CRUSTAL STRUCTURE AND KINEMATIC FRAMEWORK OF THE NORTHWESTERN PONTIAC SUBPROVINCE, QUEBEC - AN INTEGRATED STRUCTURAL AND GEOPHYSICAL-STUDY

Structural mapping, gravity and magnetic modelling, and interpretation of a deep-seismic profile in the northwestern Pontiac Subprovince outline the crustal structure and early structural development of the region. Penetrative D1 fabrics in the Pontiac Group and in the underlying Opasatica Gneiss may record south-vergent thrusting of a high-grade nappe. D2 and D3 structures record southeast-vergent folding and thrusting within the Pontiac Group. Steeply dipping northeast-trending ductile shear zones may represent oblique ramps initiated during D1. Gravity and magnetic model profiles are consistent with north-dipping structures in the shallow crust, and indicate that the Pontiac Group is a wedge underlain by north-dipping slabs of different densities and magnetic susceptibilities. Interpretation of a seismic reflection profile shows mid-crustal duplex structures overlying a deeper thrust between 16 and 24 km. From the surface to the deep crust, the structure of the northwestern Pontiac Subprovince records south- to southeast-directed thrusting and important crustal thickening during a collisional event. In light of field observations, available isotopic ages suggest that D1 deformation began no earlier than 2694 Ma, and that deformation continued until at least 2668 Ma.