EXHUMATION AND FAULTING IN HELLS CANYON, NORTH AMERICA’S DEEPEST GORGE

Here we present new low-temperature thermochronology data from Hells Canyon on the OR-ID border. This canyon is the deepest fluvially incised gorge in North America, where relief from valley-floor to the top of the adjacent Seven Devil Mountains is nearly 2700 m. Canyon carving must post-date the 16.4-15.9 Ma Grande Ronde flows of the Columbia River Basalts (CRB) through which it cuts, but the exact age of the canyon and mechanism behind its formation are unclear. Previous researchers surmise that canyon formation likely coincided with the late Miocene to early Pliocene draining of Lake Idaho and subsequent integration of the Snake River into the Columbia River watershed. Exhumation and landscape evolution in this region may also relate to crustal and mantle dynamics linked to the Miocene passage of the Yellowstone Plume (e.g., lithospheric foundering).

In order to study exhumation across this region, apatite (AHe) and zircon (ZHe) U-Th/He low-temperature thermochronometry data were collected from 13 samples in a vertical transect spanning 2 km along the deepest section of Hells Canyon. Apatite cooling ages reveal a period of early to mid-Cenozoic exhumation post-dating activity along the Late Cretaceous Western Idaho Shear Zone and predating the formation of Hells Canyon. The highest elevation samples yield Late Cretaceous and early Paleogene AHe cooling ages. The sample collected at the bottom of the canyon has a mean AHe age of 32 ± 2 Ma. ZHe ages show no age-elevation trend. A mean age of 98 Ma is representative of the ZHe sample suite. We interpret the younger AHe age to mean that by the early Oligocene rocks at the bottom of the canyon were likely within the upper 2 km of the crust. Structural relief on CRB flows across the canyon indicates ~1 km of down-to-the-west differential uplift, which suggests that a west-dipping normal fault may be responsible for much of the striking relief that we see today within Hells Canyon. Subsequent fluvial erosion would then have contributed at most 1.5 km to the overall relief of the canyon. Neither post-CRB faulting, nor fluvial incision appear to be recorded directly by our current thermochronometric data, although potential still lies in the use of geomorphic incision indicators to reveal the true age of Hells Canyon.