Tectonophysics 392 (2004) 279 – 301
www.elsevier.com/locate/tecto
Four-dimensional transform fault processes: progressive evolution
of step-overs and bends
John Wakabayashi a,*, James V. Hengesh b, Thomas L. Sawyer c
b
a
Wakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USA
William Lettis and Associates, 999 Anderson Dr., Suite 140, San Rafael, CA 94901, USA
c
Piedmont Geosciences, Inc., 10235 Blackhawk Dr., Reno, NV 89506, USA
Available online 10 June 2004
Abstract
Many bends or step-overs along strike – slip faults may evolve by propagation of the strike – slip fault on one side of the
structure and progressive shut-off of the strike – slip fault on the other side. In such a process, new transverse structures form,
and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the
progressive asymmetric development of a strike – slip duplex. Consequences of this type of step-over evolution include: (1)
the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits
are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary
kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault
system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault
system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the PlioPleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike)
along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic
trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples
of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the
Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along
strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino
triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world,
including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins
of New Zealand.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Strike – slip faults; Transform faults; Transpression; Transtension; Pull-apart basins; San Andreas fault
* Corresponding author. Tel.: +1-510-887-1796; fax: +1-510-887-2389.
E-mail address: wako@tdl.com (J. Wakabayashi).
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.04.013
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J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
1. Introduction
Bends and step-overs are common structures along
strike – slip systems (e.g., Crowell, 1974a,b; ChristieBlick and Biddle, 1985). Geologic features that form
as a consequence of these step-overs and bends
include those of transtensional character, such as
pull-apart basins and negative flower structures associated with releasing bends or step-overs, and those of
transpressional character, such as oblique fold and
thrust belts, push up structures, and positive flower
structures associated with restraining bends or stepovers (e.g., Crowell, 1974a,b; Christie-Blick and
Biddle, 1985). For the purposes of discussion, we
can classify structures within a step-over or bend
region as: (1) main strike – slip faults or bounding
faults also known as principal displacement zones or
PDZs, and (2) transverse structures that accommodate
the transfer of slip between the PDZs on either side of
the step-over region (Fig. 1). Geologic features related
to step-overs and bends have received considerable
attention from researchers (e.g., Mann et al., 1983;
Christie-Blick and Biddle, 1985; Westaway, 1995;
Dooley and McClay, 1997), yet critical aspects of
the long-term evolution of these structures are not
well understood. Studies of the evolution of step-over
features have considered step-overs as features that
Fig. 1. Simple classification of some structures associated with stepovers or bends along strike – slip faults. An idealized releasing stepover (A), and restraining step-over (B), are shown.
progressively increase in structural relief and size
(e.g., Mann et al., 1983; Dooley and McClay, 1997;
Dooley et al., 1999; McClay and Bonora, 2001).
Existing models, however, do not explain some enigmatic field relations associated with step-overs and
bends, such as: (1) Why are deposits, interpreted to
have been laid down in pull-apart basins associated
with releasing step-overs, found outside of the active
basins? and (2) Why is the structural relief associated
with restraining step-overs commonly smaller than
expected for the amount of slip transfer accommodated by the them?
The well-known dextral San Andreas fault system
of coastal California accommodates 75– 80% (38 – 40
mm/year) of present Pacific– North American plate
motion (e.g., Argus and Gordon, 1991), and 70%
(540 –590 km) of the dextral displacement that has
accumulated across the plate boundary over the last 18
Ma (Atwater and Stock, 1998). The San Andreas fault
system forms the boundary between the Pacific plate
and the Sierra Nevada microplate, whereas the Basin
and Range province and Walker Lane separate the
Sierra Nevada microplate from stable North America
(Argus and Gordon, 1991). The northern part of the
San Andreas fault system strikes slightly oblique to
the direction of relative motion between the Pacific
Plate and Sierra Nevada microplate, resulting in a
small component of fault-normal convergence (generally less than 10% of the strike –slip velocity) and
regional transpression in the California Coast Ranges
(Zoback et al., 1987; Argus and Gordon, 1991, 2001).
Because the regional fault-normal convergence is
relatively minor, most of the more prominent transpressional features along the northern San Andreas
fault system are driven by restraining bends or stepovers along strike slip faults (e.g., Aydin and Page,
1984; Bürgmann et al., 1994, Unruh and Sawyer,
1995) rather than regional transpression.
Prior to the development of the transform plate
margin, the western margin of North America in (what
has become) California was the site of over 140
million years of continuous subduction that resulted
in the formation of the Franciscan subduction complex, the primary basement rock type in the California
Coast Ranges (e.g., Wakabayashi, 1992). Initial interaction between an oceanic spreading ridge and the
subduction zone occurred at about 28 Ma, but the San
Andreas transform system did not become established
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
until about 18 Ma (Atwater and Stock, 1998). Since 18
Ma, San Andreas fault system has progressive lengthened and accumulated dextral displacement, as the
Mendocino triple junction (the north end of the
system), migrated northwestward and the Rivera triple
junction migrated southeastward (e.g., Atwater and
Stock, 1998). The San Andreas fault system consists
of several major dextral faults, including the San
Andreas fault. The distribution of active structures
and dextral slip rates has shifted irregularly throughout
the comparatively brief history of the transform fault
system; some faults increased or decreased in slip rate,
some faults initiated movement, whereas other faults
became dormant (Wakabayashi, 1999). Although the
Franciscan subduction complex comprises most of the
basement in the California Coast Ranges, dextral
displacement along strands of the San Andreas fault,
as well as earlier syn-subduction dextral displacement,
have emplaced a continental arc fragment, the Salinian
block, between two belts of subduction complex rocks
(e.g., Page, 1981). Consequently, major dextral faults
of the northern San Andreas fault system: (1) cut
Franciscan Complex basement, (2) form the boundary
between Franciscan Complex and Salinian basement,
or, (3) cut Salinian basement. Of the examples of stepovers we present, all but one occur along dextral faults
cutting Franciscan basement; the lone exception are
the structures associated with the Olema Creek Formation, formed along a reach of the San Andreas fault
that separates Franciscan and Salinian rocks. Deposition of late Cenozoic sedimentary and volcanic rocks
has occurred during the development of the transform
margin (e.g., Page, 1981). These late Cenozoic deposits provide most of the evidence for the processes
discussed in this paper.
There are many examples of transtensional (pullapart) basins and local transpressional structures related to step-overs and bends along the various
strike –slip faults of the San Andreas fault system
(e.g, Crowell, 1974a,b, 1987; Aydin and Page, 1984;
Nilsen and McLaughlin, 1985; Yeats, 1987). Local
transpressional and transtensional geologic features
along strike– slip faults occur across a range of scales,
from tens of km for large basins and push-up regions,
to meters for sag ponds and small pressure ridges
(Crowell, 1974a).
