TESTING FOR UPLIFT BY THE YELLOWSTONE HOTSPOT USING GALLATIN RIVER TERRACES, SW MONTANA

The Yellowstone region is affected by a premier example of a continental hotspot, providing a setting where hypotheses can be tested on how a mantle plume drives tectonic deformation and influences landscape evolution. Pierce and Morgan’s synthesized model of transient deformation around the hotspot -- with uplift along the Yellowstone Crescent of High Terrain (YCHT) and subsidence along the Snake River Plain (SRP) -- has yet to be tested empirically. Recent work analyzing Pleistocene fluvial terraces on the south flank seems to confirm baselevel fall downstream in the Snake River Plain (SRP), but not necessarily uplift along the YCHT. Our study transect of the Gallatin River is directed off the north flank of the Yellowstone Plateau, not into the Snake River Plain. In this case, Pierce and Morgan’s model predicts uplift of the Gallatin headwaters. More rapid headwater incision rates in response to this should cause Pleistocene terraces to converge downstream. To test for this, we map the river corridor, correlate terraces along 110 km of the upper Gallatin River, and develop a luminescence chronology of deposits. The most extensive and traceable terrace deposits are T2 and T3, which initial age results suggest date to marine isotope stage 2 (the last glacial maximum) and 4, respectively. The T2 basal strath is never exposed above the modern river channel. The T3 strath is consistently 6-12 m above the channel in the few places it is exposed, and observations suggest that the T3 may increase slightly in height downstream, instead of converging downstream. The upper treads and overall thickness of these terrace deposits also remain relatively consistent, with the exception of deposits being inflated in the narrowest part of the Gallatin Canyon due to coarse sediment input from outwash and landslide activity. Initial results also indicate that Late Pleistocene incision rates are relatively low along the Upper Gallatin–less than 100 m/Myr in the extent of time that the terraces record. Overall, our results do not match the conceptual model, which predicts measurably higher incision rates upstream and downstream terrace convergence. The implication of this work is that the pattern of active (Late Pleistocene) uplift across the landscape due to the hotspot differs from a horseshoe-shaped crescent coinciding with the YCHT.