A laterally extensive zone of southeast-dipping marble mylonites in the Central Metasedimentary Belt (CMB) of the southern Ontario Grenville (the "Bancroft shear zone") records southeast-directed extension and a displacement of about 10-12 km. Calcite-graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca. 450 °C. In contrast, carbon isotope fractionations (Δ13Ccc-gr) within the marble protolith consistently give temperatures of 650-700 °C. The Bancroft shear zone postdates 1060 Ma and older high-grade shear zones in the Central Gneiss Belt and the CMB that are associated with northwest-directed thrusting. Comparison of the temperature data from the retrograde marble mylonites with cooling curves for the CMB gives a late Grenvillian age range of 935-1010 Ma for the extensional event. The geometric relation between thrusting and normal faulting in the CMB is comparable to that in parts of the Himalayas and southern Tibet, and similarly we interpret late normal faulting in the CMB to be a result of gravitational collapse of a thrust thickened crust. In contrast to the Himalaya-Tibet region, however, accretion in the Grenville becomes older in the direction of regional thrusting.