listric thrusts in the western transverse ranges, california

7/30/2019 Listric Thrusts in the Western Transverse Ranges, California

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ABSTRACT

Some of the main faults accommodating cur-

rent shortening in the western Transverse

Ranges are probably listric because (1) they are

associated with progressive tilting, and (2) they

may be preexisting normal faults that accom-

modated Miocene extension. These faults have

been reactivated in the PlioceneQuaternary

transpressive regime. We propose a listricthrust model where slip is proportional to

backlimb dip. This model requires relatively lit-

tle fault slip to account for progressive tilting

and for wide (in the dip direction) and gently

dipping backlimbs. In contrast, widely applied

fault-bend fold and fault-propagation fold

models relate fault slip to limb width alone and

typically predict more shortening by the blind

thrusts that can be accounted for by folding in

the cover above them. We trace the southern-

most structural high in the Transverse Ranges

from the Santa Monica Mountains through the

southern Santa Barbara Channel. The north-dipping backlimb of this anticline is 2030 km

wide and 220 km long; its presence suggests a

very large north-dipping thrust that could gen-

erate very large earthquakes. The slip rate for

this fault, however, is substantially lower for a

listric thrust model than for a single-step ramp-

flat model.

Keywords: fault-related folds, fold-and-thrust

belts, listric faults, Santa Barbara Channel,

Santa Maria basin, Santa Monica Mountains.

INTRODUCTION

The strike of the right-lateral San Andreas

transform fault through the Transverse Ranges is

more westerly than the Pacific plate motion vec-

tor in southern California,and forms a restraining

bend (Fig. 1). This bend is thought to be respon-

sible for the PlioceneQuaternary transpressive

regime and for the belt of west-trending folds and

faults in the Transverse Ranges (e.g., Atwater,

1989). Most of the damaging earthquakes in this

portion of the plate boundary during the past

50 yr have been on faults other than the San An-

dreas fault, and have had large components of

thrusting (e.g., Dolan et al., 1995). Rapid short-

ening and related seismicity occur over a broad

belt in southern California that includes densely

populated urban areas. It is thus important to

identify potential sites of future damaging earth-quakes in this belt. Location, size, geometry, and

late Quaternary slip rate are critical parameters of

seismogenic faults. Slip rates on blind faults can

be determined from characteristics of related

folds, but it is dependent on the assumed shape of

these faults.

Most of the damaging earthquakes in the

Transverse Ranges have originated from struc-

turally subtle and relatively short fault segments

(e.g., Hauksson, 1990; U.S. Geological Survey

and Southern California Earthquake Center,

1994). The upper limit on the magnitude of pos-

sible earthquakes might be higher than any of thehistoric earthquakes, particularly if regional

faults, much larger than any of the historic fault

ruptures, were demonstrably active. The exis-

tence and slip history of large blind thrust faults

is inferred from the existence and growth history

of large, continuous anticlines. We interpret a

220-km-long anticline along the southern front of

the western Transverse Ranges (Fig. 1) to have

folded above a midcrustal thrust-fault system of

similar dimensions.

Recognition of faults that can generate damag-

ing earthquakes in the Transverse Ranges is ham-

pered by two factors: (1) the complexity of the

fault system, which is characterized by a largenumber of faults (Fig. 1) with low to moderate

displacement rates; and (2) the existence of nu-

merous blind thrust faults that are manifested

only by folding in the shallow crust. Worldwide,

the geometry of deep thrust faults has been in-

ferred from shallow structure by applying fault-

related fold models (e.g., Suppe, 1983). Some of

these models have been used to infer large thrust

flats and ramps beneath the Los Angeles and

Santa Barbara basins and their margins (Davis

et al., 1989; Shaw and Suppe, 1994, 1996). Th

application of fault-related fold models in the

Transverse Ranges has recently been subjected to

scrutiny, kinematic tests, and criticism (Kamer

ling and Nicholson, 1996; Huftile and Yeats

1995; Stone, 1996; Sorlien et al., 2000b). Ou

concern is that the range of models applied in thi

area has been unduly limited. In particular, could

some of the thrust faults in the Transverse Rangebe listric? Different versions of listric thrust mod

els have been successfully applied to wide and

gently dipping fold limbs of the Wyoming fore

land (Stone, 1993; Erslev, 1986). Such listri

fault models may be useful also in southern Cali

fornia because wide and gently dipping fold

limbs are common there, particularly in the off

shore area (in this paper, a limb is wide in the dip

direction). Recognizing the range of structure

active in the current tectonic regime of the Trans

verse Ranges is particularly important at thi

stage of earthquake hazard studies in southern

California so that geodetic and geomorphic datacan be correctly modeled. For example, an im

portant task of the high-resolution Global Posi

tioning System (GPS) array planned for that area

is to distinguish diffused elastic deformation

from fault slip, and then to distinguish between

thick- and a thin-skin fault array (Prescott, 1996)

A comparison of surface deformation for al

plausible fault models is critical for this task.

The purpose of this paper is to present a listric

thrust-fault model that better accounts for the de

formation characteristics of some regional fold

in the western Transverse Ranges than do ramp

flat or thick-skin models. Differences between

these sets of models are discussed with emphasion slip predicted on blind faults. Using represen

tative data from the western Transverse Ranges

we show examples of progressive fold limb rota

tion, wide and very gently dipping panels, and

Miocene basins inverted into anticlines. We pro

pose that thrust reactivation of listric Miocene

normal faults can account for many of these com

mon features. This paper, however, does not rig

orously document particular structures or strati

1067

Listric thrusts in the western Transverse Ranges, California

Leonardo Seeber* Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

Christopher C. Sorlien Institute for Crustal Studies, University of California, Santa Barbara, California 93106, USA

GSA Bulletin; July 2000; v. 112; no. 7; p. 10671079; 5 figures.

*E-mail: [email protected].


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graphy. We present a simple listric thrust model

as representative of a class of models wherein

slip is proportional to limb dip. This model is

tested by retrodeforming a well-known structure

in the Santa Maria basin and is applied for esti-

mating fault slip associated with a major regional

structure marking the southern flank of the Trans-

verse Ranges.

MODELS FOR BLIND THRUST FAULTS

AND RELATED FOLDS

Fault-bend fold and fault-propagation fold

models define the geometry and slip of blind

faults from the structure of overlying folds. The

models relate the width of fold limbs to slip on

the faults (e.g., Suppe and Medwedeff, 1990;

Geiser, 1988; Mitra, 1990). These fault models

comprise planar segments separated by kink

bends, and they typically include gently dipping

flats and more steeply dipping ramps (Fig. 2, A

and B). The predictions of these models have

been successfully tested in many settings (e.g.,

Suppe et al., 1992). In the western Transverse

Ranges, PlioceneQuaternary uplift of long con-

tinuous mountain ranges and island chains has

been interpreted in terms of such ramp-flat mod-

els (Namson and Davis, 1988, 1992; Davis and

Namson, 1994a; Shaw and Suppe, 1994; Dolan

et al., 1995).

The following predictions are made by classi-

cal ramp-flat models (i.e., with a single step and

where bed length is preserved; Suppe, 1983;

Fig. 2), which have been widely applied to the

Transverse Ranges.

1. Sediments acquire dip instantaneously, with

no progressive tilting (Suppe et al., 1992). Thus,

the uplift rate above fault ramps depends on ramp

dip and slip rate, but not on position (uplift rate is

uniform above fault ramps).

2. In fault-bend folds, fault slip is equal to or

greater than the width of the backlimb for time

SEEBER AND SORLIEN

1068 Geological Society of America Bulletin, July 2000

? ?

?

??

?

? ??

