STRUCTURAL STYLE AND EXHUMATION HISTORY OF THE FRISCO THRUST SHEET, SEVIER FOLD-THRUST BELT, UTAH

The Sevier fold-and-thrust belt in western North America is a prime example of a retro-arc fold-thrust system and an ancient analog for thrust belts around the globe. Although this belt is well studied, across portions of the Basin and Range province, challenges exist in understanding the structural development and evolution of individual thrust sheets. This is a particular problem in the western Utah, where Cenozoic volcanic cover and overprint by several generations of normal faults makes along-strike correlations of thrust faults difficult. These issues are well exemplified by the Frisco thrust in the San Francisco Mountains, which lies between the better studied and presumably correlative Canyon-Range-Willard thrust to the northeast and Wah Wah thrust to the southwest. Differences in the magnitude of shortening and sequence of fault development associated with these thrusts suggest there may be complications with these correlations. Alternative correlations have also been proposed, linking the Frisco thrust south with the Beaver Lake Mountains thrust. This study explores the geometry, kinematics, and sequence of fault development in the San Francisco Mountains through 1:24,000-scale geologic mapping. The new map data and structural analyses reveal six sets of faults. The oldest faults include the Frisco thrust and related contractional faults that are east vergent and west dipping. A set of synchronous strike-slip faults are also documented and confined to the hanging wall of the Frisco thrust. Four sets of normal faults cut the thrust: (1) north-south striking low-angle normal faults; (2) high-angle normal faults that are buried, in places, by Cenozoic conglomerates; (3) high-angle normal faults that cut the approximately 31 Ma Horn Silver Andesite; and (4) range-bounding Quaternary faults responsible for tilting of the range to its current orientation. The fault sets and associated folds reveal two episodes of Mesozoic contraction and at least three episodes of Cenozoic extension. The map data will be used to test thrust correlations and are complimented by parallel efforts to more quantitatively constrain the exhumation and erosion history of the region through (U-Th)/He dating of the hangingwalls of the Frisco and Wah Wah thrusts and detrital zircon U-Pb dating of prevolcanic (pre-35-31 Ma) sediments.