Thrust-Induced Collapse of Mountains—

An Example from the “Big Bend” Region of 

the San Andreas Fault, 

Western Transverse Ranges, California

By Karl S. Kellogg

Scientific Investigations Report 2004–5206

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

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iii

Contents

Abstract ................................................................................................... 1

Introduction ................................................................................................ 1

Geology of the Mount Pinos and Frazier Mountain Region .......................................... 3

Fracturing of Crystalline Rocks in the Hanging Wall of Thrusts .................................... 5

Worldwide Examples of Gravitational Collapse ...................................................... 6

A Spreading Model for Mount Pinos and Frazier Mountain ....................................... 6

Conclusions ................................................................................................ 8

Acknowledgments ....................................................................................... 8

References ................................................................................................ 8

Illustrations

 1. Regional geologic map of the western Transverse Ranges of southern 

California ....................................................................................... 2

 2. Simplified geologic map of the Mount Pinos-Frazier Mountain region ............... 2

 3. View looking southeast across the San Andreas rift valley toward 

Frazier Mountain .............................................................................. 3

 4. View to the northwest of Mount Pinos, the rift valley (Cuddy Valley) 

of the San Andreas fault, and the trace of the Lockwood Valley fault .................. 3

 5. View to the northeast of Frazier Mountain, showing the trace 

of the Frazier Mountain thrust ............................................................... 4

 6. Photograph, facing north, of hills underlain by Pleistocene 

landslide deposits ........................................................................... 4

 7. Photograph showing typical roadcut along Mount Pinos Road 

that exposes the highly fractured, cataclastically deformed, 

and argillically altered biotite gneiss ...................................................... 4

 8. Equal-area, lower-hemisphere, stereographic projection of 

(A) poles to fractures measured along Mount Pinos Road and 

(B) poles to foliation measured throughout the Mount Pinos Massif .................. 5

 9. Three models for gravitational spreading of mountains after 

(A) Beck (1968), (B) Mahr (1977), and (C) Kellogg (2001b) ................................. 7

 10. Proposed models for gravitational spreading of the Mount Pinos 

and Frazier Mountain massifs ............................................................... 7

 11. View toward Liebre Mountain facing southeast from Interstate Highway 5 ......... 8
Abstract

Mount Pinos and Frazier Mountain are two prominent 

mountains just south of the San Andreas fault in the western 

Transverse Ranges of southern California, a region that has 

undergone rapid Quaternary contraction and uplift. Both 

mountains are underlain, at least in part, by thrusts that place 

granitic and gneissic rocks over sedimentary rocks as young as 

Pliocene. Broad profiles and nearly flat summits of each mountain 

have previously been interpreted as relicts of a raised erosion 

surface. However, several features bring this interpretation 

into question. First, lag or stream gravels do not mantle the 

summit surfaces. Second, extensive landslide deposits, mostly 

pre-Holocene and deeply incised, mantle the flanks of both 

mountains. Third, a pervasive fracture and crushed-rock network 

pervades the crystalline rocks underlying both mountains. The 

orientation of the fractures, prominent in roadcuts on Mount 

Pinos, is essentially random. “Hill-and-saddle” morphology 

characterizes ridges radiating from the summits, especially on 

Mount Pinos; outcrops are sparse on the hills and are nonexistent in the saddles, suggesting fractures are concentrated in the 

saddles. Latest movement on the thrusts underlying the two 

mountain massifs is probably early Quaternary, during which 

the mountains were uplifted to considerably higher (although 

unknown) elevations than at present. A model proposes that 

during thrusting, ground accelerations in the hanging wall, 

particularly near thrust tips, were high enough to pervasively 

fracture the hanging-wall rocks, thereby weakening them and 

producing essentially an assemblage of loose blocks. Movement 

over flexures in the fault surface accentuated fracturing. The 

lowered shear stresses necessary for failure, coupled with deep 

dissection and ongoing seismic activity, reduced gravitational 

potential by spreading the mountain massifs, triggering flanking 

landslides and producing broad, flat-topped mountains. This 

study developed from mapping in the western Transverse Ranges 

as part of the U.S. Geological Surveyʼs Southern California 

Areal Mapping Project (SCAMP).

