Distinguishing fault offset from climatically modulated channel deflections: Insights from the Pearblossom slip-rate site, Mojave San Andreas fault, California, USA

Emery O. Anderson-Merritt, Eric Cowgill, Amanda Keen-Zebert, Elaine K. Young, & Mary Dailey

Awaiting Publication December 7, 2023, SCEC Contribution #13380

Fault slip histories are essential for understanding seismic hazards and regional fault system development and depend on correctly identifying, dating, and reconstructing displaced markers. Here we use a case study of the Pearblossom site along the Mojave section of the San Andreas fault (MSAF) to show how pulses of sediment aggradation during wet periods can complicate such reconstructions by producing “imposter offsets” – landforms that develop with an initial deflection that is easily misread as tectonic displacement caused by fault slip. Specifically, we use optically stimulated luminescence dating to both reconstruct a beheaded channel with a genuine offset of 24-49 m since 1.44 ± 0.43 ka to 1.27 ± 0.18 ka, yielding a slip rate of 16-38 mm/yr, and to date an alluvial fan deposited at ~0.6 ka with a channel that has an imposter offset of 36-88 m. The imposter offset formed during a pulse of aggradation that coincides with a wet period in Southern California precipitation records. We attribute formation of the imposter offset and the alluvial fan into which it incised to climatically modulated deposition at the site. Comparing precipitation records with charcoal ages compiled from multiple Mojave-region locations suggests that other slip-rate sites may be similarly affected. Although climatic effects can complicate slip-rate studies, we show that the morphology and upstream position of the deflected channel can indicate whether a site likely records useful information about fault slip.

Citation
Anderson-Merritt, E. O., Cowgill, E., Keen-Zebert, A., Young, E. K., & Dailey, M. (2023). Distinguishing fault offset from climatically modulated channel deflections: Insights from the Pearblossom slip-rate site, Mojave San Andreas fault, California, USA. Geosphere, (awaiting publication).