A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction – wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac–Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir–Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12–15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010–980 Ma, 1080–1060 Ma, and 1170–1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25–30 km corresponding to the top of a high-velocity lower crustal layer.