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

Fault slip histories are essential for understanding seismic hazard and regional fault system development but fundamentally depend on 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 in California (USA) 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 document two channels on the downstream side of the fault: a subtle one that we interpret to have been beheaded and displaced 24- 49 m from a source channel on the upstream side of the fault, and a second and more prominent one that we interpret as an imposter offset of 36-88 m. Using optically stimulated luminescence dating, we determine that the source channel incised between 1.44 +/- 0.43 ka and 1.27 +/- 0.18 ka with a subsequent phase of alluvial fan aggradation at similar to 0.6 ka, when the channel with the imposter offset formed. Because the pulse of fan deposition coincides temporally 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 Desert 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.