Ventura

Santa Barbara

Los Angeles

SanAndreasfault

Northridge

*

Santa Barbara Channel

A

SantaMariabasin

P

O

893

Fig. 3ab

Fig. 5 SCrSRIF

SCrIF

Fig.

4

SWCF

SMF

SCF

ORF

USGS

-105

LHF

PD

ORFMCT

M

SF

MCF

S1S2

North dip

OverprintedProgressive North Tilt

25 kmN

Faults

SMM

SR

34N

35

120 119 118 W

SM

Figure 1. Traces of main faults in the western Transverse Ranges and their offshore extension in southern California (compiled from Jennings,

1994; Sorlien et al., 2000b; Kamerling and Sorlien, 1999). The western Transverse Ranges are the province of east-westoriented faults and folds.

The Santa Monica Mountains (SMM) and the northern Channel Islands (SMSan Miguel,SRSanta Rosa, SCrSanta Cruz,AAnacapa Is-

lands) are part of a continuous 220-km-long topographic and structural high that forms the southern boundary of the western Transverse Ranges.We refer to this feature as the Santa Monica MountainsChannel Islands anticline. Darker shading delineates the north-dipping limb of this an-

ticline. This is interpreted to be a backlimb of a fold associated with a major buried north-dipping thrust fault. Much of the northern part of this

limb is overprinted by other structures (lighter shading), including backlimbs of south-dipping thrust faults. O, POrcutt, Purisima anticlines;

heavy black plus signOcean Drilling Program Site 893; faults: LHFLions Head, MCFMalibu Coast, MCTMid Channel trend, ORF

Oak Ridge, SCFSan Cayetano, SCrIFSanta Cruz Island, SFSimi, SMFSanta Monica, SRIFSanta Rosa Island, SWCFSouthwest

Channel faults. PD, MPoint Dume, Malibu. Reflection profiles in Figures 4A and 5A are located by thick lines, and the depth section in Fig-

ure 4B and USGS-105 are located by thin lines. S1 and S2 are industry reflection profiles shown in Sorlien (2000). The dotted curve southeast of

Anacapa Island represents the base of the forelimb of a fold above a blind, very gently north-dipping fault. The south edge of the progressive north

tilt through the islands is shown near the last evidence for Quaternary tilting. The actual south edge may be farther south, and modification of this

map awaits the results of surveying coastal terraces by N. Pinter and his students. The south edge of progressive tilt is shown north of San Miguel

Island where the Quaternary strata onlap (Fig. 5), and the map does not preclude a wider area of tilt.


3/13

intervals corresponding to the age of each syn-

thrust stratigraphic horizon (Fig 2A; Suppe

et al., 1992). In fault-propagation folds, slip is

equal to backlimb width only for the syn-thrust

strata within the backlimb growth triangle

(Fig. 2B).

The geometry of certain folds in the western

Transverse Ranges is not consistent with these

predictions. Post-Miocene strata commonly dip

more steeply with depth and increasing age

wherever they are preserved (mostly in the off-

shore part of the Transverse Ranges; Figs. 1

and 3). The drape and filling of a preexisting

basin are consistent with progressive increase of

dip with depth, and has this been suggested for

the southern margin of the Santa Barbara basin

(K. Mueller, 1998, personal commun.). We pre-

sent evidence that supports instead that this

geometry to be due to progressive tilting during

folding. Wide and gently dipping fold limbs are

also common along the southern margin of the

western Transverse Ranges and the outer Califor

nia Continental Borderland (Fig. 1; Namson and

Davis, 1992; Davis and Namson, 1994b). Som

of these structures would require astonishingly

large slips in classical ramp-flat models (as much

as 30 km). Alternatively, these structures can b

accounted for by relatively little slip if fault dis

placement is proportional to limb dip. Progres

sive tilting of forelimbs is predicted by recently

proposed ramp-flat and detachment thrust mod

els where bed lengths are not preserved (e.g.

Wickham, 1995; Hardy and Poblet, 1994). Pro

gressive tilting of forelimbs and backlimbs is also

predicted by certain detachment thrust models

(Epard and Groshong, 1995) and by listric thrus

models (Erslev, 1986). However, only listri

thrust models require minimal or no change in

bed length (Fig. 2C). Multibend kink-fold mod

els (Medwedeff and Suppe, 1997) can result inprogressive tilting and a backlimb much wide

than slip if the distance between bends is much

less than the slip. In this case, however, the multi

bend model is a more complex approximation o

a listric model. We argue that listric fault model

are particularly appropriate because Plio

ceneQuaternary shortening in this area is ac

commodated in part by reactivation of Miocene

normal faults (e.g., Sorlien et al., 2000a; Clark

et al., 1991; Huftile and Yeats, 1996).

LISTRIC MIOCENE NORMAL

FAULTS REACTIVATED AS

PLIOCENEQUATERNARY THRUSTS

According to interpretation of paleomagnetic

data, the east-westtrending western Transverse

Ranges have rotated clockwise about a vertica

axis more than 90 during Neogene time from an

originally north-south orientation (Kamerling

and Luyendyk, 1985; Hornafius, 1985). This ro

tation and related extension occurred above ma

jor low-angle normal faults (Yeats, 1976, 1987

Crouch and Suppe, 1993; Sorlien et al., 2000a

Nicholson et al., 1994). Many such faults hav

been inferred from seismic reflection profile

along the California margin (Crouch and Suppe1993; Nicholson et al., 1993; Bohannon and

Geist, 1998; Clark et al., 1991; McCulloch

1989). Folds that are hundreds of kilometers

long, forming submerged banks and islands off

shore southern California (Davis and Namson

1994b),may be generally related to thrust reacti

vation of the normal faults interpreted to under

lie the region (e.g., Bohannon and Geist, 1998)

If so, relatively little shortening could produce

the large structures. Alternatively, substantia

slip would be required on such large structure

LISTRIC THRUSTS IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA

Geological Society of America Bulletin, July 2000 106

SS

S

S

S

T

R R-T

W

W

SS

Fault-bend fold

Fault-propagation fold

Listric thrust

Pre-thrust strata

Syn-thrust strata Inactive axial surface

Active axial surface(A & B only)

W'

growth

pregrowth

A

B

C

Figure 2. (A) Fault-bend fold (Suppe, 1983). Slip is greater than or equal to the backlimb

width, uplift rate is constant between the active axial surfaces, and only a small part of the slip

is absorbed in the fold. (B) Fault-propagation fold (Suppe and Medwedeff, 1990; Mitra, 1990).

Slip is equal to the width of the fault between the intersections of the active axial surfaces with

the fault, and all slip is absorbed in the fold. The pre-thrust strata were modeled in A and B us-

ing the program Rampe of Eric Mercier. (C) A circular listric thrust-fault model. In the exam-

ple shown, sedimentation rate is faster than uplift rate, and the fault is planar above the footwall

cutoff of the gray layer. The age of the top of the gray layer coincides with onset of thrusting. In

this simple model, the hanging-wall block rotates rigidly about a horizontal axis. Resulting

space problems may be accounted for by localized shortening and dilation as indicated by dou-

ble arrows (see text and Erslev, 1986). See text for explanation of variables.


4/13

from scaling arguments alone (e.g., Scholz,

1990, p. 110). Large-displacement normal faults

are generally observed to be listric (i.e., cross-

sectional trace is curved), either directly from re-

flection profiles or field exposures, or indirectly

from dip panels and rollover anticlines in growth

sediments (e.g., Xiao and Suppe, 1992; Yin and

Dunn, 1992). Wide panels of strata tilted at a

constant dip are common (e.g., Fig. 4) and sug-

gest rotation about a horizontal axis with little

internal deformation. Such rotation can be ac-

complished by slip on a fault in the shape of a

partial cylinder that has the same axis as the axis

of rigid rotation (circular listric fault). Tilting by

a circular listric fault occurs either in extension

(e.g., Dula, 1991) or in contraction (e.g., Erslev,

1986). Real faults are expected to have slip gra-

dients and complex shapes, and to deform dur-

ing tectonism. However, progressive tilting and

increasing dip with increasing slip will occur as

a result of slip on any curved, concave-up fault.

