The evolution of the middle Paleozoic subduction in the South Armorican Massif (southern Brittany, France) is compared with that of the Petite Sole-Cordoba fault (western Spain and Portugal). This comparison suggests that this fault acted as a transform fault during Ordovician and Devonian times. The gravity and magnetic data compiled on both sides of the North Atlantic indicate that the southern end of the Cordoba fault links with a 2,400-km-long mafic body which extended from the South Portuguese zone to New England (U.S.A.) before the opening of the present Atlantic Ocean. The mafic body may be considered as the suture of the Theic Ocean. However, the closing of the South Armorican and Theic oceans did not occur at the same time; the South Armorican subduction ended in Late Devonian time, whereas the Theic closed in early Carboniferous time. This diachronous evolution could be explained by the existence of an Iberian (Spanish) microplate.