Palinspastic reconstructions based on balanced sections that are constrained by deep crustal structure, as outlined by seismic refraction, gravity anomaly, magnetic anomaly, and geomagnetic depth sounding studies, show that: (1) the Cordilleran miogeocline, a northeast-tapering wedge of craton-derived sedimentary strata, more than 15 km thick, accumulated outward from the rifted(?) edge of a 35-km thick slab of early Proterozoic continental crust, on a basement of oceanic crust and/or attenuated continental crust; (2) the miogeocline was compressed, detached from underlying crustal rocks, and displaced more than 200 km northeast as two successive collages of small allochthonous terranes from the adjacent ocean basin collided with North America; (3) the overthrust belt is tectonically prograded shallow accretionary prism that formed during the subduction of the basement of the miogeocline, as supracrustal rocks were scraped off the underriding continental craton and accreted to the overriding miogeoclinal prism; (4) subsidence and molasse sedimentation in the northeastward-migrating foreland basin were a result of isostatic flexure of the lithosphere in response to the weight of the encroaching accretionary prism, and of the molasse itself; and (5) burial of source rocks, and hydrocarbon generation, migration and entrapment are indirect results of the subduction of the lithosphere that formerly lay beneath the miogeocline.

The first collision (Late Jurassic and Early Cretaceous Columbian orogeny) involved outward-verging thrusting and folding on either side of the uplifted core of the miogeocline, and produced a thick wedge of molasse (Kootenay-Blairmore) that extended over the western part of the craton. Mid-Cretaceous granitic plutons truncate Columbian structures. The second collision (latest Cretaceous and Paleocene Laramide orogeny) marked the final phase of convergence during which the reservoir structures associated with northeast-verging listric thrust faults and folds developed in the Canadian Rockies. Source rocks were buried to depths of 13 km under the Lewis thrust sheet in southeastern British Columbia, and 5 or 6 km under the plains, as a wedge of Laramide molasse (Brazeau-Paskapoo) was pr graded northeastward in front of the overthrust belt.

Intracontinental transform faulting, involving 450 km of righthand strike slip on the Tintina-Northern Rocky Mountain Trench fault system, was partly taken up by thrust faulting in the Rocky Mountains south of 56°N lat. during the Laramide orogeny, but during the Eocene it was linked to the en echelon Fraser River fault zone by ductile stretching of the intervening lithosphere. This stretching is expressed, over an area of about 150,000 km² in south-central British Columbia and adjacent parts of the United States, at a shallow level, by listric normal faults and Eocene dike swarms, and at a deep level, by boudinage of the whole crust. Supracrustal rocks moved into the necked zones between the boudins as the metamorphic core complexes emerged in northeast-trending domal culminations with K-Ar mica cooling ages of about 50 Ma. A different pattern of regional extension, involving uplift and partial unroofing of deeply buried source rocks in the southern part of the over-thrust belt and adjacent foreland basin, was established in early Oligocene time.

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