Crustal structure and kinematic framework of the northwestern Pontiac Subprovince, Quebec: an integrated structural and geophysical study

Abstract

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.