www.elsevier.com/locate/tecto

Tectonophysics 392 (2004) 279–301

Four-dimensional transform fault processes: progressive evolution

of step-overs and bends

John Wakabayashia,*, James V. Hengeshb, Thomas L. Sawyerc

aWakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USAbWilliam Lettis and Associates, 999 Anderson Dr., Suite 140, San Rafael, CA 94901, USA

cPiedmont 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 Plio-

Pleistocene 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

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2004.04.013

* Corresponding author. Tel.: +1-510-887-1796; fax: +1-510-887-2389.

E-mail address: wako@tdl.com (J. Wakabayashi).

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301280

1. Introduction

Bends and step-overs are common structures along

strike–slip systems (e.g., Crowell, 1974a,b; Christie-

Blick 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 asso-

ciated 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 step-

overs (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 step-

overs or bends along strike–slip faults. An idealized releasing step-

over (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 enig-

matic 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 accommodat-

ed 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 (gener-

ally 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 trans-

pressional features along the northern San Andreas

fault system are driven by restraining bends or step-

overs along strike slip faults (e.g., Aydin and Page,

1984; Burgmann 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 com-

plex, the primary basement rock type in the California

Coast Ranges (e.g., Wakabayashi, 1992). Initial inter-

action between an oceanic spreading ridge and the

subduction zone occurred at about 28 Ma, but the San

Andreas transform system did not become established

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).

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 281

until about 18 Ma (Atwater and Stock, 1998). Since 18

Ma, San Andreas fault system has progressive length-

ened 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 step-

overs we present, all but one occur along dextral faults

cutting Franciscan basement; the lone exception are

the structures associated with the Olema Creek For-

mation, formed along a reach of the San Andreas fault

that separates Franciscan and Salinian rocks. Deposi-

tion of late Cenozoic sedimentary and volcanic rocks

has occurred during the development of the transform

margin (e.g., Page, 1981). These late Cenozoic depos-

its provide most of the evidence for the processes

discussed in this paper.

There are many examples of transtensional (pull-

apart) basins and local transpressional structures re-

lated 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 step-

overs and bends from the northern San Andreas fault

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

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301282

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 conse-

quence of releasing step-overs and bends in an other-

wise 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 kilo-

meters, 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 =Ter-

tiary and older rocks, FTB: Northeast Santa Cruz Mountains fold

and thrust belt; Q = undifferentiated Quaternary deposits, Tp = Plio-

cene 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) 279–301 283

(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 releas-

ing 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 well-

known 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 (Waka-

bayashi 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

apparent separation of the eastern graben-bounding

faults (fault F3) near the ground surface (i.e., reflect-

ing 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 defor-

mation 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 exca-

vated 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. Colluvi-

um 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 de-

formation, 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 forma-

tion, and subsequent uplift and contractional defor-

mation 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 geophys-

ical 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

Fig. 4. Trench log of trench across the Miller Creek fault at Big Burn Road.

J.Wakabayashiet

al./Tecto

nophysics

392(2004)279–301

284

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.

J.Wakabayashiet

al./Tecto

nophysics

392(2004)279–301

285

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301286

east side of the San Andreas fault on the San Fran-

cisco 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 com-

prises 1700 m of partly lithified sand and silt depos-

ited 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 (Sarna-

Wojcicki 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), pro-

vides 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 tran-

gressive–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 For-

mation 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 exten-

sional 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 pull-

apart, 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 (Hen-

gesh 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) 279–301 287

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, move-

ment 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 shut-

off 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

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 prox-

imity 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 scarp-

derived 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 For-

mation, 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

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301288

basin in an otherwise transpressional region is some-

what 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 contrac-

tional 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 under-

going 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 re-

gime. 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 struc-

tures accommodated all of the slip during the evolu-

tion of the restraining bend region, then considerable

structural relief should result, associated with signif-

icant contractional deformation, exhumation, and rock

uplift.

