mendocino fault at about long 125° W. 

along Mattole Canyon (figs. 17A, 29). This Mattole Canyon fault follows a topographic bench along the base of a 

prominent escarpment on the northeast side of the Mendocino Ridge. The faulted bench is bathymetrically higher· 

than the base of the escarpment aligned with the more 

east-west trending Mendocino Canyon. 

The Mattole Canyon fault projects shoreward south of 

the mouth of the Mattole River, to align with N. 69° W.-

trending faults that form the northern part of the suture 

between the King Range and Coastal terranes. The anomalously high and rugged relief of the King Range terrane, 

which reaches elevations of 1,220 m within 5 km of the 

coast, aligns closely with that along the crest of the. Mendocino Ridge offshore, at the north boundary of the Vizcaino 

block. Thus, there appears to be structural and physiographic continuity between the northern part of the Vizcaino 

block and the King Range terrane. 

The San Andreas fault has traditionally been projected 

northward of Point Delgada offshore from the King 

Range, to intersect with the Mendocino Fracture Zone 

along the trend of the Mattole Canyon fault (Curray and 

Nason, 1967; Brown and Wolf, 1972). The San Andreas 

fault, however, has never been delineated in the offshore 

area north of Point Delgada (McCulloch, 1987a), and so 

its presence between Point Delgada and the north side of 

the King Range terrane is speculative. The east side of the 

fault zone appears to lie inland ~long the east side of the 

King Range terrane, although as-yet-unconfirmed traces of 

the fault system may also lie offshore, separating the King 

Range terrane from the east side of the Vizcaino block. 

Fold axes and shear fabric in matrix rocks of the Coastal terrane north of Petrolia are oriented northwest-southeast, 

and foliations in the shear fabric characteristically are inclined northeastward (figs. 17A, 29). It also has been shown 

that the orientation of subduction-related structures (folds 

and thrusts) in the offshore accretionary prism above the 

subducted and deformed Gorda plate, including the basal 

thrust of the Cascadia subduction zone (Clarke, 1988, 1992; 

McLaughlin and others, 1988b ); are coparallel and co linear 

with onshore structures of the Coastal terrane. Pliocene or 

Structural Relations of the Modern Mendocino Triple Junction 51 
40° 

30' 

40° 

00' 

N38°W N7°E Orientation of 

40° 

30' 

40° 

.. · .'. 

maximum regional 

compression 

Orientation of 

maximum regional 

shear strain 

.·.· .·.·. ·.· .·.·.· . ·.·. · . ·.·.·.· . ·.···B . •••• 0 •• 0 •••••••••• 0 •• 

: · .. ·· · .. 

·:. 0 : ... · . 10 ·: : . . 20 . . 30 KILOMETERS· 

00' ~----~~--~~~~~--~--~~--~~~ 

' 

" ' \ 

\ 

A 

\ 

\ 

\ 

" 

52 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple )unction, Northwestern California 
c Area of anomalously high Quaternary uplift rates 

A 

KILOMETERS 

5 

PACIFIC OCEAN 

'-

~ 

ii: 

.Q? 

~ 

~ 

'--

~-eii:S 

.Q? ~ 

ge 

~~ 

~'- ex: 

~ .Q? 

::::ct: ~ '- -c::.Q? ~ ~ g~ ~ ii: 

JS~ ~ '-cu 

Q) 

LlJ :<: co 

.-? 

