STRESSES BENEATH A RAMPING THRUST SHEET

There is a systematic progression of both the timing of motion of thrust sheets and the spacing of thrust ramps during the development of some thrust-dominated foreland fold-and-thrust belts. On the basis of these observations, we suggest that a regional mechanical process that controls frontal ramp formation may dominate local effects such as stratigraphic pinchouts and interaction with basement. To investigate this process we have developed elastic, plane strain models of the state of stress in the footwall of the last formed, or most forward thrust ramp. These models include the load of an overriding thrust sheet and a synorogenic sedimentary cover. The effect of emplacing a thrust sheet or developing a synorogenic sedimentary cover above an incipient thrust sheet is to stabilize the footwall toward the hinterland with respect to failure, and thereby move failure more to the foreland. Failure is localized where there is a favorable trade-off between the spatial rates at which mean stress and differential stress decrease toward the foreland. The models also predict that frontal ramps nucleate well above the basal decollement and propagate upward and downward eventually linking with the regional decollement. Fault trajectories predicted by using a Coulomb criterion are subhorizontal in the hinterland portion of the model and curve upward to the foreland resulting in a curved 'flat and ramp' geometry. These analyses show that each successive thrust sheet would be thinner, as well as shorter, than the previous thrust sheet. Based on these results, we propose a model for the formation of the first 'thin-skinned' thrust sheet in the external portions of orogenic belts. The synorogenic sediments that form during uplift of the internal zones of mountain belts will cover the preorogenic strata to the foreland. As deformation progresses to the foreland, and deformation becomes more shallow and more brittle in nature, the preorogenic strata will interact with the synorogenic strata and failure will occur creating the first 'brittle' thrust sheet. The location of this first thrust will be controlled by the mechanisms outlined in our models. To the foreland of this point, each successive thrust fault will localize as a result of the combined effects of an overriding thrust sheet and synorogenic deposits.