We present examples of features related to stepovers and bends from the northern San Andreas fault
281
Fig. 2. The northern San Andreas fault system, showing major
dextral strike – slip faults and other features discussed in text. Note
the Northeast Santa Cruz Mountains fold and thrust belt (FTB) is
not the only fold and thrust belt in this area, but this specific belt is
shown because it is specifically discussed in the text. Adapted from
Wakabayashi (1999).
system (Fig. 2) at scales from meters in a paleoseismic
trench, to tens of kilometers in major basins or fold and
thrust belts, and show that it is necessary to evaluate
the evolution of these structures in four dimensions
(three spatial dimensions and time) to best understand
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them. The common attribute of all of the examples we
present is that the step-over has apparently migrated
with respect to the deposits that were formerly affected
by it. For releasing structures, such as pull-apart
basins, the basins have migrated along bounding
strike – slip faults (PDZs) with respect to their former
basinal deposits. For restraining structures, such as
fold and thrust belts or transpressional welts, the
features have migrated along bounding strike – slip
faults with respect to deposits that were deformed by
these structures. In addition to examples from the San
Andreas fault system, we also review some examples
from other plate boundaries that may have followed a
similar type of tectonic evolution. Following the
presentation of field examples we discuss a model
for the evolution of such step-overs and bends.
2. Migration of pull-apart basins with respect to
basinal deposits
As noted above, regional plate motions result in
regional transpression within California Coast
Ranges. Because transtensional tectonics within the
northern San Andreas fault system are a local consequence of releasing step-overs and bends in an otherwise transpressional setting, the migration of such a
step-over away from pull-apart basin deposits results
in the subsequent uplift and contractional deformation
(locally driven inversion) of such deposits. Below we
describe pull-apart basins that have migrated with
respect to their deposits at three scales: tens of kilometers, kilometers, and meters.
2.1. Tomales Bay depocenter and Olema Creek
formation
The Olema Creek Formation is about 130 ka old
and crops out in a 3.5 km long by 0.5 km wide belt
along the San Andreas fault, south of Tomales Bay
Fig. 3. Distribution of the Merced Formation (QTm), Colma
Formation (Qc), Olema Creek Formation (Qoc), and offshore
features along the San Andreas fault. Other abbreviations B = Tertiary and older rocks, FTB: Northeast Santa Cruz Mountains fold
and thrust belt; Q = undifferentiated Quaternary deposits, Tp = Pliocene Merced-like rocks. Merced and Colma Formation distribution
from Wagner et al. (1990), Olema Creek Formation from Grove et
al. (1995) and offshore features from Bruns et al. (2002).
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
(Grove et al., 1995) (Fig. 3). The Olema Creek
Formation was deposited in estuarine and fluvial
environments similar to modern Tomales Bay and
its associated feeder streams, but is now exposed at
elevations up to 70 m above sea level and deformed
by contractional deformation, with beds tilted to dips
of up to 65j (Grove et al., 1995). The Olema Creek
formation regionally dips northward (‘‘shingles’’) so
that deposits young northwestward (Grove et al.,
1995). There is no evidence that a regional change
in plate motions took place after 130 ka, so the change
in tectonic regime the affected the Olema Creek
Formation must have been a local and progressive
one. The Olema Creek Formation was originally
deposited in a subsiding environment along the San
Andreas fault that may have been related to a releasing bend or step-over. The depocenter apparently
migrated northward to Tomales Bay, leaving basinal
deposits outside and south of the present depocenter
and subject to contractional deformation (Grove et al.,
1995).
Unlike the deposits associated with many wellknown strike – slip basins (e.g., Crowell, 1974a,b;
Crowell, 1982a,b; Nilsen and McLaughlin, 1985),
no fault scarp breccias have been identified within
the Olema Creek Formation (Grove et al., 1995).
This may due to several factors: (1) The subsidence
associated with the depocenter is not rapid and
there is comparatively gentle topography associated
with the boundaries of the basin; (2) sedimentation
has kept pace with basin subsidence, so that slopes
of the basin margins are gentle. Coarse gravels,
interpreted as alluvial fan deposits make up part of
the Olema Creek Formation adjacent to its western
boundary, the San Andreas fault (Grove et al.,
1995). These deposits are shed-off of moderately
steep western valley wall.
2.2. Inverted graben along the Miller Creek fault
The Miller Creek fault is a dextral-reverse fault
between the Calaveras and Hayward faults (Wakabayashi and Sawyer, 1998a) (Fig. 2). A paleoseismic
trench across this fault revealed a graben, filled with
late Quaternary colluvium, that is bounded by late
Miocene bedrock (graben is bounded by faults F3 and
F2 in Fig. 4) (Wakabayashi and Sawyer, 1998a,b).
The trench was excavated in a ridge-crest saddle. The
283
apparent separation of the eastern graben-bounding
faults (fault F3) near the ground surface (i.e., reflecting most recent movement) is reverse, rather than
normal (Fig. 4). Thus there has been a reversal of
separation sense in the late Pleistocene on this fault.
The fault bounding the graben on its west side (fault
F2) has been inactive for some time (possible deformation of colluvial unit C5, but not C3), whereas the
eastern fault (fault F3) is still active. The flanks of the
ridge upon which the paleoseismic trench was excavated are mantled with landslides that cover the trace
of the fault both north and south of the trench site.
The sediment accumulation rate from such landslides
would swamp the subsidence rate of any minor
graben (sag pond) along the fault. This is the likely
reason why the active depocenter corresponding to the
colluvial graben in the trench has not been found
along strike. Although the active (migrated) graben
has not been found, the local inversion noted in the
trench suggests that the graben deposits are a small
scale analog of the Olema Creek Formation. Colluvium was apparently deposited in a small graben
located at a right step-over or bend along the fault.
This releasing step-over migrated with respect to
those deposits, leaving the deposits behind. Outside
of the local transtensional environment, the deposits
were subjected to a contractional component of deformation, and inversion of the colluvial graben
occurred.
2.3. Merced/San Gregorio depocenter and the Merced
Formation
The Plio-Pleistocene Merced Formation of the San
Francisco Bay area (Bay Area) records basin formation, and subsequent uplift and contractional deformation along the San Andreas fault. The relationship
between the Merced Formation and its depocenter is a
more complicated example of an evolving step-over
region because two major strike– slip faults, the San
Gregorio and San Andreas (as well as at least one
other minor fault), are involved instead of one (Figs. 3
and 5). However, a variety of geologic and geophysical data is available for the Merced Formation and
environs, and the tectonic evolution appears similar to
that of other step-over regions along a single fault.
