FRACTURE ANALYSES AND ORIGIN OF NORMAL FAULTS, ROCK SPRINGS UPLIFT, SW WYOMING

The Rock Springs Uplift (RSU) is a N-S-trending, doubly-plunging anticline that underwent deformation during the Laramide orogeny along a blind, east-dipping reverse fault. Cretaceous strata in the arch culmination are cut by faults, which vary in strike from NE to nearly E-W from west to east. Fractures were measured to determine paleo-stress and strain orientations in order to understand structural mechanisms (e.g. outer-arc stretching by normal faulting during folding versus syn-Laramide oblique–slip faulting).

On the eastern flank, the backlimb, fracture conjugate pair geometries (n=549) indicate normal faulting with average sigma 1 nearly vertical, average sigma 2 oriented 086°, and average sigma 3 oriented 350°. Slip orientations indicated by 10 slickenlines (rakes of 75°-86°) indicate down-dip fault movement perpendicular to fault traces.

On the SW flank, the forelimb, two fracture conjugate pair geometries are present. The first geometry (n=39) indicates normal faulting with an average sigma 1 nearly vertical, average sigma 2 trending 059°, and average sigma 3 trending 160°. The second geometry indicates strike-slip faulting (n=13) with an average sigma 1 oriented 058°, average sigma 2 nearly vertical, and average sigma 3 trending 154°. No striations were found in either geometry. The extensional direction (sigma 3) from both strike-slip and normal faulting is perpendicular to Laramide compression.

Because the nearly N-S RSU is oblique to the general Laramide NE compression direction, the master thrust fault below the RSU may contain a dextral strike-slip component. Oblique slip on the blind thrust fault could create oblique extension in the forelimb, resulting in hanging-wall normal and strike-slip faults oriented parallel to the compression direction and extending perpendicular to compression. This pattern of faulting has been seen in clay analog models evaluating oblique slip thrusting. On the eastern flank, distal from the blind thrust, normal faults and fractures are hinge-perpendicular, suggesting outer-arc extension during folding, a geometry seen in doubly-plunging folds in the Rockies (e.g., Teapot Dome). Thus, surficial faults of the RSU are interpreted to have formed by hinge-perpendicular extension combined with oblique shear in the forelimb.