Q) 

Ol 

-o A' 0: 

Q5 KILOMETERS > 

0: 5 (ij 

Q) 

(]) Eel River Basin 

SEA LEVEL 

5 

10 

\\ II ~ II \\ ~ T 1 ~ \\ II ~ 

\\ ~ If "' If "' .-? .-? If \\ .-? \\ ~ ~ II II .-? 15 

EXPLANATION 

~ Upper Cenozoic rocks 

1 · - · - · - ·j North American plate-Includes Coastal 

and Yager terranes 

[: -- ·_ . :. ) Pacific plate-Includes Vizcaino block and 

King Range terrane 

Gorda plate- Consists of: 

111111111111111 1 Offscraped sedimentary rocks from 

Coastal and Yager terranes 

I """ I! "I Oceanic crust 

.___ __ ___,! Region of high rate of coastal-terrace uplift 

and (or) fluvial-terrace incision-At 

least 1 mm/yr since 120 ka; generally 

includes area of coseismic uplift of 

4/25/92 earthquake 

Contact 

~ A Fault-Dashed where inferred; queried 

~ T where uncertain. Arrows indicate 

direction of relative movement. A, 

away from observer; T, toward 

observer (shown in cross section only) 

Figure 29. Mendocino triple junction area (MTJ, circled). A, 

Generalized map of faults that define modern Mendocino triple 

junction, showing area of active uplift associated with compression across triple-junction region, and locations of epicenters of recent earthquakes (Richard Lester, U.S. Geological 

Survey, oral commun., 1992). Area of uplift deduced from data 

of Lajoie and others (1982), Mclaughlin and others (1982), 

Bickner (1985), Hagemann (1985), Lajoie (1986), and Merritts 

• • • 

•• 

---500---

~4 25 92 M = 7.1 

Thrust fault- Dashed where inferred . 

Sawteeth on upper plate 

Basal thrust of subduction zone-Dashed 

where inferred; queried where 

uncertain. Sawteeth on upper plate 

Inferred axis of regional upwarpingShowing direction of plunge. 

Queried where uncertain 

Bathymetric contour-In meters 

Epicenter (in cross section, focus) of recent 

earthquake-Showing date and 

magnitude (M ) of shock 

Blind thrust fault (shown in cross section 

only}-Interpreted from preferred 

first-motion solution for recent earthquake. Dashed and (or) queried where 

extent of rupture is inferred from 

aftershock distribution. Arrows show 

direction of relative movement 

(1986). Orientation of maximum regional compression (inset) 

from trilateration data of Breen and others (1987). FZ, fault zone; 

SZ, shear zone. 8, Plate-tectonic model of Mendocino triple 

junction area. C, Schematic cross section across Mendocino 

triple junction area, showing inferred subsurface geometry of 

tectonic plates and locations of blind thrust faults associated with 

recent earthquakes. KRT, King Range terrane; KRTF, King Range 

thrust fault; SAF, San Andreas fault; VIZ, Vizcaino structural block. 

Structural Relations of the Modern Mendocino Triple Junction 53 
younger strata are cut by faults of this orientation and affected by the folding onshore and offshore. In addition, 

paleobathymetric data from isolated areas of the Neogene 

strata affected by these folds and faults onshore show that 

these strata were deposited at lower-bathyal to abyssal 

depths, consistent with the offshore setting of the lower part 

of the accretionary prism. The basal thrust of the accretionary prism strikes nearly east-west and aligns with a steep 

fault that bounds the north side of a remnant of late Miocene 

lower-slope strata near the mouth of Davis Creek northwest 

of Petrolia. In combination with fault relations which suggest that the obducted King Range terrane is part of the 

Pacific plate, these data suggest that the present-day Mendocino triple junction (the common contact between the 

Pacific, Gorda, and North American plates) lies onshore 

between the mouth of Davis Creek and the north side of the 

King Range terrane (fig. 29). 