The Merced Formation crops out along a narrow
( V 2.5 km wide) belt that extends 19 km along the
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Fig. 4. Trench log of trench across the Miller Creek fault at Big Burn Road.
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
285
Fig. 5. Relationship of the Merced Formation to processes along the San Andreas and San Gregorio faults. Basin formation is the consequence of two releasing step-overs that have
migrated with respect to affected deposits. These step-overs are from the Potato Patch fault to the San Andreas fault and from the San Andreas fault to the Golden Gate fault. The
Merced Formation was deposited in the depocenter related to the San Andreas – Golden Gate fault step-over.
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east side of the San Andreas fault on the San Francisco Peninsula (Brabb and Pampeyan, 1983).
The best exposures, and type locality, of the
Merced Formation are in sea cliffs extending from
southern San Francisco to the San Andreas fault (Fig.
3). At the type locality, the Merced Formation comprises 1700 m of partly lithified sand and silt deposited over a wide range of environments that include
shelf, nearshore, foreshore, backshore, eolian, fluvial,
coastal embayment, and marsh (Clifton et al., 1988).
Clifton et al. (1988) estimated the age of the basal
Merced strata as 1.2– 1.6 Ma based on fossils and
regional correlations. Alternatively, Sr isotopic dating
of the deposits suggests an age of 2.4 to 4.8 Ma for the
base of the Merced Formation (Ingram and Ingle,
1998); these authors concluded that it was difficult to
resolve ages within this time range owing to the lack
of variability of sea water strontium isotopic ratios for
that period. Several widespread tephra layers in the
San Francisco Bay region range in age from 4.1 to 2.0
Ma, including two layers of 2.4 and 2.0 Ma (SarnaWojcicki et al., 1991). If the Merced Formation was
older than 2 Ma, such layers might be found in the
Merced, however, none of them are. The absence of
these widespread tephra layers suggests that the base
of the Merced Formation is less than 2 Ma. On the
basis of these observations we consider the base of the
Merced to be 1.5 to 2 Ma.
A prominent ash, which has been correlated to the
Rockland Ash by Sarna-Wojcicki et al. (1985), provides geochronologic control for the upper part of the
Merced section; the Rockland Ash has been dated at
about 600 ka (Lanphere et al., 1999). The top of the
Merced Formation exposed at the type locality is
about 150 m above the Rockland Ash, and may be
about 500 ka based on averaged sedimentation rates
for the upper part of the formation and on strontium
isotopic data (Ingram and Ingle, 1998). The Merced
Formation is unconformably overlain by the sand of
the Colma Formation, deposits that are interpreted to
have been deposited in a nearshore environment at 75
to 135 ka (Hall, 1965). The Merced Formation and the
overlying basal part of the Colma Formation are
interpreted as sediments that were deposited in trangressive –regressive cycles related to eustatic sea level
fluctuations within a subsiding basin, with deposition
keeping pace with tectonic subsidence (Clifton et al.,
1988).
The Merced is now exposed at elevations up to 200
m on the San Francisco Peninsula at the present time,
which is also a high sea level stand (Brabb and
Pampeyan, 1983; Bonilla, 1971). Folding of the
Merced Formation has resulting in dips ranging up
to vertical (Bonilla, 1971). The younger Colma Formation is present at elevations of up to 60 to 90 m on
the San Francisco Peninsula (Bonilla, 1971;
Schlocker, 1974); these strata dip as steeply as 17j
(Hengesh and Wakabayashi, 1995a). Because there is
no documented evidence for a regional-scale change
in plate motions at 100 ka or younger, the change in
vertical tectonics recorded by the Merced and Colma
Formations appears to be a relatively local one.
Deposition of the Merced Formation appears to be
related to a step-over along the San Andreas fault.
Slip on the San Andreas fault steps 3 km right to the
Golden Gate fault forming an active basin off of the
Golden Gate (Bruns et al., 2002) (Fig. 5). The Golden
Gate fault appears to be the offshore continuation of
the San Bruno fault. Zoback et al. (1999) showed a
basin in the right step region, based on seismic
reflection and gravity studies, and identified extensional focal mechanisms in the area. Bruns et al.
(2002) conducted detailed offshore seismic reflection
studies, suggested correlations between offshore
reflectors and onshore stratigraphy, and concluded
that the offshore pull-apart basin is controlled by
right transfer of slip from both the San Gregorio
and San Andreas faults. In addition, they showed that
the San Andreas fault directly south of the present
pull-apart did not begin movement until the late
Quaternary, whereas the Golden Gate fault appears
to be a feature with greater accumulated offset.
Magnetic data also suggest little cumulative offset
on the San Andreas directly south of the present pullapart, as well as greater cumulative offset on the
Golden Gate fault (Jachens and Zoback, 1999). The
onshore, southern continuation of the Golden Gate
fault, the San Bruno fault, is no longer active (Hengesh and Wakabayashi, 1995a; McGarr et al., 1997).
It is unclear whether the southern continuation of the
Golden Gate fault strictly follows the location of the
San Bruno fault defined in previous studies (e.g.,
Bonilla, 1964, 1971), or whether it diverges from
the named San Bruno fault (Wakabayashi, 1999); in
any case the southern extension of the Golden Gate
fault is inactive. For purposes of discussion, however,
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
we will refer to the onshore part of the Golden Gate
fault as the San Bruno fault.
Hengesh and Wakabayashi (1995b) proposed that
the Merced Formation was deposited in a pull-apart
basin, formed at a right step along the San Andreas
fault, that had subsequently migrated northward to
the Golden Gate leaving Merced Formation deposits
on the Peninsula, outside of the active basin. The
San Andreas fault, south of the pull-apart, appears
to have propagated northward and the Golden Gate/
San Bruno fault appears to have progressively shut
down (Fig. 5).
Right separation of basement and late Cenozoic
features along the San Andreas fault on the San
Francisco Peninsula (Peninsula San Andreas fault)
is 22 to 36 km (Wakabayashi, 1999). This segment of
the San Andreas fault has a late Holocene slip rate of
about 17 mm/year (Hall et al., 1999). Based on the
22 to 36 km of total displacement and assuming a
long-term slip rate equal to the Holocene rate, movement began on the Peninsula San Andreas fault at 1.3
to 2.1 Ma. This age is similar to the estimate for the
age of the base of the Merced Formation. The
southern limit of the Merced Formation on the San
Francisco Peninsula is presently about 27 km south
of the active pull-apart basin, similar to the total
offset on the Peninsula San Andreas fault. This
relationship suggests that the basin in which the
Merced Formation was deposited began to form at
about the same time slip began on the Peninsula San
Andreas fault.
