Post-Laramide Extension and Erosion in the Madison and Gallatin Ranges of Southwest Montana from Apatite (U-Th-Sm)/He Thermochronometry

Abstract Since the end of the Laramide Orogeny (~50 Ma), southwest Montana has experienced complex tectonic, climatic, volcanic, and mantle dynamic processes that have left an imprint on the landscape. Here, we examine the impact of post-orogenic and recent hotspot-related processes on the landscape by quantifying the Cenozoic exhumation history of the Madison and Gallatin Ranges, located on the northern flank of the Yellowstone hotspot (YSH) in southwest Montana. We apply the apatite (U-Th-Sm)/He (AHe) thermochronometer to Cretaceous and Paleogene intrusions from three transects to constrain the Cenozoic cooling history. We also present three new zircon U-Pb crystallization ages. AHe dates from 16 samples produced dates ranging from 67 ± 8.3 Ma to 6.2 ± 0.76 Ma. Most dates are between 45 and 20 Ma and younger than their crystallization age. Samples from the elevation transect with the largest relief display a positive relationship between AHe date and elevation, and thermal history modeling shows a phase of exhumation from ~30–23 Ma. AHe dates in the Madison Range young as they approach the Madison Fault, the range-bounding normal fault, and we ascribe most of the exhumation in the Madison Range to extension and tectonic exhumation due to footwall uplift. We interpret the ~30–23 Ma cooling to represent fault initiation and a phase of Oligocene extension that shows that post-orogenic extensional faulting and collapse propagated into the Laramide domain at that time. Late Miocene AHe dates near the fault represent a renewed phase of motion in the Miocene to recent, though our data lack the resolution to constrain the specific timing. Erosional exhumation due to YSH-driven regional uplift appears to be minimal.