THE COLUMBIA PLATEAU REVEALED; LOW TEMPERATURE THERMOCHRONOLOGY ACROSS THE CANADIAN CORDILLERA

Active deformation and mountain building in the Canadian Cordillera ceased in the Paleogene, a period during which this orogen was among the highest on earth. The present morphology, relief and altitude of the mountains and the foothills in the adjacent foreland belt are the result of a feedback relationship between erosion and isostatic compensation. Our low temperature thermochronology data (U-Th/He ages on zircon and apatite) were collected from 5 sample-transects spanning the Intermontane, Omineca, Foreland, and Foothills belts along a Jasper-Vancouver transect. The data show rapid cooling (>15℃/Ma) from >180℃ to <70℃ in the interior belts to the west of the Rocky Mountain Trench (RMT) during the Eocene (56 – 33 Ma). In the Foreland and Foothills belts to the east of the RMT the data show modest cooling (~4℃/Ma) during the latest Cretaceous to Early Eocene (~80Ma – 55 Ma) followed by rapid cooling (~15℃/Ma) between ~55 Ma and ~45 Ma transitioning to slower cooling (~1℃/Ma) after 45 Ma. This cooling pattern is consistent with rapid regional uplift and exhumation across the interior belts of the Cordillera during a regional transition from contraction to extension in the early to middle Eocene. At the same time in the Foreland and Foothills belts, during a period of accelerated cooling, contraction was continuing. The combination of rapid exhumation and extension in the interior belts and rapid exhumation and contraction in the Foreland and Foothills belts is similar to what is observed around the Altiplano and Tibetan plateaus. This suggests that the Canadian Cordillera is a fossil plateau (Columbia Plateau) that formed in the Eocene. The lack of a mantle lithosphere beneath the interior belts of the Canadian Cordillera (west of the RMT) and the regional extent of rapid exhumation suggests that the Columbia Plateau formed in response to a large-scale lithosphere removal event; possibly wholesale delamination.