The Rinconada and Related Faults in 

the Southern Coast Ranges, California, 

and Their Tectonic Significance

GEOLOGICAL SURVEY PROF-ESSIONAL PAPER 981
The Rinconada and Related Faults in 

the Southern Coast Ranges, California, 

and Their Tectonic Significance

By THOMAS W. DIBBLEE, JR.

GEOLOGICAL SURVEY PROFESSIONAL PAPER 981

A study of the Rinconada fault and its 

relation to other nearby major faults 

and to the tectonics of the southern 

Coast Ranges

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Sun-tan

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Dibblee, Thomas Wilson

The Rinconada and related faults in the southern coast ranges, California, and their tectonic significance.

(Geological Survey Professional Paper 981)

Bibliography: p. 53-55.

Supt. of Docs, no.: I 19.16:981

1. Faults (Geology) California Coast Range. I. Title: The Rinconada and related faults in the southern coast ranges,

California. . . II. Series: United States Geological Survey Professional Paper 981. 

QE606.5.U6D5 551.8'7 76-608324

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-02908-1
CONTENTS

Page

Abstract __________________________________________________ 1

Introduction ____________________________________________ 1

Location of the Rinconada fault __________________ 1

Purpose and scope _______________________________ 2

Acknowledgments-_______________________________ 2

Major geologic relations of the Rinconada fault ______________ 2

Nomenclature and definition__________________________ 2

Regional relations _______________________________ 6

Concealed position of southwestern border of the Salinian

block__________________________________ 8 

Rock units ________________________________________ 9

Crystalline rocks ______________________________ 9

Franciscan rocks __________________________________ 9

Upper Jurassic(?) and Cretaceous marine sedimentary sequence _______________________________________ 12

Upper Cretaceous and lower Tertiary marine sedimentary 

sequence ___________________________________ 12

Middle Tertiary sedimentary sequence ______________ 13

Upper Tertiary sedimentary sequence ________________ 16

Valley sediments ____________________________ 16

Surficial deposits ______________________________ 19

Segments of the Rinconada fault __________________________ 19

Rinconada fault (Nacimiento segment) from San Rafael 

Mountains to Pozo area ___------._______________-_.____ 19

Other faults near and possibly related to the Nacimiento 

segment of the Rinconada fault ____________________ 20

Rinconada fault (Rinconada segment) from Rinconada mine

to Paso Robles _________________________ 21 

Rinconada fault (San Marcos segment) from Paso Robles to 

San Antonio Reservoir ____________________________ 23

Rinconada fault (Espinosa segment) from San Antonio Reservoir to Reliz Canyon ______________________ 25

Page

Segments of the Rinconada fault Continued

Relationship of Rinconada fault to Jolon and related 

faults __________________ _ 29

Geophysical expression of the Rinconada fault ____ ____ 32

Reliz fault ________________________________________________ 32

King City fault ______________________________________ 32

Reliz fault in Arroyo Seco area________________________ 33

Reliz fault at base of Sierra de Salinas __________________ 34

Possible northwestward extension of the Reliz fault _____ 34

Regional evidence of large strike-slip movement on the Rinconada-Reliz fault zone ____________________________ 36

Movements in Quaternary time _______________________ 36

Movements in Pliocene time _________________________ 36

Movements since early Miocene time ____________________ 36

Movements in Oligocenel?) time ___________________ 38

Possible movements in pre-Late Cretaceous time ______ 38

Strike-slip movement as suggested by configuration of 

buried surface of basement complex ________________ 40

Regional tectonics along and near the Rinconada-Reliz fault

zone ________________________-_-_--__-__--_--____---- 40

Concepts of tectonic evolvement of the Coast Ranges ____ 40

Tectonic pattern _______________________________ 42

Tectonic features of the east strip ____________________ 44

Tectonic features of the west strip _________________ 44

Regional tectonic analysis ______________________________ 46

Regional significance of the Rinconada-Reliz fault zone _ 48 

Effect of Rinconada-Reliz fault zone on physiography and drainage ____________________________________ 52 

Possible seismic activity___________________________ 52

References cited _________________________________ 53

ILLUSTRATIONS

FIGURE 1.

2.

3.

4.

5.

6. 

7-21.

Page

Index map showing positions of the Rinconada and Reliz faults relative to the Salinian block and the San Andreas

fault ______________________________________________________________ 2

Map showing physiographic features and drainages along and near the Rinconada and Reliz faults ____ __ __ 3

Map showing nomenclature of some major faults in part of southern Coast Ranges ____ ____________ _ 5

Map showing geologic setting of the Rinconada and Reliz faults ______________________________________________ 7

Diagram showing major rock units of the Coastal block within area of figures 1, 2 and 4 _____ ________________ 10

Diagram showing major rock units of the Salinian block within area of figures 1, 2 and 4__-__-__-_____ __ ---_ 11

Maps showing:

7. Geology along and near the Rinconada fault (Nacimiento segment) from Big Pine Mountain to Timber

Peak _____________________________________________________________ 17

8. Geology along and near the Rinconada fault (Nacimiento segment) from Timber Peak to Pozo ____________ 18

9. Geology along and near the Rinconada fault (Rinconada segment) from the Rinconada mine to Paso Robles ____ 22

10. Geology along and near the Rinconada fault (San Marcos segment) from Paso Robles to San Antonio Reservoir ______________________________________________________________________________________________ 24

11. Stream-channels deflected possibly by right-lateral movement on the Rinconada fault (San Marcos segment)

northwest of Paso Robles ________________________________________________ ____ __ __ 26

12. Geology along and near the Rinconada fault (Espinosa segment) between Lockwood and Salinas Valleys __ 27

in
IV CONTENTS

FIGURES 7-21. Maps showing Continued

Page

13. Geology along and near the Rinconada fault (Espinosa segment) and Reliz fault, near King City and

Greenfield __________________________________________________________________________________ 28

14. Stream channels of canyons deflected possibly by right-lateral movement of the Rinconada fault (Espinosa

segment) near Lockwood ____________________________________________________________________ 30

15. Geology of the northern Santa Lucia Mountains, a segment of the Salinian block elevated on the Reliz fault

and many other parallel faults ____-___-_____-___-______________-______________---_----_-____-___ 35

16. Inferred displacement of lower Pliocene (unit 1, Pancho Rico Formation) and upper Miocene (unit 2, Santa

Margarita Formation) by right lateral strike-slip movement on the Rinconada fault______________ 37

17. Pre-Oligocene rocks of the Salinian block, possibly displaced by right-lateral movement of the RinconadaReliz fault zone _______________________________________________------_---__-____-___________ 39

18. Inferred configuration of buried surface of crystalline basement complex along Rinconada-Reliz fault zone

from Paso Robles to Greenfield ________________________________________ 41

19. Right-slip tectonics and compressive upheaval along the Espinosa segment of the Rinconada fault ___ __ 42

20. Major tectonic features of the southern Coast Range region in vicinity of the Rinconada-Reliz fault zone __ 43

21. Geology of the upper Nacimiento River area and vicinity ___________________________ _ 47

22. Block diagram illustrating mechanics of conversion of strike-slip movement along a high angle fault into compressive

uplift of one block as fault terminates ____--_---_-__-______-_____-__-__-________________________________ 43

23. Map showing inferred tectonic movements in vicinity of northern Santa Lucia Mountains showing conversion of 

right-slip movements near ends of Rinconada-Reliz fault zone and San Gregorio fault into crustal compression 

and uplift of intervening area to form these mountains ________________________________________________ 50

24. Map showing inferred tectonic movements in vicinity of Sierra Madre Mountains and Pine Mountain, showing 

inferred conversion of right-slip movements near ends of the Rinconada and San Gabriel faults to crustal compression to form these ranges ______________________________________________-_--_-__-__-___-------_-____ 51
THE RINCONADA AND RELATED FAULTS

IN THE SOUTHERN COAST RANGES, 

CALIFORNIA, AND THEIR TECTONIC SIGNIFICANCE

By THOMAS W. DIBBLEE, JR.

ABSTRACT

The Rinconada fault near Santa Margarita, as depicted on the 

Geologic Map of California, is a northwest-trending high-angle fault 

about 40 km (26 mi) long that in part separates terranes with different basement complexes. The terrane on the northeast is part of the 

Salinian block, which is composed of a Mesozoic and older crystalline 

basement complex of plutonic and metamorphic rocks, overlain unconformably by a thick Upper Cretaceous and lower Tertiary marine 

sedimentary sequence. In contrast, the terrane on the southwest is 

part of the Coastal block, composed of a basement complex of Franciscan eugeosynclinal rocks overlain depositionally or tectonically by 

a thick Cretaceous marine sedimentary sequence. These blocks are 

separated by the Sur-Nacimiento fault zone to the northwest.

Southeastward from Santa Margarita the Rinconada fault extends 

continuously into a fault formerly thought to be the Nacimiento fault 

across the Cuyama River gorge to intersect the Big Pine fault in the 

San Rafael Mountains. Northwestward, the Rinconada fault passes 

concealed near Paso Robles into high-angle faults, locally called the 

San Marcos and Espinosa faults, nearly to Reliz Canyon west of King 

City, and is within the Salinian block. All these alined faults are now 

mapped as parts of a major fault 250 km (160 mi) long. The name 

Rinconada is applied to the entire fault. It is 3 km (2 mi) from the 

alined Reliz fault to the northwest along the northeastern base of 

Sierra de Salinas.

The Rinconada fault, as herein defined, is nearly parallel to and is 

located about 34 km (22 mi) southwest of the San Andreas fault. 

Southeastward from Santa Margarita the Rinconada fault presumably coincides with the southwestern boundary of the Salinian block, 

but northwestward from that town it extends into this block.

The Coastal and Salinian blocks were juxtaposed along the SurNacimiento fault zone, possibly by subduction, probably in Late Cretaceous or possibly early Tertiary time. The Rinconada-Reliz fault 

zone formed largely within the Salinian block in late Cenozoic time, 

although the southern part of the Rinconada fault followed the 

former Sur-Nacimiento fault zone. The Rinconada-Reliz fault zone is 

not presently active.

Major movement on the Rinconada fault is interpreted to have 

been right lateral strike slip. This is suggested by numerous severely 

compressed drag folds in the sedimentary rocks along and near the 

fault and the persistent slightly more east-west trend of their axes as 

compared to the northwest trend of the fault. This movement is also 

suggested by displaced stratigraphic units; those of late Miocene and 

early Pliocene ages are about 18 km (11 mi) apart near Paso Robles, 

and those of Late Cretaceous and early Tertiary age nearly 60 km 

(40 mi) apart.

Movement on the Reliz fault is in part right lateral, as suggested 

by structural relations similar to those along the Rinconada fault, 

and in part vertical, having elevated the southwest block to form the 

Sierra de Salinas uplift.

The northern part of the strip within the Salinian block east of the 

Rinconada-Reliz fault zone reacted to stress during late Cenozoic 

time as a rigid block of crystalline basement that was tilted slightly 

southwestward from an axis adjacent to the San Andreas fault to 

form the Gabilan uplift and Salinas Valley. The southern part of this 

east strip, where the basement complex deepens southward under a 

thick sedimentary cover, was compressed to form the La Panza, 

Caliente, and Sierra Madre Ranges.

In contrast, the strip that includes part of the Salinian and Coastal 

blocks west of the Rinconada-Reliz fault zone yielded to stress during 

late Cenozoic time as a series of parallel compressive uplifts and is 

deformed throughout to form the San Rafael and Santa Lucia Mountains and related hills. The northern part of this west strip, despite 

its rigid crystalline basement, was upheaved by severe compression 

to form the high northern Santa Lucia Mountains. The crustal shortening involved in this part, compared to nearly none on the east 

strip, presumably has absorbed the large amount of right-lateral 

movement on the fault zone as it dies out northwestward near 

Salinas.

The Rinconada fault and other geologic features were probably 

displaced about 14 km (9 mi) by left slip on the east-trending Big 

Pine fault. The Pine Mountain fault north of Sespe Creek may be the 

reactivated displaced counterpart of the Rinconada fault south of the 

Big Pine fault.

INTRODUCTION

LOCATION OF THE 

RINCONADA FAULT

Mapping of the regional geology of the southern 

Coast Ranges of California within 46 km (30 mi) west 

of the San Andreas fault has revealed the presence of a 

major high-angle fault, traceable for some 250 km (160 

mi) or possibly 290 km (190 mi), nearly parallel to and 

about 34 km (22 mi) southwest of the San Andreas 

fault (fig. 1). Various segments of this parallel major 

fault have been mapped at different times, by different 

workers, who did not recognize their continuity, and so 

each segment was designated by a local name.

The present investigation reveals that all or most of 

these segments are parts of a nearly continuous single 

major fault, designated herein as the Rinconada fault 

extending from the San Rafael Mountains on the 

southeast into the hills west of King City. The physiographic features of this and associated major faults are 

shown in figure 2.
120

119

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FIGURE 1._Positions of the Rinconada and Reliz faults relative to the Salinian block and the San Andreas fault. Dashed outlines indicate locations 

8, 9, 10, 12, 13, 15, and 21 showing geology along Rinconada and Reliz faults.
INTRODUCTION
RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

PURPOSE AND SCOPE

The purpose of this report is to provide data that 

suggest (1) the continuity of the Rinconada fault as 

herein designated; (2) the direction, magnitude, and 

time of movement; (3) the relation of this fault to other 

nearby major faults and to folding; (4) the relation of 

this fault to tectonics of the southern Coast Ranges; 

and (5) the relation of this fault to the physiography 

and drainage of this area.

This report is based on geologic mapping in the 

southern Coast Ranges southwest of the San Andreas 

fault since 1965 for the U.S. Geological Survey as part 

of its National Earthquake Research Project. The geology of the area southeastward from Paso Robles is 

largely from Dibblee (1972a, 1972b, 1974a, 1974c); 

Vedder (1970); Vedder, Gower, Clifton, and Durham 

(1967); Vedder and Brown (1968); Hall and Corbato 

(1967); Page (1970a, 1970b); and Vedder, Dibblee, and 

Brown (1974). The geology of the area from Paso 

Robles to Monterey Bay was modified from mapping by 

Trask (1926); Fiedler (1944); Reiche (1937); Bowen 

(1965); Gilbert (1971 thesis); Durham (1963, 1964, 

1965a, 1966, 1968a, 1968b, 1970); Compton (1966a, 

1966b, and unpublished data); Page (1970a, 1970b, 

1972); Gilbert and Dickinson (1970); and from additional mapping, remapping, and field checking 

(Dibblee, 1971, 1974b).

This report is a more comprehensive description of 

the geology, movements, and tectonic analysis of this 

important fault zone first recognized and discussed in a 

preliminary paper (Dibblee, 1972a).

ACKNOWLEDGMENTS

I am indebted to B. M. Page, Stanford University, for 

loan of his detailed field mapping north of San Luis 

Obispo; E. M. Hart, California Division of Mines and 

Geology, for his detailed mapping of the AtascaderoSanta Margarita area; and C. A. Hall, University of 

California, Los Angeles, for his detailed mapping of the 

Morro Bay-Cambria area northwest of San Luis 

Obispo. Discussions with these workers, and also with 

R. R. Thorup of Monterey and O. E. Bowen of Corralitos, have been very helpful. I acknowledge T. H. 

Nilsen and J. G. Vedder of the U.S. Geological Survey 

for critical review of the manuscript.

MAJOR GEOLOGIC RELATIONS OF 

THE RINCONADA FAULT

NOMENCLATURE AND DEFINITION

The Rinconada fault is depicted on the geologic map 

of California (Jennings, 1959) as a northwest-trending 

fault extending from the head of Rinconada Creek near 

Santa Margarita for 40 km (25 mi) northwest toward

Paso Robles (figs. 2, 3A). This fault is nearly vertical 

and in the vicinity of Rinconada Creek separates an 

area in which Mesozoic granitic basement is extensively exposed in the northern La Panza Range on the 

northeast from an area in which Mesozoic Franciscan 

basement is widespread in the southern Santa Lucia 

Mountains on the southwest. As depicted thereon 

(Jennings, 1959), and verified by mapping I have done, 

the Rinconada fault is continuous southeastward into 

the fault formerly thought to be the southeast extension of the Nacimiento fault that crosses Cuyama 

River gorge to intersect the Big Pine fault in the San 

Rafael Mountains 150 km (90 mi) southeast of Santa 

Margarita (fig. 3A), and so both are parts of one major 

high-angle fault (figs. 1, 2).

Mapping northwestward from Paso Robles indicates 

that the San Marcos and Espinosa faults depicted on 

the Geologic Map of California (Jennings 1959; Jennings and Strand, 1958) are also parts of a major 

high-angle fault and are alined and probably continuous with the Rinconada fault (fig. 3A). This condition 

suggests that all three of these segments also may be 

parts of one major fault, although the part near Paso 

Robles between the Rinconada and San Marcos segments is concealed for about 5 km (3 mi).

I conclude that the Nacimiento fault across Cuyama 

River gorge, the Rinconada, San Marcos, and Espinosa 

faults (fig. 3A) are local segments of one continuous 

major high-angle fault traceable for some 250 km (160 

mi) within the southern Coast Ranges of California 

(fig. 1). In order to simplify this complex nomenclature, 

it is proposed to apply one name to this combined major 

fault as shown in figure 3B as well as in figures 1 and 2. 

The local names shown in figure 3A are retained for 

the segments, but the fault as a whole is herein designated as the Rinconada fault. The name Rinconada is 

adopted because the segment shown as Rinconada 

fault (Jennings, 1959) is the one along which the crystalline basement and Franciscan basement are almost 

adjacent at the surface. This segment is designated as 

the type segment.

The Rinconada fault is defined herein as a highangle fault that extends from its intersection with the 

Big Pine fault near Big Pine Mountain in the San 

Rafael Mountains northwestward for some 250 km 

(160 mi) to a point where its surface trace dies out in 

the hills 11 km (7 mi) west of King City (fig. 35). Relative vertical displacements are upward on one side 

along some parts and on the opposite side along others. 

These may be only apparent or oblique slip; the major 

component may be strike slip.

There are few places where this fault is clearly exposed, but its type locality is designated at the low pass 

near the Rinconada mine, a mercury mine located
122° 

Reverse or thrust fault (sa

All other faults 

35'

FIGURE 3. Nomenclature of some major faults in part of southern Coast Ranges. A, As shown on Geologic Map of California (Jennings and Strand

1959). B, Major faults recognized and nomenclature proposed in this report.
RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

about 17 km (10 mi) southeast of Santa Margarita 

(figs. 2, 9).

About 3 km (2 mi) northwest of the point where the 

Rinconada fault dies out west of King City, the Reliz 

fault (figs. 2, 3B) is directly alined with it and extends 

along the base of the steep northeast front of Sierra de 

Salinas of the northern Santa Lucia Mountains for 

about 46 km (30 mi), then dies out northwestward (figs. 

1, 3A, 3B).

The Rinconada and Reliz faults together form a zone 

some 300 km (190 mi) long, herein designated as the 

Rinconada-Reliz fault zone. As shown in figures 1, 2, 

and 3B , this zone is nearly parallel to the San Andreas 

fault to the northeast. This parallelism, together with 

evidence of strike-slip movements, suggests that this 

zone is part of the San Andreas fault system.

REGIONAL RELATIONS

Southeastward from Santa Margarita the Rinconada 

fault as defined herein presumably forms the southwestern boundary of the Salinian block (Salinia of 

Reed, 1933, p. 12, 30), which has a crystalline basement complex of plutonic and metamorphic rocks (fig. 

1). The block on the southwest, designated by Reed 

(1933, p. 30) as the Central Franciscan area, will be 

herein referred to informally as the Coastal block, 

which lies west of the Salinian block and has a noncrystalline eugeosynclinal basement complex composed of Franciscan rocks. Northwestward from the 

Rinconada mine the Rinconada fault veers northward 

into the Salinian block.

The boundary between the two geologic blocks has 

long been regarded as the Nacimiento fault zone 

(Reed, 1933, p. 32; Taliaferro, 1943a, p. 153) or SurNacimiento fault zone (Page, 1970a, 1970b). Accordingly, the name Nacimiento has been applied to more 

than one mappable fault along this boundary and the 

faults were thought or presumed to be continuous, at 

least in the subsurface. The originally named Nacimiento fault was mapped for about 40 km (25 mi) near 

the Nacimiento River by Taliaferro and his students in 

the 1930's (Jennings, 1959) and later in more detail by 

Page (1970a, 1970b). Far to the southeast, the fault 

along the southwest border of Cuyama Valley was also 

called the Nacimiento fault by Eaton (1939), Eaton, 

Grant, and Alien (1941), and Hill, Carlson, and 

Dibblee (1958). Later it was called the South Cuyama 

fault (Jennings and Strand, 1969), and the name 

Nacimiento was applied to the fault a few kilometres 

southwest that crosses the Cuyama River gorge, on the 

assumption that this fault (1) forms part of the southwestern boundary of Salinia and (2) extends into the 

original Nacimiento fault near the Nacimiento River, 

at least in the subsurface (Dibblee, 1966, p. 64; Jennings, 1959; Jennings and Strand, 1969; Vedder and 

others, 1967; Vedder and Brown, 1968).

The first of these assumptions is presumably correct, 

but the second is questionable. The southern Nacimiento fault that crosses Cuyama River gorge is generally interpreted (Jennings, 1959) to extend northwestward from the Rinconada mine (fig. 9) via a complex 

group of faults and cross-faults along and within the 

northeast slope of the Santa Lucia Mountains for some 

50 km (30 mi) into the original northern Nacimiento 

fault west of the Nacimiento River (fig. 3A); and it is 

assumed that this alinement of faults is underlain by 

the concealed boundary between Franciscan rocks of 

the Coastal block and the crystalline basement complex of the Salinian block.

However, the faults, including those that involve 

Franciscan rocks, in the northeastern margin of the 

Santa Lucia Mountains veer westward to become 

internal faults in these mountains and in the Coastal 

block (fig. 4); there is no fault or fault zone that is 

continuous from Santa Margarita to the northern 

Nacimiento fault (fig. 4), and the position of the concealed boundary between the basement rocks of the 

Salinian and Coastal blocks through that area is not 

known. The two Nacimiento faults are thereby separated by a gap of about 48 km (30 mi; fig. 4).

