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

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

Four-dimensional transform fault processes: progressive evolution of step-overs and bends John Wakabayashi a,*, James V. Hengesh b, Thomas L. Sawyer c b

a Wakabayashi Geologic Consulting, 1329 Sheridan Lane, Hayward, CA 94544, USA William Lettis and Associates, 999 Anderson Dr., Suite 140, San Rafael, CA 94901, USA c Piedmont Geosciences, Inc., 10235 Blackhawk Dr., Reno, NV 89506, USA

Available online 10 June 2004

Abstract Many bends or step-overs along strike – slip faults may evolve by propagation of the strike – slip fault on one side of the structure and progressive shut-off of the strike – slip fault on the other side. In such a process, new transverse structures form, and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the progressive asymmetric development of a strike – slip duplex. Consequences of this type of step-over evolution include: (1) the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the PlioPleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike) along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world, including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins of New Zealand. D 2004 Elsevier B.V. All rights reserved. Keywords: Strike – slip faults; Transform faults; Transpression; Transtension; Pull-apart basins; San Andreas fault

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

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.04.013

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1. Introduction Bends and step-overs are common structures along strike – slip systems (e.g., Crowell, 1974a,b; ChristieBlick and Biddle, 1985). Geologic features that form as a consequence of these step-overs and bends include those of transtensional character, such as pull-apart basins and negative flower structures associated with releasing bends or step-overs, and those of transpressional character, such as oblique fold and thrust belts, push up structures, and positive flower structures associated with restraining bends or stepovers (e.g., Crowell, 1974a,b; Christie-Blick and Biddle, 1985). For the purposes of discussion, we can classify structures within a step-over or bend region as: (1) main strike – slip faults or bounding faults also known as principal displacement zones or PDZs, and (2) transverse structures that accommodate the transfer of slip between the PDZs on either side of the step-over region (Fig. 1). Geologic features related to step-overs and bends have received considerable attention from researchers (e.g., Mann et al., 1983; Christie-Blick and Biddle, 1985; Westaway, 1995; Dooley and McClay, 1997), yet critical aspects of the long-term evolution of these structures are not well understood. Studies of the evolution of step-over features have considered step-overs as features that

Fig. 1. Simple classification of some structures associated with stepovers or bends along strike – slip faults. An idealized releasing stepover (A), and restraining step-over (B), are shown.

progressively increase in structural relief and size (e.g., Mann et al., 1983; Dooley and McClay, 1997; Dooley et al., 1999; McClay and Bonora, 2001). Existing models, however, do not explain some enigmatic field relations associated with step-overs and bends, such as: (1) Why are deposits, interpreted to have been laid down in pull-apart basins associated with releasing step-overs, found outside of the active basins? and (2) Why is the structural relief associated with restraining step-overs commonly smaller than expected for the amount of slip transfer accommodated by the them? The well-known dextral San Andreas fault system of coastal California accommodates 75– 80% (38 – 40 mm/year) of present Pacific– North American plate motion (e.g., Argus and Gordon, 1991), and 70% (540 –590 km) of the dextral displacement that has accumulated across the plate boundary over the last 18 Ma (Atwater and Stock, 1998). The San Andreas fault system forms the boundary between the Pacific plate and the Sierra Nevada microplate, whereas the Basin and Range province and Walker Lane separate the Sierra Nevada microplate from stable North America (Argus and Gordon, 1991). The northern part of the San Andreas fault system strikes slightly oblique to the direction of relative motion between the Pacific Plate and Sierra Nevada microplate, resulting in a small component of fault-normal convergence (generally less than 10% of the strike –slip velocity) and regional transpression in the California Coast Ranges (Zoback et al., 1987; Argus and Gordon, 1991, 2001). Because the regional fault-normal convergence is relatively minor, most of the more prominent transpressional features along the northern San Andreas fault system are driven by restraining bends or stepovers along strike slip faults (e.g., Aydin and Page, 1984; 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 interaction between an oceanic spreading ridge and the subduction zone occurred at about 28 Ma, but the San Andreas transform system did not become established

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until about 18 Ma (Atwater and Stock, 1998). Since 18 Ma, San Andreas fault system has progressive lengthened and accumulated dextral displacement, as the Mendocino triple junction (the north end of the system), migrated northwestward and the Rivera triple junction migrated southeastward (e.g., Atwater and Stock, 1998). The San Andreas fault system consists of several major dextral faults, including the San Andreas fault. The distribution of active structures and dextral slip rates has shifted irregularly throughout the comparatively brief history of the transform fault system; some faults increased or decreased in slip rate, some faults initiated movement, whereas other faults became dormant (Wakabayashi, 1999). Although the Franciscan subduction complex comprises most of the basement in the California Coast Ranges, dextral displacement along strands of the San Andreas fault, as well as earlier syn-subduction dextral displacement, have emplaced a continental arc fragment, the Salinian block, between two belts of subduction complex rocks (e.g., Page, 1981). Consequently, major dextral faults of the northern San Andreas fault system: (1) cut Franciscan Complex basement, (2) form the boundary between Franciscan Complex and Salinian basement, or, (3) cut Salinian basement. Of the examples of stepovers we present, all but one occur along dextral faults cutting Franciscan basement; the lone exception are the structures associated with the Olema Creek Formation, formed along a reach of the San Andreas fault that separates Franciscan and Salinian rocks. Deposition of late Cenozoic sedimentary and volcanic rocks has occurred during the development of the transform margin (e.g., Page, 1981). These late Cenozoic deposits provide most of the evidence for the processes discussed in this paper. There are many examples of transtensional (pullapart) basins and local transpressional structures related to step-overs and bends along the various strike –slip faults of the San Andreas fault system (e.g, Crowell, 1974a,b, 1987; Aydin and Page, 1984; Nilsen and McLaughlin, 1985; Yeats, 1987). Local transpressional and transtensional geologic features along strike– slip faults occur across a range of scales, from tens of km for large basins and push-up regions, to meters for sag ponds and small pressure ridges (Crowell, 1974a). We present examples of features related to stepovers and bends from the northern San Andreas fault

