Ramp initiation and spacing in a homogeneous thrust wedge

Abstract

Fold‐and‐thrust belts grow largely through sequential initiation of thrust faults and subsequent motion on them from hinterland to foreland, i.e., away from the plate boundary. While perturbations in basal geometry, stratigraphy, excess fluid pressure, folding, and surface slope have all been suggested as mechanisms for thrust ramp nucleation, we show that preexisting inhomogeneities are not necessary. We present a mechanically homogeneous finite element wedge model to assess the effects of variations in yield stress, dilation angle, and basal friction angle on foreland fold‐and‐thrust belt evolution. Computer experiments produce a broad foreland dipping plastic strain band that marks the topographic inflection produced by the previous ramp. This band migrates toward the rigid base, where the plastic strain is preferentially concentrated in a thrust ramp. Subsequent ramps develop toward the foreland in a self‐similar fashion. The ramp formation process is systematic. Ramps form at the same location with respect to the wedge origin regardless of significant variations in yield stress, dilation angle, or basal friction. However, the backstop displacement required to initiate a new ramp is dependent on these variables. We conclude that the wedge geometry is the primary factor in determining both thrust ramp location and spacing. Syntectonic erosion and deposition, or any other process that changes wedge geometry, may in some cases control the location of thrust ramps.