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Using B4 LiDAR And CRN Age Data To Constrain Slip Rates Along The San Andreas Fault System At Millard Canyon, San Gorgonio Pass


Fault scarps cut a series of Holocene alluvial fan surfaces in Millard Canyon, within the San Gorgonio Pass (SGP). These fault scarps are likely the result of coseismic slip along the San Andreas Fault system during potentially large magnitude (Mw7+) earthquakes. Here we provide new ages for Holocene surfaces Qf2, Qf3, and Qf4. Charcoal fragments beneath Qf2 limits the surface to 1270 ± 80 years before present (ybp) and new 10Be exposure ages from the two older Holocene surfaces provide age constraints of 4800 ± 1600 ybp for Qf3 and 6800 ± 550 ybp for Qf4. These new ages provide limits on the timing of slip through the San Gorgonio Pass. Airborne LiDAR from the B4 dataset was used to identify and measure preserved scarps that cut the terrace surfaces. The northernmost fault (F1) with an observed northward dip of 45 degree vertically offsets units Qf2 and Qf3 by 1.4 ± 0.7m and 2.9 ± 0.5m respectively. The southern fault (F2), a 30 degree north dipping active oblique strike slip thrust fault, vertically offsets units Qf1 and Qf4 by 1.5 ± 0.6m, and 12.7 ± 1.4m respectively. Geomorphic evidence suggests a roughly 3:1 lateral to vertical slip relationship, and with this we mathematically resolve these vertical slip parameters onto their respective fault plane geometries to evaluate the strike slip component of motion. The strike slip component, in conjunction with the age constraints gives the following Holocene strike slip rates: northern fault (F1): 1.6 ± 1.1mm/yr; southern fault (F2): 5.4 ± 1.1mm/yr. Summation of these rates across the study area yields 7.0 ± 2.2 mm/yr for the Holocene strike slip rate through the San Gorgonio Pass. These faults, suspected of carrying the majority of San Andreas motion through the SGP are interpreted to release interseismic strain during large magnitude earthquakes of Mw 7 or greater (Yule and Sieh, JGR 2003).