MECHANICS OF OBLIQUE CONVERGENCE

The distribution and magnitude of the strike-parallel component of vel ocity in an obliquely converging thrust wedge or accretionary prism ar e determined by the geometry and mechanical properties of the wedge. A mechanical analysis based on the assumption of a critical or stable g eometry of the wedge, for which the rate of cross-strike deformation i s zero, leads to the following conclusions for different bulk rheologi es. (1) In a linear viscous wedge, the strike-parallel motion relative to the underthrust slab decreases exponentially away from the rear an d is effectively concentrated in a shear zone with a width comparable to the thickness of die wedge at the rear. The wedge also deforms by c orner flow, producing a circulation in the cross-strike plane. The str ike-parallel and corner flow velocities depend on the thickness and vi scosity of the wedge and on the shear stresses applied to its lower an d rear boundaries. Convergence at the wedge front is normal to strike. (2) A critically tapered perfect plastic wedge moves coherently witho ut internal deformation. For low and moderate obliquities of the conve rgence vector, the wedge moves at the same velocity as the backstop (u pper plate). For high angles of obliquity, the wedge moves laterally r elative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries, so that it is separated from the upper plate by a strike-slip fault, defining a for earc sliver. No geometrical configuration exists that allows the strik e-parallel motion to be distributed through the wedge. (3) A noncohesi ve Coulomb wedge behaves in much the same way as a plastic wedge, but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries, and they are independent of sca le.