EVIDENCE OF ACTIVE REVERSE FAULTING AND A LEFT-STEPPING SAN ANDREAS FAULT (MISSION CREEK STRAND) FROM THE LITTLE SAN BERNARDINO MOUNTAINS TO YUCAIPA RIDGE

This study presents new bedrock and geomorphic field mapping to evaluate fault geometry and slip transfer of the southern San Andreas fault, specifically the Mission Creek and San Bernardino strands within the San Gorgonio Pass region. Previous studies in the region indicate that as the Coachella Valley segment of the San Andreas fault splits into the Mission Creek, Banning, and Garnet Hill 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. In this currently favored kinematic model of the southern San Andreas fault, slip along the Mission Creek strand decreases significantly northwestward toward San Gorgonio Pass and the San Bernardino Mountains. Here, the Mission Creek strand is considered inactive to less active because deformation along the restraining bend of the San Andreas fault has transferred slip to the south, towards the Banning, Garnet Hill, and San Gorgonio Pass strands, the San Jacinto fault zone, and northeastward, to the Eastern California Shear Zone. Our new mapping shows evidence of multiple reverse and strike-slip faults in a region adjacent to the Mission Creek strand where previous mapping shows no faulting. We observe uplifted and tilted mid to late Pleistocene alluvial deposits juxtaposed on Cretaceous igneous-metamorphic basement rock that suggests active reverse faulting. Additionally, we also observe laterally offset ridges and drainages along an unmapped trace of the Mission Creek strand. These observations, in combination with detailed sedimentary provenance analysis, suggest the geometry of faulting in this region is partitioned into a series of left-stepping strike-slip and subsidiary reverse faults.