MIOCENE REJUVENATION OF THE BOISE MOUNTAINS, IDAHO, FROM NEW APATITE (U-TH)/HE DATA

The geomorphology of the Boise Mountains, Idaho, with deeply incised canyons cutting through low relief mountain tops has led many to suggest that the region has been affected by recent landscape uplift and incision. During Miocene, the eruption of the Columbia River Basalt and/or the passage of the Yellowstone Hotspot in the area could have potentially triggered the uplift and incision in the region, possibly through lithospheric delamination, thermal uplift, or underplating. Additionally, relative base level drop from the opening of the Western Snake River Plain and draining of Paleo-lake Idaho might also have contributed to the rejuvenation of the Boise Mountains. The timing and spatial variation of uplift and relief change from these three proposed geodynamic mechanisms are different, thus the landscape exhumation pattern can be utilized to test which of them are responsible for the landscape rejuvenation. Previous work found evidence for Miocene incision in the Middle Fork Boise River based on limited apatite (U-Th)/He (AHe) thermochronology data (Wetzel and Stanley, 2022). Therefore, we acquired more AHe data to investigate the Miocene cooling histories in the Boise Mountains. The South Fork Payette River and the Middle Fork Boise River are two of the major rivers forming steep canyons in the Boise Mountains. We collected samples of granitoid bedrock from the Cretaceous Idaho Batholith and Eocene-aged intrusions along these two rivers, and an elevation transect from within the South Fork Payette River. Our new apatite samples have average AHe dates are dominantly Miocene and range from 29.9 ± 3.1 Ma to 5.1 ± 0.7 Ma. Both the horizontal transect and the vertical transect at the South Fork Payette River yield strong positive age-elevation correlation, implying rapid cooling and steady or increasing relief during the Miocene, supporting the idea of Miocene landscape rejuvenation in Boise Mountains. Meanwhile, the horizontal transect in the Boise River doesn’t have an obvious age-elevation correlation, which could be due to a complex local thermal profile caused by abundant hot springs in the valley and/or faulting. Ongoing thermokinematic modeling is exploring the compatibility between the cooling patterns predicted by the proposed geodynamic mechanisms and the pattern of the data we collected.