The Iapetus model: a plate tectonic concept for the Variscan belt of Europe

Abstract

A plate tectonic model is presented explaining the evolution of the Variscan belt of Europe. It regards the Iapetus Ocean and its branch, the Tornquist Sea, as the only ocean involved in the Caledonian, Variscan and Alleghenian mega-orogen. The Iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence: 1. (1) northeastern Iapetus including the Tornquist Sea (Caledonian orogeny), 2. (2) central Iapetus (Acadian orogeny), 3. (3) adjacent marginal basins (Variscan orogeny) and 4. (4) southwestern Iapetus (Alleghenian orogeny). The area of the Variscides was part of the continental margin of Gondwana made up of ridges and ensialic basins. No ocean lay within the Variscan area during the Palaeozoic, although patches of oceanic crust probably existed in rift basins (e.g. the Lizard ophiolite). After the Silurian-Early Devonian closure of northwestern and central Iapetus the Variscan area was surrounded by cratons on three sides in a manner similar to that around the modern Mediterranean Sea. The subsequent Variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of Gondwana toward Laurentia. After the Variscan “collision”, Gondwana rotated clockwise to the west by moving along a strike-slip zone. This rotation during the Carboniferous opened the Palaeo-Tethys in the east and caused the Alleghenian collision in the west. The Middle Ordovician to Permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions. Avalonia is interpreted as the Ordovician volcanic island arc of Gondwana, separated from mainland Gondwana by an oceanic gulf or a back-arc basin. The Iberian arc may have originated by a “continental escape” of Iberia from the center of collision into the oceanic gulf. The Variscan Austro-Alpine and Southern Alpine units are today located near their earliest Early Carboniferous position.