The southern Nacimiento fault is continuous into 

the Rinconada fault and is not part of the original 

northern Nacimiento fault at the surface, even though 

both Nacimiento faults form parts of the boundary between the Salinian and Coastal blocks. Therefore, if 

this boundary were once a single fault plane throughout its extent, there may have been no movement on 

the segment concealed under the Cenozoic sedimentary cover since middle or possibly early Tertiary time.

This boundary between the Salinian block of crystalline basement and the Coastal block of Franciscan 

basement in this area was designated by Page (1970a, 

p. 667, fig. 1) as the Sur-Nacimiento fault zone, which 

"extends southeasterly from Point Sur, crosses the 

Santa Lucia Range obiquely, and reaches the Transverse Ranges of southern California." This definition is 

accepted.

In order to avoid confusion, the name Nacimiento 

should be applied only to the fault originally mapped 

as the Nacimiento fault near the Nacimiento River 

(fig. 4). It is illogical and confusing in my opinion to 

apply this name to any other fault into which that fault 

cannot be mapped. Therefore, it is incorrect to apply 

this name to the southern Nacimiento fault southeastward from Santa Margarita. Because that fault is 

the southeastward extension of the Rinconada fault. I 

consider it appropriate to apply the name Rinconada to 

that entire fault.
MAJOR GEOLOGIC RELATIONS OF THE RINCONADA FAULT

<A

O
RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

CONCEALED POSITION OF SOUTHWESTERN BORDER 

OF THE SALINIAN BLOCK

Between the Santa Margarita area, where Franciscan rocks west of the Rinconada fault are very near the 

granitic basement of the Salinian block on the east side 

of this fault, and the northern Santa Lucia Mountains 

about 25 km (15 mi) west of Jolon, where Franciscan 

rocks are in contact along the Nacimiento fault with 

the crystalline basement complex of the Salinian 

block, the position of the boundary between the Salinian and Franciscan rocks is not known because it is 

concealed beneath younger sedimentary rocks for a 

distance of about 100 km (62 mi). This concealed boundary could not be detected by gravity geophysical surveys, owing to lack of density contrast between these 

two basement complexes (Burch, 1971; Hanna, 1970) 

and its depth below sedimentary rocks.

Where the boundary between the Franciscan and 

crystalline basement is exposed in the northern Santa 

Lucia Mountains along the Nacimiento fault west of 

Jolon (fig. 21), the fault dips steeply northeast to vertical. On the northeastern block I found that the crystalline basement complex is overlain unconformably by 

the southward-dipping Upper Cretaceous and lower 

Tertiary sedimentary sequence. Southeastward this 

sequence conceals the crystalline basement complex on 

the Salinian block and is in contact along the fault 

with Franciscan rocks of the relatively elevated 

southwestern or Coastal block for 50 km (31 mi).

These conditions suggest, if not indicate, that the 

concealed boundary of the crystalline basement of the 

Salinian block with Franciscan rocks of the Coastal 

block is along the entire northern Nacimiento fault, 

as generally believed. However, Burch and Durham 

(1970, p. B13), Burch (1971, p. A12), and Durham 

(1974, p. 59) suggested that this boundary may be 

cross-faulted far to the east, to their postulated Jolon 

fault near the San Antonio River, because of gravity 

gradients in that area and almost none along the 

Nacimiento fault. This interpretation, although it 

cannot be ruled out, is considered improbable.

From the geologic relations described and as shown 

in figure 21, it is more probable that this concealed 

boundary continues southeastward along the length of 

the Nacimiento fault (fig. 1) from the segment along 

which it is exposed, or along the fault within 2 km 

(1.3 mi) northeast, because of the following structural 

evidence: Franciscan and overlying rocks exposed 

southwest of these faults in the central Santa Lucia 

Mountains are severly deformed throughout and elevated along these faults against the Upper Cretaceous 

and lower Tertiary sequence and the middle Tertiary

sedimentary sequence of the Nacimiento River area to 

the northeast, which are much less deformed except 

adjacent to the fault zone. These conditions suggest 

that the zone of weakness along which the Santa Lucia 

Mountains were elevated, including the Nacimiento 

fault zone, formed presumably along this concealed 

boundary and that the comparatively less deformed 

sedimentary sequences to the northeast are probably 

underlain by the crystalline basement complex that is 

comparatively more rigid than the Franciscan basement and more resistant to deformation.

There is no structural discordance either at the surface (fig. 21), or in the gravity pattern (Burch, 1971, 

pi. 1; Hanna, 1970, p. B72), to indicate that the boundary of the basement complex with Franciscan rocks is 

cross-faulted or deviated from its exposed position 

along the Nacimiento fault eastward to Lockwood Valley as suggested by Burch and Durham (1970) and 

Durham (1974). If it is, it would underlie the Upper 

Cretaceous and lower Tertiary sedimentary sequence, 

and if it is a fault contact, it would predate that sequence.

It is difficult to interpret the position of this concealed 

boundary from the geophysical data. Along the 

Nacimiento fault, including the segment along which the 

Franciscan and crystalline rocks are in contact at the 

surface west of Jolon, there is little if any gravity gradient, other than the regional one of progressively decreasing gravity values northeastward as shown by 

Burch (1971, pi. 1) and Hanna (1970, p. B72-B73). The 

very slight gradient at the segment along which Franciscan rocks are in contact with the thick Upper Cretaceous 

and lower Tertiary sedimentary sequence suggests that 

there is very little density contrast between this sequence, the Franciscan rocks, and the crystalline rocks. 

Only where one or all of these three rock units are overlain by thick middle and late Cenozoic sedimentary rocks 

of lower density, such as in the Lockwood Valley area, is 

there a defined gravity gradient. Burch (1971, p. B12), 

Hanna (1970, p. B74), and Loney (1970) suggested that 

the concealed Franciscan crystalline basement boundary may be slightly east of the Nacimiento fault or that 

the contact dips northeastward, but it is probably too 

deep to be detectable in any geophysical pattern.

An airborne magnetometer survey (Hanna, 1970, 

p. B68-B69) did not definitely detect the position of the 

concealed Franciscan-crystalline basement boundary, 

but magnetic variations are locally very strong where 

Franciscan rocks are exposed, especially over serpentine bodies, and are almost nonexistent eastward to the 

Rinconada fault. This condition suggests that the concealed boundary is near the easternmost exposures of 

Franciscan rocks, although Hanna (1970, p. B74)
ROCK UNITS 9

suggested that it may be within 8 km (5 mi) east of 

these exposures.

Beyond the southeast end of the Nacimiento fault 

(fig. 10), the position of this concealed boundary is a 

matter of speculation. Presumably it may continue to 

extend southeastward (fig. 1), possibly under the syncline axis north of the Las Tablas fault (fig. 10) to join 

the Rinconada fault near Atascadero (fig. 9).

Southeastward from Santa Margarita the concealed 

boundary of the crystalline basement of the Salinian 

block with Franciscan rocks is presumably along the 

Rinconada fault to and possibly beyond the Big Pine 

fault (fig. 1), because east of the Rinconada fault the 

thick Upper Cretaceous and lower Tertiary sedimentary sequence is underlain by the crystalline basement 

in the La Panza Range (figs. 4, 8, 9) and presumably 

throughout the extent of this sequence, whereas west 

of this fault the Cretaceous sedimentary sequence is 

underlain by Franciscan rocks and is comparatively 

more intensely deformed.

ROCK UNITS

The rock units of the Salinian and Coastal geologic 

blocks within the area of figures 1 and 2 are shown in 

figures 5 and 6. Their areal distribution is shown 

generalized in figure 4 and in more detail in figures 7, 

8, 9, 10, 12, 13, 15, and 21. The basement complexes of 

the two blocks are totally different. The overlying 

sedimentary sequences now recognized (Dibblee, 1973) 

are similar in general but different in detail.

CRYSTALLINE ROCKS

The crystalline basement complex (Dibblee, 1973) 

of the Salinian block is composed of granitic plutonic 

rocks that engulf pendants of metamorphic rocks 

(Wiebe, 1970a, 1970b; Compton, 1966b; Ross, 1972, 

1974, 1976). The granitic rocks range in composition 

from quartz diorite to quartz monzonite, with granodiorite predominant. They locally include small bodies 

of fine-grained or aplitic alaskite granite, and 

hornblende diorite or gabbro.

The more widespread plutonic rocks of the Santa 

Lucia and La Panza Mountains have been radiometrically dated (K-Ar method) at about 70 to 80 m.y. 

(million years) or Late Cretaceous (Curtis and others, 

1958; Evernden and Kistler, 1970). However, it is 

doubtful that this is the age of emplacement or crystallization of the rocks dated, because the overlying 

unmetamorphosed sedimentary rocks are also Late 

Cretaceous, from paleontologic evidence. Compton 

(1966b, p. 287) and Evernden and Kistler (1970, p. 22)

suggested that this may be the age of the earliest uplift 

since emplacement, or some other postemplacement 

event, and that the plutons were emplaced probably in 

mid-Cretaceous time.

The metamorphic rocks are sedimentary rocks regionally metamorphosed under high temperature and 

pressure. They are older than the plutonic rocks of 

Mesozoic age that engulf them. Two lithologic units 

are recognized. The most widespread is micaceous 

schist to gneiss of great thickness that includes many 

lenses of marble, hornfels, and feldspathic quartzite. 

These rocks are intricately intruded by plutonic rocks 

in most exposures. This metamorphic unit, originally 

designated the Sur Series by Trask (1926), is extensive 

in the northern Santa Lucia Mountains from the Big 

Sur area southeastward (Ross, 1976) and in most exposures dips steeply northeastward. It is present also in 

the Gabilan Mountains as small scattered pendants 

(Ross, 1972). It is similar to metasedimentary rocks of 

Paleozoic and early Mesozoic age in the western 

Mojave Desert and Tehachapi Mountains (Dibblee, 

1967). The less widespread metamorphic unit is more 

homogeneous and is composed almost entirely of 

mica-plagioclase-quartz schist. It is injected by plutonic rocks at its margins but not internally. These 

metamorphic rocks are exposed in much of the Sierra 

de Salinas, where they dip regionally southwest, and 

at the south end of the Gabilan Range (Ross, 1972, 

1974) just north of Greenfield.

FRANCISCAN ROCKS

Franciscan rocks (the Franciscan Complex of Berkland and others, 1972) form the basement complex of 

the Coastal block in which these rocks are extensively 

exposed in the central and southern Santa Lucia and 

southwestern San Rafael Mountains. This complex is 

composed of severely deformed slightly metamorphosed eugeosynclinal rocks characteristic of Franciscan rocks as described by Bailey, Irwin and Jones 

(1964). In the central Santa Lucia Mountains (vicinity 

of Alder Peak and northwestward, fig. 21) the Franciscan is composed of massive to bedded flyschlike hard 

graywacke sandstone and minor interbedded micaceous shale. Elsewhere the Franciscan in most places 

is composed of a melange (mixture) of numerous monolithic fragments of graywacke, greenstone, varicolored 

layered chert, jasper, and blue glaucophane schist in a 

pervasively sheared "matrix" of dark claystone. Small 

intrusions of ultramafic rocks such as pyroxenite and 

gabbro are locally present. Lenticular sill-like injections of serpentine or serpentinite are common within 

and at the top of the Franciscan complex, especially in 

the melange.
10 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

SEQUENCE

Surficial 

sediments*

.rv/x/x/x/xrx/x/x/x/x^

Valley 

sediments*

Middle

Tertiary 

sedimentary

sequence*

/x/xyxj^yx/x/x/x/x/xr

Cretaceous

(and Upper

Jurassic?) +

marine

sedimentary

sequence

Franciscan

Complex

FORMATION

Alluvium 

and older 

alluvium

\yxr\ja/x/x/x/x/x/x/x

Paso Robles 

Formation

vyx/x/xrxrx/xrx/x_/-\/x_

Santa Margarita 

Formation

Monterey Shale

Volcanic

rocks

Vaqueros Formation

Red beds, 

conglomerate

^x^x/x/x_/x/xrx/x/x/x^

Sandstone, 

shale, and 

conglomerate

Shale and

sandstone

Franciscan

rocks*

LITHOLOGY

AND

THICKNESS

0-1 50 ft

Terrestrial 

gravel 

0-1 200 ft

Marine 

sandstone 

600-5000 ft

Marine

siliceous

shale

600-5000 ft

Tuff, dacite, and

basalt, 0-2000 ft

Marine sandstone 

0-3200 ft

Terrestrial deposits 

0-600 ft

Marine deposits 

10,000± ft

Marine deposits

0-3000 ft

Basalt and chert

Graywacke,

shale.

chert, and

greenstone

AGE

Holocene 

and late 

Pleistocene

Pleistocene 

and 

Pliocene?

Late

Miocene

7

Middle

Miocene

 p 

Early

Miocene

 p 

Oligocene

Late 

Cretaceous

Early

Cretaceous

(and Late

Jurassic ?)

Cretaceous

and

Late

Jurassic

NARY

CC

LU

1-

D

a

 fV. 

CC

<

P

CC 

LU1-

CO

D

0

LU

O

K

LU

CC 

O

\

\

O \

w ^

W

CC

D -j

O

O 

N

O

LU 

O

O

0

N 

0

LU

'Regional unconformity at base 

^Injected by serpentine, mafic rocks 

+Ages in parentheses apply to only very 

small part of unit and only local

FIGURE 5. Major rock units of the Coastal block within area of figures 1, 2 and 4.
ROCK UNITS 11

SEQUENCE

Surf icial 

deposits*

S\J\J\J^S\J~\S\S\S\S\

Valley 

sediments*

/\/\/Ay\yx/\y\/\/\/\

Upper 

Tertiary

sedimentary 

sequence

Middle 

Tertiary 

sedimentary

sequence**

J\S\S\S\S\S\S\S\S\J~

Upper 

Cretaceous 

and lower 

Tertiary 

marine 

sedimentary 

sequence*

ru\s\s\s\s\s\s\s\s\

Crystalline 

basement

FORMATION

Alluvium and 

older alluvium

/x/\^\rx/x/\y\/x/x/\/\/\/x,

Paso Robles 

Formation

Morales 

Formationx

Quatal 

Formationx

Upper 

unitx

Pancho Rico

Formationx . Lower

unitx

Santa 

Margarita 

Formation

Monterey

Shale 

Branch Canyon 

Sandstonet

Basalt

Vaqueros 

Formation

Simmler and Berry 

Formations

U\J\S\S\J\TU\J^J\S\S\S\S\

Undifferentiated 

sandstone, 

shale, and 

conglomerate

s\j\j\j\s\j\s\j\j^u\j^s\s\.

Plutonic 

rocks

Metamorphic 

rocks

LITHOLOGY AND 

THICKNESS

0-300 ft

Terrestrial 

gravel, sand, and 

clay 0-3000 ft

Terrestrial 

gravel, sand, and 

clay 0-5000 ft

Terrestrial 

clay and sand 

0-1 000 ft

Marine sandstone 

0-700 ft

Marine 

mudstone, 

diatomite, and 

sandstone 

0-2000 ft

Marine 

sandstone 

0-2000 ft

Marine 

siliceous shale 

0-7000 ft

Marine sandstone 

0-3000 ft

Flows and dikes

Marine 

sandstone and 

shale 0-7000 ft

Terrestrial 

strata 

0-3000 ft

Marine 

strata 

25,000 ft

Granite to 

diorite and 

gabbro, 

mainly 

granodiorite

Marble, 

quartzite, 

schist, and gneiss

AGE

Holocene 

and late 

Pleistocene

Pleistocene 

and 

Pliocene

Early 

Pliocene

Late 

Miocene

Late, middle 

and early" 

Miocene

Middle 

Miocene

Early 

Miocene and 

late Oligocene

Oligocene

Eocene, 

Pa I eocene, 

and 

Late 

Cretaceous

Cretaceous 

and older(?)

Mesozoic 

or 

older

i

tL > 

PC 

£ < 3z

a 

 r^. 

>-

TERTIAF

CRETACEOUS TERTIARY AND 

a 

o

N

o

to

LU

2

ENOZOIC

O

MESOZOIC ANDCENOZOIC

* Regional unconformity at base

x. Unconformity at base along basin margins

f Intertongued with Monterey Shale

Only lowest part in few places of this age

Includes Caliente Formation, easternmost terrestrial equivalent

of all marine units of this sequence

FIGURE 6. Major rock units of the Salinian block within area of figures 1, 2 and 4.
12 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

The age of the Franciscan rocks of the Coastal block 

is inferred to range from latest Jurassic (Tithonian) to 

early Late Cretaceous (Turonian) by Bailey, Irwin, and 

Jones (1964). Near Stanley Mountain (fig. 6) these 

rocks yielded fossils considered diagnostic of Late 

Jurassic (Tithonian) age (Easton and Imlay, 1955). 

Shale samples from Franciscan rocks west of the 

Nacimiento River yielded spores and dinoflagellates 

inferred to be Early Cretaceous (W. R. Evitt, in Page, 

1970a, p. 674, 687).

A predominantly volcanic unit at the top of or above 

the Franciscan is exposed in two areas: one in the vicinity of Stanley Mountain (fig. 6) as mapped by Hall 

and Corbato (1967) who included it in the Franciscan, 

and the other north of San Luis Obispo (fig. 9) as 

mapped by Fairbanks (1904) and Page (1972). In both 

areas this unnamed unit is composed of basaltic to andesitic volcanic rocks partly altered to greenstone, as 

thick as 610 m (2,000 ft), associated intrusive diabase 

and peridotite partly serpentinized, and an interval up 

to 92 m (300 ft) thick of light-gray chert, and locally by 

a little shale and graywacke at the top. This igneous 

unit is excluded from the Franciscan by Taliaferro 

(1943a), Gilbert and Dickinson (1970), and Page 

(1972). It is now regarded as the ophiolite-ultramafic 

succession of igneous rocks as recognized by Bailey, 

Blake, and Jones (1970). The age of this volcanic unit 

is either latest Jurassic or earliest Cretaceous. In both 

areas this unit may be in fault contact on top of Franciscan rocks or serpentine and is overlain by the lower 

unit (shale) of the Cretaceous (and Upper Jurassic?) 

marine sedimentary sequence.

UPPER JURASSIC(P) AND CRETACEOUS 

MARINE SEDIMENTARY SEQUENCE

In the Coastal block, the Franciscan rocks are overlain by, or in fault contact (Brown, 1968; Page, 1970a, 

1970b) with a thick marine sedimentary sequence of 

shale, sandstone, and local conglomerate, primarily if 

not entirely of Cretaceous age. Although Taliaferro 

(1943a, 1944) recognized within this sequence three 

major units separated by unconformities, only two 

units (fig. 6) have been mapped by Page (1970a, 1970b, 

1972), who referred to this sequence as the "Great 

Valley-type Mesozoic rocks," Hall and Corbato (1967), 

and by myself (Dibblee, 1971).

The lower unit of this sequence is as thick as 1,219 m 

(4,000 ft) and is predominantly dark-gray shale that 

contains thin interbeds of dark-brown wacke sandstone. This unit was called the Toro Formation by 

Fairbanks (1904) and Page (1972) in the San Luis 

Obispo quadrangle; "Knoxville shales" and Marmolejo 

Formation by Taliaferro (1944) in the Santa Lucia 

Mountains; Jollo Formation by Hall and Corbato

(1966) in the Nipomo quadrangle; and Espada Formation (of Dibblee) by Vedder, Gower, Clifton, and 

Durham (1976) and Vedder and Brown (1968) in the 

San Rafael Mountains. Near Stanley Mountain (fig. 8), 

and also west of Santa Margarita (fig. 9), this unit lies 

depositionally (unconformably?) on the unnamed volcanic unit at the top of or above Franciscan rocks (Hall 

and Corbato, 1967; Page, 1972). Near Stanley Mountain and elsewhere the lower unit contains fossils considered diagnostic of Early Cretaceous age (Tiliaferro, 

1943a; Easton and Imlay, 1955; Hall and Corbato, 

1967; Vedder and Brown, 1968, p. 248). Elsewhere it 

contains fossils of that age and of inferred Late Jurassic age in a few places.

The upper unit of this sequence consists of interbedded gray micaceous shale, buff arkosic sandstone, and 

locally dark-gray cobble conglomerate with clasts of 

granitic rocks, andesitic porphyry, and quartzite. This 

unit is as thick as 4,572 m (15,000 ft). Shale predominates in the lower part, and sandstone in the middle 

and upper parts. It is exposed extensively in the San 

Rafael and Santa Lucia Mountains southwest of the 

Rinconada fault. In the Santa Lucia Mountains this 

unit was named the Atascadero Formation by Fairbanks (1904). Taliaferro (1943a, 1944) did not adopt 

this name but divided this unit in ascending order, into 

the Jack Creek Formation (mostly shale) and Asuncion 

Group (mostly sandstone), which he thought were 

separated by an unconformity. In the Nipomo quadrangle (fig. 8) this unit was named the Carrie Creek 

Formation by Hall and Corbato (1967). Other workers 

(Vedder and others, 1967; Vedder and Brown, 1968; 

Page, 1970a, 1970b) applied no names to this unit. Fossils found in this unit are diagnostic of Late Cretaceous 

age, as reported by all the authors mentioned above.

In the San Rafael Mountains (fig. 7) the upper unit 

(unnamed) has a thick basal conglomerate that lies 

without discordance on the lower unit (Espada Formation of Dibblee, 1966). Near Stanley Mountain (fig. 8), 

as mapped by Hall and Corbato (1967), the upper unit 

(Carrie Creek Formation) lies unconformably on the 

lower unit (Jollo Formation). In the Santa Lucia 

Mountains the upper unit is either unconformable on 

both the lower unit and Franciscan rocks (Fairbanks, 

1904; Taliaferro, 1943a, 1944) or is thrust presumably 

westward over them (Page, 1970a, 1970b).

