Mechanical stratigraphy and fault–fold interaction, Absaroka thrust sheet, Salt River Range, Wyoming

Abstract

A distinct contrast in deformation style from tightly folded and faulted Cambrian through Ordovician strata to broadly folded and faulted Mississippian and younger strata is displayed in the hanging wall of the Absaroka thrust, Salt River Range, Wyoming. Based on style, the Paleozoic section can be divided into two main structural lithic units. Each unit is characterized by a relatively weak, ductile, anisotropic lower section and a relatively strong, brittle, more isotropic upper section. Where the two units were stacked within the thrust sheet, inverted fault–propagation folds formed in the center of each unit, and the overall transition upward from close- to wide-spaced folds and imbricate faults developed in the multilayer. Where the upper unit was isolated, deformation was dominated by imbricate faulting with little associated folding, and inverted fault–propagation folds did not form. Observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response. It is note-worthy that the boundary between the two structural lithic unts, in part defined by the roof thrust of the Stewart passive roof duplex, occurs within a dolostone rather than in overlying shale of the Darby Formation. I propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions. Cross-cutting relations suggest that the Absaroka sheet was cut by large imbricate faults during late-stage movement over a major dip ramp. A high deviatoric and low mean compressive stress state just beyond the upper ramp–flat corner promoted break-back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation.