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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.

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An idealized plate tectonic model for the pre-Carboniferous development of the Canadian Appalachians explains the 400 km dextral offset of tectonostratigraphic zones from Quebec and northern New Brunswick to Newfoundland and the up to 600 km offset of oppositely verging belts of Acadian deformation from the Gaspé Peninsula to eastern Newfoundland. It is proposed that these offsets, which occur at the St. Lawrence promontory, result from the collision of an irregular North American passive continental margin with island arc and continental crust to the east, along an east-dipping subduction zone. The line of subduction is assumed to have been linear and the subducting slab to have maintained its mechanical integrity during collision. A "jigsaw fit" of the opposite sides of the Iapetus Ocean is made unnecessary by invoking lithospheric delamination and tectonic wedging during the Acadian orogeny in Newfoundland. The model is consistent with surface geology and recent deep seismic reflection observations from north of Newfoundland.

Three seismically defined lower crustal blocks (LCBs) have been recognized to underlie the familiar tectonic‐stratigraphic zones of the Canadian Appalachians. Tectonic development of the orogen is inferred to have been strongly influenced by collision of the two outboard LCBs (Central and Avalon LCBs) against the sharply irregular North American craton (Grenville LCB), as evidenced by an abrupt offset in the trend of the orogen localized at this irregular edge. The LCBs are restored to a precollisional configuration by (1) removing Carboniferous strike‐slip motion on major faults linking through the Magdalen pull‐apart basin; and (2) removing Devonian(?) offset along a hypothesized fault through the present‐day Strait of Canso (Canso fault) to realign the Central and Avalon LCBs, presumed to have been continuous prior to accretion to North America. The restoration leads to (1) a qualitative explanation for deep seismic reflection observations across the Magdalen basin which suggest that portions of the underlying crust do not correspond to any of the three principal LCBs; and (2) a plate tectonic model for the orogen which suggests the backarc basin separating the Taconian (Ordovician) arc from the eastern margin of the early Paleozoic Iapetus ocean was at least as wide (>450 km) as the offset in the irregular Grenville LCB, inherited from rifting of Iapetus. The present‐day Southwest Newfoundland Transform Margin may be a segment of the Canso fault reactivated during Mesozoic opening of the Atlantic ocean.