Burial and Exhumation History of the Rocky Mountains from Low-Temperature Thermochronology, Wyoming and Montana, Usa
We dated borehole and surface samples from Laramide-age, basement-cored uplifts of the Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) using the apatite (U-Th)/He (AHe) and apatite fission-track (AFT) systems. Comparison of these results to previously published AFT data, along with the incorporation of new He diffusion models, reveals several new insights into the shallow exhumation histories of these ranges.
Two to 8 apatite crystals for AHe and 30 crystals for AFT were dated per sample. Mean AHe ages typically range from late Cretaceous to Paleocene within ~1 km of the surface, to Miocene and younger ages at depths greater than ~2 to 2.5 km. At least one sample per borehole shows a correlation between AHe age and effective uranium (eU) content ([U] + 0.235[Th]) of the crystal, indicating that radiation damage has affected helium diffusivity, and hence AHe age. Thus an appropriate model for the diffusion of helium in radiation-damaged apatite (e.g. Shuster et al., 2006) should be used for modeling ages.
To interpret thermal histories of these ranges we forward model all available AHe and AFT data. Our goal is to understand post-Laramide burial and exhumation history, so previously-published geologic and age data are used to constrain the timing of Laramide exhumation. We first model a thermal history for a sample that shows good correlation between AHe age and eU. These temperatures are extrapolated to other elevations by assuming a geothermal gradient based on present-day corrected bottom-hole temperatures. Fossil partial retention and annealing zones in vertical profiles provide constraints on the maximum temperatures attained. The thermal model and/or geothermal gradient are iteratively adjusted to better match the age-elevation profile.
AHe and AFT ages from wells through the hanging walls of the Wind River and Beartooth thrusts can only be reproduced using models with both Laramide-age and post-Laramide-age cooling. Thermal histories that best approximate the borehole data require Phanerozoic burial until the Paleocene-Eocene of ~3-4 km, followed by two phases of cooling and exhumation. The first and larger cooling event of several tens of degrees (~3-4 km of exhumation) occurred during the Paleocene-Eocene, followed by a smaller cooling event of a few tens of degrees (~1 km of exhumation) during the Miocene. Bighorn Range data can be matched by thermal models with and without post-Laramide exhumation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009