MORPHOLOGIC DATING OF SCARPS FORMED BY REPEATED SLIP EVENTS ALONG THESAN-ANDREAS FAULT, CARRIZO PLAIN, CALIFORNIA

Morphologic dating of fault scarps determines late Cenozoic fault acti vity 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 bound ed 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 sc arp southeast of Wallace Creek along the San Andreas Fault (SAF) in th e Carrizo Plain, California. The scarp has been exposed by right-later al offset of a southeast doping shutter ridge located on the southwest side of the SAF and by vertical offset related to secondary deformati on. We assume that all the observed offset occurs after initial exposu re of the scarp by passage of the shutter ridge. Forward modeling of p rofile development yielded a kappa(mass diffusivity) of 8.6+/-0.8 m(2) kyr(-1). Normal fault slip rates were determined for two graben-bound ing faults in the Northern Elkhorn Hills in the southeastern Carrizo P lain by applying the calibrated kappa to the degradation of the scarps to determine the scarp age. One fault scarp began to form about 12 ky r ago and the other about 63 ky: ago. Those ages and estimates of the dip slip along the faults result in slip rates of 1-2 mm yr(-1).