A seismic array extended from the Grenville Province into the Abitibi and Opatica subprovinces of the Superior Province, Canada. We use P-to-S converted waves in seismograms of distant earthquakes to probe boundaries in seismic properties, and demonstrate the presence of a prominent seismic feature that dips NNW into the mantle beneath the Grenville Front. We use two variants of receiver function (RF) analysis, a common conversion-point (CCP) imaging that combines seismic signals from multiple paths connecting sources and receivers, and harmonic decomposition of RF data that separates signals according to their direction in the cone of paths from which the signals have come. CCP images show the crust beneath the Grenville Province to be thicker than beneath the Superior Province, and identify a NNW-dipping boundary beneath the Grenville Front. Harmonic RF analysis shows that this feature is associated with seismic anisotropy in upper-mantle peridotite, and that this anisotropic feature extends to depths of at least 70 km. We interpret the feature as the result of a NNW-dipping shear zone formed within the mantle lithosphere during the Grenvillian continental collision. It could have accommodated internal deformation within Laurentian lithosphere or, more speculatively, mark the contact with the impacting Amazonian lithosphere.