REANALYSIS OF APATITE FISSION TRACK THERMOCHRONOLOGY IN THE WIND RIVER RANGE PRODUCES REVISED LARAMIDE EXHUMATION HISTORY

The cooling history of the Wind River Range has been a key piece of evidence used to constrain the initial timing of thick-skinned, Laramide-style deformation. A reanalysis of apatite fission track (AFT) samples from vertical transects in the Wind River Range in west-central Wyoming provides a revised exhumation history of Laramide-style basement cored uplifts. This study reanalyzes the work of Cerveny and Steidtmann (1993) using the same samples from two vertical transects across the range plus an additional vertical transect from Gannet Peak. These early low-temperature thermochronology studies suggested that the basement-cored uplifts of the Wind River Range experienced punctuated exhumation between 62-57 Ma and at ~40 Ma. While the external detector method of fission track analysis was used in both studies, we use an updated, standardized etching procedure that avoids over-etching the spontaneous track grains. This modification coupled with a larger sample set (n=20) produces new AFT ages that revise the interpretation of Laramide exhumation history in this region. New ages and track length data are modeled using HeFTy showing a uniform three-phase exhumation history along each transect. While cooling begins in the late Cretaceous with a steady apparent exhumation rate of ~0.04 mm/yr, between 65- 50 Ma exhumation accelerates to as high as .33 mm/yr. By mid-Eocene exhumation has slowed to .01 mm/yr. Updated AFT sample-preparation procedures and thermal modeling provide critical revisions to this exhumation story that directly address the tectonic history of the region. Reanalysis of these AFT samples constrain proposed Oligocene burial and exhumation to <1km. The three phase cooling history identified in this study is consistent with multiple data sets including sedimentation rates in adjacent basins, paleoaltimetry measurements, and geophysical modeling of plateau subduction. This study demonstrates that advances in AFT thermochronology procedures and modeling do warrant reevaluation of some early AFT studies.