SURFACE PROCESSES DRIVEN BY PLUME-LITHOSPHERE INTERACTION: USING COSMOGENIC 10BE RADIONUCLIDES WITH A RIVER INCISION MODEL TO STUDY LATE MIOCENE LANDSCAPE EVOLUTION IN CENTRAL IDAHO

Rivers respond to tectonic and geologic events that affect their base-level. Transient landscapes result when changes in base-level send propagating waves of incision through a drainage network. Understanding the timing, pace, and regional extent of transient adjustment can therefore provide insight into tectonic and geologic events of the past. The mountains of central Idaho offer an opportunity to implement such an investigation. Throughout the tributaries of the Salmon River watershed, there is an observable break in slope that separates low relief headwaters from the high relief main stem of the Salmon River gorge. We use this contrast in topography and erosion rates derived from cosmogenic radionuclide concentrations to calibrate a river incision model that simulates the past evolution of channel long-profiles. Our model results suggest that the canyon-forming 3-fold increase (from 0.05 mm/yr to 0.15 mm/yr) in the incision rate of the Salmon River began roughly 9.3 Ma ± 1.5 and has continued at this elevated rate. The similarity of our model results with other long-term regional exhumation data implies a regional driver of landscape evolution. Additional channel long-profile analyses reveal a trend of decreasing exhumation rates that is congruent with both decreasing surface elevation and a decrease in the number of thermal springs sites along a transect from the northern flank of the Snake River Plain to the panhandle of Idaho. To explain these phenomena, we suggest that detachment of a lithospheric root facilitated by the arrival of the Yellowstone Plume caused a flexural-isostatic uplift that reaches ~400km to the north of the Snake River Plain.