For simplicity, we consider only circular listricfaults, with the understanding that other shapes

are likely.

SANTA MARIA BASIN

After the tectonic regime of the western Trans-

verse Ranges evolved from extensional to con-

tractile in early Pliocene time, many of the

Miocene basins were inverted into anticlines

(e.g., Fig. 4). These inverted basins have been

widely interpreted to reflect the reactivation of

Miocene normal (separation) faults as thrust

(separation) faults (Clark et al., 1991; Sorlien

et al., 2000a; Huftile and Yeats, 1996). These

faults have probably retained a listric shape

through the reversal of their dip-slip components.

The Santa Maria basin, located just north of the

western Transverse Ranges (Fig. 1), is a good ex-

ample of an extensional basin now being short-

ened (Fig. 4). Wide dip panels and both normal

and reverse separation faults have long been rec-

ognized in the area of this basin (e.g., Woodring

and Bramlette, 1950). A moderately north dip-

ping fault was interpreted beneath the Purisima

anticline by Krammes and Curran (1959). Ten-nyson (unpublished northeast striking cross sec-

tion intersecting the northern end of the north-

south section in Fig. 4A) and we interpret the

same structure to be a major listric north-dipping

fault that controlled the growth of the Santa

Maria basin during extension (Fig. 4). Miocene

and early Pliocene strata are much thicker on the

northern or hanging-wall side of this fault (see

Stanley et al., 1996). This fault is along the trend

of the Lions Head fault as mapped by McLean

(1992). In this paper, the protoLions Head fault

is the ancestral Miocene fault below the Purisima

anticline.

Miocene and early Pliocene strata are thickest

through the crest of the Purisima anticline and they

gradually thin out to the north, forming a 25km-

wide panel of growth strata in the preshortening

profile (Fig. 4C). The regular increase in thickness

to the south is consistent with a very large back-

tilted block rotating on a deep-reaching fault of ap-

proximately circular listric shape (e.g., Dula, 1991).

The Purisima anticline formed after deposition of

the late Mioceneearly Pliocene Sisquoc Forma-

tion, when the protoLions Head fault was reacti-vated in shortening. The north-verging Orcutt anti-

cline is interpreted to form above a back thrust, also

SEEBER AND SORLIEN


500 m

0.1s

75m

Vertical exaggeration = 5:1 at sea floor

Pliocene unconformity

0.5

0.6

0.7

0.8

0.9

Two-Way

Traveltime(s)

South NorthSanta Cruz Island fault

1 km

0.1s

Vertical Exaggeration ~6:1 at Sea Floor

0.5 s

0.2 s

1.0 s

~50 ka

~110 ka

Pliocene unconformity

M

M

M

M

M

U

f

0.7 s

USGS-B108

Figure 3. Progressive tilting on the north limb of the Channel

Islands anticline displayed in two 7 kJ sparker seismic reflection

profiles from U.S. Geological Survey (USGS) data set 17200. Note

that these nonmigrated reflection profiles understate the pro-

gressive tilting. Both migration and depth conversion tend to

steepen deep reflections more than shallow ones, if velocity in-

creases with depth. Locations of A and B are shown in Figure 1.

Dated strata are correlated from Ocean Drilling ProgramSite 893 (Kennett, 1995), located on the northern continuation of

USGS-B108. Shades of gray on B108 show lowstand systems

tracts and a transgressive systems tract (f is flooding surface; U is

the time-transgressive MioceneQuaternary unconformity,

which becomes a Pliocene unconformity in the basin; M is water-

bottom multiple). Note that U is more tilted than f, which is in

turn more tilted than the seafloor. The interval between U and f

may comprise more than one lowstand systems tract.

A

B


5/13


6/13

during post-Miocene time. A listric shape for the

protoLions Head fault is suggested directly by

gently dipping reflections downdip from the mod-

erately dipping near-surface part of the fault

(Fig. 4A). We assume a circular listric shape for the

reconstruction (Fig.4C). This shape is predicted by

some models if the dip of precompression layering

is uniform (e.g., inclined shear; Dula, 1991). The

deviation from such a uniform dip in our recon-

struction (Fig.4C) suggests misfit of the simple cir-

cular listric model. Another likely complication is

motion in and out of the section in Figure 4 caused

by strike-slip components on any of the faults. In

this discussion of the Santa Maria basin structures

and in the rest of this paper, we consider only dip-

slip components that are in the plane of the sec-

tions. Area balancing of this motion is meaningful

only if the structures can be assumed to be uniform

along strike (i.e., cylindrical) over distances larger

than the strike-slip components. Also, models such

as fault-bend folds and listric thrusts that relate limb

width, or limb width and dip, to slip need not bearea balanced in order to determine how far up a

thrust ramp material has been displaced. Generally,

strike and dip components in the transpressional

regime of the Transverse Ranges tend to be parti-

tioned on distinct subparallel faults (e.g., Seeber

and Armbruster, 1995; Pinter et al., 1998a). Other

probable complications include formation of a

forelimb and layer-parallel shortening (volume

loss), either of which can lead to displacement gra-

dients on the faults. The fault array in Figure 4C

may be incomplete, particularly for north-dipping

faults beneath the Orcutt fault (e.g., Woodring and

Bramlette, 1950; Namson and Davis,1990).

Structural features of the offshore Santa Maria

basin are very similar to those represented in Fig-

ure 4 (Clark et al., 1991; Sorlien et al., 2000a).

Our reconstruction of the precompressional struc-

ture in the Santa Maria basin and the interpreta-

tion of many similar extensional basins inverted

during the current compressional deformation

support our contention that Miocene extension

and subsequent contractile reactivation of the ex-

tensional structures are regional phenomena.

DIP OVERPRINTING

Shallow crustal structure may be the result ofmultiple underlying faults operating synchro-

nously or successively (Shaw and Suppe, 1994;

Novoa, 1998). The regional south tilt of the Santa

Maria basin block (Fig.4) is probably the result of

two successive and opposite tilt events. A

southerly tilt occurred during growth sedimenta-

tion in Miocene time. We argued above that a

listric fault was responsible for this southward tilt-

ing during extension. Subsequently,the same fault

was reactivated during PlioceneQuaternary

shortening when the Purisima and Orcutt anti-

clines formed. A regional tilt reversal is expected

to accompany the slip reversal on the listric fault,

but it is only seen in the Purisima anticline be-

cause the regional rotation is small (3 in Fig. 4C).

Furthermore, this regional PlioceneQuaternary

down-to-the-north tilting was accompanied lo-

cally by southward tilting on the backlimb of the

Orcutt anticline. Thus, we interpret two deforma-

tion phases and two distinct structures to have

overprinted each other on the south limb of the

Orcutt anticline. Multiple overprinting is common

in the western Transverse Ranges and compli-

cates the task of inferring buried structures and

their slip rates from folds (e.g., Novoa, 1998).

Namson and Davis (1990) ascribed part of the re-

gional southward tilt in Figure 4 to postearly

Pliocene fault-bend folding and fault-propagation

folding. They related part of this tilt to slip on cur-

rently active and possibly seismogenic faults. In

contrast, we ascribe much of the regional tilt to

Miocene extension and the folding in the Orcutt

and Purisima anticlines to PlioceneQuaternaryslip on the regional buried fault and a backthrust.

