APATITE (U-TH)/HE COOLING DATES FROM THE MADISON RANGE, SOUTHWEST MONTANA AND IMPLICATIONS FOR EXHUMATION DRIVEN BY THE YELLOWSTONE HOTSPOT

Deep mantle processes can play an important role in crustal processes and topography, particularly at intraplate hotspots. As an active center for plume activity and magmatism, the Yellowstone hotspot (YSH) in Wyoming presents an appropriate opportunity to study and quantify intraplate landscape evolution influenced by mantle processes. The purpose of this study is to determine the magnitude of landscape evolution on the northern flank of the high topography surrounding Yellowstone in the Madison Range of southwest Montana, with the approach and evolution of the YSH. We hypothesize that the eastward progression of the hotspot to its current location caused regional uplift and incision as the hotspot advanced. To address this research question, we apply the apatite (U-Th)/He (AHe) low-temperature thermochronometer to bedrock samples collected along two elevation transects located along the Gallatin River to establish any differences in exhumation rates and magnitudes. Samples within each transect were collected at ~200m elevation intervals from Tertiary-Cretaceous igneous intrusions. Preliminary analyses of the first transect within the Gallatin Gateway produced a range of cooling ages (in order of ascending elevation; 70.5+7.9 Ma, 23.9+1.7 Ma, 29.9+6.0 Ma, 32.0+2.2 Ma, 44.3+2.7 Ma). Ages between ~23.9-44.3 Ma may reflect possible exhumation or reheating of grains with the outlier age of ~70.5 Ma reflecting a possible intrusion emplacement age for that sample. Potential sources of reheating influence may include the nearby Huckleberry Ridge Tuff with Absaroka Volcanics as a possible source for the region’s intrusive complex. None of the samples have experienced sufficient recent exhumation driven by the YHS to yield Pleistocene or Quaternary dates suggesting the erosion driven by YHS is not extreme. However most published low temperature thermochronometry from the region reflects Laramide cooling, and our <45 Ma dates record some post-Laramide exhumation that could be low levels of YSH driven erosion or associated with Eocene-present extensional structures. Ongoing work focuses on acquiring more AHe dates and inverse modeling of grain ages using HeFTy software to estimate possible time-temperature history paths for each transect.