DEEP-CRUSTAL DUCTILE EXTENSION FOLLOWING THRUSTING IN THE SOUTHWESTERN GRENVILLE PROVINCE, ONTARIO

Two lithotectonic assemblages in southern Britt domain have different histories of plutonism, metamorphism, structural development, and mafic dyke emplacement. These differences are accounted for by postulating that a cryptic Grenvillian thrust separates the assemblages. Amphibolite-facies extensional shear along the Central Britt shear zone (CBSZ) overprinted the thrust, obscuring kinematic evidence for its existence. The structurally lower Bayfield - Nadeau Island assemblage contains orthogneiss suites of disparate age, lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss, pre-Grenvillian and Grenvillian metamorphic assemblages, and at least three mafic dyke suites. The overlying Ojibway - Sand Bay assemblage contains only younger orthogneiss with Grenvillian metamorphic assemblages, volumetrically important supracrustal rocks that are younger than the youngest orthogneiss, and lacks cross-cutting mafic dykes. Comparable tectono-stratigraphic changes are present across the thrust boundary separating the Ojibway - Sand Bay assemblage and the basal Parry Sound assemblage. Extensional shearing did not strongly overprint this boundary and it therefore serves as a relatively unmodified analogue of the overprinted boundary. Extension on the CBSZ overlapped formation of transverse ductile folds (hinges parallel to the extension - transport direction). These folds and the CBSZ dominate the crustal architecture at this level and are interpreted to be late orogenic structures formed during a gravity-assisted shape adjustment of the orogenic wedge. Thermal softening of the lower crust caused by thrust thickening may have allowed this to occur.