UPPER CRETACEOUS AND LOWER TERTIARY 

MARINE SEDIMENTARY SEQUENCE

In the Salinian block, the deeply eroded surface of 

the basement complex on both sides of the Rinconada 

fault is overlain unconformably by a thick marine clastic sedimentary sequence of Late Cretaceous and early 

Tertiary age, designated as the Upper Cretaceous and
ROCK UNITS 13

lower Tertiary marine sedimentary sequence (Dibblee, 

1973, p. 9-10; 1974a). Because of the scarcity of fossils, 

continuous deposition, and lithologic similarity, the 

position of the Cretaceous-Tertiary boundary within 

this sequence is questionable. This sequence is composed of interbedded light-buff arkosic sandstone, 

micaceous gray shale, and some cobble conglomerate. 

The conglomerate is composed of rounded cobbles 

mostly of hard granitic rocks, porphyritic to aphanitic 

rhyolite to andesite, and quartzite. This sequence is 

exposed extensively in the La Panza and Sierra Madre 

Mountains east of the Rinconada fault, and in the 

Nacimiento River and San Marcos Creek drainage 

areas west of this fault, or between this fault and the 

Nacimiento fault.

East of the Rinconada fault this sequence is estimated to be about 9,144 m (30,000 ft) thick and contains a basal conglomerate as thick as 610 m (2,000 ft), 

resting on granitic basement in the La Panza Range 

(Dibblee, 1973, p. 9-10). According to Vedder and 

Brown (1968) the lowest 1,524 m (5,000 ft) of this sequence just east of this fault yielded fossils diagnostic 

of Late Cretaceous age, the overlying strata in the La 

Panza Range yielded fossils of probable Paleocene age, 

and in the Sierra Madre Mountains, fossils of Eocene 

age. Taliaferro (1943a, 1944) included exposures of this 

sequence in the La Panza Range in his Upper Cretaceous Asuncion Formation. Chipping (1972, p. 485, 

492) informally called the whole sequence the Sierra 

Madre sequence.

In the Nacimiento River-San Marcos Creek area 

west of the Rinconada fault this sequence is at least 

2,454 m (8,000 ft) thick and also has a basal conglomerate locally, in places more than 305 m (1,000 ft) 

thick, on the crystalline rocks in exposures north of the 

Nacimiento River. Fossils found at a few localities in 

this sequence are diagnostic of Late Cretaceous age, 

and at other places of Paleocene age (Taliaferro, 1943a; 

Durham, 1965a). Taliaferro (1943a, 1944, p. 512-517) 

mapped the inferred Late Cretaceous part of this sequence as the Asuncion Formation and the inferred 

Paleocene part as the Dip Creek Formation. However, 

the position of the Cretaceous-Tertiary boundary 

within this continuously deposited sequence has not 

been ascertained, because of the scarcity of fossils, 

structural complications, and partial cover by younger 

rock units.

The similarity of this sequence in lithology, stratigraphic relations, and age in the La Panza Range area 

east of the Rinconada fault and in the Nacimiento 

River-San Marcos Creek area west of the fault 

suggests that these two widely separated areas of these 

rocks on opposite sides of the fault may have been displaced by large right-slip movement on the fault. This

is discussed in more detail under "Regional Evidence of 

Large Strike-Slip Movement on the Rinconada-Reliz 

Fault Zone."

In the northern Santa Lucia Mountains the crystalline basement complex is overlain unconformably by 

remnants of this seuqence at a number of places. In the 

mountains north of the upper Nacimiento River area 

the largest remnant contains about 1,829 m (6,000 ft) 

of predominantly sandstone, with sparse fossils of Late 

Cretaceous age in the lower part and of Paleocene and 

Eocene age in the upper part (Compton, 1966a, 1966b). 

Other remnants occur farther northwest adjacent to 

the Sur and Palo Colorado faults (fig. 15) and are 

thought to be of Late Cretaceous age (Trask, 1926; 

Reiche, 1937). Northward in these mountains small 

remnants (fig. 15) that contain less than 610 m 

(2,000 ft) of sandstone and shale strata include those of 

Eocene age in the vicinity of Junipero Serra Peak 

(Thorup, 1943; Compton, 1966a, 1966b), the southwest 

side of the Church Creek and Miller Creek faults (Dickinson, 1959), and small ones of Paleocene age near 

Carmel (Bowen, 1965; Clark and others, 1974). These 

remnants suggest a gradual northward thinning of the 

whole sequence probably by wedging out of the lower 

strata against the basement complex.

MIDDLE TERTIARY SEDIMENTARY SEQUENCE

Throughout both the Salinian and Coastal blocks, 

the pre-Oligocene rock units are overlain unconformably by the middle Tertiary sedimentary sequence 

(figs. 5, 6) (Dibblee, 1973). The unconformity at the 

base of this sequence is of regional extent and indicates 

that almost the entire region within the area of figures 

2 and 4 northward from the Santa Ynez Mountains 

was affected by a great orogenic episode of uplift and 

erosion probably during Oligocene time. In the Salinian block, the middle Tertiary sequence rests with 

great angular discordance upon the beveled surface of 

the Upper Cretaceous and lower Tertiary marine 

sedimentary sequence, overlapping it northward onto 

the crystalline basement complex. On the Coastal 

block the middle Tertiary sequence rests on the eroded 

surface of the Upper Jurassic(?) and Cretaceous 

marine sedimentary sequence and Franciscan rocks, 

both of which had been severely deformed previously.

The lower or basal unit of the middle Tertiary sedimentary sequence is composed of terrestrial deposits, 

mostly red to gray sandstone and conglomerate, of inferred Oligocene age. In the Coastal block it is only 

locally present, mostly southward from Santa Margarita, and is not more than a few tens of metres thick.

In the Salinian block east of the Rinconada fault the 

basal terrestrial unit is widespread. It crops out in the 

Caliente Range and underlies the Carrizo Plain, where
14 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

it is called the Simmler Formation (Hill and others, 

1958; Dibblee, 1973), which is as thick as 1,067 m 

(3,500 ft) and is composed primarily of bedded 

reddish-gray sandstone. In the La Panza and Sierra 

Madre Mountains this unit, also called the Simmler 

Formation, is composed of coarse conglomerate and 

sandstone derived from the underlying older rocks and 

is only locally present but is as thick as 914 m (3,000 ft) 

at the Cuyama River gorge (fig. 8). Northwestward 

from these areas this unit thins to only a few tens of 

kilometres of red beds under the Salinas Valley and 

eventually thins out northward, as indicated from 

well data.

In the part of the Salinian block west of the Rinconada fault the basal terrestrial unit is absent in 

outcrop except in the Santa Lucia Mountains west of 

King City, where it was called the Berry Conglomerate 

by Thorup (1943), and later the Berry Formation by 

Durham (1974). It is as thick as 335 m (1,100 ft) and is 

composed of buff-gray to locally red arkosic sandstone 

and conglomerate. Small isolated exposures of basal 

red beds that are either equivalent to or younger than 

the Berry Formation occur near the Nacimiento River, 

northwest of the reservoir, and in the northern Santa 

Lucia Mountains west of Greenfield and south of 

Carmel.

The presence of the basal terrestrial unit farther 

north on the west side of the Rinconada-Reliz fault 

zone than on the east side is somewhat suggestive of 

right-lateral displacement on this fault zone.

The basal terrestrial unit is overlain by a marine 

unit of probable late Oligocene to middle Miocene age. 

It is generally called the Vaqueros Sandstone or Formation where its age is within the Zemorrian and 

Saucesian Stages, of late Oligocene and early Miocene 

age. Where the basal terrestrial unit is absent, this 

marine predominantly sandstone unit forms the basal 

unit of the middle Tertiary sedimentary sequence and 

rests unconformably on the pre-Tertiary rocks.

On the Coastal block the basal marine sandstone 

unit attains a maximum thickness of about 610 m 

(2,000 ft) in a large syncline in the San Rafael Mountains (fig. 7), where it includes some claystone and 

ranges in age from the Zemorrian to Relizian Stages, 

late Oligocene to middle Miocene (Vedder and Brown, 

1968). Northward this unit thins to a few tens of 

metres of sandstone in the Huasna Creek area and 

southern Santa Lucia Mountains.

On the Salinian block east of the Rinconada fault the 

lowest marine unit attains its maximum thickness of 

about 2,134 m (7,000 ft) in the Caliente Range and 

Carrizo Plain, where it is called the Vaqueros Formation. It is divided into three members and contains 

faunas diagnostic of the Zemorrian and Saucesian

Stages (Vedder, 1970; Dibblee, 1973). In the La Panza 

Range this unit is about 518 m (1,700 ft) thick, is 

nearly all sandstone on the northeast flank, and is locally absent on the southwest flank. Northwestward 

under the Salinas Valley this sandstone thins to only a 

few tens of metres, and together with the basal terrestrial unit abuts northeastward against the basement 

complex along an imaginary line that extends eastsoutheast and northwest from Bradley (fig. 2).

In the Salinian block west of the Rinconada-Reliz 

fault zone the basal marine sandstone unit is present 

nearly throughout, although its age may vary. Southward from Arroyo Seco Creek to Lockwood Valley it 

contains fossils diagnostic of the Zemorrian and 

Saucesian Stages and is generally called the Vaqueros 

Formation (Thorup, 1943; Bramlette and Daviess, 

1944; Compton, 1966a; Dibblee, 1973; Durham, 1974). 

From Arroyo Seco Creek to the Nacimiento River it is 

from 310 (1,000) to 671 m (2,200 ft) thick. Southward 

from Lockwood Valley to Paso Robles this sandstone is 

thin in outcrop, generally less than 100 m (300 ft). 

However, west of the San Antonio Reservoir it is as 

thick as 800 m (2,400 ft) and contains lenses of granitic 

boulder conglomerate in the upper part (figs. 10, 21).

In the Santa Lucia Mountains northwest of Arroyo 

Seco Creek to Monterey the basal marine sandstone 

unit of the middle Tertiary sedimentary sequence may 

be younger than the Vaqueros Sandstone to the southeast, but its stratigraphic position above red beds, 

where present, and below the Monterey Shale is 

similar.

The middle Tertiary sequence is the only one besides 

the Franciscan that contains volcanic rocks in this region. The oldest are the dacitic and rhyolitic plugs and 

dikes in the San Luis Obispo area in the Coastal block 

and the rhyolitic plugs, dikes, and tuff-breccia (Pinnacles Formation of Andrews, 1936, p. 9-22; Wilson, 

1943, p. 217-218) of the Pinnacles area of the Gabilan 

Range in the Salinian block. Those in the San Luis 

Obispo area yielded K-Ar radiometric dates between 

24 and 26.9 m.y. or Oligocene (Turner and others, 

1970b). Those in the Pinnacles area yielded K-Ar 

dates of about 22 m.y. (Turner and others, 1970a). On 

the Coastal block rhyolitic tuff-breccia (Obispo Tuff of 

Bramlette, 1946, p. 22-23; Obispo Formation of Hall 

and Corbato, 1967, p. 569-570), together with flows of 

dacitic felsite, basalt, and sills of diabase, is extensive 

just below the Monterey Shale in the San Luis 

Obispo-Huasna area and is assigned to the upper 

Saucesian and Relizian Stages, Miocene by the above 

authors. The Obispo Formation yielded K-Ar radiometric dates of about 16.5 m.y. or early Relizian 

(Turner and others, 1970b).

On the Salinian block the middle Tertiary sedimen-
ROCK UNITS 15

tary sequence contains volcanic rocks in several other 

areas. East of the Rinconada fault these are flows, sills, 

and dikes of basalt, mostly in the Miocene part of this 

sequence, in the Caliente and eastern La Panza 

Ranges (Dibblee, 1973). West of the Rinconada fault 

these include flows of basalt near the base of the Monterey Shale a few kilometres west of Paso Robles and 

flows of basalt and andesite near the top of the basal 

marine sandstone unit in and near the Carmel Valley 

in the northern Santa Lucia Mountains.

The Monterey Shale, an unusual marine formation 

of siliceous and organic shale primarily of middle and 

late Miocene age, overlies the Vaqueros or basal 

sandstone unit where present and makes up the major 

part of the middle Tertiary sedimentary sequence. This 

distinctive shale formation is present on and once covered nearly the entire Coastal block and most of the 

Salinian block. On both blocks this formation varies 

greatly in thickness, indicating that the sea floor upon 

which it accumulated was undergoing movements.

On the Coastal block the Monterey Shale attains a 

maximum thickness of about 1,524 m (5,000 ft) in the 

San Rafael Mountains and Huasna area, but elsewhere 

to the northwest it is much tinner.

On the Salinian block the Monterey Shale is very 

thin or locally absent in western Cuyama Valley and 

southwestern La Panza Range but is very extensive 

under and west of Salinas Valley (fig. 2), where it is 

about 914 m (3,000 ft) in average thickness. Eastward 

under the southern part of this valley it extends nearly 

to the San Andreas fault, thinning eastward. In the 

central part from Bradley to Greenfield (fig. 2) it abuts 

northeastward through strandline sandstones against 

the basement complex along the King City hinge line 

described by Gribi (1963). Under this valley northwestward from Greenfield, the Monterey Shale presumably intertongues with arkosic sandstone and terrestrial deposits that likewise abut northeastward 

against the basement complex, as suggested from logs 

of several wells and from exposures in the hills southwest of Salinas.

The thickness of the Monterey Shale within the 

Salinian block varies greatly along both sides of the 

Rinconada-Reliz fault zone, as indicated from outcrop 

and well data. On the east side of the Rinconada fault 

near Bradley, this shale is as much as, if not more 

than, 2,134 m (7,000 ft) thick but is much thinner west 

of this fault. On the west side of the Reliz fault in the 

Arroyo Seco Creek area the Monterey Shale is about 

1,829 m (6,000 ft) thick but is much thinner east of this 

fault. This condition suggests possible displacement of 

the very thick section, if once continuous, by right slip 

on the Rinconada-Reliz fault zone.

From the Arroyo Seco Creek area the Monterey

Shale extends northwestward as remnants in the 

northern Santa Lucia Mountains into the Monterey 

Bay.

In most areas on both blocks the Monterey Shale is 

composed of two distinct parts, the lower part, of argillaceous to siliceous shale, and the upper part, of hard 

siliceous shale.

The lower part (Sandholdt Shale of Bramlette and 

Daviess, 1944; Sandholdt Member of Durham, 1974; 

Point Sal Formation of Hall and Corbato, 1967) of the 

Monterey Shale is generally about 610 m (2,000 ft) 

thick or less and is locally absent. In some areas of the 

Salinian block the lowest 185 m (600 ft) is claystone 

(Sandholdt Formation of Thorup, 1943) that contains 

a microfauna of the upper Saucesian and Relizian 

Stages. The remainder is composed of thin-bedded fissile organic shale, siliceous shale, and thin carbonate 

strata and contains a rich microfauna of the Relizian 

and Luisian Stages, middle Miocene. This part (Saltos 

and Whiterock Bluff Shale Members of Hill and others, 

1958) makes up all the Monterey Shale in the Cuyama 

Valley and the Caliente and La Panza Ranges east of 

the Rinconada fault, and the major part west of this 

fault from Atascadero northwestward to the San Antonio River (fig. 2), suggesting possible right-laternal 

displacement of this unit of the fault.

The upper part (Hames Member of Durham, 1974) of 

the Monterey Shale is composed of white-weathering 

hard platy brittle porcelaneous siliceous shale. It averages about 610 m (2,000 ft) in thickness but in some 

areas is as thick as 2,029 m (6,000 ft). It contains a 

microfauna of the Mohnian Stage, late Miocene. East 

of the Rinconada-Reliz fault zone this part makes up 

the major part of the Monterey Shale in areas northwestward from the Paso Robles area and in areas 

northwestward from Lockwood Valley west of this 

fault zone. Accordingly, the areas in which the thickness of the upper part far exceeds that of the lower part 

extend farther south on the east side of the Rinconada 

fault zone than on the west side, suggesting displacement by right slip on the fault.

The upper part of the Monterey Shale presumably 

grades laterally southeastward and eastward into the 

Santa Margarita Sandstone in the southern part of the 

Salinas Valley, on both sides of the Rinconada fault. 

Farther southeast in the Cuyama Valley, Caliente 

Range, and Carrizo Plain, the lower part of the Monterey Shale intertongues laterally eastward into the 

Branch Canyon Sandstone. In those areas that sandstone, together with the overlying Santa Margarita 

and much of the underlying Vaqueros Formation, 

intertongue eastward with red beds of the terrestrial 

Caliente Formation (Hill and others, 1958; Vedder, 

1970; Dibblee, 1973) toward the San Andreas fault.
16 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

The Santa Margarita Sandstone overlies the Monterey Shale in the southeastern parts of both the 

Coastal and Salinian block and is composed of white 

sandstone from 310 (1,000 ft) to 457 m (1,500 ft) in 

thickness. It contains bioclastic reefs of fossils diagnostic of late Miocene age. East of the Rinconada fault it 

extends from the Cuyama Valley to the Paso Robles 

area (fig. 2). West of this fault this sandstone extends 

about 18 km (11 mi) farther northwest to the San Antonio River, suggesting right-lateral displacement of 

this sandstone on the fault (fig. 18). The Santa Margarita Sandstone is not definitely known to occur 

farther northwest except in exposures east of Monterey 

(Bown, 1965) and possibly in the San Ardo oil field.

UPPER TERTIARY SEDIMENTARY SEQUENCE

With the exception of exposures south of San Luis 

Obispo and in Santa Barbara County, the upper Tertiary sedimentary sequence is present only on the 

Salinian block. In the Salinas Valley this sequence is 

marine, as thick as 610 m (2,000 ft). Its stratigraphy 

has been interpreted in various ways and is much confused as reviewed by Durham and Addicott (1965, 

p. A3-A6) because of lack of adequate geologic mapping. On the east side of the Salinas Valley this sequence was casually called the Poncho Rico Formation 

by Reed (1925, p. 591, 606), and Clark (1929), with no 

reference to a type section. Later Durham and Addicott 

(1965, p. A2) formally defined this entire marine sequence as the Pancho Rico Formation and considered 

the fauna throughout as early Pliocene. However, 

many other investigators and I recognized two distinctive lithologic units: a lower mudstone unit and an 

upper sandstone unit, as indicated below and in 

figure 5.

The lower unit (Santa Margarita or McLure Shale of 

Taliaferro, 1943a, 1943b; McLure Formation of Kilkenny, 1948, p. 2260; Pancho Rico Formation of Baldwin, 1950; Pancho Rico Shale of Gribi, 1963, p. 18; 

Pancho Rico Formation of Dibblee, 1971), is a 

creamy-white fine-grained chalky diatomaceous mudstone about 363 m (1,200 ft) thick. It is widespread 

northeast of the Salinas River where it is primarily 

diatomaceous mudstone that rests with a basal sandstone on the basement complex over much of this area. 

Elsewhere it is conformable, and commonly gradational, on the Monterey Shale. Northwest of Jolon the 

lower unit is diatomaceous to sandy siltstone. Near 

and southeast of Jolon, and west and southwest of 

Bradley, this unit becomes impure chalky diatomite. 

This diatomite was included in the Monterey Formation by Durham (1965a, 1974) as the Buttle Member, 

but its lithology and stratigraphic position indicate 

that it is probably the lower unit of Pancho Rico 

Formation.

The upper unit (Etchegoin of Taliaferro, 1943b; 

Jacalitos of Taliaferro, 1943a; Pancho Rico Formation 

of Kilkenny, 1948, p. 2260; Etchegoin Formation of 

Gribi, 1963, p. 18; unnamed sandstone of Dibblee, 

1971) is composed of yellowish-gray fossiliferous sandstone and is included in the Pancho Rico Formation by 

Durham and Addicott (1965). It is thickest (about 

185 m (600 ft) in Salinas Valley in the vicinity of San 

Ardo, where it is gradational through interbeds into 

both the underlying lower unit (of the Pancho Rico 

Formation) and into the overlying Paso Robles Formation. However, beyond this area, stratigraphic relations change rapidly toward the margins of Salinas 

Valley. Northward this sandstone unit thins to only a 

calcareous fossil reef that together with the Paso Robles Formation, laps over the lower unit onto granitic 

basement north of King City. Eastward this sandstone 

grades laterally into terrestrial gravel like that of the 

Paso Robles Formation and unconformably overlaps 

the lower unit onto the middle Tertiary sedimentary 

sequence near the San Andreas fault. Southwestward 

to and beyond the San Antonio River, this sandstone 

thins to less than 61 m (200 ft) and, together with the 

Paso Robles Formation, unconformably overlaps diatomite of the lower unit onto the Monterey Shale (fig. 

10). This unconformable relation was recognized by 

Taliaferro (1943b, p. 460), but not by Durham and Addicott (1965).

I mapped and designated the upper unit as an unnamed sandstone (Dibblee, 1971) and excluded it from 

the Pancho Rico Formation, despite the lower Pliocene 

fossils it contains, because of the stratigraphic reasons 

indicated above, and as pointed out by Gribi (1963, 

p. 16), "it certainly is a recognizable formation." However, because this unit is included in the Pancho Rico 

Formation by Durham and Addicott (1965, p. A-2) it is 

herein reluctantly included within that Formation.

In the Carrizo Plain, Caliente Range, and Cuyama 

Valley, the upper Tertiary sedimentary sequence is all 

terrestrial (Dibblee, 1973). It is as thick as 1,676 m 

(5,500 ft) and is composed of two units. A lower unit, 

the Quatal Formation, of Pliocene age, is about 152 m 

(500 ft) thick and is mostly claystone, and an upper 

unit, the Morales Formation, of Pliocene age, is as 

thick as 1,524 m (5,000 ft) and is interbedded alluvial 

conglomerate, sandstone, and clay. This sequence rests 

unconformably on the Santa Margarita Sandstone or 

on older formations and is overlain unconformably by 

the Paso Robles Formation.

