Progressive Evolution of Step-Overs and Bends

Home , Concord Fault, Hope Fault, San Gregorio Fault, Transform fault, Transpression

Tectonophysics 392 (2004) 279–301 www.elsevier.com/locate/tecto

Four-dimensional transform fault processes: progressive evolution of step-overs and bends

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

a Wakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USA b 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 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

* Corresponding author. Tel.: +1-510-887-1796; fax: +1-510-887-2389. E-mail address: [email protected] (J. Wakabayashi).

1. Introduction progressively increase in structural relief and size (e.g., Mann et al., 1983; Dooley and McClay, 1997; Bends and step-overs are common structures along Dooley et al., 1999; McClay and Bonora, 2001). strike–slip systems (e.g., Crowell, 1974a,b; Christie- Existing models, however, do not explain some enig- Blick and Biddle, 1985). Geologic features that form matic field relations associated with step-overs and as a consequence of these step-overs and bends bends, such as: (1) Why are deposits, interpreted to include those of transtensional character, such as have been laid down in pull-apart basins associated pull-apart basins and negative flower structures asso- with releasing step-overs, found outside of the active ciated with releasing bends or step-overs, and those of basins? and (2) Why is the structural relief associated transpressional character, such as oblique fold and with restraining step-overs commonly smaller than thrust belts, push up structures, and positive flower expected for the amount of slip transfer accommodat- structures associated with restraining bends or step- ed by the them? overs (e.g., Crowell, 1974a,b; Christie-Blick and The well-known dextral San Andreas fault system Biddle, 1985). For the purposes of discussion, we of coastal California accommodates 75–80% (38–40 can classify structures within a step-over or bend mm/year) of present Pacific–North American plate region as: (1) main strike–slip faults or bounding motion (e.g., Argus and Gordon, 1991), and 70% faults also known as principal displacement zones or (540–590 km) of the dextral displacement that has PDZs, and (2) transverse structures that accommodate accumulated across the plate boundary over the last 18 the transfer of slip between the PDZs on either side of Ma (Atwater and Stock, 1998). The San Andreas fault the step-over region (Fig. 1). Geologic features related system forms the boundary between the Pacific plate to step-overs and bends have received considerable and the Sierra Nevada microplate, whereas the Basin attention from researchers (e.g., Mann et al., 1983; and Range province and Walker Lane separate the Christie-Blick and Biddle, 1985; Westaway, 1995; Sierra Nevada microplate from stable North America Dooley and McClay, 1997), yet critical aspects of (Argus and Gordon, 1991). The northern part of the the long-term evolution of these structures are not San Andreas fault system strikes slightly oblique to well understood. Studies of the evolution of step-over the direction of relative motion between the Pacific features have considered step-overs as features that 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 step- overs along strike slip faults (e.g., Aydin and Page, 1984; Bu¨rgmann 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 inter- Fig. 1. Simple classification of some structures associated with step- action between an oceanic spreading ridge and the overs or bends along strike–slip faults. An idealized releasing step- subduction zone occurred at about 28 Ma, but the San over (A), and restraining step-over (B), are shown. Andreas transform system did not become established 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 deposits 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- Fig. 2. The northern San Andreas fault system, showing major lated to step-overs and bends along the various dextral strike–slip faults and other features discussed in text. Note strike–slip faults of the San Andreas fault system 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 (e.g, Crowell, 1974a,b, 1987; Aydin and Page, 1984; shown because it is specifically discussed in the text. Adapted from Nilsen and McLaughlin, 1985; Yeats, 1987). Local Wakabayashi (1999). 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, system (Fig. 2) at scales from meters in a paleoseismic to meters for sag ponds and small pressure ridges trench, to tens of kilometers in major basins or fold and (Crowell, 1974a). thrust belts, and show that it is necessary to evaluate We present examples of features related to step- the evolution of these structures in four dimensions overs and bends from the northern San Andreas fault (three spatial dimensions and time) to best understand 282 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 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 = 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 apparent separation of the eastern graben-bounding Formation was deposited in estuarine and fluvial faults (fault F3) near the ground surface (i.e., reflect- environments similar to modern Tomales Bay and ing most recent movement) is reverse, rather than its associated feeder streams, but is now exposed at normal (Fig. 4). Thus there has been a reversal of elevations up to 70 m above sea level and deformed separation sense in the late Pleistocene on this fault. by contractional deformation, with beds tilted to dips The fault bounding the graben on its west side (fault of up to 65j (Grove et al., 1995). The Olema Creek F2) has been inactive for some time (possible defor- formation regionally dips northward (‘‘shingles’’) so mation of colluvial unit C5, but not C3), whereas the that deposits young northwestward (Grove et al., eastern fault (fault F3) is still active. The flanks of the 1995). There is no evidence that a regional change ridge upon which the paleoseismic trench was exca- in plate motions took place after 130 ka, so the change vated are mantled with landslides that cover the trace in tectonic regime the affected the Olema Creek of the fault both north and south of the trench site. Formation must have been a local and progressive The sediment accumulation rate from such landslides one. The Olema Creek Formation was originally would swamp the subsidence rate of any minor deposited in a subsiding environment