RECORDS FROM COAST TO COAST: NEW TOPOGRAPHIC INSIGHTS ACROSS THE PALEOGENE CORDILLERAN HINTERLAND PLATEAU

Despite covering a relatively small portion of Earth’s surface, high elevation plateaus greatly influence global climate and regional hydrology. In the U.S. Cordilleran hinterland, west of the Sevier thrust front, previous structural analysis, drainage reconstructions, and paleoaltimetry show that Mesozoic shortening produced thickened crust and a region of high topography by the Late Cretaceous. But the Cenozoic evolution and dynamics of this region are widely debated, particularly the onset of mantle lithospheric delamination and initiation of collapse to modern Basin and Range elevations. To reconstruct the topography and morphology of the region, we use hydrogen isotope ratios (δD) of paleo-precipitation preserved in volcanic glass from widespread air-fall ashes and ignimbrites. These data span the Cordilleran system from the Pacific to Gulf of Mexico shorelines and are paired with new sanidine ⁴⁰Ar/³⁹Ar geochronology from the sampled units, providing a novel approach to interpreting changes in δD values over space and time.

Volcanic glass from samples proximal to the Sevier thrust front in western Wyoming have δD values ~30‰ lower than other western U.S. locations. Glass from 48-44 Ma samples record progressive δD value decreases totaling 91–102‰ from the paleo-Pacific coast to the southern Absaroka province, a much larger change than we observe today. These substantially D-depleted waters indicate a major topographic culmination in the northern Rockies, coincident with regions of early Eocene volcanism. To the west, from Utah to eastern Nevada, δD values from 42–28 Ma glasses do not change significantly (±3‰) for over 220 km paleo-distance, suggesting substantial moisture recycling on a long-lived high-elevation plateau. To the east, δD values from 35-28 Ma samples decrease by 20-28‰ to the northwest of Florissant, CO, while a comparison of paleo-Gulf coast and central Rockies δD values suggests air mass mixing across the Laramide province. Our data show that 1) a high-elevation plateau existed across the Cordilleran hinterland by early Eocene and controlled regional moisture transport, 2) the plateau edge was defined by a topographic apex near the Sevier thrust front, and 3) hinterland elevations increased by ~500 m due to slab rollback and delamination and remained high throughout the Paleogene.