The distribution and magnitude of the strike-parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge. A mechanical analysis based on the assumption of a critical or stable geometry of the wedge, for which the rate of cross-strike deformation is zero, leads to the following conclusions for different bulk rheologies. (1) In a linear viscous wedge, the strike-parallel motion relative to the underthrust slab decreases exponentially away from the read and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear. The wedge also deforms by corner flow, producing a circulation in the cross-strike plane. The strike-parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries. Convergence at the wedge front is normal to strike. (2) A critically tapered perfect plastic wedge moves coherently without internal deformation. For low and moderate obliquities of the convergence vector, the wedge moves at the same velocity as the backstop (upper plate). For high angles of obliquity, the wedge moves laterally relative 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 forearc sliver. No geometrical configuration exists that allows the strike-parallel motion to be distributed through the wedge. (2) A noncohesive 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 scale. ¿ American Geophysical Union 1993