Thus, our interpretations and those of Namson

and Davis (1990) differ drastically on the signifi-

cance of the regional tilt in terms of slip rates and

earthquake hazard.

LISTRIC THRUST MODEL

Erslev (1986) proposed rigid rotation on circu-

lar listric thrusts for the basement uplifts of the

Rocky Mountains foreland. We consider similar

listric-thrust fold models to account for progres-

sive tilting, tilt-dependent fault slip, and reactiva-

tion of preexisting listric normal faults. In the sim-

plest model, we assume undeformed horizontal

layering and a circular listric fault connected tan-

gentially to a layer-parallel detachment (Fig. 2C).

The hanging-wall block rotates about a horizontal

axis; this rotation can take place without internal

deformation except near the anticlinal and syncli-

nal axial surfaces because the fault is an arc of a

circle. Then, fault slip S = R, where R is the ra-

dius of curvature of the fault,and is the cumula-

tive rotation angle (in radians) of the hanging-wall

block (i.e., dip of the backlimb; Erslev, 1986). By

expressing R in terms of different combinations of

measurable quantities, we obtain

S = W/sin (1)

= T/(1 cos) (2)

= [(W2 + T2)/2T] (3)

where W, T, and are width of the backlimb,

depth to detachment, and dip of the fault, respec-

tively, as measured from the same prethrusting

layer (Fig. 2C).

The rotation of the hanging-wall block in Fig-

ure 2C is driven by a uniform transport velocity

above the detachment. With this simplest of all pos-

sible kinematic boundary conditions, rigid hori-

zontal-axis rotation can occur with localized exten-

sion and shortening at the upper and lower ends of

the backlimb (double arrows in Fig. 2C; Erslev,

1986). Conservation of bed length at the base of the

backlimb is possible with a specific nonuniform

kinematic boundary. Generally,however, layer-par-

allel slip and/or other deformation is required in the

hanging-wall block for a transport velocity that in-

creases upward. Equations 13 require rigid rota-

tion, but are still valid if layer-parallel slip is taking

place in the hanging-wall block, provided W is

measured and not W (Fig. 2C).

Wide forelimbs can also form synchronously

with wide backlimbs above thrust faults, and

these forelimbs are expected if the fault is blind

(Sibson,1995). Deformation of the footwall block

implies changes in the shape of the fault (e.g.,

Dula, 1991; Ramsay, 1992). Footwall collapsecaused by loading of the uplifted hanging-wall

block would rotate the upper reaches of the fault

to a lower dip (counterclockwise in Fig. 2C).

Forelimbs are then created by rigid block rotation

above the resulting convex-up parts of the fault

(Erslev, 1986). Such a convex-up fault segment

beneath a wide forelimb is imaged on profile S1,

located in Figure 1 and shown in Sorlien (2000).

Internal deformation is superimposed on this

rigid-rotation model to fully explain wide fore-

limbs. Slip is expected to propagate updip on a

normal fault that is reactivated as a thrust. The

resulting displacement gradient is generally ac-

counted for by folding of the hanging-wall block

creating a wide progressively tilted forelimb

(Sibson, 1995; Sorlien and Seeber, 1997). Forma-

tion of a forelimb by such a displacement gradient

can force formation development of a backlimb

(Fig. 9c in Wickham, 1995). This backlimb will

be superimposed on that formed by rigid-block

rotation in Figure 2C. As we propose for back-

limbs, the width of forelimbs is related to the

width of the map-view projection of the under-

lying fault, and forelimb dip is related to fault slip.

SANTA MONICA

MOUNTAINCHANNELISLANDS THRUST

A prominent topographic high, the southern-

most ridge of the western Transverse Ranges, is

continuous westward from the Santa Monica

Mountains to the northern Channel Islands and

beyond (Fig. 1). The anticlinal nature of parts of

this ridge has long been recognized. Like Davis

and Namson (1994a), we stress the continuity of

the anticlinal structure over the length of the topo-

graphic high, and we refer to this structure as the

SEEBER AND SORLIEN



7/13


SWCF

SRIF

?

Southwest

?

?

??

?

?

5s

4s

3s

2s

1s

Two-wayTravelTime

?=Stratigraphyuncertain

,based

onseafloorgeology

Reflectionordiscontinuity

(interpretedasfault)

?=Faultinterpretationun

certain

SCrIF

?

M

M

M

?

?

?

?

SMCIA

Northeast

5km

1.0s

2.0s

3.0s

4.0s

5.0s

M

iocenesediments(includesSisquocFormation)

No

verticalexaggerationat3km/s

SantaBarbaraChannel

progressivetilting

?

Two-wayTravelTime

0 2 4 6 8 10

12 1

416

0 2 4 6 8 10

12 1

416

SW

NE

5km

SM

CIA

SWCF

=8

=5

1.5km/s

1.8km/s

3.0km/s

T

Depthinkm

Displacement

Gradient

NotModeled

S

3.2

-5.9

km

5+km

13

+k

m

Miocene

W

Figure5.(A)ReflectionprofileacrossthewesternterminusoftheSantaMon-

icaMountainsChann

elIslandsanticline(SMCIA;profilelo

catedinFig.1).

SeafloorgeologyisfromVedder(1990).Thisprofileanditsapproximateinter-

sectionwiththemoredetailedstratigraphicinterpretationofUSGS-105islo-

catedonFigure1.Faults;SWCFSouthwestChannel,SRIF

SantaRosaIs-

land,SCrIFSantaCr

uzIsland.(B)Asimpledepthsection(1:1

)oftheprofilein

Aassuming1.5km/sinthewaterlayer,1.8km/sinpost-Mio

cenerocks,and

3.0km/sinMiocenerocks.ParametersW,T,S,,andrefertothecircular

listricmodelinFigure

2C.Afault-bendmodelwithtworampsseparatedbya

flatthatcouldaccount

fortheoverallshapeofthefoldisalsosketchedin.The

parametersoftheSWCF,includingareverseslipof3.25.9kmandadetach-

mentdepthof1219k

m,arederivedfromthedipofthefaultintheMiocene

layer(4555),thedip

ofthebacklimb(58),andthewidthoftheSantaMon-

icaMountainsChannelIslandanticline(30km).

B

A


8/13

Santa Monica MountainsChannel Island anti-

cline. The anticline has an asymmetric profile,

with a gentle dip to the north and a steeper dip to

the south (e.g., Fig. 5; Davis and Namson,1994a).

The southern limb forms the north margin of the

Los Angeles basin and of offshore basins.

We used a closely spaced (800 m) grid of seis-

mic reflection data over most of Santa Barbara

Channel, and published and unpublished struc-

ture-contour maps (e.g., Heck, 1998; Sorlien et al.,

2000b) to map the north-dipping limb of the Santa

Monica MountainsChannel Island anticline

along the southern margin of the Santa Barbara

Channel (Fig.1). Our primary grids of seismic re-

flection data were the U.S. Geological Survey

(USGS) 17200 and 19236 sets,described in Rich-

mond et al. (1981) and in Burdick and Richmond

(1982; see also reflection data published in Junger,

1979). These data were supplemented by a few

USGS multichannel profiles (Sorlien et al., 1998,

2000a) and industry multichannel profiles (Fig. 5;

Sorlien etal., 2000b). Dense grids of industry mul-tichannel data and many wells constrained the sub-

surface structure contour map of a ca. 6 Ma hori-

zon in Sorlien et al. (2000b) and in Heck (1998).

We traced the north-dipping limb of the Santa

Monica MountainsChannel Island anticline on-

shore by using data from numerous 1:24000 scale

geologic maps in the Santa Monica Mountains

area (e.g., Dibblee and Ehrenspeck,1993), as well

as from cross sections and structure-contour maps

in an industry study (Hopps et al., 1995; Nicholson

et al., 1997; see also the Web site http://quake.

crustal.ucsb.edu/hopps).

