LATE-QUATERNARY SLIP-RATE OF THE SOUTHERN SAN ANDREAS FAULT INFERRED FROM LANDSCAPE MODELING OF SHEARED DRAINAGES

Major strike-slip fault systems are common at plate boundaries and create significant seismic hazard. Understanding the magnitude of the hazard and the interplay of slip distribution on the various fault strands requires constraints on modern and past fault slip rates. Slip rate data has been obtained from measurements on offset of discrete, dateable geomorphic features across a fault. However, for many faults, the low number and preservation potential of such features precludes collection of this data at high spatial and temporal densities. We investigate whether basin and river channel topology can be used to constrain slip rate data on geological timescales, taking the San Andreas Fault (SAF) of southern California as an example. We examine a series of river basins that have been deformed by right lateral distributed shear in the Mecca Hills, CA. The basins show a consistent change in trend of their main channels: the headwaters tend to be oriented perpendicular to the main trace of the SAF, whereas lower reaches become more closely fault-parallel with increasing proximity to the fault. We attempt to extract information on the long-term slip rate since ~700 ka of the SAF by simulating these drainages with the Landlab 1.0 modeling framework. Using Landlab, we forward model river basin evolution in a landscape of diffusive hillslopes and stream power channel incision experiencing both uplift and simple shear. The orientation of the basins seems to reach a quasi-steady condition in which the deformation increasing basin curvature is countered by stream piracy, which serves to straighten networks. We quantify the change in curvature over time using geomorphic metrics derived from hillslope aspect, total relief, and mean elevation. The simulations reveal a correlation between the geomorphic metrics and the slip rate, which implies that extraction of slip-rate information is possible if rate parameters such as fluvial erosion can be constrained.