LOW-TEMPERATURE THERMOCHRONOLOGY OF THE LARAMIDE RANGES AND EASTWARD TRANSLATION OF SHORTENING IN THE SEVIER BELT, WYOMING, UTAH AND MONTANA
AuthorPeyton, Sara Lynn
AdvisorReiners, Peter W
DeCelles, Peter G
Committee ChairReiners, Peter W
DeCelles, Peter G
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThis dissertation contains two studies that investigate the Mesozoic and Cenozoic tectonics of the western USA. The first study investigates shortening in the Sevier thrust belt of northeast Utah and southwest Wyoming. Cross section restoration suggests that there was ∼8-14 km of pre-Absaroka-thrust shortening above the Jurassic Preuss salt detachment (PSD), but not below it, in the hanging wall of the Absaroka thrust. Reflection seismic data show that the Crawford thrust is not offset along the PSD, indicating that the additional shortening on the Absaroka plate was transferred east before main movement on the Crawford thrust. Integration of surface and subsurface geology suggests slip from the Willard or Lost Creek thrust was transferred several tens of kilometers east along the PSD between ∼102-90 Ma.The second study investigates the low-temperature thermochronology of the Laramide Ranges. We dated 91 borehole and surface samples from basement-cored uplifts of the Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges), and the Uncompahgre Uplift, using the apatite (U-Th)/He system. (U-Th)/He ages generally increase with increasing elevation. Most samples show age dispersion of tens to hundreds of Myr. Several samples show correlations between (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]), indicating that radiation damage has affected (U-Th)/He age. Many surface and near-surface samples have (U-Th)/He ages that are older than apatite fission-track ages.Forward and inverse modeling using a radiation damage diffusion model showed that (U-Th)/He ages may be widely dispersed, and may be older than apatite fission-track ages within a fossil partial retention zone. Most samples, however, do not exhibit the predicted (U-Th)/He age-eU correlation. We show that the effects of grain size can obscure (U-Th)/He age-eU correlations. Best-fit thermal histories from the inversion of age-eU pairs were extrapolated to other elevations to create model age-elevation plots. "Too-old" (U-Th)/He ages that are not within a fossil partial retention zone are likely due to He implantation from high-eU phases. Inverse modeling of (U-Th)/He age data suggests that rapid exhumation within the Laramide province began earlier in the Bighorn Mountains (before ∼71 Ma) than the Beartooth Range (before ∼58 Ma).