Stream Channel Offset and Late Holocene Slip Rate of the San Andreas Fault at the Van Matre Ranch Site, Carrizo Plain, California

Abstract

Sets of well-preserved channels offset across the San Andreas fault (SAF) at the Van Matre Ranch (VMR) site in the northwestern Elkhorn Hills area of the Carrizo Plain offer the opportunity to measure slip rate and examine geomorphic development of the channels. The fault zone and offset channels were exposed by excavation in one fault-perpendicular and five fault-parallel trenches. The geomor- phology and stratigraphy in the channels reveal a record of filling by fluvial sedi- mentation, lateral colluviation, and pedogenesis. The buried thalweg of the currently active channel is offset 24.8 1 m, while the geomorphic channel is offset approx- imately 27.6 1 m. Seventeen samples were collected from channel margin deposits for 14 C dating. An OxCal model of the radiocarbon dates with stratigraphic control suggests that the oldest date for channel incision was A.D. 1160. Minimum and maximum slip rates ranging from 29.3 to 35.6 mm/yr are derived from different assumptions about the timing of channel incision and offset. The resulting slip rates at VMR agree well with the late-Holocene slip rate of 33.9 2.9 mm/yr at Wallace Creek, approximately 18 km to the northwest, and imply that within measurement uncertainty the 30-37 mm/yr velocity gradient across the SAF from decadal time- scale geodetic measurements is accommodated across the several-meter-wide SAF zone at VMR over the last millennium.