SHEAR ZONES AND OTTAWAN DEFORMATION IN THE CENTRAL METASEDIMENTARY BELT OF THE GRENVILLE PROVINCE OF SOUTHERN ONTARIO, CANADA

In the Grenville Province of Ontario, Canada, the Central Metasedimentary Belt consists of several terranes made up of arc rocks and associated sedimentary basins amalgamated during the Mesoproterozoic. Our work focuses on the belt’s northwestern boundary and adjacent terranes: the Bancroft Terrane, and the Harvey-Cardiff and Belmont Domains of the Elzevir Terrane. The northwestern boundary, the Central Metasedimentary Belt boundary zone, is a 10 km thick, 200 km long crustal-scale shear zone made up of crystalline thrust sheets stacked by northwest-directed thrusting. The boundary shear zone is polymetamorphic and contains anastomosing amphibolite-facies ductile shear zones that accommodated juxtaposition of the metasedimentary belt against the underlying granulite facies Central Gneiss Belt. In addition to traces of earlier tectonic and orogenic events, the boundary shear zone enjoyed Ottawan (i.e. Geon 10) crustal thickening and collapse. Southeastwards from the boundary zone, and structurally above it, metamorphic grade decreases from amphibolite facies conditions in the Bancroft Terrane to greenschist facies conditions in the Tudor Township region of the Belmont Domain. Calcite-graphite carbon isotope thermometry results from marble samples across the study area show a gradual decrease in peak metamorphic temperature from over 700°C in the Central Metasedimentary Belt boundary zone to under 500°C in Tudor Township in the east. These temperatures probably correspond to peak metamorphism during the Ottawan orogeny. A lack of significant thermal discontinuities at terrane boundaries and shear zones, including both the Central Metasedimentary Belt boundary zone itself and the spectacularly mylonitic Bancroft Shear Zone, indicates that the entire region remained largely intact during and after the Ottawan peak metamorphic event. This large area may not have any distinct large-displacement extensional features, but instead, perhaps many small-offset shear zones (in marble) with shallow, southeast-directed extension. This structural geometry differs from the eastern Central Metasedimentary Belt (including the Adirondack Lowlands of New York), and the belt is structurally asymmetric.