OLIGOCENE-MIOCENE SURFACE UPLIFT OF THE SOUTHERN ROCKIES BASED ON VOLCANIC GLASS HYDROGEN ISOTOPES

The uplift history of the southern Rocky Mountains (Rockies) remains enigmatic because of consecutive Cenozoic deformations. Previous studies to the surface uplift history can be classified into two schools, including that the southern Rockies has gained its modern or a higher-than-modern elevation during the Laramide Orogeny by crustal shortening and thickening, and that the southern Rockies only gained part of its modern elevation from the Laramide Orogeny and experienced Neogene uplift because of changes in lithosphere density, thermal buoyance, or both. Here we first compile modern river water stable isotope data to understand spatial patterns and their controlling factors, then apply the understanding to the late Eocene-Miocene surface water hydrogen isotope compositions (δD) reconstructed from δD values of hydrated volcanic glass samples from the southern Rockies, its adjacent Great Plains and southern Texas to constrain the surface uplift history of the southern Rockies. We find that the lowest modern river water δD has a latitudinal gradient of -3 ‰/degree in the studied portion of the Great Plains, lower than North American average. Our reconstructed surface water δD values increase between ~36-6 Ma in the three studied regions, consistent with the records in the central Rockies and its adjacent Great Plains. This trend reflects gradual drying during the middle-late Cenozoic global cooling. The amount of surface water δD increase in the southern Rockies is ~20‰ smaller than that in southern Texas, which we suggest to be caused by that surface uplift reduced δD increase. We apply local modern isotopic gradient and consider the influence of climate change when reconstruct the paleoelevation of the southern Rockies. Our results show that the southern Rockies were >1 km lower than its modern elevation during the late Eocene, experienced rapid uplift of ~750-1000 m during the early Oligocene and gradual uplift of ~500 m during the late Oligocene-Miocene. We attribute the rapid uplift to crustal inflation associated with ignimbrite flare-up and associated removal of the Farallon flat slab, and the gradual uplift to changes in thermal buoyancy associated with crustal thinning and thermal heating during the opening of the Rio Grande Rift.