From the middle Mesozoic (approximately million year ago) to the present, the topography of the western Cordillera of North America evolved in response to a diverse history of uplift, exhumation, and down-drop, due in part to first convergence and then transform motion along the western margin of North America. This time interval also saw many dramatic changes in global climate, including the transition from a Mesozoic-Paleogene greenhouse to a Neogene world with ice on the poles. The role of climate change in the tectonic evolution of the western Cordillera has been much discussed, as has the influence of topography on paleoclimate. In this project, two cutting-edge techniques from stable isotope geochemistry will be used to determine past temperatures and paleohydrology, which will allow reconstruction of past elevations across the northern Basin and Range Province. Variations in past temperature and hydrology will be mapped during three key time intervals - the warm Late Cretaceous (approximately 70 million years ago) and Early Eocene (approximately 55 million years ago), and the relatively cool Late Miocene (approximately 10 million years ago). By mapping isotopic variations at different time slices, past elevations at interior sites can be estimated while controlling for the effects of global climate change on geochemical records. This approach will address outstanding questions about the tectonic history of western North America, such as: Was there a very high plateau in western Nevada in the Late Cretaceous, analogous to the current South American Altiplano? Was the region east of the Sierra Nevada Mountains at low or high elevation during the Eocene? The answer should be clear in temperature and isotope hydrology profiles across the region.
Elevation is a fundamental parameter in tectonics. Reconstructing past elevation is therefore critical for testing geodynamic models that have been proposed to explain the tectonic evolution of the western Cordillera. In addition to constraints on the tectonic evolution of the western U.S., this study will add to the growing body of continental paleoenvironmental data that are critical for comparison to output from computer climate simulations. As new simulations are developed at finer spatial scales, it is essential to have paleoclimatic data at a similar scale, particularly in regions with great topographic relief.
