Possible Correlations of 

Basement Rocks Across the 

San Andreas, San GregorioHosgri, and RinconadaReliz-King City Faults,

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1317
Possible Correlations of 

Basement Rocks Across the 

San Andreas, San GregorioHosgri, and RinconadaReliz-King City Faults, 

California

By DONALD C. ROSS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1317

A summary of basement-rock relations 

and problems that relate to possible 

reconstruction of the Salinian block 

before movement on the San Andreas fault

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1984
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Boss, Donald Clarence, 1924-

Possible correlations of basement rocks across the San Andreas, San Gregrio-Hosgri, and Rinconada-Reliz-King City 

faults, California

(U.S. Geological Survey Bulletin 1317)

Bibliography: p. 25-27

Supt. of Docs, no.: 119.16:1317

1. Geology, structural. 2. Geology California. 3. Faults (geology) California. I. Title. II. Series: United States.

Geological Survey. Professional Paper 1317. 

QE601.R681984 551.8'09794 84-600063

For sale by the Distribution Branch, Text Products Section, 

U.S. Geological Survey, 604 South Pickett St., Alexandria, VA 22304
CONTENTS

Page

Abstract _____________________________________________________________ 1

Introduction __________________________________________________________ 1

San Gregorio-Hosgri fault zone ___________________________________________ 3

San Andreas fault zone _________________________________________________ 8

Rinconada-Reliz-King City fault zone ______________________________________ 16

Problems of Salinian-block reconstruction that still remain.____________________ 18

Cordilleran miogeoclinal rocks_____________________________---_-------_ 18

Bean Canyon Formation _____________________________________________ 19

Temblor clasts _____________________________________________________ 19

Sierran quartzites __________________________________________________ 19

Metallic-mineral deposits.____________________________________________ 19

Barrett Ridge slice.________________________________________________ 20

Montara Mountain and Gualala granitic clasts ___________________________ 21

Europium-anomaly contrast __________________________________________ 21

Unlocated west flank of the Salinian block. ______________________________ 23

Conclusions.__________________________________________________________ 23

References cited _______________________________________________________ 24

Supplemental information _______________________________________________ 27

Comparison of the porphyritic granodiorites of Point Reyes and Monterey _____ 27

Comparison of the granitic rocks near the Pinnacles and Neenach Volcanic

Formations of Matthews (1976) ______________________________________ 31

ILLUSTRATIONS

Page

FIGURE 1. Index map of central and southern California, showing selected geologic and geographic features __________ 2

2. Map showing postulated realinement of some basement-rock units along the San Gregorio-Hosgri fault zone 3

3. Generalized geologic map of the Gualala area ____________________________________________________ 4

4. Generalized geologic map of the northern Salinian block ___________________________________________ 6

5. Generalized geologic map of the central Salinian block _____________________________________________ 7

6. Generalized geologic map showing location of a possible north-south-trending fault connecting the Ben

Lomond and La Honda faults ______________________________________________________________ 8

7. Map showing probable offset of the west margin of the Salinian block by the San Gregorio-Hosgri fault zone 8

8. Map showing locations of wells to basement between the Chalone Creek and San Andreas faults ___________ 9

9. Generalized geologic map of the southernmost Sierra Nevada and the westernmost Mojave Desert _________ 10

10. Generalized geologic map showing locations of sheared rocks along the Vergeles fault.___________________ 11

11. Map showing extrapolation from well-core data of some basement-rock units in the central Salinian block ___ 15

12. Generalized geologic map of the southern Salinian block ___________________________________________ 17

13. Triangular diagram showing comparison of modally analyzed specimens of the porphyritic granitic rocks of

Thermal Canyon and the porphyritic granodiorite-granite of the La Panza Range.___________________ 18

14. Maps showing possible reconstructions of the northern and central Salinian block before San Andreas fault

movement-_____________________________________________________________________________ 22

15. Generalized geologic maps showing locations of modally and chemically analyzed samples of the porphyritic

granodiorites of Point Reyes and Monterey ___________________________________________________ 27

16. Triangular diagrams showing modes of the porphyritic granodiorites of Point Reyes and Monterey _________ 28

17. Histograms showing chemical analyses of the porphyritic granodiorites of Point Reyes and Monterey _______ 28

18. Triangular diagrams showing chemical compatibility of the porphyritic granodiorites of Point Reyes and

Monterey ______________________________________________________________________________ 30

19. Triangular diagrams showing modes of the Tomales Point and Inverness granitic bodies _________________ 30

20. Maps showing locations of granitic rocks of the Pinnacles and Neenach areas and of modally and chemically

analyzed samples._______________________________________________________________________ 31

21. Triangular diagrams showing modes of granitic rocks of the Pinnacles and Neenach areas ________________ 34

22. Histograms of chemical analyses of samples of granitic rocks of the Pinnacles and Neenach areas __________ 35

23. Triangular diagram showing modes of chemically analyzed samples of granitic rocks of the Pinnacles and

Neenach areas __________________________________________________________________________ 36

24. Triangular diagrams showing chemical compatibility of granitic rocks of the Pinnacles area with those of the

Neenach area___________________________________________________________________________ 36

ill
IV CONTENTS

Page

FIGURE 25. Histograms showing semiquantitative-spectroscopic-analysis "midpoints" of trace-element concentrations in 

samples of the tonalite and granodiorite of Johnson Canyon, the granodiorite of the Fairmont Reservoir, 

the granite of Bickmore Canyon, and the felsic variant of the Fairmont Reservoir body _______________ 37

TABLES

Page

TABLE 1. Chemical analyses of selected granitic rocks from the Point Reyes and Monterey areas, California-_________ 29

2. Modes of selected granitic-rock samples from the Neenach area, California-____________________________ 33

3. Chemical analyses of selected samples of the tonalite and granodiorite of Johnson Canyon, the granodiorite 

of the Fairmont Reservoir, the granite of Bickmore Canyon, and the felsic variant of the Fairmont 

Reservoir body._________________________________________________________________________ 35
POSSIBLE CORRELATIONS OF BASEMENT ROCKS

ACROSS THE SAN ANDREAS, SAN GREGORIO-HOSGRI,

AND RINCONADA-RELIZ-KING CITY FAULTS, CALIFORNIA

By DONALD C. Ross

ABSTRACT

A reconstruction that treats the Salinian block as a southward extension of the Sierra Nevada is supported by (1) the 

possible correlation of several basement-rock features of the 

central Salinian block and the south end of the Sierra Nevada, 

and (2) reversal of right-lateral movement on the San GregorioHosgri fault zone that significantly telescopes and shortens the 

north end of the Salinian block. However, currently accepted 

and documented movements on the San Gregorio-Hosgri and 

San Andreas fault zones are not great enough to allow either 

the Barrett Ridge slice or the K-feldspar-rich Cretaceous and 

lower Tertiary sedimentary rocks of the Gualala area to be 

derived from the southern Sierra Nevada. Additional fault displacement or transportation of exotic blocks from considerable 

distances would seem to be required. A model that treats the 

Salinian block solely as a fault-derived fragment of the southern 

Sierra Nevada does not appear to completely resolve the reconstruction problem. Yet the similarities between the basement rocks of the Salinian block and the southern Sierra Nevada are great enough that strong data would be required to 

support alternative models of Salinian-block origin.

INTRODUCTION

During the past several years, I have gathered 

considerable information on the distribution, petrography, and chemical composition of the granitic and metamorphic rocks of the California Coast 

and Transverse Ranges, particularly in the Salinian block. On the basis of that information, I previously noted (Ross, 1978) that a parent terrane for 

the Salinian block is not evident and that the southernmost Sierra Nevada is not a likely source terrane from which the Salinian block could have been 

derived. Three strong objections were made (Ross, 

1977a, 1978) to a reconstruction that would place 

the Salinian block adjacent to the southern Sierra 

Nevada. First, hornblende-rich high-grade metamorphic rocks, which are widespread in the southernmost Sierra Nevada, appear to be absent from 

the Salinian block. Second, the Salinian block is 

practically barren of metallic mineralization, 

whereas metallic-mineral deposits are common in 

the southern Sierra Nevada. Third, there is no evidence in the Salinian block of metasedimentary 

rocks that could be correlative with the thick

quartzite and marble units of the Cordilleran miogeocline units that almost certainly reach the 

San Andreas fault southeast of the Sierra Nevada.

Two recent developments, however, lend support 

to a reconstruction model that would derive the 

Salinian block from the southern Sierra Nevada 

region. First, my mapping in the east-west-trending tail of the Sierra Nevada (San Emigdio Mountains) has revealed a major structural break that 

juxtaposes oceanic basement1 on the north against 

continental basement on the south. The VergelesZayante fault zone in the central Salinian block 

may be a continuation of that fault zone in the 

Sierra Nevada.

Second, Graham and Dickinson (1978a) and, more 

recently, Clark and others (1984) have suggested 

that significant right-lateral offset may have taken 

place along the San Gregorio-Hosgri fault zone. A 

reconstruction that took account of such offset 

could significantly telescope the northern part of 

the Salinian block (Bodega Head to Ben Lomond) 

and help resolve the longstanding contrast between suspected offsets on the San Andreas fault 

in southern and northern California. About 300 km 

of right-lateral offset on the San Andreas fault is 

documented by the offset of several Tertiary and 

basement-rock units across the fault in central and 

southern California. More than 500 km of rightlateral offset has been suggested in northern California on the assumption that the northernmost 

granitic outcrops in the Salinian block were once 

adjacent to granitic terrane on the northeast side 

of the San Andreas fault. The telescoping effect of 

the San Gregorio-Hosgri fault zone, inferrable from 

the the data of Graham and Dickinson (1978a, p. 

17) and Clark and others (1984), removes more than 

half of the offset discrepancy.

In this report, I review data from selected basement-rock units in the Salinian block, the south-

*As used in this report, "oceanic basement" includes not only greenstone, basalt, 

and gabbro derived from oceanic crust, but also the Franciscan assemblage and 

associated rocks, most of which were deposited on, and are closely associated with, 

oceanic crustal material.
POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

ernmost Sierra Nevada, and nearby areas in relation to the problem of offset on the San GregorioHosgri, San Andreas, and Rinconada-Reliz-King 

City fault zones. I also discuss problems concerning 

the Barrett Ridge slice and the Gualala granitic 

clasts problems that are difficult, if not impossible, to resolve on the basis of 300 to 350 km of basement movement between the central Salinian 

block and the southern Sierra Nevada.

By their very nature, basement-rock correlations 

are equivocal; for this reason, I place little faith in 

"one-to-one" basement-rock correlations. The comparison of several basement-rock units to several 

similar basement-rock units is much more convincing, and such a comparison is more apt to provide a genuine contribution to a correlation model;

122"

however, the correlations of Cordilleran granitic 

suites and their surrounding metamorphic rocks 

are particularly difficult because in this region similar rocks are widespread and recurrent. The work 

then becomes a search for unusual rock types that 

may be, if not unique, at least rare enough to be 

regionally significant. The younger, fossiliferous 

rocks, whose correlation parameters (such as 

shorelines and lithofacies pinchouts) are more definitive, give us a better chance for unequivocal 

correlations, but these younger rocks cannot be 

used to resolve fundamental offset problems concerning events that predate their existence. Thus, 

I have for the most part consciously ignored the 

younger sedimentary, volcanic, and alluvial units 

that are inherently part of any reconstruction

FIGURE 1. Index map of central and southern California, showing selected geologic and geographic features and locations for

figures 3, 4, 5, 9, and 12.
SAN GREGORIO-HOSGRI FAULT ZONE

model; my studies have been concerned solely with 

the basement rocks and with the maximum movements of fault blocks, as recorded by these rocks. 

A series of generalized geologic maps of parts of 

the Salinian block, the southern Sierra Nevada, 

and nearby related areas is included in this report 

to show the distribution of those basement-rock 

units that are pertinent to the discussion. Figure 

1 shows the areas covered by these geologic maps.

SAN GREGORIO-HOSGRI FAULT ZONE

Graham and Dickinson (1978a, b) suggested as 

much as 115 km of right-lateral slip along the northern part of the San Gregorio-Hosgri fault zone, on 

the basis of correlation of several features across 

the fault zone, including: similar Cretaceous and 

Tertiary sedimentary sections, two similar anomalous K-feldspar-bearing tectonic slabs of the Franciscan assemblage, a matching pair of Mesozoic 

ophiolite patches, an offset gravity feature, and a 

tectonic contact of the Franciscan terrane against 

Salinian-block basement between Bodega Head 

and the Gualala Basin (see fig. 4) that has been 

observed to match certain relations along the Pilarcitos fault. If these Cretaceous, Tertiary, and 

ophiolite correlations are valid, then the northern 

Salinian-block basement rocks must also have been 

dismembered and moved a comparable distance. 

Graham and Dickinson (1978a) noted that their 

pairs of offset geologic features are not "tightly 

constrained" and that they "show probable offset 

ranges." More recent work by Clark and others 

(1984) indicates that a larger right-lateral offset on 

the San Gregorio fault of about 150 km is called for 

by the present distribution of Tertiary sedimentary rocks of the Point Reyes, Santa Cruz Mountains, and Monterey areas. The granitic basement 

of this same region is also compatible with such an 

offset (see supplementary section below entitled 

"Comparison of the Porphyritic Granodiorites of 

Point Reyes and Monterey").

A basement reconstruction using either the 115- 

km offset of Graham and Dickinson (1978b) or the 

150-km offset of Clark and others (1984), is permissible, considering the vagaries of shapes of granitic 

bodies. The larger offset, however, in my opinion, 

permits a better "onstrike" alignment of a number 

of possibly related tonalitic bodies, as well as an 

alignment of the porphyritic granodiorites of Point 

Reyes and Monterey (fig. 2). Either reconstruction 

would place the granitic rocks of Bodega Head opposite Montara Mountain a permissible correlation because both masses are dominantly hornblende-biotite tonalite. This adjustment would also 

place all known onshore and offshore outcrops of 

the northern Salinian block (Bodega Head, Point 

Reyes area, Cordell Bank, and the Farallon Islands) to the south of the Pilarcitos fault (see fig. 

4). Furthermore, it follows that if, as Silver and 

others (1971) suggested, the Gualala Basin is underlain by Franciscan basement, then a bluntended Salinian block rests against Franciscan 

basement on the north. The presence of Franciscan 

basement is suggested by a small fault-bounded 

outcrop of spilite (fig. 3) near Black Point (Wentworth, 1968). Such spilitic basement rocks are unknown in the Salinian block but are found in the 

Franciscan basement.

Even if the Gualala area is underlain by Franciscan basement, granitic basement rocks must be 

relatively close by to have supplied a source terrane for a thick section of Cretaceous and lower 

Tertiary K-feldspar-bearing arkose and conglomerate that is rich in clasts of granitic rock. If the 

Gualala Basin is floored by the Franciscan assemblage, then this basin cannot be too wide because 

granitic basement must be present to the west or 

south; that is, the Salinian block probably extends 

northward from its exposures at Bodega Head and 

Cordell Bank, possibly as far as Point Arena (fig. 

