Paper No. 38-2
Presentation Time: 8:30 AM-6:00 PM
STRUCTURAL STYLE AND EXHUMATION HISTORY OF THE FRISCO THRUST SHEET, SEVIER FOLD-THRUST BELT, UTAH
The Sevier fold-thrust belt contains thrust sheets that are overprinted by extensional faults, distorting correlations of thrust sheets in the region. The San Francisco Mountains in west-central Utah exposes the Frisco thrust, but correlations with the better studied Wah Wah thrust to the south and Canyon Range-Willard thrust to the north are not well established. The Wah Wah thrust has a backward breaking sequence in its footwall, a minimum of 38 km of shortening, but timing is poorly constrained (Friedrich and Bartley, 2003). In contrast, 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 is a continuation of the same thrust sheet, has a minimum of 60 km of shortening, and timing is constrained to 125-92 Ma (Yonkee et al., 2019). This study tests these thrust correlations by examining the geometry, kinematics, and timing of the Frisco thrust. Methods to test the correlations include new, detailed (1:24,000 scale) geologic mapping and zircon and apatite (U-Th)/He thermochronology. Preliminary map data reveal five fault sets and four distinct episodes of faulting. The oldest faults include the Frisco thrust and related faults that are west dipping (5-20 degrees) with a minimum displacement of ~37 km. Three sets of normal faults cut the thrust: (1) NS-striking normal faults, originally interpreted as thrusts on 1:48,000 scale maps, but have clear younger over older relationships; (2) NE-striking normal faults that are buried, in places, by Cenozoic conglomerates that correlate with the Paleocene Flagstaff Formation; and (3) NS-striking normal faults that cut the ~31 Ma Horn Silver Andesite. The study area also contains a set of NS-striking strike-slip faults, confined in the Frisco thrust hanging wall, but relative age is unknown. To constrain the timing of thrusting, 11 thermochronology samples were collected in an ~E-W transect, with ~700 m sample spacing, in the hanging wall of the Frisco thrust. The thermochronology data will be combined with balanced cross sections derived from the new map data. The timing and magnitude of shortening for the Frisco thrust will be compared to thrusts to the south and north to establish correlations.