TWO MODES OF TERRANE ACCRETION IN THE CANADIAN CORDILLERA REVEALED BY LITHOPROBE SEISMIC SURVEYING

Lithoprobe seismic reflection and refraction surveys traverse the Canadian Cordillera in southern and northern British Columbia and the southern Yukon. Prominent reflectivity can be followed from outcrop in the foreland westwards, beneath the accreted terranes as far west as the eastern Coast Mountains, and indicates that pre-Cordilleran North American crust extends beneath most of the Cordillera. The inner terranes are primarily confined to the crust above these reflections, leading to the conclusion that they were decoupled from their lithospheric roots and obducted onto North America. In northern B.C. and the southern Yukon, Quesnellia and Yukon-Tanana rocks occur in thin thrust sheets and klippen, and in southern B.C. Quesnellia forms a thin infrastructure above ductile-deformed rocks of the North American margin. Farther west, Cache Creek terrane is poorly reflective and is confined to the upper crust. Stikinia may occupy most of the crust and is characterized by prominent reflectivity, the origin of which is uncertain.

The outer terranes underlie most of the Coast Mountains. They appear to have been thrust beneath North America and were attached by tectonic underplating, or subcretion. The outer terranes appear to have been stacked such that their age of accretion decreases westward and with increasing depth. Beneath Vancouver Island and the southern Coast Mountains, reflections increase in easterly dip with increasing depth. Accordingly, this fan-shaped pattern may be an indication of subduction rollback. Eastward-dipping mantle reflections between Skagway and Carcross likely delineate the relict Kula plate.