THE APPLICATION OF ZIRCON (U-TH)/HE THERMOCHRONOLOGY TO DETERMINE THE TIMING AND SLIP RATE ON THE WILLARD THRUST, SEVIER FOLD AND THRUST BELT, NORTHERN UTAH

Although the Sevier belt is one of the best studied fold-thrust systems in the world, timing of motion on dominant western thrust sheets that carry thick Neoproterozoic to Paleozoic strata, including the Willard thrust sheet (WTS) of northern Utah, remains incompletely understood. Interpretations for the age of initial slip on the WTS vary widely from 140 to 115 Ma, reflecting ambiguous relations with foreland basin strata and limited geochronologic data. Large displacement on the WTS (~60 km net slip), wide range of exposed levels (>8 km vertical structural relief), and wide aerial extent (>60 km horizontal length) provided an ideal setting for application of zircon (U-Th)/He thermochronology (ZHe) to evaluate thrust timing. Samples were collected along three pseudo-vertical transects that spanned the eastern leading, east-central, and central parts of the WTS (with ~0.5 to 1 km sample spacing) and along a stratigraphic-parallel (pseudo-horizontal) traverse at an intermediate level across the sheet. Due to relatively slow cooling rates and multi-kinetic zircon populations, grains were prescreened based on similar U-Pb ages and U/Th contents, with 6 grains selected for ZHe analysis at each sample site. Vertical transects capture an Early Cretaceous partial retention zone (PRZ) with slow cooling starting at ~130 Ma. Cooling ages increase systematically upward in the east-central and central transects, yielding an average exhumation rate of 0.12 mm/yr; only part of the PRZ is preserved along the eastern leading edge of the sheet. Detrital zircon analysis of Aptian to Turonian (~125-90 Ma) synorogenic strata also record protracted, slow exhumation of upper levels of the WTS. The horizontal traverse yields an average slip rate of 1.7 mm/yr from ~125 to 90 Ma (assuming a constant geothermal gradient), consistent with estimated net slip. ⁴⁰Ar/³⁹Ar UV laser ablation ages of mica in the western, basal part of the WTS are ~145-130 Ma, recording early alteration and internal deformation that preceded large-scale thrust slip. The WTS was subsequently passively uplifted and exhumed during Late Cretaceous development of the Wasatch anticlinorium. Additional work is underway to increase sample density and conduct thermo-kinetic modeling to evaluate potential temporal variations in exhumation and slip rates.