Comparison of thrust fault rock models to basement-cored folds in the Rocky Mountain foreland
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Nonscaled rock models of the ramp regions of thrust faults develop structures at a late stage of shortening that are similar in geometry, relative mechanical properties, and deformation mechanisms to some basement-cored folds in the Rocky Mountain foreland. Based on these similarities, the models suggest that a ramp anticline in crystalline basement could evolve into a foreland-style fold. The configuration consists of a 20 dip-ramp in a single, thick layer of sandstone overlain by a layered veneer of limestone and mica. The models are deformed at a confining pressure of 50 MPa and displacement rate of 8(10) -3 cm s -1 . Although designed to simulate thin-skinned tectonics, the configuration allows comparison with any layered sequence overlying a low-angle reverse fault. The late-stage structure produced in the model is a flat-topped, asymmetric fold with a near-vertical forelimb overlying an uplifted, faulted sandstone layer. Critical to the development of this structure is the change in mechanisms of shortening from translation of the hanging wall along a decollement to deformation within the hanging wall. The latter enhances the vertical displacement field. The change in mechanisms appears to be caused by deformation-induced changes in rheology and geometry of the layers that weaken the hanging wall and promote locking of the decollement. The comparison presented here may be particularly applicable to structures in the interaction zone between the foreland and thrust belt. Ramp anticlines and basement uplifts are end members in a continuum of potential structures that may be present in this zone, and evolution from one style to the other may be possible.
