A viscoelastic model of crustal deformation suggests that the formation and evolution of strike-slip fault systems are strongly influenced by rheologic contrasts between the upper and lower crust. When deformation is driven by a narrow zone of high shear in the mantle, the presence of a low-viscosity lower crustal layer underlying a primarily elastic upper crust widens the deformation zone with time and promotes the formation of a broadly distributed network of interacting faults within the upper crust. In contrast, the deformation zone in a primarily elastic crust is narrow, encompassing a single, plate-bounding fault. Patterns of surface strain rate and seismicity are thus significantly more complex in the presence of a low-viscosity lower crust, due to interactions between faulting in the upper crust at short time scales and viscous behavior in the lower crust at long time scales. ¿ 1998 American Geophysical Union