Constraining long term slip rates along the San Andreas Fault System using B4 LiDAR and Cosmogenic Beryllium-10 dating methods at Millard Canyon, San Gorgonio Pass, California

Fault scarps cut a series of Holocene alluvial fan surfaces in Millard Canyon, within San Gorgonio Pass (SGP). These fault scarps are likely the result of coseismic slip along the San Andreas Fault system during potentially large (Mw >7) earthquakes. Here I provide a new age for Holocene surface Qf4, mapped by Yule and Sieh (2003). Charcoal fragments beneath Qf2 limits the surface to 1270 ± 80 years before present (ybp) and new 10Be exposure data provides 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 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) is a 45° north dipping active oblique strike slip thrust fault that vertically offsets units Qf2 and Qf3 by 1.8 ± 0.1 m and 3.1 ± 0.7 m respectively. The southern fault (F2), a 25° north dipping active thrust fault vertically offsets units Qf1 and Qf4 by 1.9 ± 0.2 m and 12.7 ± 1.4 m respectively. I mathematically resolve these vertical throw components into a set of fault plane kinematics and rates including uplift rate, dip slip rate and contraction rates using the new age constraints. I then interpret likely per event slip rates on each fault by incorporation of know recurrence intervals for faults in Millard Canyon (McBurnett, 2011). The northern fault F1 is shown to have a long-term dip slip rate of 2.0 ± 0.1 mm/yr and an interpreted per event net slip of 2.8 ± 0.3 meters. The southern fault shows evidence of primarily dip slip at a rate of 4.4 ± 0.6 mm/yr and a per event dip slip rate of 2.2 ± 0.3 meters. Summation of this local SGP slip yields 6.4 ± 0.6 mm/yr for the Holocene dip slip rate through SGP. Summation of the observed uplift yields 3.3 ± 0.3mm/yr through the Holocene. These faults are the primary conduit for slip through SGP and are interpreted to release interseismic strain during large magnitude earthquakes of Mw >7 (Yule and Sieh, 2003).