In our interpretation, the north limb of the


cline is 2030km wide and 220km long and gen-

erally displays uniform gentle northward dips of

520. A prominent exception is a 20-km-wide

panel of Miocene rocks in the Santa Monica

Mountains portion of the fold limb with gentle to

steep northerly dips (Dibblee, 1982; Dibblee and

Ehrenspeck, 1993). These steeper dips could par-

tially reflect tilting associated with south-dipping

Miocene extensional faults (e.g., Huftile and

Yeats, 1996, see also Campbell et al., 1966). Lo-

calized outcrops of postextensional rocks support

this hypothesis. For example, northeast of Point

Dume late Miocene strata are gently north dip-ping above moderately north dipping middle

Miocene rocks (Fig. 1; Dibblee, 1993).

We ascribe deviations from a planar geometry

of the Santa Monica MountainsChannel Island

anticlines backlimb to thrust faults that may be

secondary and shallow relative to the fault associ-

ated with the anticline. In particular, we interpret

folding associated with south-dipping faults as an

overprint on to regional north tilt. In this interpre-

tation, the south-dipping Oak Ridge fault and

faults beneath the Mid Channel trend are anti-

thetic to the thrust fault associated with the Santa

Monica MountainsChannel Island anticline, as

the thrust fault associated with the Orcutt anticline

is antithetic to the protoLions Head fault

(Fig. 4). Onshore, the Simi fault (Fig. 1) is also as-

sociated with a narrow belt of south dips. The area

mapped as overprinted south of the offshore and

coastal Oak Ridge fault (Fig. 1) is characterized

by short-wavelength folding in post-Miocene lay-

ers and by gentle dips, mostly to the north, of a

6 Ma horizon (Heck, 1998; Huftile and Yeats,

1995; Sorlien et al., 2000b). The onshore over-

printed area north and northeast of Point Dume

(Fig. 1) is characterized by northerly dips, but it is

separated from the Santa Monica Mountains by a

prominent belt of south dips. This structural break

narrows westward and disappears north-north-

west of Point Dume. Near the west end of the


cline, a less pronounced structural break between

north-dipping panels displayed in Figure5A is in-

terpreted to reflect a north-dipping,shallow thrust.In summary, we see strong evidence for a regional

gently dipping Santa Monica MountainsChan-

nel Island anticline fold limb. This north-dipping

fold limb is at least as wide as the darker shaded

area in Figure1, but could include part or all of the

overprinted area as well.

We believe the Santa Monica MountainsChan-

nel Island anticline and its wide and gently dipping

backlimb to be controlled by a major north-dip-

ping thrust fault, which we refer to as the Santa

Monica MountainsChannel Islands thrust. Parts

of the thrust coincide with thrust faults proposed

by others (e.g., Keller and Prothero, 1987; mid-

crustal detachment of Novoa, 1998; the Channel

Island thrust of Shaw and Suppe,1994; the Elysian

Park thrust of Davis and Namson, 1994a; the part

of the Elysian Park thrust beneath the Santa Mon-

ica Mountains was renamed the Santa Monica

Mountains thrust by Dolan et al., 1995). We argue

that the Santa Monica MountainsChannel Island

anticline defines a regionally continuous active

fold formed by slip on a regional master thrust

fault (also Davis and Namson, 1994a). This pro-

posed large active fault could produce large earth-

quakes, but not necessarily one large enough to

rupture the entire length of the fault. The structure

is likely to be segmented, particularly at the inter-sections with active northwest-southeast right-lat-

eral faults (Fig.1).

Early and middle Miocene rocks are thicker on

the islands than along the axis of western Santa

Barbara Channel (Fig. 5; Sorlien et al., 2000a;

Fig. 3 in Weaver, 1969; Redin et al., 1998). This

observation is consistent with the hypothesis that

the axis of the Santa Monica MountainsChannel

Island anticline coincides with a Miocene basin,

and that the Santa Monica MountainsChannel

Island thrust is a reactivated normal fault. A

715-km-wide south-dipping forelimb is present

along much of the length of the Santa Monica

MountainsChannel Island anticline (e.g., the

eastern Santa Monica Mountains, and offshore;

Sorlien and Seeber, 1997; Fig. 5). This wide fore-

limb could have been partially created during ini-

tial thrust reactivation, while the deep fault was

slipping and the shallow fault was locked (see

also Sibson, 1995). The residual south dip be-

neath Santa Maria Valley in the early Pliocene re-

construction of Santa Maria basin (Fig. 4C) can

be explained by similar nonrigid deformation

during initial reactivation of a Miocene normal

fault. Thus, although the Santa Monica Moun-

tainsChannel Island anticline and the structure

in the Santa Maria basin differ in size and other

important respects, they appear similar in their

evolution.

East-weststriking faults mapped south of the

northern Channel Islands (Fig. 1; Crouch and

Suppe,1993; Bohannon and Geist, 1998) may be

the most direct surface manifestation of the SantaMonica MountainsChannel Island thrust. The

Santa Monica fault bounds the Santa Monica

Mountains to the south. This fault may be analo-

gous to the Santa Rosa Island fault (Figs. 1

and 5A) in cutting the Santa Monica Moun-

tainsChannel Island thrusts hanging-wall block

at the forelimb of the Santa Monica Moun-

tainsChannel Island anticline and in having a

significant component of left slip (Sorlien et al.,

1998; Dolan et al., 1995). The Santa Monica fault

and associated faults to its south have been inter-

preted to be reactivated normal faults (Schneider

et al., 1996) and may have been coupled with the

Santa Monica MountainsChannel Island thrust

before the onset of thrusting.

PROGRESSIVE TILTING

The western half of the north-dipping back-

limb of the Santa Monica MountainsChannel

Island anticline is characterized by Plio-

ceneQuaternary strata that have a north dip that

increases with depth, and by older sequence

boundaries that are more steeply north dipping

than the younger ones (Figs.1, 3, and 5). Velocity

logs in Santa Barbara Channel show that velocity

increases with depth,and therefore vertical exag-geration on the time sections in Figures 3 and 5

decreases with depth. Depth-converted profiles

near Figure 5 (USGS-105, located in Fig. 1 and

shown in Sorlien et al., 2000a) and north of Santa

Rosa Island (Sorlien et al., 1998) show dip to in-

crease with depth. Differential subsidence due to

greater compaction in the north than the south

and/or drape and filling of a preexisting basin

may contribute a nontectonic component of north

tilt. However, two factors support tectonic pro-

gressive north tilt.

SEEBER AND SORLIEN



9/13

First, a time-transgressive unconformity under-

lying the shelf and slope manifests a period of ero-

sion (Figs.3,A and B). Posterosional compaction

is not expected in the rocks below the unconfor-

mity because more material was removed by ero-

sion than has been redeposited. The dip of onlap-

ping strata just above the unconformity, therefore,

is not affected by compaction. Preunconformity

strata in Figure 3 dip more steeply than does the

unconformity, which in turn dips more steeply

than the strata above it. Younger onlapping strata

are progressively flatter upsection. This geometry

is not consistent with differential compaction.

Furthermore,deep Pliocene erosion beneath what

is now the deepest part of the basin (Shaw and

Suppe, 1994) is inconsistent with gradual filling

of a basin. It suggests instead an increase in

seafloor relief along the basin margin. We con-

clude that progressive tilting in the Santa Barbara

channel is at least in part tectonic. This tilting is

most prominent along the base of the slope along

the south margin of Santa Barbara basin, where itaffects strata (dated by the Ocean Drilling Pro-

gram, Site 893) that are younger than 50 ka

(Fig. 3, A and B; Kennett, 1995; Junger, 1979).