VALLEY SEDIMENTS

Through the Salinas Valley the upper Tertiary 

sedimentary sequence is overlain by a widespread severely dissected valley formation as thick as 310 m 

(2,000 ft) known as the Paso Robles Formation (Fair-
EXPLANATION

SOUTHWEST OF

SOUTH OF BIG P

t Morales Fofmation

: Santa Margarita Sandstone

Monterey Shale

Sandstone and claystone Sandstone 

.. T . 77 Lower Tertiary

11!__11! sedimentary rocks

; Lower Ter

sedimen

] Upper Cre

i sedimen

°Kc° 0 Cong

 v, Lowe

PINE

OUNTAIN\ AMADULC E 

PEAK v. 3>< 

B'IG 

34°45' -

2468 KILOMETRES 

__ 

120' 119° 45' 119° 30'

FIGURE 7. -Geology along and near the Rinconada fault (Nacimiento segment) from Big Pine Mountain to Timber Peak (modified from Vedder and B
EXPLANATION 

NORTHEAST OF RINCONADA FAULT

Santa Margarita Formation

Middle 

Tertiary 

sediment

sequence

Monterey Shale

Vaqueros Sandstone

Upper Cretaceous

sedimentary rocks

Basal conglomerate

Metamorphic 

SOUTHWEST OF 

RINCONADA FAULT

Santa Margarita 

Sandstone

_SfANLEY 

^MOUNTAIN

Vaqueros 

Sandstone

Ks" ' Upper Cretaceous 

sedimentary 

rocks

1 20° 1 5' 120° 

FIGURE 8. Geology along and near the Rinconada fault (Nacimiento segment) from Timber Peak to Pozo (modified from Vedder and Brown
SEGMENTS OF THE RINCONADA FAULT 19

banks, 1904; Taliaferro, 1943a,b; Bramlette and 

Daviess, 1944; Durham, 1965a, 1966, 1974; Galehouse, 

1967, Dibblee, 1973). It is composed of weakly indurated pebble-gravel, sand, and clay. A large percentage 

of the pebbles are derived from the Monterey Shale.

In the central Salinas Valley area near San Ardo 

and San Miguel, the Paso Robles Formation is conformable on the sandstone unit of the Pancho Rico 

Formation of Pliocene age. However, along the southwestern margin of this valley from Lockwood Valley 

southeastward through Paso Robles, Atascadero, and 

the northeast foothills of the La Panza Range, the Paso 

Robles Formation is unconformable with great discordance on the middle Tertiary sedimentary sequence 

and the granitic basement.

The top of the Paso Robles Formation is a surface of 

deposition. In most areas this is severely dissected or 

destroyed by erosion, but locally remnants of it are 

preserved as flat-topped mesas. The Carrizo Plain is in 

large part the preserved deposition surface of the Paso 

Robles Formation. West of Carrizo Plain the Paso Robles Formation is exposed as dissected southwardthinning alluvial fans that terminate against granitic 

basement of the La Panza Range.

The Paso Robles Formation is primarily Pleistocene 

in age (Dibblee, 1973), but northeast of San Miguel the 

lowest part intertongues westward with the upper 

sandstone unit of the Pancho Rico Formation, which 

contains mollusks assigned an early Pliocene age by 

Durham and Addicott (1965). A similar relation may 

occur in the subsurface northward from Atascadero, as 

suggested by the occurrence near there of Pliocene 

mollusks at the base of the Paso Robles Formation 

(Galehouse and Addicott, 1973).

The Paso Robles Formation accumulated as alluvial 

deposits in the ancestral Salinas Valley when this valley was much more extensive than it is now and was 

derived from areas to the northeast and southwest that 

were elevated to form mountains. It was deposited almost entirely on the Salinian block, on which it is 

widespread, on both sides of the Rinconada-Reliz fault 

zone. It extends into the Coastal block only in the vicinity of Santa Margarita.

SURFICIAL DEPOSITS

Alluvial deposits of late Pleistocene and Holocene 

age fill all modern valleys and flood plains of streams. 

Older alluvium, deposited when this region was somewhat lower than it is now, is dissected into terrace 

remnants. None are affected by deformation except 

locally along the San Andreas fault.

SEGMENTS OF THE RINCONADA FAULT

RINCONADA FAULT (NACIMIENTO SEGMENT) FROM 

SAN RAFAEL MOUNTAINS TO POZO AREA

The southeastern 110 km (70 mi) of the Rinconada 

fault, or the part heretofore called the Nacimiento 

fault (Jennings, 1959; Hall and Corbato, 1967; Vedder 

and others, 1967; Vedder and Brown, 1968), and the 

related geology are shown in figures 7 and 8. This segment is not to be confused with the original 

Nacimiento fault west of the Nacimiento River far to 

the northwest, even though early movements on both 

faults may have been related. Because this segment 

(figs. 7, 8), designated as part of the Rinconada fault 

(Dibblee, 1973, fig. 2), has been previously referred to 

the Nacimiento fault, it is designated herein the 

Nacimiento segment of the Rinconada fault.

This segment is interpreted to be the former position 

of the southeastern part of the Nacimiento fault along 

which the Salinian and Coastal blocks presumably are 

in contact. This segment is mapped as part of the Rinconada fault because it is continuous northwestward 

into the Rinconada fault (Rinconada segment) and not 

into the Nacimiento fault near and northwest of Atascadero where it is deeply buried and inactive since 

Miocene time. If the Nacimiento segment of the Rinconada fault was once the southeastern part of the 

Nacimiento fault as interpreted, that part has been 

displaced southeastward from the buried position of 

this fault near Atascadero by right slip on the Rinconada fault.

The Nacimiento segment of the Rinconada fault is 

generally nearby vertical, but in many places, especially southeast of the Cuyama River gorge, it dips 

northeast at varying angles at the surface. At one 

place it dips steeply southwest. This fault is poorly expressed topographically, but the terrain is generally 

higher on the northeast and in many places, especially 

southeastward from Cuyama River gorge, rises abruptly from the fault trace. This condition suggests relative upward displacement of the northeastern block, 

which is further indicated at two places where Miocene 

strata on the southwest side are against lower Tertiary 

strata on the northeast (figs. 7, 8). However, this condition may be in part the effect of differential erosion of 

the southwest block composed largely of intensely deformed weakly resistant Cretaceous clay shale, compared to the northeast block composed of comparatively less deformed, more resistant lower Tertiary 

sandstone and conglomerate.

Lack of deviation or offsetting of the many stream 

channels that cross this segment of the fault indicates
20 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

that no lateral movement occurred along it in late 

Quaternary time.

If the latest vertical displacement on this segment 

was relatively upward on the northeast side, then, as 

interpreted by Vedder and Brown (1968, p. 247, 252), 

this movement was reversed from earlier movements 

during which the southwestern block was displaced 

relatively upward, as indicated by (1) the older or Cretaceous age of the sedimentary rocks on the southwest 

block with respect to the early Tertiary and Oligocene 

age of those sedimentary rocks on the northeast block 

along and throughout this segment and (2) the upward 

bending of strata of the northeast block adjacent to the 

fault (fig. 7; fig. 8 northwest of Cuyama River gorge).

If movement was by reversals of vertical displacements, then it must be assumed that strata of Paleocene and early Eocene ages once overlay the Cretaceous strata of the southwest block but were eroded off 

before middle Eocene time, as suggested by the occurrence of strata of inferred middle Eocene age that lie 

directly on Upper Cretaceous strata on the southwest 

block near Big Pine Mountain. The probable total absence of strata of Paleocene and early(?) Eocene age on 

the southwest block suggests that they were probably 

never present there, at least not in great thickness.

The juxtaposition of different rocks or rocks of different ages along this segment is difficult to account for 

by simple vertical movements. As indicated in figures 

7 and 8 and in figure 1, this segment follows the old 

Sur-Nacimiento fault zone along which the crystalline 

basement complex of the Salinian block was juxtaposed against the Franciscan basement of the Coastal block, either before or during deposition of thick 

overlying Cretaceous and lower Tertiary sedimentary 

sequences. The Rinconada fault was active much later, 

after these strata and the middle Tertiary sedimentary 

sequence accumulated. These later movements may 

have been transcurrent, as discussed under "Regional 

Evidence of Large Strike-Slip Movement on the 

Rinconada-Reliz Fault Zone."

The only local evidence of strike-slip movement on 

this segment of the Rinconada fault is the fold pattern 

adjacent to and near it, especially on the southwest 

block (figs. 7, 8; Vedder and Brown, 1968, p. 245). The 

folds in this area trend slightly more westerly than 

does the fault, and some terminate against it. This pattern, distinct also along other nearly parallel highangle faults, may be partly the effect of right-lateral 

drag movements along this fault in the underlying 

basement complex.

OTHER FAULTS NEAR AND POSSIBLY RELATED TO THE 

NACIMIENTO SEGMENT OF THE RINCONADA FAULT

The Sierra Madre Mountains were elevated between

the Rinconada fault and two southwest-dipping thrust 

or reverse faults, the South Cuyama and Ozena faults 

(fig. 7), along the northeastern base of this mountain 

block. The block was thrust up or elevated along these 

two faults toward Cuyama Valley in Pleistocene time, 

and possibly earlier. The range may have been thrust 

up also on the Rinconada fault to the southwest, although it is not known whether this happened during 

Pleistocene time or earlier. The South Cuyama and 

Ozena faults dip toward the Rinconada fault and either 

splay from it at depth or branch from it near Garcia 

Mountain (fig. 8). The thrusting movement on these 

faults, as well as the folding of the thick sedimentary 

sequences of the mountain block, indicate that severe 

compression is involved in movements along the Rinconada fault.

The Stanley Mountain area west of the Rinconada 

fault (fig. 8) was in large part elevated on a fault (East 

Huasna fault of Hall and Corbato, 1967) just west of 

this mountain, probably in Pleistocene time. This fault 

is practically vertical. Movement on the East Huasna 

fault is up on the northeast and right lateral, as indicated by the northwestward bending of the westtrending rock units and syncline on the east block, 

nearly parallel to the fault.

The higher part of the La Panza Range was elevated 

on a northeast-dipping thrust or reverse fault, the La 

Panza fault (fig. 8), also in Pleistocene time. Although 

on the map this fault is nearly parallel to the Rinconada fault, it dips northeast away from that fault 

and is therefore probably unrelated, even though both 

may have been activated by the same regional stress. 

There are no offset canyons nor offset geologic contacts 

to indicate lateral movement on the La Panza fault, 

although Durham (1974, p. 71) suggested 7 or 8 km 

(4 or 5 mi) of right-lateral movement on its northwestern part (locally known as the Huer Huero fault, 

fig. 3A), because the contact of the middle Tertiary sequence on granitic basement is farther northwest on 

the southwest block than on the northeast block. However, this contact is exposed only on the northeast 

block and is concealed under the Paso Robles Formation on the southwest block within 2 km (1.2 mi) of the 

fault, and most if not all the apparent offset could be 

accounted for by the probable westerly strike of this 

concealed contact southwest of the fault.

The San Juan fault (fig. 8) is a nearly-vertical fault 

that probably splays southeastward on the west side of 

the San Andreas fault (figs. 3J5, 4). Movement may be 

in large part right lateral, as suggested by the comparatively more easterly trend of the fold axes of the 

Caliente Range on the east side. This range was elevated by the squeezing up of the middle Tertiary sequence where it is tremendously thick to form several
SEGMENTS OF THE RINCONADA FAULT 21

large southwestward-leaning anticlines on northeastdipping thrust faults that splay eastward from the San 

Jaun fault (Dibblee, 1974a). These structures appear to 

be the effect of right-lateral drag movement on the San 

Juan fault and are probably related more to compressive right-lateral movements on the San Andreas fault 

than to those on the Rinconada fault.

Although the Red Hills (41 km or 25 mi east of Paso 

Robles) and the Caliente Range have been elevated 

northeast of the San Juan fault, little topographic relief is present on the fault. This condition, together 

with the slightly more easterly trend of the folds and 

thrust faults of the Caliente Range, suggests that 

movement on this fault is primarily right lateral. If so, 

a right slip of some 13 km (8 mi) is suggested by that 

much offset of the basal conglomerate and contact of 

the Upper Cretaceous and lower Tertiary marine 

sedimentary sequence with the crystalline basement 

along upper San Juan Creek (fig. 8), if this is not the 

result of relative vertical upward displacement of that 

sequence.

Schwade, Carlson, and O'Flynn (1958, p. 85) suggested that this contact may be laterally displaced as 

much as 37 km (23 mi) on this fault zone. Possible 

additional earlier transcurrent movement is suggested 

by the difference of the crystalline basement on opposite sides of the fault (fig. 8). Further evidence of 

strike-slip movement is suggested by the farther 

northwestward extent on the southwest side of this 

fault in Cuyama Valley of the Branch Canyon 

Sandstone that intertongues westward with the Monterey Shale (Dibblee, 1973, fig. 12).

The San Juan fault was active in late Cenozoic time, 

mostly after deposition of the Santa Margarita Formation and before that of the unconformably overlying 

Paso Robles Formation. It was recurrently active after 

deposition of the Paso Robles, but there is no indication 

of movements during Holocene time.

The Nacimiento segment of the Rinconada fault 

terminates southeastward against the Big Pine fault 

(figs. 1, 7), which is a nearly-vertical transverse fault 

with left-lateral movement (Hill and Dibblee, 1953; 

Vedder and others, 1974). As suggested by Vedder and 

Brown (1968, p. 254), the possible offset continuation of 

the Nacimiento segment may be the Pine Mountain 

fault (figs. 1, 7). If this fault was once part of the Rinconada fault, it has been displaced about 15 km (9 mi) 

eastward by that much left slip on the Big Pine fault 

since Pliocene(?) time. Other geologic features displaced in the same manner are (1) the synclinal structure of the lower Tertiary sedimentary sequence and 

(2) a distinctive fades within this sequence (Vedder 

and Brown, 1968; Vedder and others, 1974). The Pine 

Mountain fault (Jennings and Strand, 1969) is traceable for about 47 km (30 mi) east-southeastward from 

the Big Pine fault and is a major thrust or reverse fault 

dipping 33° to 50° northeast, along which the Pine 

Mountain Range was elevated in Pleistocene time. If 

the Rinconada and Pine Mountain faults were once 

continuous and later displaced out of line by the Big 

Pine fault, the Big Pine fault is younger, or mainly of 

Quaternary age.

RINCONADA FAULT (RINCONADA SEGMENT) 

FROM RINCONADA MINE TO PASO ROBLES

The San Luis Obispo folio was the first geologic map 

published that includes the Rinconada fault; no faults 

were shown on the map, but a fault (designated years 

later as the Rinconada fault) was shown on three cross 

sections and was discussed (Fairbanks, 1904, p. 8). The 

earliest mention of the Rinconada fault was by Reed 

(1933, p. 30), presumably from unpublished mapping 

by N. L. Taliaferro of the northwest-trending fault recognized earlier by Fairbanks (1904) near Santa Margarita. The first published map that shows it as Rinconada fault is the San Luis Obispo sheet of the 

Geologic Map of California (Jennings, 1959) on which 

it is about 40 km (26 mi) long. This fault is designated 

herein as the Rinconada or type segment of the Rinconada fault.

The geology along the Rinconada fault in the vicinity of Santa Margarita and Atascadero is as shown in 

figure 9. This is the segment along which the granitic 

basement of the Salinian block and Franciscan basement of the Coastal block are closest at the surface. 

The Nacimiento segment is continuous into this segment from the southeast.

The Rinconada segment is nowhere well exposed and 

is poorly expressed topographically. As along the 

Nacimiento segment, the terrain is generally higher 

on the northeast. There are no deviated nor offset 

stream channels, and the Rinconada segment appears 

to be inactive. However, at a place 5 km (3 mi) east of 

Atascadero, there is a small trenchlike feature nearly 

a kilometre (half a mile) long in alluvium that may 

have been formed by recent movement on this segment.

Near Paso Robles the Rinconada fault is concealed 

by alluvium deposited by the Salinas River, and so its 

position there can only be inferred. The Paso Robles 

Formation nearby is nearly unaffected. The two hot 

springs at Paso Robles are close to the inferred position 

of a possible fault that may extend beneath the alluvium as a possible north-trending offshoot of the 

Rinconada fault (fig. 10).

As shown in figure 8 and partly in figure 9, the premiddle Tertiary rock units and their structures on op-
RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

EXPLANATION

Alluvium

Paso Robles Formation 

anta rit nd N 0 o o

 *.- > .

0 f-W o

1SI

.jTempleton

35° 30' Ks

>°pj ° Paso Robles Formation

iJsmXv Santa Margarita Sandstone

Middle Tertiary II Monterey Shale ^

sedimentary - Tb Basalt 

sequence y TV'-.-'. Vaqueros Sandstone

°o'oTc°'°c Nonmarine conglomerate

Southwest of Rinconada fault Northeast of Rinc° d* fault

Tes Lower Tertiary 

 sedimentary rocks _

I | ' v ' ' Upper Cretaceous 

Ks uPPer Cretaceous . |SS?, sedimentary rocks

 ' sedimentary rocks ^J^ gasal conglomerate

 ^~ Lower Cretaceous ?-l / <V C 

~- -~ sedimentary rocks ~\--^f'^ 

^ =b ^ Basaltic rocks " /^gr>^ Granitic rocks

~- - - sedimentary 

** =b *^ Basaltic rocks

Serpentinite 

p??f^: Franciscan rocks

>O> /^O/J

'^7\

\ \'

A I.

' \ / 

Ks

°K°cS

ii . 25

1

.50,

.Ksv

25*

ill

m ^^. San Luis u \^. 

ObispoX a

3 4 MILES ^ 

6 KILOMETRES

Kl

 25

120° 45' 120° 30'

FIGURE 9. Geology along and near the Rinconada fault (Rinconada segment) from the Rinconada mine to Paso Robles. Geology of 

parts of area west of Rinconada fault modified from Page (1972) and E. M. Hart (written commun., 1972).
SEGMENTS OF THE RINCONADA FAULT 23

posite sides of this segment of the Rinconada fault are 

vastly different. This condition indicates this fault was 

of great magnitude prior to middle Tertiary time. As 

shown thereon, the pre-middle Tertiary terrane on the 

east side is one of granite basement overlain unconformably by the very thick Upper Cretaceous and 

lower Tertiary marine sedimentary sequence that dips 

southerly and strikes west into the fault and is sharply 

truncated by it. In contrast, the terrane on the west 

side is one of Franciscan rocks, serpentine, basaltic 

rocks, and the Cretaceous marine sedimentary sequence, all severely compressed into structures that 

trend northwesterly nearly parallel to the Rinconada 

fault. The Franciscan rocks and serpentine are exposed 

extensively on the southwest side of the fault at the 

Rinconada mine, and as two very small slices on the 

fault 8 and 9.4 km (5 and 6 mi) northwest. All these 

rocks are close to granitic basement east of the fault. 

These are the only places along this fault where these 

contrasting basement rocks are so near each other at 

the surface, and so there this segment forms the local 

boundary between the Salinian and Coastal blocks.

The middle Tertiary sedimentary sequence, which is 

unconformable on all older rocks, is greatly involved in 

the severe deformation west of the fault. On the east 

side it is also intensely folded near the fault, even 

though it lies directly upon rigid granitic basement. If 

this deformation is the effect of movements on the 

fault, the fault was active after Miocene time. The Paso 

Robles Formation, which is unconformable on the 

Miocene and older rocks, is comparatively much less 

deformed, indicating comparatively little movement 

on this segment since deposition of the Paso Robles 

Formation.

Because of relations as shown in figure 9, it is difficult to determine the character and movement on 

this segment of the Rinconada fault. The nearly 

straight trace of this segment through hilly terrain 

indicates it to be vertical or nearly so. The somewhat 

higher terrain and relatively older rocks on the northeast side between the Rinconada mine and Templeton 

suggest relative upward displacement of that side, but 

southeast of the mine these conditions are reversed. 

The geologic relations as shown in figure 9 and just 

summarized cannot be accounted for by mere vertical 

displacements, especially for the pre-middle Tertiary 

rocks. These relations can probably be explained only 

by horizontal juxtaposition along the fault of areas 

once far apart, suggesting a large amount of strike-slip 

movement prior to middle Tertiary time and possibly a 

moderate amount since. If such movements happened, 

they must have been right lateral, as suggested by the 

following evidence: (1) More easterly trends of fold 

axes in all formations as compared with trend of fault,

especially in the Monterey Shale adjacent to part of 

fault southeast of Paso Robles and their truncation by 

the fault, and (2) sharp truncation of the very thick 

west-striking south-dipping Upper Cretaceous and 

lower Tertiary sedimentary sequence and of the underlying granitic rocks by the fault.

Where are the displaced rocks mentioned in item 2 

above, on the west side of the fault? In order to answer 

this question, it is necessary to go far to the northwest 

to find these rocks. (See section on "Regional Evidence 

of Large Strike-Slip Movement on the Rinconada-Reliz 

Fault Zone.")

RINCONADA FAULT (SAN MARCOS SEGMENT) FROM 

PASO ROBLES TO SAN ANTONIO RESERVOIR

Extensive pioneer mapping by Taliaferro (1943a, 

1943b, 1944) and his students in the hills northwest of 

Paso Robles revealed a northwest-trending fault which 

he called the San Marcos thrust zone, after San Marcos 

Creek. It was later called the San Marcos fault (Kilkenny, 1948; Walrond and Gribi, 1963; Jennings, 1959; 

Page, 1970a). It is designated herein as the San Marcos 

segment of the Rinconada fault, because from my own 

mapping I conclude that it is nearly alined with and is 

probably continuous with the Rinconada segment to 

the southeast. The San Marcos segment extends from 

Paso Robles to the San Antonio Reservoir for a distance of about 23 km (15 mi). The geology along this 

segment, as I have mapped it (Dibblee, 1972b), is as 

shown in figure 10. This segment is continuous northwestward into that generally known as the Espinosa 

fault (Jennings, 1959).

The San Marcos segment is a complex zone of faults 

or strands as shown. Between Paso Robles and San 

Marcos Creek this segment is composed of one major 

vertical (?) fault, but northwestward from that creek it 

is composed generally of two major strands. In most 

places both are nearly vertical, but within 5 km (3 mi) 

northwest of San Marcos Creek both dip southwest at 

indeterminate angles; one strand is well exposed in 

San Marcos Creek where it dips about 50° southwest in 

Monterey Shale. Northwest of the Nacimiento River, 

the northeast strand appears to be vertical. Northwest 

of the San Antonio Dam, the southwest strand becomes 

the northeast-dipping San Antonio fault.