South of Davis Creek and, notably, in the area between McNutt Gulch and the mouth of the Mattole River, 

the structural grain in matrix rocks of the Coastal terrane 

dips south, subparallel to faults bounding the King Range 

terrane. Lower-bathyal to abyssal Pliocene strata are cut 

by these thrusts north of Petrolia. We interpret these Pliocene or younger thrusts to be associated with active northsouth-oriented compression between the Pacific and North 

American plates. This association suggests to us that the 

triple junction may be very close to the hamlet of Petrolia 

(figs. 17A, 29). Thus, the area of imbricated Coastal terrane and lower-slope Neogene sedimentary deposits between Davis Creek and the north side of the obducted 

King Range terrane may correspond to highly deformed 

hemipelagic sediment and acoustic basement that are compressed against the Gorda Escarpment at the south side of 

Gorda Basin offshore (Clarke, 1992). On the basis of this 

interpretation, these imbricated rocks north of the King 

Range terrane were scraped off the Gorda plate and 

squeezed between the North American and Pacific plates, 

and are currently accreting to the Pacific plate. 

Seismicity 

Seismic activity is concentrated along the Mendocino 

fault zone and within the Gorda plate (Bolt and others, 1968; 

Seeber and others, 1970; Eaton, in pre~s). The depth to the 

seismogenic zone within the Gorda plate is shallow in the 

offshore area, but it deepens to below 12 km to the east, 

where the Gorda plate is subducted beneath the California 

margin (Jachens and Griscom, 1983). First-motion studies 

suggest left-lateral slip on northeast-oriented faults within 

the Gorda plate (Eaton, in press), and right-lateral and upto-the-north thrusting along east-west-trending faults of the 

Mendocino fault zone west of long 125° W. These data argue 

that north-south-oriented compression between the Gorda 

and Pacific plates, in conjunction with the east-west component of relative motion with North America, has contributed 

substantially to Gorda plate deformation (Wilson, 1986; 

Stoddard, I987) and to its partial coupling to the North 

American plate. This interpretation is consistent with trilateration data for the region (Breen and others, I987) which 

show that maximum compression is oriented about N. 7° E. 

Focal mechanisms for earthquakes deeper than 12 km 

along the Mendocino fault zone east of long 125° W. suggest right-lateral slip between the Gorda and Pacific plates. 

However, numerous shallow earthquakes occur onshore and 

are diffusely aligned along the north and east sides of the 

King Range terrane. These shallow earthquakes define a 

zone that displays a moderate to steep southwestward dip 

(Eaton, in press); and are reasonably well constrained as 

occurring above IO km (R. McPhearson, oral commun., 

1988). No focal mechanisms have been deduced for the 

shallow earthquakes associated with the King Range terrane 

boundary, but their southwest-dipping depth distribution 

suggests that they could be associated with southwest-sideup thrusting-indicative of the relative motion between the 

Pacific and North American plates. 

Quaternary Uplift 

Studies of Quaternary marine and inland fluvial deposits 

(Lajoie and others, I982; McLaughlin and others, 1983; 

Bickner, 1985; Hagemann, 1985; Carver and others, 1986; 

Lajoie, I986; Merritts, 1986) yield relatively high rates of 

tectonic uplift of 1 to 4 mm/yr since 120 ka along the coast 

between Cape Mendocino and the mouth of Whale Gulch 

southeast ofPoint:Delgada (fig. 29A). Comparably high incision rates for fluvial terraces persist inland for at least 50 km 

along the Mattole River (Merritts, I986) and may be as high 

as I mm/yr neat Garberville (Bickner, 'I985). South of 

Whale Gulch and north of Cape Mendocino, uplift rates diminish to 0.3 mQ:Liyr or less toward the center of the Eel 

River basin. Thus, Quaternary uplift data are consistent with 

active northeast-southwest-oriented compression (fig. 29B) 

and suggest that a significant component of the motion between the North American and Pacific plates has been resolved through vertical motion for at least the past 120 ka. 

This uplift, centered over the King Range terrane (fig. 29), is 

expressed as a broad regional warp extending northward approximately to C~pe Mendocino, southward to Bear Harbor, 

and eastward to the vicinity of Garberville. Active thrusting 

along the north and east sides of the King Range probably 

has contributed to the uplift and warping of this area, in 

addition to movement along relatively flat, blind thrusts associated with the ~outhern Cascadia subduction zone. 