along the San graben (sag pond) along the fault. This is the likely Andreas fault that may have been related to a releas- reason why the active depocenter corresponding to the ing bend or step-over. The depocenter apparently colluvial graben in the trench has not been found migrated northward to Tomales Bay, leaving basinal along strike. Although the active (migrated) graben deposits outside and south of the present depocenter has not been found, the local inversion noted in the and subject to contractional deformation (Grove et al., trench suggests that the graben deposits are a small 1995). scale analog of the Olema Creek Formation. Colluvi- Unlike the deposits associated with many well- um was apparently deposited in a small graben known strike–slip basins (e.g., Crowell, 1974a,b; located at a right step-over or bend along the fault. Crowell, 1982a,b; Nilsen and McLaughlin, 1985), This releasing step-over migrated with respect to no fault scarp breccias have been identified within those deposits, leaving the deposits behind. Outside the Olema Creek Formation (Grove et al., 1995). of the local transtensional environment, the deposits This may due to several factors: (1) The subsidence were subjected to a contractional component of de- associated with the depocenter is not rapid and formation, and inversion of the colluvial graben there is comparatively gentle topography associated occurred. with the boundaries of the basin; (2) sedimentation has kept pace with basin subsidence, so that slopes 2.3. Merced/San Gregorio depocenter and the Merced of the basin margins are gentle. Coarse gravels, Formation interpreted as alluvial fan deposits make up part of the Olema Creek Formation adjacent to its western The Plio-Pleistocene Merced Formation of the San boundary, the San Andreas fault (Grove et al., Francisco Bay area (Bay Area) records basin forma- 1995). These deposits are shed-off of moderately tion, and subsequent uplift and contractional defor- steep western valley wall. mation along the San Andreas fault. The relationship between the Merced Formation and its depocenter is a 2.2. Inverted graben along the Miller Creek fault more complicated example of an evolving step-over region because two major strike–slip faults, the San The Miller Creek fault is a dextral-reverse fault Gregorio and San Andreas (as well as at least one between the Calaveras and Hayward faults (Waka- other minor fault), are involved instead of one (Figs. 3 bayashi and Sawyer, 1998a) (Fig. 2). A paleoseismic and 5). However, a variety of geologic and geophys- trench across this fault revealed a graben, filled with ical data is available for the Merced Formation and late Quaternary colluvium, that is bounded by late environs, and the tectonic evolution appears similar to Miocene bedrock (graben is bounded by faults F3 and that of other step-over regions along a single fault. F2 in Fig. 4) (Wakabayashi and Sawyer, 1998a,b). The Merced Formation crops out along a narrow The trench was excavated in a ridge-crest saddle. The ( V 2.5 km wide) belt that extends 19 km along the 284 .Wkbysie l etnpyis32(04 279–301 (2004) 392 Tectonophysics / al. et Wakabayashi J.

Fig. 4. Trench log of trench across the Miller Creek fault at Big Burn Road. .Wkbysie l etnpyis32(04 279–301 (2004) 392 Tectonophysics / al. et Wakabayashi J.

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 285 Merced Formation was deposited in the depocenter related to the San Andreas–Golden Gate fault step-over. 286 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 east side of the San Andreas fault on the San Fran- The Merced is now exposed at elevations up to 200 cisco Peninsula (Brabb and Pampeyan, 1983). m on the San Francisco Peninsula at the present time, The best exposures, and type locality, of the which is also a high sea level stand (Brabb and Merced Formation are in sea cliffs extending from Pampeyan, 1983; Bonilla, 1971). Folding of the southern San Francisco to the San Andreas fault (Fig. Merced Formation has resulting in dips ranging up 3). At the type locality, the Merced Formation com- to vertical (Bonilla, 1971). The younger Colma For- prises 1700 m of partly lithified sand and silt depos- mation is present at elevations of up to 60 to 90 m on ited over a wide range of environments that include the San Francisco Peninsula (Bonilla, 1971; shelf, nearshore, foreshore, backshore, eolian, fluvial, Schlocker, 1974); these strata dip as steeply as 17j coastal embayment, and marsh (Clifton et al., 1988). (Hengesh and Wakabayashi, 1995a). Because there is Clifton et al. (1988) estimated the age of the basal no documented evidence for a regional-scale change Merced strata as 1.2–1.6 Ma based on fossils and in plate motions at 100 ka or younger, the change in regional correlations. Alternatively, Sr isotopic dating vertical tectonics recorded by the Merced and Colma of the deposits suggests an age of 2.4 to 4.8 Ma for the Formations appears to be a relatively local one. base of the Merced Formation (Ingram and Ingle, Deposition of the Merced Formation appears to be 1998); these authors concluded that it was difficult to related to a step-over along the San Andreas fault. resolve ages within this time range owing to the lack Slip on the San Andreas fault steps 3 km right to the of variability of sea water strontium isotopic ratios for Golden Gate fault forming an active basin off of the that period. Several widespread tephra layers in the Golden Gate (Bruns et al., 2002) (Fig. 5). The Golden San Francisco Bay region range in age from 4.1 to 2.0 Gate fault appears to be the offshore continuation of Ma, including two layers of 2.4 and 2.0 Ma (Sarna- the San Bruno fault. Zoback et al. (1999) showed a Wojcicki et al., 1991). If the Merced Formation was basinintherightstepregion,basedonseismic older than 2 Ma, such layers might be found in the reflection and gravity studies, and identified exten- Merced, however, none of them are. The absence of sional focal mechanisms in the area. Bruns et al. these widespread tephra layers suggests that the base (2002) conducted detailed offshore seismic reflection of the Merced Formation is less than 2 Ma. On the studies, suggested correlations between offshore basis of these observations we consider the base of the reflectors and onshore stratigraphy, and concluded Merced to be 1.5 to 2 Ma. that the offshore pull-apart basin is controlled by A prominent ash, which has been correlated to the right transfer of slip