STABLE ISOTOPE-BASED PALEOELEVATION RECONSTRUCTIONS IN COUPLED BASIN-DETACHMENT SYSTEMS: THE NORTH AMERICAN CORDILLERA- A CENOZOIC OROGENIC PLATEAU?

High-elevation orogenic plateaux exert a strong control on global climate and therefore, it is critical to understand their topographic history. Despite its importance for understanding the mechanisms of plateau formation and its impact on global climate relatively little is known of the Cenozoic topography of western North America. Commonly, stable isotope-based paleoelevation studies exploit changes in meteoric water composition in the near-surface record. We extend this approach by combining multi-proxy, multi-isotope data from syntectonic basins and kinematically linked extensional mylonite zones that record paleotopographic and climatic changes during Cenozoic extension of the North American Cordillera. Combined stable isotope and geochronological data from Eocene and Oligocene/Miocene extensional detachments of the Shuswap/Kettle/Pioneer (British Columbia/Washington/Idaho) and Raft River/Ruby Mountains core complexes (Utah/Nevada) indicate that temporal and spatial variations in topography were closely related to crustal-scale extension. Hydrogen, oxygen and Ar/Ar isotope data from Eocene detachments of the Shuswap/Kettle core complexes suggest mean elevations of ~ 4000 m immediately preceding extensional deformation at 49.0 - 47.0 Ma and indicate that crustal thickening prior to detachment faulting resulted in high mean elevations for the Cordillera north of the Snake River. In contrast, stable isotope data from mylonites of the Raft River and Ruby Mountains core complexes indicate a more complex elevation history with significant elevations still present in the Miocene. Our stable isotope data from coupled basin-detachment systems in the Great Basin display a remarkable consistency but also highlight some of the uncertainties associated with stable isotope-based paleoelevation reconstructions. With this complementary approach, the isotopic composition of meteoric water is determined in low and high-temperature environments, over a wide range of time scales and with variable temporal resolution, and with respect to the timing of major tectonic structures. The data thus eliminate some of the problems (e.g. climate vs. topographic forcing) associated with paleoelevation reconstructions based on a single proxy.