3).

Tonalite of Montara 

Mountain

SAN GREGORIO-HOSGRI 

FAULT ZONE

Bodega

Tomales x Head 

Point

Point \ 

Reyes

Monterey 

Peninsula

EXPLANATION

Porphyritic granodiorites 

of Monterey and 

Point Reyes

Ben TonalUes of Tomales 

Lomond Polnt Ben Lomond,

and Vergeles

O

Vergeles

60 KILOMETERS

FIGURE 2. Postulated realignment of some basement-rock 

units along the San Gregorio-Hosgri fault zone.
123"45'

POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA 

123°30 ' ,39-00'

123°15'

K-feldspar arkose 

(Tertiary) Plagioclase arkose 

(Cretaceous)

K-feldspar arkose 

(Cretaceous)

123°00'

-1 38°30'

38°15'

FIGURE 3. Generalized geologic map of the Gualala area (modified from Wentworth, 1968).
SAN GREGORIO-HOSGRI FAULT ZONE

Instead of the large block (sphenochasm) that 

Silver and others (1971) suggested for the Gualala 

Basin, there may be only a thin sliver of the Franciscan that is bounded by a fault on the west just 

offshore, as Wentworth (1968) suggested. This fault 

could well be an extension of the Pilarcitos fault 

that is, a San Andreas fault strand. If the Pilarcitos 

fault correlates with a structural contact north of 

Bodega Head and south of Fort Ross, as suggested 

by Graham and Dickinson (1978a), then granitic 

basement rocks may extend as much as 70 km 

northward of Bodega Head and offshore of Gualala 

(fig. 3). It would be interesting to know whether 

any of the Eocene rocks east of Montara Mountain 

between the Pilarcitos and San Andreas faults resemble the K-feldspar arkoses of the same age in 

the Gualala area; such a resemblance would be expected if the offset suggested by Graham and Dickinson (1978a) is valid.

Placing Bodega Head opposite Montara Mountain would leave granitic rocks north of probable 

oceanic basement. Both geologic and geophysical 

evidence suggest that oceanic basement may be 

present between the Vergeles-Zayante fault and 

the Butano fault (figs. 4, 5). The only basement 

outcrop between these two faults is the hornblende 

quartz gabbro and anorthositic gabbro near Logan 

(fig. 5). These basement-rock types are unlike any 

granitic basement in the Salinian block, and their 

chemistry and petrography suggest that they are 

probably oceanic (Ross, 1970).

West of the Logan outcrops (fig. 5), the occurrence 

of a gravity high (Clark and Rietman, 1973) suggests that gabbroic rocks extend westward of the 

present outcrop, possibly to the Vergeles fault. 

Also, Hanna and others (1972) and Brabb and 

Hanna (1981) noted that the Boulder Creek aeromagnetic anomaly north of Ben Lomond Mountain 

between the Zayante and Butano faults is one of 

the largest magnetic features in the vicinity of the 

San Andreas fault. The Boulder Creek high and 

the somewhat less impressive Corralitos aeromagnetic high (fig. 4) are most likely caused by gabbroic 

rocks similar to those exposed near Logan, where 

there is also an impressive aeromagnetic anomaly 

(Hanna and others, 1972). Seismic-refraction velocities of about 5.5 km/s have recently been obtained 

from the basement between the Zayante and Butano faults. These velocities are compatible with 

those obtained elsewhere in the region from Franciscan graywacke (A. G. Lindh, written commun., 

1982). Graywacke in this block would be as compatible as gabbro with an oceanic basement. Seismic-refraction profiles obtained recently north of

the Vergeles-Zayante fault zone suggest that the 

Logan-type gabbroic rocks may be less extensive 

than had previously been believed. The only other 

basement-rock data for the area between these two 

fault zones are from the Union Hihn #2 oil well 

(fig. 4). This well is east of Ben Lomond, and its 

plotted position is just east of the Zayante fault 

(Ross and Brabb, 1973). The core material from the 

well is much like that from the basement outcrop 

just west of the Zayante fault. The Union Hihn #2 

may actually bottom west of the Zayante fault if 

the fault dips steeply east, or if the well is somewhat deflected or either the well or the fault are 

slightly mislocated. Union Hihn #1, a few hundred 

feet northeast of Union Hihn #2, bottomed at 7,747 

ft in Miocene beds, whereas the basement samples 

from Union Hihn #2 are from depths of 3,400 to 

3,500 ft (Ross and Brabb, 1973). These data suggest 

some discordance (the Zayante fault?) between the 

two holes. Nevertheless, Salinian basement in the 

block between the Vergeles-Zayante and Butano 

faults cannot be ruled out on the basis of the present data. I first suggested (Ross, 1970) that the 

gabbroic rocks at Logan composed a thin fault 

sliver along the San Andreas fault, but later gravity and aeromagnetic data (Clark and Rietman, 

1973; Hanna and others, 1972) have suggested that 

the gabbroic rocks are much more extensive; nonetheless, the seismic-refraction velocity of 5.5 km/s 

seems to contradict all these older data. Clearly, 

more data are needed to delineate the basement of 

this block.

If the block between the Butano and VergelesZayante faults is oceanic, the Vergeles-Zayante 

fault zone may be an old strand of the San Andreas 

system, as Dibblee (1980) suggested. Alternatively, 

the terrane between the Butano and Vergeles-Zayante faults could be an exotic sliver that was 

caught up in the Salinian block. The problem with 

considering the Vergeles-Zayante fault zone to be 

a major oceanic-continental break is that the Montara Mountain granitic massif would then be marooned east of the westernmost fault of the San 

Andreas fault system. On the basis of the presence 

of the Montara mass (fig. 4), the basement between 

the Butano and Pilarcitos faults is generally 

thought to be granitic.

A northward continuation of the Ben Lomond 

fault that would tie into the La Honda fault (fig. 

6) was suggested by C. M. Wentworth (oral communication, 1982) on the basis of an abrupt change 

in the geologic grain and attitude of bedding in 

Tertiary sedimentary-rock units across this zone. 

The occurrence of such a fault would suggest that
POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

granitic basement underlies the block of country 

between the proposed La Honda-Ben Lomond fault 

and the San Gregorio fault from Montara Mountain south to Ben Lomond (fig. 6). Such a northsouth-trending fault and the accompanying block 

of granitic basement would contradict my current 

suggestion that a suture of oceanic basement between the Zayante-Vergeles and Butano faults extends westward to the San Gregorio fault.

Granitic rocks and metasedimentary rocks exposed east of the Ben Lomond fault are grossly 

similar to the basement rocks of the Ben Lomond 

Mountain area; that similarity suggests that the 

Ben Lomond fault is not a significant basement 

break and that displacement of the basement is 

modest. However, an impressive aeromagnetic

123°30' 123°15' 123°00'

Granodiorite and 

granite of Inverness

Tonalite of Cordell Bank 

/(dredge hauls sites.a)

Porphyritic granodiorite 

of Point Reyes

PACIFIC

OCEAN

Texaco Pdetti *1 

(porphyritic grenodiorite of Monterey?)

121*45'

37«OO'

FIGURE 4. Generalized geologic map of the northern Salinian block. Well symbol (-£-) shows location of undivided granitic

rocks in the subsurface.
SAN GREGORIO-HOSGRI FAULT ZONE

high between the Zayante and Butano faults (fig. 

6), which suggests the presence of gabbroic rocks 

there similar to exposed rocks near Logan (Brabb 

and Hanna, 1981), appears to terminate near the 

line of the proposed fault. The aeromagnetic-anomaly gradient drops off rapidly west of the proposed 

fault, but it is questionable whether the gradient 

change is enough to allow the interpretation that 

granitic basement is faulted there against gabbroic 

basement.

Information provided by present exposures and 

geophysical data do not appear to be sufficient for 

a unique solution to the problem of a possible northsouth-trending fault and an accompanying strip of 

granitic basement. Closer examination of the area 

near the junction of the Ben Lomond and Zayante 

faults might resolve whether the Ben Lomond

fault is truncated by the Zayante fault or vice 

versa, but probably nothing short of additional subsurface basement data can resolve this dilemma. 

Because of similar gravity patterns near Ano 

Nuevo Point (west of Ben Lomond) (fig. 1) and offshore from Point Sur (fig. 5), Silver and others 

(1971) suggested an offset of only some 90 km on 

the San Gregorio fault. They interpreted these 

data to indicate offset on the west margin of the 

Salinian block (the Nacimiento fault zone). More 

recent data, however, suggest that the Franciscan 

assemblage of the Point Sur area has been displaced from similar rocks near Cambria (fig. 1) 

along the San Gregorio-Hosgri fault zone (Graham 

and Dickinson, 1978a). Thus, the Sur fault is most 

probably a segment of the San Gregorio-Hosgri 

fault zone and not part of the Nacimiento fault

121°30'

3ligocene \ 

sedimentan/S. 121°15' 

vrocks \

'-<-x Tonalite 

*"-*granodio..~ 

"-/i'. Johnson 

Canyon

i ^ v\\ x of Monterey «T °>«>V\. N

§^^> \ X^?» %*. \ X_ V

Charnockitic 

tonalite o* 

Compton 

(1960)

120°45'

36°15'

\ \ l i^' l rrTs ,____NV'.^>| !'.'I

FIGURE 5. Generalized geologic

10 15 20 25 KILOMETERS

i___i___i

map of the central Salinian block.

36«OO'
8 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

zone. If so, the offset on the Nacimiento fault zone 

is about 130 to 135 km (fig. 7). The question then 

arises whether the gravity pattern off Point Sur 

(Silver and others, 1971) is related to the gravity 

expression of the Farallon Ridge to the north. Note 

that this offset on the Nacimiento fault zone is 

somewhat less than the 150-km offset indicated by 

data of Clark and others (1984) for the San Gregorio 

fault. The structure is complex where the Nacimiento fault intersects the shoreline south of Monterey, and more San Gregorio-Hosgri slivers may 

exist there that are analogous to the one at Point 

Sur. Thus, the seeming discrepancy may be less 

than the present figures suggest.

Recent dredging in Ascension Canyon (off the 

coast west of the Ben Lomond area) (fig. 7) has 

brought up specimens of spilitic basalt and serpentinite that are considered to be inplace Franciscan 

(Mullins and Nagel, 1981). This dredged material 

provides a definite tie point to limit the westward 

extent of the Salinian block at that latitude.

SAN ANDREAS FAULT ZONE

Matthews (1976) presented compelling evidence 

that his Pinnacles Volcanic Formation in the south-

\

Tonalite of 

Montara 

Mountaii

O»s

»s

o

T

BEN 

LOMON

FAULTS 

Tonalite of 

Ben Lomond

I

Metasedimentary 

rocks

20

eromagnetic 

highs

40 KILOMETERS

MONTEREYBAY

/______

FIGURE 6. Generalized geologic map showing location 

of possible north-south-trending fault (hachured line) 

connecting the Ben Lomond and La Honda faults.

ern Gabilan Range and his Neenach Volcanic Formation on the opposite side of the San Andreas 

fault (see fig. 20) are correlative and were contiguous 23 m.y. ago, in Miocene time. To directly juxtapose the Neenach and Pinnacles volcanic areas, 

Matthews suggested that the Chalone Creek fault 

(fig. 8) is an early Miocene trace of the San Andreas 

fault. He based this suggestion on the alignment 

of the Chalone Creek fault with Peach Tree Valley 

and on the absence of granitic outcrops in the sliver 

between the Chalone Creek and San Andreas 

faults. Oil wells have penetrated granitic basement 

at 2,000 to 4,300 ft within this sliver, however, and 

the core material suggests ties with the central 

Salinian block (Ross, 1974).

Core samples 34 and 45 (fig. 8) are probably of 

the schist of Sierra de Salinas. These cores are located very near the extension of the Chalone Creek 

fault proposed by Matthews, but the fact that the 

samples were recovered at depths of 5,300 and 5,800 

ft suggests that they are east of the Chalone Creek 

fault, because surface outcrops are present no 

more than 5 km to the west. If samples 34 and 45 

are of the schist of Sierra de Salinas and if sample 

27 is of the tonalite and granodiorite of Johnson 

Canyon, as its lithology suggests, it is unlikely that

124°

38°

36°

122°

Point Arena

 y

o

Cordell

Possible extension 

of Nacimiento 

fault zone Farallon 

Islands

40

Sedimentary rocks of Pigeon ' 

Point (Cretaceous!

Ascension Canyon

O

o

-fr Point Sur

120 KILOMETERS' (Franciscan*?)^ 

sssemblsge)

FIGURE 7. Probable offset of west margin of the Salinian 

block (Nacimiento fault) by the San Gregorio-Hosgri 

fault zone.
SAN ANDREAS FAULT ZONE 9

the Chalone Creek fault has had significant strikeslip displacement, and it may be dominantly dip 

slip, as Andrews (1936) suggested. The general configuration of the slice between the Chalone Creek 

and San Andreas faults more closely resembles a 

sliver along the San Andreas fault, and the wellcore data are admittedly sparse. Even if the Chalone Creek fault is not a strike-slip fault, Matthews' 

(1976) correlation is still valid. The present separation of these two volcanic-rock units compares 

well with earlier estimates of San Andreas basement offsets by Crowell and Walker (1962) and with 

various Tertiary offset features described by Addicott (1968) and Grantz and Dickinson (1968). 

These offsets would bring the Gabilan Range (fig, 

5) up against the Sierra Nevada tail (San Emigdio 

Mountains) (fig. 1). I am skeptical of this correlation for several reasons, mainly because there is 

no evidence in the Salinian block of counterparts

to the hornblende-rich high-grade metamorphic 

rocks of probable oceanic affinity that underlie 

large areas of the Sierran tail. However, the recent 

realization that the Pastoria fault zone (fig. 9) of 

Crowell (1952) may extend westward to intercept 

the San Andreas fault has caused me to speculate 

that the Pastoria fault zone may have a counterpart fault in the Salinian block the Vergeles-Zayante fault zone. The Pastoria fault zone and, by 

much inference, the Vergeles-Zayante fault zone 

may indicate a major structural discordance 

(strike-slip or reverse fault movement, or both) that 

juxtaposes oceanic and continental terranes. Certainly, the gabbroic rocks at Logan (see figs. 5, 14) 

and Eagle Rest Peak (see fig. 9,14) near these two 

fault zones are petrographically and chemically 

compatible with each other (Ross, 1970; Ross and 

others, 1973). The absence north of the VergelesZayante fault zone of representatives of the horn36°30'

'innacles Volcanic Formation 

of Matthews (1976)

Tonalite and granodiorite 

/of Johnson Canyon!?)

Granitic rocks; 

undivided

Basement penetrated at 

about 3000'; no samples

Schist of Sierra 

de Salinas

Dsrk gneiss, like gneissic 

rocks at Red Hills (?)

