Abstract

Thrust faults are capable of affecting the stress magnitudes and orientations in little-deformed strata several hundreds of kilometers in front of a thrust front. A conceptual model based on this mechanism suggests that regional fractures and the maximum horizontal compressive stress trajectories in the Frontier Formation in the central Green River Basin trend approximately north-south. This regional pattern results from the imposition of stresses onto basin-fill strata by Sevier and Laramide thrusting at the edges of the basin. The Sevier thrust belt on the western margin of the basin was active for 100 million years, but had relatively little permanent influence on stresses and fracturing in the Frontier Formation within the basin because stresses from this system were self-limiting. The east-west horizontal stress trajectories from this system were overwhelmed by north-south compression during the Laramide orogeny. Stress and fracture orientations deviate from the general north-south trend at the basin margins, in the corners of the basin, and in association with local structures. Stress-orientation data, from a limited number of oriented caliper logs, support the conceptual stress trajectories.