Second, Santa Cruz Island and its northern

shelf are tilting northward with little possible con-

tribution from either differential compaction or

drape. Progressive north tilting can be demon-

strated and quantified by comparing uplift of

dated coastal terraces on northwestern Santa Cruz

Island to subsidence of strata deposited on the

shelf north of the island above overcompacted

Miocene rocks. The paleoshoreline (shoreline an-

gle) of the stage 5e (ca. 125 ka) marine abrasion

platform is near its original elevation (~5 m) near

the northwest point of Santa Cruz Island, while

earlier uplift is required to explain the nearby

higher shorelines at 25 m and 80 m (Pinter et al.,

1998a), or 130 m a few kilometers east (Pinter

et al., 1998b). On the shelf north of Santa Cruz

and Anacapa islands, post-Miocene uplift and

erosion was followed by aggradation that accu-

mulated only a few seismic sequences (Fig. 3B).

The tops of prograding sediment packages (low-

stand systems tracts) are tangential to planar se-

quence boundaries (toplap) north of Santa Cruz

Island, indicating sea-levelcontrolled deposition

(in contrast to later erosion, which would truncatethe clinoforms). The oldest of these surfaces oc-

curs 10 km north of the uplifted 130 m pale-

oshoreline, in water as deep as 240 m, twice the

depth of the lowest eustatic sea level (Pinter et al.,

1998b). These sequences overlie the 1 Ma horizon

of Yeats (1981) near the east end of Santa Cruz Is-

land (Pinter et al., 1998b, 1998c; Junger, 1979).

The simplest explanation for uplift of the islands

and subsidence of the shelf is tectonic north tilt.

A tentative age of 400 ka has been proposed for

the 80130m terrace (Pinter etal.,1998a). Assum-

ing a constant rate,the extrapolated uplift would be

325 m in 1 m.y. We can then calculate a tilting rate

of 2.4/m.y. from the differential vertical motion

between the uplifted terraces on the island and

post1 Ma 100+ m subsidence of the shelf 10 km

to the north. At this rate, it would only take a few

million years to form the gentle north dips of the

Miocene strata on northwestern Santa Cruz Island.

Available data on coastal terraces on the north-

ern Channel Islands suggest that north tilting is re-

gional. The inner edge of the low prominent ma-

rine abrasion platform on northeast Santa Rosa

Island slopes down to the north from more than

20 m elevation near the Santa Rosa Island fault to

about 14 m elevation at the northeast point of that

island (Sorlien, 1994). The shelf break is deeper

north of Anacapa Island than south of it (Scholl,

1960), consistent with the subsidence of the shelf

north of that island (subsided shelf-edge strata

shown in Junger, 1979, sheet 3, profile K-606). In

the eastern half of the Santa Monica Moun-

tainsChannel Island anticline backlimb, we inter-pret Quaternary progressive tilting from cross sec-

tions by Dibblee (1992) and Paschall et al. (1956)

in small areas of the northern Santa Monica

Mountains where Quaternary strata are preserved

(Fig. 1). Post-Miocene strata are not preserved on

the rest of the subaerial part of the Santa Monica

MountainsChannel Island anticline backlimb.

UNIFORM EARLY KINEMATICS

OF THE SANTA MONICA

MOUNTAINSCHANNEL

ISLAND ANTICLINE

Folding of the forelimb of the Santa Monica

Mountains initiated at 5 Ma (Schneider et al.,

1996), or 4.5Ma (the entire early Pliocene Repet-

tian stage thins onto folds along northern Los An-

geles basin in cross sections in Wright, 1991).

The Repettian stage strata also onlap the base of

the north-dipping fold limb along the north-verg-

ing Oak Ridge trend of the eastern Santa Barbara

Channel (Redin et al., 1998). Initial short-wave-

length folding of the western part of the structure,

northwest of San Miguel Island, also occurred

during early Pliocene time (Sorlien et al., 2000a).

The short-wavelength folds are overlapped by a

sequence with uniform thickness and an apparentnortheast dip of 5, suggesting a flat seafloor dur-

ing deposition and later tilt. This sequence is

capped by a late Pliocene onlap surface that dates

initiation of regional tilting (Fig. 5A). Similarly,

short-wavelength, north-verging folds in south-

east Santa Barbara Channel are capped by an un-

conformity that now dips north on the order of 5

(Junger, 1979). The initiation of this regional tilt-

ing is probably late Pliocene or early Quaternary,

although this dating is hampered by absence of

Pliocene strata in this area.

IS THE SANTA MONICA MOUNTAIN

PART OF THE SANTA MONICA

MOUNTAINSCHANNEL ISLAND

ANTICLINE ACTIVE?

Some of the recent work on the Santa Monica

MountainsLos Angeles basin emphasizes lack o

evidence for current activity on blind thrust fault

in this area (Johnson et al., 1996; Foxall, 1998)

Although probably not as important to the overal

strain budget as initially thought (e.g., Davis and

Namson, 1994a), the evidence still points to sig

nificant activity on blind thrust faults. Compari

son of geodetically determined north-south con

traction to geologic-based slip estimates on fault

in the Los Angeles basin area indicates that half o

the convergence is accommodated on conjugate

strike-slip faults (Walls et al., 1998). After ac

counting for surface faults, as much as 1.5 mm/y

north-south shortening could be accommodated

by folds above blind thrust faults (Walls et al.

1998; Yeats and Huftile, 1996). Slip on blindthrust faults could be higher than 1.5 mm/yr if th

strike-slip faults in the upper crust are confined in

the hanging wall above such faults.

A ca. 125 ka marine terrace along the Malibu

Coast east of Point Dume is uplifted at 0.20.4

mm/yr, even after removing the effects of surfac

faults (Johnson et al., 1996). This surface uplif

rate can be interpreted, for example, to reflect

slip rate 0.81.5 mm/yr on a portion of a thrus

ramp dipping 15. Furthermore, this rate is a min

imum value because the uplift is probably a prod

uct of both tectonic thickening and sinking of the

crust. Meigs et al. (1999) interpreted that uplift o

the Santa Monica Mountains is balanced by ero

sion and that the mountains are therefore in iso

static balance. This interpretation is in part based

on late Quaternary rates of surface uplift being

comparable to the 5 m.y. average increase in

structural relief (Meigs et al., 1999). However

isostatic subsidence of the coastline along the

Santa Monica Mountains is suggested if increase

in structural relief has been higher during Qua

ternary time than the 5 m.y. average rate, or i

erosion rates near the coastline, where coastal ter

races are preserved, are lower than in the interio

of the Santa Monica Mountains. We interpret tha

isostatic or flexural subsidence is pervasive alongthe southern front of the Transverse Ranges

Widespread subsidence has been documented in

the Santa Barbara Channel (e.g., Pinter et al

1998b, 1998c), and is inferred for the hanging

wall block of the offshore Santa Monica fault (the

Dume fault). This upthrown side of the fault is

only lightly eroded and is now submerged; it i

probably subsiding with respect to sea level (pro

file S2 located in Fig. 1, shown in Sorlien, 2000

Davis and Namson, 1994b). We conclude that ab

solute surface uplift rates are not a reliable mea




10/13

sure of blind fault activity along the southern

front of the Transverse Ranges.

Blind thrust faulting is also suggested by fold-

ing of PliocenePleistocene strata north of down-

town Los Angeles that have absorbed 0.50.7

mm/yr of shortening (Schneider et al., 1996).

