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A detailed seismic refraction study was undertaken of the rifted continental margin southeast of the Grand Banks of Newfoundland, along a preexisting deep multichannel seismic reflection profile. The primary objectives were to determine the topography of Moho across the margin, to detect any evidence for crustal underplating, and to determine the position of the continent‐ocean boundary. Data were obtained from nine ocean bottom seismometers deployed along the profile, with an array of six 1000‐inch3 air guns as the source. Two additional refraction lines were shot parallel to structure over the continental slope. The resultant velocity model is well constrained and consistent with the observed gravity. The crust thins abruptly beneath the continental slope to a major structural transition identified as the continent‐ocean boundary. There is no significant magmatic underplating of the thinned continental crust. Seaward of the transition, the seismic velocity structure is characterized by a 50‐km‐wide zone of intermediate velocity (7.2–7.6 km/s). Beyond this is a thin (3–5 km) crust with low velocity (4.5–5 km/s) and high gradient, over mantle with a velocity of 7.7 km/s increasing to 8.0 km/s. The zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust, suggesting little or no magmatic activity during and immediately after rifting. This interpretation is consistent with results from adjacent and conjugate margins.

Wide‐angle seismic studies have determined the detailed velocity structure along a 350‐km‐long profile across the Labrador margin. Combination of this model with a previously published cross section for the southwestern Greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure. The results indicate three distinct zones across the Labrador margin, similar to the structure of the conjugate Greenland margin. Zone 1 represents 27 to 30‐km‐thick continental crust thinning gradually seaward over ∼100 km distance. Farther seaward, zone 2 is 70–80 km wide, characterized by a distinct lower crust, 4–5 km thick, in which velocity increases with depth from 6.4 to 7.7 km/s. Interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under‐plating. Zone 3 represents two‐layered, normal oceanic crust. The cross sections from both margins are reconstructed to an early drift stage at Chron 27. This demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of Chrons 31 and 33 on both margins. Zone 1 shows a marked asymmetry, with a gradual thinning of continental crust off Labrador contrasted with a rapid thinning off Greenland. The abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt.

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A seismic-refraction survey providing deep crustal structural information on the continent–ocean boundary south of Flemish Cap on the east coast of Canada was carried out using large air-gun sources and ocean-bottom seismometers. The seismic-refraction results and gravity modelling suggest that thinned continental crust extends 25 km seaward of the shelf break. The transition from continental to oceanic crust with a main crustal layer p-wave velocity of 7.3 km/s extends seaward over 100 km to the south. One refraction profile with thin (~4 km) oceanic crust was probably shot on, or very near, the trace of a fracture zone. Previous plate reconstructions have suggested that Cretaceous-age sea-floor spreading south of Flemish Cap occurred as a series of short spreading segments offset by transform fauits, or by asymmetric rifting between Iberia and Flemish Cap. This study suggests that an oblique shear margin may have formed south of Flemish Cap. possibly as a result of transcurrent motion between Flemish Cap and Iberia.