Based on the field relations presented above, the
Merced basin pull-apart has apparently migrated at the
slip rate of the Peninsula San Andreas relative to
Merced Formation deposits on the east side of the
fault (Fig. 5). This migration was accompanied by the
northward propagation of the San Andreas fault south
of the pull apart, and the associated, progressive shutoff of activity on the Golden Gate– San Bruno fault
(Fig. 5). The migration of the pull-apart has left relict
basinal deposits outside of the active basin in a
transpressional tectonic environment in which tilting,
folding, and uplift of the deposits has occurred.
Additional complexity associated with evolution of
offshore basins may have occurred as a consequence
of an additional slip transfer from the San Gregorio
and Potato Patch faults to the San Andreas fault
(Bruns et al., 2002) (Fig. 5). Deposits similar to the
287
Merced Formation are present on the Marin Peninsula,
north of the Golden Gate (Tp in Fig. 3), however,
these deposits appear to be older (Clark et al., 1984)
and they may be related to a different basin than the
one in which the Merced Formation was formed.
Similar to the Olema Creek formation, no fault
scarp breccias have been identified within the
Merced Formation (Clifton et al., 1988). Adequate
exposure of the Merced Formation exists in proximity to the western boundary, the (Peninsula) San
Andreas fault, to conclude that fault scarp breccias
do not occur along that basin boundary. The eastern
basin boundary (onshore extension of the Golden
Gate fault) is not exposed, so the presence of scarpderived breccias on that side of the basin cannot be
evaluated without direct subsurface data (borehole
samples); we are unaware of any boreholes that
have recovered such breccias. Slopes are gentle in
the vicinity of the active depocenter (Bruns et al.,
2002), possibly as a result of rapid sedimentation
that has kept up with tectonic subsidence (e.g.,
Clifton et al., 1988), so fault scarp breccias may
not be associated with the either the Merced Formation, or its modern offshore analog.
2.4. Other possible examples of migrating pull-aparts
outside of the Pacific – North American plate boundary
The Dead Sea pull-apart region, along a sinistral
part of the African –Arabian plate boundary (the
Dead Sea transform fault), is over 120 km in length
by up to 30 km in width. The pull-apart is
interpreted to have formed from the Pliocene to
the present by progressive northward migration of
the depocenter with respect to its deposits and
progressive development of transverse structures
(ten Brink and Ben-Avraham, 1989).
The Devonian Hornelen basin of Norway is 60 km
long by 20 km wide and may have also formed with
progressive development of transverse faults that left
deposits outside of the active basin and subject to
contractional deformation (Steel and Gloppen, 1980).
The Miocene to present Gulf of Paria pull-apart
basin of eastern Venezuela and Trindad (and related
features) extends over an area of approximately 150
km by 40 km in width and has developed along
strike – slip faults that transect a fold and thrust belt
(Flinch et al., 1999). The setting of this pull-apart
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basin in an otherwise transpressional region is somewhat similar to pull-apart basins along the modern San
Andreas fault system. Inversion is occurring in the
eastern and central part of the basin, whereas the most
recent history of the western part of the basin is
characterized by normal faults cutting earlier contractional structures (Flinch et al., 1999). This suggests
that transtensional basin development, superimposed
on a fold and thrust belt, is migrating westward with
respect to basinal deposits.
The Quaternary Hanmer Basin, located along the
Hope fault, a dextral fault that splays from the Alpine
fault in New Zealand, has dimensions of 10 km by 20
km (Wood et al., 1994). This basin occurs in a region
characterized by transpression. The western part of the
basin is actively subsiding, whereas shortening and
inversion of basinal deposits is occurring in the
eastern part of the basin (Wood et al., 1994). This
suggests that the active basin (pull-apart) has migrated
westward with respect to its deposits.
The Quaternary Dagg Basin, located along an
offshore reach of the dextral Alpine fault, west of
the South Island of New Zealand has dimensions of
about 5 km by 10 km (Barnes et al., 2001). The
northeastern side of the basin is actively subsiding,
whereas the southwestern end of the basin is undergoing inversion as a result of the inpingement of a
push up block (Barnes et al., 2001). Similar to the
Hanmer Basin, the Dagg Basin has formed as a
consequence of a releasing bend along a strike –slip
fault imposed on an otherwise transpresssional regime. The inversion of the southeast part of the basin
is enhanced by what appears to be the presence of a
restraining bend along the fault (Barnes et al., 2001).
In this example, it appears two step-overs comprising
a releasing and a restraining bend may have migrated
northeast along the Alpine fault with respect to
affected deposits.
3. Migration of restraining bends or step-overs with
respect to material involved in the deformation
3.1. Lack of evidence for large structural relief
associated with most restraining bends and steps
In the California Coast Ranges, the dextral faults
of the northern San Andreas fault system have dis-
placed rocks several hundred km in the late Cenozoic
(e.g., McLaughlin et al., 1996; Dickinson, 1997;
Wakabayashi, 1999). Within the fault system several
left (restraining) step-overs, slip transfers, or bends
have accommodated tens of km of slip (Wakabayashi,
1999). Examples include the Hayward –Calaveras slip
transfer zone, the northern termination of the Green
Valley fault, and the northernmost part of the San
Andreas fault system. If the same transverse structures accommodated all of the slip during the evolution of the restraining bend region, then considerable
structural relief should result, associated with significant contractional deformation, exhumation, and rock
uplift.
Hypothetical rock uplift associated with a restraining step-over is difficult to estimate because vertical
deformation may be accommodated across multiple
structures and because of the uncertainty of the
flexural response of the deformed zone. The vertical
component of fault displacement in a restraining stepover can be estimated for an idealized case of a single
transverse structure as
z ¼ xsinðtan1 ðsinhtandÞÞ
ð1Þ
where z is the vertical component of displacement, x
is the strike –slip displacement, h is the angle between
the transverse structure strike and the PDZ strike, and
d is the dip of the transverse structure. This equation
accounts only for amount of vertical displacement
predicted from strike – slip movement through the
step-over; it does not take into account the impact
of regional transpression. However, as noted earlier,
the amount of regionally driven fault-normal convergence in the northern San Andreas fault system is
likely to be much smaller than that generated by
restraining bends along the major strike– slip faults.
Any regionally derived fault-normal convergence
acting on a restraining bend area should serve to
slightly increase the predicted amount of vertical
displacement compared to the amount calculated by
Eq. (1).