from both the San Gregorio Rockland Ash by Sarna-Wojcicki et al. (1985), pro- and San Andreas faults. In addition, they showed that vides geochronologic control for the upper part of the the San Andreas fault directly south of the present Merced section; the Rockland Ash has been dated at pull-apart did not begin movement until the late about 600 ka (Lanphere et al., 1999). The top of the Quaternary, whereas the Golden Gate fault appears Merced Formation exposed at the type locality is to be a feature with greater accumulated offset. about 150 m above the Rockland Ash, and may be Magnetic data also suggest little cumulative offset about 500 ka based on averaged sedimentation rates on the San Andreas directly south of the present pull- for the upper part of the formation and on strontium apart, as well as greater cumulative offset on the isotopic data (Ingram and Ingle, 1998). The Merced Golden Gate fault (Jachens and Zoback, 1999). The Formation is unconformably overlain by the sand of onshore, southern continuation of the Golden Gate the Colma Formation, deposits that are interpreted to fault, the San Bruno fault, is no longer active (Hen- have been deposited in a nearshore environment at 75 gesh and Wakabayashi, 1995a; McGarr et al., 1997). to 135 ka (Hall, 1965). The Merced Formation and the It is unclear whether the southern continuation of the overlying basal part of the Colma Formation are Golden Gate fault strictly follows the location of the interpreted as sediments that were deposited in tran- San Bruno fault defined in previous studies (e.g., gressive–regressive cycles related to eustatic sea level Bonilla, 1964, 1971), or whether it diverges from fluctuations within a subsiding basin, with deposition the named San Bruno fault (Wakabayashi, 1999);in keeping pace with tectonic subsidence (Clifton et al., any case the southern extension of the Golden Gate 1988). 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 Merced Formation are present on the Marin Peninsula, fault as the San Bruno fault. north of the Golden Gate (Tp in Fig. 3), however, Hengesh and Wakabayashi (1995b) proposed that these deposits appear to be older (Clark et al., 1984) the Merced Formation was deposited in a pull-apart and they may be related to a different basin than the basin, formed at a right step along the San Andreas one in which the Merced Formation was formed. fault, that had subsequently migrated northward to Similar to the Olema Creek formation, no fault the Golden Gate leaving Merced Formation deposits scarp breccias have been identified within the on the Peninsula, outside of the active basin. The Merced Formation (Clifton et al., 1988). Adequate San Andreas fault, south of the pull-apart, appears exposure of the Merced Formation exists in prox- to have propagated northward and the Golden Gate/ imity to the western boundary, the (Peninsula) San San Bruno fault appears to have progressively shut Andreas fault, to conclude that fault scarp breccias down (Fig. 5). do not occur along that basin boundary. The eastern Right separation of basement and late Cenozoic basin boundary (onshore extension of the Golden features along the San Andreas fault on the San Gate fault) is not exposed, so the presence of scarp- Francisco Peninsula (Peninsula San Andreas fault) derived breccias on that side of the basin cannot be is 22 to 36 km (Wakabayashi, 1999). This segment of evaluated without direct subsurface data (borehole the San Andreas fault has a late Holocene slip rate of samples); we are unaware of any boreholes that about 17 mm/year (Hall et al., 1999). Based on the have recovered such breccias. Slopes are gentle in 22 to 36 km of total displacement and assuming a the vicinity of the active depocenter (Bruns et al., long-term slip rate equal to the Holocene rate, move- 2002), possibly as a result of rapid sedimentation ment began on the Peninsula San Andreas fault at 1.3 that has kept up with tectonic subsidence (e.g., to 2.1 Ma. This age is similar to the estimate for the Clifton et al., 1988), so fault scarp breccias may age of the base of the Merced Formation. The not be associated with the either the Merced For- southern limit of the Merced Formation on the San mation, or its modern offshore analog. Francisco Peninsula is presently about 27 km south of the active pull-apart basin, similar to the total 2.4. Other possible examples of migrating pull-aparts offset on the Peninsula San Andreas fault. This outside of the Pacific–North American plate boundary relationship suggests that the basin in which the Merced Formation was deposited began to form at The Dead Sea pull-apart region, along a sinistral about the same time slip began on the Peninsula San part of the African–Arabian plate boundary (the Andreas fault. Dead Sea transform fault), is over 120 km in length Based on the field relations presented above, the by up to 30 km in width. The pull-apart is Merced basin pull-apart has apparently migrated at the interpreted to have formed from the Pliocene to slip rate of the Peninsula San Andreas relative to the present by progressive northward migration of Merced Formation deposits on the east side of the the depocenter with respect to its deposits and fault (Fig. 5). This migration was accompanied by the progressive development of transverse structures northward propagation of the San Andreas fault south (ten Brink and Ben-Avraham, 1989). of the pull apart, and the associated, progressive shut- The Devonian Hornelen basin of Norway is 60 km off of activity on the Golden Gate–San Bruno fault long by 20 km wide and may have also formed with (Fig. 5). The migration of the pull-apart has left relict progressive development of transverse faults that left basinal deposits outside of the active basin in a deposits outside of the active basin and subject to transpressional tectonic environment in which tilting, contractional deformation (Steel and Gloppen, 1980). folding, and uplift of the deposits has occurred. The Miocene to present Gulf of Paria pull-apart Additional complexity associated with evolution of basin of eastern Venezuela and Trindad (and related offshore basins may have occurred as a consequence features) extends over an area of approximately 150 of an additional slip transfer from the San Gregorio km by 40 km in width and has developed along and Potato Patch faults to the San Andreas fault strike–slip faults that transect a fold and thrust belt (Bruns et al., 2002) (Fig. 5). Deposits similar