OROCLINAL BUCKLING OF THE ARMORICAN CONTINENTAL RIBBON: AN ALTERNATIVE TECTONIC MODEL FOR PANGEAN AMALGAMATION AND VARISCAN OROGENESIS

The Iberian segment of the Western European Variscan orogen is characterized by an S-shaped pair of coupled oroclines that accommodated >1100 km of orogen parallel shortening over a 15 m.y. period following the Late Carboniferous cessation of E-W Variscan shortening. Traditional models interpret Variscan orogenesis as a product of Pangea-forming collision between Gondwana and Laurussia following closure of the intervening Rheic ocean. However, the rapid rate of translation required for formation of the coupled Iberian oroclines (~10 cm Ÿ yr^-1), is difficult to reconcile with paleogeographic and tectonic models that place orocline formation within the core of Pangea. Paleomagnetic data indicate that northward migration of Gondwana relative to Laurussia continued through Variscan orogeny to cease coincident with formation of the Iberian coupled oroclines at the Carboniferous–Permian boundary. Palinspastic restoration of the Iberian coupled oroclines reveals a N-S trending 2100 km-long linear Variscan orogen bound on either side by ophiolite bearing allochthons. The presence of two distinct Variscan sutures is consistent with paleomagnetic data from the Gondwana derived Variscan autochthon indicating its mid-Paleozoic separation as the so-called Armorica microplate and requiring the growth of a second major (Paleotethys) ocean. Persistent sedimentological and paleontological ties between Armorica and Gondwana can be reconciled by a model within which the Armorican ribbon continent stretches north from Gondwana between Paleotethys to the west and the Rheic to the east. Our model explains Pangea amalgamation as being coincident with buckling of a linear Armorican ribbon continent caught between Gondwana to the south and Laurussia to the north during northward migration of Gondwana. In this model, Variscan orogenesis includes (1) Late Devonian accretion of the Galicia–Trás-os-Montes oceanic arc to the western Armorican margin following west-dipping subduction of oceanic lithosphere continuous with it and (2)­ Carboniferous west-dipping subduction beneath the eastern Armorican margin giving rise to accretion of the Beja Abuches ophiolite and South Portuguese continental terrane, imbrication of the east-verging Cantabrian fold and thrust belt, and an easterly-younging magmatic arc.