VOLCANIC GLASS HYDROGEN ISOTOPE EVIDENCE FOR LATEST EOCENE HIGH-ELEVATION OF THE SOUTHERN ROCKIES

The high-elevation of the southern Rocky Mountains (Rockies) is a combined result of the latest Cretaceous–Paleogene Laramide tectonics, middle Cenozoic magmatism associated with the Farallon slab removal, Neogene extension, and late Neogene mantle dynamic process. Paleoelevation history is a key to understand their relative importance to topography evolution of the southern Rockies. Here we use δD values of hydrated volcanic glass to constrain the latest Eocene–Miocene relief between the southern Rockies and its adjacent Great Plains, and the relief between the southern Great Plains and a sea-level reference site in south Texas. The volcanic glass δD values vary from -144‰ to -44‰. These values decrease gradually as elevation increases gradually along the transect of south Texas–southern Great Plains–southern Rockies, similar to the trend of modern stream water δD values. These data also show that the δD difference between the southern Rockies and south Texas during the latest Eocene is ~10‰ lower than that of the early Oligocene, and ~20‰ lower than modern difference. This gradient difference is a result of gradual increase of δD in south Texas, but stable δD in the southern Rockies from the latest Eocene to late Miocene. The persisted regional δD trend suggests that the high relief between the southern Rockies and the Great Plains and the relief between the southern Great Plains and the sea-level reference site in south Texas have been established by the latest Eocene. The smaller latest Eocene gradient can be mostly explained by smaller precipitation δD-elevation gradient in a warm climate with 2x preindustrial level pCO₂. The difference between the early Oligocene and modern gradients can be explained by that the southern Rockies gained at most ~500 m mean elevation during the Neogene. If our interpretations can be verified by more data, they suggest that the Neogene extension and late Neogene mantle dynamic process had little influence on the mean elevation of the southern Rockies.