Hornblende-biotite tonalite 

3500'

10 15 KILOMETERS

FIGURE 8. Locations of wells to basement between the Chalone Creek and San Andreas faults, showing rock types 

penetrated, sample numbers, and depths to basement. Modified from Matthews (1976, fig. 18); basement core data 

from Ross (1974).
10 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

;«»^».i»»«**'*»»»YX»«V'-^^^&*--~~ «***-. * * ***»* ** »**»*»**- *» *~~' *^-y> ±&^&i&:^ *&£?< ttift*#»v u ,v;« v; v.\<.\s.*:

&gV

w

 5
SAN ANDREAS FAULT ZONE 11

blende-rich high-grade metamorphic rocks may 

simply indicate more overlap by Eocene and Oligocene sedimentary rocks and Miocene volcanic 

rocks there than to the north of the Pastoria fault 

zone. The outcrop belt of the hornblende-rich metamorphic rocks thins markedly north of the Pastoria 

fault zone near the San Andreas fault, and so the 

absence of such rocks along the north side of the 

Vergeles-Zayante fault is not surprising.

On an earlier map of the Vergeles fault area, 

Alien (1946) showed a sliver of granitic rocks north 

of the Vergeles fault about 5 km south of San Juan 

Bautista (fig. 10). My examination of this area revealed marble, mica schist, the tonalite of Vergeles, 

and medium- to coarse-grained granite, all sparsely 

exposed along a ridge that extends from the Vergeles fault northward to the Old Stage Road, where 

tonalite and granite are exposed in a roadcut. 

These granitic rocks might have been emplaced

across the Vergeles fault zone, and they may have 

a relation analogous to that of the granite of Brush 

Mountain to the Pastoria fault zone (fig. 9). However, the granitic rocks that appear to be on the 

north side of the Vergeles fault zone are more likely 

a sliver along the fault.

Cursory examination of the rocks at several localities along the Vergeles fault zone has revealed 

only a modest amount of shearing that can be related to the fault (fig. 10). Several thin slickensided 

shear zones were observed in Tertiary sandstone 

immediately northeast of the Vergeles fault zone 

at its west limit of exposure east of U.S. Highway 

101. There, the fault takes a more northerly trend, 

and shears that strike about N. 20° W. are aligned 

with the mapped fault trend. The slickensided 

zones dip 60°-70° S. and flake off in flattened ovoid 

plates that have the appearance of paper shale. 

Sparsely exposed granitic rocks just southwest of

EXPLANATION

Strike and dip 

of foliation

Pliocene and Holocene 

sedimentary deposits

Protomylonitize 

tonalite

Sheared metased 

- imentary rocks*

FIGURE 10. Generalized geologic map showing locations of sheared rocks (circles) along the Vergeles fault. 

Geology simplified from Clark and Rietman (1973) and Dibblee (1980).
12 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

the faultline are deeply weathered and show no 

obvious evidence of shearing. Slickensides were observed, however, in some granitic float.

Shearing also occurs in Tertiary sandstone and 

in micaceous schist on both sides of the Vergeles 

fault along the San Juan Grade Road between Salinas and San Juan Bautista (about 8 km east along 

the fault from the U.S. Highway 101 locality). Slickensided shear zones, as much as 0.3 m wide, cut 

Tertiary sandstone beds that are in, or just north 

of, the Vergeles fault zone. The shear zones trend 

N. 70° W. and dip about 85° S. About 0.2 km south 

of the fault zone, shearing and granulated lenses 

are evident in micaceous schist in a roadcut. These 

shear zones also strike N. 70° W., but they dip somewhat less steeply at 70° S. Some of this so-called 

shearing, parallel to the schistose foliation, may 

merely be weathered metamorphic foliation. The 

granulated zones, however, observed in thin section, definitely postdate the metamorphic foliation 

and reflect later deformation.

Shear zones were also seen in granitic rocks 

about 0.2 km south of the Vergeles fault zone, directly south of the St. Francis Retreat (fig. 10). A 

few thin red shear zones cut the granitic rocks and 

trend N. 65° W. and dip about 80° S. Very near the 

mapped trace of the Vergeles fault in San Juan 

Canyon (about 2 km to the west of the previous 

locality), a roadcut on the south side of the canyon 

exposes Tertiary sandstone. This sandstone contains many angular quartz grains, as much as 4 

mm.across, that are strongly undulatory, granulated, and (or) slivered; some of these grains may 

be fragments of mylonite. Other clasts are polycrystalline and consist of plagioclase and deformed 

quartz. The general clast lithologies suggest that 

the source rocks were cataclastically deformed, 

probably felsic granitic rocks. Most of the sandstone that I sampled near the Vergeles fault in the 

Tertiary section probably is locally derived. By 

analogy, the sandstone containing the clasts of cataclastic(?) quartz and granitic fragments was apparently derived from deformed granitic rocks 

along the Vergeles fault.

Just south of the Old Stage Road, biotite granodiorite is exposed on a ridge on the north line of 

sec. 16, T. 13 S., R. 4 E., just south of the trace of 

the Vergeles fault (about 4 km west of the mylonite 

clast locality, fig. 10). These granitic rocks have a 

protomylonitic texture in thin section; they include 

strung-out, mosaicked, and slivered quartz as well 

as streaked-out biotite that gives an anastamosing 

fabric to the rocks. These rocks seem to be deformed 

more pervasively than are the outcrops previously

described, in which deformation is localized along 

discrete shear zones. Because of poor exposures, 

relations are unclear there, but I suspect that the 

deformed granitic rocks are in or very near the 

Vergeles fault zone. These protomylonitized granitic rocks are a likely source for the Tertiary sandstone in San Juan Canyon that contains mylonitized grains of granitic composition.

The clues of mylonitization at these latter two 

localities suggest that strong compression is part 

of the deformational pattern of the Vergeles fault. 

Mylonite would also seem to be more compatible 

with a strongly compressional strike-slip fault zone 

than with a largely extensional dip-slip fault zone, 

which the Vergeles fault had previously been considered to be. However, the basement is certainly 

downdropped north of the Vergeles fault; seismicrefraction lines across the fault suggest a downdrop of several thousand feet (W. D. Mooney, written commun., 1982).

Coppersmith (1979), in a detailed study of the 

Vergeles-Zayante fault zone, noted slickensides in 

trenches across the fault zone whose orientations 

indicated a significant horizontal component in 

most recent movements. He also noted that several 

streams crossing the fault trace had clearly been 

diverted in a right-lateral sense. Coppersmith further noted that fault-plane solutions of several seismic events that most probably were associated 

with the Vergeles-Zayante fault zone show strikeslip focal mechanisms with little or no vertical component of movement. Dupre (1975) observed that 

the Sangamon-age Watsonville terrace complex 

(100,000 years old) is deformed by faulting and that 

Holocene activity is evident in the fault-controlled 

geomorphology. He cited these observations as evidence that the Vergeles-Zayante fault zone is active and has undergone late Pleistocene and Holocene movements. Dupre (1975) also cited current 

seismic activity in the Vergeles-Zayante region as 

indicative of ongoing fault movement. These two 

fairly recent studies support the view that the Vergeles-Zayante fault zone is an active strike-slip 

fault zone.

Coppersmith (1979) also noted that along much 

of the more than 80-km trace of the Vergeles-Zayante fault zone, a main fault is difficult or impossible to identify and that there are many relatively 

short fault segments without much lateral continuity. It is difficult to reconcile this statement with 

my contention that the Vergeles-Zayante fault 

zone is a major throughgoing strike-slip fault zone. 

Poor exposures may explain the apparent absence 

of a mappable main fault; nevertheless, if Cop-
SAN ANDREAS FAULT ZONE 13

persmith's interpretation is correct, it creates difficulties for my suggestion that the Vergeles-Zayante fault zone is a major strike-slip fault zone.

Miocene volcanic rocks directly north of the Vergeles fault have a radiometric age of 22 m.y., which 

is the same as the age of similar volcanic rocks just 

north of the Pastoria fault zone in the San Emigdio 

Mountains (fig. 9) (Huffman and others, 1973). The 

suggestion that these two volcanic areas were once 

part of one volcanic field is strengthened by lithologic and petrologic similarities (Bazeley, 1961), 

and by similarities of both bulk chemical analyses 

of the volcanic rocks and trace-element concentrations in garnet and biotite from the two areas 

(Turner, 1968).

Numerous workers, the earliest of whom were 

Hill and Dibblee (1953), have noted stratigraphic 

similarities between the Eocene to Miocene sedimentary section in the western San Emigdio Mountains (fig. 9) and the interconnected depositional 

area that received sediment of the same age in the 

northern Santa Lucia Range and the northern Gabilan Range north of the Vergeles fault (fig. 5; see 

sources cited by Clarke and Nilsen, 1973, and Nilsen and Link, 1975).

In all, there is abundant evidence from such diverse groups of rocks as oceanic gabbroic rocks, 

Miocene volcanic rocks, and Eocene to Miocene sedimentary rocks for a firm tie between the northernmost Gabilan Range and the westernmost San 

Emigdio Mountains (Sierra Nevada tail). In view 

of this tie, as well as the compelling similarity between the Pinnacles Volcanic Formation in the 

southern Gabilan Range and the Neenach Volcanic 

Formation southeast of the San Emigdio Mountains, the question should be asked: Can the Salinian block basement units of the Gabilan Range be 

fitted to the basement units in the San Emigdio 

Mountains on the east of the San Andreas fault?

I have long been intrigued by the close resemblance of the granodiorite of Natividad in the Gabilan Range to the granodiorites of Lebec and GatoMontes in the Sierran tail (figs. 5, 9); in fact, I commonly referred to the granodiorites of Lebec and 

Gato-Montes early in my Sierran work as "Natividad type." All three granodiorites are medium-fine 

grained and contain distinctive scattered coarse 

biotite flakes. All three masses are distinguished 

by scattered hornblende crystals with red cores 

that indicate the presence of skeletal clinopyroxene remnants. Modally and chemically, the three 

masses are also virtually identical. Coarse-grained 

biotite granite, of the typical and common "low 

melting trough" granitic rock type, is associated

with all three granodiorites the granites of Fremont Peak (Gabilan Range), Tejon Lookout, and 

Brush Mountain (Sierran tail).

Neither the tonalite of Vergeles nor the tonalite 

and granodiorite of Johnson Canyon of the Gabilan 

Range (fig. 5) appear to have counterparts in the 

Sierra Nevada tail (fig. 1) (San Emigdio Mountains). The granodiorite of the Fairmont Reservoir, 

however, which crops out near the Neenach Volcanic Formation, is similar lithologically, modally, 

and chemically to the Johnson Canyon body, which 

crops out near the Pinnacles Volcanic Formation. 

Both the Fairmont Reservoir and Johnson Canyon 

granitic bodies contain much sphene with inclusions of opaque material in shapes resembling arabic- to runic-written characters. I have observed 

this feature in other plutonic masses in the California Coast and Transverse Ranges, but it is uncommon there, and so its abundance in both the 

Fairmont Reservoir and Johnson Canyon masses 

may be significant. More detail on the similarity 

of those two granitic masses is presented in the 

supplementary section below entitled "Comparison 

of the Granitic Rocks near the Pinnacles and Neenach Volcanic Formations of Matthews (1976)." The 

presence in the Sierra Nevada tail of equivalents 

to the Johnson Canyon and Vergeles bodies would 

greatly strengthen correlation of the tail with the 

central Salinian block, but considering the common 

shape and distribution vagaries of granitic bodies, 

the absence of such equivalents is not devastating 

to the proposed correlation.

Strontium-isotopic data available for the central 

Salinian block and the Sierra Nevada tail enable 

a general comparison between these two areas. 

Relatively high initial strontium-isotopic ratios 

(approx 0.7082) have been determined for several 

granitic samples from the Gabilan and Santa Lucia 

Ranges (Kistler and Peterman, 1973). More detailed work by J. M. Mattinson (written commun., 

1982) indicates that not all the Gabilan Range plutons are cogenetic and that there is some variance 

in initial strontium-isotopic ratios; nevertheless, 

all are relatively high. Similar high initial strontium-isotopic ratios (0.7079-0.7085) have been determined in one sample of the granodiorite of Lebec 

and in several samples of the granodiorite of GatoMontes in the Sierra Nevada tail (R. W. Kistler, 

written commun., 1981). One granitic sample from 

the La Panza Range has an initial strontium-isotopic ratio of 0.7080 (Kistler and others, 1973). 

These data, though admittedly sparse, point to 

strontium-isotopic compatibility between the central Salinian block and the Sierra Nevada tail.
14 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

Doe and Delevaux (1973) determined the leadisotopic compositions in seven granitic samples 

from the length of the Sierra Nevada for the Late 

Cretaceous part of the Sierra Nevada batholith. Of 

particular interest in their studies is the 2wp\)/2Mp\) 

ratio, which is a measure of the uranogenic or "radiogenic (J-type)" lead. The two southernmost samples analyzed from the batholith (lat approx 35°45' 

N., near Isabella Lake, fig. 1) are two of the three 

Sierran samples with the highest 206Pb/204Pb ratios 

(19.37 and 19.15). Doe and Delevaux (1973) reported 

similar relatively high 206Pb/204Pb ratios from the 

Santa Lucia Range (19.53 and 19.11) and Point 

Reyes (19.44) in the Salinian block. They suggested 

that these data support the contention that the 

Salinian block is a faulted-off part of the Sierra 

Nevada batholith. This conclusion, though permissible, is admittedly based on very limited data.

Another feature that seems to relate the two 

areas is their anomalous structural grain. The granitic contacts, and the trend and strike of much of 

the metamorphic pendant material in the northern 

part of the Gabilan Range, are generally east-west, 

markedly different from the north-southward to 

northwestward trend of most California basement. 

If the Gabilan Range is placed alongside the Sierra 

Nevada tail and if the Vergeles and Pastoria faults 

and the Neenach and Pinnacles Volcanic Formations are lined up, the combined block will have a 

generally east-west-trending grain.

Another correlation between these two areas has 

already been suggested, on the basis of the similarity of the schist (metagraywacke) of Sierra de 

Salinas (figs. 5, 11) to the schist of Portal-Ritter 

Ridge (Pelona Schist?) (fig. 9); (Ross, 1976). The exposed schist of Sierra de Salinas, together with similar schist in well cores, composes a rather impressive belt that appears to be cut off by the San 

Andreas fault. If the Gabilan Range is placed next 

to the Sierra Nevada tail, this belt aligns with outcrops of the schist of Portal-Ritter Ridge.