to the (Flinch et al., 1999). The setting of this pull-apart 288 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 basin in an otherwise transpressional region is some- placed rocks several hundred km in the late Cenozoic what similar to pull-apart basins along the modern San (e.g., McLaughlin et al., 1996; Dickinson, 1997; Andreas fault system. Inversion is occurring in the Wakabayashi, 1999). Within the fault system several eastern and central part of the basin, whereas the most left (restraining) step-overs, slip transfers, or bends recent history of the western part of the basin is have accommodated tens of km of slip (Wakabayashi, characterized by normal faults cutting earlier contrac- 1999). Examples include the Hayward–Calaveras slip tional structures (Flinch et al., 1999). This suggests transfer zone, the northern termination of the Green that transtensional basin development, superimposed Valley fault, and the northernmost part of the San on a fold and thrust belt, is migrating westward with Andreas fault system. If the same transverse struc- respect to basinal deposits. tures accommodated all of the slip during the evolu- The Quaternary Hanmer Basin, located along the tion of the restraining bend region, then considerable Hope fault, a dextral fault that splays from the Alpine structural relief should result, associated with signif- fault in New Zealand, has dimensions of 10 km by 20 icant contractional deformation, exhumation, and rock km (Wood et al., 1994). This basin occurs in a region uplift. characterized by transpression. The western part of the Hypothetical rock uplift associated with a restrain- basin is actively subsiding, whereas shortening and ing step-over is difficult to estimate because vertical inversion of basinal deposits is occurring in the deformation may be accommodated across multiple eastern part of the basin (Wood et al., 1994). This structures and because of the uncertainty of the suggests that the active basin (pull-apart) has migrated flexural response of the deformed zone. The vertical westward with respect to its deposits. component of fault displacement in a restraining step- The Quaternary Dagg Basin, located along an over can be estimated for an idealized case of a single offshore reach of the dextral Alpine fault, west of transverse structure as the South Island of New Zealand has dimensions of À1 about 5 km by 10 km (Barnes et al., 2001). The z ¼ xsinðtan ðsinhtandÞÞ ð1Þ northeastern side of the basin is actively subsiding, whereas the southwestern end of the basin is under- where z is the vertical component of displacement, x going inversion as a result of the inpingement of a is the strike–slip displacement, h is the angle between push up block (Barnes et al., 2001). Similar to the the transverse structure strike and the PDZ strike, and Hanmer Basin, the Dagg Basin has formed as a d is the dip of the transverse structure. This equation consequence of a releasing bend along a strike–slip accounts only for amount of vertical displacement fault imposed on an otherwise transpresssional re- predicted from strike–slip movement through the gime. The inversion of the southeast part of the basin step-over; it does not take into account the impact is enhanced by what appears to be the presence of a of regional transpression. However, as noted earlier, restraining bend along the fault (Barnes et al., 2001). the amount of regionally driven fault-normal conver- In this example, it appears two step-overs comprising gence in the northern San Andreas fault system is a releasing and a restraining bend may have migrated likely to be much smaller than that generated by northeast along the Alpine fault with respect to restraining bends along the major strike–slip faults. affected deposits. Any regionally derived fault-normal convergence acting on a restraining bend area should serve to slightly increase the predicted amount of vertical 3. Migration of restraining bends or step-overs with displacement compared to the amount calculated by respect to material involved in the deformation Eq. (1). For restraining step-over regions that have accom- 3.1. Lack of evidence for large structural relief modated 20 km or more of displacement (an amount associated with most restraining bends and steps that would include most of the major restraining step- overs along the northern and central San Andreas In the California Coast Ranges, the dextral faults fault system), vertical displacements of 4 km or more of the northern San Andreas fault system have dis- 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 Mount Diablo fold-and-thrust belt (MFTB) (Crane, Andreas fault system did not begin movement until 1995) (Fig. 6). Mount Diablo is the most prominent about 18 Ma (Atwater and Stock, 1998),soall topographic landmark in the San Francisco Bay vertical displacement related to restraining step-overs region with an elevation more than 500 m greater along the strands of the transform fault system must than the highest ridges outside of the step-over area. be post-18 Ma. Most areas in the California Coast The total late Cenozoic dextral slip that has trans- Ranges yield apatite fission track ages that are z 30 ferred through the Mount Diablo step-over is 18 km Ma, with only a few localities exhibiting younger or more, the combined displacement total of the ( < 18 Ma) ages that coincide with the transform Greenville fault and several faults that merge with regime (Dumitru, 1989). Geothermal gradients in it south of Mount Diablo, including the Corral the northern and central California Coast Ranges Hollow fault, and Mount Lewis trend (Wakabayashi, are z 25 jC/km in most areas (Lachenbruch and 1999) (Fig. 2). The angle between the strike of the Sass, 1980; Furlong, 1984), so transform-related main transverse structure, the Mount Diablo thrust structural relief of 2–3 km or more can be recorded fault, and strikes of the PDZs (Concord and Green- by apatite fission track data (e.g., Dumitru, 1989). ville faults) is about 38j. The Mount Diablo thrust This means that areas with >18 Ma apatite fission fault dips at 30j (Unruh, 2000). From Eq. (1) above, track ages have experienced less than 2 to 3 km of the approximate vertical displacement expected is exhumation during the transform regime. Of the about 6 km, if the Mount Diablo thrust was the restraining bend regions in the central and northern master transverse structure for the entire life of the Coast Ranges, only the left bend