Three-dimensional crustal structure of the Eastern Cordillera, southwestern Canada and northwestern United States

Abstract

LITHOPROBE and COCORP crustal seismic reflection profiles across the southwestern Foreland and Omineca belts of southern British Columbia and the adjacent United States provide regional three-dimensional coverage of orogen-parallel structural variations. In this area, North American cratonic basement and cover rocks are deformed into east-verging folds and thrusts, which increase in scale and depth to the west. This simple pattern may be complicated by out-of-sequence faulting in the upper and middle crust of the Kootenay Arc and Purcell anticlinorium in southern British Columbia. The southern Omineca belt is the structural and metamorphic hinterland of the Foreland belt and contains the Monashee-Priest River crustal-scale tectonic ramp, which records a protracted history of east-directed contraction. It is overlain by the Eagle River-Vernon-Colville crustal-scale antiform, of probable Late Cretaceous to Paleocene age, which locally may have formed above blind thrusts. An embayment in the autochthonous craton between the Monashee and Priest River ramps corresponds spatially with the Kootenay Arc structural salient and eastward deflection of the lower Paleozoic shelf-to-basin facies transition. The south flank of the Monashee and Valhalla complexes and the north flank of the Purcell anticlinorium near the international border correspond with the lateral margins of this east-trending embayment. On an even larger scale, the increasing northward tectonic overlap of accreted terranes onto thick cratonic crust from the northern United States into British Columbia and associated basement thickening coincide spatially with the northward increased width and depth of the early Tertiary foreland basin. In this part of the Cordillera, Eocene crustal extension did not completely mask older contractional fabrics and appears to have been accomplished by two mechanisms: (1) in the east, by crust- penetrating shear zones that separate the Omineca and Foreland belts, and (2) in the west, by crustal-scale boudinage within and west of the Omineca belt.