Continued slip on blind thrust faults is indicated

by a 5 south tilt of the ca. 1 Ma horizon across a

3 km width (Fig. 4 in Schneider et al., 1996). Fur-

ther evidence of ongoing deformation along the

southeastern front of the Santa Monica Moun-

tainsChannel Island anticline is a south-facing

seafloor fault or fold scarp as much as 700 m high

along the offshore Santa Monica fault west of

Point Dume (Fig. 1; Davis and Namson, 1994b;

Sorlien, 2000). Much of the seismicity below the

Los Angeles basin is characterized by east-

weststriking low-angle thrust planes (Hauksson,

1990; Geiser and Seeber, 1996).

NO LATE QUATERNARY TILT

NORTHEAST OF ANACAPA ISLAND

Northeast of Anacapa Island the Santa Monica

MountainsChannel Island anticline takes a left

bend and forms a structural saddle (Fig.1). In the

same area, post-Miocene reflectors suggest no

progressive tilting. These strata are clearly im-

aged by 12-fold stacked seismic reflection pro-

files (USGS data set 19236), as well as by indus-

try data. The reflections from post1 Ma strata

above a north-dipping unconformity are flat ly-

ing. Reflections from Miocene strata beneath the

unconformity dip uniformly to the north (Greene,

1976). The tilt of these older strata is therefore

prelate Quaternary.

This lack of late Quaternary tilting may be in-

terpreted as either evidence for thrust inactivity,

or alternatively as evidence of a change in the

shape of an active fault. A gradual decrease in

north dip of post1 Ma strata from eastern Santa

Cruz Island eastward is consistent with a gradual

flattening of the fault toward the east. Folding

southeast of Anacapa Island reveals that blind

thrust faulting has propagated south of the Santa

Monica MountainsChannel Island thrust trend

in this area (Fig. 1). The industry seismic reflec-

tion profile located as S1 in Figure 1 shows the

Santa Monica MountainsChannel Island thrustto be convex up in the upper 89 km, consistent

with the very wide south-dipping panel or fore-

limb in the area of the structural saddle (Sorlien,

2000). The left bend on the Santa Monica Moun-

tainsChannel Island anticline between the Santa

Monica Mountains and Anacapa Island is a re-

leasing bend along the Santa Monica Moun-

tainsChannel Island thrust for associated left-

oblique faults such as the Malibu Coast fault

(Fig 1; Dibblee, 1982). Such a releasing bend

could locally cancel the effect of regional short-

ening. The lack of tilting might suggest local fault

inactivity, but this presents some problems when

considering regional kinematics in map view

(Sorlien et al., 2000b).

In summary, the Santa Monica Moun-

tainsChannel Island anticline is a continuous

structure with a uniform kinematic development

from the western Channel Islands to the Santa

Monica Mountains. Much of the north limb is

progressively tilted, possibly including the Santa

Monica Mountains area. We consider this pro-

gressive tilting to reflect PlioceneQuaternary

thrust slip on the underlying master fault. This

progressive tilting is problematic for single-step

ramp-flat models and supportive of a listric fault

model for the Santa Monica MountainsChannel

Island anticline.

ACCUMULATED SLIP ON THE SANTA

MONICACHANNEL ISLAND THRUST

FAULT:WESTERN SECTION

The implications of the listric thrust model for

earthquake hazard estimates can be illustrated by

estimating slip near the western end of the Santa

Monica MountainsChannel Island thrust where

late Miocene and younger strata are present

across much of the fold. An industry seismic re-

flection profile offers a continuous section across

the west-northwesttrending part of the Santa

Monica MountainsChannel Island anticline

west of San Miguel Island (Fig. 5). The profile

crosses USGS-105, which has a detailed strati-

graphic interpretation based on wells (Fig.1; Sor-

lien et al., 2000b). The younger strata in Figure 5

are progressively tilted, as expected from fold-

limb rotation above a listric fault, according to

our model. A late Pliocene onlap surface dates

initiation of regional tilting and postdates early

Pliocene short-wavelength folding, similar in

timing and style to north-verging folds seen in the

southeast channel. Therefore, we interpret the

fold limb to be entirely the result of shortening

(Sorlien etal., 2000a). Two north-dipping panels,

13 and 57 km wide, from northeast to south-

west, are also interpreted by us to be part of the

same backlimb. This backlimb has been over-

printed by numerous structures, including a

prominent listric fault dipping north in the north-ern half of the profile. This low-angle fault is

probably at least partly responsible for the 3-km-

wide flat separating the dip panels and for the

short-wavelength folding (Fig. 5). The Santa

Rosa Island and Santa Cruz Island faults, known

to have large components of left-lateral slip

where they are exposed on the islands (e.g., Pin-

ter et al., 1998a), are imaged as steep faults on

opposite limbs of the Santa Monica Moun-

tainsChannel Island anticline and appear to con-

verge below the crest of this anticline. These

faults may be partly responsible for the folding

between them, but cannot account for the wide

north-dipping fold limb.

The northeast-dipping Southwest Channel

fault imaged at the base of the Santa Monica

MountainsChannel Island anticline forelimb

(Fig. 1) has been interpreted to be a major

Miocene normal-separation fault (Fig. 5; Sorlien

et al., 2000a). Although the Southwest Channel

fault may have been distinct during Miocene time,

the anticline in its hanging-wall block is continu-

ous with the Santa Monica MountainsChannel

Island anticline (Fig. 1). Thus we consider the

Southwest Channel fault to now be a thrust fault,

the westernmost segment of the Santa Monica

MountainsChannel Island thrust.

The Southwest Channel fault in pre-Miocene

rocks near the sea floor (Fig. 5) is estimated to

dip from 45 to 55 (assuming an interval veloc-

ity of 2.53.5 km/s). By neglecting deformation

in the hanging-wall block, which may be associ-

ated with secondary faulting, we estimate a dip of58 for the backlimb (Fig. 5B). This is consis-

tent with the 2.4/m.y. late Quaternary tilt rate in-

ferred in the Santa Cruz Island area and with late

Pliocene initiation of regional tilting. Thrust re-

activation propagated updip, and a broad fore-

limb developed while the deep fault slipped and

the shallow fault was locked. The forelimb

formed by consumption of the upper part of the

backlimb (W in Fig.2C), so that backlimb width

is likely to be less than expected from a rigid ro-

tation model. Thus we measure instead the dis-

tance W = 30 km between the base of the back-

limb and the Southwest Channel fault where it

intersects prethrusting strata (Figs. 2C and 5B).

Assuming a circular listric fault, we obtain a slip

between 3.2 and 5.9 km from equation 1 and a

detachment depth between 12 and 19 km (in-

cluding 1 km to account for water and syn-thrust

strata) from equation 2. This result is consistent

with depths of 1213 km (Keller and Prothero,

1987) and 1116 km (Nicholson et al, 1992) for

the top of a high-velocity layer interpreted to be

oceanic basement in the general area of the pro-

file in Figure 5A. Formation of a wide forelimb

by displacement gradient steepens the upper part

of the backlimb (Wickham, 1995), so that the

lower estimates for backlimb dip and thrust slipare more likely.

The structure in Figure 5 could also be inter-

preted as two fault-bend folds above two ramps

separated by a small flat (as sketched in Fig. 5B).

Fault slip in such a model would be at least as great

as the width of the wider backlimb (Fig. 2A), or

about 13 km. Such an amount of slip or shortening

in the buried structure would be several times

larger than the shortening that can be accounted

for by folding in the hanging wall, and the major-

ity of slip needs be transferred south, beyond the

SEEBER AND SORLIEN



11/13


cline. In contrast, the listric model tends to predict

relatively less displacement on buried faults re-

sponsible for wide and very gently dipping fold

limbs and can generally reconcile this slip with

shortening in shallow layers.