Introduction

The “Big Bend” region of the San Andreas fault (SAF), in the 

western Transverse Ranges of southern California (fig. 1), is a zone 

of relatively large transpression due to the left-stepping, right-slip 

fault geometry (Davis, 1983). This transpression has produced 

widespread thrusting and folding, both north and south of the SAF, 

that has significantly increased crustal thickness (for example, 

Namson and Davis, 1988). South of the SAF, the locus of 

contractional tectonism has apparently migrated south during the 

Quaternary to the Santa Barbara Channel (Rockwell, 1983), where 

folding and thrusting are currently active. In a roughly mirror-image 

scenario, thrusting north of the SAF has migrated northward with 

time, with a large component of shortening accommodated along 

the south-dipping Pleito-Wheeler Ridge fault system (Davis, 1983).

In concert with thrusting, the western Transverse Ranges have 

been uplifted rapidly during the Quaternary, with rates of uplift 

estimated at 4–8 millimeters per year (Yeats, 1983). This rapid–

uplift has been accompanied by rapid erosion and dissection, as 

shown by thick Quaternary basin-fill and mountain-flanking deposits 

adjacent to most of the mountain uplifts (see Dibblee, 1982).

The author of this paper proposes a model in which 

hanging-wall rocks were weakened by widespread fracturing 

that resulted from movement along low- to moderate-angle 

thrusts that placed brittle, crystalline rocks of Cretaceous and 

early Proterozoic age over structurally weak, mostly fluvial 

sedimentary rocks of the Miocene Caliente and Pliocene 

Quatal Formations. Uplift and erosion during and subsequent 

to thrusting and the resulting steepening of the mountain flanks, 

possibly driven by ongoing seismic activity, promoted episodic 

gravitational collapse of the mountains. 

The proposed model is speculative for several reasons, mostly 

due to the difficulty of constraining the ages of either thrusting or 

the inferred associated gravitational collapse. Despite these 

difficulties (which will be discussed further), one objective of 

this paper is to provide a testable alternative to the conventional 

paradigm for peneplained mountaintop surfaces (see Dibblee, 

1982) in active tectonic environments. This study evolved from 

mapping in the region as part of the U.S. Geological Surveyʼs 

Southern California Areal Mapping Project (SCAMP).

Thrust-Induced Collapse of Mountains—An Example from 

the “Big Bend” Region of the San Andreas Fault, 

Western Transverse Ranges, California

By Karl S. Kellogg
2 Thrust-Induced Collapse of Mountains—Transverse Ranges, California

AREA OF

FIGURE 2

San Joaquin Valley

Mount Pinos

Frazier

Mountain

Liebre Mountain

SAN ANDREAS

FAULT

San Gabriel fault

Garlock fault

Ventura

Oxnard

Santa Monica

Santa Ynez fault

Mojave

Desert

SANTA BARBARA

CHANNEL

Cuyama Valley

San Fernando

Valley

San Gabriel

Mountains

Big Pine fault

PWRF

Pine Mountain fault

Santa Barbara

Ridge

Basin

Area

of

fig. 2

F i g . 1

CALIFORNIA MAP

AREA

Quaternary

alluvium

Pliocene to

Oligocene

Basement

complex

Eocene to

Upper Cret.

0 10 20 30 KM Figure 1. Regional geologic 

map of the western Transverse Ranges of Southern 

California, showing the Big 

Bend region of the San 

Andreas fault and the location 

of Mount Pinos and Frazier 

Mountain, modified from Jennings and Strand (1969) and 

Dibblee (1982). Faults showing teeth are thrust or reverse 

faults, with the teeth in the 

hanging wall. PRWF is the 

Pleito-Wheeler Ridge fault.

119 15' W

34

52'

30"

 N

34

45'

 N

119 W

Landslide

deposits

Tertiary sedimentary 

 rocks

Basement 

rocks

Lockwood Valley

fault

0 1 2 3 4 5 KM

Strike-slip fault

Thrust fault

San Andreas

fault system 

Frazier

Mount Pinos

Cerro

Noroeste

PINE

MOUNTAIN CLUB

Alluvial deposits

(mostly fans) 

LOCKWOOD VALLEY 

Fracture, fault, scarp

Abel 

Mountain

thrust

Sawmill

trhust

Road

Mount Pinos

 Road 

4

3

5

6

7

Pelona Schist

6 Photograph site; figure number

(arrow shows direction of photograph)

San Emigdio Mesa

Mountain

thrust

CUDDY VALLEY

F i g . 2

Frazier

Mountain

San Emigdio Mountain

Figure 2. Simplified geologic map of the Mount Pinos-Cerro Noroeste region. The camera position for photograph site 

4 is about 2 kilometers southwest of position shown. Modified from Kellogg (1999, 2003) and Kellogg and Miggins (2002). 

