Palaeozoic palaeogeography: A North Atlantic viewpoint

Palaeozoic palaeogeography, highlighting the North Atlantic Caledonian evolution and the destruction of the Iapetus Ocean and the Tornquist Sea, is recapitulated with reconstruction maps from Early Ordovician to Mid-Devonian times. In the Early Ordovician (Tremadoc-Arenig), Laurentia, Siberia, and the North China Block were positioned in equatorial latitudes, Baltica was located at intermediate southerly latitudes, whilst Avalonia and the European Massifs were located together with the North African part of Gondwana in high southerly latitudes. During the Ordovician, Baltica drifted northwards and approached Siberia while undergoing counter-clockwise rotations. Avalonia rifted away from Gondwana during Arenig-Llanvirn time, and the Tornquist Sea, separating Avalonia and Baltica, narrowed gradually during the Ordovician followed by Late Ordovician 'soft docking' of Eastern Avalonia and Baltica prior to their joint collision with Laurentia. The main collisional event between Baltica and Laurentia occurred at c. 425 Ma and was marked by deep subduction of Baltican crust beneath Laurentia with concomitant eastward translation of nappes over the Baltican margin. Deep subduction was a function both of rapid motion of Baltica (8-10 cm/year) toward a stationary Laurentia and precedence of prolonged subduction of large volumes of cold lithosphere. Shortly after collision, in Emsian times, these rocks were exhumed by extensional collapse.