For restraining step-over regions that have accommodated 20 km or more of displacement (an amount
that would include most of the major restraining stepovers along the northern and central San Andreas
fault system), vertical displacements of 4 km or more
are expected for existing fault geometries, as will be
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
illustrated by examples detailed below. The San
Andreas fault system did not begin movement until
about 18 Ma (Atwater and Stock, 1998), so all
vertical displacement related to restraining step-overs
along the strands of the transform fault system must
be post-18 Ma. Most areas in the California Coast
Ranges yield apatite fission track ages that are z 30
Ma, with only a few localities exhibiting younger
( < 18 Ma) ages that coincide with the transform
regime (Dumitru, 1989). Geothermal gradients in
the northern and central California Coast Ranges
are z 25 jC/km in most areas (Lachenbruch and
Sass, 1980; Furlong, 1984), so transform-related
structural relief of 2 –3 km or more can be recorded
by apatite fission track data (e.g., Dumitru, 1989).
This means that areas with >18 Ma apatite fission
track ages have experienced less than 2 to 3 km of
exhumation during the transform regime. Of the
restraining bend regions in the central and northern
Coast Ranges, only the left bend in the Loma Prieta
region exhibits post-18 Ma apatite fission track ages
(Bürgmann et al., 1994) suggesting structural relief of
several km, consistent with interpretations of regional
structure (McLaughlin et al., 1999). The lack of
structural or thermochronologic evidence for large
amounts of structural relief associated with most
restraining bends and steps in the northern San
Andreas fault system suggests that the restraining
bends and step-overs may have migrated with respect
to rocks originally deformed in these areas, analogous
to the basin migrations discussed above. Some examples are described below.
3.2. Mount Diablo restraining step-over
The Mount Diablo restraining left step-over lies
between the Greenville fault to the south and the
Concord fault to the north (Unruh and Sawyer, 1997),
the easternmost active dextral faults of the San
Andreas fault system in the San Francisco Bay area
(Figs. 2 and 6). The transfer of dextral shear between
these bounding faults drives contractional deformation within the step-over region (Unruh and Sawyer,
1995). Mount Diablo (1173 m) marks the core of a
regional fault-propagation fold underlain by the
northeast-dipping, blind Mount Diablo thrust fault
(Unruh and Sawyer, 1997; Unruh, 2000), the largest
contractional structure in the newly recognized
289
Mount Diablo fold-and-thrust belt (MFTB) (Crane,
1995) (Fig. 6). Mount Diablo is the most prominent
topographic landmark in the San Francisco Bay
region with an elevation more than 500 m greater
than the highest ridges outside of the step-over area.
The total late Cenozoic dextral slip that has transferred through the Mount Diablo step-over is 18 km
or more, the combined displacement total of the
Greenville fault and several faults that merge with
it south of Mount Diablo, including the Corral
Hollow fault, and Mount Lewis trend (Wakabayashi,
1999) (Fig. 2). The angle between the strike of the
main transverse structure, the Mount Diablo thrust
fault, and strikes of the PDZs (Concord and Greenville faults) is about 38j. The Mount Diablo thrust
fault dips at 30j (Unruh, 2000). From Eq. (1) above,
the approximate vertical displacement expected is
about 6 km, if the Mount Diablo thrust was the
master transverse structure for the entire life of the
step-over. Apatite fission track ages from the Mount
Diablo area predate the transform regime (T.A.
Dumitru data in Unruh, 2000). Thus the vertical
exhumation (less than 2 – 3 km) is less than would
be expected if the same transverse structure had been
operative throughout the late Cenozoic history of the
step-over.
Comparable deformation rates on the PDZs and on
transfer structures within the MFTB are consistent
with the evolution of the fold and thrust belt within a
restraining step-over as originally proposed by Unruh
and Sawyer (1995). The Greenville fault has an
estimated dextral slip rate of 4.1 F 1.8 mm/year, or
less, during the Holocene (Sawyer and Unruh, 2002),
but dies out northward along the eastern edge of the
MFTB (Unruh and Sawyer, 1998). This rate is
comparable to the Holocene slip rate (3.4 F 0.3
mm/year; Borchardt et al., 1999) and historical creep
rate (approximately 3 mm/year; Galehouse, 1992) on
the Concord fault, and to the late Cenozoic average
slip rate of 1.3 to 2.4 mm/year, or more, on the
Mount Diablo thrust fault (Unruh, 2000). Morphometric and geochronologic studies of tectonically
deformed fluvial terraces within the MFTB provide
late Holocene uplift rates of 3 mm/year, or more
(Sawyer, 1999).
Foreland propagation of the MFTB into the ancestral Livermore sedimentary basin, bordered on the east
by the Greenville – Clayton– Marsh Creek fault system
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Fig. 6. Geology of the Mt. Diablo restraining step-over area. Geology from Wagner et al. (1990) and Unruh and Sawyer (1997).
(Fig. 6), has resulted in contractional deformation and
associated uplift and exhumation of a thick sequence
of basin-fill deposits, the Tassajara and Sycamore
formations (Isaacson and Anderson, 1992; Anderson
et al., 1995). Correlation or dating of tephra layers
within the Sycamore Formation that are involved in
the folding indicate that contractional deformation
began in the past 3.3 to 4.8 Ma (Unruh, 2000).
As a consequence of foreland propagation of the
MFTB, the point or fault section where slip transfers
left from the Greenville – Clayton– Marsh Creek fault
system has migrated southward, progressively shutting off the Clayton –Marsh Creek fault zone. The late
Cenozoic Clayton– Marsh Creek fault zone has been
determined in several previous studies to be inactive
(e.g., Wright et al., 1982; Hart, 1981). This model
implies southward propagation of the Concord fault,
which appears to be supported by the pattern of
faulting. The northern and central sections of the
Concord fault are continuous and remarkably linear,
whereas the southern section is discontinuous and is
comprised of short en echelon fault traces (Sharp,
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
1973; CDMG, 1974, 1977; Wills and Hart, 1992).
Following the structural development model for
strike – slip faults proposed by Wesnousky (1988,
1989), the pattern of faulting along the southern
Concord fault is consistent with a strike – slip fault
having limited cumulative offset, consistent with
southward ’younging’ of the fault zone. The field
relations in the Mount Diablo area are consistent with
southward migration of the Greenville – Concord fault
restraining step-over with respect to deposits deformed within the step-over region.
3.3. Slip transfer from the Calaveras fault to the
Hayward fault
The slip transfer from the Calaveras fault to the
Hayward fault represents a restraining step-over that
has been associated with a large amount of displacement on the associated PDZs. The Hayward fault has
about 37 to 61 km of post-10 Ma displacement, most
or all of which has been transferred from the Calaveras fault via a left step-over (Wakabayashi, 1999).
