IMPOSTER OFFSETS AND CLIMATIC MODULATION OF FAULT-CROSSING STREAMS: A CASE STUDY AT THE PEARBLOSSOM SITE ON THE MOJAVE SECTION OF THE SAN ANDREAS FAULT, CA

Pulses of sediment aggradation during wet periods can complicate the reconstruction of a slip-rate site by producing “imposter offsets” – landforms that develop with an initial offset or deflected configuration that is easily misread as tectonic offset caused by fault slip. Here we show a case study at the Pearblossom site along the Mojave section of the San Andreas fault (MSAF), where we use optically stimulated luminescence dating to 1) reconstruct a beheaded channel with a genuine offset of 24-49 m since between 1.44 ± 0.43 ka and 1.27 ± 0.18 ka, yielding a slip rate of 16-38 mm/yr, and 2) date an alluvial fan that was emplaced at ~0.6 ka with an imposter offset of 36-88 m. The imposter offset formed during a pulse of aggradation at ~0.6 ka both upstream and downstream of the fault. This pulse of aggradation coincides with a wet period in Southern California precipitation records and indicates that emplacement of the imposter fan was likely due to climatically-modulated deposition at the site. Comparing a compilation of charcoal ages from many sites in the Mojave region with precipitation records suggests that other sites may be similarly affected. Although such climatic effects can complicate slip-rate studies, we show how the morphology and upstream position of the deflected channel can provide an indication of whether a site is likely to record useful information about fault slip. A deflected channel that is narrow relative to the width of its deflection is likely to have incised in a deflected configuration. For a channel crossing a right-lateral fault and viewed looking downstream, a deflected channel incised on the left side of an alluvial fan gives a minimum offset measurement, while a deflected channel incised on the right side gives a maximum offset measurement. Determining histories of Holocene fault slip is essential for understanding seismic hazards and the development of regional fault systems. Using these criteria to assess potential slip-rate sites prior to field study can help maximize the efficiency of field time and resources.