LARAMIDE REACTIVATION OF THE CLEAR CREEK FAULT, SOUTHWEST WIND RIVER BASIN, WY: EFFECTS ON LARAMIDE FOLD GEOMETRIES

The southwestern margin of the Wind River basin contains a series of southwest verging, left stepping en-echelon folds that fold Paleozoic and Mesozoic strata dipping off the uplifted Precambrian core of the Wind River Range. The southernmost, up-plunge extension of this Laramide fold sequence can be subdivided into two structurally distinct zones, one to the north of the ENE-trending Clear Creek fault (CCF) and a second to the south of the CCF. The northern zone contains Sheep Mountain anticline (ShMA), a SW vergent structure that plunges shallowly toward ~N15°W. This fold is similar in geometry and structural orientation to other Laramide folds that extend northward along the basin-margin fold trend. Folding of ShMA is interpreted to be controlled by a blind basement-involved thrust with a vertical throw of ~190 feet. Conversely, the southern structural zone includes fundamentally different fault and fold orientations and overall fault displacements. The southern zone contains the Schoettlin Mountain anticline (ScMA), which is a variably plunging, ENE-WSW trending, basement cored anticline that is bounded to the south by the Beaver Creek thrust (BCT). The complex fold geometry associated with this fold is interpreted to be the result of fault-related folding along the BCT. In contrast with the structural orientation of ShMA and other Laramide features of the area, both the ScMA and the BCT trend dominantly E-W. Furthermore, fault displacement to the south of the CCF along the BCT is substantially larger, with an estimated vertical throw of ~3,000 feet. The clear boundary between the two structural zones, the CCF, is a steep southerly dipping, ENE-WSW trending regional reverse fault with south-side-up displacement. However, the CCF extends well to the southwest of the study area and deforms the Precambrian core of the Wind River Range in the South Pass area (NW of highway 28), where the fault exhibits a south-side-down sense of offset. Due to the observed reversal in fault-offset sense between the two locations, the CCF is interpreted to be a pre-existing discontinuity, reactivated in Laramide time. We attribute the fundamental change in Laramide fault and fold orientations and overall fault displacements between the two structural zones to stress perturbations produced during reactivation of the CCF.