Application of a new 3D Finite-element Elasto-visco-plastic Modeling Technique to Simulate Origination and Evolution of the San Andreas Fault System

The San Andreas Fault System (SAFS) in central and northern California is a complex of faults that accommodate the relative motion between the Pacific plate and the North American plate. This system began to develop about 20 Myr ago in response to the northward migration of the Mendocino triple junction. As the triple junction migrated northward along the plate boundary, the slab being subducted beneath North America was replaced by hot asthenospheric material in a slab window or slab gap and the transform deformation along the plate boundary developed simultaneously with thermal re-equilibration of the lithosphere. Large spatial (several 100 km) and temporal (20 Myr) scale, as well as essentially 3D style and strongly non-linear character of the associated brittle-ductile deformation processes require new efficient modeling technique. Our approach is based on the implicit time integration of momentum, mass and energy conservation equations and employs temperature- and stress-dependant elastoviscoplastic rheology. The code called SLIM3D ( Semi-Lagrangian Implicit Modeler) combines Lagrangian formulations and particle-based remeshing procedure. The locking-free hexahedral finite element with hourglass control is adopted to maintain robustness and efficiency of computation. The notion of consistent linearization of stress update algorithm and derivation of tangent modulus tensor is used to achieve optimal convergence rate of global Newton-Raphson equilibrium iteration at a large time step. First results of a full 3D modeling demonstrate that the general features of the SAFS can indeed result from strike-slip deformation of the continental margin subjected to the strong heating and then cooling according to the "slab-window" scenario. The key factor controlling spacing of the major faults is the amount of friction weakening achieved at the major faults, in accord with the results of our previous extended 2D models. The model-predicted faults distribution as well as deformation and thermal patterns are consistent with the SAFS, if friction coefficient at faults can be decreased at high shear strain by at least 3-5 times relative to its usual value of 0.5-0.7. We also discuss effects on SAFS deformation patterns of 3D lithospheric heterogeneity of North America margin and boundary tractions imposed by the Gorda plate.