The Bay of Biscay is bounded to the north by the North Biscay margin, which comprises the Western Approaches and Armorican segments. In the 1970s and 1980s, most researchers considered this margin typical of a non-volcanic passive margin: it is characterized by a striking succession of tilted blocks beneath which occurs the S reflector and the continent–ocean boundary is abrupt. This paper examines the Armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data (Norgasis cruise). An important result is the discovery of a 80 km wide ocean–continent transition zone that coincides with the Armorican Basin (a deep sedimentary basin). It is characterized by a high-velocity lower-crustal layer (7.4–7.5 km s−1) overlain by sediments. The other results are: (1) the main crustal thinning occurs exclusively under the narrow continental slope; (2) the tilted blocks and the S reflector are observed only at the base of the continental slope in the narrow domain called the ‘neck area’; (3) the North Biscay Ridge is a large oceanic plateau present only off the NW Armorican margin rather than a long ridge elongated off the whole North Biscay margin.

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