With the exception of the granitic basement on the 

northeast side between Paso Robles and San Marcos 

Creek, the San Marcos segment of the Rinconada fault 

at the surface is mainly within the middle Tertiary 

sedimentary sequence, with some involvement of the 

Paso Robles Formation. Along and near its strands the 

Monterey Shale is intensely folded and also severely 

shattered, brecciated, slickensided, and contorted.
sequence sedimentary Tertiary Middle 

CENOZOIC MESOZOIC

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SEGMENTS OF THE RINCONADA FAULT 25

Evidence of the direction of relative vertical slip or 

movement on the San Marcos segment of the Rinconada fault is conflicting. Between Paso Robles and 

San Marcos Creek, as shown in figure 10, granitic 

basement exposed on the northeast side against the 

middle Tertiary sedimentary sequence on the southwest side indicates relative uplift of the northeast 

block. However, the southwest block is topographically 

higher. Northwest of San Marcos Creek, Miocene 

strata of the southwest block are in fault contact with 

the Paso Robles Formation on the topographically 

lower northeast block. These conditions indicate, if 

movements were vertical, that the earliest displacement since Miocene time was up on the northeast, then 

in late Pleistocene time, up on the southwest.

These suggested reversals of vertical displacement 

may be more apparent than real. Physiographic, structural, and stratigraphic evidences suggest that the 

dominant movement was strike slip, right lateral, as 

indicated in the following paragraphs.

Physiographic evidence of right-lateral movement, 

mainly on the northeast strand, is suggested by the 

southeastward deflection or partial deflection of the 

San Antonio and Nacimiento Rivers, San Marcos 

Creek, and several small intervening channels, where 

they cross the fault (fig. 11).

Structural evidence of right-lateral movement is 

suggested by the axial trends of numerous folds along 

and near the faults as shown in figure 10. A few axes 

are almost parallel to the faults, but most trend 

slightly more west than do the faults. Adjacent to the 

faults, these folds are numerous and tightly compressed, especially in the Monterey Shale. These conditions appear to be the effect of right-lateral drag 

movement along this fault segment, combined with severe compression.

A large amount of right-lateral displacement on the 

Rinconada fault is suggested by the presence west of 

the San Marcos segment of the thick Upper Cretaceous 

and lower Tertiary marine sedimentary sequence and 

absence of this sequence east of this segment. Several 

wells drilled for oil east of it enter granitic basement 

below the middle Tertiary sedimentary sequence. A 

well drilled west of it, on a large anticline in the Upper 

Cretaceous and lower Tertiary marine sedimentary 

sequence about 3 km (2 mi) southeast of the 

Nacimiento Reservoir (fig. 10), bottomed in that sequence at a depth of 1,585 m (5,200 ft). This thick 

sandy sequence may have been displaced from that sequence east of the Rinconada segment (fig. 9), either 

before or after deposition of the middle Tertiary 

sedimentary sequence, or in both times.

Post-Miocene lateral displacement is suggested by 

different stratigraphic relations on opposite sides of

this fault segment, especially on the northeast strand. 

On the northeast side of that strand, a great thickness 

of the upper part of the Monterey Shale (upper 

Miocene) is overlain by diatomite of the Pancho Rico 

Formation, and both these units are overlain unconformably together by the thin marine sandstone of the 

Pancho Rico Formation (Pliocene) and by the Paso Robles Formation. On the southwest side, the lower part 

of the Monterey Shale (middle Miocene) is overlain by 

a thin unit of the upper part (upper Miocene), which in 

turn is overlain by the Santa Margarita Sandstone, 

and all these units are overlain unconformably by the 

Paso Robles Formation which is generally much less 

deformed than the Miocene units. These adjacent 

blocks must have been juxtaposed by right slip from 

areas once separated. This evidence is further discussed under "Regional Evidence of Large Strike-Slip 

Movement on the Rinconada-Reliz Fault Zone."

RINCONADA FAULT (ESPINOSA SEGMENT) FROM 

SAN ANTONIO RESERVOIR TO RELIZ CANYON

The San Marcos segment was extended northwestward from San Antonio Creek by Kilkenny (1948). Reconnaissance mapping by the author in the hills on the 

west side of the Salinas Valley in search of petroleum 

revealed the presence of a major fault zone alined between the San Marcos and Reliz faults. This was shown 

by Hill and Dibblee (1953, pi. 1; p. 454-455) as the 

Reliz Canyon-San Marcos fault, branching southward 

from the supposed King City fault and interpreted to 

be among "many important northwest-trending steep 

faults parallel to the San Andreas probably also 

characterized by right-lateral components of displacement."

On the geologic map of California the northern part 

(north of San Antonio Reservoir) of the San Marcos 

fault shown by Hill and Dibblee (1953, pi. 1) is shown 

as the Espinosa fault (fig. 3A} by Jennings and Strand 

(1958) and Jennings (1959).

On a geologic map (scale 1:125,000) of this area compiled by Walrond and Gribi (1963) as well as on a subsurface map by Gribi (1963, p. 26) major northwesttrending faults designated as the Reliz, Espinosa, San 

Marcos, San Antonio and Jolon faults are shown. The 

first three mentioned are alined and coincide with the 

Reliz Canyon-San Marcos fault zone recognized independently by Hill and Dibblee (1953) and are alined 

with the Rinconada fault to the southeast. Gribi (1963, 

p. 23) stated that "the Reliz and Espinosa faults are 

nearly continuous" and "all folds abutting this fault 

demonstrate drag indicating marked right lateral 

movement."

The most extensive large-scale mapping to date of 

the geology of the hills from San Marcos Creek to Ar-
26 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

120°

FIGURE 11. Stream channels deflected possibly by right-lateral movement on the Rinconada fault (San Marcos segment) northwest 

of Paso Robles. See Bradley and Adelaida, U.S.G.S. 7'/2-minute quadrangles for topography.

royo Seco Canyon was by Durham (1963, 1966, 1968a, 

1968b, 1970). He inferred faults along or near much of 

the Espinosa, San Antonio, and San Marcos faults 

mapped independently by Walrond and Gribi (1963), 

but he applied only the names San Marcos, San Antonio, and Jolon to parts of these. He also inferred 

numerous other vertical faults, forming a random pattern of intersecting dashed lines through these hills.

On the basis of field checking and additional mapping of the geology of these hills, I conclude that the 

Espinosa fault shown on the maps mentioned above is 

continuous with the Rinconada fault (San Marcos segment) to the southeast and thereby designate the Espinosa fault as the Espinosa segment of the Rinconada 

fault.

The geology of the Espinosa segment of the Rinconada fault is shown in figures 12 and 13. An arbitrary division between the Espinosa segment and the 

San Marcos segment to the southeast is designated at a 

point where a minor fault splays off northward east of 

San Antonio Reservoir (fig. 10). From that point the 

Espinosa segment extends northwest for 40 km (25 

mi), then dies out at the surface in Monroe Canyon 

about 12 km (7 mi) west of King City.

The Espinosa segment of the Rinconada fault transects the length of the unnamed range of hills, sometimes called San Antonio Hills (Reed, 1933, p. 44; 

Kilkenny, 1948, p. 2261), between the Salinas and 

Lockwood Valleys (figs. 10, 12, 13). Along and northwest of San Antonio Reservoir (fig. 10) the southwest 

strand of the San Marcos segment becomes the San 

Antonio fault, a northeast-dipping reverse or thrust
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SEGMENTS OF THE RINCONADA FAULT 29

fault along which the Monterey Shale on the northeast 

was elevated against the Paso Robles Formation and 

older alluvium of Lockwood Valley on the southwest. 

This fault is traceable for about 19 km (12 mi) northwestward; then it eventually rejoins the southwest 

strand of the Espinosa segment (fig. 12).

With the exception of the San Antonio fault strand, 

the Espinosa segment of the Rinconada fault is entirely within the Monterey Shale at the surface, making it difficult to trace. Its course is marked by an 

alinement of canyons and low divides that forms a 

trenchlike notch through these hills, especially in the 

vicinity of Espinosa Canyon. The Monterey Shale 

along this notch is brecciated, crushed, and contorted.

Because the Espinosa segment is all within Monterey Shale, relative vertical displacements along this 

segment can be determined only by difference of elevations of the adjacent blocks. Southeastward from Espinosa Canyon, the northeast block is higher, reaching 

a high point at Williams Hill. Northwestward from 

that canyon to Quinado Canyon (fig. 12) the southwestern block is slightly higher, and beyond that there 

is no difference in elevation. These differences in relative elevation may reflect relative vertical displacements on the fault.

The presumed vertical displacements may be only 

apparent. At Espinosa Canyon and a few kilometres 

southeastward, several canyons that drain southwest 

across the fault and its southwest strand are deflected 

northwestward after crossing it, and northwest of that 

canyon for a few miles several northeast-draining 

canyons are deflected southeastward as shown in figure 14. This condition suggests southeastward movement of the northeast block relative to the southwest 

block, or right-lateral movement, in very late Quaternary time.

Throughout these hills the Cenozoic strata are deformed into numerous tight folds, not only adjacent to 

the Rinconada fault but along the margins of these 

hills as well. Indeed, the hills are in large part bounded 

on both sides either by a thrust fault dipping inward 

under the hills, or by a zone of upended or intensely 

folded strata (figs. 12, 13). The Los Lobos thrust south 

of San Ardo is verified by wells in and near the San 

Ardo oil field, and it dips southwest at low angles. This 

and other thrust faults dip toward the Espinosa segment, as do thrusts near other segments of the Rinconada fault to the southeast. These conditions suggest 

severe compressive movements associated with rightslip movements along the fault.

Structural evidence of right slip along the Espinosa 

segment of the Rinconada fault, as along the San Marcos segment, consists of the slightly more westerly 

trend of the folds in the Monterey Shale as compared

with that of the fault (figs. 12, 13, 19). None of the folds 

cross the fault; those adjacent to it veer nearly parallel 

to it and terminate against or near it. Folding is especially intense between strands of this segment, including the San Antonio fault. This structural pattern 

must be the drag effect in the thick incompetent Monterey Shale of right-slip movement along the fault in 

the deeply underlying basement complex.

RELATIONSHIP OF RINCONADA FAULT 

TO JOLON AND RELATED FAULTS

Published geologic maps (Jennings, 1959; Walrond 

and Gribi, 1963; Page, 1970a) show the San Marcos 

fault dividing northwestward into three strands near 

the San Antonio River (fig. 3A). One of these is the San 

Antonio thrust fault already mentioned. The other is 

an inferred fault shown as the Jolon fault, projected 

northwest through Lockwood Valley via the San Antonio River, then through an isolated outcrop of Monterey Shale on the road 3 km (2 mi) south of Lockwood, 

to a fault traceable for 3 km (2 mi) along the southwest 

base of a low ridge northwest of Jolon, for a total distance of some 32 km (20 mi).

The only part of this supposed major fault that is 

mappable by direct field observations is that part 

within 3 km (2 mi) northwest of Jolon (fig. 12), along 

which the Monterey Shale on the northeast was displaced upward against the Paso Robles Formation on 

the southwest. Field evidence suggests that this is a 

minor northeast-dipping reverse fault that probably 

curves eastward just north of Jolon and dies out. Only 

this fault will be referred to as the Jolon fault in this 

report.

The inferred or projected 32 km (20 mi) segment of 

the Jolon fault through Lockwood Valley is not mappable by direct field observations. Other than the long 

straight course of part of the San Antonio River along 

which the fault is projected, no topographic indications 

of this fault have been observed on the ground or on 

aerial photographs. The only suggestion of such a fault 

is the isolated outcrop of Monterey Shale on the road 

by Tule Creek 3 km (2 mi) south of Lockwood that 

protrudes through the unconformably overlying Paso 

Robles Formation, which appears to be elevated anticlinally. There is no evidence of severe deformation 

along this projected fault, such as that along the entire 

course of the Rinconada fault. Deep well data in 

Lockwood Valley are insufficient to ascertain the 

existence of such a fault in the subsurface. The gravity 

data (Burch, 1971) neither affirm nor deny the existence of this supposed major fault projection.

Durham (1968a, p. 40) stated that "the Jolon fault is 

a major northwest-trending feature of the San Antonio 

River valley" but did not show it on his maps of that
30 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

121 C

FIGURE 14. Canyons deflected possibly by right-lateral movement of the Rinconada fault (Espinosa segment) near 

Lockwood. See Espinosa Canyon and Williams Hill U.S.G.S. 7Vfe-minute quadrangles for topography.
SEGMENTS OF THE RINCONADA FAULT 31

valley (Durham, 1968a, pi. 1; 1965a, pis. 1, 2), although he showed a major fault there (unnamed) on a 

small-scale map (Durham, 1965b, p. D107). He cited 

geologic evidence of such a fault as follows (Durham, 

1968a, p. 41): "In the San Antonio River Valley near 

Jolon, the Jolon fault forms the boundary between two 

distinct stratigraphic sections, one to the northeast in 

which the Vaqueros Formation lies directly on the 

pre-Tertiary basement complex, and one to the southwest in which the Vaqueros is preceded by a thick sequence of strata."

This conclusion is inferred from the presence of the 

"thick sequence of strata" (the Upper Cretaceous and 

lower Tertiary sedimentary sequence) exposed extensively below the Vaqueros Formation in the hills of the 

Nacimiento River area to the west and its total absence 

in all wells drilled to the basement complex in Lockwood Valley. This interpretation implies that the 

faulting is pre-Vaqueros. These areas are at least 6 km 

(4 mi) apart, and the pre-Vaqueros rocks are entirely 

concealed throughout the intervening area and Lockwood Valley. Therefore it is not possible to determine 

whether the Upper Cretaceous and lower Tertiary 

sedimentary sequence is in fault or depositional 

(unconformable) contact with the basement complex 

where concealed. In the exposed section north of the 

Nacimiento River, the relation is depositional, with 

the unconformable contact dipping south (fig. 21). This 

contact presumably extends south of east concealed 

under the Vaqueros Formation through the length of 

Lockwood Valley, possibly under the San Antonio 

River. If it does, no fault is required.

Part of the San Marcos segment of the Rinconada 

fault shown herein (fig. 10) is considered by Durham 

(1968a, p. 41) "to be the southeast extension of the 

Jolon fault," and accordingly, he showed the northeast 

strand as the Jolon fault (Durham, 1968a, pis. 1, 2, 3). 

Because of reasons given above, this interpretation is 

not accepted.

On his final regional geologic map of the southern 

Salinas Valley, Durham (1974, p. 58-64, pis. 1, 2) 

made the following interpretations concerning the 

Jolon-Rinconada fault: (1) It is a single major fault 

extending for more the 80 km (50 mi); (2) the Rinconada fault is the principal trace east of the Salinas 

River (southeast of Paso Robles), and the Jolon fault is 

the principal trace west of that river (northwest of Paso 

Robles); (3) the Jolon fault veers somewhat westward 

through the length of Lockwood Valley; (4) southeastward from Lockwood this fault line may be (as also 

suggested by Burch and Durham, 1970, p. B13) the 

boundary between granitic basement of the Salinian 

block and Franciscan basement of the southwest at

depth; (5) the San Marcos fault is "a major secondary 

element" west of the southern part of the Jolon fault; 

(6) the Espinosa fault zone is a group of minor faults 

branching northwestward to the right from the Jolon 

fault where it crosses the San Antonio River; (7) his 

Jolon fault extends northwest under the northeastdipping San Antonio fault and veers westward into 

Lockwood Valley; and (8) his Bee Rock fault is a low 

northeast-dipping thrust fault for about 20 km (13 mi) 

along the base of the Monterey Shale southwest of his 

San Marcos fault.

Many of these interpretations are questionable. The 

positions of the Rinconada fault and its northwestern 

extensions through the southeastern part of the Jolon 

fault and Espinosa fault segments are generally shown 

correctly, as well as that of the San Antonio fault 

(Durham, 1974, pi. I). However, there is no evidence 

that the Jolon fault shown extends under the San Antonio fault into Lockwood Valley, and the segment 

shown through the hills south of Lockwood is not 

mappable.

I was unable to discover any reliable evidence, either 

in the field or in the literature, for the Jolon fault as a 

major fault diverging from the San Marcos fault segment into and through Lockwood Valley. Therefore no 

major fault is shown through that valley (figs. 3b, 4,10, 

12, 18,21).

The northeast-dipping San Antonio fault is probably 

a westerly strand of the Rinconada fault, if it rejoins it 

to the northwest. The only part of the Bee Rock fault 

that can be verified in surface exposures is a 5-km 

(3-mi)-long segment along which tightly folded Monterey Shale (lower part) was thrust southwestward 

over the northeast-dipping basal clay shale unit of the 

Monterey Shale southeast of the San Antonio River.

Another major fault supposedly related to the Jolon 

fault has been inferred under the hills of folded Monterey Shale that border the southwest side of Lockwood 

Valley, largely from gravity geophysical data (Burch, 

1971, p. B13; Durham, 1974, p. 59, their Sam Jones 

fault). The gravity pattern there is a strong gradient 

between the extensive gravity high of the Santa Lucia 

Mountains and the Nacimiento River area and the 

gravity low of Lockwood Valley and is attributed to a 

major fault (Burch, 1971, pi. 1, p. B13). However, I 

could not verify a major fault through these hills. The 

gravity gradient is parallel to and northeast of the 

great angular unconformity between the synclinally 

folded Upper Cretaceous and lower Tertiary sedimentary sequence and the overlying generally northeastward-dipping middle Tertiary sedimentary sequence (fig. 21) of relatively lower density.

From this relationship it is more probable that this
32 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

gravity gradient is the expression of the northeastward 

dip of the middle Tertiary sequence, which may be 

steep at depth where the gradient is strong, into the 

structural basin that underlies much of Lockwood Valley. Although the exposed low-density Monterey Shale 

is apparently thin and much folded, the Vaqueros 

Sandstone dips northeastward and thickens under this 

shale, as seen in exposures further southeast (fig. 21). 

This relation, together with the presence of a thick fill 

of valley sediments under Lockwood Valley, may accentuate the gravity gradient.

GEOPHYSICAL EXPRESSION OF THE RINCONADA FAULT

The Rinconada, San Marcos, and most of the Espinosa segments of the Rinconada fault are strongly 

expressed by the patterns of gravity and magnetism, as 

shown by Burch (1971), Burch and Durham (1970), and 

Hanna (1970), which strongly suggest continuity of 

this fault alinement at depth. Their gravity maps 

show that the extensive granitic exposure east of 

Santa Margarita (fig. 9) and the small one northwest of 

Paso Robles (fig. 10), both on the northeast side of the 

Rinconada fault, are expressed as moderate gravity 

highs and both are separated by a strong gravity gradient along and parallel to the fault from weak gravity 

lows that express a shallow trough of low-density 

Cenozoic sedimentary deposits on the southwest side of 

the fault. The aeromagnetic map (Hanna, 1970) shows 

a nearly similar expression, of which the two granitic 

exposures on the northeast side of the fault are expressed as very strong magnetic highs, and both are 

separated by a strong magnetic gradient along and 

parallel to the fault from magnetic lows with very little 

magnetic relief on the southwest side of the fault, 

owing to the comparative low magnetism of the 

Cenozoic sedimentary rocks there.

The granitic rock of the small exposure northwest of 

Paso Robles is indicated by both the gravity and 

magnetic data (Burch and Durham, 1970, pi. 1; 

Hanna, 1970, fig. 2) to extend about 3 km (2 mi) southeastward under the Paso Robles Formation, probably 

to the north-trending fault questionably inferred to 

pass through the two hot springs in Paso Robles.

Northwestward from San Marcos Creek, the gravity 

variations become reversed along the Rinconada fault, 

as shown by Burch and Durham (1970, pi. 1). This 

strong gravity high produced by the granitic mass exposed south of this creek gives way northwestward to a 

large gravity low on the northeast side of the fault and 

south of San Ardo (figs. 10, 18) that apparently expresses a structural basin in which the middle Tertiary 

sedimentary sequence is more than 3,048 m (10,000 ft) 

thick. This gravity low is bounded along the Rinconada

fault by a steep gradient to an area of relatively higher 

gravity under eastern Lockwood Valley on the southwest side of the fault. This gravity feature (Lockwood 

high of Durham, 1974, pi. 3), which has little gravity 

relief, is the expression of a comparatively thin middle 

Tertiary sedimentary sequence on granitic basement. 

The gravity low northeast of the fault is also a magnetic low and is adjacent to a magnetic pattern of almost 

no relief on the southwest side (Hanna, 1970, fig. 2).

RELIZ FAULT

Southwest of Greenfield (figs. 2, 3B, 13) another 

major fault probably continuous for some 56 km (35 

mi) is directly alined northwest of the Rinconada fault. 

As explained in the following paragraphs, the northern 

part of this fault has been designated as part of the 

King City fault and the southern part as the Reliz 

fault.

KING CITY FAULT

The so-called King City fault has been interpreted in 

various ways. It was first mentioned by Clark (1929, 

p. 109-110). He did not define it specifically but showed 

it on a map as extending southeastward from Monterey 

Bay along the southwest margin of Salinas Valley past 

King City, Bradley, and beyond to intersect the San 

Andreas fault in the Carrizo Plain. It was one of many 

extensive northwest-trending faults that he hypothesized to explain his fault-block theory of the origin of 

the Coast Ranges. Reed (1933, p. 43-44) also applied 

the name King City to his inferred concealed fault. He 

concluded that southeastward from the base of Sierra 

de Salinas this fault is largely theoretical, although he 

indicated there is fair evidence that it may extend as 

far as Bradley and possibly to San Miguel.

Taliaferro (1943a, p. 158) stated:

"Physiographic evidence for the King City fault is the steep, straight 

eastern front of the Sierra de Salinas which attains an elevation of 

over 3,800 feet above the Salinas Valley within 2 miles, and which is 

cut by sharp steep-walled high-gradient canyons. Alluvial fans, 

formed at the mouths of these canyons, have coalesced into a continuous apron along the range front, burying practically all direct 

evidence of faulting."

