STRUCTURAL EVOLUTION OF GNEISSIC DOMES IN THE FOOTWALL OF THE WAKEHAM GROUP,EASTERN GRENVILLE PROVINCE

The Early Mesoproterozoic Wakeham Group is a clastic sequence interleafed with abundant hectometric amphibolite derived from gabbroic sills. In the study area, this greenschist to lower amphibolite facies rock assemblage outcrops as a regional saddle shaped synform flanked by orthogneisses complexes. On its southeastern flank, a large ductile-brittle shear zone, the Lac Caron shear zone (LCSZ), separates the Wakeham Group- amphibolite assemblage from structurally underlying orthogneiss complex. This crystalline footwall comprises three kilometres scale domes evident on aerial photographs. These domes are not the product of a single deformation event; they more lickely result from cross-folding of early regional scale antiforms. The north-plunging axes of the early folds are parallel to the regional stretching lineation defined by recrystallized elongated K-feldspar megacrysts. In places, the drawn out granoblastic augens exhibit a shape ratio in excess of 10:1:1. Penetrative extension is also manifested by ubiquitous foliation boudinage structures and by the boudinage and transposition of amphibolitized dykes. Non-coaxial flow is indicated by the geometry of sheared amphibolites dykes, asymmetrical intrafolial folds, type 1 and 2 boudins and shear bands. A newly recognized type of kinematics indicator referred to as flanking folds is also widespread. It consists of synthetically rotated foliation-boudin necks or extensional veins (filled with granitic mobilisate) that cause the deflection of the foliation while rotating. The sense of shear along these transecting mesostructures is antithetic with a diagnostic final geometry consisting of parasitic antiform in the hanging wall and synform in the footwall. The facts that: - the dextral sense of shear is regionally constant, even on the opposite folds limbs , - there is an increase in finite strain in folds cores, - L-tectonites are well developed in the hinges of minor folds, imply folding under a non-coaxial shear regime. We tentatively propose that the observed mesoscopic structures and the map scale pattern resulted from the lateral escape of granitic rocks following the gravitational collapse of the denser Wakeham Group/amphibolite assembly into thermally weakened underplating granitic complex.