Testing for Patterns of Deformation From the Yellowstone Hotspot Along the Gallatin River, SW Montana

Yellowstone has fascinated humans for thousands of years. Geologists have addressed many of the region’s mysteries, including the underlying mantle hotspot, the chain of calderas tracking the motion of the North American Plate over this hotspot, and the risks posed by the super volcano. However, the way that the hotspot impacts regional tectonics and topography remains under-studied. Influential previous work recognized the Yellowstone Crescent of High Terrain (YCHT), an arc of high topography around the Yellowstone Plateau with limbs extending to the west and south into central Idaho and northern Utah. The YCHT is also hypothesized to be the pattern of active surface uplift caused by the hotspot. To test this hypothesis and better understand the pattern of deformation, rivers that drain Yellowstone Plateau and the deposits they leave behind in the form of terraces can be studied. We focus on the Gallatin River, which flows north out of the northwest corner of Yellowstone National Park and along a transect from the greatest hypothesized hotspot-related uplift to past Bozeman, Montana, presumably at the edge of the tectonic influence of the hotspot. Thus, the Gallatin should reveal a signal of differential headwater uplift in its terrace record and topography. Instead, we find that incision rates are low across the entire transect and the river is responding to bedrock and glacial geomorphic forcings, rather than differential uplift. Results suggest that the Gallatin region has been relatively stable tectonically over the past ~500 thousand years. This supports a simplified pattern of uplift only at the northeast leading edge of the hotspot, with areas of tectonic stability to the west (around the Gallatin) and south, and then faulting and subsidence occurring in a transition zone into the Snake River Plain behind the hotspot to the southwest.