The average angle between the presently active
transverse structure, a largely or entirely blind
oblique slip fault, and the Hayward fault is about
20j, and this transverse structure dips at 75 – 80j
(Wong and Hemphill-Haley, 1992). From Eq. (1), the
approximate magnitude of predicted vertical slip for a
single transverse structure is 29 km, clearly an
unrealistic amount. Approximately 9 mm/year of
Holocene dextral slip is transferred from the southern
Calaveras fault to the Hayward fault (Lienkaemper et
al., 1991). The geometry of the slip transfer is
consistent with an 8-km left or restraining step. In
this area, Mission Peak and Mount Allison (about
800 m above sea level) attain elevations at least 400
m higher than the crests of neighboring ridges. This
elevated region extends for about 10 km along strike,
and is consistent with uplift associated with contractional deformation of the restraining step-over. The
high ridge in the step-over region, however, is
composed of late Cenozoic deposits, whereas Mesozoic basement rocks crop out in many of the surrounding, and topographically lower, areas. This
relationship indicates that long-term erosion may be
actually less in the present step-over region than
surrounding regions, the opposite of the expected
relationship. This relationship is consistent with mi-
291
gration of the restraining step-over with respect to the
deposits deformed within the step-over region.
Andrews et al. (1993) suggest that the region the
more deeply eroded region southeast of the current
step-over region exposes a former transfer structure
and that the step-over has been migrating northwestward with respect to affected deposits.
3.4. Transfer of slip from Eastern faults of the San
Andreas fault system to the Mendocino triple junction
The largest-scale restraining bend or step-over in
the northern San Andreas fault system may occur near
the northern terminus of the system at the Mendocino
triple junction. In the northernmost San Andreas fault
system, more than 250 km of dextral slip, the aggregate amount of displacement of faults east of the San
Andreas fault (eastern faults), must transfer westward
to the Mendocino triple junction (Wakabayashi,
1999). The eastern faults cannot simply propagate
northward without slip transfer, otherwise major displacement incompatibilities would result along those
faults; fault displacement for simply propagating
faults would be zero at the fault tip, and many kilometers further south. The strike of a transverse structure (or structures) connecting the eastern faults to the
present triple junction must be at least 20j more
westerly than that of the eastern faults. For any range
of transverse structure dips, this predicts an unrealistic
amount of vertical fault movement. For example, for a
transverse fault dip of 30j, the estimated vertical fault
movement is 48 km from Eq. (1). No well-defined
transverse structures have been identified in the northernmost Coast Ranges. In addition, the eastern faults,
such as the Hayward – Rodgers Creek – Maacama
trend, and the Green Valley fault, die out northward
as well-defined faults (Fig. 2). This may be because
the eastern faults are young and propagating northward. Slip on the eastern faults transfers to the
Mendocino triple junction, which moves northward
relative to a given position on these faults at the slip
rate ( f 25 mm Holocene rate; Niemi and Hall, 1992;
Prentice, 1989) of the northern San Andreas fault. In
order to transfer slip to the migrating triple junction,
new transverse faults must continue to form. Thus, the
step-over region progressively migrates so that largescale displacement or structural relief has not developed on any given transverse structure. The northern-
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most San Andreas fault system may be another
example of a restraining step-over that has migrated
with respect to the rocks deformed within the stepover. In such a model the eastern faults propagate
northward (lengthen), and new transverse structures
form as the triple junction migrates (Fig. 10 of
Wakabayashi, 1999) (Fig. 7). Note that even if some
of the eastern faults slightly overstep the triple junction, as suggested by Gulick and Meltzer (2002), a
transfer zone that migrates northward with the triple
junction is still required in order to maintain slip
compatibility along the eastern faults.
3.5. San Gregorio structural zone
Detailed seismic reflection studies of Bruns et al.
(2002) show that a late Cenozoic fold and thrust belt
is present directly west of the San Gregorio fault
offshore of the Golden Gate (Fig. 3). This fold and
thrust belt is at least 40 km long and up to 8 km wide.
The age of the onset of movement youngs progressively from north to south, but the belt does not
appear to progressively narrow in that direction. If
the transpressional deformation zone was simply
enlarging with time, the belt should progressively
narrow southward. If the fold and thrust belt were a
product of regional strain partitioning (e.g., Zoback et
al., 1987), then initiation of deformation should be
approximately synchronous along the length of the
belt. The north to south progression of deformation
suggests a direct relationship between the deformation
and the migration of a restraining bend or step-over
along the San Gregorio fault with respect to rocks
deformed within the step-over region. The left bend
that has caused the deformation may be present in the
area where the San Gregorio fault locally comes
onshore near Pillar Point (this is the bend at the
southernmost limit of Fig. 3).
3.6. Northeast Santa Cruz Mountains thrust/fold belt
and the Serra Fault
Fig. 7. Migration of the restraining transfer zone to the Mendocino
Triple Junction. Approximate past positions of the triple junction
and transverse structures shown in grayed dashed lines with
corresponding age designations. Longitude/latitude and other
reference points are valid only for present. Abbreviations, in
addition to those given in Fig. 2: BMV: Burdell Mountain
volcanics; Co/Ce: Franciscan Coastal Belt/Central Belt contact; F:
Fairfield; RC: Rodgers Creek fault; SSS: Skaggs Springs schist; TV:
Tolay volcanics. Adapted from Wakabayashi (1999).
A fold and thrust belt, informally called the Northeast Santa Cruz Mountains thrust/fold belt, occurs east
of and parallel to the Peninsula San Andreas fault
(Lajoie, 1996). This belt narrows northward from a
width about 10 km near Palo Alto to 1.5 km or less at
the sea coast. The northernmost fault in the belt, the
Serra fault (Fig. 3), is a dextral-reverse fault that
daylights along its southern reach (Bonilla, 1971;
Hengesh et al., 1996a). Its northernmost 5 km, however, is a blind feature underlying a fault-propagation
fold that shows evidence of having begun deformation
within the last several hundred thousand years (Kennedy and Caskey, 2001). Both exposed and blind parts
of the Serra fault exhibit evidence of Holocene deformation (Hengesh et al., 1996a,b; Kennedy and Caskey, 2001). In contrast, the folds and faults in the Palo
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
Alto area have apparently been active for 3– 4 my
(Angell and Crampton, 1996). These field relations
suggest that the activity along the fold and thrust belt
has propagated northward with time into materials
previously unaffected by it. Although the northward
propagation of activity may suggest the migration of
the transpressional region relative to materials affected
by it, the southern part of this belt of deformation is
associated with the Loma Prieta region that has
generated significant structural relief (Bürgmann et
al., 1994). Accordingly, this belt may be an example
of a transpressional region that is growing with time
(both along and across strike) rather than one in which
the deformation has migrated along strike. Alternatively, the initiation of contractional deformation
along the northernmost part of the fold and thrust belt
may represent a return to ‘‘background’’ transpressional conditions that characterize the Coast Ranges,
following the northward migration of local transtensional conditions associated with pull-apart basin that
had formed the Merced Formation.