The strong physical and chemical similarity of 

the schists in these two areas has already been 

established (Ross, 1976); similarity of their structural settings remains to be shown. The schist of 

Sierra de Salinas in the Gabilan Range is intruded 

by granitic rocks and has some migmatitic marginal zones. In addition, dikes and sills of volcanic 

rocks (probably related to the Pinnacles Volcanic 

Formation) are conspicuous near the south end of 

the large outcrop area of schist in the Gabilan 

Range. By contrast, the only known intrusions into 

the schist of Portal-Ritter Ridge (fig. 9) are dark 

diabasic dikes composed of labradorite, hornblende, clinopyroxene, and minor quartz. The 

source of these dikes is unknown.

The contact along the north margin of the schist 

of Portal-Ritter Ridge has been mapped as a fault 

the Hitchbrook fault of Dibblee (1960, 1961). However, this fault does not cleanly separate granitic 

rock on the north from schist on the south. For 

example, the large mass of schist at Quartz Hill is 

north of the fault, and the Hitchbrook fault, as 

mapped, passes through a spur of schist in the main 

Portal-Ritter Ridge body (Dibblee, 1961). Furthermore, Evans (1978) showed a small outcrop of granitic rock south of, and adjacent to, the Hitchbrook 

fault. This outcrop, immediately south of a distinctive hill of lithic lapilli tuff, is not shown as 

fault-bounded against schist; however, no mention 

of this outcrop is made in Evans' text. The contact 

between the schist and the granitic rocks is covered 

with alluvium along much of its length. In my brief 

examination of the rocks along the Hitchbrook 

fault in a few localities, I saw sheared granitic rock 

and no granitic apophyses into the schist. Nevertheless, I am not convinced that the Hitchbrook 

fault is a tectonic break between the schist of Portal-Ritter Ridge and the granitic terrane to the 

north. I suggest that a thorough search along this 

contact be made for intrusive relations whose occurrence would support the thesis that the schist 

terranes of the Sierra de Salinas and Portal-Ritter 

Ridge are correlative and were once contiguous.

Together with the Tertiary correlations, the possibility of basement-rock correlations suggests 

that the Gabilan Range (figs. 5, 11) may have originated opposite the area of the present Sierra Nevada tail (fig. 9). Important to this correlation is 

my interpretation that the Pastoria and VergelesZayante fault zones represent a tectonic contact 

between oceanic and continental terranes. If it can 

be determined that the basement between the Vergeles-Zayante and Butano faults is largely oceanic, 

and if similar rocks underlie the block north of the 

Butano fault, these facts would suggest a major 

break between continental and oceanic terranes 

that would greatly strengthen the correlation between the Gabilan Range and the Sierran tail.

A reconstruction that attaches the Gabilan 

Range to the Sierran tail would probably carry the 

Santa Lucia Range along with the Gabilan Range. 

The gneissic and tonalitic rocks (with associated 

migmatite of the Santa Lucia Range) might have 

their onstrike equivalents in the gneisses and tonalites of the Holcomb and Wrightwood area (fig. 1); 

(Ross, 1972a). Correlation of these two gneissic terranes was suggested to me several years ago by P.
SAN ANDREAS FAULT ZONE 15

L. Ehlig, and the idea still seems to have merit. At 

present, such a correlation must remain speculative because much of the basement between the 

Santa Lucia Range and the San Andreas fault is 

covered by younger rocks, but it is worth noting

121°

that the correlation would fit with the GabilanSierran correlations. One possible reason to reject 

this correlation is the fact that the route from the 

Santa Lucia Range to the San Andreas fault seems 

to be blocked by granitic rocks and schist similar

EXPLANATION

Location of well from 

which basement core 

was studied

GABILAN 

* RANGE ". ". 3

Limits of extrapolation 

from well-core data

»' ** 

* SANTA

« LUCIA 

" RANGE

H .. .. ..

-eppery-^ji^ , > Quarttpoor 

granite **- / ^hornblende-rich

 \ * »\ /'\ tonalite

10 20 30 KILOMETERS

FIGURE 11. Extrapolation from well-core data of some basement-rock units in the central Salinian block (modified

from Ross, 1974).
16 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

to the schist of Sierra de Salinas that are evident 

in well-core material from the San Ardo oil-field 

area (figs. 5, 11).

Hypersthene-bearingtonalite and associated hypersthene-bearing granulite-grade metamorphic 

rocks are found in both the southernmost Sierra 

Nevada and the Santa Lucia Range. The tonalites 

and associated metamorphic rocks of both terranes 

have middle Cretaceous U-Pb radiometric zircon 

ages. In addition, coarse red garnet is characteristic of both terranes. The occurrence of granulitegrade metamorphic rocks also implies greater crustal depths than is common for batholithic terranes 

in California. At the least, these two unusual, coeval terranes appear to reflect a similar metamorphic and tectonic setting. The correlation of the 

two terranes is speculative but plausible. Further 

details of the comparison of these two terranes 

were discussed by Ross (1983).

Another large area of basement rock in the central Salinian block is the porphyritic granodioritegranite of the La Panza Range (fig. 12). This body 

of rock probably extends southward, possibly discontinuously, from the San Ardo area to the La 

Panza Range and presumably even farther south 

(fig. 11), although no basement outcrops or subsurface data indicate its presence there (Ross, 

1974). Much of this large and relatively homogeneous mass is distinctly porphyritic, and I have 

suggested (Ross, 1978) that it is correlative with 

the porphyritic granodiorite of Monterey (figs. 4, 

5).

A body of porphyritic granitic rock in the Thermal Canyon area (fig. 1) was believed by Smith 

(1977) to be a possible correlative of the La Panza 

mass. I was also informed by S. E. Joseph (oral 

commun., 1979) that the rubidium-strontium data 

for the Thermal Canyon and La Panza rocks are 

similar. I have examined and sampled the granitic 

rocks in Thermal canyon and made modal analyses 

of seven stained slabs. In the field, these rocks 

strikingly resemble the granitic rocks of the La 

Panza Range. In modal composition (fig. 13), the 

Thermal Canyon samples are generally lower in 

quartz than the La Panza rocks, and the two fields 

are somewhat distinct, although they overlap. It 

is hard to evaluate the relation between these two 

bodies because I sampled only a small area in Thermal Canyon. More data are needed to confirm (or 

deny) the correlation between these two masses.

The present distance between the Thermal Canyon rocks and the nearest La Panza Range granitic 

outcrops is more than 400 km; this distance exceeds 

the amount of displacement required on the San

Andreas fault if the Gabilan Range is to be refitted 

against the Sierra Nevada tail. However, for a considerable distance south of the La Panza Range no 

basement rocks are exposed, and the extent of the 

Thermal Canyon mass is also unknown. Thus, the 

separation may be somewhat less than 400 km. It 

seems unlikely, however, that the discrepancy in 

separation can be completely compensated for in 

this way. Smith (1977) has noted the problem and 

proposed some 175 km of right-lateral offset on a 

fault within the Salinian block, but I have trouble 

reconciling the basement-rock data with such a 

fault.

One additional obstacle stands in the way of a 

La Panza-Thermal Canyon correlation. The Thermal Canyon region seems to have a framework of 

Precambrian metamorphic and granitic rocks 

(Rogers, 1965; Jennings, 1967), whereas no Precambrian rocks have been found in the central Salinian 

block. However, small outcrops of an unusual 

gneiss have been noted in American Canyon (fig. 

12) just south of the La Panza granitic outcrop. I 

have examined these outcrops and studied some 

thin sections and stained slabs from the specimens 

collected. The gneiss appears to be of relatively 

high grade and in part to have a distinctive purple 

color in weathered outcrop; at least in part, it appears to be an orthogneiss. These gneissic rocks 

resemble some of the gneissic rock at Red Hills or 

some of the "dioritized" gneiss of Mount AbelMount Pinos; their appearance is out of character 

for the central Salinian block. Further study of 

these rocks, particularly isotopic age dating, is 

needed. At this latitude, these gneisses may very 

well be Precambrian because a Gabilan RangeSierran tail reconstruction would place the La 

Panza Range adjacent to known Precambrian terranes in the San Bernardino Mountain area (fig. 

1).

RINCONADA-RELIZ-KING CITY FAULT ZONE

In previous reconstructions, the Rinconada fault 

has been considered to be a favorable structure 

along which to sliver and attenuate the Salinian 

block. For example, Smith (1977) called for some 

170 km of right-lateral offset on this fault zone. 

Dibblee (1976), in a carefully documented study of 

the Rinconada and related faults, called for a more 

modest offset of Upper Cretaceous and lower Tertiary units, of about 60 km. Graham (1978), in a 

detailed study of Tertiary units and structures in 

the central Salinian block, postulated 45 km of 

right-lateral movement on the basis of offset shore-
120°30'

120°45'

10 20 30 KILOMETERS

FIGURE 12. Generalized geologic map of the southern Salinian block. Black dots denote locations where basement core m

Ridge slice is gneiss and felsic granitic rocks, both of which are unlike basement rocks to the west in the central Sal

(1974) for description of core material.
18 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

lines and other features. He also believed that the 

distribution of the schist of Sierra de Salinas supports such an offset. On the basis of the present 

distribution of schist in the subsurface in the San 

Ardo oil field and in outcrops in the Sierra de Salinas (fig. 5), offsets of much as 50 km are possible.

I have rnoted that the major schist bodies in the 

Sierra de Salinas and in the Gabilan Range, together with all the other schist localities (except 

the isolated San Ardo area), form a somewhat discontinuous, relatively regular west-northwesttrending belt of schist that extends from the Sierra 

de Salinas to the San Andreas fault. I see no compelling reason to believe that this schist belt was 

significantly faulted or offset by the Rinconada 

fault zone.

I have long been perplexed by the isolated schist 

of the San Ardo area and its relation to the rest of 

the schist belt. The San Ardo rocks may be something other than the schist of Sierra de Salinas, 

but physically they are certainly similar. Given 

Graham's (1978) data on Tertiary shorelines, the 

basement may have been offset by the RinconadaReliz fault, though by no more than some 45 km. 

Interestingly enough, reversal of that amount of 

movement across this modest crack in the Salinian 

block would bring the porphyritic granodiorite of 

Monterey and its offshore continuation much 

closer to the northernmost outcrops of the porphyritic granodiorite in the Gabilan Range a correlation I had suggested before Graham's study 

(Rqss, 1978).

Other

Hornblende

Biotite content ranges from 

2 to 12 percent in the La 

Panza body. Less than 1 

percent hornblende in 

some specimens

Biotite

60 EXPLANATION

 La Panza

O Thermal 

Canyon

90 65 35

K-feldspar

10 

Plagioclase

FIGURE 13. Comparison of modally analyzed specimens of 

the porphyritic granitic rocks of Thermal Canyon and the 

porphyritic granodiorite-granite of the La Panza Range.

Presumably, the Rinconada-Reliz fault zone continues northwestward along the course of the 

much-discussed and sometimes controversial King 

City fault (Ross and Brabb, 1973). It may continue 

across Monterey Bay along one of several northwest-trending faults of the Monterey Bay fault 

zone of Greene and others (1973). Then, to the 

north, the Rinconada-Reliz-King City fault zone 

probably merges with the San Gregorio-Hosgri 

fault west of the basement outcrops in the Ben 

Lomond area (Ross and Brabb, 1973, fig. 4).

PROBLEMS OF SALINIAN-BLOCK 

RECONSTRUCTION THAT STILL REMAIN

Many basement-rock correlations seem to make 

good sense or, at least, provide compatible ties if 

the Bodega-Point Reyes basement is pulled southward along the San Gregorio-Hosgri fault zone to 

the Ben Lomond area and then the central block 

of the Salinian block is pulled southward along the 

San Andreas fault to the general area of the Sierra 

Nevada tail (San Emigdio Mountains). However, 

the following basement problems still persist, 

unresolved.

CORDILLERAN MIOGEOCLINAL ROCKS

The upper Precambrian and lower Paleozoic sedimentary rocks of the Cordilleran miogeocline have 

been traced southward from Nevada, through eastern California, across the Mojave Desert, and into 

the San Bernardino Mountains. Although the preservation of this belt is spotty and near its south 

limits is much disrupted by intrusive rocks and 

covered by younger rocks, there seems to be little 

doubt that it reaches, and is cut off by, the San 

Andreas fault zone. The distinctive rock types of 

this section thick pure quartzite and thick limestone have not been found in the Salinian block. 

If the Gabilan Range-Sierran tail correlation is 

valid, then such rocks might be expected to appear 

at about the latitude of the La Panza Range (figs. 

11, 12).

The La Panza Range basement is almost entirely 

granitic; it contains only traces of biotite schist and 

marble, as well as a small amount of unusual-appearing gneiss just south of the main granitic basement outcrops. Admittedly, vast areas to the north 

and south of the La Panza Range granitic outcrops 

are covered by younger deposits that conceal the 

composition of the basement; nevertheless, it 

would have been reassuring to see at least a few 

quartzite or marble pendants in the exposed base-
PROBLEMS OF SALINIAN-BLOCK RECONSTRUCTION THAT STILL REMAIN 19

ment. Of considerable interest are the strikingly 

distinctive pure quartzite (orthoquartzite) cobbles 

and pebbles that are common in the Cretaceous 

beds which lap up on the south side of the La Panza 

granitic basement. These pure quartzite cobbles 

and pebbles are of a rock type that is virtually 

unknown in the Salinian block basement.

Howell and Vedder (1978) speculated that both 

the concentration of quartzite in the erosional 

product of the basement and its rarity in the present basement may reflect differential magmatic 

stoping. They proposed that the relatively light 

quartzite is selectively floated to the upper parts 

of the magma chamber, where it becomes an early 

erosional product, leaving the pluton relatively impoverished in quartzite, even though quartzite 

may have been abundant in the original metamorphic framework. Their mechanism suggests that 

upper Precambrian and lower Paleozoic Cordilleran miogeoclinal rocks may once have been present in the central Salinian block.

BEAN CANYON FORMATION

The Bean Canyon Formation forms discontinuous pendants for almost the entire length of the 

granitic outcrop south of the Garlock fault, from 

the San Andreas fault east for about 60 km (fig. 9) 

The Bean Canyon contains distinctive metavolcanic-rock layers and dark schist that, in part, is 

rich in coarse andalusite crystals. If the Gabilan 

Range is moved up against the Sierran tail, the 

Bean Canyon Formation might be expected to occur in the central or southern Gabilan Range; however, no metavolcanic or dark andalusite-bearing 

rocks have been seen there. Note, however, that 

the westernmost large exposure of the Bean Canyon is dominantly marble and contains neither 

metavolcanic-rock layers nor dark andalusite 

schist, and that the next most easterly marble-rich 

area of outcrop contains very minor amounts of 

andalusite-bearing schist and thin layers that may, 

or may not, be metavolcanic rocks. These volcanic 

and andalusite layers are probably absent to the 

west because of a facies change and so would not 

be expectable in the Gabilan Range. Scattered marble inclusions throughout the granitic terrane of 

the central and southern Gabilan Range may represent the Bean Canyon, but no distinctive associated lithologies occur to make a convincing tie.

