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Progressive Evolution of Step-Overs and Bends

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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 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). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.04.013 280 J. Wakabayashi et al. / Tectonophysics 392 (2004) 279–301 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 (gener- ally less than 10% of the strike–slip velocity) and regional transpression in the California Coast Ranges (Zoback et al., 1987; Argus and Gordon, 1991, 2001). Because the regional fault-normal convergence is relatively minor, most of the more prominent trans- pressional features along the northern San Andreas fault system are driven by restraining bends or step- overs along strike slip faults (e.g., Aydin and Page, 1984; 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 com- plex, 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 depos- its provide most of the evidence for the processes discussed in this paper. There are many examples of transtensional (pull- apart) basins and local transpressional structures re- 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.
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