The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains

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Abstract
Summary: The thick (∼40 km) slab of Hudsonian (>1750 Ma) continental crust that extends under western Canada from the Canadian Shield can be followed westward, on the basis of its distinctive magnetic anomalies, its influence on the Bouguer gravity values, the results of deep seismic refraction experiments, and the results of geomagnetic depth sounding of the deep electrical conductivity structure, to the Kootenay Arc. The Kootenay Arc is basically a W-facing monocline of crustal dimensions, across which the change in structural level involves an aggregate stratigraphic thickness of up to 20 km. It marks the western edge of the continental craton over which the displaced supracrustal rocks have been draped.Balanced structure sections of the thrust and fold belt, which take into consideration the deep crustal structure, as constrained by the geophysical data, show that: (i) in early Campanian time the continental crust that now lies beneath the western Rocky Mountains and the Purcell anticlinorium was covered with the platformal Palaeozoic to Upper Jurassic rocks and the exogeoclinal Mesozoic rocks that now form the northeasterly verging imbricate thrust slices of the eastern Rocky Mountains; (ii) the Cordilleran miogeocline developed outboard from the edge of the continental craton, on tectonically attenuated continental crust, or oceanic crust; and (iii) tectonic shortening of about 200 km in the supracrustal rocks in the Rocky Mountains must be balanced at a deeper level, W of the Kootenay Arc, by the shortening of the oceanic or attenuated continental crust.The net convergence between the Cordilleran magmatic arc and the autochthonous cover on the continental craton is a type of intra-plate subduction that was antithetic to the SW verging subduction zone marking the boundary of the North American Plate. The basement of the back-arc or marginal basin, in which the miogeocline formed, was consumed; but the adjoining continental margin was not. The foreland thrust and fold belt is a shallow subduction complex that was tectonically prograded over the margin of the continental craton, as the supracrustal cover scraped off the down-going slab was piled up against the overriding mass, and spread laterally eastward under its own weight.