in the Loma Prieta step-over. Apatite fission track ages from the Mount region exhibits post-18 Ma apatite fission track ages Diablo area predate the transform regime (T.A. (Bu¨rgmann et al., 1994) suggesting structural relief of Dumitru data in Unruh, 2000). Thus the vertical several km, consistent with interpretations of regional exhumation (less than 2–3 km) is less than would structure (McLaughlin et al., 1999). The lack of be expected if the same transverse structure had been structural or thermochronologic evidence for large operative throughout the late Cenozoic history of the amounts of structural relief associated with most step-over. restraining bends and steps in the northern San Comparable deformation rates on the PDZs and on Andreas fault system suggests that the restraining transfer structures within the MFTB are consistent bends and step-overs may have migrated with respect with the evolution of the fold and thrust belt within a to rocks originally deformed in these areas, analogous restraining step-over as originally proposed by Unruh to the basin migrations discussed above. Some exam- and Sawyer (1995). The Greenville fault has an ples are described below. estimated dextral slip rate of 4.1 F 1.8 mm/year, or less, during the Holocene (Sawyer and Unruh, 2002), 3.2. Mount Diablo restraining step-over but dies out northward along the eastern edge of the MFTB (Unruh and Sawyer, 1998).Thisrateis The Mount Diablo restraining left step-over lies comparable to the Holocene slip rate (3.4 F 0.3 between the Greenville fault to the south and the mm/year; Borchardt et al., 1999) and historical creep Concord fault to the north (Unruh and Sawyer, 1997), rate (approximately 3 mm/year; Galehouse, 1992)on the easternmost active dextral faults of the San the Concord fault, and to the late Cenozoic average Andreas fault system in the San Francisco Bay area slip rate of 1.3 to 2.4 mm/year, or more, on the (Figs. 2 and 6). The transfer of dextral shear between Mount Diablo thrust fault (Unruh, 2000). Morpho- these bounding faults drives contractional deforma- metric and geochronologic studies of tectonically tion within the step-over region (Unruh and Sawyer, deformed fluvial terraces within the MFTB provide 1995). Mount Diablo (1173 m) marks the core of a late Holocene uplift rates of 3 mm/year, or more regional fault-propagation fold underlain by the (Sawyer, 1999). northeast-dipping, blind Mount Diablo thrust fault Foreland propagation of the MFTB into the ances- (Unruh and Sawyer, 1997; Unruh, 2000), the largest tral Livermore sedimentary basin, bordered on the east contractional structure in the newly recognized by the Greenville–Clayton–Marsh Creek fault system 290 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301

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 system has migrated southward, progressively shut- associated uplift and exhumation of a thick sequence ting off the Clayton–Marsh Creek fault zone. The late of basin-fill deposits, the Tassajara and Sycamore Cenozoic Clayton–Marsh Creek fault zone has been formations (Isaacson and Anderson, 1992; Anderson determined in several previous studies to be inactive et al., 1995). Correlation or dating of tephra layers (e.g., Wright et al., 1982; Hart, 1981). This model within the Sycamore Formation that are involved in implies southward propagation of the Concord fault, the folding indicate that contractional deformation which appears to be supported by the pattern of began in the past 3.3 to 4.8 Ma (Unruh, 2000). faulting. The northern and central sections of the As a consequence of foreland propagation of the Concord fault are continuous and remarkably linear, MFTB, the point or fault section where slip transfers whereas the southern section is discontinuous and is left from the Greenville–Clayton–Marsh Creek fault 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). gration of the restraining step-over with respect to the Following the structural development model for deposits deformed within the step-over region. strike–slip faults proposed by Wesnousky (1988, Andrews et al. (1993) suggest that the region the 1989), the pattern of faulting along the southern more deeply eroded region southeast of the current Concord fault is consistent with a strike–slip fault step-over region exposes a former transfer structure having limited cumulative offset, consistent with and that the step-over has been migrating northwest- southward ’younging’ of the fault zone. The field ward with respect to affected deposits. relations in the Mount Diablo area are consistent with southward migration of the Greenville–Concord fault 3.4. Transfer of slip from Eastern faults of the San restraining step-over with respect to deposits de- Andreas fault system to the Mendocino triple junction formed within the step-over region. The largest-scale restraining bend or step-over in 3.3. Slip transfer from the Calaveras fault to the the northern San Andreas fault system may occur near Hayward fault the northern terminus of the system at the Mendocino triple junction. In the northernmost San Andreas fault The slip transfer from the Calaveras fault to the system, more than 250 km of dextral slip, the aggre- Hayward fault represents a restraining step-over that gate amount of displacement of faults east of the San has been associated with a large amount of displace- Andreas fault (eastern faults), must transfer westward ment on the associated PDZs. The Hayward fault has to the Mendocino triple junction (Wakabayashi, about 37 to 61 km of post-10 Ma displacement, most 1999). The eastern faults cannot simply propagate or all of which has been transferred from the Cala- northward without slip transfer, otherwise major dis- veras fault via a left step-over (Wakabayashi, 1999). placement incompatibilities would result along those The average angle between the presently active faults; fault displacement for simply propagating transverse structure, a largely or entirely blind faults would be zero at the fault tip, and many kilo- oblique slip fault, and the Hayward fault is about meters further south. The strike of a transverse struc- 20j, and this transverse structure dips at 75–80j ture (or structures) connecting the eastern faults to the (Wong and Hemphill-Haley, 1992). From Eq. (1), the present triple junction must be at least 20j more approximate magnitude of predicted vertical slip for a westerly than that of the eastern faults. For any range single