An active blind thrust fault is generally expected

to propagate updip, whether it is new or is reacti-

vating a preexisting fault (e.g., Suppe and Med-

wedeff, 1990). The simple rigid circular listric

thrust model in Figure 2C does not account for a

propagating fault. Intuitively, a forelimb is ex-

pected to form above a buried fault tip (Sibson,

1995; Sorlien and Seeber, 1997). As this fault tip

propagates updip and breaches the surface, the ac-

tive forelimb is expected to narrow and eventually

stop tilting. Furthermore, the basal thrust in a clas-

sical fold-and-thrust belt propagates forward by

developing a new imbricate thrust and abandon-

ing, at least partially, the previous one (Davis et al.,

1983). As the belt of convergence widens and ma-

tures, surface shortening at a material-fixed site onthis belt is likely to be accounted for progressively

less by folding and more by faulting. A site in the

hanging wall is likely to be transported over a pro-

gressively more active basal thrust detachment if

propagation of the thrust is more rapid than slip

(e.g., Davis et al, 1983; Geiser and Seeber, 1996).

Thus, a possible decrease in the rate of tilting in the

forelimb of the Santa Monica Mountains anticline

in the past ~1 m.y. (Schneider et al., 1996) or

125 ka (Johnson et al., 1996) does not necessarily

imply a decrease in the rate of slip of thrust faults

below it. On the contrary, an increase in the slip

rate of faults below a given location could occur

even at a constant convergence rate across the

southwestern front of the Transverse Ranges. Al-

ternatively,this rate could be decreasing to balance

an increasing convergence rate across the Ventura

basin in the past 1 m.y. (Huftile and Yeats, 1995).

If so, slip rates inferred from total post-Miocene

folding of the Santa Monica MountainsChannel

Island anticline may overestimate the current de-

formation rate.

SUMMARY AND CONCLUSIONS

The shape of folds is related to the geometry

and slip of buried causative fault(s), but this rela-tion is model dependent. PlioceneQuaternary

transpression in the western Transverse Ranges

was preceded by a Miocene extensional regime

controlled by large, gently and moderately dip-

ping, listric normal faults. Regional sedimentary

growth wedges require hanging-wall block rota-

tion about horizontal axes and suggest major nor-

mal faults with nearly circular listric shapes. We

propose that some of the major active thrusts in

this area may be reactivated normal faults that

preserve their listric shape. Progressive tilting of

backlimbs that developed during the present

phase of transpression supports the contention

that the thrusts are listric. The ramp-flat fault

models that have been widely applied in the

western Transverse Ranges are inappropriate for

these structures, based on our analysis. Fault slip

is proportional to limb length and independent of

limb dip in these models. Single-step ramp-flat

models applied to wide low-angle backlimbs

commonly predict fault displacements that are

much larger than the shortening due to folding in

the shallow layer above the thrusts. Furthermore,

these models cannot account for progressive tilt-

ing of backlimbs. We propose instead a listric

fault model where fault slip is proportional to

limb dip, and expect this slip to produce progres-

sive tilting of the hanging-wall block.

The Santa Monica MountainsChannel Island

anticline is a 220-km-long structural and topo-

graphic high along the southern margin of the

western Transverse Ranges that is associated with

a 2030-km-wide north-dipping fold limb,most ofwhich has been tilted progressively. We interpret

this structure as a backlimb associated with a re-

gional north-dipping thrust fault, the Santa Monica

MountainsChannel Island thrust (as did Davis

and Namson, 1994a). This structure is superim-

posed on a Pliocene north-verging forelimb in

southeast Santa Barbara Channel. By assuming

that the shape of the Santa Monica Moun-

tainsChannel Island thrust is circular listric and

that the hanging-wall block rotates rigidly, we ob-

tain 3.25.9 km of reverse slip at the western end

of the thrust since regional tilting commenced dur-

ing late Pliocene time. We interpret north-verging

Pliocene folding in the eastern Santa Barbara

Channel to be in the roof of a thrust wedge propa-

gating south, and that much of the regional north

tilting of the continental shelf of the islands is Qua-

ternary. In the early development of the thrust re-

activation, therefore, the tip of the fault might have

moved updip south of the apex of a wedge be-

tween the reactivated fault and an antithetic thrust

fault (the Western Deep fault of Novoa, 1998,

along the Mid Channel Trend). We do not attempt

to estimate total slip or late Quaternary slip on the

Santa Monica Mountains thrust because post-

Miocene strata are not generally present. However,

published results are permissive of 12 mm/yr ofthrust slip (e.g.,Walls et al., 1998).

From our results, average slip rates on the

Santa Monica MountainsChannel Island thrust

since the inferred onset of regional tilting during

late Pliocene time are low, in the 12 mm/yr

range. Because changes in long-term slip rates

are possible, the current geological slip rate rele-

vant to earthquake hazard may be better defined

from a detailed chronology of progressive tilting.

This chronology can be determined by using ge-

omorphology (e.g., Johnson et al., 1996) and de-

tailed stratigraphy where Quaternary strata are

preserved (e.g., between Anacapa Island and the

Santa Monica Mountains).

According to our listric thrust model, the accu

mulated slip and the inferred long-term slip rate

are much less than the rate for the Santa Monica

Mountains thrust published by Davis and Nam

son (1994a) and are similar to the post 1 Ma rate

on the Channel Islands thrust proposed by Shaw

and Suppe (1994). This similarity is coincidenta

because the structural model proposed by Shaw

and Suppe (1994) is drastically different than ou

interpretation.

There are island-scale irregularities or recesse

in the fold limb (Fig 1), perhaps related to effect

of northwest-southeast right-lateral faults that in

tersect the Santa Monica MountainsChannel Is

land anticline from the south. These right-latera

faults are expected to load the Santa Monica

MountainsChannel Island thrust differentially

along strike and possibly segment this fault. A

flattening of this thrust fault east of Anacapa Island and east of the Santa CruzCatalina Ridg

segment of the San Clemente fault system may re

flect this segmentation. Despite this possible seg

mentation, the timing and evolution of fold devel

opment and inferred thrust activity appear to be

similar along the entire structure. Thus, we em

phasize the continuity, rather than the segmenta

tion, of the Santa Monica MountainsChannel Is

land anticline in terms of possible maximum-siz

earthquakes. This structure has a relatively low

slip rate and is secondary in terms of moment re

lease, but it may be a primary structure in terms o

possible earthquake size.

ACKNOWLEDGMENTS

Nicholas Pinters work on Santa Cruz Island

was instrumental in initial recognition of onshore

offshore tilting. Our interpretation of the section

in Figure 4B was influenced by Lynn Tennyson

nearby unpublished cross section. Greg Moun

tain, Peter Geiser, Milene Cormier, John Arm

bruster, Marc Kamerling, and Jim Galloway con

tributed with discussions and/or their data. We are

grateful to the petroleum industry and to UNO

CAL for the profiles in Figure 5 and in Figure 4

respectively, and to the Mineral Management Service for access to public wells and high-resolution

seismic profiles. Peter Geiser, Craig Nicholson

Art Sylvester, Lynn Tennyson, Tom Wright, Kar

Mueller, and Tom Rockwell reviewed the manu

script and offered valuable suggestions. Seebe

was supported by Southern California Earthquake

Center (SCEC) grant USCPO 569934 scope A

USGS grant 1434-95-G-2576, and National Sci

ence Foundation (NSF) grant EAR-94-16222

L. Seeber was supported by the SCEC, which i

funded by NSF Cooperative Agreement EAR




12/13

SEEBER AND SORLIEN


8920136 and USGS Cooperative Agreements 14-

08-0001-A0899 and 1434-HQ-97AG01718. This

is SCEC contribution 501, Lamont-Doherty con-

tribution 6047, and Institute for Crustal Studies

contribution 0251-67TC.

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