This paper investigates how rheologic stratification within the crust affects the formation and long-term evolution of fault systems at a strike-slip plate boundary. We present an analytic model of deformation at a strike-slip plate margin within a two-layer viscoelastic crust, with fixed shear modulus but varying viscosity in each layer. Faulting is represented by static elastic dislocations of fixed, shallow depth extent, imposed at a high critical stress threshold for fracture of a new fault and a lower one for sliding on a preexisting fault. To drive crustal deformation, we impose basal velocity boundary conditions at the Moho representing a narrow zone of high shear in the mantle. In this study we restrict attention to deformation at the surface, where simple analytic solutions exist for velocities and stresses. Our results suggest that when a primarily elastic/brittle upper crust is underlain by a low-viscosity lower crust, the deformation zone in the upper crust widens in time. At steady state the surface width of the deformation zone may be significantly greater than the prescribed, narrow mantle shear zone. The long-term width of the deformation zone increases with the viscosity contrast within the crust and with the thickness of the low-viscosity lower crust. Widening of the deformation zone is accompanied by the fracture of new faults in the upper crust, leading to the formation of a system of parallel strike-slip faults surrounding the plate boundary. For fixed plate velocity and fracture criteria, we find that the width of the brittle fault network within the total (distributed) deformation zone is primarily governed by strength properties of the upper crust and by the viscosity contrast within the crust. ¿ 2000 American Geophysical Union