Abstract 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.

ABSTRACT The North American continent consists of a set of Archean cratons, Proterozoic orogenic belts, and a Sequence of Phanerozoic accreted terranes. We present an ~1250-km-long seismological profile that crosses the Superior craton, Grenville Province, and Appalachian domains, with the goal of documenting the thickness, internal properties, and the nature of the lower boundary of the North American crust using uniform procedures for data selection, preparation, and analysis to ensure compatibility of the constraints we derive. Crustal properties show systematic differences between the three major tectonic domains. The Archean Superior Province is characterized by thin crust, sharp Moho, and low values of Vp/Vs ratio. The Proterozoic Grenville Province has some crustal thickness variation, near-uniform values of Vp/Vs, and consistently small values of Moho thickness. Of the three tectonic domains in the region, the Grenville Province has the thickest crust. Vp/Vs ratios are systematically higher than in the Superior Province. Within the Paleozoic Appalachian orogen, all parameters (crustal thickness, Moho thickness, Vp/Vs ratio) vary broadly over distances of 100 km or less, both across the strike and along it. Internal tectonic boundaries of the Appalachians do not appear to have clear signatures in crustal properties. Of the three major tectonic boundaries crossed by our transect, two have clear manifestations in the crustal structure. The Grenville front is associated with a change in crustal thickness and crustal composition (as reflected in Vp/Vs ratios). The Norumbega fault zone is at the apex of the regional thinning of the Appalachian crust. The Appalachian front is not associated with a major change in crustal properties; rather, it coincides with a zone of complex structure resulting from prior tectonic episodes, and thus presents a clear example of tectonic inheritance over successive Wilson cycles.