GEOLOGIC MAP FROM MISSION CREEK PRESERVE TO WHITEWATER RIVER, SAN BERNARDINO AND RIVERSIDE COUNTIES, CALIFORNIA: EVIDENCE FOR AN ACTIVE AND EVOLVING LEFT-STEPPING SAN ANDREAS FAULT (MISSION CREEK STRAND) IN THE EASTERN SAN BERNARDINO MOUNTAINS

This study presents a revision of mapped active and inactive faults of the southern San Andreas Fault, specifically the Mission Creek strand, at the latitude of northern Coachella Valley from Mission Creek Preserve to Whitewater River. Previous studies in the region indicate that as the Coachella Valley segment of the San Andreas Fault splits into the Mission Creek, Banning, Garnet Hill and San Gorgonio Pass strands in the southern Indio Hills, the Banning strand accommodates the majority of displacement across the plate boundary and forms a structurally complex zone with the Garnet Hill and San Gorgonio Pass strands. However, new geologic and geomorphic mapping, and a recent provenance study of displaced Quaternary sediment and ³⁶Cl/¹⁰Be burial dating indicate more recent deformation adjacent to the Mission Creek strand during the mid to late Pleistocene. Field mapping at the onset of the left-restraining bend of the Mission Creek strand reveals a complex zone of previously mapped and unmapped faults that display geomorphic features in Quaternary alluvium and deformation in the local bedrock. These faults have: (1) tilted and uplifted a mid Pleistocene alluvial fan complex; (2) preserved dip-slip displacement in mid to late Pleistocene fan surfaces along fault scarps; and (3) laterally offset stream channels that were incised during the latest Pleistocene and possibly Holocene. Most interestingly, we observe laterally offset ridges, drainages, and debris flows along an unmapped trace of the Mission Creek strand and several possible splays. Pulverized and brecciated bedrock and abrupt changes in bedding attitudes of early Pleistocene gravels are then used to project the fault trace into the San Bernardino Mountains, where it likely connects with the San Bernardino strand near Yucaipa Ridge. These observations combined with cross-sections across the mapped region suggest the geometry of faulting in this region is partitioned across several strands and has migrated across several unmapped strands, including an active and previously unmapped Mission Creek strand, and several subsidiary reverse faults adjacent to the Mission Creek left-restraining bend.