The Rise and Erosion of the South Eastern Canadian Cordillera - Cenozoic Cooling, Exhumation and Elevation of the Columbia Mountains and Southern Rocky Mountains in Western Canada from Low-Temperature Thermochronology

The North American Cordillera influences climate on a local and global scale, by forming a distinct barrier to Pacific moisture reaching the continental interior. The extent to which this climatic pattern existed in the past is uncertain, so improving our understanding of the elevation history of the Cordillera is critical to determining the controls on climate change within the Northern Hemisphere [e.g. Foster et al. 2010]. In western Canada, the Cordillera comprises two parallel mountain chains separated by a high elevation (~1100 m) intermontane plateau. The Cenozoic exhumation history of the western range, the Coast Mountains, has been well studied [e.g. Parrish, 1983; O'Sullivan and Parrish, 1995; Farley et al., 2001], while the Cenozoic history of the eastern Cordillera remains poorly constrained. This study presents a comprehensive new apatite (U-Th)/He, apatite fission track and zircon (U-Th)/He dataset of the south-eastern Canadian Cordillera and constrains the Cenozoic cooling, exhumation and elevation history of the area. Cooling ages show rapid cooling (>10°C/Ma) from peak metamorphic temperatures (>500°C) to below 120°C during the Cretaceous to Eocene (75-40 Ma) followed by a period of slow cooling (<1°C/m.y.) and a later phase of rapid cooling (>10°C/m.y.) to below 70°C since the Miocene (10-0 Ma). Corresponding exhumation phases modelled using age-elevation relationships of numerous vertical profiles show 1-5 km of erosion between 70-35 Ma and up to ~2.5 km in current valleys since ~10 Ma. Paleo-surface reconstructions and Paleo-mean elevations estimated from isostasy indicate a high elevation (~2.5 km) but low relief plateau at the end of orogeny (~45 Ma), which gradually lowered in mean elevation by ~1.5 km until ~10 Ma. A later Neogene increase lifted the peaks a further ~2.5 km to their current height, while incising up to 2.5 km of relief. The main causes for both exhumation phases are found to be a combination of lithosphere delamination, asthenosphere upwelling and thermal expansion, while the last phase of incision and surface uplift was further enhanced by glacial incision and isostasy.