listric thrusts in the western transverse ranges, california
TRANSCRIPT
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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.
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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.
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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
1070 Geological Society of America Bulletin, July 2000
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
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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
1072 Geological Society of America Bulletin, July 2000
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Geological Society of America Bulletin, July 2000 107
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
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7/30/2019 Listric Thrusts in the Western Transverse Ranges, California
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
Santa Monica MountainsChannel Island anti-
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
Santa Monica MountainsChannel Island anti-
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
1074 Geological Society of America Bulletin, July 2000
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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
LISTRIC THRUSTS IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA
Geological Society of America Bulletin, July 2000 107
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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
1076 Geological Society of America Bulletin, July 2000
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7/30/2019 Listric Thrusts in the Western Transverse Ranges, California
11/13
Santa Monica MountainsChannel Island anti-
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
LISTRIC THRUSTS IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA
Geological Society of America Bulletin, July 2000 107
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SEEBER AND SORLIEN
1078 Geological Society of America Bulletin, July 2000
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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