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Fig. 2. The northern San Andreas fault system, showing major dextral strike – slip faults and other features discussed in text. Note the Northeast Santa Cruz Mountains fold and thrust belt (FTB) is not the only fold and thrust belt in this area, but this specific belt is shown because it is specifically discussed in the text. Adapted from Wakabayashi (1999).

system (Fig. 2) at scales from meters in a paleoseismic trench, to tens of kilometers in major basins or fold and thrust belts, and show that it is necessary to evaluate the evolution of these structures in four dimensions (three spatial dimensions and time) to best understand

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them. The common attribute of all of the examples we present is that the step-over has apparently migrated with respect to the deposits that were formerly affected by it. For releasing structures, such as pull-apart basins, the basins have migrated along bounding strike – slip faults (PDZs) with respect to their former basinal deposits. For restraining structures, such as fold and thrust belts or transpressional welts, the features have migrated along bounding strike – slip faults with respect to deposits that were deformed by these structures. In addition to examples from the San Andreas fault system, we also review some examples from other plate boundaries that may have followed a similar type of tectonic evolution. Following the presentation of field examples we discuss a model for the evolution of such step-overs and bends.

2. Migration of pull-apart basins with respect to basinal deposits As noted above, regional plate motions result in regional transpression within California Coast Ranges. Because transtensional tectonics within the northern San Andreas fault system are a local consequence of releasing step-overs and bends in an otherwise transpressional setting, the migration of such a step-over away from pull-apart basin deposits results in the subsequent uplift and contractional deformation (locally driven inversion) of such deposits. Below we describe pull-apart basins that have migrated with respect to their deposits at three scales: tens of kilometers, kilometers, and meters. 2.1. Tomales Bay depocenter and Olema Creek formation The Olema Creek Formation is about 130 ka old and crops out in a 3.5 km long by 0.5 km wide belt along the San Andreas fault, south of Tomales Bay Fig. 3. Distribution of the Merced Formation (QTm), Colma Formation (Qc), Olema Creek Formation (Qoc), and offshore features along the San Andreas fault. Other abbreviations B = Tertiary and older rocks, FTB: Northeast Santa Cruz Mountains fold and thrust belt; Q = undifferentiated Quaternary deposits, Tp = Pliocene Merced-like rocks. Merced and Colma Formation distribution from Wagner et al. (1990), Olema Creek Formation from Grove et al. (1995) and offshore features from Bruns et al. (2002).

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(Grove et al., 1995) (Fig. 3). The Olema Creek Formation was deposited in estuarine and fluvial environments similar to modern Tomales Bay and its associated feeder streams, but is now exposed at elevations up to 70 m above sea level and deformed by contractional deformation, with beds tilted to dips of up to 65j (Grove et al., 1995). The Olema Creek formation regionally dips northward (‘‘shingles’’) so that deposits young northwestward (Grove et al., 1995). There is no evidence that a regional change in plate motions took place after 130 ka, so the change in tectonic regime the affected the Olema Creek Formation must have been a local and progressive one. The Olema Creek Formation was originally deposited in a subsiding environment along the San Andreas fault that may have been related to a releasing bend or step-over. The depocenter apparently migrated northward to Tomales Bay, leaving basinal deposits outside and south of the present depocenter and subject to contractional deformation (Grove et al., 1995). Unlike the deposits associated with many wellknown strike – slip basins (e.g., Crowell, 1974a,b; Crowell, 1982a,b; Nilsen and McLaughlin, 1985), no fault scarp breccias have been identified within the Olema Creek Formation (Grove et al., 1995). This may due to several factors: (1) The subsidence associated with the depocenter is not rapid and there is comparatively gentle topography associated with the boundaries of the basin; (2) sedimentation has kept pace with basin subsidence, so that slopes of the basin margins are gentle. Coarse gravels, interpreted as alluvial fan deposits make up part of the Olema Creek Formation adjacent to its western boundary, the San Andreas fault (Grove et al., 1995). These deposits are shed-off of moderately steep western valley wall. 2.2. Inverted graben along the Miller Creek fault The Miller Creek fault is a dextral-reverse fault between the Calaveras and Hayward faults (Wakabayashi and Sawyer, 1998a) (Fig. 2). A paleoseismic trench across this fault revealed a graben, filled with late Quaternary colluvium, that is bounded by late Miocene bedrock (graben is bounded by faults F3 and F2 in Fig. 4) (Wakabayashi and Sawyer, 1998a,b). The trench was excavated in a ridge-crest saddle. The

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apparent separation of the eastern graben-bounding faults (fault F3) near the ground surface (i.e., reflecting most recent movement) is reverse, rather than normal (Fig. 4). Thus there has been a reversal of separation sense in the late Pleistocene on this fault. The fault bounding the graben on its west side (fault F2) has been inactive for some time (possible deformation of colluvial unit C5, but not C3), whereas the eastern fault (fault F3) is still active. The flanks of the ridge upon which the paleoseismic trench was excavated are mantled with landslides that cover the trace of the fault both north and south of the trench site. The sediment accumulation rate from such landslides would swamp the subsidence rate of any minor graben (sag pond) along the fault. This is the likely reason why the active depocenter corresponding to the colluvial graben in the trench has not been found along strike. Although the active (migrated) graben has not been found, the local inversion noted in the trench suggests that the graben deposits are a small scale analog of the Olema Creek Formation. Colluvium was apparently deposited in a small graben located at a right step-over or bend along the fault. This releasing step-over migrated with respect to those deposits, leaving the deposits behind. Outside of the local transtensional environment, the deposits were subjected to a contractional component of deformation, and inversion of the colluvial graben occurred. 2.3. Merced/San Gregorio depocenter and the Merced Formation The Plio-Pleistocene Merced Formation of the San Francisco Bay area (Bay Area) records basin formation, and subsequent uplift and contractional deformation along the San Andreas fault. The relationship between the Merced Formation and its depocenter is a more complicated example of an evolving step-over region because two major strike– slip faults, the San Gregorio and San Andreas (as well as at least one other minor fault), are involved instead of one (Figs. 3 and 5). However, a variety of geologic and geophysical data is available for the Merced Formation and environs, and the tectonic evolution appears similar to that of other step-over regions along a single fault. The Merced Formation crops out along a narrow ( V 2.5 km wide) belt that extends 19 km along the

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

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Fig. 5. Relationship of the Merced Formation to processes along the San Andreas and San Gregorio faults. Basin formation is the consequence of two releasing step-overs that have migrated with respect to affected deposits. These step-overs are from the Potato Patch fault to the San Andreas fault and from the San Andreas fault to the Golden Gate fault. The Merced Formation was deposited in the depocenter related to the San Andreas – Golden Gate fault step-over.

