FRACTURE ANALYSIS IN OBLIQUE RAMPS OF THE WYOMING SALIENT, SEVIER FOLD-THRUST BELT: EVOLUTION FROM EARLY LAYER PARALLEL SHORTENING TO FINAL OROGENIC CURVATURE

To better understand the nature of elastico-frictional deformation and its relationship to strain and vertical-axis rotations in the Wyoming salient, fracture analysis was conducted within the Triassic Ankareh (redbeds) and Jurassic Twin Creek (limestone) Formations exposed in the Absaroka, Prospect and Hogsback thrusts. While fracture morphologies varied with lithology, fracture orientations in both units indicate that elastico-frictional deformation probably occurred throughout the development of the Wyoming salient. For instance, the trend of one fracture set (consistently at high angles to bedding), fan around the salient, and are perpendicular to orogenic strike, suggesting that layer parallel shortening surfaces may have ‘relaxed' over time. Another primary set of fractures, which are steep to vertical, trend parallel to the strike of the orogen, showing evidence for widespread tangential extension and appearing to be related to development of tectonic curvature. Complications occur in regions where map patterns suggest the presence of oblique/lateral ramps at depth. In the Fort Hill area, there is a ~90º change in the orientation of the ‘LPS fractures' with the cross fractures showing a change of ~50º around the structure. In the Beaver Creek area, the fracture sets show slight changes (10-30º) in their orientation with respect to the map patterns. Lastly, the LaBarge Creek area, which is comprised of a number of small bends in map view, displays little orientation variation of the ‘LPS fractures' and more variation (10-30º) of the cross fractures around the structure. Possible explanations for the fracture orientation changes include motion over oblique/lateral ramps, late-stage vertical-axis rotations, or a combination of both mechanisms. By integrating our fracture analysis with strain and paleomagnetic data, we try to resolve the temporally and spatially distributed three-dimensional displacement field associated with the evolution of the Wyoming Salient.