We examine the Pleistocene tectonic reorganization of the Pacific-North American plate boundary in the Salton Trough of southern California with an integrated approach that includes basin analysis, magnetostratigraphy, and geologic mapping of upper Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills. These deposits preserve the earliest sedimentary record of movement on the San Felipe and San Jacinto fault zones that replaced and deactivated the late Cenozoic West Salton detachment fault. Sandstone and mudstone of the Brawley Formation accumulated between ∼1.1 and ∼0.6-0.5 Ma in a delta on the margin of an arid Pleistocene lake, which received sediment from alluvial fans of the Ocotillo Formation to the west-southwest. Our analysis indicates that the Ocotillo and Brawley formations prograded abruptly to the east-northeast across a former mud-dominated perennial lake (Borrego Formation) at ∼1.1 Ma in response to initiation of the dextral-oblique San Felipe fault zone. The ∼25-km-long San Felipe anticline initiated at about the same time and produced an intrabasinal basement-cored high within the San Felipe-Borrego basin that is recorded by progressive unconformities on its north and south limbs. A disconformity at the base of the Brawley Formation in the eastern San Felipe Hills probably records initiation and early blind slip at the southeast tip of the Clark strand of the San Jacinto fault zone. Our data are consistent with abrupt and nearly synchronous inception of the San Jacinto and San Felipe fault zones southwest of the southern San Andreas fault in the early Pleistocene during a pronounced southwestward broadening of the San Andreas fault zone. The current contractional geometry of the San Jacinto fault zone developed after ∼0.5-0.6 Ma during a second, less significant change in structural style.

Structural and stratigraphic analyses along the western margin of the Salton Trough show that the San Andreas fault system was reorganized in early Pleistocene time from a system dominated by two fault zones (the San Andreas fault and the West Salton detachment fault) to a network of dextral faults that include the SanAndreas and at least four dextral faults to the southwest. The San Felipe fault zone, one of these dextral faults, has ~5.8 ± 2.8 km of right separation and consists of three principal faults in the Peninsular Ranges. These are the San Felipe fault in the WNW, Sunset fault in the middle, and Fish Creek Mountains fault in the ESE. They form a left-stepping array and bound domains in which the Sunset Conglomerate, the older West Salton detachment fault, and Cretaceous mylonitic rocks below the detachment are folded about WNW-trending folds. A complex fl ower structure within the left-stepovers probably produced this faultparallel folding. Because all the rocks within stepovers of the San Felipe fault zone, from Cretaceous to Pleistocene, are deformed about WNW-trending folds and record broadly similar shortening strains, we infer a Quaternary age of deformation. Parts of the San Felipe fault zone cut latest Pleistocene to Holocene surfi cial deposits, and the fault zone is likely active. Evidence for early Pleistocene initiation of the San Felipe fault zone is preserved in conglomerate NE of the Sunset fault. Poorly sorted angular boulder conglomerate and pebbly sandstone of the Sunset Conglomerate are ~600 m thick and lie in angular unconformity on the Pliocene Palm Spring Group. The conglomerate coarsens upward and toward the fault, and is dominated by plutonic clasts derived from SW of it. Conglomerate beds contain up to 10% sandstone clasts recycled from older basin fi ll and accumulated in proximal to medial alluvial fans that were shed to the NE from uplifted rocks along the then-active Sunset fault. Based on lithologic, stratigraphic, structural, and compositional similarities, we correlate the Sunset Conglomerate to the Pleistocene Ocotillo Formation. Clasts of recycled sandstone record erosion of detachmentrelated basin fi ll that predates the San Felipe fault and once covered the Vallecito and Fish Creek mountains. These crystalline-cored mountain ranges fi rst emerged from beneath basin fi ll during early slip above the nascent San Felipe fault ca. 1.1-1.3 Ma. Later, the San Felipe fault zone cut upward, folded, cut across, and deactivated the West Salton detachment fault within a ~9-km-wide contractional bend and pair of left-steps. Areas that accumulated sediment within this stepover zone between ca. 1.1 and ca. 0.6 Ma are currently being inverted and folded. Initiation of the San Felipe fault in early Pleistocene time was a signifi cant event in the reorganization of the southern San Andreas fault system. The Quaternary dextral faults broadened the plate boundary zone southwestward from roughly 25 km (during coeval slip on the San Andreas fault and West Salton detachment fault) to 50-70 ...

on June 12, 2015 specialpapers.gsapubs.org Downloaded from 2 Janecke et al. ABSTRACTThe San Jacinto right-lateral strike-slip fault zone is crucial for understanding plate-boundary dynamics, regional slip partitioning, and seismic hazards within the San Andreas fault system of southern California, yet its age of initiation and long-term average slip rate are controversial. This synthesis of prior and new detailed studies in the western Salton Trough documents initiation of structural segments of the San Jacinto fault zone at or slightly before the 1.07-Ma base of the Jaramillo subchron. The dextral faults changed again after ca. 0.5-0.6 Ma with creation of new fault segments and folds. There were major and widespread basinal changes in the early Pleistocene when these new faults cut across the older West Salton detachment fault. We mapped and analyzed the complex fault mesh, identifi ed structural segment boundaries along the Clark, Coyote Creek, and San Felipe fault zones, documented linkages between the major dextral faults, identifi ed previously unknown active strands of the Coyote Creek fault 5 and 8 km NE and SW of its central strands, and showed that prior analyses of these fault zones oversimplify their complexity. The Clark fault is a zone of widely distributed faulting and folding SE of the Santa Rosa Mountains and unequivocally continues 20-25 km SE of its previously inferred termination point to the San Felipe Hills. There the Clark fault zone has been deforming basinal deposits at an average dextral slip rate of ≥ ≥10.2 +6.9/−3.3 mm/yr for ~0.5-0.6 m.y. Five new estimates of displacement are developed here using offset successions of crystalline rocks, distinctive marker beds in the late Cenozoic basin fi ll, analysis of strike-slip-related fault-bend folds, quantifi cation of strain in folds at the tips of dextral faults, and gravity, magnetic, and geomorphic data sets. Together these show far greater right slip across the Clark fault than across either the San Felipe or Coyote Creek faults, despite the Clark fault becoming "hidden" in basinal deposits at its SE end as strain disperses onto a myriad of smaller faults, strike-slip ramps and fl ats, transrotational systems of cross faults with strongly domain patterns, and a variety of fault-fold sets. Together the Clark and Buck Ridge-Santa Rosa faults accumulated ~16.8 +3.7/−6.0 km of right separation in their lifetime near Clark Lake. The Coyote Ridge segment of the Coyote Creek fault accumulated ~3.5 ± 1.3 km since roughly 0.8-0.9 Ma. The San Felipe fault accumulated between 4 and 12.4 km (~6.5 km preferred) of right slip on its central strands in the past 1.1-1.3 Ma at Yaqui and Pinyon ridges. Combining the estimates of displacement with ages of fault initiation indicates a lifetime geologic slip rate of 20.1 +6.4/−9.8 mm/yr across the San Jacinto fault zone (sum of Clark, Buck Ridge, and Coyote Creek faults) and about ~5.4 +5.9/−1.4 mm/yr across the San Felipe fault zone at Yaqui and Pinyon ridges. The NW Coyote Creek fault has a lifetime slip r...

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