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east side of the San Andreas fault on the San Francisco Peninsula (Brabb and Pampeyan, 1983). The best exposures, and type locality, of the Merced Formation are in sea cliffs extending from southern San Francisco to the San Andreas fault (Fig. 3). At the type locality, the Merced Formation comprises 1700 m of partly lithified sand and silt deposited over a wide range of environments that include shelf, nearshore, foreshore, backshore, eolian, fluvial, coastal embayment, and marsh (Clifton et al., 1988). Clifton et al. (1988) estimated the age of the basal Merced strata as 1.2– 1.6 Ma based on fossils and regional correlations. Alternatively, Sr isotopic dating of the deposits suggests an age of 2.4 to 4.8 Ma for the base of the Merced Formation (Ingram and Ingle, 1998); these authors concluded that it was difficult to resolve ages within this time range owing to the lack of variability of sea water strontium isotopic ratios for that period. Several widespread tephra layers in the San Francisco Bay region range in age from 4.1 to 2.0 Ma, including two layers of 2.4 and 2.0 Ma (SarnaWojcicki et al., 1991). If the Merced Formation was older than 2 Ma, such layers might be found in the Merced, however, none of them are. The absence of these widespread tephra layers suggests that the base of the Merced Formation is less than 2 Ma. On the basis of these observations we consider the base of the Merced to be 1.5 to 2 Ma. A prominent ash, which has been correlated to the Rockland Ash by Sarna-Wojcicki et al. (1985), provides geochronologic control for the upper part of the Merced section; the Rockland Ash has been dated at about 600 ka (Lanphere et al., 1999). The top of the Merced Formation exposed at the type locality is about 150 m above the Rockland Ash, and may be about 500 ka based on averaged sedimentation rates for the upper part of the formation and on strontium isotopic data (Ingram and Ingle, 1998). The Merced Formation is unconformably overlain by the sand of the Colma Formation, deposits that are interpreted to have been deposited in a nearshore environment at 75 to 135 ka (Hall, 1965). The Merced Formation and the overlying basal part of the Colma Formation are interpreted as sediments that were deposited in trangressive –regressive cycles related to eustatic sea level fluctuations within a subsiding basin, with deposition keeping pace with tectonic subsidence (Clifton et al., 1988).

The Merced is now exposed at elevations up to 200 m on the San Francisco Peninsula at the present time, which is also a high sea level stand (Brabb and Pampeyan, 1983; Bonilla, 1971). Folding of the Merced Formation has resulting in dips ranging up to vertical (Bonilla, 1971). The younger Colma Formation is present at elevations of up to 60 to 90 m on the San Francisco Peninsula (Bonilla, 1971; Schlocker, 1974); these strata dip as steeply as 17j (Hengesh and Wakabayashi, 1995a). Because there is no documented evidence for a regional-scale change in plate motions at 100 ka or younger, the change in vertical tectonics recorded by the Merced and Colma Formations appears to be a relatively local one. Deposition of the Merced Formation appears to be related to a step-over along the San Andreas fault. Slip on the San Andreas fault steps 3 km right to the Golden Gate fault forming an active basin off of the Golden Gate (Bruns et al., 2002) (Fig. 5). The Golden Gate fault appears to be the offshore continuation of the San Bruno fault. Zoback et al. (1999) showed a basin in the right step region, based on seismic reflection and gravity studies, and identified extensional focal mechanisms in the area. Bruns et al. (2002) conducted detailed offshore seismic reflection studies, suggested correlations between offshore reflectors and onshore stratigraphy, and concluded that the offshore pull-apart basin is controlled by right transfer of slip from both the San Gregorio and San Andreas faults. In addition, they showed that the San Andreas fault directly south of the present pull-apart did not begin movement until the late Quaternary, whereas the Golden Gate fault appears to be a feature with greater accumulated offset. Magnetic data also suggest little cumulative offset on the San Andreas directly south of the present pullapart, as well as greater cumulative offset on the Golden Gate fault (Jachens and Zoback, 1999). The onshore, southern continuation of the Golden Gate fault, the San Bruno fault, is no longer active (Hengesh and Wakabayashi, 1995a; McGarr et al., 1997). It is unclear whether the southern continuation of the Golden Gate fault strictly follows the location of the San Bruno fault defined in previous studies (e.g., Bonilla, 1964, 1971), or whether it diverges from the named San Bruno fault (Wakabayashi, 1999); in any case the southern extension of the Golden Gate fault is inactive. For purposes of discussion, however,

