Morphologic dating of scarps formed by repeated slip events along the San Andreas Fault, Carrizo Plain, California

Abstract

Morphologic dating of fault scarps determines late Cenozoic fault activity by comparing observed topographic profiles with those determined using a calibrated hillslope development model. We postulate that the material transport rate along the profile is a function only of local slope and is transport‐limited. Morphologic dating of hillslopes bounded by continuously dropping boundaries or cut by continuously slipping faults is used to determine the material transport rate constant, the time since the downdrop was initiated, or the fault began to slip. We calibrated the hillslope development model on the southwest facing scarp southeast of Wallace Creek along the San Andreas Fault (SAF) in the Carrizo Plain, California. The scarp has been exposed by right‐lateral offset of a southeast sloping shutter ridge located on the southwest side of the SAF and by vertical offset related to secondary deformation. We assume that all the observed offset occurs after initial exposure of the scarp by passage of the shutter ridge. Forward modeling of profile development yielded a κ (mass diffusivity) of 8.6±0.8 m2 kyr−1. Normal fault slip rates were determined for two graben‐bounding faults in the Northern Elkhorn Hills in the southeastern Carrizo Plain by applying the calibrated κ to the degradation of the scarps to determine the scarp age. One fault scarp began to form about 12 kyr ago and the other about 63 kyr ago. Those ages and estimates of the dip slip along the faults result in slip rates of 1–2 mm yr−1.