ABSTRACT

The Western Canada Sedimentary Basin (WCSB) is the thinner part of the same northeast-tapering wedge of supracrustal rocks that forms the eastern part of the Cordillera. The deformed southwest part of the wedge is an accretionary prism. It was scraped off the under-riding North American plate, and was tectonically prograded northeastward while an over-riding collage of tectonic flakes, comprising various displaced terranes, converged obliquely with North America. Palinspastic reconstructions of the accretionary prism provide the framework for stratigraphic analysis of the pre-collisional tectonic history of the continental margin, and of its implications for the concurrent tectonic evolution of the WCSB.

Subsidence and uplift in the basin were controlled mainly by isostatic flexure of the Early Proterozoic continental lithosphere in response to "loads" created by changing plate tectonic regimes at the continental margin. Late Proterozoic rifting defined the configuration of the margin. A

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sub-Cambrian "break-up" unconformity marks the rift-drift transition. Cambrian to Middle Ordovician passive margin thermal subsidence ended with Middle Ordovician to Late Devonian unstable shelf deformation (block faulting with >5 km of local sub-Fairholme stratigraphic relief), which was accompanied by widespread epeirogenic basin and arch formation on the North American craton, and was associated with the establishment of an unstable outboard volcanic arc and marginal basin regime that persisted until terrane accretion began.

Westward subduction of the floor of the marginal basin and of the outboard part of the continental terrace sedimentary prism during Early? and Middle Jurassic time resulted in the collapse of the marginal basin and the obduction of marginal basin and oceanic arc strata, but had little effect on the WCSB. Late Jurassic-Early Cretaceous left-hand oblique terrane convergence involved tectonic wedging of displaced terranes between the North American supracrustal rocks and their crystalline basement. It produced tectonic thickening along the continental margin, isostatic flexure of the North American lithosphere, and a broad foreland basin that extended from Montana to Alaska, and far into the WCSB. Late Cretaceous to earliest Eocene right-hand convergence between North America and the displaced terranes was transformed, north of 54°N, into strike-slip on the Tintina-Northern Rocky Mountain Trench fault system. A large accretionary prism, involving substantial crustal thickening and deep foreland basin subsidence, formed south, but not north of 54°N. Erosional unloading and isostatic rebound of the lithosphere beneath the accretionary prism and the foreland basin was greatest during the Early and Middle Eocene (59-42 Ma) when the strike-slip faulting was transformed into right-hand transtension of the Cordillera south of 54°N. However, it also continued after 42 Ma when the North American plate boundary had shifted to the vicinity of the present continental margin.

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