THE TORNQUIST SEA AND BALTICA-AVALONIA DOCKING

Abstract

Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania–Bornholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude=19°N, longitude=051°E) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tornquist Sea.Tornquist Sea closure and the Baltica–Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds.In Scania, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: latitude=4°S, longitude=012°E). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440–450 Ma), and sheds new light on the Baltica–Avalonia docking.