Ramp initiation in a thrust wedge.

Abstract

Collisional mountain belts are characterized by fold and thrust belts that grow through sequential stacking of thrust sheets from the interior (hinterland) to the exterior (foreland) of the mountain belt. Each of these sheets rides on a fault that cuts up through the stratigraphic section on inclined ramps that join a flat basal fault at depth. Although this stair-step or ramp-flat geometry is well known, there is no consensus on why a particular ramp forms where it does. Perturbations in fault shape, stratigraphy, fluid pressure, folding, and surface slope have all been suggested as possible mechanisms. Here we show that such pre-existing inhomogeneities, though feasible causes, are not required. Our computer simulations show that a broad foreland-dipping plastic strain band forms at the surface near the topographic inflection produced by the previous ramp. This strain band then migrates towards the rigid base, where the plastic strain is preferentially concentrated in a thrust ramp. Subsequent ramps develop toward the foreland in a similar fashion. Syntectonic erosion and deposition may strongly control the location of thrust ramps by enhancing or removing the surface point of initiation.