Concerning the southward extension (later called the 

Reliz fault) of this fault, he stated (p. 159): 

"The Sierra de Salinas and its wide alluvial apron end just north of 

Arroyo Seco, and a mile east of Paraiso Springs a fault emerges 

beneath the alluvial apron. This trends due south for about 2 miles 

and then turns to the southeast, crossing both Arroyo Seco and Reliz 

Canyon and dying out in Miocene sediments. This is thought to be 

a branch of the King City fault, although it may be its southern 

end. The King City fault may extend to the southeast along the 

edge of the valley either as a fault in the bedrock or as a zone of sharp 

flexing in the Tertiary sediments ***. There is no surface evidence 

*** that the King City fault extends as far as King City."

Kilkenny (1948, p. 2264-2266) considered the King
RELIZ FAULT 33

City fault to be a zone of southwest-dipping faults 

through the Salinas Valley, separating the northeastern block, in which the basement surface is shallow 

and the Miocene sequence is either very thin or absent, 

from the southwestern block, in which the basement 

surface is very deep and overlain by a thick Miocene 

sequence. Kilkenny (1948, p. 2266) concluded that 

"the King City fault system is a definitely established feature extending from the Sierra de Salinas to Bradley along the west side of 

the Salinas River; there were two periods of faulting along the King 

City line. The first, in Eocene time, created the scarp which underwent periodic rejuvenation in Miocene time; the second, in Pliocene 

and Pleistocene time, was of a thrust nature in the opposite direction, with some lateral movement in contrast to the early normal 

faulting and located along the same trend, but not necessarily coincidental with it."

Schwade, Carlson, and O'Flynn (1958, p. 86, fig. 4) accepted Kilkenny's interpretation and inferred the King 

City fault to extend southeastward via the La Panza 

fault to the Cuyama Valley.

The numerous wells drilled to develop the San Ardo 

oil field since its discovery in 1947, and many others 

drilled in and near the Salinas Valley, provided much 

data on the subsurface geology of this area, particularly in relation to the King City fault. There is no 

evidence from the well data of a single present-day 

fault of mainly vertical movement under this valley 

southeastward from the base of Sierra de Salinas. Instead, the basement surface slopes southwestward, 

under this valley, in places rather abruptly, from the 

Gabilan mesa or shelf to a deeply subsided basin filled 

with a thick middle Tertiary sedimentary sequence.

This slope trend, formerly interpreted by Kilkenny 

(1948, p. 2264-2266) as the King City fault, was reinterpreted by Gribi (1963, p. 17, 23) from the well data 

as the King City hinge line, along which a great thickness of incompetent Monterey Shale abuts northeastward against the rigid basement, has been folded 

against it by compressive stress, then thrust toward 

the basement buttress. Gribi (1963) cited as an outstanding example the Los Lobos fault along the foothills west of San Ardo for about 17 km (10 mi) northwestward. However, I suggest that this fault may be 

related to the Rinconada fault (Espinosa segment) if it 

splays from it at depth (fig. 18). Gribi (1963) suggested 

that the King City hinge line may have originated as a 

basement fault.

The so-called King City fault southeastward from 

the Sierra de Salinas to Bradley is now generally considered nonexistent and is not shown in the Salinas 

Valley on geologic maps compiled by Jennings and 

Strand (1959), Walrond and Gribi (1963), Walrond, 

Thorup, Gribi, and Rogers (1967), Martin and Emery 

(1967), nor on maps and sections by Durham (1963, 

1964, 1965b, 1966, 1970), although it is mentioned by

him (1970, p. 24), nor Dibblee (1971), and none of these 

authors applied this name to any fault shown.

Gribi (1967, p. 91) and Walrond, Thorup, Gribi, and 

Rogers (1967) interpreted the fault formerly called the 

King City fault along the base of Sierra de Salinas as 

continuous with that mapped as the Reliz thrust by 

Schombel (1943, p. 462) and designated the combined 

fault as the Reliz fault. This designation is accepted, as 

shown in figures SB, 4, 13, and 15, even though Jennings (1973) applied the name King City fault to the 

part along the base of Sierra de Salinas.

RELIZ FAULT IN ARROYO SECO AREA

The Reliz fault of the Arroyo Seco area was defined 

by Nickell (1931) and Schombel (1943). The geology 

along this part is as shown in figure 13. This part is 

considered to be the southeastern segment of the major 

concealed fault along the base of Sierra de Salinas. The 

Reliz fault is directly alined with the Rinconada fault 

to the southeast but is separated from it at the surface 

by a gap of only about 3 km (2 mi) in Monroe Canyon 

west of King City.

The Reliz fault is well exposed on the west side of 

Reliz Canyon and on the north bank of Arroyo Seco 

(Durham, 1970, p. 25). In both places the fault dips 

about 70° southwest. With the exception of the syncline 

across Reliz Canyon, the southwest block is elevated to 

juxtapose tightly folded Monterey Shale against 

northeast-dipping Pancho Rico and Paso Robles Formations of the northeast block (fig. 13). Vertical displacement increases northward as successively lower 

(older) Tertiary units on the southwest side are elevated against the Paso Robles Formation on the northeast side.

A right-lateral component of movement on this segment of the Reliz fault is suggested by the pattern of 

east-trending fold axes in the Tertiary strata on the 

southwest side of the fault. The large syncline across 

Reliz Canyon is displaced slightly right laterally, but 

the folds in the Monterey Shale to the northwest are 

terminated against the fault (fig. 13).

The Reliz fault was active primarily after deposition 

of the Paso Robles Formation, for this formation as 

well as the underlying strata are truncated by and 

tilted up by the fault. It is considered presently inactive because there is no indication of movement after 

deposition of the older alluvium and alluvial fans that 

cover this segment of the fault. However, 3 km (2 mi) 

northeast of Paraiso Hot Springs or 10 km (6 mi) west 

of Greenfield and 1.6 km (1 mi) northeast of the Reliz 

fault, an old alluvial fan is broken by a small 

southwest-facing scarp that was undoubtedly formed 

by faulting.
34 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

Paraiso Hot Springs (fig. 13) issue from basal red 

beds of the middle Tertiary sedimentary sequence in 

contact with granitic basement just west of the Reliz 

fault. The hot water presumably rises along or near the 

fault and breaks through just west of it, possibly on a 

subsidiary fracture.

RELIZ FAULT AT BASE OF SIERRA DE SALINAS

Near Paraiso Hot Springs the Reliz fault is concealed 

by alluvial fans. Farther northwest, the base of the 

steep northeast slope of Sierra de Salinas is undoubtably underlain by a major fault, as postulated by Clark 

(1929), Reed (1933), Taliaferro (1943a), and Kilkenny 

(1948), and designated by them as the King City fault. 

This major fault is considered to be part of the Reliz 

fault (Walrond and Gribi, 1963; figs. 13, 15 herein).

The Sierra de Salinas is eroded from pre-Tertiary 

basement complex elevated on the southwest side of 

this fault. The base of this steep mountain front is 

moderately straight for a distance of nearly 12 km 

(20 mi), but the fault trace is almost entirely covered 

by the upper margins of the northeast-sloping dissected Pleistocene coalescent alluvial fans, some of which 

extend into canyons of this range. However, about 8 

km (5 mi) northwest of Paraiso Hot Springs the alluvial fans have been eroded through to expose the unconformably underlying Paso Rboles Formation on the 

northeast side of the fault.

Upward vertical displacement of the Sierra de 

Salinas block on the Reliz fault relative to the valley 

block on the northeast must be many hundreds of 

metres, equal to the depth to the top of the buried surface of the basement complex under the valley block 

adjacent to the fault, plus the present altitude of Sierra 

de Salinas, plus the amount of basement complex 

above this altitude removed by erosion. This displacement may be as much as 3,048 m (10,000 ft) or more. 

Several deep wells drilled for oil in the valley area 

indicate that the buried surface of the basement complex slopes southwest from its exposures in the Gabilan Range toward the base of Sierra de Salinas. The 

position of the "trough line" of this buried surface is 

undetermined but is probably close to, if not at, the 

Reliz fault. Owing to the concealment of the Reliz fault 

along the base of Sierra de Salinas, the horizontal 

component of movement on this segment is not known.

A northeast-southwest gravity profile (Fairborn, 

1963) across Salinas Valley about 9 km (6 mi) southeast of Salinas indicates that the buried surface of the 

basement complex slopes about 10° southwest under 

the valley from exposures at its northeast margin, to a 

depth of about 2,438 m (8,000 ft) below sea level at its 

southwest margin adjacent to the Sierra de Salinas

where basement complex rises as high as 610 m (2,000 

ft) above sea level. The steep gravity gradient near the 

mountain front indicates the probable existence of the 

Reliz (King City) fault with a possible vertical displacement of 3,048 m (10,000 ft) at this line.

Farther northwest, another northeast-southwest 

gravity profile (Sieck, 1964) across Salinas Valley 

about 1.3 km (1 mi) northwest of Salinas indicates that 

the buried basement surface likewise slopes southwestward to a low point of about 1,890 m (3,700 ft) at 

the southwest margin of the valley, either on a fault or 

a possible steep northeast flank of an arched basement 

surface. If the latter is so, the Reliz fault dies out 

northwestward near this gravity profile line.

The direction and angle of dip of Reliz fault along the 

base of Sierra de Salinas are not known but are suspected to be southwestward and steep judging from the 

70° SW. dip of the part exposed in the Arroyo Seco area.

The alluvial fans that cover the Reliz fault are not 

broken by the fault, which indicates that it was not 

active since they were deposited. However, in one 

small area about 14 km (9 mi) southeast of Salinas 

there are several northeast-facing scarps in the fans 

parallel to the fault. These appear to be scarps produced by faulting, but they are more likely former 

stream-cut banks of the Salinas River.

Near the north end of Sierra de Salinas the Reliz 

fault splays into several faults, which presumably die 

out northwestward (fig. 15). Accordingly, the basement 

complex elevated on these faults becomes concealed 

under the Cenozoic sediments that flatten northwestward toward Monterey Bay.

POSSIBLE NORTHWESTWARD EXTENSION 

OF THE RELIZ FAULT

Martin and Emery (1967, fig. 6, p. 2302) postulated 

that the Reliz fault extends northwestward across 

Monterey Bay via their Gabilan fault to join the San 

Gregorio fault (of Branner and others, 1909), with a 

branch extending northwest to the Ben Lomond fault 

northwest of Santa Cruz. Ross and Brabb (1973) also 

suggested that their King City fault may extend across 

Monterey Bay to join the San Gregorio fault onshore 

northwest of Santa Cruz, largely on the basis of possible differences in types of plutonic basement rocks on 

opposite sides of this postulated extension. However, 

there are no surface indications onshore there of this 

extension (Branner and others, 1909, p. 9; Brabb, 1970; 

Clark, 1970; fieldwork by myself in 1948-49).

An acoustical survey of Monterey Bay by Greene 

(1970) and Greene, Lee, McCullock, and Brabb (1973) 

showed evidence of several parallel discontinuous local 

northwest-trending faults and small folds in the Monterey Shale farther west, just north of Monterey (fig.
122°

36 37'30" -

121°45' 121°30'

36 30' -

36 22'30" -

V-C, V\^ rv/-x\ '-NX. i '-\*>X''<xrx \CON

0 \. %tf.^^'s£7.fei

e. '.YT^^T^^/^'-X^ZNO"'^

/XOv/w.-iv-;^ 

^V^i>-T^

FIGURE 15. Geology of the northern Santa Lucia Mountains, a segment of the Salinian block elevated on the Reliz fault and many other 
36 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

22), but none in the bay area underlain by Pliocene and 

Pleistocene sediments through which the Reliz (or 

King City) fault has been projected.

REGIONAL EVIDENCE OF LARGE STRIKE-SLIP

MOVEMENT ON THE RINCONADA-RELIZ

FAULT ZONE

From the data recorded I concluded that movement 

on the Rinconada fault was predominantly strike slip 

by which the terrane to the southwest has moved 

northwestward relative to that on the northeast, with 

local or apparent relative vertical displacements, and 

that movement on the Reliz fault was probably oblique 

slip with the southwestern block elevated and displaced northwestward relative to the northeast block. 

Right-slip movement is strongly suggested by the severe deformation of the sedimentary rocks into folds 

with axes that persistently trend slightly more eastwest than the faults along nearly the entire course of 

the Rinconada fault and the part of the Reliz fault that 

transects those rocks.

MOVEMENTS IN QUATERNARY TIME

There are no indications that the Rinconada and 

Reliz faults are active presently or were during Holocene time. Evidence of the inferred Holocene movement on the Rinconada fault north of Santa Margarita 

is slight and inconclusive. Deflection of canyons, possibly by right-lateral movement on the San Marcos and 

Espinosa segments of this fault, as mentioned, may 

have happened in very late Pleistocene time, just prior 

to Holocene time, because there are no breaks in the 

alluvium. At the west base of Williams Hill there is 

suggestive evidence that the Monterey Shale was elevated along the east strand of this segment against 

older alluvium to the southwest. Elsewhere in this 

area both strands are in places covered by this 

alluvium.

The Paso Robles Formation, primarily of Pleistocene 

age, is the youngest formation definitely involved in 

displacements and deformation along both the Rinconada and Reliz faults. Therefore these faults were 

active in late Pleistocene time. Along the Rinconada 

fault, vertical displacements that involve this formation are not large, generally less than a few hundred 

metres (1,000 ft). Along the Reliz fault north of Arroyo 

Seco they are greater. The amount of lateral displacement of this formation is not measurable.

MOVEMENTS IN PLIOCENE TIME

The unconformable relation of the Paso Robles Formation (and of the Pliocene upper sandstone unit of the 

Pancho Rico Formation where present) to the more severely deformed Miocene formations along and near 

the Rinconada fault suggests that this fault was 

strongly active in Pliocene time.

A large amount of lateral displacement since early 

Pliocene time is suggested on the San Marcos segment 

by differences in the Miocene and Pliocene stratigraphy on opposite sides of that segment. Durham (1965b) 

pointed out that the northernmost extent of the Santa 

Margarita Formation and southernmost extent of the 

Pancho Rico Formation nearly coincide and that both 

have been displaced about 18 km (11 mi) by right-slip 

movement on an undesignated northwest-trending 

fault.

This interpretation of that amount of right-lateral 

movement is accepted herein, but it applies to the San 

Marcos segment of the Rinconada fault, as shown in 

figure 16. On the northeast side of this fault, logs of 

exploratory wells drilled for oil or gas indicate that the 

Pancho Rico Formation extends far south of the northern limits of the Santa Margarita Formation. From 

these relations (fig. 16) it is probable that the San Marcos segment, if not adjacent segments, of the Rinconada fault were active since early Pliocene time with 

right-lateral movement that amounted to as much as 

18 km (llmi). Most of this movement probably preceded deposition of the Paso Robles Formation and possibly the upper sandstone unit of the Pancho Rico Formation.

Additional evidence of right-lateral displacement on 

the Rinconada fault since deposition of the Monterey 

Shale is suggested by the areal extent farther northwest of the thick upper part of the Monterey Shale. The 

absence of the Santa Margarita Sandstone, overlying 

the Monterey Shale, on the west side of the fault also 

suggests this displacement.

MOVEMENTS SINCE EARLY MIOCENE TIME

The occurrence of the basal terrestrial unit of the 

middle Tertiary sedimentary sequence farther northwest on the west side of the Rinconada fault than on 

the east side could be the result of right-lateral displacement on the Rinconada fault (see "Rock Units"). 

More data on the subsurface extent of this unit are 

needed to test this hypothesis.

At Harris Valley about 3 km (2 mi) west of San Antonio Reservoir west of the Rinconada fault, the 

sandstone of the upper part of the Vaqueros Formation 

contains several thick lenses of conglomerate composed of unsorted cobbles and boulders almost entirely 

of granitic detritus (fig. 10). These lenses pinch out 

southwestward, and their northeastward extent is concealed. They must have been derived from a granitic 

terrane, despite the fact that the exposed Vaqueros 

Formation throughout this area is underlain by the
121°30* 120° 30' 

40 KILOM

EXPLANATION

Unit 1 exposed 

Unit 2 exposed 

Unit 1 covered 

Unit 2 covered 

Units 1 and 2 covered

Rocks older than unit 2, 

exposed

Rocks older than unit 2 covered 

by rocks younger than unit 1

Covered extents of units 1 and 2 

inferred from well data

x x x x x x Inferred southeasternmost extent of unit 1 

oooooooo Inferred northwesternmost extent of unit 2 

X Probably displaced from vicinity of Y

Santa Matgarita

FIGURE 16. Inferred displacement of lower Pliocene (unit 1, Pancho Rico Formation) and upper Miocene (unit 2, Santa Margarita Formatio

strike-slip movement on the Rinconada fault (in part modified after Durham, 1965b).
38 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

thick Upper Cretaceous and lower Tertiary marine 

sedimentary sequence exposed to the west and south. 

The granitic source area must have been to the northeast, presumably across the Rinconada fault.

According to D. C. Ross (oral commun., 1974) the 

granitic detritus is subporphyritic granodiorite-quartz 

monzonite similar to that exposed on the east side of 

the Rinconada fault northwest of Paso Robles (fig. 10) 

and exposed extensively in the La Panza Mountain 

area. This granitic rock may underlie a large part of 

the Salinas Valley at least as far northwest as San 

Ardo; therefore, the granitic source area of these clasts 

could have been anywhere between the La Panza 

Range and San Ardo northeast of the Rinconada fault. 

However, the source must have been an area that was 

undergoing erosion when the Vaqueros Formation was 

being deposited or an area where this granitic basement is overlain by sediments younger than the Vaqueros. Such an area may be the exposure northwest of 

Paso Robles, which is overlain by the Paso Robles 

Formation. If this was the source area of the granitic 

clasts, it has been shifted some 23 km (15 mi) southeastward on the Rinconada fault since early Miocene 

time. Another possible source is the large granitic exposure of the La Panza Range area near Santa Margarita, on which the Vaqueros is very thin or locally 

absent. If this were the source, it has been shifted as 

much as 56 km (35 mi) southeastward on the Rinconada fault since early Miocene time.

MOVEMENTS IN OLIGOCENE(P) TIME

Movement on the Rinconada fault before deposition of the middle Tertiary sedimentary sequence is 

suggested from the possibly displaced areal extent of 

the very thick Upper Cretaceous and lower Tertiary 

sedimentary sequence that overlies the crystalline 

basement complex of the Salinian block. As shown in 

figure 4, and as discussed previously, on the southwest 

side of the fault this sequence extends as far north as 

the Nacimiento River area, whereas on the northeast 

side, it extends no farther north than the La Panza and 

perhaps the Caliente Range. Deep wells drilled for oil 

or gas throughout Salinas Valley north of La Panza 

Range and in the Carrizo Plain enter the crystalline 

basement directly below the middle Tertiary sedimentary sequence.

These conditions suggest that the basement complex 

and the overlying thick southward-dipping sedimentary sequence may have been displaced by a large 

amount of right-slip movement on the Rinconada fault, 

just prior to or possibly during or after Oligocene time. 

It is also possible that this could have resulted from 

relative upward vertical displacement of the northeast

block of the fault prior to Oligocene time. However, the 

granitic basement and the overlying thick sedimentary sequence so extensively exposed in the vicinity of 

the La Panza Range east of parts of the Nacimiento 

and Rinconada segments of this fault are abruptly 

truncated by the fault and have no counterparts directly across it (figs. 8, 9). Instead, the pre-Oligocene 

rocks exposed west of these segments are Franciscan 

rocks, overlain by a relatively thin sequence of Cretaceous strata structurally unlike the sequence east of 

the fault. These conditions cannot be accounted for by 

vertical displacement, especially if this fault is nearly 

vertical. The only explanation seems to be that the 

opposing blocks were juxtaposed by lateral movement 

along the fault from areas originally far apart.

The westward extensions of the granitic basement 

complex and overlying thick Upper Cretaceous and 

lower Tertiary sedimentary sequence of the La Panza 

Range area on the east side of the Rinconada fault 

(figs. 8, 9) are probably the same as the granitic and 

metamorphic basement complex and the overlying 

thick Upper Cretaceous and lower Tertiary sedimentary sequence exposed in the San Marcos CreekNacimiento River area west of the San Marcos and 

Espinosa segments of the Rinconada fault (figs. 10, 17). 

Supporting evidence is that in both areas the sedimentary sequence is lithologically identical, has a similar 

basal cobble conglomerate locally, and dips generally 

southward off the basement complex into a synclinal 

structure. All these features seem to be displaced along 

the fault as much as 60 km (38 mi), suggesting that 

much right-lateral movement on the Rinconada fault 

since early Tertiary time. Schwade, Carlson, and 

O'Flynn (1958, p. 85, fig. 4) recognized evidence of displacement of this magnitude when they concluded that 

the contact of Cretaceous strata with the granitic 

basement of the Salinian block exposed in the northern 

Santa Lucia Mountains and in the La Panza Range 

was laterally displaced 72 km (45 mi) on the Rinconada 

fault (which they designated as the San Marcos fault) 

even though no detailed geologic mapping had been 

done by that time to demonstrate this interpretation. 

In addition to the basic geologic mapping to date, detailed geologic and petrologic work may be needed to 

test this hypothesis.

POSSIBLE MOVEMENTS IN PRE-LATE CRETACEOUS TIME

There is no direct evidence to indicate that the Rinconada or Reliz faults existed prior to deposition of the 

Upper Cretaceous and lower Tertiary sedimentary 

sequence.

However, the pre-Upper Cretaceous crystalline 

basement complex exposed west of both the Rinconada
.'O/o '.a-'

'? :&%

££'«&

36°

35°

EXPLANATION

PRE-OLIGOCENE ROCKS OF SAL

.v.-.'-rx.-j

i.;.;-\v\V| Upper Cretaceous and 

marine sedimentary s

( o o o -basal conglo

Basement complex of 

metamorphic rocks (

older)

Rock units covered

A, Unconformable contact of basal 

Upper Cretaceous and lower T

sequence on the basement com

placed from B

C, Probable major synclinal axis in 

and lower Tertiary sedimenta

sibly displaced from D or E

10 30 fvl IL ES

0 10 20 30 40 KILOMETRES

122° 121 120 119

FIGURE 17. Pre-Oligocene rocks of the Salinian block, possible displaced by right-lateral movement of the Rinconada-Reliz fault zon
40 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

and Reliz faults is composed mostly of metamorphic 

rocks with the exception of the northern Santa Lucia 

Mountains where the basement is plutonic, whereas 

that exposed and struck in many deep wells east of 

those faults is mostly plutonic. This condition is 

suggestive of large displacement, either vertical or lateral, on a possible fault zone that may have existed 

prior to Late Cretaceous time and that evolved into the 

Rinconada and Reliz fault alinement, as suggested by 

Nilsen and Clarke (1975, p. 35).

