PHYSICAL MODELING OF PRIMARY AND PROGRESSIVE OROGENIC CURVATURE

Displacement paths of curved orogenic systems are based on field observations of timing relationships between thrusting and vertical axis rotations. In the present study, physical analog models permit systematic investigation of the evolution of primary and progressive curvature, and identification of characteristic structures associated with these curvature types.

Models were constructed of colored sand layers overlying thin, rigid, plastic sheets. In primary curve models, layers in the hanging wall moved over an arc-shaped footwall ramp. For progressive curve models, differential displacement of parallel plastic strips caused thrust faulting to nucleate in the center of each model and propagate laterally, resulting in growth of an arcuate thrust salient. Models with straight footwall ramps perpendicular to the shortening direction served as experimental controls. Surface deformation was tracked throughout each experiment using a colored sand grid with 2 cm spacing, and models were dissected after deformation to reveal serial cross sections.

Primary curve deformation produced insignificant surface rotation, whereas the progressive curve models experienced surface rotation in the arc corners. Tear structures and wrench faults near the axes of rotation developed only in the progressive curve models. In both primary and progressive curve models, thrust fault dips decreased from the center of the salient to the edges. Thrust faults at the center of the arc of the primary curve models formed at dip angles similar to the straight, control model thrust faults.

Conceptual models of curved orogens have predicted vertical axis rotation associated with progressive curves, and an absence of rotation associated with primary curves. Our experiments substantiate this interpretation, and they reveal structures and characteristics that may be useful to differentiate between curvature processes in the field.