Kinematics of the Uinta Fault System (Southern Wyoming and Northern Utah) during the Laramide Orogeny

Abstract

The origin of the Late Cretaceous-Early Tertiary Uinta uplift created during the Laramide orogeny has long been problematic, because the structure is E-trending nearly parallel to the ENE-WSW direction of regional shortening. Early workers have proposed that the Uinta uplift is bounded by a large pop-up thrust system, which suggests that uplift was induced by N-S contraction. The geometric relationship between E-trending Laramide faults that bound the Uinta uplift and nearly N-S–trending folds and thrusts at the eastern end of the uplift implies that the formation of the Uinta uplift could have been related to strike-slip faulting. In order to test whether the Uinta uplift was created by either N-S compression or E-W strike-slip faulting, we performed detailed field mapping, kinematic analysis of fault zones, and construction of detailed cross sections along the E-trending Uinta fault system in the northern Uinta Mountains. The results of our study suggest that the Uinta fault is a reverse left-slip structure that has accommodated ENE-WSW contraction during the Laramide orogeny. Our study also shows that although individual faults in the system have variable kinematics and orientations, their average transport direction is remarkably consistent, either in the ENE or WSW direction. These orientations are consistent with the ENE-WSW direction of regional shortening in the overall Late Cretaceous to Early Tertiary Laramide orogenic belt in the western United States and suggest that the stress field was rather uniform in the central U.S. Rocky Mountains during the Laramide orogeny.