TEMBLOR CLASTS

Directly related to the discussion of the Bean 

Canyon Formation is the problem of the Temblor

clasts. Bouldery beds in the Miocene Santa Margarita Formation in the Temblor Range (fig. 1) appear to be cut off on the west by the San Andreas 

fault. Huffman (1972) suggested that the most plausible source terrane for these clasts is the northern 

Gabilan Range. In examining the Temblor clasts, 

I noted many metavolcanic and dark andalusitebearing rocks (Ross, 1980). I have seen neither rock 

type in the Gabilan Range or anywhere in the Salinian block, but these two rock types generally 

characterize the Bean Canyon Formation. This dilemma will persist until these two rock types are 

found in the Gabilan Range basement. My studies, 

which admittedly were scattered and concentrated 

on the granitic rocks, leave much room for more 

detailed investigation, particularly of the metamorphic rocks of the Gabilan Range.

SIERRAN QUARTZITES

Within the metasedimentary rocks of the southern Sierra Nevada, pure quartzite (orthoquartzite) 

is distinctive and abundant enough to be noted and 

mapped in various localities. I have suggested that 

the absence of pure quartzite in the Salinian block 

was one of the reasons for questioning a match 

between the Salinian block and the southernmost 

Sierra Nevada. However, I have come to think that 

this distinction may have been overemphasized. 

Certainly, only minor amounts of pure quartzite 

occur near the San Andreas fault in pendants and 

inclusions in the granodiorite of Lebec and in the 

Bean Canyon Formation, whereas more seems to 

occur in the Kernville and Isabella Lake areas (fig. 

1) farther to the north and east. The difference may 

be solely a matter of facies change or is an indication of irregular variation in a discontinuously 

exposed sedimentary section. I no longer think that 

the difference between the metasedimentary rocks 

of the Gabilan and Santa Lucia Ranges and those 

of the southernmost Sierra Nevada is great enough 

to argue against correlating these two terranes.

METALLIC-MINERAL DEPOSITS

In an earlier report (Ross, 1978), I noted the abundance and variety of metallic-mineral deposits in 

the southernmost Sierra Nevada, in contrast to the 

virtual absence of such deposits in the Salinian 

block. The Salinian block still appears to be anomalously barren.However, the number of developed 

and productive properties drops off sharply south 

of the latitude of Tehachapi (fig. 1), and mineral 

deposits are notably sparse in the Sierran tail 
20 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

that is, in the area where the proposed correlative 

join with the barren Gabilan Range would be. The 

difference in the amount of mineralization is considerably less if the Sierran tail alone is compared 

with the Salinian-block rocks. The normal vagaries 

of distribution of mineral deposits in similar terranes may also be a factor here.

The problems discussed thus far can all be accommodated, at least in part, by the proposed reconstruction. Four more serious problems remain, 

involving: (1) the Barrett Ridge slice (fig. 12), (2) 

Montara Mountain and the K-feldspar-rich sedimentary rocks of the Gualala area (figs. 3, 4), (3) a 

europium-anomaly contrast between the Salinian 

block and the southernmost Sierra Nevada, and (4) 

the unlocated west margin of the Salinian block.

BARRETT RIDGE SLICE

The Barrett Ridge slice (Ross, 1972a) the anomalous basement terrane east of the Red Hills-San 

Juan-Chimeneas fault (fig. 12) poses possibly the 

greatest problem for a fault-reconstruction model 

based only on reversal of currently acceptable displacements on the San Andreas and San GregorioHosgri fault zones. A recapitulation here of the 

data from the Red Hills, Barrett Ridge, Mount 

Abel-Mount Pinos, and several well cores should 

help explain why the Barrett Ridge slice presents 

a problem.

In the Red Hills, a few square kilometers of gneissic basement rocks pokes up through the Cenozoic 

cover and appears to be truncated on the west 

against the Red Hills-San Juan-Chimeneas fault. 

Most of these outcrops consist of hornblende-biotite gneiss of tonalitic composition that is, in part, 

homogenized to a massive granitic rock. Less common are outcrops of augen gneiss, biotite-rich 

gneiss, hornblende schist, amphibolite, marble, 

and metasiltstone.

Barrett Ridge is an elongate rib of basement 

made up largely of quartzofeldspathic gneiss that 

is locally rich in biotite but poor in hornblende. The 

gneiss is thinly layered and strongly folded, in part 

ptygmatically. Near the south end of the ridge, a 

thick interbed of relatively pure quartzite and a 

plug of alaskite are present. Alaskite-aplite dikes 

are common in the gneiss and, at least in part, 

appear to be sweated out of the gneissic rocks. Several wells drilled north and south of Barrett Ridge 

have pierced the basement. Studies of core material from these wells (Ross, 1974) show the basement to be dominantly dark gneiss and granofels, 

with lesser amounts of amphibolite and felsic granite. In other words, the suite of subsurface samples 

generally resembles the outcrop areas of Barrett 

Ridge and Red Hills.

The inferred south end of the Barrett Ridge slice, 

and the southernmost basement exposure of the 

Salinian block, is the massif of Mount Abel-Mount 

Pinos (fig. 12). Although the geology is complex in 

detail, the overall pattern is simple. A southern 

belt is dominated by felsic granitic rocks. A central 

belt of high-grade banded gneiss, augen gneiss, 

migmatite, and amphibolite is characterized by 

gradations of the gneiss to homogeneous granitic 

rocks of tonalite and granodiorite compositions, 

much like those of the Red Hills. The central belt 

overlies a northern belt of the Pelona Schist along 

a thrust contact marked by an impressive mylonite 

zone which may be the westernmost exposure (on 

the southwest side of the San Andreas fault) of the 

Vincent thrust (Ehlig, 1968).

It is clear from this description that the Barrett 

Ridge slice differs significantly from the central 

Salinian block to the west. Plutons of tonalite and 

granodiorite seem to be absent to rare in the Barrett Ridge slice, and the granitic rocks generally 

contain patches of ghost gneiss and grade into 

gneissic rocks. Kistler and others (1973) obtained 

an anomalously high initial 87Sr/86Sr ratio of 0.7164 

from a sample of homogenized gneiss from Mount 

Abel; they concluded that the gneiss is probably 

Precambrian. They also obtained an initial 87Sr/86Sr 

ratio of 0.7095 from homogenized gneiss from the 

Red Hills a value significantly higher than from 

samples from the central Salinian block (west of 

the Barrett Ridge slice), but comparable to those 

obtained from gneissic samples from the San Gabriel Mountains to the southeast. These data confirm what the petrography had suggested that a 

strong discordance exists between the Barrett 

Ridge slice and the rest of the Salinian block.

If rn attempt is made to slide the Barrett Ridge 

slice back along the San Andreas fault zone, finding a parent terrane poses a real problem. Southeastward from the Portal-Ritter Ridge area (fig. 1) 

on the northeast side of the San Andreas fault, the 

Holcomb and Wrightwood basement rocks (Ross, 

1972a) might be able to accommodate part of the 

Barrett Ridge slice, but surely the large plutonic 

bodies of the San Bernardino Mountains (fig. 1) and 

their probable Cordilleran miogeoclinal roof pendants would be strange company for the Barrett 

ridge slice. The basement sample from the Barrett 

Ridge slice is small; however, a good petrographic 

sample is available from the Red Hills along the 

west side of the slice to the Mount Abel-Mount Pi-
PROBLEMS OF SALINIAN-BLOCK RECONSTRUCTION THAT STILL REMAIN 21

nos area (fig. 12). These rocks are relatively similar 

and are distinctive over a large area.

If, instead of pushing the Barrett Ridge slice 

back along the San Andreas fault, that slice and 

the adjacent Mount Frazier area were to be pushed 

back along the south side of the San Gabriel lozenge and molded around it along the San GabrielSierra Madre fault zone (fig. 1), a better match of 

rock types would be obtained. Also, the Pelona 

Schist at Mount Abel-Mount Pinos would end up 

south of, and almost adjacent to, the main outcrops 

of the Pelona Schist in the eastern San Gabriel 

Mountains. Although this reconstruction makes 

good lithologic sense, it brings about a structural 

problem that is virtually insurmountable. If the 

San Gabriel basement block originated opposite 

the Chocolate and Orocopia Mountains before San 

Andreas fault movement, as Crowell and Walker 

(1962) convincingly demonstrated on the basis of a 

correlation of several basement-rock units, then, 

to reach its present position, the Barrett Ridge 

slice would have had to travel some 200 km farther 

than the central block of the Salinian block. This 

movement would be necessary for both the correlations of Crowell and Walker (1962) and the one 

proposed here for the Gabilan Range-Sierra Nevada to be satisfied, and clearly it is implausible.

Howell and others (1980) suggested that the Red 

Hills-San Juan-Chimeneas fault is part of an old 

collision zone between two basement terranes that 

were accreted to the North American Continent at 

a latitude somewhere in southern Mexico. These 

two terranes drifted northward as a structural 

unit, beached in southern California, and subsequently were transported some 300 km by movement on the San Andreas fault system. This proposal nicely solves the apparent need for additional 

movement on the Barrett Ridge slice, but it suggests that we should see evidence of an impressive 

collision in the area of the Red Hills-San Juan-Chimeneas fault zone. Small samples of basement 

rocks from the Red Hills, San Juan Canyon, and 

Barrett Ridge have not revealed the types of major 

contortions and cataclastic deformation that I 

would expect to find associated with such a structural event. Nonetheless, it seems reasonable to 

view the Red Hills-San Juan-Chimeneas fault zone 

as a major structural discordance (possibly a 

strike-slip fault) that predates the San Andreas 

fault. The Barrett Ridge slice and its west-bounding fault present a serious if not insurmountable problem for a simple 300 to 350 km of basement reconstruction.

MONTARA MOUNTAIN AND GUALALA GRANITIC CLASTS

The proposed reconstruction that places the Gabilan Range opposite the Sierra Nevada tail also 

presumes that oceanic basement is present north 

of the Vergeles-Zayante fault. This reconstruction 

(fig. 14) leaves the granitic basement of Montara 

Mountain marooned considerably north of the Bodega Head-Ben Lomond granitic basement. This 

anomaly calls for the Montara mass to be either a 

structurally isolated block or an intrusion into 

oceanic basement.

The K-feldspar arkose of Gualala (fig. 3), with its 

coarse granitic debris, must have been derived 

from a continental source. According to my reconstruction, however, these sedimentary rocks are 

also cut off from a granitic basement source. Both 

the granitic rocks of Montara Mountain and the Kfeldspar-rich sedimentary rocks of the Gualala 

area can be accommodated more easily if the basement is continental north of the Butano fault; however, such a situation would cast considerable 

doubt on the correlation of the Pastoria and Vergeles-Zayante fault zones.

EUROPIUM-ANOMALY CONTRAST

Trace-element patterns for the granitic rocks of 

the Salinian block and the southernmost Sierra 

Nevada present possibly significant contrasts. Europium anomalies in the rocks of the southernmost 

Sierra Nevada not only occur in the granites, as 

they do in most granitic suites, but also are surprisingly common in the granodiorites and even in 

some of the tonalites. In the Salinian block as well 

as in the central Sierra Nevada (Dodge and others, 

1982), pronounced europium anomalies are found 

only in the granites. A graphic comparison of the 

trace-element patterns of these three regions was 

shown by Ross (1982).

The Salinian-block data, though sparse (14 samples), cover a wide range of chemical compositions 

and a wide geographic range, and so they are probably representative. The sampling in the southernmost Sierra and, even more so, in the central 

Sierra is much more extensive and surely reflects 

an actual difference in trace-element patterns.

The contrast in trace-element patterns between 

the Salinian block and the southernmost Sierra 

Nevada is incompatible with the correlation of 

these two terranes. At present, however, the full 

significance of this contrast is not fully understood. 

More trace-element data for the gap between the 

central and southernmost Sierra Nevada are
PASTORIA FAULT 

Eagte Rest Peak 

Gold

HOS^^CTz5Ni

G

S

Con

B

Weste

Of S

basa

Franciscan^'/ / 

assemblage //,

/

Point / «x 

Reyes 

Ben 

Lomon

Franciscan 

assemblage

FIGURE 14. Possible reconstructions of the northern and central Salinian block before San Andreas fault movement. A, Distribution 

after commonly accepted movements on the San Andreas and San Gregorio-Hosgri fault zones are removed. B, Major oroclinal 

and Davis (1980) and their interpretation of the distribution of selected areas of basement rocks of the Salinian block before mo

fault. Simplified from Burchfiel and Davis (1980, fig. 9-9).
PROBLEMS OF SALINIAN-BLOCK RECONSTRUCTION THAT STILL REMAIN 23

needed to determine the geographic extent of europium anomalies and their values over a broad 

range of granitic rock compositions.

UNLOCATED WEST FLANK OF THE SALINIAN BLOCK

The Salinian block, as presently exposed, is notably thinner than other parts of the Cordilleran 

plutonic arc, from which arc it is generally assumed 

to have been derived. For example, in comparison 

with the southern California batholith or the central Sierra Nevada batholith, the Salinian block 

appears to lack a west flank that elsewhere in the 

Cordilleran plutonic arc includes abundant tonalite, trondhjemite, and gabbro as well as metamorphic framework rocks rich in volcanic material and 

volcanoclastic debris. Page (1982), in his comprehensive review of the migration history of the Salinian block, emphasized its anomalous thinness 

and suggested that the disappearance of the west 

flank is the result of either (1) megatransport at 

the Earth's surface or (2) destruction by piecemeal 

subduction (tectonic erosion).

If the present west limit of the Salinian block 

(the Nacimiento fault) marks a zone where oceanic 

material has collided with a continental margin, 

the surviving continental-margin rocks would be 

expected to preserve some evidence of this strongly 

compressive event. However, the westernmost granitic exposures in the La Panza Range, adjacent 

to the Nacimiento fault zone, do not appear to be 

strongly deformed or melanged, and Page (1982) 

noted that Salinian granite, essentially in contact 

with oceanic and pelagic sedimentary rocks, shows 

no evidence of interaction. It seems unlikely that 

piecemeal subduction of lighter continental material downward into denser oceanic material could 

have occurred here (or anywhere) on a large scale 

without leaving evidence of such an impressive 

event.

The abrupt west termination of the granitic basement of the Salinian block against oceanic material, seen in both the La Panza and Santa Lucia 

Ranges, more likely reflects a strike-slip fault zone. 

If the missing western part of the original Salinian 

block has, indeed, been stripped away by megatransport to a site as yet unknown, it is plausible 

that as Howell and others (1980) proposed the 

east margin of the block may also have undergone 

megatransport, of which modest movements along 

the San Andreas and San Gregorio-Hosgri fault 

zones are only the most recent events. However, 

the possibilities of correlation between the central 

Salinian block and the southernmost Sierra Nevada cannot be lightly dismissed. As we continue 

to search for the source of the Salinian block, so 

also must we continue to search for its sheared-off 

west margin.

