NEW TOOL FOR 3D THERMO-MECHANICAL MODELING OF LITHOSPHERIC SCALE DEFORMATION WITH APPLICATION FOR EVOLUTION OF SAN ANDREAS FAULT SYSTEM

Modeling of highly complex deformation of the San Andreas Fault System (SAFS) requires development of modelling technique capable of handling both brittle and ductile flows at extreme strains within single computational domain. To approach this problem we use fully implicit backward Euler algorithm to simultaneously integrate equations of mass, energy and momentum conservation using large time step and standard first order hexahedral finite elements. To resolve extreme strains, we utilize Arbitrary Lagrangian Eulerian kinematical formulation. We track stresses, material phases and other related variables between old and new computational mesh by particle-in-cell method. Computational mesh is completely decoupled from material flow allowing for any type of boundary conditions, including open boundaries, moving calculation window etc. We employ realistic, highly complex non-linear temperature- strain- and stress-dependent elasto-visco-plastic rheology. Brittle (plastic) component includes non-associative Drucker-Prager type model with pressure- and strain-dependent yield stress. Ductile (viscous) rheology assumes simultaneous activity of three thermally activated creep mechanisms: Newtonian, Power Law and Peierls. Among other rheological effects we include elasticity and thermal expansion.

We apply this modeling technique to simulate evolution of SAFS in central and northern California during the last 20 Myr due to the northward migration of the Mendocino triple junction. The model domain includes opening slab window overplayed by the North America plate with the subducted Gorda plate at the model northern boundary, Pacific plate margin at the western boundary and the Great Valley block at the eastern boundary. Massive numerical experiments are just underway, their results being systematized to be reported in this presentation. Preliminary results are very encouraging, showing, for instance, that advection of asthenosphere within the slab window and motion of the Gorda plate can cause significant deformation of the North American lithosphere overlaying slab window, explaining major features of its surface topography and variations of crustal thickness.