Award Date


Degree Type


Degree Name

Master of Science in Geoscience



First Committee Member

Michael L. Wells

Second Committee Member

Adolph Yonkee

Third Committee Member

Andrew Hanson

Fourth Committee Member

Paul Forster

Number of Pages



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 (>8 km vertical structural relief), and the wide aerial extent (>60 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.


Thermochronology; Thrust faults (Geology); Willard Thrust



File Format


Degree Grantor

University of Nevada, Las Vegas




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