Linking exhumation, paleo-relief, and rift formation to magmatic processes in the western Snake River Plain, Idaho, using apatite (U-Th)/He thermochronology

The western Snake River Plain (WSRP) in southwest Idaho has been char-acterized as an intracontinental rift basin but differs markedly in topography and style from other Cordilleran extensional structures and structurally from the down-warped lava plain of the eastern Snake River Plain. To investigate mechanisms driving extension and topographic evolution, we sampled granit-oid bedrock from Cretaceous and Eocene-aged plutons from the mountainous flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates from seventeen samples range from 7.9 +/- 1.4 Ma to 55 +/- 10 Ma. Most cooling dates from Cretaceous plu-tons adjacent to the WSRP are Eocene, while Eocene intrusions from within the Middle Fork Boise River canyon similar to 35 km NE of the WSRP yield Miocene cooling dates. The AHe dates provide evidence of exhumation of the Idaho batholith during the Eocene, supporting a high relief landscape at that time, followed by decreasing relief. The Miocene AHe dates show rapid cooling along the Middle Fork Boise River that we take to indicate focused river inci-sion due to base level fall in the WSRP. Eocene AHe dates limit magnitudes of exhumation and extension on the flanks of the WSRP during Miocene rift formation. This suggests extension was accommodated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. We favor a model where WSRP extension was related to Columbia River Flood Basalt eruption and enhanced by later eruption of the Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent difference with other Cordilleran extensional structures.

Automated summary

The western Snake River Plain in southwest Idaho has unique topography and style compared to other Cordilleran extensional structures. By using apatite (U-Th)/He thermochronometry, researchers were able to determine the exhumation history of the region's granitoid bedrock. The results suggest that the uplift of the area occurred during the Eocene period and was influenced by later volcanic eruptions.