South Armorican Shear Zone and Continental Fit before the Opening of the Bay of Biscay

Geological similarity between Hercynian zones from Brittany and the Iberian Peninsula has led various authors [l-31 to propose that these zones were once continuous. Bard et ai. [3], for example, reconstmct the Hercynian 1bero.Armorican belt to find the position of the Iberian peninsula before the opening of the Bay of Biscay (1 50 to 200 my ago). They assume continuity of Hercynian structures of the Armorican massif over hundreds of kilometers across the continental shelf. This supposes that Late of Post Hercynian horizontal displacements have not significantly modified the original framework of the Hercynian massif. The purpose of this note is to discuss, on the basis of new results, the importance of the main Late or Post Hercynian horizontal movements which have taken place in Brittany and on the adjacent continental shelf and their implication for the fit of continents based on the reconstruction of the Ibero-Armorican belt. The principal horizontal movements which may have affected the Ibero-Armorican belt after the main Hercynian orogeny are those which have occurred during Late Hercynian tirne dong the south Armorican shear zone [ l , 41. The south Armorican shear zone, which is an important Late Hercynian feature (figs. 1, 2), is roughly parallel to the main structurai units in Brittany. The horizontal motion associated with the