COMBINING NEW AIRBORNE LIDAR DATA AND PROVENANCE OF ALLUVIAL FAN DEPOSITS TO CONSTRAIN LONG-TERM OFFSETS ALONG THE ELSINORE FAULT IN THE COYOTE MOUNTAINS, IMPERIAL VALLEY, CALIFORNIA

The Elsinore-Laguna Salada fault is part of the San Andreas fault (SAF) system in southern California, extending 250 km from the Los Angeles Basin southeastward into Mexico. The slip rate on the southern Elsinore fault is believed to be low, based on recent InSAR observations, although a recent study near Fossil Canyon suggests a rate in the range of 1-2 mm/yr. However, the fault is well-expressed in the geomorphology in the Coyote Mountains, which argues for a reassessment of the long-term rate along this branch of the SAF system. For this study, we used the new, recently acquired airborne LiDAR dataset (EarthScope Southern & Eastern California, SoCal) to map alluvial offsets in the Coyote Mountains in western Imperial Valley. We reprocessed the point clouds to produce DEMs with 0.5m and 0.25m grids. We varied the hill shade and insolation angles to illuminate and map the various fault strands and alluvial deposits. This allowed us to detect not only the major offsets along the primary fault strands, but also the many secondary minor faults. We identified numerous offset features, such as rills, channel bars, channel walls, alluvial fans, beheaded channels and small erosional basins that varied in displacement from 1 to 350 m. These are compared to measurements on many of the same features in the field. For the larger offsets, we recognize that older alluvium that is offset greater amounts is commonly buried beneath younger pulses of alluvial fan deposition, and are separated by buried soils. To determine the actual source canyon of various alluvial elements, we quantified the clast assemblage of each source basin and each alluvial fan on both sides of the fault. To accomplish this, we used a portable grid and classified more than 300 clasts at each of more than 35 sites along the fault. We found a very good fit between displaced alluvial fan elements and their inferred source canyons, but a poor match with the alluvium from neighboring canyons, which allows us to resolve long-term offset. Future dating of the pedogenic carbonate associated with these buried soils will allow resolution of the long term slip rates over multiple time frames to test the constancy of fault slip rate during the late Quaternary, as well as to test the lateral variations in rate along the length of the fault.