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 mafi c dyke emplacement. These differences are accounted for by postulating that a cryptic Grenvillian thrust separates the assemblages. Amphibol ite-facies extensional shear along the Central Britt shear zone (CBSZ) overprinted the thrust, obscuring kinematic evidence for its existenc e. The structurally lower Bayfield - Nadeau Island assemblage contains orthogneiss suites of disparate age, lesser amounts of supracrustal r ocks deposited before intrusion of the youngest orthogneiss, pre-Grenv illian and Grenvillian metamorphic assemblages, and at least three maf ic dyke suites. The overlying Ojibway - Sand Bay assemblage contains o nly younger orthogneiss with Grenvillian metamorphic assemblages, volu metrically important supracrustal rocks that are younger than the youn gest orthogneiss, and lacks cross-cutting mafic dykes. Comparable tect ono-stratigraphic changes are present across the thrust boundary separ ating the Ojibway - Sand Bay assemblage and the basal Parry Sound asse mblage. Extensional shearing did not strongly overprint this boundary and it therefore serves as a relatively unmodified analogue of the ove rprinted boundary. Extension on the CBSZ overlapped formation of trans verse ductile folds (hinges parallel to the extension - transport dire ction). These folds and the CBSZ dominate the crustal architecture at this level and are interpreted to be late orogenic structures formed d uring a gravity-assisted shape adjustment of the orogenic wedge. Therm al softening of the lower crust caused by thrust thickening may have a llowed this to occur.