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we will refer to the onshore part of the Golden Gate fault as the San Bruno fault. Hengesh and Wakabayashi (1995b) proposed that the Merced Formation was deposited in a pull-apart basin, formed at a right step along the San Andreas fault, that had subsequently migrated northward to the Golden Gate leaving Merced Formation deposits on the Peninsula, outside of the active basin. The San Andreas fault, south of the pull-apart, appears to have propagated northward and the Golden Gate/ San Bruno fault appears to have progressively shut down (Fig. 5). Right separation of basement and late Cenozoic features along the San Andreas fault on the San Francisco Peninsula (Peninsula San Andreas fault) is 22 to 36 km (Wakabayashi, 1999). This segment of the San Andreas fault has a late Holocene slip rate of about 17 mm/year (Hall et al., 1999). Based on the 22 to 36 km of total displacement and assuming a long-term slip rate equal to the Holocene rate, movement began on the Peninsula San Andreas fault at 1.3 to 2.1 Ma. This age is similar to the estimate for the age of the base of the Merced Formation. The southern limit of the Merced Formation on the San Francisco Peninsula is presently about 27 km south of the active pull-apart basin, similar to the total offset on the Peninsula San Andreas fault. This relationship suggests that the basin in which the Merced Formation was deposited began to form at about the same time slip began on the Peninsula San Andreas fault. Based on the field relations presented above, the Merced basin pull-apart has apparently migrated at the slip rate of the Peninsula San Andreas relative to Merced Formation deposits on the east side of the fault (Fig. 5). This migration was accompanied by the northward propagation of the San Andreas fault south of the pull apart, and the associated, progressive shutoff of activity on the Golden Gate– San Bruno fault (Fig. 5). The migration of the pull-apart has left relict basinal deposits outside of the active basin in a transpressional tectonic environment in which tilting, folding, and uplift of the deposits has occurred. Additional complexity associated with evolution of offshore basins may have occurred as a consequence of an additional slip transfer from the San Gregorio and Potato Patch faults to the San Andreas fault (Bruns et al., 2002) (Fig. 5). Deposits similar to the

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Merced Formation are present on the Marin Peninsula, north of the Golden Gate (Tp in Fig. 3), however, these deposits appear to be older (Clark et al., 1984) and they may be related to a different basin than the one in which the Merced Formation was formed. Similar to the Olema Creek formation, no fault scarp breccias have been identified within the Merced Formation (Clifton et al., 1988). Adequate exposure of the Merced Formation exists in proximity to the western boundary, the (Peninsula) San Andreas fault, to conclude that fault scarp breccias do not occur along that basin boundary. The eastern basin boundary (onshore extension of the Golden Gate fault) is not exposed, so the presence of scarpderived breccias on that side of the basin cannot be evaluated without direct subsurface data (borehole samples); we are unaware of any boreholes that have recovered such breccias. Slopes are gentle in the vicinity of the active depocenter (Bruns et al., 2002), possibly as a result of rapid sedimentation that has kept up with tectonic subsidence (e.g., Clifton et al., 1988), so fault scarp breccias may not be associated with the either the Merced Formation, or its modern offshore analog. 2.4. Other possible examples of migrating pull-aparts outside of the Pacific – North American plate boundary The Dead Sea pull-apart region, along a sinistral part of the African –Arabian plate boundary (the Dead Sea transform fault), is over 120 km in length by up to 30 km in width. The pull-apart is interpreted to have formed from the Pliocene to the present by progressive northward migration of the depocenter with respect to its deposits and progressive development of transverse structures (ten Brink and Ben-Avraham, 1989). The Devonian Hornelen basin of Norway is 60 km long by 20 km wide and may have also formed with progressive development of transverse faults that left deposits outside of the active basin and subject to contractional deformation (Steel and Gloppen, 1980). The Miocene to present Gulf of Paria pull-apart basin of eastern Venezuela and Trindad (and related features) extends over an area of approximately 150 km by 40 km in width and has developed along strike – slip faults that transect a fold and thrust belt (Flinch et al., 1999). The setting of this pull-apart

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basin in an otherwise transpressional region is somewhat similar to pull-apart basins along the modern San Andreas fault system. Inversion is occurring in the eastern and central part of the basin, whereas the most recent history of the western part of the basin is characterized by normal faults cutting earlier contractional structures (Flinch et al., 1999). This suggests that transtensional basin development, superimposed on a fold and thrust belt, is migrating westward with respect to basinal deposits. The Quaternary Hanmer Basin, located along the Hope fault, a dextral fault that splays from the Alpine fault in New Zealand, has dimensions of 10 km by 20 km (Wood et al., 1994). This basin occurs in a region characterized by transpression. The western part of the basin is actively subsiding, whereas shortening and inversion of basinal deposits is occurring in the eastern part of the basin (Wood et al., 1994). This suggests that the active basin (pull-apart) has migrated westward with respect to its deposits. The Quaternary Dagg Basin, located along an offshore reach of the dextral Alpine fault, west of the South Island of New Zealand has dimensions of about 5 km by 10 km (Barnes et al., 2001). The northeastern side of the basin is actively subsiding, whereas the southwestern end of the basin is undergoing inversion as a result of the inpingement of a push up block (Barnes et al., 2001). Similar to the Hanmer Basin, the Dagg Basin has formed as a consequence of a releasing bend along a strike –slip fault imposed on an otherwise transpresssional regime. The inversion of the southeast part of the basin is enhanced by what appears to be the presence of a restraining bend along the fault (Barnes et al., 2001). In this example, it appears two step-overs comprising a releasing and a restraining bend may have migrated northeast along the Alpine fault with respect to affected deposits.

3. Migration of restraining bends or step-overs with respect to material involved in the deformation 3.1. Lack of evidence for large structural relief associated with most restraining bends and steps In the California Coast Ranges, the dextral faults of the northern San Andreas fault system have dis-

placed rocks several hundred km in the late Cenozoic (e.g., McLaughlin et al., 1996; Dickinson, 1997; Wakabayashi, 1999). Within the fault system several left (restraining) step-overs, slip transfers, or bends have accommodated tens of km of slip (Wakabayashi, 1999). Examples include the Hayward –Calaveras slip transfer zone, the northern termination of the Green Valley fault, and the northernmost part of the San Andreas fault system. If the same transverse structures accommodated all of the slip during the evolution of the restraining bend region, then considerable structural relief should result, associated with significant contractional deformation, exhumation, and rock uplift. Hypothetical rock uplift associated with a restraining step-over is difficult to estimate because vertical deformation may be accommodated across multiple structures and because of the uncertainty of the flexural response of the deformed zone. The vertical component of fault displacement in a restraining stepover can be estimated for an idealized case of a single transverse structure as z ¼ xsinðtan1 ðsinhtandÞÞ

