QUATERNARY TECTONICS ALONG THE FLANK OF YELLOWSTONE FROM TERRACES ALONG THE HOBACK AND SNAKE RIVER

The Yellowstone hotspot generates one of Earth’s largest active supervolcanos and is thought to be among the most dynamically deforming areas in North America. Motion of the North American plate across the plume produced a sequence of northeast-younging volcanic centers and models predict a corresponding wave of transient and dynamically supported uplift and extension of the upper crust. Moreover, geologic evidence suggests that the Snake River Plain (SRP) in the wake of the plume has been subsiding 4 – 8 km due to a combination of crustal densification and loading by basaltic magmatism. Studying patterns and rates of Quaternary incision of rivers flowing off the Yellowstone plateau can test models about the distribution and timing of active uplift, subsidence, and faulting, improving our understanding of the geodynamics and the hazards in the region.

We present results from surveying and optically stimulated luminescence (OSL) dating of river terraces along the Hoback and upper Snake rivers (western Wyoming and southeastern Idaho), which provide a study transect from the hypothetical Yellowstone crescent of high terrain, across zones of changing fault activity, into the subsiding SRP. Upstream of Alpine, WY, we find that strath terraces record incision rates of 0.1 – 0.3 mm/y along the deeply incised Alpine Canyon and the Hoback River. Instead of decreasing downstream off the axis of hypothesized uplift, incision rates instead vary coherently in relation to apparently active normal faults, providing estimates of their slip. Moreover, incision peaks in a knickzone just above the Grand Valley fault at the mouth of Alpine Canyon, which previous workers indicated has not moved in the Quaternary. Downstream and approaching the SRP, both late Quaternary fill-terraces and older Quaternary basalt-flow markers indicate progressively slower incision rates. These initial results seem to broadly support the expectation of downstream SRP subsidence and upstream Yellowstone uplift, yet the detailed pattern of incision and deformation deviates from previous knowledge. Future study utilizing markers along rivers around Yellowstone will further illuminate the patterns of Quaternary deformation in the region.