A new model for the Hercynian Orogen of Gondwanan France and Iberia

The Hercynian Orogen of Gondwanan France and Iberia is a collage of disparate terranes separated by faults. Plate reconstructions indicate that the orogen developed during a massive dextral transpression as Laurentia slid along the boundary of Gondwana, and as Laurentia, Baltica, and East Avalonia rotated clockwise to form Pangea. It is proposed, therefore, that the collage is the result of this transpression, and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the Gondwanan shelf, with its stable platform Ordovician–Devonian sedimentary sequence, and the neighbouring Rheic Ocean. Specifically, it is proposed that: (1) the slice of the Gondwanan shelf that now forms north Brittany was displaced dextrally from NW Africa, around Iberia, by more than 2000 km; (2) the ophiolites, oceanic material, and volcanic arcs of south Armorica and the Massif Central represent slices of the Rheic Ocean, displaced dextrally along the Gondwana margin and shuffled in amongst slices of Gondwanan shelf; (3) the convergent component of the transpression is represented by thrusts throughout the region, the Léon Terrane of France, and ophiolite-bearing klippe in NW Iberia. The Ibero-Armorican arc was formed by wrapping the mobile dextral shear belts about a rigid Iberian basement block. The dextral Porto-Tomar Shear Zone represents the more important shearing in Iberia, and the Iberian sinistral shears are interpreted as bookshelf-type structures, subsidiary to the dextral shearing.