transverse structure is 29 km, clearly an of transverse structure dips, this predicts an unrealistic unrealistic amount. Approximately 9 mm/year of amount of vertical fault movement. For example, for a Holocene dextral slip is transferred from the southern transverse fault dip of 30j, the estimated vertical fault Calaveras fault to the Hayward fault (Lienkaemper et movement is 48 km from Eq. (1). No well-defined al., 1991). The geometry of the slip transfer is transverse structures have been identified in the north- consistent with an 8-km left or restraining step. In ernmost Coast Ranges. In addition, the eastern faults, this area, Mission Peak and Mount Allison (about such as the Hayward–Rodgers Creek–Maacama 800 m above sea level) attain elevations at least 400 trend, and the Green Valley fault, die out northward m higher than the crests of neighboring ridges. This as well-defined faults (Fig. 2). This may be because elevated region extends for about 10 km along strike, the eastern faults are young and propagating north- and is consistent with uplift associated with contrac- ward. Slip on the eastern faults transfers to the tional deformation of the restraining step-over. The Mendocino triple junction, which moves northward high ridge in the step-over region, however, is relative to a given position on these faults at the slip composed of late Cenozoic deposits, whereas Meso- rate ( f 25 mm Holocene rate; Niemi and Hall, 1992; zoic basement rocks crop out in many of the sur- Prentice, 1989) of the northern San Andreas fault. In rounding, and topographically lower, areas. This order to transfer slip to the migrating triple junction, relationship indicates that long-term erosion may be new transverse faults must continue to form. Thus, the actually less in the present step-over region than step-over region progressively migrates so that large- surrounding regions, the opposite of the expected scale displacement or structural relief has not devel- relationship. This relationship is consistent with mi- oped on any given transverse structure. The northern- 292 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 most San Andreas fault system may be another junction is still required in order to maintain slip example of a restraining step-over that has migrated compatibility along the eastern faults. with respect to the rocks deformed within the step- over. In such a model the eastern faults propagate 3.5. San Gregorio structural zone northward (lengthen), and new transverse structures form as the triple junction migrates (Fig. 10 of Detailed seismic reflection studies of Bruns et al. Wakabayashi, 1999) (Fig. 7). Note that even if some (2002) show that a late Cenozoic fold and thrust belt of the eastern faults slightly overstep the triple junc- is present directly west of the San Gregorio fault tion, as suggested by Gulick and Meltzer (2002),a offshore of the Golden Gate (Fig. 3). This fold and transfer zone that migrates northward with the triple 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

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; Fig. 7. Migration of the restraining transfer zone to the Mendocino Hengesh et al., 1996a). Its northernmost 5 km, how- Triple Junction. Approximate past positions of the triple junction ever, is a blind feature underlying a fault-propagation and transverse structures shown in grayed dashed lines with fold that shows evidence of having begun deformation corresponding age designations. Longitude/latitude and other within the last several hundred thousand years (Ken- reference points are valid only for present. Abbreviations, in nedy and Caskey, 2001). Both exposed and blind parts addition to those given in Fig. 2: BMV: Burdell Mountain volcanics; Co/Ce: Franciscan Coastal Belt/Central Belt contact; F: of the Serra fault exhibit evidence of Holocene defor- Fairfield; RC: Rodgers Creek fault; SSS: Skaggs Springs schist; TV: mation (Hengesh et al., 1996a,b; Kennedy and Cas- Tolay volcanics. Adapted from Wakabayashi (1999). 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 can move passively along a fault, provided another (Angell and Crampton, 1996). These field relations fault (fault b in Fig. 8A) is involved in its translation. suggest that the activity along the fold and thrust belt If slip transfer continues to occur, the basin will has propagated northward with time into materials lengthen (Fig. 8B). Some combination of the mecha- previously unaffected by it. Although the northward nisms illustrated in Fig. 8A and B has been proposed propagation of activity may suggest the migration of for the development of many basins related to strike– the transpressional region relative to materials affected slip faults (e.g., Crowell, 1982a, 1987; Mann et al., by it, the southern part of this belt of deformation is 1983; May et al., 1993). For the mechanisms illus- associated with the Loma Prieta region that has trated in Fig. 8A and B, deposits will not become generated significant structural relief (Bu¨rgmann et separated from the basin proper unless new transverse al., 1994). Accordingly, this belt may be an example structures are formed, as in the case of Fig. 8C. of a transpressional region that is growing with time Migration of pull-aparts with respect to their (both along and across strike) rather than one in which deposits is illustrated in Fig. 8C. In addition to the the deformation has migrated along strike. Alterna- creation of new transverse structures, such migration tively, the initiation of contractional deformation also indicates that the strike–slip fault on one side of along the northernmost part of the fold and thrust belt the pull-apart will propagate in the direction of basin may represent a return to ‘‘background’’ transpres- migration (PDZ1 in Fig. 8C), whereas the fault on the sional conditions that characterize the Coast Ranges, other side of the basin will progressively shut off following the northward migration of local transten- (PDZ2 in Fig. 8C). The propagation and shut-off of sional conditions associated with pull-apart basin that bounding strike–slip faults is consistent with the had formed the Merced Formation. seismic reflection interpretation of Bruns et al. (2002) for the relationship of the Merced Formation 3.7. Other possible example outside of the Pacific– to offshore pull-apart structures. Shut-off of trans- North American plate boundary verse structures left outside of the active basin should also occur (the