DENSITY, TEMPERATURE, AND RHEOLOGICAL MODELS FOR THE SOUTHEASTERN CANADIAN CORDILLERA - IMPLICATIONS FOR ITS GEODYNAMIC EVOLUTION

Two-dimensional density, temperature, and rheological models are constructed for a 350 km northeast-trending transect of the southeastern Canadian Cordillera. All models highlight several major physical differences between foreland and hinterland lithosphere. Significant features of the density model are the presence of an anomalously low-density (3.10 x 10(3) kg . m-3) layer, with a maximum thickness of 12 km, beneath the Moho in the hinterland; the similar densities of the Monashee Terrane and the cratonic crust of the foreland; and an increase in crustal thickness beneath the Southern Rocky Mountain Trench. The temperature model shows steeper gradients and higher Moho temperatures beneath the hinterland than beneath the foreland. In the rheological model the hinterland is characterized by a thin, brittle, upper crust beneath which the entire lithosphere is hot, weak, and ductile. In contrast, the foreland is composed of a thick, brittle, upper crust, with an additional brittle zone in the upper mantle. The Moho is a large strength discontinuity beneath the foreland, and the total lithospheric strength there is an order of magnitude larger than in the hinterland. The models are constrained and supported by geological mapping and a number of independent geophysical data sets. Palinspastic cross sections, together with paleotemperature and paleopressure information, are used to generate a time series of one-dimensional paleorheological profiles at a number of times during deformation. This sequence of profiles indicates that the foreland and hinterland have been rheologically distinct since pre-Late Cretaceous times. The profiles are used to clarify the geodynamic evolution of the area and to explain why deformation remained thin skinned in the foreland whereas in the hinterland the entire lithosphere was deformed.