LITHOSPHERIC CROSS SECTION OF SOUTHWESTERN CANADA AND THE NORTHWESTERN UNITED STATES

A synthesis of deep seismic data, potential field data and detailed geological information in southwestern Canada and the northwestern United States allows for construction of a regional lithospheric–scale cross section from the foreland basin (Western Canada Sedimentary Basin) to the coast. West of the foreland basin the cross section traverses the Rocky Mountains, the Purcell anticlinorium, the Kootenay arc, the extended Okanagan, and the Coast Mountains. Key results include the following: 1) North American crustal and mantle rocks can be followed in the subsurface at least as far west as the Okanagan; 2) The base of contractional deformation, which in the east carries the Rocky Mountains, projects westward to the lower crust beneath the Kootenay arc and Okanagan regions above the relatively flat Moho; 3) Rocks that have previously been interpreted as accreted terranes in the Kootenay arc are thin and overlie the North American strata along faults which lose displacement into stratigraphic contacts north of the cross section. Mesoproterozoic strata (Aldridge Formation) of the Purcell anticlinorium can be followed in the subsurface beneath these faults as far west as the Eocene Kettle River fault, where they terminate. Stratigraphic, structural and geochronological considerations indicate that the Aldridge strata most likely end because this locality represents the west edge of the Belt-Purcell basin; 4) Extensional faults (e.g., Kettle River fault, Granby fault, Greenwood fault, etc.) have relatively small displacments (a few km each) and are not crustal-scale features; 5) Geochronological data indicate that upper mantle rocks along the cross section are Proterozoic, may have affinity with the North American craton, and, if so, indicate that North American subcrustal lithosphere projects beneath most of the Cordillera, from the foreland to the coast.