TECTONIC TRANSECT OF THE SEVIER FOLD-THRUST BELT: EVOLUTION OF A COMPLEX OROGENIC WEDGE

The Sevier fold-thrust belt contains two distinct thrust systems: (1) the western Willard-Paris-Meade thrusts that were active during the Early Cretaceous and carried thick passive margin strata above a basal decollement in Neoproterozoic micaceous rocks; and (2) the eastern Crawford, Absaroka, and Hogsback thrusts that were active during the Late Cretaceous to Paleogene and carried thinner Paleozoic strata above a basal decollement in Cambrian shale. The western system is marked by widely spaced thrusts with large displacements and by variable internal deformation with limited layer-parallel shortening (LPS) at upper levels, combined with increasing thrust-parallel shear, vertical thinning, volume loss, and local thrust-parallel extension toward the base of the sheet. Analysis of strained polymict diamictites and fault rocks shows that mica alteration, grain size reduction, hydrolytic weakening, and high fluid pressure contributed to a very weak base that favored large thrust slip and low taper. Geochronologic data record early fluid influx, alteration, and internal deformation from ~140-130 Ma, possibly related to initial thickening in the hinterland, followed by large-scale thrusting at ~125-90 Ma. The eastern system is marked by closer spaced thrusts with smaller displacements, and by widespread LPS that displays a final radial pattern around the Wyoming salient. Early ~E-directed LPS in the future Crawford and Absaroka sheets, recorded by spaced cleavage, was favored by increased temperature during foredeep sedimentation, fluid flow, and stress along the leading edge of the western thrust system. Subsequently, during the Late Cretaceous, deformation became concentrated along weak fault zones, with large-scale thrusting, folding, and vertical-axis rotations resulting in curved structural trends. Early LPS in the future Hogsback sheet, associated with emplacement of the Absaroka sheet, was followed by Paleogene large-scale thrusting and interaction with Laramide foreland uplifts. A model is presented that relates primary sedimentary wedge architecture, early internal deformation from enhanced temperature, fluid flow, and stress, subsequent large-scale thrusting along weak faults, and vertical-axis rotations during propagation of the fold-thrust wedge.