The Lockwood Valley fault was formerly known as the eastern end of the Big Pine fault (Minor and Kellogg, 1997).
Geology of the Mount Pinos 

and Frazier Mountain Region

Mount Pinos and Frazier Mountain are two prominent, 

flat-topped mountain masses just south of the SAF (fig. 1). 

Cerro Noroeste is a westward high point, connected by a ridge 

to Mount Pinos (fig. 2). North of the SAF, the San Emigdio 

Mountains form a low (up to 2,210 m), rounded ridge parallel 

to the SAF. The SAF system occupies a broad east-striking 

valley (Cuddy Valley) that diverges from the overall northeaststriking trend of the fault system (Dibblee, 1982; Davis and 

Deubendorfer, 1987), thereby forming the “Big Bend” in the 

fault system. The “Big Bend” reach of the SAF has ruptured at 

least three times in the last 500 years (Davis, 1983; Davis and 

Deubendorfer, 1987), the last time being the large-magnitude 

1857 Fort Tejon earthquake (estimated minimum M=8.25; 

Sieh, 1978). Scarps and pressure ridges as high as 8 m, 

probably associated with the 1857 earthquake, are prominent 

features in Cuddy Valley along the SAF (fig. 3). 

Mount Pinos (fig. 4) and Frazier Mountain (figs. 3 and 5) 

rise to 2,605 m and 2,365 m, respectively; each is characterized 

by a wide, low profile and large (several square kilometers), 

nearly flat summit. The mountain flanks are mostly gentle, 

typically less than 20o. Exceptions are slopes on the north 

sides, facing the SAF, which are steep and locally form cliffs 

as high as about 50 m. Extensive landslide deposits cover 

large areas on the flanks of both mountain massifs. Although 

historical landslides have occurred at several places on Mount 

Pinos (Kellogg and Miggins, 2002), most landslide deposits on 

both mountain massifs are long inactive; extensive landslide 

deposits on Frazier Mountain are Pleistocene in age (Zhou, 1990). 

The deposits lack identifiable breakaway zones, transverse 

ridges, or scarps and are deeply dissected (fig. 6). They are 

“relict” slides in the terminology of Cruden and Varnes (1996). 

The relatively flat summits have been interpreted as 

remnants of a late Tertiary erosional surface (Dibblee, 1982; 

Davis, 1983), but there are reasons to doubt this interpretation 

(Kellogg, 2001a). The surface on Mount Pinos lacks lag gravels, which does not preclude the existence of an erosional 

surface, but neither does it support such an interpretation. 

Deposits interpreted as relict Pleistocene gravels have been mapped 

on the flanks of Frazier Mountain (Zhou, 1990), but these 

gravels have a provenance similar to gravels in the Miocene 

Caliente Formation, which is locally exposed on the flanks 

of the mountain (Carman, 1964; Kellogg, 2003). The clasts 

within the gravel consist of a bimodal population of (1) 

locally derived, angular augen gneiss, biotite gneiss, and vein 

quartz as long as 1 m; and (2) well-rounded, fine- to mediumgrained granitoid rocks, intermediate volcanic rocks, and chert 

clasts as long as 20 cm. The second clast population, most 

of which does not have a local source, is similar to that of the 

Caliente Formation in Lockwood Valley and on the west side 

of Frazier Mountain (Carman, 1964). This similarity in clast 

type and lack of local source terrains suggest that the well-rounded, 

Caliente-like gravel clasts may be second generation, derived 

from erosion of the Caliente Formation, and not relicts of a 

late Tertiary erosion surface. 

Thrusting is presently inactive in the Mount Pinos and 

Frazier Mountain region; the latest movement was during the 

Figure 3

F r a z i e r M o u n t a i n

scarp

San Andreas fault

Lockwood Valley fault

Mount Pinos

San Emigdio Mountain

northeast end, Lockwood Valley

Figure 3. View looking southeast across 

the San Andreas rift valley toward Frazier 

Mountain. A scarp probably produced during the 1857 Fort Tejon earthquake is visible 

at the extreme right side of the photograph.

Figure 4. View to the northwest of Mount 

Pinos, the rift valley (Cuddy Valley) of the 

San Andreas fault, and the trace of the 

Lockwood Valley fault (formerly the eastern 

end of the Big Pine fault) (Minor and Kellogg, 1997). San Emigdio Mountain is on the 

right side of the photograph, immediately 

north of the San Andreas fault. Pleistocene 

landslide deposits, a close-up of which is 

shown in figure 6, underlie the hummocky 

hills in the foreground.

Geology of the Mount Pinos and Frazier Mountain Region 3
4 Thrust-Induced Collapse of Mountains—Transverse Ranges, California

Frazier Mountain thrust

Frazier Mountain thrust system

F r a z i e r M o u n t a i n

Proterozoic augen gneiss

Quatal Formation

Lockwood Valley

Caliente Formation

F i g u r e 5 .

Figure 6

Figure 5. View to the northeast of 

Frazier Mountain, showing the trace 

of the Frazier Mountain thrust.

Figure 6. Photograph, facing north, of 

hills underlain by Pleistocene landslide 

deposits that locally contain clasts of Proterozoic augen gneiss as long as several 

meters. The morphology of the hills is 

largely due to post-landslide dissection. 

Figure 7. Photograph showing typical 

roadcut along Mount Pinos Road that 

exposes the highly fractured, cataclastically deformed, and argillically altered 

biotite gneiss. The distance across 

the photograph at road level is about 6 

meters. 
early Pleistocene (Dibblee, 1982). Early Proterozoic augen 

gneiss (and lesser amounts of biotite schist and gneiss and 

felsic gneiss) that underlies most of Frazier Mountain is thrust 

over rocks as young as Pliocene along the Frazier Mountain 

thrust (Carman, 1964; Crowell and others, 1964; Kellogg, 

1999) (fig. 5). Cretaceous granitic intrusive rocks, Cretaceous 

gneiss, and the Paleocene Pelona Schist (Ehlig, 1974; Kellogg 

and Miggins, 2002) that underlie the Mount Pinos massif are 

likewise thrust westward along the Abel Mountain thrust over 

Miocene terrigenous sediments of the Caliente Formation 

(Crowell and others, 1964; Davis, 1983; Kellogg and Miggins, 

2002). The southern extent of the Abel Mountain thrust is 

concealed beneath the surface of San Emigdio Mesa and its 

continuation southeastward on the south side of the massif 

(fig. 2) is uncertain. 

Fracturing of Crystalline Rocks in the 

Hanging Wall of Thrusts

Crystalline rocks in the hanging wall of both the Frazier 

Mountain thrust and the Abel Mountain thrust display a 

pervasive fracture network, which is generally not exposed 

at the surface on either Mount Pinos or Frazier Mountain. 

However, in fresh roadcut exposures along Mount Pinos Road, 

the fractures are well exposed and form wide (some more than 

10 m) zones of gouged rock and intervening zones of rock 

containing fractures in a seemingly random orientation (fig. 7). 

All gouge zones consist of crushed crystalline rock that 

has largely altered to clay and is moderately oxidized to an 

orange-tan color. The spacing of the fractures in the roadcuts 

is at millimeter to meter scale. The ground surfaces above the 

roadcuts contain mostly grus-rich soil and colluvium, although 

isolated tors of variably weathered granitic and gneissic rocks 

crop out in a scattered and irregular array. The relationships 

in the roadcuts suggest that the grussy areas between these 

outcrops are underlain by intensely fractured and sheared rock 

that is susceptible to deep weathering.

