(A) Magnetic signature superimposed on crustal domains. (B) Segmentation, crustal domains and major structures of the Bay of Biscay region. Crustal domains defined by magnetic signatures: Oceanic (I), Ocean-Continental Transition (IIa, b, c); (III) Western Approaches Margin; (IV) North-Armorican Margin; (V) South Armorican Margin: (a) off south Brittany; (b) off Oléron city; (VI) Aquitaine Margin; (VII) Aquitaine Basin; (1) Oceanic crust; (2) Oceanic crust with underplated serpentinized mantle; (3) OCT with exhumed serpentinized mantle; (4) OCT with exhumed serpentinized mantle with a possible intruded magmatism (with strong magnetic anomalies) (from this study in accordance with Pedreira et al., 2015); (5) hyper-thinned continental crust; (6) thinned continental crust; (7) seamount volcanic edifices on seismic data coinciding to magnetic anomalies; (8) Strong magnetic anomalies associated to subvolcanic rocks (ophites) in the Variscan basement (Taramis well); (9) Strong magnetic anomalies often associated to the exhumed serpentinized mantle or lower crust; (10) Observed major crustal structures with magnetic signatures; (11) Observed major structures; (12) Major magnetic lineament uncorrelated to known structures; (13) Segmentation of continental crustal domains and (14) their extension in the OCT: Ouessant system (Os), Sizun system (Sis) and Loire system (Ls); (15) A34 magnetic anomaly (Seton et al., 2014); (16–17) Observed and supposed boundary of the Biscay oceanic crust; (18–19) Observed and supposed boundary between the OCT and the continental domain; (20–21) Observed and supposed boundary between the hyper-thinned and thinned continental domain. Triangle: Ophites sampled by wells (112 ODP leg 12; Taramis); blue names indicate: localities (Cap Sizun, Nantes city, Yeu, Oléron). AMR: Antin-Matourget ridge; BTz: Barlanes transfer Zone; CAF: Celtaquitain flexure (redrawn from the new magnetic compilation); CFS: Concarneau Fault System linked to N140 Kerforne fault; CIZ: Central Iberian Zone; ECL: Eastern Crustal Lineament; JA: Jonzac Anticline; M: Meriadzek Terrace; OAf: Ouessant-Aurigny Fault (Evans 1990, or Iroise Fault according to Le Roy et al., 2011); PTz: Pamplona Transfer Zone; SASZ and NASZ: South and North Armorican Shear Zone; SBS: Saintes-Berbézieux Syncline; STz: Santander Transfer Zone (from Roca et al., 2011); Vf: Ventaniella Fault; WALZ: West Asturian-Leonese Zone. The dashed rectangle correspond to the location of Figure 9.