Stratigraphic record of Pleistocene initiation and slip on the Coyote Creek fault, Lower Coyote Creek, southern California

The Coyote Creek fault is a major strand of the San Jacinto fault zone in southern California. Pleistocene sediments and sedimentary rocks exposed in the lower Coyote Creek area preserve a record of surface deformation, stream reorganization, and erosion that resulted from initiation and slip on the Coyote Badlands strand of the Coyote Creek fault. A well-exposed section of conglomerate and sandstone contains the 760 ka Bishop Ash and reveals (1) complete reversal of paleocurrents from northwest-directed (opposed to modern drainages) to southeast-directed (consistent with modern drainages); (2) fanning dips and a progressive unconformity bounded by the Coyote Creek and Box Canyon faults; (3) a thick gravel unit (Qg) that caps the fanning-dip section and accumulated between ca. 700 and 600 ka; and (4) post-Qg offset and deep erosion of the entire section. The fanning dips and reversal of paleocurrents are interpreted to record initiation of the Box Canyon and Coyote Creek faults by dip-slip (normal) displacement beginning at 750 ka. Strike-slip offset of Qg is equal to total offset on the Coyote Creek fault in the study area ( 6 km), indicating that strike-slip motion on the fault began after deposition of Qg, after ca. 600 100 ka. This gives a time-averaged slip rate in Coyote Creek of 10 mm/yr. Two alternative models for Pleistocene fault evolution are considered: (1) prior to ca. 600 ka, the Clark and southern Coyote Creek faults were connected via a releasing bend that produced a pull-apart basin in the Borrego Badlands, and initiation of the Coyote Badlands strand at 600 ka represents northwestward propagation of the Coyote Creek fault; or (2) the Coyote Creek fault was initiated along most or all of its length at ca. 600 ka, establishing the modern link to plate-boundary faults in the Imperial Valley. Existing data are equivocal on this question. Dorsey, R.J., 2002, Stratigraphic record of Pleistocene initiation and slip on the Coyote Creek fault, Lower Coyote Creek, southern California, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 251–269. *E-mail: [email protected] INTRODUCTION The San Jacinto fault zone is a seismically active system of strike-slip fault segments, segment-bounding discontinuities, and related zones of contractional and extensional deformation in southern California (Sharp, 1967; Wesnousky, 1986, 1988; Sanders, 1989; Sanders and Magistrale, 1997). The San Jacinto splays off from the San Andreas fault in a complex zone of diffuse strike-slip faults northwest of San Bernardino (Fig. 1; Matti et al., 1992; Morton and Matti, 1993), and southeast of there the two fault zones accommodate most of the motion on the Pacific-North American plate boundary (Fig. 1; DeMets et al., 1994; DeMets, 1995). Although recent studies of historical seismicity provide insights into modern fault behavior and interactions, relatively little is known about the Pleistocene history of fault slip, sedimentation, and deformation in the San Jacinto fault zone. The evolution of the fault zone at geologic time scales of 0.5–2.0 Ma has been largely overlooked in