3.7. Other possible example outside of the Pacific–
North American plate boundary
Eusden et al. (2000) has described a tranpressional
duplex structure that has migrated northeast relative to
deposits it has deformed along the Hope fault of New
Zealand. This duplex structure is 13 km along strike
by 1.3 km wide. The southwest of the active duplex,
former duplex structures are now collapsing, undergoing a reversal of slip to normal faults (negative
inversion).
4. Discussion
The step-over and bend regions reviewed above
apparently have migrated with respect to deposits that
were originally within the step-over or bend regions.
For this to have occurred, new transverse structures
associated with the bend or step-over regions must
have progressively formed in the direction of the
migration. If the same transverse structures migrated
instead of new structures forming, the material
bounded by them must have migrated with them. This
is most easily illustrated for the case of pull-aparts
along releasing bends (Fig. 8). Once formed, a basin
293
can move passively along a fault, provided another
fault (fault b in Fig. 8A) is involved in its translation.
If slip transfer continues to occur, the basin will
lengthen (Fig. 8B). Some combination of the mechanisms illustrated in Fig. 8A and B has been proposed
for the development of many basins related to strike –
slip faults (e.g., Crowell, 1982a, 1987; Mann et al.,
1983; May et al., 1993). For the mechanisms illustrated in Fig. 8A and B, deposits will not become
separated from the basin proper unless new transverse
structures are formed, as in the case of Fig. 8C.
Migration of pull-aparts with respect to their
deposits is illustrated in Fig. 8C. In addition to the
creation of new transverse structures, such migration
also indicates that the strike – slip fault on one side of
the pull-apart will propagate in the direction of basin
migration (PDZ1 in Fig. 8C), whereas the fault on the
other side of the basin will progressively shut off
(PDZ2 in Fig. 8C). The propagation and shut-off of
bounding strike – slip faults is consistent with the
seismic reflection interpretation of Bruns et al.
(2002) for the relationship of the Merced Formation
to offshore pull-apart structures. Shut-off of transverse structures left outside of the active basin should
also occur (the gray transverse structures labeled in
Fig. 8C). An essentially identical model for migration
of a pull-apart with progressive propagation and shutoff of PDZs and progressive development of transverse structures has been proposed for the Dead Sea
by ten Brink and Ben-Avraham (1989). The model
proposed by Steel and Gloppen (1980) for the development of the Hornelen basin in Norway is also
similar in that it proposes the progressive development of transverse faults. In some (or many) cases,
transverse structures may not be discrete faults, but
may be zones of distributed deformation (perhaps
above blind normal or oblique faults) accommodating
the differential displacement between reaches of a
given bounding faults with differing senses of movement (for example pure strike – slip vs. normaloblique). This may explain why major vertical separation and extension has been interpreted to be
associated with bounding faults that are the alongstrike continuation of strike –slip faults in the cases of
several basins including the San Gregorio Basin
(Bruns et al., 2002) and the Gulf of Elat (BenAvraham and Zoback, 1992). We suggest that the
Olema Creek formation and the graben deposits
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J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
found in the Miller Creek fault paleoseismic trench
followed an evolution similar to Fig. 8C. The
Merced Formation is somewhat more complex
because two faults and two step-overs are involved.
We suggest that the evolution of the Merced
Formation followed a combination of Fig. 8C and
F (compare these schematics with the sequence
illustrated in Fig. 5).
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
295
Fig. 8. Progressive evolution of releasing and restraining step-over and bend regions along strike – slip faults. The star in each diagram is a
reference point that represents a point within deposits affected by the step-over. For ‘‘D’’, ‘‘E’’, ‘‘G’’, ‘‘H’’, and ‘‘I’’, the contours (solid, fine
lines) in the restraining step-over regions schematically depict elevation. Diagrams A, B, and D depict interactions in which the step-over does
not migrate with respect to affected deposits, whereas Diagrams C, E, F, G, H, and I depict migration of step-overs with respect to affected
deposits. For the latter, migration of the step-over occurs with progressive development on new transfer structures (and progressive shut-off of
old ones) in the direction of step-over migration. For a single fault, the fault strand on one side of the step-over propagates in the direction of
step-over migration, whereas the other fault strand shuts off. Variations in the propagation/shut-off direction of the PDZs are possible in the
cases with multiple faults as shown in Diagrams F and G. The effect of the evolution of a series of step-overs is shown in Diagram I.
Both the evolution of pull-apart basins shown in
Fig. 8C (migration with respect to deposits) and
progressive enlargement of a pull apart (Fig. 8B)
can produce the ‘‘shingling’’ observed in several
strike – slip basins (e.g., Crowell, 1974a; Steel and
Gloppen, 1980; Nilsen and McLaughlin, 1985). For
basins that migrate with respect to their deposits, the
younging direction is the direction of relative migration of the depocenter.
The restraining bend or step-over case is analogous
to that of pull-apart basin migration. In order for a
restraining step or bend to migrate with respect to
deposits deformed by it, new transverse structures
must form (Fig. 8E); if the transverse structure does
not migrate with respect to the deformed deposits
structural relief progressively as illustrated in Fig. 8D.
For migrating restraining step-overs, one of the faults
involved in the step-over should propagate, whereas
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J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
the other should shut-off (Fig. 8E). The along-strike
propagation of faulting has occurred along the San
Gregorio structural zone (Bruns et al., 2002) and the
Mount Diablo restraining step-over region as
reviewed above. The Calaveras –Hayward fault slip
transfer is somewhat different in that two faults are
involved (schematically shown in Fig. 8G). The
Calaveras– Hayward step-over appears to have migrated northward, resulting in the shut-off of the
southern Hayward fault (schematically illustrated the
progressive shut-off of PDZ 1 in Fig. 8G). It is likely
that there are similar two-fault slip transfer zones in
which PDZ1 propagates rather than shuts off. Progressive restraining step-over migration may have
affected the eastern part of the northernmost San
Andreas fault system, in which the eastern faults of
the system propagate northwestward (Wakabayashi,
1999, Fig. 10), although the ‘northern half’ of the
step-over is not present because the plate margin north
of the Mendocino triple junction is a subduction zone
rather than a transform margin (Fig. 7; schematic
block illustration in Fig. 8H).
The model for the migration of some step-overs
and bends described above is essentially the asymmetric progressive development of strike – slip duplexes (Woodcock and Fischer, 1986). Unlike thrust
duplexes, where faulting generally propagates in the
foreland direction, there may not be a ‘preferred’
direction of propagation for strike –slip duplexes because the two fault-normal directions are essentially
the same with respect to gravity. The direction of
migration may be driven by the presence of mechanical differences on either side of the structure, such as
the locally greater strength of the crust on one side of
the fault. In cross section transpressional duplexes
(associated with migrating restraining step-overs of
bends) form positive flower structures, whereas transtensional duplexes (associated with migrating releasing bends or step-overs) form negative flower
structures in cross sectional view (Woodcock and
Fischer, 1986).
