3-DIMENSIONAL CRUSTAL STRUCTURE OF THE EASTERN CORDILLERA, SOUTHWESTERN CANADA AND NORTHWESTERN UNITED-STATES

LITHOPROBE and COCORP crustal seismic reflection profiles across the s outhwestern Foreland and Omineca belts of southern British Columbia an d the adjacent United States provide regional three-dimensional covera ge of orogen-parallel structural variations. In this area, North Ameri can cratonic basement and cover rocks are deformed into east-verging f olds and thrusts, which increase in scale and depth to the west. This simple pattern may be complicated by out-of-sequence faulting in the u pper and middle crust of the Kootenay Arc and Purcell anticlinorium in southern British Columbia. The southern Omineca belt is the structura l and metamorphic hinterland of the Foreland belt and contains the Mon ashee-Priest River crustal-scale tectonic ramp, which records a protra cted history of east-directed contraction. It is overlain by the Eagle River-Vernon-Colville crustal-scale antiform, of probable Late Cretac eous to Paleocene age, which locally may have formed above blind thrus ts. An embayment in the autochthonotts craton between the Monashee and Priest River ramps corresponds spatially with the Kootenay Arc struct ural salient and eastward deflection of the lower Paleozoic shelf-to-b asin facies transition. The south flank of the Monashee and Valhalla c omplexes and the north flank of the Purcell anticlinorium near the int ernational border correspond with the lateral margins of this east-tre nding embayment. On an even larger scale, the increasing northward tec tonic overlap of accreted terranes onto thick cratonic crust from the northern United States into British Columbia and associated basement t hickening coincide spatially with the northward increased width and de pth of the early Tertiary foreland basin. In this part of the Cordille ra, Eocene crustal extension did not completely mask older contraction al fabrics and appears to have been accomplished by two mechanisms: (1 ) in the east, by crust-penetrating shear zones that separate the Omin eca and Foreland belts, and (2) in the west, by crustal-scale boudinag e within and west of the Omineca belt.