Work on the crystalline basement complex of the 

Salinian block is in progress (Ross, 1972; 1974).

STRIKE-SLIP MOVEMENT

AS SUGGESTED BY CONFIGURATION

OF BURIED SURFACE OF BASEMENT COMPLEX

The inferred configuration of the surface of the crystalline basement complex under the sedimentary cover 

of the central Salinas Valley region is about as shown 

in figure 18. The positions of contours near wells and 

basement exposures are deemed moderately accurate 

but elsewhere are inferred from the geology.

It should be noted that this is one of several alternative contour interpretations, some of which show no 

displacement on the Rinconada fault zone (Durham, 

1974, pi. 3). In my opinion the one shown seems to be 

the most probable interpretation on the basis of the 

available data.

As shown in figure 18, the partly exposed basement 

high near Paso Robles (or possibly the basement exposure of the La Panza Mountains to the southeast) east 

of the Rinconada fault may have been displaced at 

least 38 km (25 mi) right laterally on the fault from the 

buried shallow basement high (Lockwood high of 

Durham, 1974, p. 13, pi. 3) near Lockwood.

The estimated 3,658-km (12,000-ft)-deep trough 

west of Bradley on the east side of the Rinconada fault 

may be displaced by that same amount from the 3,048 

km (10,000-ft)-deep trough in the Arroyo Seco area 

west of the Rinconada-Reliz fault alinement. If this 

interpretation is correct, these faults are continuous at 

depth, even though there is a gap between the Rinconada and Reliz faults at the surface.

The inferred continuous southward increase in depth 

to the basement complex on the west side of the Rinconada fault northwest of Paso Robles (fig. 18) is the 

effect of the southward slope of the basement surface 

under the thick regionally southward-dipping Upper 

Cretaceous and lower Tertiary sedimentary sequence. 

A similar condition must exist east of this fault southeastward from the La Panza Range (southeast of Paso 

Robles). Displacement of this southward-sloping 

basement surface is nearly 60 km (38 mi), in accordance with the offset shown in figure 17.

REGIONAL TECTONICS ALONG AND NEAR 

THE RINCONADA-RELIZ FAULT ZONE

CONCEPTS OF TECTONIC EVOLVEMENT 

OF THE COAST RANGES

The tectonic evolvement of the Coast Ranges and 

intervening valleys to their present form has been interpreted in various ways by geologists who have done 

or directed extensive geologic work within them. Willis 

(1925, p. 270-271) interpreted the Coast Ranges as 

crustal segments elevated primarily by northeastward 

thrusting on major faults that bound some of the 

ranges on the northeast and curve southwestward 

under them to eventually extend under the Pacific 

Ocean at depth. He interpreted the San Andreas and 

Sierra Nevada faults as two of these southwest-curving 

faults. Clark (1929) interpreted the coast ranges and 

valleys to have been formed as fault blocks bounded by 

high angle faults, with the elevated blocks forming the 

ranges and the depressed blocks the valleys, and supposed that each block acted independently. Reed (1933, 

p. 55-58) questioned Clark's fault-block theory and 

suggested that the ranges were elevated primarily by 

folding and then became faulted along one or both 

margins as deformation progressed. Taliaferro (1943a, 

p. 157-158) concurred with Reed (1933) that the Coast 

Ranges, such as the Santa Lucia Mountains, were 

formed by uplift on one or both sides by compressive 

folding and eventually by thrusting outward toward 

the adjacent valley or valleys.

Hill and Dibblee (1953, p. 455) hypothesized that the 

Coast Range structures are genetically related to the 

San Andreas and other high-angle faults along which 

movements have been strike slip during late if not all 

of Cenozoic time, in accordance with the law of uniformitarianism, with as much as 560 km (350 mi) of 

cumulative right-lateral movement on the dominating 

San Andreas fault. They asserted that the folds and 

accompanying thrust and reverse faults in the Coast 

and Transverse Ranges are shallow structures generated by strike-slip movements on the deep-seated 

high-angle faults. Crowell (1952, 1962) independently 

arrived at similar conclusions regarding large strikeslip movements 'on the San Andreas, San Gabriel, and 

other major high-angle faults in southern California.

Christensen (1965, p. 1118-1119) interpreted the 

Coast Ranges to have been elevated vertically as the 

result of forceful plutonic intrusions at depth and 

suggested that the folding and thrust faulting along 

their margins may be the effect of downward movement of the elevated mountain masses by gravity.

During the last decade many publications have appeared, including those by Crowell (1968), Atwater 

(1970), and Nilsen and Clarke (1975), that relate tec-
REGIONAL TECTONICS ALONG AND NEAR THE RINCONADA-RELIZ FAULT ZONE 41

121'

36C

Exposed basement complex

 -2000 

Contours on top of buried

basement complex

Contour interval 2000 feet 

Datum is mean sea level

Well that reaches basement 

complex

10 MILES

10 15 KILOMETRES

FIGURE 18. Inferred configuration of buried surface of crystalline basement complex along Rinconada-Reliz fault zone from 

Paso Robles to Greenfield. In part based on well data from Burch and Durham (1970), Burch (1971), and Durham (1974), 

and much modified from Gribi (1963, p. 26).
42 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

tonics along the San Andreas fault system to the global 

theory of plate tectonics and continental drift. In so 

doing, nearly all accept the concept of large cumulative 

right-lateral movement on the San Andreas fault system and related tectonics advanced by Hill and Dibblee 

(1953) and Crowell (1962).

From the extensive geologic mapping I have done 

since 1938 within 50 km (35 mi) of much of the San 

Andreas fault, including the geologic data presented in 

the foregoing sections of this report, I conclude that the 

Coast Ranges have evolved in accordance with the tectonic genesis postulated by Hill and Dibblee (1953) and 

Crowell (1952, 1962). This concept therefore serves as 

a basis for the tectonic interpretations presented in 

this section.

TECTONIC PATTERN

The tectonic pattern along the Rinconada-Reliz fault 

zone is fairly well denned. The tectonic pattern and 

upheaval along the Espinosa segment of the Rinconada 

fault are shown diagramatically in figure 19. This condition is general in varying degrees along the entire 

course of the Rinconada-Reliz fault zone. The persistent slightly more east-west trend of folds in the

sedimentary rocks as compared to the trend of both 

faults is interpreted to be the drag effect of rightlateral movement on the faults in the underlying 

basement complex. Thrust and reverse faults that dip 

inward toward the Rinconada fault from one or both 

sides probably evolved from large ruptured folds and, 

together with the intensity of the associated folding, 

indicate severe compression of one block against the 

other, or resistance to lateral slip. This resulted in 

severe squeezing along one or both sides of the 

Rinconada-Reliz fault zone.

On a regional scale, the major tectonics of the southern Coast Ranges are basically similar to the smaller 

scale tectonics along the Rinconada-Reliz fault zone. 

The major tectonic features of the area of figures 1 to 5 

are as interpreted in figure 20. All the present uplifts 

shown were formed during late Cenozoic (primarily 

Quaternary) time. Throughout this area, in both the 

Salinian and Coastal block, the axes of major uplifts 

(except the Gabilan uplift) and the intervening troughs 

trend slightly west of northwest, as do the thrust faults 

and fold axes. The consistency of this trend indicates 

crustal squeezing or shortening normal to this trend 

from a compressive stress that acted accordingly.

FIGURE 19. Right-slip tectonics and compressive upheaval along the Espinosa segment of the Rinconada fault.
REGIONAL TECTONICS ALONG AND NEAR THE RINCONADA-RELIZ FAULT ZONE 43

These parallel structures are contemporaneous with 

and may be subsidiary to right-lateral movements of 

blocks on the major faults, such as the San Andreas, 

Rinconada, Reliz, and other lesser faults shown in 

figure 20 which trend slightly north of northwest.

The Rinconada-Reliz fault zone divides the region 

southwest of the San Andreas fault into two geologic 

blocks of strips, the east strip and the west strip, which

differ somewhat structurally and physiographically.

The east strip, wholly within the Salinian block, is 

structurally and physiographically simple, in general. 

The northern part is almost unfaulted and includes the 

major part of the Salinas Valley, which merges eastward into extensive areas of hills and low mountains 

including the Gabilan Range, which rises to about 

650 m (2,000 ft). The east strip becomes increasingly

36° -

35" -p

EXPLANATION

Exposed Franciscan Exposed crystalline

basement complex basement complex

of Coastal block of Salinian block

High-angle strike-slip fault

Reverse or thrust fault 

(sawteeth on elevated block)

MAJOR FAULTS

RE Reliz fault SJ San Juan fault 

RI Rinconada fault BP Big Pine fault 

NA Nacimiento fault

Approximate crest line of late 

Quaternary uplift (not necessarily anticlinal)

Approximate trough line of 

areas between uplifts (generally synclinal)

Direction of extensive late 

Quaternary tilt

UPLIFTS TROUGHS ,/ 

Northeast of Rinconada-Reliz fault zone 

Gabilan 1. North Salinas 

Caliente 2. South Salinas 

La Panza 3. Cuyama 

Sierra Madre 4. Madulce syncline 

Pine Mountain (pre-Miocene)

Southwest of Rinconada-Reliz fault zone 

Sierra de Salinas 5. Arroyo Seco 

North Santa Lucia 6. Lockwood 

Junipero Serra Dome 1'. Nacimiento 

Coast Ridge (largely pre-Miocene) 

South Santa Lucia 8. Huasna 

San Rafael Mountain

122° 121° 120° 

FIGURE 20. Major tectonic features of the southern Coast Range region in vicinity of the Rinconada-Reliz fault zone.

119°
44 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

complex southeastward as parts of it were elevated 

along faults to form the La Panza and Caliente Ranges, 

which separate the Carrizo Plain from Cuyama Valley, 

and the Sierra Madre Range, elevated on faults south 

of that valley. The Sierra Madre Range, the highest of 

this strip with altitudes of more than 1,600 m 

(5,000 ft), is physiographically part of the west strip, 

but it lies east of the Rinconada fault. The east strip 

includes Mount Pinos at its easternmost tip with an 

altitude of 2,600 m (8,000 ft).

In contrast, the west strip, which includes parts of 

both the Salinian and Coastal blocks, is structurally 

complex nearly throughout. It is an area of ranges of 

mountains and hills and includes only a few very small 

valleys. It is made up largely of the Santa Lucia-San 

Rafael Mountain belt, which is structurally the most 

complex and mobile part of the southern Coast Range 

province west of the San Andreas fault. The northern 

part of the west strip is elevated along the Reliz fault 

against the Salinas Valley part of the east strip. The 

southwest border of the west strip is poorly defined, but 

it is arbitrarily placed at the southwestern base of the 

Santa Lucia-San Rafael Mountain belt because this is 

the only continuous physiographic lineament. Northward this strip terminates at the high northern Santa 

Lucia Mountains with altitudes of more than 1,600 m 

(5,000 ft). Southward it terminates either at the high 

part of the San Rafael Mountains north of the Big Pine 

fault with altitudes of more than 2,000 m (6,500 ft) or 

farther southeast at the Santa Ynez fault. This strip 

thereby attains its greatest altitudes at its northern 

and southern extremities.

TECTONIC FEATURES OF THE EAST STRIP

The northwestern part of the east strip is made up 

primarily of the Gabilan uplift (A, fig. 20). This is a 

stable mass of crystalline basement complex (mostly 

plutonic) in large part overlain by a thin cover of middle and late Cenozoic sedimentary deposits. This uplift 

is slightly arched along an axis near the San Andreas 

fault and is tilted gently southwest toward the Salinas 

trough (1 and 2, fig. 20). The thin sedimentary cover is 

generally undeformed, except along the margins where 

it is folded probably by inferred right-lateral drag on 

the San Andreas and Rinconada-Reliz fault zones.

In the southeastern part of the east strip the surface 

of the basement complex deepens southward as the 

middle and late Cenozoic sedimentary cover thickens. 

The narrow wedge east of the San Juan fault within 

this part of the east strip was dragged southward along 

this fault and compressed into several southwardleaning and southward-thrust large anticlinal folds to 

form the Caliente Range uplift (B, fig. 20).

The La Panza uplift (C, fig. 20) was formed by arching and northeastward tilt on the northeast-dipping La 

Panza thrust fault. South of the La Panza and Caliente 

uplifts the east strip is increasingly deformed southward, as the top of the basement complex deepens even 

more so under a great thickness of the Upper Cretaceous and lower Tertiary sedimentary sequence, now 

exposed in the Sierra Madre Mountains where it is 

synclinally folded adjacent to the Rinconada fault. A 

long wedge of this thick sequence adjacent to this fault 

was severely compressed and elevated along the 

southwest-dipping south Cuyama and Ozena thrust 

faults toward Cuyama Valley to form the Sierra Madre 

uplift (D, fig. 20) during late and possibly middle 

Cenozoic time.

The east-trending Big Pine fault was active in 

Quaternary time as indicated by left-lateral displacement and deflection of some canyons that cross it (Hill 

and Dibblee, 1953) and may have been active earlier. 

Major left-lateral movements on this transverse fault 

dispalced the Sierra Madre uplift (D, fig. 20), Madulce 

trough (4, fig. 20), Ozena fault, and possibly the Rinconada fault, all about 14 km (9 mi), indicating that 

this transverse fault is in large part younger than all 

those features. The Pine Mountain uplift (E, fig. 20), 

the probable displaced counterpart of the Sierra Madre 

uplift, was elevated in Quaternary time on the northeast-dipping Pine Mountain fault. This fault may have 

been reactivated on an earlier, displaced extension of 

the Rinconada fault on which there may have been 

much right-lateral movement in late Tertiary time. 

Evidence suggesting this movement is the similarity of 

the Eocene stratigraphy of the Pine Mountains uplift 

to that of the southeastern part of the Sierra Madre 

uplift and its dissimilarity to that of the area south of 

the Pine Mountain fault.

TECTONIC FEATURES OF THE WEST STRIP

The northern part of the west strip is made up of a 

wedge of the Salinian block that is severely deformed 

to form the high northern Santa Lucia Mountains from 

which several major ridges extend southeastward toward the Rinconada-Reliz fault zone. This wedge is the 

most intensely deformed part of the Salinian block and 

is composed of four major uplifts and two intervening 

troughs (F, G, H, I, 5 and 6, fig. 20). The middle Tertiary sedimentary sequence that once covered this area 

is preserved only as remnants in the troughs, which 

indicates that the uplifts formed after deposition of this 

sequence, or in late Cenozoic time.

The Sierra de Salinas uplift (F, fig. 20; fig. 15) was 

formed largely as a southwest-tilted block of basement 

complex elevated on the Reliz fault. On its southwest
REGIONAL TECTONICS ALONG AND NEAR THE RINCONADA-RELIZ FAULT ZONE 45

flank it merges through complex structures as shown 

in figure 20 with the north Santa Lucia uplift.

The Arroyo Seco trough (5, fig. 20; fig. 13) preserves 

much of the thick middle Tertiary sedimentary sequence that is compressed into a complex synclinal 

structure that plunges east toward the Reliz fault. This 

trough might have been displaced on the RinconadaReliz fault from the south Salinas trough (2, fig. 20; 

fig. 18) of the east strip. Northwestward the Arroyo 

Seco trough once extended into a shallow syncline in 

Carmel River valley south of Monterey and into Monterey Bay. The middle Tertiary sedimentary sequence 

of this trough has been severely folded, disrupted by 

faulting, and involved with the adjacent uplifts to form 

part of the northern Santa Lucia Mountains during 

Quaternary time and is now undergoing erosion.

The north Santa Lucia uplift (G, fig. 20; fig. 15) constitutes the major mass of the northern Santa Lucia 

Mountains. It is in large part bounded on the northeast 

by the southwest-dipping Tularcitos fault and the 

Arroyo Seco trough and on the southwest by the 

northeast-dipping Sur fault zone. Northward it terminates in the granitic mass on the Monterey Peninsula 

and undersea to the northwest. Southeastward it extends into the Junipero Serra Dome (H, fig. 20). Structurally the north Santa Lucia uplift is primarily a 

group of subparallel northeast-tilted fault blocks, each 

elevated on a fault at its southwest border. At least two 

of these faults (Miller Creek and Church Creek faults, 

fig. 15) dip steeply northeast; two others to the northwest near the coast seem to deflect canyons right laterally. This uplift was apparently elevated on these 

internal faults as well as on those that bound it.

The position of the north Santa Lucia uplift, together 

with the Sierra de Salinas uplift, west of the Reliz fault 

may be tectonically significant. Because the Reliz fault 

is at the northwest end of the Rinconada-Reliz fault 

zone and because this mass of mountains is directly 

across it from the little disturbed part at Salinas Valley and the Gabilan uplift, it appears that these mountains formed by severe crustal compression or as a 

pile-up of basement terrane on only the west side of 

this fault zone as it dies out northwestward. If this is 

so, the Rinconada-Reliz fault zone acted somewhat as a 

tear fault between the stable, rigid east strip and the 

relatively northward moving west strip, and the crustal compression or shortening that formed the mountains on the west side has absorbed the right-lateral 

movement at the northwest end of its extensive fault 

zone, as shown in figure 22.

Southeast of Arroyo Seco the north Santa Lucia uplift narrows into a localized mass of high mountains, or 

the Junipero Serra Dome of Compton (1966a, p. 1367; 

H, fig. 20). This uplift, which includes the highest

peaks of the Santa Lucia Mountains, is structurally 

similar to the main part of the north Santa Lucia uplift 

and was elevated as several parallel northeast-tilted 

blocks on northeast-dipping reverse faults, as mapped 

by Compton (1966a). Although these elevated blocks 

and the structure of the metamorphic basement rocks 

within them trend northwest, the long axis of this 

mountain uplift trends east-west nearly through its 

highest peaks. This indicates an overall north-south 

crustal shortening. The northwest trend of the fault 

blocks and their somewhat west-east en echelon arrangement suggests a possible right-lateral component 

of movement on the bounding faults, but proving evidence is lacking. Only the eastermost of these faults 

shows structural evidence of small right-lateral drag 

movement (fig. 13).

The position of the Junipero Serra Dome and its 

fault blocks nearly west of the "gap" between the Rinconada and Reliz faults (figs. 13, 20) suggests that this 

uplift formed as a secondary crustal compression or 

pile-up of basement terrane and that the 18 km (11 mi) 

of right-slip movement on the Rinconada fault during 

the late Cenozoic time was absorbed by the crustal 

shortening involved in the Junipero Serra uplift west 

of the area where the Rinconada fault dies out northwestward. At the "gap," what remains of this strikeslip movement was absorbed by intense folding of the 

sedimentary rocks.

Lockwood Valley, the largest valley within the west 

strip, has remained depressed and little deformed relative to the adjacent areas. This valley is structurally a 

trough, designated as the Lockwood trough (6, fig. 20), 

in the middle and upper Cenozoic sedimentary sequences, which are underlain directly by the crystalline basement complex, according to well data (fig. 18). 

The sedimentary sequences are intensely folded along 

the northern margin of this valley as the result of 

impingement by the Junipero Serra Dome and by the 

hills elevated along the Rinconada fault (Espinosa 

segment, fig. 12), and the sequences are moderately 

folded along the southwestern margin of this valley. 

The western extension of the trough has become 

involved in uplift of the northern Santa Lucia 

Mountains.

The Lockwood trough separates the Junipero Serra 

Dome from the south spur of the north Santa Lucia 

uplift, designated here as the Coast Ridge uplift 

(J, fig. 20). The Sur-Nacimiento fault zone may be 

taken as the southwest geologic boundary of this uplift, 

but it does not form a distinct physiographic boundary 

with the adjacent south Santa Lucia uplift (K, fig. 20). 

The Coast Ridge uplift forms a high crest of crystalline 

basement but decreases rapidly in height southeastward as it passes between the Lockwood and
46 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

Nacimiento troughs and as the basement complex becomes covered by the overlying sedimentary rocks of 

these troughs. The structure of this uplift is complex 

but appears to be in large part arched. This uplift may 

have been displaced from the La Panza uplift 

(C, fig. 20) of the east strip, if originally continuous, on 

the Rinconada fault.

The Nacimiento trough (7, fig. 20) is a synclinal 

structure composed of several en echelon axes in the 

thick Upper Cretaceous and lower Tertiary sedimentary sequence that overlies crystalline basement. It is 

so named for the conspicuous syncline along much of 

the upper Nacimiento River (fig. 10, 21). This sequence 

was deformed and eroded prior to deposition of the 

unconformably overlying middle Tertiary sedimentary 

sequence. During late Cenozoic time both sequences 

were deformed when the Rinconada fault was active 

and the Nacimiento trough became involved with the 

adjacent uplifts to form an extensive area of hills. Only 

the southeastern part of this trough remained somewhat depressed to form the narrow valley through 

Templeton and Santa Margarita (fig. 9). The synclinal 

structure in the Upper Cretaceous and lower Tertiary 

sedimentary sequence might have been displaced as 

much as 60 km (38 mi) on the Rinconada fault from the 

Madulce syncline in that sequence (figs. 4, 7, 20) of the 

east strip, if originally continuous.

The southern part of the west strip is part of the 

coastal block of Franciscan basement and forms a 

structurally complex belt of mountains, designated as 

the Santa Lucia-San Rafael Mountain belt, composed 

of the central and southern Santa Lucia Mountains, 

San Rafael Mountains, and intervening lower ranges 

of hills.

The south Santa Lucia uplift (K, fig. 20) consists of 

intensely deformed Franciscan rocks and overlying 

Cretaceous sedimentary rocks squeezed up southwest 

of the Nacimiento and Las Tables faults and farther 

south against the Nacimiento trough and the Rinconada fault.

