DYNAMIC TOPOGRAPHY AND TECTONICS DURING THE DEVELOPMENT OF THE SNAKE RIVER PLAIN-BOISE MOUNTAINS TOPOGRAPHIC CONTRAST: EVIDENCE FROM LOW-T THERMOCHRONOLOGY

The distinctive topographic contrast between the western Snake River Plain (WSRP) and the nearby Boise Mtns may be explained with some combination of dynamic topography, basin and range extension, flexure, and erosion. This study seeks to understand the timing, degree and spatial extent of exhumation that led to the unique topography of the WSRP and Boise Mtns. We present apatite (U-Th)/He (AHe) and apatite fission-track (AFT) apparent ages from Cretaceous and Eocene granitoids along a transect through the Boise Mtns from the edge of the WSRP northeast to the Sawtooth Mtns, and over an elevation range spanning ~ 2 km. The analyses give Eocene to Miocene AHe and AFT apparent ages. Samples proximal to the WSRP generally yielded older ages (AHe ages = 33.9-21.0 Ma, AFT ages = 41.0-23.7 Ma) for the Boise Ridge and (AHe ages 16.8-11.6 Ma, AFT ages = 32.1-20.9 Ma) for the Trinity Mtn. area. These results suggest that the areas near the WSRP have been at shallow crustal levels (<3km) since the Late Eocene - Early Miocene. The spatial distribution of ages indicates that rather than being a major rift flank, the topographic contrast between the WSRP and Boise Mtns may be due to down flexing in the WSRP. Down flexing could be induced by the emplacement of mafic magmas to the crust, followed by cooling and subsidence. Samples from the distal end of the transect, within the footwall of the Sawtooth block, yielded ages (AHe ages of 11.5–9.9 Ma, AFT ages of 13.8–13.7 Ma) with apparently smaller age gaps than the more proximal samples. These data suggest rapid cooling occurred during ~13-10 Ma due to exhumation at about 0.6 mm/year. The AHe apparent ages partially overlap the closest pass of Yellowstone hotspot volcanism to the transect. These age distributions could be explained by down flexing in the SRP (tied to emplacement of mafic magmas, cooling and subsidence), coupled with flexural uplift in the distal portion of the transect. Our initial findings suggest that while exhumation timing and rates vary significantly along different faults, dynamic topography induced by passage of North America over the Yellowstone hotspot, along with flexure may have had a significant impact on the exhumation history of the Boise Mtns. This study offers rare insight into the response of the upper crust to impingement of a mantle hotspot with the continental lithosphere.