MAPPING OF PLEISTOCENE TO HOLOCENE DEXTRAL FAULTING IN THE SAN GORGONIO PASS REGION

This study presents recent mapping of previously unidentified active right lateral strike-slip faults at the latitude of 34 N within the San Gorgonio Pass region of the San Andreas Fault in southern California. Geologic and geomorphic field mapping across an approximately 12-km-wide region in Banning, Oak Glen and Mile High canyons and Yucaipa Ridge reveals multiple fault strands that offset geomorphic features in Quaternary alluvium and local bedrock. In Banning Canyon, we observe a stream incised into an alluvial fan that is offset by ~40 m. In Oak Glen, we observe a deflected stream and fault scarps offsetting alluvial fans near 34.0313 N, approximately 3 km southwest from the current mapped active fault strand. In Mile High Canyon, detailed mapping reveals a flight of uplifted terraces abandoned from their source, likely Banning Canyon. Additionally, fault gouge is observed in Mile High and Banning canyons where these fault strands are mapped in the local bedrock. Further mapping of these newly identified fault strands, Quaternary geochronology and provenance studies of sediment samples from alluvial fans, terraces, and active washes will be used to determine fault displacement over multiple Quaternary timescales and site locations. This work will provide new insights into the Quaternary evolution of the San Andreas Fault in the San Gorgonio Pass region of southern California.