MESOPROTEROZOIC BASIN SINKING AND SOLID-STATE DIAPIRISM IN THE EASTERN GRENVILLE PROVINCE, CANADA

The Early Mesoproterozoic Wakeham Group is an epicontinental clastic sequence interleaved with abundant hectometric amphibolitized gabbroic sills. This greenschist to lower amphibolite facies rock assemblage outcrops as a regional trough (Davy Synform) flanked by orthogneiss complexes that are characterized by kilometer-scale domal structures evident on aerial photographs. Amphibolite dykes, presumed to be correlative with the gabbroic sills, are ubiquitous in these complexes. On its southeastern flank, the trough is separated from the structurally underlying orthogneissic complex (Boulain Complex) by a major ductile-brittle shear zone (Lac Caron shear zone). Detailed study of a region located in the crystalline footwall of the Lac Caron shear zone which comprises two domal structures gives the opportunity to test three dome-forming mechanisms: cross-folding, doming of an initial planar anisotropy and solid-state diapirism. Our structural observations are best interpreted by solid-state diapirism. These include: -the absence of mesoscopic interference pattern; -the absence of an axial planar foliation in the dome cores, the composite metamorphic fabric being sub-horizontal; -pre-existence of an earlier planar anisotropy is impossible to demonstrate using the symmetry principle; -the stretching lineation defined by recrystallized elongated K-feldspar megacrysts defines a radial pattern around the dome, and; -tectonic foliation trajectories are everywhere parallel to lithological contacts. On a regional scale, the intrusion of gabbroic sills in the Wakeham Group (ca.1177 Ma) immediately preceded the emplacement of the adjacent Havre St-Pierre anorthositic-mangerite-charnockite-granite complex (1126-1062 Ma). Accordingly, while a wealth of hot mafic magma pounded at the base of the crust, the intrusion of abundant gabbroic sills in the upper crust led to the development of a thermal blanket. Overall, it significantly increased the geothermal gradient and set-up ideal conditions for the gravitational collapse of the denser gabbro-sediment assemblage into the thermally softened underlying granitic crust which rises by solid-state diapirism to shoulder the newly formed trough.