Evolution of deep-crustal normal faults: Constraints from thermobarometry in the Grenville Orogen, Ontario, Canada

The Robertson Lake shear zone (RLSZ) is a low-angle (similar to 30 degrees dip), plastic to brittle extensional shear zone located in the southeastern Grenville Orogen, Ontario, Canada. Thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients. Upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall. However, steep gradients in metamorphic pressure are present adjacent to the Robertson Lake shear zone. In the footwall pressures increase from 550 MPa to 800 MPa within 8 km of the RLSZ and in the hanging wall pressures decrease from 900 MPa to 500 MPa within 12 km of the RLSZ. The pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone. The degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60-90 degrees, which suggests reactivation of an initially high-angle terrane boundary. Previous studies provide constraints of a depth extent of 40 km for the RLSZ and crustal thickness of 60 km during extension. Thus, extensional rotations were accommodated by decoupling and regional-scale flow in the lower third of the crust.