TWO EXHUMATION EVENTS OF THE WHEELER PASS THRUST SHEET IN THE SOUTHERN SEVIER OROGEN FROM (U-TH)/HE ZIRCON THERMOCHRONOLOGY

The Wheeler Pass (WP) thrust sheet is a major structural feature that played a key role in the development of the southern Sevier orogenic belt, now discontinuously exposed across several mountain ranges in the Mojave Desert. This regionally extensive sheet of Neoproterozoic and Paleozoic rocks from the passive margin of western North America experienced ~6 km of stratigraphic throw and >30 km of lateral displacement. The timing of motion of the WP thrust fault is poorly bracketed by stratigraphy between the Pennsylvanian and Late Cenozoic. Proposed correlative thrusts to the south, including the Winters Pass and Pachalka thrusts, permit additional constraints that late motion on the fault system occurred at ~146 Ma. The large thickness (9+ km) of the WP sheet and stratigraphic throw permit the application of zircon (U-Th)/He (ZHe) thermochronometry to date cooling related to erosional exhumation due to rock uplift over thrust ramps. Here we present new ZHe data from two key sections of the WP thrust sheet that represent different stratigraphic depth intervals: Neoproterozoic to Devonian strata in the NW Spring Mountains, and Paleoproterozoic basement through Cambrian strata in the southernmost Nopah Range. Our results indicate that the WP thrust sheet in the northwest Spring Mountains underwent moderate cooling rates in the Late Jurassic at ~155 Ma, followed by slow cooling and development of a ZHe Partial Retention Zone (PRZ) between ~150 Ma and ~50 Ma. In contrast, ZHe results from the southern Nopah Range section show a preserved Cretaceous PRZ followed by moderate cooling rates beginning at ~100 Ma. We interpret these two seemingly disparate cooling histories as reflecting separate cooling events from different structural levels of the thrust sheet and different initial positions relative to a complex WP thrust ramp. ZHe thermochronometry of the WP thrust sheet indicates Late Jurassic motion on the WP thrust followed by mid Cretaceous passive uplift and exhumation due to thrusting along a structurally lower ramp related to the frontal Keystone and Red Springs-Wilson Cliffs-Contact thrust system that lies to the east.