Abstract

The Precambrian crystalline basement of western Canada commonly is hypothesized to have been reactivated during the Phanerozoic and "basement control" is used as an explanation for a variety of anomalous features of the sedimentary section including abrupt facies changes, the orientation of reef trends and clastic strandlines, development of fracture porosity, the localization of hydrothermal fluids and the accumulation of hydrocarbons. The inference of basement control is based almost universally on spatial coincidence of patterns observed on potential field maps and satellite images with those seen on maps of the sedimentary section. Lithoprobe seismic reflection data provide a framework for testing these concepts by first unravelling the nature and geometry of Precambrian basement structures and then examining their relationship to Phanerozoic tectonostratigraphic elements. Based on interpretations of Lithoprobe crustal seismic profiles, aeromagnetic anomaly data and comparisons with Phanerozoic structure in the Alberta Basin, we can place an upper limit on the degree of direct control (in terms of seismically resolvable displacements) of Phanerozoic patterns by reactivation of Precambrian structures. Faults and shear zones that formed during the collisional assembly of the basement in the Precambrian can be mapped with both regional aeromagnetic anomaly data and crustal reflection profiles. These structures rarely are reactivated in the Phanerozoic Alberta Basin except in close proximity to the present Cordillera (e.g. Vulcan structure) and locally on the crest of the Peace River Arch area of northwest Alberta. With the exception of a region on the southeast flank of the Peace River Arch, the orientations of faults in the sedimentary section do not appear to coincide with underlying basement structures. These Phanerozoic faults exhibit little spatial collocation with antecedent fabrics within the basement and are associated only locally with an offset of the basement-cover contact. Thus, direct basement control of the presence and orientation of faults in the sedimentary section seems unlikely. We attribute the general paucity of basement reactivation in the Alberta Basin to the thermal history and consequent strength of the region's lithospheric mantle, which has acted as the main, load-bearing layer in the lithosphere and has limited stress transmission into the crust. This contrasts with areas of western Canada where the thermal structure of the mantle has been perturbed by Phanerozoic tectonic events, such as initiation of the Williston Basin ca. 500 Ma and the break-up of western Canada ca. 700 Ma, where basement structures clearly are reactivated.

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