Palaeozoic palaeogeography: A North Atlantic viewpoint

Palaeozoic palaeogeography, highlighting the North Atlantic Caledonian evol ution and the destruction of the Iapetus Ocean and the Tornquist Sea, is re capitulated with reconstruction maps from Early Ordovician to Mid-Devonian times. In the Early Ordovician (Tremadoc-Arenig), Laurentia, Siberia, and t he North China Block were positioned in equatorial latitudes, Baltica was l ocated at intermediate southerly latitudes, whilst Avalonia and the Europea n 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. Avalo nia rifted away from Gondwana during Arenig-Llanvirn time, and the Tornquis t Sea, separating Avalonia and Baltica, narrowed gradually during the Ordov ician followed by Late Ordovician 'soft docking' of Eastern Avalonia and Ba ltica prior to their joint collision with Laurentia. The main collisional e vent between Baltica and Laurentia occurred at c. 425 Ma and was marked by deep subduction of Baltican crust beneath Laurentia with concomitant eastwa rd translation of nappes over the Baltican margin. Deep subduction was a fu nction both of rapid motion of Baltica (8-10 cm/year) toward a stationary L aurentia and precedence of prolonged subduction of large volumes of cold li thosphere. Shortly after collision, in Emsian times, these rocks were exhum ed by extensional collapse.