CENOZOIC TOPOGRAPHIC EVOLUTION OF THE CENTRAL ROCKY MOUNTAINS CONSTRAINED FROM STABLE ISOTOPE GEOCHEMISTRY

The central Rocky Mountains in the western U.S. is a sequence of isolated basement-cored mountain ranges up to 4 km high and intermontane basins of 1.5-2 km high. The region was near sea level at ~80 Ma ago. The history of subsequent vertical movement to present day topography is less well constrained, yet is crucial for understanding the tectonic processes and driving forces that caused the high topography. Recent applications of stable isotope paleoaltimetry have provided a rich amount of paleoelevation data that can be used to test the geodynamic models of the formation of the central Rocky Mountains. Reconstructed paleowater δ¹⁸O values from fossil bivalves, fluvial carbonate cements, and soil carbonates show that during the early Eocene, the basement-cored ranges had been exhumed to an elevation similar to today, but the basin floors remained low (~0.5 km). The elevation gain of the ranges seem to mainly occur during the late Paleocene – early Eocene, following a southeastward younging trend. This trend suggests the uplift of the basement-cored ranges was the result of eastward roll-back or detachment of the Farallon flat slab. Reconstructed paleowater δD values from hydrated volcanic glass in the last ~37 Myr show that the paleowater δD values increase eastward to the adjacent Great Plains. This gradient is similar to that of present day meteoric water and was established before 37 Ma. Given that the eastward increase of precipitation isotope values is a result of decreasing mean surface elevation from the central Rocky Mountains to the Great Plains, the establishment of the present day water isotope gradient as early as the early Oligocene suggests that the high central Rocky Mountains has been a major topographic feature before the early Oligocene. The uplift of the basin floors may have occurred during the late Eocene when a regional unconformity was formed between the uppermost Cretaceous and lowermost Oligocene strata in Nebraska and South Dakota, and the depositional hiatus or erosion of the late Uintan-Duchesnean age occurred in Wyoming. Late Eocene uplift to modern topography in the central Rocky Mountains do not support any large-magnitude (>0.5 km) uplift after the early Oligocene.