Regionally, the fractures or faults appear to form a cobweblike mosaic of crushed rock or gouge zones, the individual 

strands of which are short (generally less than 1 meter to 

several hundred meters). Numerous ridges that form hills and 

saddles also suggest the widespread fracture network. The 

summits of the hills commonly contain scattered outcrops, 

although outcrops are extremely rare on the grus-covered 

saddles. In addition, the “hill-and-saddle” morphology of the 

ridges is rarely linear in plan view and the saddles are rarely 

aligned, which suggests a lack of control by throughgoing 

faults. Speculatively, the saddles are interpreted to contain 

dense fractures on a scale of meters to tens of meters, whereas 

fractures under the hills are considered to be much less dense. 

The extensive fractures were previously recognized but 

were attributed to undefined “regional stresses” (Dibblee, 1982). 

In order to test whether there is a correlation between fracture 

orientations and regional stress, 104 fracture orientations 

were measured in roadcuts along Mount Pinos Road 

(fig. 8A), from near its intersection with Cuddy Valley 

Road to a point about two-thirds of the way to the summit. 

The host rocks are medium-grained, foliated, granitic 

rocks and biotite orthogneiss. A preferred fracture orientation 

might be correlated to some regional stress, or it may 

reflect exploitation of consistently oriented, older planes 

of weakness, such as foliation. Random orientations collected 

over a large region would be more consistent with rock 

movement in numerous directions, as would occur during 

gravitational spreading.

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Figure 8. Equal-area, lower-hemisphere, stereographic projection of (A) poles to fractures measured along Mount Pinos Road 

and (B) poles to foliation measured throughout the Mount Pinos 

massif. Contoured on 2-, 4-, 6-, and 8-percent poles per 1-percent 

area. N is the number of observations.

A

B

Fracturing of Crystalline Rocks in the Hanging Wall of Thrusts 5
6 Thrust-Induced Collapse of Mountains—Transverse Ranges, California

The results show a strongly dispersed spread of 

orientations, with a very weak development of a steep, 

east-northeast-striking girdle. The girdle orientation is oblique 

to the regional foliation, which is variable but generally 

strikes east-west with low to moderate dip (typically 20o-60o; 

fig. 8B). The significance, if any, of the subtle east-northeast 

fracture trend is unclear. 

A mappable network of fractures, scarps, and faults is 

exposed on Mount Pinos and Frazier Mountain. Many of 

these features are discernible on aerial photographs, and 

the more prominent ones are shown in figure 2. Some may 

be throughgoing joints with little offset, while others may 

reflect the gneissic foliation. However, many lineations are 

faults or fractures with demonstrable offset (Kellogg and 

Miggins, 2002). 

Subtle linear scarps or sackungen (Beck, 1968; Varnes 

and others, 1989), both uphill- and downhill-facing and as 

long as several hundred meters, are locally preserved. The 

presence of sackungen indicates deep-seated rock creep, 

although all sackungen are eroded and appear to be inactive. 

Locally on some slopes, scarps bound relatively horizontal 

surfaces on the downhill side, suggesting deep-seated 

rotational slumps. 

Some scarps are located on the nearly flat summit surface 

near the northern steep face of the Mount Pinos-Cerro Noroeste 

massif and are clearly related to the gravitationally induced 

extension adjacent to the north face (fig. 2). The scarps may 

represent the locus of future landslide movement. Large 

landslide deposits have accumulated at the base of the north 

face, and active landsliding within the past few decades is 

evident in some places on the east face.

Worldwide Examples of 

Gravitational Collapse

Gravitational spreading has been recognized at a variety 

of scales, from entire orogens to individual mountain ranges. 

As an example of orogen-scale collapse, the Tibetan plateau 

is apparently spreading in an approximate east-west direction 

concurrent with north-south contraction and lithospheric thickening caused by underthrusting of the Indian subcontinent 

beneath the Tibetan plateau (England and Houseman, 1989; 

Coleman and Hodges, 1995). Similarly, during the middle 

Eocene to early Miocene, the Cordilleran fold-and-thrust belt 

of western North America apparently collapsed gravitationally 

following Late Cretaceous and early Tertiary thrusting and 

crustal thickening (Constenius, 1996).