ð1Þ

where z is the vertical component of displacement, x is the strike –slip displacement, h is the angle between the transverse structure strike and the PDZ strike, and d is the dip of the transverse structure. This equation accounts only for amount of vertical displacement predicted from strike – slip movement through the step-over; it does not take into account the impact of regional transpression. However, as noted earlier, the amount of regionally driven fault-normal convergence in the northern San Andreas fault system is likely to be much smaller than that generated by restraining bends along the major strike– slip faults. Any regionally derived fault-normal convergence acting on a restraining bend area should serve to slightly increase the predicted amount of vertical displacement compared to the amount calculated by Eq. (1). For restraining step-over regions that have accommodated 20 km or more of displacement (an amount that would include most of the major restraining stepovers along the northern and central San Andreas fault system), vertical displacements of 4 km or more are expected for existing fault geometries, as will be

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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 (Bu¨rgmann et al., 1994) suggesting structural relief of several km, consistent with interpretations of regional structure (McLaughlin et al., 1999). The lack of structural or thermochronologic evidence for large amounts of structural relief associated with most restraining bends and steps in the northern San Andreas fault system suggests that the restraining bends and step-overs may have migrated with respect to rocks originally deformed in these areas, analogous to the basin migrations discussed above. Some examples are described below. 3.2. Mount Diablo restraining step-over The Mount Diablo restraining left step-over lies between the Greenville fault to the south and the Concord fault to the north (Unruh and Sawyer, 1997), the easternmost active dextral faults of the San Andreas fault system in the San Francisco Bay area (Figs. 2 and 6). The transfer of dextral shear between these bounding faults drives contractional deformation within the step-over region (Unruh and Sawyer, 1995). Mount Diablo (1173 m) marks the core of a regional fault-propagation fold underlain by the northeast-dipping, blind Mount Diablo thrust fault (Unruh and Sawyer, 1997; Unruh, 2000), the largest contractional structure in the newly recognized

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Mount Diablo fold-and-thrust belt (MFTB) (Crane, 1995) (Fig. 6). Mount Diablo is the most prominent topographic landmark in the San Francisco Bay region with an elevation more than 500 m greater than the highest ridges outside of the step-over area. The total late Cenozoic dextral slip that has transferred through the Mount Diablo step-over is 18 km or more, the combined displacement total of the Greenville fault and several faults that merge with it south of Mount Diablo, including the Corral Hollow fault, and Mount Lewis trend (Wakabayashi, 1999) (Fig. 2). The angle between the strike of the main transverse structure, the Mount Diablo thrust fault, and strikes of the PDZs (Concord and Greenville faults) is about 38j. The Mount Diablo thrust fault dips at 30j (Unruh, 2000). From Eq. (1) above, the approximate vertical displacement expected is about 6 km, if the Mount Diablo thrust was the master transverse structure for the entire life of the step-over. Apatite fission track ages from the Mount Diablo area predate the transform regime (T.A. Dumitru data in Unruh, 2000). Thus the vertical exhumation (less than 2 – 3 km) is less than would be expected if the same transverse structure had been operative throughout the late Cenozoic history of the step-over. Comparable deformation rates on the PDZs and on transfer structures within the MFTB are consistent with the evolution of the fold and thrust belt within a restraining step-over as originally proposed by Unruh and Sawyer (1995). The Greenville fault has an estimated dextral slip rate of 4.1 F 1.8 mm/year, or less, during the Holocene (Sawyer and Unruh, 2002), but dies out northward along the eastern edge of the MFTB (Unruh and Sawyer, 1998). This rate is comparable to the Holocene slip rate (3.4 F 0.3 mm/year; Borchardt et al., 1999) and historical creep rate (approximately 3 mm/year; Galehouse, 1992) on the Concord fault, and to the late Cenozoic average slip rate of 1.3 to 2.4 mm/year, or more, on the Mount Diablo thrust fault (Unruh, 2000). Morphometric and geochronologic studies of tectonically deformed fluvial terraces within the MFTB provide late Holocene uplift rates of 3 mm/year, or more (Sawyer, 1999). Foreland propagation of the MFTB into the ancestral Livermore sedimentary basin, bordered on the east by the Greenville – Clayton– Marsh Creek fault system

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

(Fig. 6), has resulted in contractional deformation and associated uplift and exhumation of a thick sequence of basin-fill deposits, the Tassajara and Sycamore formations (Isaacson and Anderson, 1992; Anderson et al., 1995). Correlation or dating of tephra layers within the Sycamore Formation that are involved in the folding indicate that contractional deformation began in the past 3.3 to 4.8 Ma (Unruh, 2000). As a consequence of foreland propagation of the MFTB, the point or fault section where slip transfers left from the Greenville – Clayton– Marsh Creek fault

system has migrated southward, progressively shutting off the Clayton –Marsh Creek fault zone. The late Cenozoic Clayton– Marsh Creek fault zone has been determined in several previous studies to be inactive (e.g., Wright et al., 1982; Hart, 1981). This model implies southward propagation of the Concord fault, which appears to be supported by the pattern of faulting. The northern and central sections of the Concord fault are continuous and remarkably linear, whereas the southern section is discontinuous and is comprised of short en echelon fault traces (Sharp,

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1973; CDMG, 1974, 1977; Wills and Hart, 1992). Following the structural development model for strike – slip faults proposed by Wesnousky (1988, 1989), the pattern of faulting along the southern Concord fault is consistent with a strike – slip fault having limited cumulative offset, consistent with southward ’younging’ of the fault zone. The field relations in the Mount Diablo area are consistent with southward migration of the Greenville – Concord fault restraining step-over with respect to deposits deformed within the step-over region. 3.3. Slip transfer from the Calaveras fault to the Hayward fault The slip transfer from the Calaveras fault to the Hayward fault represents a restraining step-over that has been associated with a large amount of displacement on the associated PDZs. The Hayward fault has about 37 to 61 km of post-10 Ma displacement, most or all of which has been transferred from the Calaveras fault via a left step-over (Wakabayashi, 1999). The average angle between the presently active transverse structure, a largely or entirely blind oblique slip fault, and the Hayward fault is about 20j, and this transverse structure dips at 75 – 80j (Wong and Hemphill-Haley, 1992). From Eq. (1), the approximate magnitude of predicted vertical slip for a single transverse structure is 29 km, clearly an unrealistic amount. Approximately 9 mm/year of Holocene dextral slip is transferred from the southern Calaveras fault to the Hayward fault (Lienkaemper et al., 1991). The geometry of the slip transfer is consistent with an 8-km left or restraining step. In this area, Mission Peak and Mount Allison (about 800 m above sea level) attain elevations at least 400 m higher than the crests of neighboring ridges. This elevated region extends for about 10 km along strike, and is consistent with uplift associated with contractional deformation of the restraining step-over. The high ridge in the step-over region, however, is composed of late Cenozoic deposits, whereas Mesozoic basement rocks crop out in many of the surrounding, and topographically lower, areas. This relationship indicates that long-term erosion may be actually less in the present step-over region than surrounding regions, the opposite of the expected relationship. This relationship is consistent with mi-

