The kinematics and mechanics of oblique ramp deformation within fold-and-thrust belts

Abstract

The South Fork thrust, a hanging wall imbricate of the Absaroka thrust sheet, southwest Wyoming, has a high angle (~70°), small displacement (~900 ft.), oblique ramp in the hanging wall and footwall. Two penetrative deformation events were recorded by folding, stylolitic cleavage, and calcite twinning strains. The first, with east-directed "regional" shortening, is attributed to early layer-parallel shortening. The second event suggests northeast-directed shortening unique to the oblique ramp, nearly perpendicular to its strike. A systematic change in the orientation of bedding and fractures near the oblique ramp suggests that the hanging wall also underwent a quasi-rigid body rotation as it was pinned near its western terminus. 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 behavior: vertical shear and layer-parallel shear. In the former case, material is sheared vertically and displacements remain within the tectonic transport plane. In the later case, material is deflected out of the transport plane. The deflection and out-of-plane shear strains are zero for frontal and lateral ramps, and maximum at an intermediate oblique orientation, depending on ramp dip. At frontal ramp-oblique ramp intersections, the deflection may cause local strike-parallel extension or shortening. Hanging wall motion over footwall topography has been modeled by solving a 3-D boundary value problem, in which the hanging wall is represented by a linear viscous half-space sliding over a low dip, frictionless, sinusoidal interface. One interface resembles a series of basins and domes, and is the locus of out-of-plane displacements, velocities, and stresses. In a second model, the basic velocity is at an oblique angle to the strike of the axis of the cylindrical interface. The maximum and minimum compressive stress and perturbed velocity field are always oriented in a plane perpendicular to the cylinder axis, and their magnitudes are a maximum for dip-slip, and minimum for strike-slip. The total velocity field is locally deflected out of the transport plane. The magnitude of the deflection is zero for dip-slip and strike-slip, and a maximum at an intermediate value.