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 fold-thrust belt is one of the best-studied foreland systems in the world, timing of motion on the dominant western thrust sheets that carry thick Neoproterozoic to Paleozoic strata remains incompletely understood. Zircon (U-Th)/He thermochrometry (ZHe) studies of the hanging wall of the Willard thrust sheet (WTS) in northern Utah are used to constrain the timing and rate of thrust slip. Previous interpretations of the age of initial slip on the Willard thrust vary widely from 150 to 115 Ma, reflecting ambiguous relations with foreland basin strata and limited geochronologic data. The large displacement on the Willard thrust (~60 km net slip), the wide range of exposed levels (\u3e8 km vertical structural relief), and the wide aerial extent (\u3e60 km horizontal length) provide an ideal setting for application of ZHe to evaluate thrust timing. Samples were collected and analyzed for 40 sites along three pseudo-vertical transects that spanned the eastern, east-central, and central parts of the WTS (with sample spacing of ~0.5 to 1 km structural depth) and along three stratigraphic-parallel (pseudo-horizontal) traverses at intermediate levels 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 captured an Early Cretaceous partial retention zone (PRZ) with slow cooling starting at ~130 Ma. Cooling ages are systematically younger downward in the east-central and central vertical transects, yielding an average exhumation rate of 0.12 mm/yr (assuming steady state geothermal conditions) from ~125 to 90 Ma; only part of the PRZ is preserved along the eastern leading edge of the sheet. Cooling ages are systematically younger westward along a horizontal traverse near the base of the Geertsen Canyon Formation, yielding an average slip rate of 1.7 mm/yr from ~125 to 90 Ma. The estimated exhumation rate is consistent with uplift and erosion above a moderately (average ~10°) dipping, composite thrust ramp, and the estimated slip rate and duration are roughly consistent with a net slip of ~60 km. The WTS was subsequently passively uplifted and exhumed during Late Cretaceous development of the Wasatch anticlinorium, recorded by cooling ages at deeper levels

Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

Zircon (U‐Th)/He (ZHe) and zircon fission track thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier fold‐thrust belt record enhanced cooling and exhumation during major thrust slip spanning approximately 125–90 Ma. ZHe and zircon fission track age‐paleodepth patterns along structural transects and age‐distance relations along stratigraphic‐parallel traverses, combined with thermo‐kinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/Myr from 125 to 105 Ma, increasing to ~3 km/Myr from 105 to 92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/Myr. Local cooling ages of approximately 160–150 Ma may record a period of regional erosion, or alternatively an early phase of limited... (see full abstract in article)