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gration of the restraining step-over with respect to the deposits deformed within the step-over region. Andrews et al. (1993) suggest that the region the more deeply eroded region southeast of the current step-over region exposes a former transfer structure and that the step-over has been migrating northwestward with respect to affected deposits. 3.4. Transfer of slip from Eastern faults of the San Andreas fault system to the Mendocino triple junction The largest-scale restraining bend or step-over in the northern San Andreas fault system may occur near the northern terminus of the system at the Mendocino triple junction. In the northernmost San Andreas fault system, more than 250 km of dextral slip, the aggregate amount of displacement of faults east of the San Andreas fault (eastern faults), must transfer westward to the Mendocino triple junction (Wakabayashi, 1999). The eastern faults cannot simply propagate northward without slip transfer, otherwise major displacement incompatibilities would result along those faults; fault displacement for simply propagating faults would be zero at the fault tip, and many kilometers further south. The strike of a transverse structure (or structures) connecting the eastern faults to the present triple junction must be at least 20j more westerly than that of the eastern faults. For any range of transverse structure dips, this predicts an unrealistic amount of vertical fault movement. For example, for a transverse fault dip of 30j, the estimated vertical fault movement is 48 km from Eq. (1). No well-defined transverse structures have been identified in the northernmost Coast Ranges. In addition, the eastern faults, such as the Hayward – Rodgers Creek – Maacama trend, and the Green Valley fault, die out northward as well-defined faults (Fig. 2). This may be because the eastern faults are young and propagating northward. Slip on the eastern faults transfers to the Mendocino triple junction, which moves northward relative to a given position on these faults at the slip rate ( f 25 mm Holocene rate; Niemi and Hall, 1992; Prentice, 1989) of the northern San Andreas fault. In order to transfer slip to the migrating triple junction, new transverse faults must continue to form. Thus, the step-over region progressively migrates so that largescale displacement or structural relief has not developed on any given transverse structure. The northern-

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most San Andreas fault system may be another example of a restraining step-over that has migrated with respect to the rocks deformed within the stepover. In such a model the eastern faults propagate northward (lengthen), and new transverse structures form as the triple junction migrates (Fig. 10 of Wakabayashi, 1999) (Fig. 7). Note that even if some of the eastern faults slightly overstep the triple junction, as suggested by Gulick and Meltzer (2002), a transfer zone that migrates northward with the triple

junction is still required in order to maintain slip compatibility along the eastern faults. 3.5. San Gregorio structural zone Detailed seismic reflection studies of Bruns et al. (2002) show that a late Cenozoic fold and thrust belt is present directly west of the San Gregorio fault offshore of the Golden Gate (Fig. 3). This fold and thrust belt is at least 40 km long and up to 8 km wide. The age of the onset of movement youngs progressively from north to south, but the belt does not appear to progressively narrow in that direction. If the transpressional deformation zone was simply enlarging with time, the belt should progressively narrow southward. If the fold and thrust belt were a product of regional strain partitioning (e.g., Zoback et al., 1987), then initiation of deformation should be approximately synchronous along the length of the belt. The north to south progression of deformation suggests a direct relationship between the deformation and the migration of a restraining bend or step-over along the San Gregorio fault with respect to rocks deformed within the step-over region. The left bend that has caused the deformation may be present in the area where the San Gregorio fault locally comes onshore near Pillar Point (this is the bend at the southernmost limit of Fig. 3). 3.6. Northeast Santa Cruz Mountains thrust/fold belt and the Serra Fault

Fig. 7. Migration of the restraining transfer zone to the Mendocino Triple Junction. Approximate past positions of the triple junction and transverse structures shown in grayed dashed lines with corresponding age designations. Longitude/latitude and other reference points are valid only for present. Abbreviations, in addition to those given in Fig. 2: BMV: Burdell Mountain volcanics; Co/Ce: Franciscan Coastal Belt/Central Belt contact; F: Fairfield; RC: Rodgers Creek fault; SSS: Skaggs Springs schist; TV: Tolay volcanics. Adapted from Wakabayashi (1999).

A fold and thrust belt, informally called the Northeast Santa Cruz Mountains thrust/fold belt, occurs east of and parallel to the Peninsula San Andreas fault (Lajoie, 1996). This belt narrows northward from a width about 10 km near Palo Alto to 1.5 km or less at the sea coast. The northernmost fault in the belt, the Serra fault (Fig. 3), is a dextral-reverse fault that daylights along its southern reach (Bonilla, 1971; Hengesh et al., 1996a). Its northernmost 5 km, however, is a blind feature underlying a fault-propagation fold that shows evidence of having begun deformation within the last several hundred thousand years (Kennedy and Caskey, 2001). Both exposed and blind parts of the Serra fault exhibit evidence of Holocene deformation (Hengesh et al., 1996a,b; Kennedy and Caskey, 2001). In contrast, the folds and faults in the Palo

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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 (Bu¨rgmann et al., 1994). Accordingly, this belt may be an example of a transpressional region that is growing with time (both along and across strike) rather than one in which the deformation has migrated along strike. Alternatively, the initiation of contractional deformation along the northernmost part of the fold and thrust belt may represent a return to ‘‘background’’ transpressional conditions that characterize the Coast Ranges, following the northward migration of local transtensional conditions associated with pull-apart basin that had formed the Merced Formation. 3.7. Other possible example outside of the Pacific– North American plate boundary Eusden et al. (2000) has described a tranpressional duplex structure that has migrated northeast relative to deposits it has deformed along the Hope fault of New Zealand. This duplex structure is 13 km along strike by 1.3 km wide. The southwest of the active duplex, former duplex structures are now collapsing, undergoing a reversal of slip to normal faults (negative inversion).

