LEVERAGING STRUCTURAL, SEISMIC, AND LOW-TEMPERATURE THERMOCHRONOLOGY DATASETS TO DETERMINE THE MECHANISMS AND RATES OF FORELAND BASIN EXHUMATION, CRAZY MOUNTAINS, USA

The Crazy Mountains, located in southwestern Montana, record growth and erosion of the Cordilleran fold-thrust belt, accumulation in the adjacent foreland basin, and subsequent exhumation of igneous, sedimentary, and metamorphic rocks including some of the youngest foreland basin stratigraphy preserved in western North America. This range defines a unique example of topographic uplift and relief development east (craton-ward) of the locus of major fold-thrust belt shortening, in a region that has also not been greatly affected by later extension. We integrate structural, seismic, and new apatite (U-Th)/He (AHE) and fission track (AFT) data to evaluate the timing, rates, and mechanisms of magmatic and exhumational cooling in the Crazy Mountains. New and published structural measurements were combined with industry seismic data to create a 1:100,000-scale cross section across the Battle Ridge Monocline, Big Timber Stock, and surrounding foreland basin. Steeply-dipping foreland basin strata are observed along the Battle Ridge Monocline and within ~5–10 km of the Big Timber Stock, but structural data otherwise indicate minimal deformation of the basin. AHe and AFT data collected along the same profile are characterized by: (1) AFT ages from foreland basin deposits that are younger than sample depositional age but older than the Eocene Big Timber Stock, consistent with post-depositional magmatic (or burial) reheating to less than ~120ºC. (2) AFT ages from the Big Timber Stock are within error of the timing of stock emplacement, while AHe ages are systematically younger than AFT ages for the same samples. These data suggest that our Big Timber Stock sample locations were emplaced at depths shallower than the AFT closure isotherm, and that subsequent cooling below the AHe threshold was driven by exhumational rather than magmatic processes. Finally, we leverage positive age-elevation trends in the Ft. Union AFT data and Big Timber Stock AHe data to calculate apparent exhumation rates of ~0.03–0.10 km/Myr and ~1.2–2.1 km/Myr, respectively. These results provide new insights into the thermal history of the Crazy Mountains, including preliminary constraints on the magnitude and rates of post-orogenic exhumation and topographic relief development.