Kinematic models of deformation at an oblique ramp

Abstract

Kinematic models for the deformation of hanging wall material moving over a footwall oblique ramp are developed by considering two end members of assumed mechanical behaviour, vertical shear and layer-parallel shear. In the former case, material is sheared vertically and displacements remain within the tectonic transport plane; the deformation is accommodated by thinning of the hangingwall over the ramp. In the later case, material is deflected out of the transport plane such that the pitch angle of the hangingwall particle path in the plane of the oblique ramp is equal to the angle between the transport direction and the strike of the oblique ramp. As a result, shear strains above the oblique ramp are non-zero in both the transport and transport-normal planes. The deflection and transport-normal shear strains are a minimum for the special cases of pure frontal and lateral ramps, and maximum at an intermediate orientation, depending on oblique ramp dip. Fault-bend folds are similar in most respects for both vertical shear and layer-parallel shear mechanisms. At frontal ramp — oblique ramp intersections, synformal or antiformal multiple bends in the footwall generate, respectively, second order hangingwall synclines or anticlines, which terminate along strike into simple fault-bend folds. For the layer-parallel shear mechanism along the pure oblique ramp, deflected hangingwall material passes through the transport plane, conserving area and volume. At the rearward intersection zone (concave toward the transport direction), hangingwall material diverges resulting in local strike-parallel extension. This extension may be a mechanism for the generation of transverse faults (or ‘tear faults’) in the hangingwall. At the forward intersection zone (convex toward the transport direction), displacement paths converge resulting in local strike-parallel shortening. The attitude of the oblique ramp and the amount of displacement significantly affect the map geometry and magnitude of lateral strains.