Characterizing the Quaternary slip history of an unmapped strand of the southern San Andreas fault near San Gorgonio Pass through sedimentary provenance and stratigraphy

Leigh M. Musick, Kim D. Blisniuk, Julie C. Fosdick, & Patrick O'Connor

Submitted August 30, 2026, SCEC Contribution #15516, 2026 SCEC Annual Meeting Poster #TBD

This study presents the mid to late Quaternary slip history of a recently identified unmapped strand of the San Andreas fault in the San Gorgonio Pass region of Southern California through detailed stratigraphy and sedimentary provenance. This recently identified and mapped fault strand may be an important structure in accommodating slip in this geometrically complex region of the San Andreas fault, often referred to as the San Gorgonio Knot. Here, a major restraining bend along the fault contributes to the complex fault geometry and partitioning of slip across a number of fault strands in the region. Current kinematic models show that the majority of slip is accommodated by the Banning and San Gorgonio Pass fault strands, however, these models do not account for slip along this recently identified fault strand. This study presents a sediment provenance analysis of stranded debris flow deposits that have been offset from their source along this unmapped fault strand. Modal petrographic point count data show that Pleistocene debris flow deposits near Banning Canyon have similar composition to active wash deposits in Whitewater River 20 km to the east, and dissimilar composition to the nearest sediment transport systems. Cumulative age distributions of detrital zircon ages show that Pleistocene debris flow sediments contain zircons that are dissimilar in age to zircons found in Banning Canyon active wash sediments, suggesting that the debris flows were originally sourced from drainages to the east of Banning Canyon.

Citation
Musick, L. M., Blisniuk, K. D., Fosdick, J. C., & O'Connor, P. (2026, 08). Characterizing the Quaternary slip history of an unmapped strand of the southern San Andreas fault near San Gorgonio Pass through sedimentary provenance and stratigraphy. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology