We provide information on the physical properties across two important morphogeological regions of the southeastern Canadian Cordillera, the Omineca and Intermontane Belts, and tie together two previously interpreted wide-angle seismic lines which have separately probed each zone along strike. The data analyzed here, Line 7 of Lithoprobe's 1990 Southern Cordillera Refraction Experiment in southern British Columbia is along a 410-km profile from the Fraser River to Radium Hot Springs near the Rocky Mountain Trench. Iterative forward and inverse modeling of the seismic travel times, constrained by seismic amplitude characteristics and earlier cross-line interpretations, defines a thin crust with relatively low velocities. The upper crust has an average thickness of 15.3 km and an average P wave velocity of less than 6.1 km s-1 with velocities at the bottom reaching 6.2--6.3 km s-1. It is further characterized by velocity gradients which decrease markedly with depth and by low-velocity trends which correlate with the traces of crustal-scale shear zones. A high-velocity region at the west end of the profile is coincident with a major uplift structure. The midcrust has an average thickness of 9.2 km with velocities below 6.1 km s-1 except for the extreme west end of the profile. This defines a midcrustal low-velocity zone (LVZ) in the Omineca Belt and partly in the Intermontane Belt. The lower crust, from 24 km to 34 km depth, has velocities averaging 6.6--6.7 km s-1.

Higher velocities in the deep crust may mark the westward extend of North American cratonic rocks at the Okanagan Valley. The upper mantle velocity has an average 8.0 km s-1 increasing to 8.2 km s-1 at the maximum depth of ray penetration at 48 km. The model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles. The velocity structure of the crust, especially the LVZ, closely correlated with the regional isotherms. While there seems to be no well-defined velocity boundary between the Intermontane and Omineca Belts, there are distinct velocity differences between the Intermontane and neighboring Coast Plutonic Complex to the west. The low-velocity characteristics of the Omineca and much of the Intermontane belts may be a consequence of temperature control of the physical parameters of the crust. The lithosphere in the southeastern Canadian Cordillera is clearly anomalous with respect to much of northwest North America, and in terms of crustal thickness and velocity structure, it is similar only to the Basin and Range province of the United States. ¿ American Geophysical Union 1995