EXAMINING THE LATE CENOZOIC EXHUMATION HISTORY OF THE GALLATIN RIVER CATCHMENT AND ITS RELATIONSHIP TO THE YELLOWSTONE HOTSPOT, GALLATIN COUNTY, SOUTHWEST MONTANA

The Yellowstone hotspot has a large impact on the topography of the northern Rocky Mountain region, creating a topographic swell centered on the hotspot. This swell is thought to be the result of thermal and dynamic buoyancy from a mantle-sourced plume. However, it has been difficult to quantify exhumation and uplift temporally and spatially in regions surrounding the hotspot. We examine the exhumation history of the Gallatin River catchment in southwest Montana using apatite (U-Th/He) (AHe) and ⁴He/³He thermochronometry to determine how the catchment has responded to the arrival of the hotspot. Previous work using bulk AHe data from two elevation transects in the central part of the catchment yielded dates ranging from 14.5 ± 2.2 to 70.5 ± 3.4 Ma. Thermal history modeling suggests that major cooling occurred in the Eocene or Oligocene with post 6 Ma cooling limited to <50ºC. Cooling and exhumation related to the hotspot appears to be minimal, but the bulk AHe method lacks the sensitivity to resolve the lowest temperature portion of the history in detail. Assuming a low geothermal gradient of 30˚C/km, the hypothesized 0.5 – 1.5 km of erosional exhumation would result in as little as 15-45˚C of cooling. More resolution is needed to resolve the lowest temperature portion of the thermal histories, so we apply the ⁴He/³He technique on the two transects. Additionally, we present one additional transect for AHe analysis in the southernmost portion of the catchment closest to the hotspot to determine the spatial relationship of exhumation in the catchment.