Structural style of the San Francisco Mountains, Sevier fold-thrust belt, Utah
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The Sevier fold-thrust belt contains thrust sheets that are overprinted by extensional faults, distorting correlations of geologic structures in the region. The Frisco thrust is exposed within the San Francisco Mountains in west-central Utah. Correlations with the better studied Canyon Range-Willard thrust to the north and the Wah Wah thrust to the south are not well established. The Canyon Range thrust to the north has a forward-breaking sequence, a minimum of 100 km of shortening, and timing is constrained to 145-100 Ma (Pujols et al., 2020). The Willard thrust farther north, has a minimum of 60 km of shortening, and timing is constrained to 125-92 Ma (Yonkee et al., 2019). By comparison, the Wah Wah thrust has a backward-breaking sequence in its footwall, a minimum of 38 km of shortening, and timing that is poorly constrained (Friedrich and Bartley, 2003). This study tests these thrust correlations by examining the geometry, kinematics, and timing of the Frisco thrust through new, detailed (1:24,000 scale) geologic mapping, balanced cross-section construction, and structural analysis techniques. Map data reveal five distinct episodes of faulting. The Frisco thrust and related contractional faults that are east vergent and west dipping. Synchronous strike-slip faults are 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 results suggest that the Frisco thrust correlates with the Wah Wah thrust due to its backward-breaking structural style of thrusting. Direct correlations are unclear due to lack of timing data. Complete structural and timing data are required to directly correlate the Frisco thrust.
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M.S.