Evidence for an active and evolving left-stepping San Andreas fault (Mission Creek fault strand) from the Little San Bernardino Mountains to Yucaipa Ridge

Jesse Waco, Kimberly D. Blisniuk, & Julie C. Fosdick

Published August 15, 2017, SCEC Contribution #7849, 2017 SCEC Annual Meeting Poster #162

This study presents new detailed field mapping and high-resolution topographical data to provide evidence for an alternative interpretation of fault geometry and slip transfer for the southern San Andreas fault zone, specifically the Mission Creek and San Bernardino fault strands, through the San Gorgonio Pass region. Present geologic and geomorphic studies in the region indicate that as the southern San Andreas fault splits into the Mission Creek and Banning fault strands in the southern Indio Hills, the Banning fault strand accommodates the majority of lateral displacement across the San Andreas fault zone as it merges, into a zone of complexity, with the Garnet Hill and San Gorgonio Pass fault system. In this currently favored kinematic model of the southern San Andreas fault zone, slip along the Mission Creek fault strand decreases significantly northwestward toward San Gorgonio Pass and the San Bernardino Mountains. Along this restraining bend, the Mission Creek fault strand is considered to be less active to inactive since the late- to mid-Quaternary (~500-150 kya) due to the transfer of plate boundary strain westward to the Banning, Garnet Hill, and San Gorgonio Pass fault strands, the San Jacinto fault zone, and northeastward, to the Eastern California Shear Zone. Here, we present a revised interpretation of fault geometry from observed translated and uplifted mid- to late-Quaternary alluvial deposits that indicate a left-stepping Mission Creek fault strand that connects with the San Bernardino strand of the San Andreas fault near Yucaipa Ridge. Our observations, in combination with detailed sedimentary provenance analysis, suggest a dominant dextral strike-slip Mission Creek fault strand and a series of adjoining reverse faults that may connect with the Cox Ranch fault zone. We hypothesize that continuous large–scale translation of this structure, the Mission Creek fault strand, has occurred throughout its history into the present-day. Accordingly, the Mission Creek fault strand is active and likely a primary plate boundary fault through the San Gorgonio Pass at this latitude.

Key Words
active tectonics, neotectonics, geomorphology, san andreas fault, san gorgonio pass, mission creek fault, central transverse ranges, san bernardino mountains, yucaipa ridge

Citation
Waco, J., Blisniuk, K. D., & Fosdick, J. C. (2017, 08). Evidence for an active and evolving left-stepping San Andreas fault (Mission Creek fault strand) from the Little San Bernardino Mountains to Yucaipa Ridge. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)