Recently assembled geological and geophysical evidence from southern Brittany reveals the organization of submerged structures off the southern Brittany coast. Fragments of a high-pressure–low-temperature belt are caught between a northern high-temperature zone of middle Paleozoic age, and a southern zone of intermediate metamorphism. It is not known whether the rocks in the high-pressure–low-temperature belt belong to a middle Paleozoic allochthonous slab or whether they are parautoch-thonous and the result of Carboniferous obduction. Farther to the south a major geophysical discontinuity is interpreted as a suture, the trace of which cuts across the sedimentary facies zonation established between the Armorican Massif and Spain. It is therefore suggested that the formation of the Iberian-Armorican arc predates the Hercynian orogeny. A geodynamic model is proposed that attempts to synthesize the orogenic mechanisms in Spain and southern Brittany during middle and late Paleozoic time.