Isostatically Driven Flexural Uplift Following the “Head to Tail” Transition Along the Yellowstone Hotspot Track: Evidence from Low-Temperature Thermochronology in the Boise Mountains

Abstract:

The distinctive topographic contrast between the western Snake River Plain and the nearby Boise Mountains may be explained with some combination of dynamic and tectonic processes including: Basin and Range extension, thermal uplift, flexural uplift, and erosion. We present apatite (U-Th)/He (AHe), apatite fission-track (AFT) data, and inverse thermal models from Cretaceous and Eocene granitoids along a transect in the Boise Mountains. The transect spans from the edge of the western Snake River Plain ~100 km northeast to the Sawtooth Mountains with an elevation range ~ 2 km. These analyses give Eocene to Miocene AHe and AFT apparent ages. Samples proximal to the western Snake River Plain yielded late Eocene to early Miocene ages, while samples progressively young away from the western Snake River Plain. Samples from the distal end of the transect, within the footwall of the Sawtooth normal fault, yielded the youngest apparent ages (AHe ages of 11.5-9.9 Ma, AFT ages of 13.8-13.7 Ma) and underwent rapid cooling ~10 Ma. The spatial distribution of these ages, along with cooling histories derived from inverse modelling, suggest that the areas proximal to western Snake River Plain have experienced monotonic cooling and have been at shallow crustal levels (<3km) since the Late Eocene-Early Miocene, while those distal to the western Snake River Plain experienced progressively more rapid late Miocene cooling and exhumation. These exhumation histories are consistent with flexural uplift of the Boise Mountains, away from the western Snake River Plain, enhanced by normal faulting during Basin and Range extension. Flexural uplift was likely driven by the emplacement and cooling of dense mafic magmas in the western Snake River Plain, during the “head to tail” transition of the Yellowstone hotspot at ~12-9 Ma.