gray transverse structures labeled in Eusden et al. (2000) has described a tranpressional Fig. 8C). An essentially identical model for migration duplex structure that has migrated northeast relative to of a pull-apart with progressive propagation and shut- deposits it has deformed along the Hope fault of New off of PDZs and progressive development of trans- Zealand. This duplex structure is 13 km along strike verse structures has been proposed for the Dead Sea by 1.3 km wide. The southwest of the active duplex, by ten Brink and Ben-Avraham (1989). The model former duplex structures are now collapsing, under- proposed by Steel and Gloppen (1980) for the devel- going a reversal of slip to normal faults (negative opment of the Hornelen basin in Norway is also inversion). similar in that it proposes the progressive development of transverse faults. In some (or many) cases, transverse structures may not be discrete faults, but 4. Discussion may be zones of distributed deformation (perhaps above blind normal or oblique faults) accommodating The step-over and bend regions reviewed above the differential displacement between reaches of a apparently have migrated with respect to deposits that given bounding faults with differing senses of move- were originally within the step-over or bend regions. ment (for example pure strike–slip vs. normal- For this to have occurred, new transverse structures oblique). This may explain why major vertical sepa- associated with the bend or step-over regions must ration and extension has been interpreted to be have progressively formed in the direction of the associated with bounding faults that are the along- migration. If the same transverse structures migrated strike continuation of strike–slip faults in the cases of instead of new structures forming, the material several basins including the San Gregorio Basin bounded by them must have migrated with them. This (Bruns et al., 2002) and the Gulf of Elat (Ben- is most easily illustrated for the case of pull-aparts Avraham and Zoback, 1992). We suggest that the along releasing bends (Fig. 8). Once formed, a basin Olema Creek formation and the graben deposits 294 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 found in the Miller Creek fault paleoseismic trench We suggest that the evolution of the Merced followed an evolution similar to Fig. 8C.The Formation followed a combination of Fig. 8C and Merced Formation is somewhat more complex F (compare these schematics with the sequence because two faults and two step-overs are involved. 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 The restraining bend or step-over case is analogous Fig. 8C (migration with respect to deposits) and to that of pull-apart basin migration. In order for a progressive enlargement of a pull apart (Fig. 8B) restraining step or bend to migrate with respect to can produce the ‘‘shingling’’ observed in several deposits deformed by it, new transverse structures strike–slip basins (e.g., Crowell, 1974a; Steel and must form (Fig. 8E); if the transverse structure does Gloppen, 1980; Nilsen and McLaughlin, 1985). For not migrate with respect to the deformed deposits basins that migrate with respect to their deposits, the structural relief progressively as illustrated in Fig. 8D. younging direction is the direction of relative migra- For migrating restraining step-overs, one of the faults tion of the depocenter. involved in the step-over should propagate, whereas 296 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 the other should shut-off (Fig. 8E). The along-strike zone, such as the San Andreas and Hayward faults, propagation of faulting has occurred along the San where the zone of long-term (late Cenozoic) displace- Gregorio structural zone (Bruns et al., 2002) and the ment is several km wide, although the Holocene trace Mount Diablo restraining step-over region as occupies a much more limited width (Wakabayashi, reviewed above. The Calaveras–Hayward fault slip 1999). The braided nature of strike–slip faults noted transfer is somewhat different in that two faults are by Crowell (1974a,b) apparently describes the same involved (schematically shown in Fig. 8G). The type of relationship. Calaveras–Hayward step-over appears to have mi- The field examples we have presented from the grated northward, resulting in the shut-off of the San Andreas system illustrate progressive develop- southern Hayward fault (schematically illustrated the ment of step-over features along a strike slip fault progressive shut-off of PDZ 1 in Fig. 8G). It is likely system with a minor component of fault normal that there are similar two-fault slip transfer zones in convergence. Because of the regionally imposed which PDZ1 propagates rather than shuts off. Pro- transpressional regime, basin deposits left outside of gressive restraining step-over migration may have active pull-apart basins are subject to contractional affected the eastern part of the northernmost San deformation and local positive inversion, even in the Andreas fault system, in which the eastern faults of absence of the interaction of these deposits with a the system propagate northwestward (Wakabayashi, restraining bend. In contrast, a transform fault system 1999, Fig. 10), although the ‘northern half’ of the with no fault-normal convergence would require step-over is not present because the plate margin north interaction of a restraining step-over with basinal of the Mendocino triple junction is a subduction zone deposits to cause contractional deformation of them rather than a transform margin (Fig. 7; schematic (in the absence of a regional change to transpression) block illustration in Fig. 8H). (as illustrated in Fig. 8I). Behavior of deposits asso- The model for the migration of some step-overs ciated with step-overs in a transform fault system and bends described above is essentially the asym- with a component of extension (regional transten- metric progressive development of strike–slip dup- sional) should be opposite of the examples reported lexes (Woodcock and Fischer, 1986). Unlike thrust from the transpressional San Andreas system. Along duplexes, where faulting generally propagates in the a transtensional transform system, rocks involved in foreland direction, there may not be a ‘preferred’ contractional deformation driven by a restraining direction of propagation for strike–slip duplexes be- step-over may be affected by negative inversion as cause the two fault-normal directions