Late-stage N-S shortening and fold offset during Laramide deformation along the southeastern margin of the Wind River Mountains, Wyoming

Abstract

Uplift of the basement-cored Wind River Mountains during the Laramide orogeny (ca. 75-45 Ma) produced a series of subsidiary NW-SE trending folds along the southwestern margin of the Wind River Basin. These folds are consistent with the regional NE-SW shortening that produced the uplift. However, individual folds along the trend display a local left-stepping en echelon pattern that becomes more complex in the southeastern most folds of the series. The interchange between Sheep Mountain Anticline (SMA) and Derby Dome (DD), the two southernmost folds in this sequence, contains a complex series of faults and smaller scale folds that are consistent with a late stage of N-S Laramide shortening. Faults associated with the folding of SMA, to the south of the interchange between SMA and DD, trend NW-SE, while several structures, ranging in scale, trend E-W. E-W trending structures near the southern termination of SMA indicate the N-S shortening is not constrained to the interchange zone, but occurs both in the northern and southern terminations of SMA. Results from geologic mapping and structural analysis of deformation features in the northern closure of SMA, balanced cross-sections, fracture analysis, and a 2-D seismic reflection and refraction experiment are all consistent with N-S-directed shortening and late-stage enhancement of the offset between north SMA and south DD. The study indicates that recently recognized N-S Laramide shortening to the south of the study area extends well to the north and may be associated with regionally significant late-stage N-S Laramide shortening.