Crustal rheology and faulting at strike-slip plate boundaries 2. Effects of lower crustal flow

We present a numerical model of deformation at a strike-slip plate boundary within a linear viscoelastic crust, which is driven by far-field plate mot ions and basal mantle velocities. The crust is assumed to have uniform elas tic properties but continuously varying viscosity as a function of depth. B rittle faulting is represented by static elastic dislocations that are impo sed when stresses exceed a critical threshold for fracture or frictional sl iding. The locations and depth extents of faults in this model are not pres pecified but, instead, are governed by stress evolution within the crust. W e find that when a primarily elastic upper crust is underlain by a low-visc osity lower crustal layer, the deformation zone broadens in time to encompa ss many parallel strike-slip faults in an interacting network. In contrast, when the entire crust behaves elastically, the deformation zone remains na rrow and focused on a single plate-bounding fault, reflecting imposed mantl e motions. Surface strain rate patterns within the interacting fault networ k are complex and reflect significant faulting-related strain rate perturba tions that decay over timescales of postseismic relaxation in the lower cru st (10-100 years). The fault network has a characteristic spacing, with com plex fault interactions and with the depth extents of faults increasing wit h time to a maximum depth governed by crustal rheology. The maximum depth o f faults is limited by stress relaxation and large-scale viscous flow in th e lower crust, which confines brittle failure to shallow and midcrustal lev els.