are essentially the restraining step-over migrates away from them, the same with respect to gravity. The direction of even if no interaction with a releasing bend occurs. migration may be driven by the presence of mechan- For strike–slip faults in a neutral case (without ical differences on either side of the structure, such as regional transpression or transtension), inversion can the locally greater strength of the crust on one side of occur with a migrating pair of restraining or releasing the fault. In cross section transpressional duplexes bends (Fig. 8I). Alternating subsidence and uplift (associated with migrating restraining step-overs of may occur along a strike–slip fault with a migrating bends) form positive flower structures, whereas trans- series of step-overs (Fig. 8I). Crowell (1974a) de- tensional duplexes (associated with migrating releas- scribed such behavior and suggested that it was a ing bends or step-overs) form negative flower consequence of progressive interaction of irregulari- structures in cross sectional view (Woodcock and ties along a strike–slip fault (fault convergences and Fischer, 1986). divergences, and step-overs and bends) with contin- Progressive migration of step-overs suggests that ued slip. the zone of long-term displacement associated with As noted in the field examples, migration of step- major strike–slip faults may be several km wide, overs with respect to affected deposits appears to bounded by the former lateral boundaries (PDZs) of cause tectonic inversion along strike–slip faults. Such migrating pull-apart basins or transpressional welts tectonic inversion is locally driven by the strike–slip (Fig. 8). This conclusion is consistent with observa- fault geometry, in contrast to the traditional model in tions along the major strands of the San Andreas fault 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 modeled in analog experiments (e.g., Dooley and change from transtensional to transpressional relative McClay, 1997; Dooley et al., 1999). Why one type motion between the Pacific plate and the Sierra of behavior occurs instead of the other is not clear. Nevada microplate (the part of the North American The migrating type of step-overs we have reviewed plate margin bounded on the west by the San Andreas occur along strike–slip faults cutting a variety of fault system and on the east by the Basin and Range basement types, from deformed subduction complex Province) is thought to have occurred between 8 and 6 rocks to crystalline basement. Within the northern Ma (Atwater and Stock, 1998; Argus and Gordon, and central San Andreas fault system the two exam- 2001). Such a change in plate motion may have ples of long-lived restraining bends, the Big Bend resulted in regional, simultaneous, inversion of many and Loma Prieta areas, may be associated with of the Tertiary basins of coastal California, but it gabbroic basement (Griscom and Jachens, 1990). cannot explain the more local structural inversions Such mafic basement is stronger than quartz-bearing associated with the field examples presented in this basement rocks, as such rocks maintain brittle behav- paper. Rotation of crustal blocks may also result in ior to greater depth at equivalent temperatures. The local basin inversion (Nicholson et al., 1986; Christie- greater depth of crustal seismicity associated with Blick and Biddle, 1985; implied in Crowell, 1974a), these two restraining bend areas is consistent with the but it should not produce the progressive along-strike deeper brittle–ductile transition associated with these migration of deposition or deformation for the exam- mafic rocks (e.g., Hill et al., 1990); it is not clear ples presented herein. whether such a relationship fits other step-overs of As illustrated by the examples, the step-over evo- this type. lution described herein can apply to different scales of Based on our studies of the San Andreas fault features from large pull-apart basins and push up system and review of published literature from other structures to pressure ridges and sag ponds. The regions, it appears that migration of step-overs with model presented for migration of step-overs and bends respect to their affected deposits is an important does not include all such features, however. The tectonic process. This process has largely escaped Salton trough area, where young spreading centers notice until this time. In orogenic belts around the may be forming (e.g., Elders et al., 1972; Crowell, world, many locally observed transitions in structural 1974a,b; Irwin, 1990), appears to be an example of a style, which have been ascribed to regional changes, releasing bend that has evolved associated with pro- in tectonic regime may require reexamination. The gressive enlargement of a pull-apart basin along the model for step-over development we have described southern San Andreas fault. As noted previously, can be rigorously tested by additional field studies, transverse structures associated with the restraining because the detailed field relations predicted by this bend in the Loma Prieta area have been comparatively model contrast markedly with field relations predicted long lived and have developed significant structural by other models. 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 Acknowledgements structural relief has developed (e.g., Namson and Davis, 1988; Huftile and Yeats, 1995; Blythe et al., Parts of this research were supported by the U.S. 2000; Spotila et al., 2001). Note, however, that some Geological Survey (USGS), Department of the of the contractional deformation in the Big Bend Interior, under USGS award numbers 1434-94-G- region may be at least partially a result of block 2426, 1434-95-G-2549, and 1434-HQ-97-GR-03141. rotation (e.g., Hornafius et al., 1986; Nicholson et The views and conclusions in this document are those al., 1994). of the authors and should not be interpreted as It appears that some step-overs and bends along necessarily representing the official policies, either strike–slip faults migrate with respect to deposits expressed or implied, of the U.S. Government. We affected by them, whereas others progressively grow thank J. Dewey and J. Lewis for reviews of the in structural relief, in a manner similar to that manuscript. We also thank S.J. Caskey and K. Grove 298 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 for reviews of earlier versions of the paper. 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