CONCLUSIONS

Figure 14A, which shows selected key units of 

basement rock of the Salinian block reconstructed 

along the southwest side of the present San Andreas fault, indicates a possible distribution of 

basement before movement along the San Gregorio-Hosgri and San Andreas fault zones. Figure 

145 shows a similar reconstruction, including the 

oroclinal bend proposed by Burchfiel and Davis 

(1980). Their reconstruction connects the Nacimiento fault around the oroclinal bend with the 

contact, now buried, between Sierran basement on 

the east and the Franciscan-assemblage basement 

on the west. Note that the reconstruction by Burchfiel and Davis (1980) places the Santa Lucia Range 

about 50 km south relative to the Gabilan Range, 

so as to take account of the movement proposed by 

Graham (1978) across the Rinconada-Reliz-King 

City fault (fig. 5). Either way (fig. 14A or 145), there 

is a good juxtaposition of the schists of Sierra de 

Salinas and Portal-Ritter Ridge. The combined 

Pastoria and Vergeles-Zayante fault zone would 

presumably bend southward, with the oroclinal 

flexure somewhere seaward of Bodega Head and 

Cordell Bank to join the Nacimiento fault zone. 

However, the dilemma of the isolation of the Gualala area and Montara Mountain north of the other 

continental (granitic) basement remains, and its 

solution demands more data on the characteristics 

of the basement at the north end of the Salinian 

block.

A reconstruction that depends on some 300 to 

350 km of right-lateral offset on the San Andreas 

fault and some 130 km of similar movement on the 

San Gregorio-Hosgri fault brings into alignment 

several basement-rock units and structures in the 

central Salinian block with similar features in the 

southernmost Sierra Nevada. Correlations of individual granitic and metamorphic basement rocks 

can never be truly unequivocal because similar 

basement-rock types and textures are apt to recur. 

However, the occurrence of a cluster of correlative 

basement-rock units in the Gabilan-Santa Lucia 

Ranges and the southern Sierra Nevada region 

cannot be dismissed lightly.

In summary, that cluster of possibly correlative 

units and features includes (1) an unusual and distinctive metagraywacke (schists of Sierra de Sali-
24 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

nas and Portal-Ritter Ridge), (2) mineralogically 

unusual hornblende quartz gabbros and coarse anorthositic gabbros (in the Logan, Gold Hill, and Eagle Rest Peak areas), (3) several granitic units, (4) 

a similar east-west structural grain, and (5) a structural break between oceanic and continental basement terranes (the Vergeles-Zayante and Pastoria 

fault zones).

The reconstruction that fits the central part of 

the Salinian block to a potentially correlative terrane is far more uncertain for the northern part 

of the Salinian block. There, the composition of the 

basement rocks is of critical importance, but large 

areas are now covered by sedimentary rocks and 

ocean water. Furthermore, whether this northern 

basement is oceanic or continental, some correlation problems still remain. To the south is an even 

greater problem the Salinian block-Sierra Nevada reconstruction places part of the Barrett 

Ridge slice opposite the San Bernardino Mountains in an untenable juxtaposition of dissimilar 

basement rocks.

By themselves, movements on the San Andreas 

and San Gregorio-Hosgri fault zones do not seem 

to resolve the problem of the origin of the Salinian 

block. Such movements solve some problems, but 

they create other, greater ones. As already discussed, Howell and others (1980) proposed much 

larger movement for a composite block of coastal 

California that includes the Salinian block. This 

movement, premised in part on the paleomagnetic 

studies by Champion and others (1980), would predate San Andreas movement. Early in the preparation of this report, when I first heard this model 

described, I was nearly convinced that data on offsets of the San Gregorio-Hosgri fault zone (Graham 

and Dickinson, 1978a, b), my new ideas concerning 

an ancient ocean margin in the Sierra Nevada tail, 

and the conventionally accepted amounts of movement on the San Andreas fault zone would enable 

a resolution of the origin of the Salinian block. 

Therefore, I gave the model of Howell and others 

(1980) short shrift. However, as I pursued the problem, it became apparent that my initial model was 

inadequate and that theirs might be right.

Nevertheless, the match of several basementrock units and features in the central Salinian 

block with the southern Sierra Nevada region (fig. 

14) cannot be readily dismissed. Are all of those 

correlations only the fortuitous superpositions of 

undiagnostic features? Or do all of the correlative 

features occur within a large, composite, exotic 

coastal block that was offset only by the later San 

Andreas movement? For example, the schists of

Sierra de Salinas and Portal-Ritter Ridge could 

both be part of a composite coastal terrain that 

was split apart by later San Andreas movement. 

However, the granitic rocks, structures, and structural grain in the Sierran tail are firmly tied to the 

main Sierra Nevada block and are separate from 

any composite coastal block. The presence, which 

I have suggested, of correlatives of these features 

in the central Salinian block could pose problems 

for the model of Howell and others (1980). Their 

model would then call for the present Gabilan 

Range to be beached (after its long journey from 

southern Mexico) adjacent to some nearly identical 

rocks and structures in the southern Sierra Nevada and that would imply a degree of coincidence that is hard to credit.

In conclusion, the problem of the origin of the 

Salinian block does not seem to be resolved. I had 

hoped that this study would resolve the problem; 

instead, these reconnaissance studies have merely 

provided tantalizing clues that need to be pursued. 

Neither the model that calls for some 300 km of 

San Andreas fault movement supplemented by San 

Gregorio-Hosgri fault movement, nor the model of 

Howell and others (1980), which calls for a pre-San 

Andreas movement of a large composite terrane, 

provides all the answers. Both models create some 

new problems even as they appear to solve others. 

Further detailed basement-rock studies of both the 

central Salinian block and the Sierran tail should 

help decide whether there is a valid correlation 

between these two areas or merely a juxtaposition 

of several of similar-appearing but unrelated basement-rock units.

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Petroleum Geologists Bulletin, v. 62, no. 11, p. 2214-2231.

Graham, S. A., and Dickinson, W. R., 1978a, Apparent offsets of 

on-land geologic features across the San Gregorio-Hosgri 

fault trend: California Division of Mines and Geology Special Report 137, p. 13-23.

 1978b, Evidence for 115 kilometers of right slip on the San 

Gregorio-Hosgri fault trend: Science, v. 199, no. 4325, p. 179- 

181.

Grantz, Arthur, and Dickinson, W. R., 1968, Indicated cumulative offsets along the San Andreas fault in the California 

Coast Ranges, in Dickinson, W. R., and Grantz, Arthur, eds., 

Proceedings of conference on geologic problems of the San 

Andreas fault system: Stanford University Publications in 

the Geological Sciences, v. 11, p. 117-120.

Greene, H. G., Lee, W. H. K., McCulloch, D. S., and Brabb, E. E., 

1973, Faults and earthquakes in the Monterey Bay region, 

California: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-518, scale 1:200,000.

Hanna, W. F., Brown, R. D., Ross, D. C., and Griscom, Andrew, 

1972, Aeromagnetic reconnaissance along the San Andreas 

fault between San Francisco and San Bernadino, California: U.S. Geological Survey Geophysical Investigation Map 

GP-815, scale 1:250,000.

Hill, M. L., and Dibblee, T. W., Jr., 1953, San Andreas, Garlock, 

and Big Pine faults, California: A study of the character, 

history, and tectonic significance of their displacements: 

Geological Society of America Bulletin, v. 64, no. 4, p. 443- 

458.

Howell, D. G., and Vedder, J. G., 1978, Late Cretaceous paleogeography of the Salinian block, California, in Howell, D. 

G., and McDougall, K. A., eds., Mesozoic paleogeography of 

the western United States: Pacific Coast Paleogeography 

Symposium 2: Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 523-534.

Howell, D. G., McLean, Hugh, and Vedder, J. G., 1980, Late 

Cretaceous suturing and translation of the Salinian and 

Nacimiento blocks, California [abs.]: Eos (American Geophysical Union Transactions), v. 61, no. 46, p. 948.

Huffman, O. F., 1972, Lateral displacement of Upper Miocene 

rocks and the Neogene history of offset along the San Andreas fault in central California: Geological Society of America Bulletin, v. 83, no. 10, p. 2913-2946.

Huffman, O. F., Turner, D. L., and Jack, R. N., 1973, Offset of 

Late Oligocene- early Miocene volcanic rocks along the San 

Andreas fault in central California, in Kovach, R. L., and 

Nur, Amos, eds., Proceedings of the Conference on tectonic
26 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

problems of the San Andreas fault system: Stanford University Publications in the Geological Sciences, v. 13, p. 368- 

373.

Jennings, C. W., compiler, 1967, Salton Sea sheet of Geologic map 

of California: California Division of Mines and Geology, 

scale 1:250,000.

Kistler, R. W., and Peterman, Z. E., 1973, Variations in Sr, Rb, 

K, Na, and initial Sr^/Sr86 in Mesozoic granitic rocks and 

intruded wall rocks in central California: Geological Society 

of America Bulletin, v. 84, no. 11, p. 3489-3512.

Kistler, R. W., Peterman, Z. E., Ross, D. C., and Gottfried, David, 

1973, Strontium isotopes and the San Andreas fault, in Kovach, R. L., and Nur, Amos, eds., Proceedings of the conference on tectonic problems of the San Andreas fault system: Stanford University Publications in the Geological 

Sciences, v. 13, p. 339-47.

Matthews, Vincent, III, 1973, Geology of the Pinnacles Volcanic 

Formation and the Neenach Volcanic Formation and their 

bearing on the San Andreas fault problem: Santa Cruz, 

University of California, Ph.D. thesis, 214 p.

 1976, Correlation of Pinnacles and Neenach volcanic formations and their bearing on San Andreas fault problem: 

American Association of Petroleum Geologists Bulletin, v. 

60, no. 12, p. 2128-2141.

Moore, J. G., 1959, The quartz diorite boundary line in the western United States: Journal of Geology, v. 67, no. 2, p. 64-68.

Mullins, H. T., and Nagel, D. K., 1981, Franciscan-type rocks off 

Monterey Bay, California: Implications for western boundary of Salinian block: Geo-Marine Letters, v. 1, no. 2, p. 135- 

139.

Nilsen, T. H., and Link, M. H., 1975, Stratigraphy, sedimentology 

and offset along the San Andreas fault of Eocene to lower 

Miocene strata of the northern Santa Lucia Range and the 

San Emigdio Mountains, Coast Ranges, central California, 

in Weaver, D. W., Hornaday, G. R., and Tipton, Ann; eds., 

Paleogene Symposium and selected technical papers: Conference on Future Energy Horizons of the Pacific Coast: 

American Association of Petroleum Geologists-Society of 

Exploration Geophysicists-Society of Economic Paleontologists and Mineralogists, Pacific Sections, Annual Meeting, 

Long Beach, Calif., 1975, Proceedings, p. 367^00.

Page, B. M., 1982, Migration of Salinian composite block, California, and disappearance of fragments: American Journal 

of Science, v. 282, p. 1694-1734.

Rogers, T. H., compiler, 1965, Santa Ana sheet of Geologic map 

of California: California Division of Mines and Geology, 

scale 1:250,000.

Ross, D. C., 1970, Quartz gabbro and anorthositic gabbro: Markers of offset along the San Andreas fault in the California 

Coast Ranges: Geological Society of America Bulletin, v. 81, 

no. 12, p. 3647-3662.

 1972a, Petrographic and chemical reconnaissance study 

of some granitic and gneissic rocks near the San Andreas 

fault from Bodega Head to Cajon Pass, California: U.S. Geological Survey Professional Paper 698, 92 p.

 1972b, Geologic map of the pre-Cenozoic basement rocks, 

Gabilan Range, Monterey and San Benito Counties, California: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-357, scale 1:125,000.

 1974, Map showing basement geology and locations of 

wells drilled to basement, Salinian block, central and southern Coast Ranges, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-588, scale 1:500,000.

 1975, Modal and chemical data for granitic rocks of the 

Gabilan Range, Central Coast Ranges, California: National 

Technical Information Service Report PB-242 458/AS, 42 p.

 1976, Metagraywacke in the Salinian block, central Coast 

Ranges, California and a possible correlative across the 

San Andreas fault: U.S. Geological Survey Journal of Research, v. 4, no. 6, p. 683-696.

 1977a, Pre-intrusive metasedimentary rocks of the Salinian block, California a tectonic dilemma, in Stewart, J. H., 

Stevens, C. H., Fritsche, A. E., eds., Paleozoic paleogeography of the western United States: Pacific Coast Paleogeography Symposium 1: Los Angeles, Society of Economic 

Paleontologists and Mineralogists, Pacific Section, p. 371- 

380.

 1977b, Maps showing sample localities and ternary plots 

and graphs showing modal and chemical data for granitic 

rocks of the Santa Lucia Range, Salinian block, California 

Coast Ranges: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-799, 3 sheets.

 1978, The Salinian block a Mesozoic granitic orphan in 

the California Coast Ranges, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western 

United States: Society of Economic Paleontologists and 

Mineralogists, Pacific Coast Paleogeography Symposium 2, 

p. 509-522.

 1980, A tectonic mystery basement rock clasts in the 

Temblor Range, San Luis Obispo and Kern Counties, California: California Geology, v. 33, no. 7, p. 153-157.

 1982, Results of instrumental neutron activation analyses 

for selected plutonic samples from the Salinian block, California Coast Ranges: U.S. Geological Survey Open-File Report 82-935, 6 p. 

-1983, Hornblende-rich, high-grade metamorphic terranes

in the southernmost Sierra Nevada, California, and implications for crustal depths and bathlith roots: U.S. Geological 

Survey Open-File Report 83-465, 51 p.

Ross, D. C., and Brabb, E. E., 1973, Petrography and structural 

relations of granitic basement rocks in the Monterey Bay 

area, California: U.S. Geological Survey Journal of Research, v. 1, no. 3, p. 273-282.

Ross, D. C., Wentworth, C. M., and McKee, E. H., 1973, Cretaceous 

mafic conglomerate near Gualala offset 350 miles by San 

Andreas fault from oceanic crusutal source near Eagle Rest 

Peak, California: U.S. Geological Survey Journal of Research, v. 1, no. 1, p. 45-52.

Silver, E. A., Curray, J. R., and Cooper, A. K., 1971, Tectonic 

development of the continental margin off central California: Geological Society of Sacramento Annual Field Trip 

Guidebook, p. 1-10.

Smith, D. P., 1977, The San Juan-St. Francis fault and the Rinconada-Jolon fault: proposed middle Tertiary right-lateral 

faults in southern California: Geological Society of America 

Abstracts with Programs, v. 9., no. 4, p. 501-502.

Turner, D. L., 1968, Potassium-argon dates concerning the Tertiary foraminiferal time scale and San Andreas fault displacement: Berkeley, University of California, Ph.D. thesis, 

99 p.

