Crustal rheology and faulting at strike-slip plate boundaries 1. An analytic model

This paper investigates how rheologic stratification within the crust affec ts 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-sl ip plate margin within a two-layer viscoelastic crust, with fixed shear mod ulus but varying viscosity in each layer. Faulting is represented by static elastic dislocations of fixed, shallow depth extent, imposed at a high cri tical stress threshold for fracture of a new fault and a lower one for slid ing on a preexisting fault. To drive crustal deformation, we impose basal v elocity 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 stres ses. Our results suggest that when a primarily elastic/brittle upper crust is underlain by a low-viscosity lower crust, the deformation zone in the up per crust widens in time. At steady state the surface width of the deformat ion zone may be significantly greater than the prescribed, narrow mantle sh ear zone. The long-term width of the deformation zone increases with the vi scosity contrast within the crust and with the thickness of the low-viscosi ty lower crust. Widening of the deformation zone is accompanied by the frac ture 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 p late 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 con trast within the crust.