Near San Luis Obispo the south Santa Lucia uplift 

divides southeastward into two narrow uplifts of hills 

separated by the Huasna trough (8, fig. 20), a complex 

synclinal structure in the middle Tertiary sedimentary 

sequence that unconformably overlies the previously 

deformed Cretaceous sedimentary sequence and Franciscan rocks. This trough probably formed a valley in 

early Quaternary time but has since become involved 

with the adjacent uplifts to form part of the Santa 

Lucia-San Rafael Mountain belt.

The San Rafael Mountain uplift (L, fig. 20) rises 

south of the Huasna trough to more than 2,000 m 

(6,000 ft) against the Big Pine and other faults to the 

south.

REGIONAL TECTONIC ANALYSIS

The Salinian and Coastal blocks were juxtaposed, 

presumably by a large amount of movement along a 

great shear plane, such as the Sur-Nacimiento fault 

zone. How they juxtaposed is not known. It might have 

been by a large amount of lateral slip (Curtis and 

others, 1958). However, Page (1970a, 1970b) postulated that it was by thrusting (subduction) of the 

oceanic Coastal block northeastward under the Salinian block, which he interpreted as part of the continental plate, in accordance with the plate tectonic 

theory. This inferred plane of underthrusting was designated as the Coast Ranges subduction zone by Hill 

(1971, p. 2959). If this postulation is correct, this 

movement resulted presumably from a direct northeast-southwest compressive stress normal to the subduction zone.

The time of subduction, or whatever mechanism by 

which these blocks were juxtaposed, was prior to 

Oligocene time (Page 1970b). As suggested by Hill 

(1971, p. 2959), it may have been just before or during 

deposition of the coarse sandy sediments of Late Cretaceous age, because these are the oldest arkosic sediments on both blocks and because they lie unconformably on the basement rocks of the Salinian block and 

also on the Lower Cretaceous shale unit of the Coastal 

block near Stanley Mountain (Hall and Corbato, 1967) 

and possibly in the San Rafael Mountains. In the Santa 

Lucia Mountains the Upper Cretaceous unit may be 

unconformable on this shale unit and the Franciscan 

rocks (Taliaferro, 1944) or may have been, but now in 

most if not all places is, in fault relation (Brown, 1968; 

Page, 1970a, 1970b, 1972) possibly by later or continued subduction.

Eventually, the Franciscan rocks of the Coastal 

block, which presumably were deposited in an offshore 

trench in Mesozoic time, became squeezed and upheaved against the advancing continental block of 

crystalline rocks by Oligocene time. During that time, 

the supposed subduction tectonics, if they were the 

actual mechanism, were converted to right-lateral 

strike-slip tectonics on high-angle northwest-trending 

shear zones such as the San Andreas fault, as suggested by Atwater (1970, p. 3525-3529), along which 

blocks were carried past each other laterally as well as 

compressed against each other. These tectonics prevailed throughout middle and late Cenozoic time; they 

may have started before Oligocene time if the San Andreas fault first formed as a strike-slip fault before that 

time, as suggested by Wentworth (1968, p. 139-140), 

Ross (1970, p. 3661-3662), and Clarke and Nilsen 

(1973, p. 302).

Within the area of figures 1 and 20 west of the San
121°30' 121° 15*

Alluvium

Paso Robles Formation

Marine sedimentary 

sequence; basal 

conglomerate (eg)

_ Lower Tertiary and 

Upper Cretaceous

serpentine, gabbro (g)

01234567 KILOMETRES

FIGURE 21. Geology of the upper Nacimiento River area and vicinity (in part modified from Page, 1970a).
48 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

Andreas fault, the Rinconada-Reliz fault zone is the 

largest strike-slip shear zone along which right-lateral 

movement took place in middle(?) and late Cenozoic 

time, as indicated by the evidence presented herein. 

Much of this movement was distributed and taken up 

by other lesser faults of similar trend, as well as by 

folding and thrust faulting within this area.

In Cenozoic time right-lateral movements on the 

southern part of the Rinconada fault followed the old 

Mesozoic Sur-Nacimiento fault zone or Coast Ranges 

subduction zone of Hill (1971). In this respect, the Rinconada fault bears the same relation to that zone as the 

late Cenozoic Newport-Inglewood zone bears to the 

Mesozoic southern California subduction zone as postulated by Hill (1971, p. 2959).

Since Oligocene time, the Sur-Nacimiento fault zone 

was apparently inactive northwestward from the Rinconada mine where the Rinconada fault veers northward from that zone, except for small local movements. 

This inactivity is indicated by the burial of that zone 

by the middle Tertiary sedimentary sequence between 

that point and the southeast end of the Nacimiento 

fault.

The Rinconada-Reliz fault zone is inferred to have 

formed along the old Mesozoic subduction(?) zone as far 

northwest as the Rinconada mine and then to have 

veered away northward into the Salinian block, parallel to the San Andreas fault and slipped laterally in the 

same manner. Thereby, during late Cenozoic time, the 

west strip was carreid northward relative to the east 

strip along the Rinconada-Reliz fault zone.

The east strip of the Salinian block between the San 

Andreas and Rinconada-Reliz fault zones reacted to 

stress as a generally rigid mass, compared to the adjacent blocks. The northern part, along which the crystalline basement is exposed in the Gabilan Range and 

buried under a thin middle and late Cenozoic sedimentary cover elsewhere, resisted deformation and crustal 

shortening during middle and late Cenozoic time to 

form a stable area with a surface near sea level. The 

only deformation of this strip was partial uplift by 

arching along an axis near the San Andreas fault to 

form the Gabilan Range (fig. 20).

The southern part of the east strip, where the buried 

basement surface deepens southward under sedimentary sequences, yielded somewhat to become increasingly deformed southward and was impinged southward against part of the Transverse Ranges. This may 

account for the intense compressive deformation of the 

very thick sedimentary sequences of those ranges.

The west strip, in contrast to the east strip, yielded 

to stress almost throughout its length during the middle and late Cenozoic diastrophism to form the complex 

series of parallel uplifts along the Santa Lucia-San

Rafael mountain belt, as described. The southern part 

of this strip, that within the Coastal block, is the most 

intensely deformed part because much of the Franciscan basement is melange with little internal strength. 

The overlying Cretaceous marine strata are severely 

shattered and broken, forming poor exposures in contrast to the prominent unshattered exposures of the 

Upper Cretaceous and lower Tertiary marine sedimentary sequence on the Salinian block.

The northern part of the west strip, even though it is 

within the Salinian block which has a rigid crystalline 

basement, is nearly as intensely deformed as the 

southern part. The northwesternmost part was compressed and upheaved between the Reliz and Sur faults 

and on a number of reverse faults within it to form the 

high, rugged northern Santa Lucia Mountains during 

late Cenozoic time. This upheaval formed by crustal 

compression or pile-up west of where the RinconadaReliz fault zone dies out and involves several 

kilometres of north-south crustal shortening of the 

west strip compared to almost none of the east strip 

across this fault zone. This shortening on only one side 

of this fault zone thereby presumably absorbed the 

large right-lateral slip on the Rinconada-Reliz fault 

zone as it dies out northwestward, as demonstrated in 

figure 22.

The lowlands and Monterey Bay north of this compressive uplift were only slightly affected by crustal 

movements and thereby remained compartively stable 

to form part of the still-depressed North Salinas trough 

(1, fig- 20).

REGIONAL SIGNIFICANCE OF 

THE RINCONADA-RELIZ FAULT ZONE

I conclude that the Rinconada-Reliz fault zone is the 

most extensive late Cenozoic transcurrent fault zone in 

the southern Coast Ranges west of the San Andreas 

fault and that the tectonic and physiographic features 

of the area traversed by the Rinconada-Reliz fault zone

FIGURE 22. Block diagram illustrating mechanics of conversion of 

strike-slip movement along a high angle fault into compressive 

uplift of one block as fault terminates. Block B, which moved to 

the right along fault (f) relative to block A was compressed and 

bulged upward at C near end of fault. Northern Santa Lucia 

Mountains were elevated probably in large part in this manner as 

the Rinconada-Reliz fault zone dies out northward.
REGIONAL TECTONICS ALONG AND NEAR THE RINCONADA-RELIZ FAULT ZONE 49

(figs. 2, 20) are probably drag features formed by or 

controlled in part by northwestward movements of the 

west strip relative to the east strip of the fault zone, 

together with right-lateral movements of the more active San Andreas fault. The Rinconada-Reliz fault zone 

is therefore a major subsidiary or companion fault of 

the San Andreas, but with a somewhat different history of movement.

The position of the Rinconada fault is en echelon to 

two other major faults of similar extent, trend, and 

movement: the San Gregorio fault to the northwest 

and the San Gabriel fault to the southeast. The en 

echelon position of the Rinconada-Reliz fault zone between these two major faults and the similar movement history of all of them suggest that all are tectonically related as faults subsidiary to the San Andreas as 

indicated in the interpretations that follow.

Northwestward the Rinconada-Reliz fault zone dies 

out probably south of Monterey Bay, but the San Gregorio fault (Branner and others, 1909; Jennings and 

Burnett, 1961) along the southwestern margin of the 

Santa Cruz Mountains is nearly parallel to and northwest of the Reliz fault zone but is farther west. The San 

Gregorio fault had a comparably large amount of 

right-slip movement, as suggested by dissimilarity of 

rock units and structures mapped on opposite sides 

(Brabb, 1970; Clark, 1970; fieldwork by myself in 

1948-49). According to recent work by Greene, Lee, 

McCullock, and Brabb (1973), that fault extends 

southeastward offshore across subsea Monterey Canyon and onshore into either the Palo Colorado or Sur 

fault or both (fig. 15), along which its right-slip movement eventually dies out. Consequently, the right-slip 

movement of the Rinconada-Reliz fault zone must have 

been transferred through the many large northwesttrending faults in the northern Santa Lucia Mountains 

to the San Gregorio fault northwestward, and so the 

right-slip movement on both faults was absorbed in the 

crustal compression or pile-up that formed these 

mountains between the extremities of these two en 

echelon major faults (fig. 23). A small part of this 

movement was probably absorbed by the many small 

right-slip faults in Monterey Bay (Greene and others, 

1973).

Southeastward the Rinconada fault terminates 

against the Big Pine fault, but its extension beyond 

that fault may be the Pine Mountain fault (fig. 4), as 

suggested previously, even though the Pine Mountain 

fault is a north-dipping reverse fault on which Pine 

Mountain was elevated. If the Pine Mountain fault 

were once part of the Rinconada fault, it may have 

originally trended southeastward, but since it was displaced by left slip on the Big Pine fault, it probably has 

been dragged into a more easterly trend by left slip on

that fault to become parallel to the more easterly 

trending part of the San Andreas fault and evolve into 

a north-dipping reverse fault as the Pine Mountain 

block was squeezed upward on it. The Pine Mountain 

fault curves southeastward and dies out near the San 

Gabriel fault to the east.

The San Gabriel fault extends about 144 km (90 mi) 

and dips steeply northeast. This fault was active during late Cenozoic time but mostly during Pliocene 

time, with an inferred maximum cumulative right slip 

of about 40 km (25 mi) (Crowell, 1952; Dibblee, 1968). 

It is therefore related to the San Andreas fault, but its 

history of movement was similar to that of the Rinconada fault. Consequently, the inferred right-lateral 

movements on both faults may have been absorbed by 

conversion to compressive vertical movements between or near their extremities to form the Sierra 

Madre Mountains and Pine Mountain-Alamo Mountain uplift (fig. 24), in much the same way that the 

northern Santa Lucia Mountain were formed between 

the Reliz fault and the probable southeastern extension of the San Gregorio fault.

During Quaternary time left slip on the Big Pine 

fault, which is apparently younger than the Rinconada 

fault, not only offset the Rinconada fault but also probably impeded or locked right-slip movements on it, and 

so as stress continued the lateral movements became 

converted to thrusting movements on the South 

Cuyama, Ozena, Pine Mountain, and Little Pine 

faults.

The influence of lateral movements of the blocks or 

strips bounded by the San Andreas fault, RinconadaPine Mountain fault combination, and the San Gabriel 

fault during late Cenozoic time extended far to the 

southwest. Assuming that the east strip of the Salinian 

block, which has a relatively rigid crystalline basement complex, between the San Andreas fault and the 

Rinconada, Pine Mountain, and San Gabriel faults was 

stationary relative to the adjacent blocks, the block 

east of the San Andreas fault was moving relatively 

southeastward, and the block west of the Rinconada, 

Pine Mountain, and San Gabriel faults was moving 

relatively northwestward (fig. 24), or actually northward, against this intervening strip.

The continued irresistable force of these movements 

caused severe impingement northward against this 

strip of the Salinian block by the Coastal block which 

contains the very thick sedimentary accumulations of 

the Santa Barbara trough that in large part overlie 

Franciscan basement. This impingement resulted in 

intense buckling and upthrusting of this thick 

sedimentary crust to form the San Rafael, the Santa 

Ynez-Topatopa, and Santa Susana Mountains, and 

possibly the Santa Monica Mountains. The north-
50 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

dipping thrust or reverse faults under Frazier Mountain, Pine Mountain, Topatopa Mountains, and the 

Santa Susana Mountains (fig. 24) may be the effect of 

counterimpingement from the east strip of the Salinian block, as well as compressive movements along 

the San Gabriel fault.

Another significant tectonic effect of the

123° 122°

Rinconada-Reliz fault zone, together with other major 

right lateral faults, is their elongation of the Salinian 

block. Johnson and Normark (1974, p. 11-14) postulated that prior to early Tertiary time the Salinian 

block, which is part of the borderland of the North 

American plate, fitted into the 300 km (185 mi) gap 

between the Sierra Nevada and Peninsular Ranges

121

38°

37 C

36C

EXPLANATION

Elevated area

Half arrows indicate direction of inferred lateral movement of 

fault-bounded blocks relative to the block bounded by the 

San Andreas, Rinconada-Reliz, and San Gregorio faults that 

is assumed to be relatively stationary

Monterey Bay 

and Canyon

NORTHERN SANTA 

LUCIA MOUNTAINS

10i 20

, i

30 MILES

20

 i 

40 KILOMETRES

FIGURE 23. Inferred tectonic movements in vicinity of northern Santa Lucia Mountains showing conversion of right-slip movements 

near ends of Rinconada-Reliz fault zone and San Gregorio fault into crustal compression and uplift of intervening area to form 

these mountains.
RECIONAL TECTONICS ALONG AND NEAR THE RINCONADA-RELIZ FAULT ZONE 51

basement terranes and that it bulged westward; during Neogene time, the Salinian block was displaced 

northwestward to its present position by right slip on 

the San Andreas fault and the block has been sliced 

and elongated to about 600 km (370 mi) or about twice 

its orginal length by right slip on the Rinconada-Reliz 

and other major longitudinal faults within this block

as it became attached to the relatively northward moving Pacific plate. They also claimed that there is no 

need for a proto-San Andreas fault within the Salinian 

block.

These views differ somewhat from those by Nilsen 

and Clarke (1975, pi, 48-52), who postulated, from 

their sedimentology studies of early Tertiary strata

Reverse or thrust fault

(sawteeth on elevated block)

All other faults

are high-angle

La Panza Range 

Caliente Range 

Santa Lucia Mountains 

San Rafael Mountains 

Sierra Madre Mountains 

Mount Pinos 

Pine Mountain 

Alamo Mountain 

Santa Ynez Mountains 

Topatopa Mountains 

Santa Susana Mountains 

Santa Monica Mountains 

San Gabriel Mountains

LOS ANGELES

Half arrows indicate direction of inferred lateral movement of 

fault-bounded blocks relative to the block bounded by the 

San Andreas, Rinconada, Pine Mountain, and San Gabriel 

0 10 20 30 40 KILOMETRES faults that is assumed to be relatively stationary

34° -

120 119 118°

FIGURE 24. Inferred tectonic movements in vicinity of Sierra Madre Mountains and Pine Mountain, showing inferred conversion of 

right-slip movements near ends of the Rinconada and San Gabrel faults to crustal compression to form these ranges.
52 RINCONADA AND RELATED FAULTS, COAST RANGES, CALIFORNIA

and the comparatively greater displacement of basement rocks of the Salinian block than the lower Tertiary sedimentary rocks, that the Salinian block was 

displaced northwestward from the continental landmass on a proto-San Andreas fault prior to early Tertiary time; and that clastic sediments were being 

deposited in early Tertiary time in deep marine basins 

formed perhaps as rhombochasms between rising 

source areas within the Salinian block from movements related presumably to right-lateral movements 

on the proto-San Andreas fault.

Evidence of pre-early Tertiary displacement of the 

Salinian block presented by Nilsen and Clark (1975) is 

reasonably convincing. If there is a proto-San Andreas 

fault in the Salinian block, it is probably close to or 

at the present San Andreas fault. The slicing or displacement of at least one large sedimentary basin, the 

Sierra Madre basin, in the Salinian block during 

Neogene time by the Rinconada fault is one evidence 

that the Salinian block has been elongated during 

Neogene time by faults parallel to the San Andreas as 

postulated by Johnson and Normark (1974), even 

though parts of the fault strips have been shortened by 

compressive movements as indicated.

EFFECT OF RINCONADA-RELIZ FAULT ZONE 

ON PHYSIOGRAPHY AND DRAINAGE

The physiographic features and drainage of the area 

of figure 2 evolved directly from the tectonic features 

(fig. 20) formed in late Cenozoic time. These in turn are 

genetically related in large part to movements on the 

Rinconada-Reliz fault zone and related faults. The elevated areas evolved into the present ranges of mountains and hills and are undergoing erosion. The intervening troughs that remained depressed are the valley 

areas through which the major streams from the elevated areas drain. These conditions indicate that the 

tectonic features are very young and are still forming.

The mountain ranges uplifted adjacent to or near the 

Rinconada-Reliz fault zone form one of the major 

drainage divides of the southern Coast Ranges (fig. 2). 

Southeastward from the Rinconada mine, the principal 

divide is formed by the Sierra Madre and La Panza 

Ranges elevated northeast of the fault. Northwestward 

from that point, the Santa Lucia Mountains elevated 

west of the Rinconada-Reliz fault zone from a drainage 

divide adjacent to the coast for 170 km (110 mi).

The drainage pattern of the two strips opposite 

the Rinconada-Reliz fault zone are notably different 

(fig. 2). On the east strip, the major streams such as the 

Salinas and Cuyama Rivers drain northwestward, and 

most of their tributaries normally drain directly into 

them. In contrast, the major streams on the west strip

that drain the east slope of the Santa Lucia Mountains, 

such as the Arroyo Seco, San Antonio, and Nacimiento 

Rivers, drain southeastward or eastward through 

structural troughs. Farther south, as this range decreases in height and as the principal drainage divide 

shifts east of the fault, drainage is normally southwestward to the sea.

The southeastward-draining San Antonio and 

Nacimiento Rivers are considered by Baldwin (1963) to 

be remnants of a drainage system in southern Salinas 

Valley that emptied eastward into San Joaquin Valley. 

However, I consider this theory improbable because 

both these streams follow troughs between uplifts of 

the west strip, and after crossing the Rinconada fault, 

both turn abruptly northeastward to flow directly to 

the northwestward-flowing Salinas River, which was 

established probably either during or after deposition 

of the Paso Robles Formation.

POSSIBLE SEISMIC ACTIVITY

No earthquakes attributed to the Rinconada or Reliz 

faults, faults branching from them, or thrust or reverse 

faults considered related to the Rinconada fault have 

been recorded during historic time. The area northward from Greenfield has been seismically monitored 

from 1969 to 1972 (Greene and others, 1973). The results of this work showed numerous minor epicenters 

in the Gabilan Range just southwest of the San Andreas fault. Only a few with magnitudes up to 4 (Richter scale) were located on or just southwest of the Reliz 

fault southwest of Gonzales. If these are located accurately, they suggest possible minor activity on this 

fault. The area of the Rinconada fault has not been 

monitored.

Compared to the San Andreas fault, the Rinconada 

and Reliz faults pose little if any seismic hazards and 

are probably inactive because no trenches, scarps, 

shutter ridges, or recently offset minor drainage channels in the alluvium or even in the older alluvium have 

been positively identified along them. The Paso Robles 

Formation, the youngest geologic unit definitely truncated by the faults, is probably not younger than several hundred thousand or possibly a million years old. 

Except possibly at a few places, there are no surface 

indications that either fault has moved since deposition of the older alluvium, which is estimated to be 

about 50,000 to 500,000 years old.

Suggestive evidence of the latest movements on the 

Rinconada and Reliz faults are (1) deviation of canyons 

and stream channels locally on the Rinconada fault 

(figs. 11, 14) (2) possible fault contact of the Monterey 

Shale against older alluvium on Rinconada fault near 

Williams Hill (fig. 13); (3) shallow depression on Rin-
REFERENCES CITED 53

conada fault 4 km (2Vfe mi) north of Santa Margarita, 

due either to fault movement or to former stream erosion; (4) northwest-facing low scarps in older alluvium 

on Reliz fault southwest of Chualar (fig. 15), due either 

to vertical fault displacement or to erosion by a former 

course of the Salinas River; and (5) southwest-facing 

scarp on older alluvium on a minor fault near Reliz 

fault east of Paraiso Spring (fig. 15).

Inasmuch as the Rinconada and Reliz faults presumably have been inactive so long, it is not likely that 

they would generate an earthquake, although the possibility exists. It is possible that some of the related 

thrust or reverse faults may generate an earthquake 

because several of these, such as the Los Lobos thrust, 

San Antonio fault, South Cuyama, Ozena, and Pine 

Mountain faults, as well as those at the base of the La 

Panza and Caliente Ranges, are at the base of steep 

probably actively rising mountain fronts.

The Pine Mountain fault is potentially active because it offsets left laterally some canyons that cross it 

(Hill and Dibblee, 1953, p. 452) and could generate an 

earthquake. The Sierra Madre and Pine Mountain 

areas are potential earthquake areas because these 

mountains are probably actively rising on the faults 

that bound them.

Another likely area in which a major earthquake 

may originate is in the northern Santa Lucia Mountains, because this is an area of rigid basement complex that is being elevated on a number of major faults 

(fig. 15). Some of these faults may be active and near 

the coast may offset several canyons in a right-lateral 

sense. Several of the many minor northwest-trending 

faults under Monterey Bay are thought to displace the 

sea floor and, if so, are active (Greene and others, 

1973).

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