Slip Transfer and the Growth of the Indio and Edom Hills, Southern San Andreas Fault

Abstract:

Emerging work on the southern San Andreas Fault reveals a complex pattern of deformation as the fault splays into the Mission Creek, Banning, and Garnet Hill strands where the fault system bounds the Indio Hills and Edom Hill. Work at several sites along the Mission Creek strand show that Quaternary through Holocene slip rates are high (14-21mm/yr; Behr et al., 2010; Blisniuk et al., 2014). Holocene slip rates are low and fairly constant, both at a site 8 km from the SW end of the Banning (2-6 mm/yr, this study) and at another site 30 km to the NW (4-5 mm/yr, Gold et al., in press), suggesting slip is efficiently and completely transferred onto the Banning strand. No slip rates exist for the Garnet Hill strand. We compare the horizontal slip rates with structural and geomorphic indices of uplift and deformation in the Indio Hills and Edom Hill using (1) long-term Tertiary bedrock exhumation rates, (2) short-term 10Be-derived, catchment-averaged erosion rates, (3) catchment relief and area and (4) channel steepness, normalized for drainage area. Each metric suggests progressive increase in uplift to the NW along the southwestern 15 km of the Indio Hills; at the NW end of the hills, the abrupt tapering of elevation suggests a termination of uplift. This is consistent with geophysical studies that confirm no resolvable bedrock in the area (Catchings et al., 2009; Fuis et al., 2012). Long-term exhumation and geomorphic indices indicate uplift of Edom Hill is relatively high (10Be rates are pending), so we hypothesize that the location of Edom Hill just W of the peak in uplift of the Indio Hills reflects complete transfer of vertical slip off the Banning strand onto the Garnet Hill strand which dips below Edom Hill. Overall, the fault geometry and uplift patterns are consistent with the more western orientation of each successive splay; the orientation produces a geographically restricted pulse in bedrock uplift concentrated within the intersection of the fault strands.