CRUSTAL VELOCITY STRUCTURE OF THE OMINECA AND INTERMONTANE BELTS, SOUTHEASTERN CANADIAN CORDILLERA

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 inter preted wide-angle seismic lines which have separately probed each zone along strike. The data analyzed here, Line 7 of Lithoprobe's 1990 Sou thern Cordillera Refraction Experiment in southern British Columbia is along a 410 km profile from the Eraser River to Radium Hot Springs ne ar the Rocky Mountain Trench. Iterative forward and inverse modeling o f the seismic travel times, constrained by seismic amplitude character istics and earlier cross-line interpretations, defines a thin crust wi th 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) w ith velocities at the bottom reaching 6.2 - 6.3 km s(-1). It is furthe r characterized by velocity gradients which decrease markedly with dep th and by low-velocity trends which correlate with the traces of crust al-scale shear zones. A high-velocity region at the west end of the pr ofile 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 lo w-velocity zone (LVZ) in the Omineca Belt and partly in the Intermonta ne Belt. The lower crust, from 24 km to 34 km depth, has velocities av eraging 6.6 - 6.7 km s(-1). Higher velocities in the deep crust may ma rk the westward extent of North American cratonic rocks at the Okanaga n Valley. The upper mantle velocity has an average 8.0 km s(-1) increa sing 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 struc ture of the crust, especially the LVZ, closely correlates with the reg ional isotherms. While there seems to be no well-defined velocity boun dary between the Intermontane and Omineca Belts, there are distinct ve locity differences between the Intermontane and neighboring Coast Plut onic Complex to the west. The low-velocity characteristics of the Omin eca and much of the Intermontane belts may be a consequence of tempera ture control of the physical parameters of the crust. The lithosphere in the southeastern Canadian Cordillera is clearly anomalous with resp ect to much of northwest North America, and in terms of crustal thickn ess and velocity structure, it is similar only to the Basin and Range province of the United States.