CONCLUSIO~S 

I. From 90 to 521Ma, the tectonic setting of the Cape Mendocino region was dominated by oblique dextral convergence between the North American plate and a poorly 

54 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 
defined, fast-moving oceanic plate (McLaughlin and others, 1988a). The dextral component of motion between 

these plates contributed to the formation and resulted in 

the northward translation of melange in the Central belt of 

the Franciscan Complex. This melange includes igneous 

and pelagic rocks scraped off the fast-moving, northwardtranslating oceanic plate, as well as slabs and blocks of 

rock derived from previously accreted and metamorphosed parts of the Franciscan Complex (Eastern belt), the 

Coast Range ophiolite and overlying Great Valley sequence, and other land-derived sedimentary sequences deposited with and on top of far-traveled oceanic rocks 

during their northward translation. 

2. Between about 60 Ma (late Paleocene) and 52 Ma (early 

Eocene), the Central belt ceased moving northwestward, 

was accreted to the northern California margin, was uplifted and eroded, and was depositionally overlapped by nonaccretionary marine littoral, shelf, and slope deposits 

(McLaughlin and others, 1988a). Simultaneously, the active accretionary margin stepped westward to the Cape 

Mendocino region, as the North American plate began 

interacting with the Farallon plate. Between about 55 and 

41 Ma, Farallon-North American plate motion was 

strongly convergent, with a much smaller northward component than before accretion of the Central belt. The 

Coastal and Yager terranes of the Coastal belt were deposited largely during this time interval on the slope and 

trench-slope of the Farallon-North American plate margin, north of lat 30° N. A smaller but significant component of the Coastal terrane, consisting of basalt, pelagic 

limestone, and locally interbedded arkosic sandstone, 

formed and was deposited at low paleolatitudes on the 

Farallon plate. The basaltic and pelagic rocks formed between 82 and 69 Ma near the triple junction of the Pacific, 

Farallon, and Kula plates and north of the Mendocino 

Fracture Zone. Arkosic sandstones interbedded with the 

basalts probably were derived from the Late Cretaceous 

continental margin of northern Mexico and subsequently 

translated northeastward on the Farallon plate. These strata deposited on the Farallon plate moved at poleward rates 

of 7.1 to 7.2 crn/yr, until they contacted the California 

margin near the south end of the Diablo Range at about 49 

Ma. Between 49 and 41 Ma, Coastal belt sediment eroded 

and transported from the North American plate interior 

and from uplifted, older parts of the Franciscan Complex 

were deposited on the slope and trench-slope, above the 

Farallon-North American plate subduction margin. Accretion of the Yager and Coastal terranes of the Coastal belt 

occurred between the latest Eocene (ca. 40 Ma) and latest 

Oligocene (ca. 24 Ma). At this time, the subduction margin 

again stepped westward, as poleward translation of the 

Coastal terrane slowed to less than 2 crn/yr, and the Coastal and Yager terranes were compressed and uplifted concurrently with westward-directed thrusting of the Central 

belt along the Coastal belt thrust. 

3. After assembly of the Yager and Coastal terranes, the 

subduction zone between the Farallon and North American plates stepped westward, creating the site of deposition of Miocene clastic strata in the King Peak subterrane. 

Rocks of the Central belt may partly underlie these clastic 

strata. Basalt, overlain by radiolarian chert of middle Miocene age, suggests that part of the Farallon plate was 

incorporated into the King Peak subterrane in the region of 

the trench. We conclude that the depositional site of strata 

of the King Range terrane was along the Faralloo-North 

American plate subduction margin and as much as 435 km 

south of the Cape Mendocino area. According to this interpretation, the King Range terrane was transferred to, and 

translated with, the Pacific plate beginning about 13.8 Ma, 

as the Mendocino triple junction propagated northward 

past about lat 36.5° N. We suggest that at this time, the 

King Peak subterrane was rifted away from the North 

American plate margin as magma intruded concurrently 

and hydrothermal circulation was initiated above a 

slabless window (slab gap) filled with asthenosphere behind and adjacent to the northward-propagating triple 

junction. At the time of this rifting and accretion to the 

Pacific plate, the King Peak subterrane came into contact 

with the Point Delgada subterrane, which had previously 

been accreted to the Vizcaino structural block. The transfer of the King Range terrane to the Pacific plate was 

accommodated by eastward extension of the Mendocino 

Fracture Zone and a concurrent eastward jump of the San 

Andreas transform. 

