In 1986, 1181 km of marine seismic reflection data was collected to 18–20 s of two-way traveltime in the Gulf of St. Lawrence area. The seismic profiles sample all major surface tectono-stratigraphic zones of the Canadian Appalachians. They complement the 1984 deep reflection survey northeast of Newfoundland. Together, the seismic profiles reveal the regional three-dimensional geometry of the orogen.Three lower crustal blocks are distinguished on the seismic data. They are referred to as the Grenville, Central, and Avalon blocks, from west to east. The Grenville block is wedge shaped in section, and its subsurface edge follows the form of the Appalachian structural front. The Grenville block abuts the Central block at mid-crustal to mantle depths. The Avalon block meets the Central block at a steep junction that penetrates the entire crust.Consistent differences in the seismic character of the Moho help identify boundaries of the deep crustal blocks. The Moho signature varies from uniform over extended distances to irregular with abrupt depth changes. In places the Moho is offset by steep reflections that cut the lower crust and upper mantle. In other places, the change in Moho elevation is gradual, with lower crustal reflections following its form. In all three blocks the crust is generally highly reflective, with no distinction between a transparent upper crust and reflective lower crust.In general, Carboniferous and Mesozoic basins crossed by the seismic profiles overlie thinner crust. However, a deep Moho is found at some places beneath the Carboniferous Magdalen Basin.The Grenville block belongs to the Grenville Craton; the Humber Zone is thrust over its dipping southwestern edge. The Dunnage Zone is allochthonous above the opposing Grenville and Central blocks. The Gander Zone may be the surface expression of the Central block or may be allochthonous itself. There is a spatial analogy between the Avalon block and the Avalon Zone. Our profile across the Meguma Zone is too short to seismically distinguish this zone from the Avalon Zone.