Modeling evolution of the San Andreas Fault system in northern and central California

We present a three-dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the San Andreas Fault system (SAFS) in northern and central California over the past 20 Myr. More specifically, we investigate the mechanisms responsible for the eastward (landward) migration of the San Andreas plate boundary over time, a process that has largely determined the evolution and present structure of SAFS. Two possible mechanisms had been previously suggested. One mechanism suggests that the Pacific plate first cools and captures uprising mantle in the slab window, subsequently causing accretion of the continental crustal blocks. An alternative scenario attributes accretion to the capture of plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Here we test both these scenarios numerically using a recently developed lithospheric-scale code, SLIM3D, that employs free surface, nonlinear temperature-and stress-dependent elastoviscoplastic rheology and allows for self-generation of faults. Modeling suggests that microplate capture is the primary driving mechanism for the eastward migration of the plate boundary, while the slab window cooling mechanism alone is incapable of explaining this phenomenon. We also show that the system evolves to the present day structure of SAFS only if the coefficient of friction at mature faults is low (0.08 for the best fit model). Thus, our model provides an independent constraint supporting the "weak fault in a strong crust" hypothesis for SAFS.