Several models for gravitational spreading at individualmountain scales are outlined in figure 9. In some cases such 

spreading has been directly related to seismic activity. For 

example, ridge-crest spreading and widespread landsliding 

in the Santa Cruz Mountains of California were directly 

induced during the 1989 Loma Prieta earthquake (Ponti and 

Wells, 1991). Ridge-crest grabens and uphill-facing scarps 

(sackungen) associated with spreading of the Stillwater 

Range in Nevada were formed during large earthquakes in 

both 1915 and 1954 (McCalpin, 1999). In the central Italian 

Alps, features attributed to gravitational spreading (including 

uphill- and downhill-facing scarps, ridge-crest grabens, and 

linear troughs) have been attributed to paleoseismic events as 

young as late Holocene (Onida and others, 1999).

An excellent analogue to the Mount Pinos and Frazier 

Mountain region is in the Williams Fork Mountains of 

central Colorado (fig. 9C). In this case, highly fractured 

Precambrian crystalline rocks in the hanging wall of a large, 

low-angle, Late Cretaceous to early Tertiary (“Laramide”) 

thrust (Williams Range thrust) overlie incompetent, mostly 

argillaceous Cretaceous rocks (Tweto and others, 1978; 

Kellogg, 2001b). The Williams Fork Mountains have a 

generally rounded morphology, the crystalline rocks are 

highly fractured, and active, uphill-facing scarps and ridge-top 

trenches (sackungen) are common. These features indicate 

ongoing gravitational spreading and collapse (Varnes and 

others, 1989). A model for formation of the widespread 

fracturing proposes that Laramide thrusting moved brittle 

hanging-wall rocks across a flexure in the thrust surface 

at approximately the crystalline cover-rock contact in the 

footwall and placed them above the relatively incompetent 

Cretaceous rocks (Kellogg, 2001b). After uplift and 

erosion, the weakened, fractured hanging-wall rocks spread 

gravitationally, resulting in widespread landslide deposits 

along the mountain flanks. Most of the deposits are currently 

inactive but may have been active in the late Pleistocene 

when greater effective moisture increased pore-water pressure 

in bedrock fractures. 

A Spreading Model for Mount Pinos 

and Frazier Mountain

The model proposes that, during late Pliocene and early 

Pleistocene thrusting along the Frazier Mountain and Abel 

Mountain thrusts, ground accelerations in the hanging 

wall created an environment conducive to rock shattering 

and crushing. Such ground accelerations are particularly 

intense in the hanging wall near thrust tips (Brune, 1999), the 

region where the thrust surfaces. Fracturing may have been 

accentuated by possible (but not currently recognized) flexures 

along the thrust surface upon which the brittle crystalline 

rocks in the hanging wall may have moved. Seismic shaking was 

enhanced by the juxtaposition of brittle rocks over soft, deformable 

sedimentary rocks of the Caliente and Quatal Formations.

The hanging-wall region was rapidly uplifted during 

the late Pliocene to early Pleistocene episode of thrusting 

at a rate that was considerably more rapid than at present. 

Ground acceleration during thrusting may have been 

augmented by strike-slip movement along the SAF. 

However, because most of the flanking landslide deposits 

appear to be considerably older than Holocene, most 
shattering and fracturing was probably related to the 

currently inactive thrusting.

Due to the lowered shear stresses necessary for failure, 

coupled with ongoing seismic activity, the gravitational 

potential of the uplifting mountain masses was significantly 

reduced by gravitational spreading, which was accentuated 

by deepening of the flanking valleys by erosion. The result 

was broad, nearly flat-topped mountains with numerous 

flanking landslide deposits. Figure 10 shows schematically 

this model for gravitational spreading.

Many questions regarding the proposed model remain 

unanswered, and until further work is done the proposed 

mechanism for gravitational collapse will remain speculative. 

Important questions revolve around (1) dating the exact 

timing of thrusting (Pliocene to probably early Pleistocene), 

(2) dating the timing of the pervasive fracturing that weakened the crystalline rocks underlying the mountains, and (3) 

establishing that these two events are coeval. Nonetheless, 

the observations are entirely consistent with a gravitationalspreading origin for Mount Pinos and Frazier Mountain and 

suggest that a similar mechanism may operate wherever 

brittle crystalline rocks have been thrust above weaker sedimentary rocks. Conversely, some observations, such as the 

lack of lag gravels on summit surfaces, are inconsistent with 

a peneplanation model.

