CONSTRAINING THE TIMING OF DEFORMATION IN SOUTHWESTERN MONTANA BASEMENT CORED RANGES USING LOW TEMPERATURE THERMOCHRONOLOGY

Basement cored uplifts in southwestern Montana have a similar structural style to the classic “Laramide” province of Wyoming. However, constraints on the timing of deformation from syntectonic sedimentation and preliminary low temperature thermochronologic data suggest that the timing of deformation in these basement cored uplifts preceded the inception of shallow subduction along the classic Laramide “corridor” from southern California to Wyoming. We apply low temperature thermochronology to investigate the pattern of exhumation (uplift and erosion) associated with shortening in the Idaho-Montana fold-thrust belt along a transect from central Idaho into southwest Montana. Here, we use forward and inverse modeling of zircon U-Th)/He (ZHe) data to assess whether the timing of basement exhumation overlaps with that of more interior thrust sheets in the Idaho fold-thrust belt. Existing ZHe data from these interior thrust sheets suggest thrust-related exhumation lasted from >94-58 Ma. Our new data (n=43 dates) from basement cored uplifts in southwest Montana display strong negative date-effective uranium (eU) correlations and demonstrate the limited utility of weighted mean dates for constraining the timing of specific cooling events in this region. We thus utilize pediment mean dates and thermal history models, which better constrain the timing of thrust-related cooling: 54.9 ±0.65 Ma for the Gallatin uplift and 73-65 Ma for the Madison-Gravelly arch. These ages of major, thrust-related exhumation within southwest Montana basement cored ranges are both contemporaneous with and younger than exhumation constraints for more hinterland thrust sheets in the Idaho-Montana fold-thrust belt. These results suggest that the activation of basement uplifts did not require the influence of flat-slab subduction. Additional sampling in the Idaho fold-thrust belt (including the Hawley Creek and Pioneer thrusts) will further test these findings.