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 motions and basal mantle velocities. The crust is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth. Brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding. The locations and depth extents of faults in this model are not prespecified but, instead, are governed by stress evolution within the crust. We find that when a primarily elastic upper crust is underlain by a low-viscosity lower crustal layer, the deformation zone broadens in time to encompass many parallel strike-slip faults in an interacting network. In contrast, when the entire crust behaves elastically, the deformation zone remains narrow and focused on a single plate-bounding fault, reflecting imposed mantle motions. Surface strain rate patterns within the interacting fault network are complex and reflect significant faulting-related strain rate perturbations that decay over timescales of postseismic relaxation in the lower crust (10--100 years). The fault network has a characteristic spacing, with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology. The maximum depth of faults is limited by stress relaxation and large-scale viscous flow in the lower crust, which confines brittle failure to shallow and midcrustal levels. ¿ 2000 American Geophysical Union