PROFILE OF A PALEOGENE ANDES IN THE NORTH AMERICAN WEST

Earth's major mountain belts reflect a combination of the plate boundary, internal, and mantle-derived stresses that formed them. Evidence of Cretaceous-Paleogene subduction-driven orogenesis in the North American Cordillera stretches from the Pacific to the Rocky Mountains. Although many of the same subduction-related features exist from north to south, there are fundamental differences between the Cenozoic deformational histories of the regions to the north and south of the Mendocino Triple Junction and Snake River Plain. And these differences extend far beyond Neogene plate boundary changes. Across the northwestern U.S., Eocene sedimentary basins, metamorphic core complexes, and widespread volcanism record a complex tectonic transition as Farallon subduction waned. To determine possible controls on the distribution and progress of extension, we reconstruct the magnitude and distribution of elevations from the Pacific coast across the Rocky Mountains to the Gulf of Mexico coast in the Paleogene.

Here we present new paleoaltimetry data from hydrated Paleogene volcanic glasses (55-30 million years old) that provide a record of the ancient topography of the American west. δD value profiles document moisture transport from known low-elevation coastal locations to the Rocky Mountains. δD values progressively decrease from central Oregon to western Wyoming, showing that the highest elevations in the region were in the area of the Absaroka volcanic province, and not at the Idaho batholith/Challis volcanic province as previously hypothesized. δD values across Wyoming and Colorado exhibit a prominent inverse isotopic gradient, as Pacific-derived moisture mixed with less D-depleted moisture sourced from the Gulf of Mexico, mimicking a pattern seen across the western Andes today. Comparisons to 1D and 3D climate-isotope distillation models confirm that the highest elevations were not located at the location of maximum upper crustal shortening, but at the spatial/temporal intersection between Sevier thrust sheets, Laramide basement-cored uplifts, and the arrival of volcanism related to the rollback of the Farallon slab. Paleogene elevation profiles resemble the central Altiplano-Andes in elevation and morphology and would have had a major effect on climate and atmospheric circulation.