Hypothetical rock uplift associated with a restrain-

ing 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 step-

over can be estimated for an idealized case of a single

transverse structure as

z ¼ xsinðtan�1ðsinhtandÞÞ 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 conver-

gence 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 accom-

modated 20 km or more of displacement (an amount

that would include most of the major restraining step-

overs 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) 279–301 289

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

(Burgmann 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 exam-

ples 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 deforma-

tion 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

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 trans-

ferred 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 Green-

ville 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 4.1F1.8 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 (3.4F 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). Morpho-

metric 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 ances-

tral Livermore sedimentary basin, bordered on the east

by the Greenville–Clayton–Marsh Creek fault system

Fig. 6. Geology of the Mt. Diablo restraining step-over area. Geology from Wagner et al. (1990) and Unruh and Sawyer (1997).

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301290

(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 shut-

ting 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) 279–301 291

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 de-

formed 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 displace-

ment 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 Cala-

veras 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 contrac-

tional deformation of the restraining step-over. The

high ridge in the step-over region, however, is

composed of late Cenozoic deposits, whereas Meso-

zoic basement rocks crop out in many of the sur-

rounding, 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-

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 northwest-

ward 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 aggre-

gate 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 dis-

placement incompatibilities would result along those

faults; fault displacement for simply propagating

faults would be zero at the fault tip, and many kilo-

meters further south. The strike of a transverse struc-

ture (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 north-

ernmost 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 north-

ward. 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 large-

scale displacement or structural relief has not devel-

oped on any given transverse structure. The northern-

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301292

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 step-

over. 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 junc-

tion, as suggested by Gulick and Meltzer (2002), a

transfer zone that migrates northward with the triple

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).

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 progres-

sively 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

A fold and thrust belt, informally called the North-

east 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, how-

ever, is a blind feature underlying a fault-propagation

fold that shows evidence of having begun deformation

within the last several hundred thousand years (Ken-

nedy and Caskey, 2001). Both exposed and blind parts

of the Serra fault exhibit evidence of Holocene defor-

mation (Hengesh et al., 1996a,b; Kennedy and Cas-

key, 2001). In contrast, the folds and faults in the Palo

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 293

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 (Burgmann 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. Alterna-

tively, the initiation of contractional deformation

along the northernmost part of the fold and thrust belt

may represent a return to ‘‘background’’ transpres-

sional conditions that characterize the Coast Ranges,

following the northward migration of local transten-

sional 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, under-

going 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

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 mecha-

nisms 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 illus-

trated 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 trans-

verse 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 shut-

off of PDZs and progressive development of trans-

verse 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 devel-

opment of the Hornelen basin in Norway is also

similar in that it proposes the progressive develop-

ment 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 move-

ment (for example pure strike–slip vs. normal-

oblique). This may explain why major vertical sepa-

ration and extension has been interpreted to be

associated with bounding faults that are the along-

strike 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 (Ben-

Avraham and Zoback, 1992). We suggest that the

Olema Creek formation and the graben deposits

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301294

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).

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.

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 295

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 migra-

tion 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

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301296

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 mi-

grated 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. Pro-

gressive 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 asym-

metric progressive development of strike–slip dup-

lexes (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 be-

cause the two fault-normal directions are essentially

the same with respect to gravity. The direction of

migration may be driven by the presence of mechan-

ical 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 trans-

tensional duplexes (associated with migrating releas-

ing 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 observa-

tions 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) displace-

ment 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 develop-

ment 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 asso-

ciated with step-overs in a transform fault system

with a component of extension (regional transten-

sional) 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) de-

scribed such behavior and suggested that it was a

consequence of progressive interaction of irregulari-

ties along a strike–slip fault (fault convergences and

divergences, and step-overs and bends) with contin-

ued slip.

As noted in the field examples, migration of step-

overs 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) 279–301 297

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; Christie-

Blick and Biddle, 1985; implied in Crowell, 1974a),

but it should not produce the progressive along-strike

migration of deposition or deformation for the exam-

ples presented herein.

As illustrated by the examples, the step-over evo-

lution 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 pro-

gressive 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

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 exam-

ples 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 behav-

ior 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-G-

2426, 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

J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301298

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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