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

There is a pronounced increase in heat flow and lithosphere temperatures ac ross the transition from the stable North American Craton to the southeaste rn 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 he at flow data in the Cordillera but measurements in the Western Canada Sedim entary Basin overlying the adjacent Craton are in petroleum exploration wel ls with inherent low accuracy and are affected by hydrological effects. How ever, the deep thermal boundary is well defined based upon contrasts in sev eral other temperature-sensitive geophysical parameters. The boundary is lo cated 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. Temperature s 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 these estimated from the temperature dependence of uppermost mantle seis mic 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 tem peratures just below the Moho are 400-500 degrees C for the Craton and 900- 1000 degrees C beneath the Cordillera based upon all three constraints. Two temperature-sensitive changes across the thermal boundary are examined. Th ere is a westward decrease in crustal thickness from 40-50 km for the Crato n to 32-34 km for the Cordillera with no significant change in average elev ation nor in gravity. Isostatic balance is maintained by thermal expansion and density reduction in the high-temperature Cordillera lithosphere. The a verage temperature to a depth of 150 km is about 400 degrees C higher for t he Cordillera compared to the Craton. There also is a pronounced westward d ecrease 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 r esult, the foreland belt deep crust and upper mantle have remained undeform ed during late Mesozoic to early Tertiary tectonics. Shortening occurred pr imarily in overlying sedimentary thrust sheets, mediated by high pore fluid pressures. In the Cordillera hinterland, the high temperatures result in t he rheological lithosphere with significant strength being Limited to the u pper 10-15 km of the crust. Mesozoic-Cenozoic shortening deformation and su bsequent extension included this whole thin lithosphere. (C) 1999 Elsevier Science B.V. All rights reserved.