Progressive migration of step-overs suggests that
the zone of long-term displacement associated with
major strike – slip faults may be several km wide,
bounded by the former lateral boundaries (PDZs) of
migrating pull-apart basins or transpressional welts
(Fig. 8). This conclusion is consistent with observations along the major strands of the San Andreas fault
zone, such as the San Andreas and Hayward faults,
where the zone of long-term (late Cenozoic) displacement is several km wide, although the Holocene trace
occupies a much more limited width (Wakabayashi,
1999). The braided nature of strike –slip faults noted
by Crowell (1974a,b) apparently describes the same
type of relationship.
The field examples we have presented from the
San Andreas system illustrate progressive development of step-over features along a strike slip fault
system with a minor component of fault normal
convergence. Because of the regionally imposed
transpressional regime, basin deposits left outside of
active pull-apart basins are subject to contractional
deformation and local positive inversion, even in the
absence of the interaction of these deposits with a
restraining bend. In contrast, a transform fault system
with no fault-normal convergence would require
interaction of a restraining step-over with basinal
deposits to cause contractional deformation of them
(in the absence of a regional change to transpression)
(as illustrated in Fig. 8I). Behavior of deposits associated with step-overs in a transform fault system
with a component of extension (regional transtensional) should be opposite of the examples reported
from the transpressional San Andreas system. Along
a transtensional transform system, rocks involved in
contractional deformation driven by a restraining
step-over may be affected by negative inversion as
the restraining step-over migrates away from them,
even if no interaction with a releasing bend occurs.
For strike – slip faults in a neutral case (without
regional transpression or transtension), inversion can
occur with a migrating pair of restraining or releasing
bends (Fig. 8I). Alternating subsidence and uplift
may occur along a strike – slip fault with a migrating
series of step-overs (Fig. 8I). Crowell (1974a) described such behavior and suggested that it was a
consequence of progressive interaction of irregularities along a strike – slip fault (fault convergences and
divergences, and step-overs and bends) with continued slip.
As noted in the field examples, migration of stepovers with respect to affected deposits appears to
cause tectonic inversion along strike –slip faults. Such
tectonic inversion is locally driven by the strike – slip
fault geometry, in contrast to the traditional model in
which inversion results from a regional (far field)
J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301
change in tectonic regime (e.g, Cooper et al., 1989). A
change from transtensional to transpressional relative
motion between the Pacific plate and the Sierra
Nevada microplate (the part of the North American
plate margin bounded on the west by the San Andreas
fault system and on the east by the Basin and Range
Province) is thought to have occurred between 8 and 6
Ma (Atwater and Stock, 1998; Argus and Gordon,
2001). Such a change in plate motion may have
resulted in regional, simultaneous, inversion of many
of the Tertiary basins of coastal California, but it
cannot explain the more local structural inversions
associated with the field examples presented in this
paper. Rotation of crustal blocks may also result in
local basin inversion (Nicholson et al., 1986; ChristieBlick and Biddle, 1985; implied in Crowell, 1974a),
but it should not produce the progressive along-strike
migration of deposition or deformation for the examples presented herein.
As illustrated by the examples, the step-over evolution described herein can apply to different scales of
features from large pull-apart basins and push up
structures to pressure ridges and sag ponds. The
model presented for migration of step-overs and bends
does not include all such features, however. The
Salton trough area, where young spreading centers
may be forming (e.g., Elders et al., 1972; Crowell,
1974a,b; Irwin, 1990), appears to be an example of a
releasing bend that has evolved associated with progressive enlargement of a pull-apart basin along the
southern San Andreas fault. As noted previously,
transverse structures associated with the restraining
bend in the Loma Prieta area have been comparatively
long lived and have developed significant structural
relief. An even larger-scale example of a long-lived
restraining bend (or bends) is the Big Bend area along
the southern San Andreas fault, where considerable
structural relief has developed (e.g., Namson and
Davis, 1988; Huftile and Yeats, 1995; Blythe et al.,
2000; Spotila et al., 2001). Note, however, that some
of the contractional deformation in the Big Bend
region may be at least partially a result of block
rotation (e.g., Hornafius et al., 1986; Nicholson et
al., 1994).
It appears that some step-overs and bends along
strike – slip faults migrate with respect to deposits
affected by them, whereas others progressively grow
in structural relief, in a manner similar to that
297
modeled in analog experiments (e.g., Dooley and
McClay, 1997; Dooley et al., 1999). Why one type
of behavior occurs instead of the other is not clear.
The migrating type of step-overs we have reviewed
occur along strike – slip faults cutting a variety of
basement types, from deformed subduction complex
rocks to crystalline basement. Within the northern
and central San Andreas fault system the two examples of long-lived restraining bends, the Big Bend
and Loma Prieta areas, may be associated with
gabbroic basement (Griscom and Jachens, 1990).
Such mafic basement is stronger than quartz-bearing
basement rocks, as such rocks maintain brittle behavior to greater depth at equivalent temperatures. The
greater depth of crustal seismicity associated with
these two restraining bend areas is consistent with the
deeper brittle– ductile transition associated with these
mafic rocks (e.g., Hill et al., 1990); it is not clear
whether such a relationship fits other step-overs of
this type.
Based on our studies of the San Andreas fault
system and review of published literature from other
regions, it appears that migration of step-overs with
respect to their affected deposits is an important
tectonic process. This process has largely escaped
notice until this time. In orogenic belts around the
world, many locally observed transitions in structural
style, which have been ascribed to regional changes,
in tectonic regime may require reexamination. The
model for step-over development we have described
can be rigorously tested by additional field studies,
because the detailed field relations predicted by this
model contrast markedly with field relations predicted
by other models.
Acknowledgements
Parts of this research were supported by the U.S.
Geological Survey (USGS), Department of the
Interior, under USGS award numbers 1434-94-G2426, 1434-95-G-2549, and 1434-HQ-97-GR-03141.
The views and conclusions in this document are those
of the authors and should not be interpreted as
necessarily representing the official policies, either
expressed or implied, of the U.S. Government. We
thank J. Dewey and J. Lewis for reviews of the
manuscript. We also thank S.J. Caskey and K. Grove
298
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for reviews of earlier versions of the paper. We have
benefited from discussions on this subject with many
of our colleagues, particularly T. Bruns, U.S. ten
Brink, M.L. Zoback, C. Busby, and S.J. Caskey.
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