4He/3He THERMOCHRONOMETRY REVEALS THE LATE CENOZOIC EXHUMATION HISTORY OF THE GALLATIN RIVER CATCHMENT, SW MONTANA

The Yellowstone hotspot impacts the topography of the northern Rocky Mountain region, creating a topographic swell ~0.5 km higher than the surrounding region. 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 the magnitude of the swell and its impact on spatial and temporal patterns of exhumation surrounding the hotspot. We examine the exhumation history of the Gallatin River catchment which drains the northern flank of the Yellowstone Plateau 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. The thermal models allow for cooling after 6 Ma, but cooling is limited to <50ºC and not required. Any post 6 Ma cooling could be attributed to the hotspot, but the bulk AHe method lacks the sensitivity to resolve the lowest temperature portion of the history in detail. We present new ⁴He/³He thermochronometry from a subset of the previously studied samples to constrain the lowest temperature portion of the cooling histories. Inverse thermal history modeling of ⁴He/³He data for five samples agrees with major cooling in the Eocene or Oligocene and better resolves the post-Eocene cooling history. All good fitting thermal histories show 30-40˚C after 30 Ma. A small set of good-fit thermal histories show slow consistent cooling from ~30 Ma to the present, while a larger subset of cooling histories show little to no cooling from 30 to <20 Ma, followed by 30-40˚C of rapid cooling. The exact timing of this mid-to-late Miocene cooling is difficult to resolve, but we suggest this 30-40˚C could be the result of erosional exhumation due to the Yellowstone hotspot or the Middle Miocene Climatic Optimum (17-15 Ma). We favor the Yellowstone hotspot interpretation due to the short time span of the Middle Miocene Climatic Optimum and carbonate isotope data in the adjacent valley that suggest stable climate during this time. The magnitude of cooling is in-line with hypothesized magnitudes of erosional exhumation due to the arrival of a Yellowstone-driven topographic swell in the region.