NEW TERRESTRIAL LIDAR AND COSMOGENIC RADIONUCLIDE CONSTRAINTS on THE LITTLE LAKE FAULT, EASTERN CALIFORNIA SHEAR ZONE

An ever-expanding inventory of fault slip-rates spanning various time intervals reveals contrasting spatial and temporal patterns of strain for active faults within the Eastern California shear zone (ECSZ). Comparison of these data, derived from geologic, geomorphic, and paleoseismic records, with geodetic estimates of fault loading affords unparalleled opportunity to investigate the dynamics of earthquake processes and the evolution of an intracontinental plate boundary fault system. We focus here on the Little Lake fault, which occurs along the western margin of the ECSZ between the Sierra Nevada and the Coso Range in east-central California. The fault accommodates between 10-20% of the total dextral motion within the ECSZ at this latitude and a smaller fraction of the relative motion between the Pacific and North American plates. Relatively high rates of decadal fault loading described for the Little Lake fault zone from GPS measurements and InSAR data suggest potential discrepancies with longer, late Quaternary records of geologic strain. To address this inconsistency, we targeted a series of fluvial terraces related to overtopping and outflow from pluvial Owens Lake that cross the fault and record dextral offset since Late-Pleistocene time. High-resolution topographic surveying using a tripod-mounted laser scanner suggests between 33 and 38 m of reconstructed right-lateral displacement of two individual terrace risers over this time period. Coupled with cosmogenic ¹⁰Be dating of intact, meter-scale outwash boulders preserved on terrace treads bounding each riser, detailed characterization of these geomorphic features provides a robust geologic slip-rate estimate for the Little Lake fault zone at the ~10⁴ year timescale. Preliminary correlation of terrace surfaces with available exposure dating of fluvially scoured basalt upstream at Fossil Falls suggests a right-lateral displacement-rate on the order of ~2 mm/yr. Although more than double the previously reported rate estimate (≤ 1 mm/yr) based on older, poorly constrained basalt flows, this result tentatively suggests that decadal rates of loading (≥ ~5 mm/yr) on the Little Lake fault zone are not sustained geologically over longer periods of time.