The Canadian Shield in Southern Quebec – Some Outstanding Issues
The Canadian Shield in Southern Quebec – Some Outstanding Issues
Previously Published Material: This is partly, but not entirely a review, from several sources over the past decade. It was requested by the organizers.
Abstract ID#: 33552
English Abstract:
Tectonics N of the St-Lawrence is dominated by the Mesoproterozoic Grenville Orogen, a 500-km-wide belt which overprints Archean terranes of the Superior province in E Ontario and W Quebec, and Paleoproterozoic terrains farther E. The Archean terranes comprise the Abitibi and Opatica subprovinces, consisting respectively of predominantly juvenile mafic volcanic rocks and associated plutons, and homogeneous tonalitic gneisses. The Abitibi/Opatica boundary is a N-vergent thrust. The Abitibi and Opatica subprovinces have been interpreted as composite oceanic terranes and an associated subduction complex, juxtaposed at a subduction zone. A prominent N-dipping seismic reflector in the upper mantle lies beneath the Opatica subprovince and provides some support for Neoarchean subduction. Clear linkage between the mantle reflector and the Opatica/Abitibi boundary has however not been demonstrated, nor is the reflector well characterized. Origins other than subduction, such as crustal delamination, remain viable.
The Grenville Orogeny (1080-980 Ma) resulted in deep burial of rocks of the Laurentian margin. Numerical models show much of Grenvillian architecture may result from development of a broad plateau similar to that in modern Tibet. Although most Grenvillian faults show thrust kinematics, later normal and transtensional faults are common and interpreted as due to collapse in the later stages of plateau development. The Grenville Front marks the N limit of Mesoproterozoic tectonism and was active only late (1020-980 Ma). Crustal architecture along the front ranges from S-dipping faults soling into a flat Moho in E Ontario, through large-scale Moho offset with crustal thinning in W Quebec, to substantial crustal thickening in E Quebec. The crustal thinning in W Quebec indicates extension of the orogen as a whole. The gravity field suggests there may be other extensional features S of the Grenville Front whose trends indicate the stress orientations during late extension.