GEOLOGIC MAPPING AND PRELIMINARY STRUCTURAL ANALYSIS OF THE BACKLIMB OF THE MINERS MOUNTAIN UPLIFT, TWIN ROCKS 7.5 MINUTE QUADRANGLE, CAPITOL REEF NATIONAL PARK, UTAH

Completion of the geologic map of the Twin Rocks Quadrangle marks the last of a three part NPS-funded project to map the most heavily visited areas of Capitol Reef National Park, Utah at 1:24,000 scale. Located in the northwestern portion of the Park, the Twin Rocks Quadrangle displays a variety of geologic features including the Goosenecks at Sulphur Creek, Chimney Rock, the Castle, the Western Escarpment, and the entrance to Spring Canyon. The quadrangle lies on the backlimb of the Miners Mountain uplift, a Laramide-age southwest verging, asymmetrical, doubly-plunging anticline. This structure sets up some of the most popular vistas observed in the park including the Western Escarpment. The uplift is thought to have developed from left-lateral oblique-reverse motion at depth, along a reactivated basement normal fault. Detailed analysis of deformation bands in the forelimb (Bump and Davis, 2003) shows a principle compressive stress nearly 45 degrees clockwise from perpendicular to the trend of the fold axis. Measurements of fracture sets and small-scale compressional folds in the backlimb of the uplift reveal a principle stress orientation approximately perpendicular to the trend of the fold axis. Although not a unique solution, this data suggests that stress transmitted through the basement is partitioned and rotated in the backlimb, likely due to decoupling and differential slip in strata with low shear strength.