Wentworth, C. M., 1968, Upper Cretaceous and lower Tertiary 

strata near Gualala, California and inferred large right lateral slip on the San Andreas fault, in Dickinson, W. R., and 

Grantz, Arthur, eds., Proceedings of conference on geologic 

problems of the San Andreas fault system: Stanford University Publications in the Geological Sciences, v. 11, p. 130- 

143.
SUPPLEMENTAL INFORMATION 27

SUPPLEMENTAL INFORMATION

COMPARISON OF THE PORPHYRITIC GRANODIORITES OF 

POINT REYES AND MONTEREY

The granitic rocks at Point Reyes and on the 

Monterey Peninsula (fig. 15) are generally similar 

in appearance. Both contain conspicuous euhedral 

K-feldspar phenocrysts, but the rocks are not

everywhere porphyritic (seriate). The phenocrysts 

in the Point Reyes granitic rocks do not exceed 5 

cm in length, whereas those in phenocrysts of the 

Monterey granitic rocks are as much as 15 cm long. 

This extreme development of coarse phenocrysts, 

however, occurs only locally in the Monterey mass; 

commonly, the phenocryst size in both masses is 

similar. The modal mineralogies of the two masses 

are also similar (fig. 16); the variations are well 

within the limits found in normal plutonic units.

123°

38°

100 KILOMETERS

S& Area of 

rY_ .figure 15C 

Point I A I 

Reyesif/A

San Francisco

\

\ ^ I

Area of 7

figure 156

122«00'

\

\

\ \ 

\ (Offshore extent \

\ based on

123°

38"

5 KILOMETERS

Tonalite of' 

Tomales Point

Granodiorite and 

granite of Inverness

Porphyritic granodiorite 

of Point Reyes

38°

121°45'

Porphyritic grano 

diorite of Monterey DR-X»» 

1973

B

123°

DR-533-3

ife Boat 

station

532A

FIGURE 15. Locations of the porphyritic granodiorites of Point Reyes and Monterey. A, Locations of study areas at Point Reyes 

and Monterey. B, Locations of modally analyzed samples (dots) and chemically analyzed samples (circles) of the porphyritic 

granodiorite of Monterey. C, Location of the porphyritic granodiorite of Point Reyes. D, Locations of modally analyzed 

samples (dots) and chemically analyzed samples (circles) of the porphyritic granodiorite of Point Reyes. Numbers refer to 

samples listed in table 2.
28 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

One possibly significant difference is that the specimens of the Point Reyes mass that I have examined contain about 1 percent of hornblende, 

whereas I have seen no hornblende in the Monterey mass. Also, the Monterey mass has what appears to be primary muscovite, which is absent in 

the Point Reyes mass. However, the granodiorite 

of Cachagua, which I consider to be in gradational 

contact with the Monterey mass, contains hornblende, as does a suspected correlative of the Monterey mass in the La Panza Range (fig. 12). Thus, 

both the overall physical appearance and the 

modal mineralogy suggest that these two masses 

are compatible and are potential correlatives.

Chemically, the two masses also appear to be approximately comparable (fig. 17; table 1). Although

Number of superposed samples

Hornblende

K-feldspar 90 65 35 10 Plegioclase 

PORPHYRITIC GRANOOIORITE OF POINT REYES

Hornblende Biotite

K-feldspar 90 65 35

PORPHYRITIC GRANOOIORITE OF MONTEREY

10 Plagioclase

FIGURE 16. Modes of the porphyritic granodiorites of Point 

Reyes and Monterey.

|

£ 16 2 o°

 _«M

2 15 tO

O

uj 14

O

X

O

70

CM 

O

in

65

60

CM

Q 1 t0

4

2 -

I

Point 

Reyes

Monterey

FIGURE 17. Histograms showing chemical analysis 

of the porphyritic granodiorites of Point Reyes and 

Monterey.
SUPPLEMENTAL INFORMATION 29

only two analyses are available from the Point 

Reyes mass, they probably are representative of 

the small exposed part of the mass. Oxide contents 

are comparable to the average value for the four 

specimens from the Monterey mass. The Point 

Reyes mass is somewhat richer in CaO and poorer 

in SiO2, but these differences do not seem to be 

significant. The chemical similarity of the two 

masses is further accentuated by the ternary plots 

Q-Or-(Ab+An), Or-Ab-An, and Alk-F-M (fig. 18). Of 

possible significance is the fact that both masses 

have normative corundum: 1.4 percent for the 

Point Reyes and about 3.0 percent for the Monterey. The average abundance of normative quartz 

is higher in the Monterey mass (34 percent), than 

in the Point Reyes mass (31 percent), but values 

for Monterey samples overlap those for the Point 

Reyes samples.

The exposures of granitic rocks on Point Reyes 

lie 10 km from the nearest granitic exposures of 

the Inverness area. I had originally (Ross, 1972a) 

considered the Point Reyes granitic rocks to be part 

of the granodiorite and granite of Inverness (fig. 

15). The porphyritic texture of the Point Reyes granitic rocks, and their hornblende content, indicate 

that they are at least a separate facies of the Inverness granitic rocks; they could even represent 

a separate granitic body.

Just east of the Fish Docks is an exposure of

darker granitic rock (sample 533-3, fig. 19) that contains conspicuous abundant hornblende and numerous small ellipsoidal mafic inclusions. The dark 

rocks sharply contact the lighter Point Reyes granitic rocks (which here are not porphyritic), but the 

age relation is not obvious. These dark rocks, which 

contain 11 percent biotite and 9 percent hornblende, I interpret to be a large inclusion of the 

tonalite of Tomales Point within the porphyritic 

granodiorite of Point Reyes. The granitic rocks of 

Point Reyes may well be connected physically at 

depth with the tonalite of Tomales Point and the 

granodiorite and granite of Inverness. However, 

the physical and chemical compatibility of the 

Point Reyes and Monterey granitic rocks leaves 

open the possibility of their correlation and of a 

structural break between the Point Reyes and Inverness masses. The dark rocks near the Fish 

Docks could as well be an inclusion of the tonalite 

of Soberanes Point, which is widely exposed in the 

Santa Lucia Range south of Monterey.

Nothing at Point Reyes rules out a correlation 

with the granitic rocks at Monterey. I think that 

the granitic rocks at Point Reyes are related to the 

Tomales Point-Inverness basement. At Kehoe 

Beach, about 17 km north of Point Reyes, darker 

rocks resemble some of the rocks at Point Reyes. 

Also, near Inverness, hornblende-bearing granitic 

rocks occur that are physically and modally nearly

TABLE 1. Chemical analyses of selected granitic rocks from the Point Reyes and Monterey areas, California

[All values in weight percent; n.d., not determined. Sources of chemical data: sample 

DR-532A, Ross (1972a); sample LPR-3, Clark and others (1984); samples Mo-IB, DR-1973, 

DR-1974, and Mop-1, Ross (1977b). See figure 15 for sample locations]

Point Reyes

Sample 

Si02 

A1 2 D3 

Fe ? 0o 

FeD-- 

MgO 

CaO 

Na 2 0 

K

7\

H~cr 

i/n+

TiOo 

p 0 5 - 

MnODR-532A

70.2 

15.4 

.74 

1.6 

.97 

3.0 

3.3 

3.2 

.09 

.64 

.59 

.14 

.11

LPR-3

71.02 

15.00

1 2.80

.75 

2.50 

3.27 

3.63 

n.d. 

n.d. 

.43 

.10 

.04

Average

70.6 

15.2

} 2 '6

.86 

2.8 

3.3 

3.4 

.1 

.6 

.5 

.12 

.07

Mo-IB

73.5 

14.4 

.53 

.83 

.33 

2.1 

3.7 

3.3 

.09 

.68 

.20 

.05 

.08

DR-1973

69.2 

16.5 

1.2 

1.7 

.56 

2.6 

3.8 

2.8 

.24 

.67 

.43 

.10 

.06

Monterey

DR-1974

72.3 

15.6 

.80 

1.0 

.42 

2.2 

3.3 

3.1 

.24 

.50 

.26 

.06 

.06

Mop-1

73.2 

15.3 

1.9 

.08 

.45 

2.1 

2.4 

3.2 

.39 

.34 

.23 

.06 

.06

Average

72.1 

15.5 

1.1 

.9 

.44 

2.3 

3.3 

3.1 

.2 

.6 

.3 

.07 

.07

Total 100.00 99.54 100.2 99.9 99.7 100.0
30 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

FIFeO+2 Fe2O3 +MnO) Other

Alk (Ns,O+K,O) 50

50

Ab+An

Ab 50

EXPLANATION

O Monterey 

 Point Reyes

M(MgO)

Normative 

minerals

An

FIGURE 18. Chemical compatibility of the porphyritic 

granodiorites of Point Reyes and Monterey.

10 

Tonalite of 

Tomates Point

Hornblende

\ \

K-feldspar 90 65 35

Other

_ 6 Number of super 

V-3- posed samples

10 Ptogioclase

20

Granodiorite and 

granite of Inverness

Hornblende

K-feldspar 90 65

10 

35 10 Plagioctese

Other

20 

Hornblende

f\^ 10 points

Alaskite samples 

within the 

Inverness body

Biotite

/ 7

K-feldsper 90 65 35 10 Ptagioclese

FIGURE 19. Modes of the Tomales Point and Inverness 

granitic bodies.
SUPPLEMENTAL INFORMATION 31

identical to those at Point Reyes. This whole discussion illustrates the frustrations of attempting 

to make positive correlations of granitic basement 

terranes.

COMPARISON OF THE GRANITIC ROCKS NEAR THE

PINNACLES AND NEENACH VOLCANIC FORMATIONS OF

MATTHEWS (1976)

The Pinnacles and Neenach Volcanic Formations 

of Matthews (1976) (fig. 20A) are both closely associated with various granitic rocks. In the Pinnacles area (fig. 20B) the tonalite and granodiorite 

of Johnson Canyon, the granite of Bickmore Canyon, and associated felsic rocks crop out over a 

large area adjacent to the Pinnacles Volcanic Formation. Petrographically and chemically comparable granitic bodies, the granodiorites of Fairmont Reservoir and Burnt Peak, and a felsic 

variant(?) are exposed near the Neenach Volcanic 

Formation (fig. 20C).

Granitic data from the Pinnacles area were collected early in the 1970's and summarized by Ross 

(1975). Data from the Neenach area are a combination of an earlier reconnaissance study of the 

Transverse Ranges (Ross, 1972a) and more detailed 

mapping and sampling near the Neenach Volcanic 

Formation in 1982 (fig. 20C; table 2). Present

12V 120° 119°

37°

36"

35°

34°

Cenozoic sedimentary 

deposits, undivided

EXPLANATION

LOCATION OF SAMPLES

Chemical 

analysis

. A DR-1555

Granite of Bickmore Canyon Grenitic rocks 

undivided

Tonalite and granodiorite 

of Johnson Canyon

Tonalite and granodiorite 

of Johnson Canyon

Aplite, alaskite 

pegmatite

Pinnacles Volcanic : 

Formation of '. ' 

Matthews (1976)

Granite of Bick 

more Canyon

. ranite of Bick- 

 ' ' ' 'more Canyon

Schist of Sierra. 

de Salinas

FIGURE 20. Granitic rocks of the Pinnacles and Neenach areas. A, Locations of generalized geologic maps of the Pinnacles and 

Neenach areas. B, Locations of modally and chemically analyzed samples of the Pinnacles area. Numbers refer to samples 

listed in table 3. C, Locations of modally and chemically analyzed samples of the Neenach area. Numbers refer to samples 

listed in tables 2 and 3; unnumbered sample locations are from Ross (1972a).
32 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

modal data probably are sufficient to characterize the various granitic units, but more details are 

needed to better define the relation between the 

Gabilan Range units (Johnson Canyon and Bickmore Canyon) and the Neenach units (Burnt Peak 

and Fairmont Reservoir). Chemical data are admittedly sparse and certainly need augmenting, as 

do the radiometric ages and Rb/Sr analyses.

The modal field of the Johnson Canyon mass (fig. 

21A) in the Pinnacles area is similar to the field of 

the combined Fairmont Reservoir and Burnt Peak 

units (figs. 21B, 21C) in the Neenach area. The 

modal field of the Johnson Canyon body is somewhat more spread out, but considering that it represents samples taken from a much wider area 

than that of the Fairmont Reservoir-Burnt Peak 

pair, this spread is not surprising in such varying 

plutonic bodies. Particularly noteworthy is the horizontal trend of these modal fields, which reflects 

a generally similar quartz content over a wide 

range of plagioclase/K-feldspar ratios.

My recent (1982) work in the Neenach area confirmed that as I had suspected earlier (Ross, 

1972a) there are two mappable granodiorite bodies (the Burnt Peak and Fairmont Reservoir). The 

dashed contact shown separating them on figure 

20C gives a picture that may appear more definitive than I would desire. The contact was not seen, 

nor were any diking relations seen to confirm the 

occurrence of two separate intrusive bodies. Instead, the observable evidence includes gradational lithologies and suggests that this may be a

single granodiorite pluton with a somewhat fine 

grained core (Fairmont Reservoir). Certainly the 

Burnt Peak and Fairmont Reservoir masses are 

petrographically nearly identical and are closely 

related.

The granite of Bickmore Canyon in the Pinnacles 

area (fig. 20B) is a felsic rock that was mapped 

separately from the Johnson Canyon unit. No contacts were seen, however, and the only diking relation of the Bickmore Canyon into the Johnson 

Canyon that suggested the occurrence of two separate bodies may have been an alaskite rather than 

a Bickmore Canyon dike. Aplite, alaskite, and pegmatite are abundant in the Bickmore Canyon area; 

and distinctions, particularly between the alaskite 

and the granite of Bickmore Canyon, are difficult. 

Furthermore, darker rocks within the Bickmore 

Canyon mass show considerable modal overlap 

with the Johnson Canyon mass (fig. 21C). The Johnson Canyon-Bickmore Canyon pair may be a zoned 

pluton somewhat analogous to the Burnt PeakFairmont Reservoir pair.

Another general similarity between the Pinnacles and Neenach areas is the abundance of aplite, 

alaskite, and pegmatite in both areas. Mostly, 

these rock types occur in isolated dikes or dike 

swarms and are not mappable, but just west of the 

Pinnacles Volcanic Formation (fig. 20B) a large 

mass could be delineated. The granite of Antelope 

Buttes and numerous other felsic rocks in the 

Neenach area (fig. 20C; table 3) are also similar in 

this respect. Aplite, alaskite, and pegmatite are

118°45' 118°30'

.57.'*5>.'5746-'

Neenach Volcanic 

Formation of 

Matthews (1976) 

Granodiorite of 

Burnt Peak 

5756 

5758C

.- ' ' '.Granodiorite of 

 ' ' ''. '/isj'Fairmont Reservoir

.. .-.- ' Granite of :' : ;' ' ' 

 '.'.-. .' '; Antelope Buttes :': '

34°45'

5748 

5744A 

5744B . ... 5771 

DR-593.5770A 

-590

5767 Granodiorite of 

Fairmont Reservoir

5 7 50 A 

5750C 

DR-614

DR-602 

10 KILOMETERS

EXPLANATION

Sample location

.5762 A DR-590 

Modal analysis Chemical analysis

Schist of PortalRitter Ridge

FIGURE 20. Continued
SUPPLEMENTAL INFORMATION 33

widespread in varying amounts in many granitic 

terranes; it is only their relative abundance and 

even local dominance in the Neenach and Pinnacles areas that is supportive of correlation.