4. The Vizcaino structural block represents either the north 

end of an extinct paleosubduction margin between the 

North American and Farallon plates, or an extinct convergent margin between the Pacific and North American 

plates, driven from the southeast by Oligocene extension 

and rifting above subducted segments of the Pacific-Farallon Ridge system. The Vizcaino block was accreted to the 

Pacific plate after initial contact of the Pacific-Farallon 

Ridge with the North American plate between 30 and 

28 Ma. Between about 28 and 16 Ma, fragmented remnants of the Farallon plate south of the Mendocino Fracture Zone may have been caught between segments of the 

Pacific-Farallon Ridge that were becoming extinct, and 

the North American subduction margin. Alternatively, 

westward-directed thrusting of the North American plate 

over the Pacific plate may have begun south of the Mendocino Fracture Zone, as a result of northwest-southeast 

extension and rifting initiated above the subducted Pacific-Farallon ridge after its collision with the North American plate margin during Oligocene time. Between about 

16 and 14 Ma, the Mendocino Fracture Zone extended 

eastward across this convergent margin, forming the north 

boundary of the Vizcaino block. Eastward extension of 

the Mendocino Fracture Zone probably was concurrent 

with and, possibly, resulted from a reorientation of the 

Pacific-Farallon Ridge at its point of contact with the 

Conclusions 55 
North American plate. This reorientation caused the west 

edge of the North American plate, including the Vizcaino 

block, to rift and take up northward motion of the Pacific 

plate. The modern San Andreas fault system was thereby 

initiated, connecting the newly extended segment of the 

Mendocino Fracture Zone to the newly oriented, active 

Pacific-Farallon Ridge system. 

5. The modern-day Mendocino triple junction probably is 

situated onshore near Petrolia, rather than in the offshore 

region where it has traditionally been located. The 

present-day triple-junction region is characterized by high 

rates of Quaternary uplift (1-4 mm/yr) that define an area 

of anomalous upwarping extending from Cape Mendocino on the north to Bear Harbor on the south and, at least, to 

Garberville on the east. This uplift is attributable to northnortheastward-directed blind thrust faults in rocks as 

young as Pliocene along the north and northeast sides of 

the King Range terrane, and to more broadly defined regional warping above north-northeast-dipping blind thrust 

faults north of Petrolia. These features apparently accommodate northeast-southwest compression between the 

North American and Pacific plates. In addition, north of 

the hamlet of Petrolia, shear fabric in the Coastal terrane 

of the Franciscan Complex parallels north-northeast-dipping, northwest-trending active thrusts in the offshore accretionary wedge overlying the subducting Gorda plate. 

The faulted base of the accretionary prism offshore bends 

east and trends onshore in the vicinity of Petrolia. Together, these data suggest that the south end of the Cascadia 

subduction zone is bent eastward to accommodate motion 

between the North American and Pacific plates and that 

partial coupling is occurring between the North American 

and Gorda plates (fig. 29). The Mendocino triple junction 

is expressed today as a region of northeastward-directed 

thrusting related to motion between the North American 

and Pacific plates, superposed on the northwest-southeastoriented, northeast-dipping, convergent structures resulting from motion between the North American and Gorda 

plates. 

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60 Tectonostratigraphic Terranes of the Franciscan Complex and Evolution of the Mendocino Triple Junction, Northwestern California 

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