4. Discussion The step-over and bend regions reviewed above apparently have migrated with respect to deposits that were originally within the step-over or bend regions. For this to have occurred, new transverse structures associated with the bend or step-over regions must have progressively formed in the direction of the migration. If the same transverse structures migrated instead of new structures forming, the material bounded by them must have migrated with them. This is most easily illustrated for the case of pull-aparts along releasing bends (Fig. 8). Once formed, a basin

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can move passively along a fault, provided another fault (fault b in Fig. 8A) is involved in its translation. If slip transfer continues to occur, the basin will lengthen (Fig. 8B). Some combination of the mechanisms illustrated in Fig. 8A and B has been proposed for the development of many basins related to strike – slip faults (e.g., Crowell, 1982a, 1987; Mann et al., 1983; May et al., 1993). For the mechanisms illustrated in Fig. 8A and B, deposits will not become separated from the basin proper unless new transverse structures are formed, as in the case of Fig. 8C. Migration of pull-aparts with respect to their deposits is illustrated in Fig. 8C. In addition to the creation of new transverse structures, such migration also indicates that the strike – slip fault on one side of the pull-apart will propagate in the direction of basin migration (PDZ1 in Fig. 8C), whereas the fault on the other side of the basin will progressively shut off (PDZ2 in Fig. 8C). The propagation and shut-off of bounding strike – slip faults is consistent with the seismic reflection interpretation of Bruns et al. (2002) for the relationship of the Merced Formation to offshore pull-apart structures. Shut-off of transverse structures left outside of the active basin should also occur (the gray transverse structures labeled in Fig. 8C). An essentially identical model for migration of a pull-apart with progressive propagation and shutoff of PDZs and progressive development of transverse structures has been proposed for the Dead Sea by ten Brink and Ben-Avraham (1989). The model proposed by Steel and Gloppen (1980) for the development of the Hornelen basin in Norway is also similar in that it proposes the progressive development of transverse faults. In some (or many) cases, transverse structures may not be discrete faults, but may be zones of distributed deformation (perhaps above blind normal or oblique faults) accommodating the differential displacement between reaches of a given bounding faults with differing senses of movement (for example pure strike – slip vs. normaloblique). This may explain why major vertical separation and extension has been interpreted to be associated with bounding faults that are the alongstrike continuation of strike –slip faults in the cases of several basins including the San Gregorio Basin (Bruns et al., 2002) and the Gulf of Elat (BenAvraham and Zoback, 1992). We suggest that the Olema Creek formation and the graben deposits

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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).

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Fig. 8. Progressive evolution of releasing and restraining step-over and bend regions along strike – slip faults. The star in each diagram is a reference point that represents a point within deposits affected by the step-over. For ‘‘D’’, ‘‘E’’, ‘‘G’’, ‘‘H’’, and ‘‘I’’, the contours (solid, fine lines) in the restraining step-over regions schematically depict elevation. Diagrams A, B, and D depict interactions in which the step-over does not migrate with respect to affected deposits, whereas Diagrams C, E, F, G, H, and I depict migration of step-overs with respect to affected deposits. For the latter, migration of the step-over occurs with progressive development on new transfer structures (and progressive shut-off of old ones) in the direction of step-over migration. For a single fault, the fault strand on one side of the step-over propagates in the direction of step-over migration, whereas the other fault strand shuts off. Variations in the propagation/shut-off direction of the PDZs are possible in the cases with multiple faults as shown in Diagrams F and G. The effect of the evolution of a series of step-overs is shown in Diagram I.

Both the evolution of pull-apart basins shown in Fig. 8C (migration with respect to deposits) and progressive enlargement of a pull apart (Fig. 8B) can produce the ‘‘shingling’’ observed in several strike – slip basins (e.g., Crowell, 1974a; Steel and Gloppen, 1980; Nilsen and McLaughlin, 1985). For basins that migrate with respect to their deposits, the younging direction is the direction of relative migration of the depocenter.

The restraining bend or step-over case is analogous to that of pull-apart basin migration. In order for a restraining step or bend to migrate with respect to deposits deformed by it, new transverse structures must form (Fig. 8E); if the transverse structure does not migrate with respect to the deformed deposits structural relief progressively as illustrated in Fig. 8D. For migrating restraining step-overs, one of the faults involved in the step-over should propagate, whereas

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the other should shut-off (Fig. 8E). The along-strike propagation of faulting has occurred along the San Gregorio structural zone (Bruns et al., 2002) and the Mount Diablo restraining step-over region as reviewed above. The Calaveras –Hayward fault slip transfer is somewhat different in that two faults are involved (schematically shown in Fig. 8G). The Calaveras– Hayward step-over appears to have migrated northward, resulting in the shut-off of the southern Hayward fault (schematically illustrated the progressive shut-off of PDZ 1 in Fig. 8G). It is likely that there are similar two-fault slip transfer zones in which PDZ1 propagates rather than shuts off. Progressive restraining step-over migration may have affected the eastern part of the northernmost San Andreas fault system, in which the eastern faults of the system propagate northwestward (Wakabayashi, 1999, Fig. 10), although the ‘northern half’ of the step-over is not present because the plate margin north of the Mendocino triple junction is a subduction zone rather than a transform margin (Fig. 7; schematic block illustration in Fig. 8H). The model for the migration of some step-overs and bends described above is essentially the asymmetric progressive development of strike – slip duplexes (Woodcock and Fischer, 1986). Unlike thrust duplexes, where faulting generally propagates in the foreland direction, there may not be a ‘preferred’ direction of propagation for strike –slip duplexes because the two fault-normal directions are essentially the same with respect to gravity. The direction of migration may be driven by the presence of mechanical differences on either side of the structure, such as the locally greater strength of the crust on one side of the fault. In cross section transpressional duplexes (associated with migrating restraining step-overs of bends) form positive flower structures, whereas transtensional duplexes (associated with migrating releasing bends or step-overs) form negative flower structures in cross sectional view (Woodcock and Fischer, 1986). Progressive migration of step-overs suggests that the zone of long-term displacement associated with major strike – slip faults may be several km wide, bounded by the former lateral boundaries (PDZs) of migrating pull-apart basins or transpressional welts (Fig. 8). This conclusion is consistent with observations along the major strands of the San Andreas fault