TABLE 2. Modes of selected granitic-rock samples from the 

Neenach area, California

[All values in modal percent. Others: 0, opaque minerals; S, sphene. All samples 

were collected in August 1982 except samples BP-2, 590, 592, 595B, 600A, 617, and 

618, which are from Ross (1972a). See figure 20 for locations of samples]

Sample plagioclase K-feldspar Quartz Biotite Hornblende Others

Granodiorite of the Fai rraont Reservoir

Standard 

deviation.

Average (Ross, 

1972a).

Grand averageStandard 

deviation.

Grand 

average. 

Standard 

deviation.

49 

48 

47 

48 

52 

43 

54 

46 

45 

52 

53 

48 

49 

3.4

51 

50

33 

50 

33 

53 

46 

50 

45 

51 

46 

50 

50 

43 

41 

46 

6.5

42 

42 

36 

40

45 

6.4

Felsic rocks intruding the

Grand average --

31 

20 

26 

15 

13 

52 

26

32 

29 

36 

30 

32

29

15 

13 

16 

18 

9 

21 

7 

19 

23 

7 

7 

13 

14 

5.6

10 

12

Granodiorite

27 

9 

25

9 

12 

13 

9 

14 

12 

9 

18 

16 

13 

7.1

20 

11 

25 

19

14 

6.6

yranodiorites

34

44 

41 

46 

36 

14 

36

28 

25 

31 

33 

29

33

26 

26 

27 

26 

27 

23.5 

26 

26 

21 

24 

23 

23 

25 

1.9

24 

24

of Burnt Peak

30 

29 

29 

18 

23 

21 

20 

24 

21 

26 

24 

26 

27 

25 

3.8

25 

26 

26 

26

25 

3.4

9 0.5 

11 1 

9 

8 

11 

9.5 2 

10 2 

8 

11 

15 1 

15 2 

12 3 

10.5 1 

2.4 1.1

11 2

11 2

7 3 

8 3 

7 6 

17 12 

17 5 

14 3 

11 11 

9 6 

13 5 

8 2 

11 6 

9 4 

11 5 

11 5 

3.5 3.0

6 6 

15 5 

7 4 

9 5

10 5 

3.7 2.7

of the Fairmont Reservoir and

34 

35 

33 

38 

50 

32 

37

34 

42 

30 

33 

35

36

1

1

.5 

1 

2 <1 

1 <1

6 

3 

3 

4 

4

2 <1

0.5(S)

Ko, s)

KO.S) 

<KO) 

1(0, S) 

KS) 

KO,S)

KS)

<KO) 

1(0, S)

KS)

.5

.5

2

1

<KS)

KS)

<KS)

<Ks)

<KS) 

<KS) 

<KS)

KS) 

KS)

2(S) 

<KS) 

<KS) 

<KS)

KS) 

KS)

2(S)

KS) 

KS)

 7(S)

Burnt Peak

<1(0) 

.5(0)

1(0, S)

Granite of Antelope Buttes

29 

33 

22 

28

36 

31 

39 

35

33 

34 

38 

35

2 

2 

1 

2 ..

The granitic rocks of the Neenach and Pinnacles 

areas are not petrographically unusual, but they 

do have some features that may increase their correlative possibilities. Small salmon-pink K-feldspar 

phenocrysts, as much as 2 cm long, are noteworthy 

in the Bickmore Canyon, Burnt Peak, and Fairmont Reservoir bodies and are present, but less 

common, in the Johnson Canyon mass. More distinctive may be the subhedral to euhedral sphene 

crystals that contain opaque-mineral inclusions in 

intriguing shapes that resemble arabic or runic 

written characters. These distinctive sphene crystals are present in the Johnson Canyon, Burnt 

Peak, and Fairmont Reservoir bodies. By itself, the 

occurrence of these crystals is probably only of 

small correlative merit because I have seen similar 

inclusion-laden sphene crystals in other granitic 

bodies. Nevertheless, the feature is sufficiently uncommon to support the correlation of these otherwise-compatible bodies. Metallic opaque minerals are also rare or absent in many samples of 

granitic rocks from these two areas that contain 

abundant biotite and hornblende, although opaque 

minerals would be expected.

The chemical analyses of selected specimens 

from the Fairmont Reservoir and Johnson Canyon 

bodies are much alike (fig. 22, table 3). Two samples 

of the Johnson Canyon mass contain more FeO and 

MgO than the other samples, indicating the higher 

content of mafic minerals in that mass. The other 

two samples of the Johnson Canyon body, however, 

are similar in their contents of those two oxides to 

the Fairmont Reservoir samples.

The Bickmore Canyon samples are similar in oxide content to the one felsic rock sample from the 

Fairmont Reservoir mass. The two sets of comparisons (the Fairmont Reservoir with the Johnson 

Canyon and the Bickmore Canyon with the Fairmont Reservoir felsic variant rocks) are by no 

means conclusive for correlation, but the data certainly are compatible and permit correlation. Similarly, a comparison of the modes of the analyzed 

specimens (fig. 23) clearly shows a grouping of the 

two pairs of bodies, which supports correlation.

Three selected ternary plots also accentuate the 

compatibility of the tonalite and granodiorite of 

Johnson Canyon with the granodiorite of Fairmont 

Reservoir, and the compatibility of the granite of 

Bickmore Canyon with the felsic variant of the 

Fairmont Reservoir body (fig. 24). The Alk-F-M diagram shows the higher alkali content of the granite 

of Bickmore Canyon and the felsic variant of the 

Fairmont Reservoir mass relative to the two granodiorite bodies. Both of the normative-mineral
34 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

Other Other

(Biotite) granite of 

Bickmore Canyon

(Hornbtende-biotite)

tonalite and granodiorite

of Johnson Canyon

K-feldspar

K-feldspar

rzx

65 35 10 Plagioclase K-feldspar 65 35 10 Plagioclase

Other Other

(Hornblende-biotite)

grenodiorite of

Burnt Peak

(Biotite) granodiorite 

of Fairmont Reservoir

\ \

65 35 10 Plegioclase K-feldspar 10 Plagioclase

Other

9 (Number of superposed samples)

Granite of Antelope Buttes (0) 

and related felsic rocks (A) 

that intrude the granodiorites 

of Fairmont Reservoir and 

Burnt Peak

/ 7

K-feldspar 90 65 35 10 Plegioclese K-feldspar

'/rocks Canyon \ \

]*- ^ I ,Johnson\Canyon

,~\J---. L -VF»irmFairmont Reservoir 

Burnt Peak

10 Plagioclaae

FIGURE 21. Modes of granitic rocks of the Pinnacles and Neenach areas. A, Modal plots of granitic rocks of the Pinnacles area. 

B, Modal plots of granitic rocks of the Neenach area. C, Comparison of modal fields of granitic rocks of the Pinnacles and 

Neenach areas.
SUPPLEMENTAL INFORMATION 35

* 5

?

-

[U,

PL

Fairmont 

Reservoir

Johnson 

Canyon

Bickmore 

Canyon

Felsic variant of 

Fairmont Reservoir

FIGURE 22. Comparisons of chemical analyses of samples of 

the tonalite and granodiorite of Johnson Canyon with those 

of samples of the granodiorite of Fairmont Reservoir, and of 

chemical analyses of one sample of the felsic variant of the 

Fairmont Reservoir body with those of samples of the granite 

of Bickmore Canyon.

^ g s. £* -

C 

R i

00 0)

CM *3- t ( CM CO CO

00 ' C\J ID in r^ LD 

T I i I CO CO i I V

i T i r-«. c\j r*^. ID

' O r-» CM O in 

r^vOt «CMi icororo f-^ v

iniOi-ICMt-l^3-COCM

COf^t ICMt

CMI-~t-<CM,

oo o o cri in

COl£>t-HCOCM^3-COCV)

CM oo 00 oo if> ID

l£)l£)i ICMr ICOCOCO

mint ICMCM^-COCO

CM^Dt-HCOCMincOCM

i ll£)r-ICOCMinCO, 1 , I
36 POSSIBLE CORRELATIONS OF BASEMENT ROCKS ACROSS FAULTS IN CALIFORNIA

plots, Q-Or-(Ab+An) and Or-Ab-An, clearly show 

grouping and separation of the two pairs of bodies. 

The ternary diagrams accentuate the picture of 

chemical compatibility that was evident in the histogram (fig. 22).

The trace-element abundances for the chemically analyzed samples are plotted here on a histogram for easy reference and comparison (fig. 25). 

The data were derived from semiquantitative spectroscopic analyses. The data plotted in figure 25 

are arbitrary midpoints of the steps shown, from 

which the actual concentration may vary by one 

or two steps. The comparison of values between 

the Johnson Canyon and Fairmont Reservoir 

masses clearly shows similarity. Levels of nickel 

(Ni) and, to a lesser extent, scandium (Sc) and chromium (Cr) are somewhat higher in the Johnson 

Canyon samples. This relatively high concentration probably reflects the generally higher maficmineral content of the Johnson Canyon mass Ni, 

Sc, and Cr are all relatively concentrated in mafic 

minerals. Expectably, the samples from the Bickmore Canyon body and the one felsic sample from 

the Fairmont Reservoir area are generally about 

the same, or somewhat lower, in most trace elements than are the granodiorite samples. Exceptions are barium, which is slightly higher in the 

granites (probably because they contain relatively 

more K-feldspar), and lead, which is also slightly 

higher in the granite specimens. Such differences of one or, at most, two steps are probably 

not significant in semiquantitative spectroscopic

Other

10 

v 2(Number of superposed samples)

a

EXPLANATION

o Johnson Canyon 

o Fairmont Reservoir

 Fairmont Reservoir 

(felsic)

Bickmore Canyon

K-feldspar 90 65 35 10 Plagioclase

FIGURE 23. Modes of chemically analyzed samples of the 

granodiorite of Fairmont Reservoir, the granite of Bickmore Canyon, the tonalite and granodiorite of Johnson 

Canyon, and the felsic variant of the Fairmont Reservoir 

body.

FIFeO+2Fe2O3 +Mr»O)

EXPLANATION

o Johnson Canyon 

o Fairmont Reservoir

 Fairmont Reservoir 

(felsic)

A Bickmore Canyon

50 50

Alk(Na,O+K,O) M(MgO)

50 Ab+An

FIGURE 24. Chemical compatibility of the tonalite and granodiorite of Johnson Canyon with the granodiorite of Fairmont Reservoir, and of the granite of Bickmore Canyon with 

the felsic variant of the Fairmont Reservoir body.
SUPPLEMENTAL INFORMATION 37

CONCENTRATION, IN PARTS PER MILLION 

o 

o ooooo o 

.» ooooooo ooooo o ' o moo ooom oo ooo m oooo o

! ! 1 1 1 1 II i i 1 MINI INN!

J

R Zr

a

 n

a 

i 1 1

Yb

Y

i i i innj

r i i V

ffl

CD 

rfl

t-r-H

Fairmont -~i i 

Reservoir f ^

Johnson 

Canyon x,-, ,-R

Sr

Sc

Pb

Ni

- n

B a Nb

1 1 1

|~ I ""]

CD La

LJ -1

B a Cu

a cj

a

i i i

 ZJZZJ

B n

m

Cr

Co

Be

Fairmont Reservoir i | I JM, Fels'c 

^variant 

Bickmore Canyon i i B 

Johnson Canyon i i i r i

1 1 1 1 1 1 M II 1 1 1 1 1 1 1 1 1 1 1 1

oooooooooooo-jo^-;,-;,-; 0 ,-; 0000 °°°°°°°°666o d

o .00 oooo 

° CONCENTRATION, IN WEIGHT PERCENT

FIGURE 25. Histograms showing semiquantitative-spectroscopic-analysis "midpoints" of trace-element concentrations 

in samples of the tonalite and granodiorite of Johnson Canyon, 

the granodiorite of the Fairmont Reservoir, the granite of 

Bickmore Canyon, and the felsic variant of the Fairmont Reservoir body.

data. Nevertheless, comparison of the trace-element concentrations in the Johnson Canyon and 

Fairmont Reservoir masses, and of the Bickmore 

Canyon mass with felsic specimens from the Fairmont Reservoir area, supports the contention that 

these two pairs are correlative.

Sparse Rb/Sr data suggest compatibility between 

the granitic rocks of the Neenach and Pinnacles 

areas, although, clearly, more work is needed. 

Three samples from the Johnson Canyon body and 

one from the Bickmore Canyon unit give an initial 

strontium-isotopic ratio of 0.7082 (R. W. Kistler, in 

Ross, 1972b). A single sample from the Fairmont 

Reservoir mass gives an initial strontium-isotopic 

ratio of 0.7083 (Kistler and others, 1973).

Dikes of Tertiary volcanic rocks that are almost 

certainly satellitic to the main intrusive center of 

the Pinnacles Volcanic Formation are abundant 

and widespread in granitic rocks of the Pinnacles 

area. The relation leaves little doubt that the volcanic rocks intruded the surrounding granitic 

rocks. By contrast, I have seen no volcanic dikes 

in granitic rocks of the Neenach area. I do not know 

the reason for this anomaly. Could it be caused by 

structural discordance between the volcanic and 

granitic rocks of the Neenach area? The pattern 

that is apparent on the map (fig. 20C) does not 

suggest an abrupt break between the two rock 

types. However, exposures in this area are very 

poor for example, most of the area underlain by 

granitic rocks and surrounded by volcanic rocks 

near the west end of the Neenach map area is a 

low rolling grassy slope with scattered felsic granitic rock fragments and a small exposure of the 

granodiorite of Burnt Peak at its south tip. Matthews (1973) noted that poor exposures hindered 

delineation of the volcanic and granitic units, and 

he resorted to showing vertical fault contacts on a 

cross section. This expedient fails to solve the problem; it merely points out that a problem exists. My 

reconnaissance work with the granitic rocks suggests that they are relatively cohesive and that 

the faults shown within them (Dibblee, 1967) do not 

seriously disrupt the granitic rocks. Nevertheless, 

the question of why volcanic dikes are absent in 

the granitic rocks of the Neenach area needs an 

answer!

In conclusion, the granitic rocks near the Pinnacles Volcanic Formation are compatible, both 

modally and chemically, with the granitic rocks 

near the Neenach Volcanic Formation. The expectable uncertainties of correlating granitic masses 

persist, but the further work suggested in this report may reduce these uncertainties considerably.
GPO 587-OW10016