CONSTRAINING EXHUMATION OF THE SAWTOOTH AND WHITE CLOUD MOUNTAINS BY (U-TH)/HE ZIRCON THERMOCHRONOLOGY

The Sawtooth Valley of central Idaho is bounded by normal faults on both sides and separates the Sawtooth and White Cloud Mountains to the W and E, respectively. New (U-Th)/He zircon ages were obtained from rocks exposed in this area to determine the cooling and exhumation history. These data were collected as part of the IDOR EarthScope project in order to provide a Tertiary tectonic framework along the east-central edge of the 430-km long active seismic line.

Two samples from the White Cloud Mountains yield ages of 44.5 ± 0.7 Ma and 62.7 ± 1.0 Ma. The younger age corresponds to a Challis volcaniclastic unit exposed at 2502 m, while the older age corresponds to a sample from the Idaho batholith granite exposed at 2914 m. In contrast, samples from the Sawtooth Mountains to the west yield younger ages ranging from 20.2 ± 0.3 to 34.2 ± 0.5 Ma. We note the samples in the Sawtooth Mountains cross lithologic boundaries between Sawtooth (Challis age: Eocene) pink granite and Cretaceous Idaho batholith. Five (U-Th)/He zircon ages were obtained from the Sawtooth Range. Three sample localities occur at similar structure relief, near the top of Sawtooth fault block, and yield ages of 33.3 ± 0.5 to 34.2 ± 0.5 Ma. This range is consistent with (U-Th)/He zircon ages obtained from other localities in the Atlanta lobe of the Idaho batholith. In contrast, the two samples at lower structural levels in the Sawtooth Mountains give younger ages of 20.2 ± 0.3 Ma and 25.0 ± 0.4 Ma.

The results indicate that the rocks exposed in the While Cloud Mountains cooled below 200°C ~20 m.y. earlier than the Sawtooth Mountains. We interpret this differential cooling to reflect significantly more Late Tertiary exhumation on the W side of Sawtooth Valley. Further, the Challis-age date from the low elevation site in the White Cloud Mountains suggests that these rocks were never significantly buried after the Eocene. We also interpret the younger (U-Th)/He ages in the structurally lower parts of the Sawtooth Mountains to result from exhumation along the Sawtooth fault. These ages suggests that exhumation began before ~20 Ma, prior to initiation of Basin and Range tectonism.