zone, such as the San Andreas and Hayward faults, where the zone of long-term (late Cenozoic) displacement is several km wide, although the Holocene trace occupies a much more limited width (Wakabayashi, 1999). The braided nature of strike –slip faults noted by Crowell (1974a,b) apparently describes the same type of relationship. The field examples we have presented from the San Andreas system illustrate progressive development of step-over features along a strike slip fault system with a minor component of fault normal convergence. Because of the regionally imposed transpressional regime, basin deposits left outside of active pull-apart basins are subject to contractional deformation and local positive inversion, even in the absence of the interaction of these deposits with a restraining bend. In contrast, a transform fault system with no fault-normal convergence would require interaction of a restraining step-over with basinal deposits to cause contractional deformation of them (in the absence of a regional change to transpression) (as illustrated in Fig. 8I). Behavior of deposits associated with step-overs in a transform fault system with a component of extension (regional transtensional) should be opposite of the examples reported from the transpressional San Andreas system. Along a transtensional transform system, rocks involved in contractional deformation driven by a restraining step-over may be affected by negative inversion as the restraining step-over migrates away from them, even if no interaction with a releasing bend occurs. For strike – slip faults in a neutral case (without regional transpression or transtension), inversion can occur with a migrating pair of restraining or releasing bends (Fig. 8I). Alternating subsidence and uplift may occur along a strike – slip fault with a migrating series of step-overs (Fig. 8I). Crowell (1974a) described such behavior and suggested that it was a consequence of progressive interaction of irregularities along a strike – slip fault (fault convergences and divergences, and step-overs and bends) with continued slip. As noted in the field examples, migration of stepovers with respect to affected deposits appears to cause tectonic inversion along strike –slip faults. Such tectonic inversion is locally driven by the strike – slip fault geometry, in contrast to the traditional model in which inversion results from a regional (far field)

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change in tectonic regime (e.g, Cooper et al., 1989). A change from transtensional to transpressional relative motion between the Pacific plate and the Sierra Nevada microplate (the part of the North American plate margin bounded on the west by the San Andreas fault system and on the east by the Basin and Range Province) is thought to have occurred between 8 and 6 Ma (Atwater and Stock, 1998; Argus and Gordon, 2001). Such a change in plate motion may have resulted in regional, simultaneous, inversion of many of the Tertiary basins of coastal California, but it cannot explain the more local structural inversions associated with the field examples presented in this paper. Rotation of crustal blocks may also result in local basin inversion (Nicholson et al., 1986; ChristieBlick and Biddle, 1985; implied in Crowell, 1974a), but it should not produce the progressive along-strike migration of deposition or deformation for the examples presented herein. As illustrated by the examples, the step-over evolution described herein can apply to different scales of features from large pull-apart basins and push up structures to pressure ridges and sag ponds. The model presented for migration of step-overs and bends does not include all such features, however. The Salton trough area, where young spreading centers may be forming (e.g., Elders et al., 1972; Crowell, 1974a,b; Irwin, 1990), appears to be an example of a releasing bend that has evolved associated with progressive enlargement of a pull-apart basin along the southern San Andreas fault. As noted previously, transverse structures associated with the restraining bend in the Loma Prieta area have been comparatively long lived and have developed significant structural relief. An even larger-scale example of a long-lived restraining bend (or bends) is the Big Bend area along the southern San Andreas fault, where considerable structural relief has developed (e.g., Namson and Davis, 1988; Huftile and Yeats, 1995; Blythe et al., 2000; Spotila et al., 2001). Note, however, that some of the contractional deformation in the Big Bend region may be at least partially a result of block rotation (e.g., Hornafius et al., 1986; Nicholson et al., 1994). It appears that some step-overs and bends along strike – slip faults migrate with respect to deposits affected by them, whereas others progressively grow in structural relief, in a manner similar to that

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modeled in analog experiments (e.g., Dooley and McClay, 1997; Dooley et al., 1999). Why one type of behavior occurs instead of the other is not clear. The migrating type of step-overs we have reviewed occur along strike – slip faults cutting a variety of basement types, from deformed subduction complex rocks to crystalline basement. Within the northern and central San Andreas fault system the two examples of long-lived restraining bends, the Big Bend and Loma Prieta areas, may be associated with gabbroic basement (Griscom and Jachens, 1990). Such mafic basement is stronger than quartz-bearing basement rocks, as such rocks maintain brittle behavior to greater depth at equivalent temperatures. The greater depth of crustal seismicity associated with these two restraining bend areas is consistent with the deeper brittle– ductile transition associated with these mafic rocks (e.g., Hill et al., 1990); it is not clear whether such a relationship fits other step-overs of this type. Based on our studies of the San Andreas fault system and review of published literature from other regions, it appears that migration of step-overs with respect to their affected deposits is an important tectonic process. This process has largely escaped notice until this time. In orogenic belts around the world, many locally observed transitions in structural style, which have been ascribed to regional changes, in tectonic regime may require reexamination. The model for step-over development we have described can be rigorously tested by additional field studies, because the detailed field relations predicted by this model contrast markedly with field relations predicted by other models.

Acknowledgements Parts of this research were supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award numbers 1434-94-G2426, 1434-95-G-2549, and 1434-HQ-97-GR-03141. The views and conclusions in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. We thank J. Dewey and J. Lewis for reviews of the manuscript. We also thank S.J. Caskey and K. Grove

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for reviews of earlier versions of the paper. We have benefited from discussions on this subject with many of our colleagues, particularly T. Bruns, U.S. ten Brink, M.L. Zoback, C. Busby, and S.J. Caskey.

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