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.