T163 Late Cretaceous–Eocene (pre-Cascadia) Tectonics from the Greater Pacific Northwest Margin to the Rocky Mountains (Posters)

Tuesday, 12 October 2021
9:00 AM - 1:00 PM
Exhibit Hall A

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

Paper number: T163-9
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

West-central Utah preserves thrust sheets of the Sevier belt that are overprinted by extensional faults, making correlations of various thrusts in the region challenging. One such area is the San Francisco Mountains, east-central Utah, where the Frisco thrust is exposed and correlations with the better studied Canyon Range-Willard thrust to the north and Wah Wah thrust to the south are not well established. The forward breaking Canyon Range thrust has a minimum of 100 km of shortening and timing constrained to 145-110 Ma (Pujols et al. 2020). The Willard thrust, a continuation of the same thrust, has a minimum of 60 km of shortening and timing constrained to 125-92 Ma (Yonkee et al., 2019). In contrast, the Wah Wah thrust, to the south, is backward breaking and has a minimum of 38 km of shortening, but its timing is poorly constrained (Friedrich and Bartley, 2003). This study tests these correlations by examining the geometry, kinematics, and timing of Frisco thrust using new, detailed (1:24,000 scale) geologic mapping and zircon and apatite (U-Th)/He thermochronology. Preliminary map data reveal five sets of faults and three distinct episodes of faulting. The oldest faults are west-dipping (5-15 degrees) thrusts faults of the Frisco thrust stack, with a minimum displacement of ~37 km. The thrusts are cut by three sets of high-angle (~65 degree dips) normal faults. One set includes NE-striking normal faults, some of which were interpreted as thrusts on 1:48,000 scale maps, but have clear younger on older relationships. Faults in this set are buried, in places, by Cenozoic conglomerates that may correlate to the Paleocene Flagstaff Formation. Another set of normal faults strikes NS and cuts the ~31 Ma Horn Silver Andesite. Lastly, a set of NS-striking strike-slip faults are observed to be confined in the hanging wall of the Frisco thrust, but are of unknown relative age. In addition to the new map data, 11 thermochronology samples were collected in an ~E-W transect across the hanging wall of the Frisco thrust. The thermochronology data will be combined with retrodeformable cross sections to further constrain the timing and magnitude of deformation, and the relationship of the Frisco thrust with thrusts to the north and south.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-370516

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