SHEDDING LIGHT ON HIDDEN LINKAGES IN SEVIER FOLD-THRUST BELT: CORRELATING THE FRISCO THRUST USING (U-Th)/He THERMOCHRONOMETRY

The Sevier fold-and-thrust belt marks the easternmost extent of the Sevier orogenic system in the western North American Cordillera. While this belt has been extensively studied in portions of the Basin and Range province, understanding the evolution of individual thrust sheets and their along-strike linkages remains a challenge. In western Utah, this problem is further compounded by overprinting of multiple episodes of extension and associated volcanism. The Frisco thrust in the San Francisco Mountains is a prime example of this complexity, where new mapping reveals that the area is distorted by at least three separate episodes of Cenozoic extension. The FT has been linked to the better studied Canyon Range-Willard thrust to the northeast and to the Wah Wah thrust to the southwest due to similarities in hanging wall units, however, differences in structural style, magnitude of shortening, and timing constraints suggest that correlations are not straightforward. To test correlations and more quantitatively constrain the timing and magnitude of exhumation, a thermochronology transect was collected across the northern San Francisco Mountains. Eleven samples were collected for (U-Th)/He dating from Precambrian to Cambrian quartzites in the hanging wall of the Frisco thrust. Thirty selected zircon grains from each sample underwent prescreening using laser ablation depth profiles to determine parent isotope concentrations and U-Pb ages. From each batch of prescreened grains, up to six were selected based on the lowest effective uranium content, resulting in sixty-three new dates. After excluding three unreliable dates, eleven new ages were calculated. Cooling ages increase from west to east across the transect, consistent with eastward transport of the hanging wall. The results suggest major thrust slip from 136.7 ± 12.7 Ma to 88.4 ± 6.3 Ma aside from for a single 161.6 ± 75.1 Ma age which may record erosional exhumation or an early phase of slip along the thrust. Cooling ages align well with published timing estimates of both the Canyon Range (146-100 Ma) and Willard (125-92 Ma) thrusts to the north, however, estimates of the WWT are too broad and need to be refined to make a more concrete correlation. To further distill correlations, ages will be integrated into paleodepth reconstructions and thermal models to understand the time-temperature evolution of the thrust.