KINEMATIC ANALYSIS OF THE STRUCTURALLY COMPLEX, EASTERNMOST WIND RIVER BASIN USING FRACTURE CHARACTERIZATION AND 3D RESTORATION

The origin of the structures surrounding the eastern Wind River Basin has been problematic due to their diverse orientations. Current hypotheses explaining their diverse orientations include (1) single-stage NE- directed shortening with reactivation of some pre-existing weaknesses and (2) sequential multi-directional shortening. This study used 3D modeling and restoration with integrated fracture analysis on the southeastern margin of the Wind River Basin to determine the structural significance of these features. A better understanding of the kinematics of diversely oriented structures can be applied to improve predictions of hydrocarbon reservoir geometry and fracture characteristics in similar subsurface systems.

Fracture analysis was conducted throughout the study area to characterize the Laramide and post-Laramide strain and then used to constrain stress evolution. Regional faults cutting pre-Laramide rocks are dominated by NW-SE striking thrust faults and E-W and NE-SW strike-slip faults, consistent with an ENE-trending regional Laramide compression. Similarly, ENE striking joint sets in pre-Laramide rocks are also consistent with regional Laramide compression. A regional NW-SE joint set occurs in most of the syn- and post-Laramide rocks.

Uniform Laramide ENE-WSW shortening indicated by fracture data was successfully tested by palinspastic restoration of structures in the study area at the level of the crystalline/sediment interface. 3D restoration requires significant left-lateral slip between the southwest-verging Casper Arch and the east-verging Laramie Range along E-W faults bracketing Casper Mountain and the NE-SW striking Muddy Mountain fault. Also, the restoration shows a 1.5° counterclockwise rotation during NE-SW shortening of Casper Mountain during deformation. Observations during restoration of Casper Mountain suggest that it served as an accommodation block in a small transfer zone between Casper Arch and the Laramie Range during a single-stage of NE- directed shortening. Thus, this area at the intersection between Casper Arch and the Laramie Range is a transfer zone with oblique left-lateral convergence and compatible counterclockwise rotation of the Casper Mountain block.