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Structural and Mechanical Analyses of Thrust Ramp Development in Mechanically Stratified Sequences

Abstract

We examine alternative end-member models of thrust fault formation in which thrust ramps initiate deformation in relatively stronger rocks within a sedimentary package that has significant vertical strength variations, versus initiation on thrust flats. We evaluate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, and fault geometry by examining exposed thrust fault footwall-hanging wall structures with field studies, kinematic modeling of two-dimensional cross section reconstructions, and two-dimensional finite element modeling. Field data and retro-deformable cross sections support the hypothesis that thrust faults at the Ketobe Knob, Utah site nucleated as ramp-first structures within the competent sandstone layers. The faults propagated upward and downward and formed fault propagation folds at both fault tips within weaker units. Finite element models that use this structure as the base case indicate that in a compressional setting, stresses are elevated in competent rock layers and are increased in the areas above and below fault tips. These results support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with the ramp-first kinematic model and development of these structures can be significantly influenced by mechanical stratigraphy. Stress concentrations at fault tips and bends develop in finite element models that allow layer-parallel slip, additionally models with displacement boundary conditions result in fault-propagation folds at both fault tips. We compare these results with other field sites of structures at a range scales as well as several published seismic reflection data sets to suggest that in some cases, thrust fault structures may form by nucleation as thrust ramps.