ELEVATING THE WEST: IS THE ANSWER IN PLAINS SITES?

The diversity of tectonic and magmatic events in the western U.S. has permitted a wealth of hypotheses to flourish to explain the modern elevation of the region, from dynamic topography to crustal thickening to magmatic underplating to mantle depletion to lower crustal flow to vertically non-uniform extension to oblique transform motion to delamination/de-blobbing to mantle hydration to lithospheric erosion. Difficulty in determining ancient lithospheric structure and uncertainty in modern density structures has clouded efforts to resolve between these possibilities. Recent work building off of new surface wave and receiver function models suggests that the Cordillera overall are elevated by buoyant mantle but intra-Cordilleran variations are largely crustal in origin. However, some ambiguity exists in the eastern parts of this analysis, where one elevated region is plausibly free of many of these effects: the High Plains just east of the Rocky Mountains and, to a lesser degree, the high plateaus of Wyoming. Existing work from the Deep Probe line north from Wyoming and gravity and seismic data along a line east out of northern Colorado have suggested that the differences in elevations are generated within the crust and yet these differences must be post-Cretaceous in age. Xenoliths near the Deep Probe line suggest a progressive hydration in the lower crust from north to south, and new dates from a hydrated xenolith on the Colorado Plateau indicate substantial crustal hydration during the Laramide orogeny. Hydration of ~10 km thick layer of garnet-rich (30% modal garnet) lower crust that consumes all the garnet could produce ~0.8 km of uplift. If this underlies the Great Plains and Wyoming, it might also be an unrecognized contributor to topography farther west. The difficulty in fully testing this hypothesis lies in identifying and characterizing high-wavespeed lower crust as well as demonstrating a more widespread hydration event in early Cenozoic time.