Exhumation Across Hells Canyon and the Arc-continent Boundary of Idaho-Oregon

Abstract:

Hells Canyon is located along the Idaho-Oregon border. It is proximal to the Salmon River suture zone, the Cretaceous-age western margin of North America that juxtaposes accreted terranes to the west and cratonic North America to the east. We applied (U-Th)/He zircon and apatite thermochronometry to samples along an EW transect across Hells Canyon. (U-Th)/He zircon and apatite ages record the time at which rocks cool below ~ 200 and 60 °C, respectively, providing information on both the timing and rate at which rocks cooled. Samples were collected with respect to structural position relative to the basal Columbia River basalt flow (Imnaha), dated at ~ 17.4 Ma, with most samples taken <100 m below the contact. Given that all localities were at the Earth’s surface - and thus cooled below 60˚C - at ~ 17.4 Ma, the variation in obtained ages are assessed relative to this common datum.

The easternmost sites were taken on the western margin of the Idaho batholith at Lava Buttes, ID at ~2,700 m elevation: The (U-Th)/He zircon and apatite ages are 64.9±4.6 Ma and 53.8±4.9 Ma, respectively. The westernmost sites occur in the Wallowa Mountains, Oregon, where the base of the Imnaha flow exists at ~3,000 m: The (U-Th)/He zircon and apatite ages are 136.2±42.8 Ma and 21.7±10.0 Ma. Additionally, the basal basalt contact occurs at ~900 m and ~600 m at the bottom of the Salmon River Canyon and Hells Canyon respectively. The (U-Th)/He zircon and apatite ages are 73.1±14.6 Ma and 20.0±7.4 Ma, respectively, for the Salmon River Canyon and 88.6±2.4 Ma and 3.4±0.6 Ma, respectively, for Hells Canyon. The data indicate that: 1) The western Wallowa (accreted) terrane cooled below ~200 °C prior to the formation of the Idaho batholith; 2) The western side of the Idaho batholith shows a rapid and consistent cooling between ~200 °C and ~60 °C in the Paleogene; and 3) Samples at low elevation in Hells Canyon cooled below 60˚C in the Pliocene, which requires reburial of the rocks after deposition of the basal portions of the Miocene Columbia River basalts. The young cooling age in Hells Canyon is consistent with recent geomorphological studies constraining incision rates.