One locality in particular, Liebre Mountain, about 

25 km east of Frazier Mountain and just south of the San 

Andreas fault (fig. 1), appears to share many structural and 

geomorphic characteristics with Mount Pinos and Frazier 

Mountain (J.C. Crowell, University of California at Santa 

Barbara [Emeritus], oral commun., 2001) and may present 

a future opportunity to test the model for gravitational 

collapse. Liebre Mountain is broad with a wide, flat-topped 

summit region (fig. 11) and is bounded on the south and west 

by thrusts that place mostly Cretaceous granitic rocks above 

the Pliocene sedimentary rocks of the Ridge Basin (Jennings 

and Strand, 1969). 

TT

crushed

 rock

after Beck (1968) 

gneiss

quartz diorite

after Mahr (1977)

antislope

scarps

(sackungen)

 antislope

 scarps

(sackungen)

antislope

scarps (sackungen)

argillite,

graywacke

after Kellogg (2001b)

(A)

(B)

(C) fractured

rocks landslide

deposits

Williams

Range

thrust

crystalline 

basement rocks

Mesozoic 

sedimentary

rock

syn-thrust

 surface

Figure 9. Three models for gravitational spreading of 

mountains, after (A) Beck (1968), (B) Mahr (1977), and (C) 

Kellogg (2001b). Distance across each diagram is ~5–10 

kilometers.

Figure 10 

Tertiary sedimentary 

 rocks

crystalline basement 

 rocks

San Andreas

 fault zone

landslide and

fan deposits

syn-thrust

 surface

Figure 10. Proposed model for gravitational spreading of the 

Mount Pinos and Frazier Mountain massifs.

A

B

C

A Sreading Model for Mount Pinos and Frazier Mountain 7
8 Thrust-Induced Collapse of Mountains—Transverse Ranges, California

Conclusions

A number of observations outlined in this paper support a 

model for gravitational spreading: (1) Mount Pinos and Frazier 

Mountain are rounded, nearly flat-topped mountain massifs, 

flanked by extensive, mostly pre-Holocene landslide deposits; 

(2) lack of direct evidence, such as lag gravels, indicates that 

the summits may not be remnants of old erosion surfaces, as has 

been previously suggested; (3) the crystalline rocks underlying 

both mountain massifs are pervasively fractured and crushed; 

(4) the crystalline rocks in most places along the margins of 

the mountains are thrust above easily deformable Miocene and 

Pliocene sandstone, mudstone, and shale; and (5) regionally, the 

area has undergone extremely rapid (4–8 mm/yr) uplift rates.

These observations lead to the following proposed tectonic 

history: (1) the pervasive shattering of the crystalline rocks resulted 

from early Quaternary and late Pliocene thrusting, perhaps as a result 

of high accelerations resulting from movement over nonplanar 

surfaces in the thrust plane; (2) the weakened crystalline rocks spread 

gravitationally in response to local, thrust-induced uplift followed 

by rapid dissection, a process augmented by regional uplift; and 

(3) spreading and mountain “flattening” may have been augmented 

by ongoing seismic activity along the SAF.

Acknowledgments

This study evolved as part of the U.S. Geological Surveyʼs 

Southern California Areal Mapping Project (SCAMP). I am 

indebted to R.G. Bohannon, P.C. Carrara, S.A. Minor, R.R. 

Shroba, E.A. Keller, and I. Cemen for careful reviews of 

the manuscript that clarified numerous points. In particular, 

additional discussions with Minor and Keller encouraged me 

to proceed with presenting the ideas in this paper.

References

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Geophysics, v. 11, p. 191–199.

Brune, J.N., 1999, Rock shattering vs. crushing and shearing—Evidence for extreme ground motion on the hanging wall of thrust sheets: Geological Society of America 

Abstracts with Programs, v. 31, no. 6, p. A41.

Carman, M.F., Jr., 1964, Geology of the Lockwood Valley 

area, Kern and Ventura Counties, California: Sacramento, 

California Division of Mines and Geology Special Report 

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Liebre Mountain

Figure 11. View toward Liebre Mountain 

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References 9
10 Thrust-Induced Collapse of Mountains—Transverse Ranges, California

Manuscript approved for publication September 28, 2004

Published in the Central Region, Denver, Colorado

Edited by Mary Kidd

Page layout, photocomposition by Amber Swallow

Graphics by the author