Geophysical consequences of the Cordillera–Craton thermal transition in southwestern Canada

Abstract There is a pronounced increase in heat flow and lithosphere temperatures across the transition from the stable North American Craton to the southeastern Canadian Cordillera. The heat flow increases from 40–60 mW m −2 in the Craton to 80–100 mW m −2 in the Cordillera. There are numerous reliable heat flow data in the Cordillera but measurements in the Western Canada Sedimentary Basin overlying the adjacent Craton are in petroleum exploration wells with inherent low accuracy and are affected by hydrological effects. However, the deep thermal boundary is well defined based upon contrasts in several other temperature-sensitive geophysical parameters. The boundary is located 100 km west of the mountain front in the region of the Rocky Mountain Trench, and it must occur over a distance of less than 200 km. Temperatures in the deep crust and upper mantle are first computed from the heat flow and radioactive heat generation data. These temperatures are then compared to those estimated from the temperature dependence of uppermost mantle seismic velocity, Pn, and from xenoliths in kimberlites for the Craton, and in Tertiary volcanics for the Cordillera. Pn velocities decrease from about 8.2 km s −1 in the exposed Craton to 7.8 km s −1 in the Cordillera. The temperatures just below the Moho are 400–500°C for the Craton and 900–1000°C beneath the Cordillera based upon all three constraints. Two temperature-sensitive changes across the thermal boundary are examined. There is a westward decrease in crustal thickness from 40–50 km for the Craton to 32–34 km for the Cordillera with no significant change in average elevation nor in gravity. Isostatic balance is maintained by thermal expansion and density reduction in the high-temperature Cordillera lithosphere. The average temperature to a depth of 150 km is about 400°C higher for the Cordillera compared to the Craton. There also is a pronounced westward decrease in lithosphere strength and thus deformation style. In the Craton, the crust and upper mantle are very strong to at least 100 km depth. As a result, the foreland belt deep crust and upper mantle have remained undeformed during late Mesozoic to early Tertiary tectonics. Shortening occurred primarily in overlying sedimentary thrust sheets, mediated by high pore fluid pressures. In the Cordillera hinterland, the high temperatures result in the rheological lithosphere with significant strength being limited to the upper 10–15 km of the crust. Mesozoic–Cenozoic shortening deformation and subsequent extension included this whole thin lithosphere.