Late-Grenvillian channel flow in the central Grenville Province (Manicouagan Reservoir area): New constraints from a structural and geochronological study of the Allochthon Boundary Thrust

Abstract

Crucial to reconstructing the tectonic evolution of the Grenvillian Orogeny is the characterisation of the Allochthon Boundary Thrust (ABT) separating the Parautochthonous from the structurally overlying Allochtonous belts. Combined with previously published data, our structural analysis complemented with an analysis of geophysical lineaments conducted south of the Manicouagan Reservoir, revealed that top-to-the-NNE shearing in the hanging wall occurred several ten's of myrs prior to top-to-the-NNW shearing in its footwall. The newly mapped down-to-the-SE Thachic shear-zone cuts down section from the hanging wall, crosscutting the ABT and deforming Parautochtonous Belt rocks in its footwall. Timing of this normal-sense shearing event is constrained from syn-kinematic leucosome and leucogranite between 993 ± 3 Ma and 986 ± 0.5 Ma, respectively. This timing overlaps that of reverse-sense shearing in the underlying Parautochtonous Belt documented previously. Coeval normal-, coaxial, and reverse-sense shearing at upper, middle and lower structural levels, respectively, of a partially molten rock-panel, favours a channel flow (or extrusion) rather than an orogenic wedge model. By incorporating a slice of allochtonous rocks, with diverging lineation at its roof and by moulding the shape of a rigid pluton, the flow within this channel was heterogenous, as predicted in previously proposed numerical models.