Palaeomagnetic constraints upon the palaeogeographic position of the Baltic Shield in the Ordovician

The palaeomagnetic analysis of the Ordovician limestones in Southern Scandinavia (75 sites) has given the following results: (1) in the southern part of the Oslo Region, all Ordovician sediments were remagnetized during the Lower Permian, by a thermo-chemical mechanism triggered by the emplacement of the Permian volcanic and intrusive suite; (2) in the northern part of the Oslo Region, the Upper Caradoc Mjøsa Limestone carries a high-temperature component (decl. = 53°, incl. = −40°, α95 = 25°, 4 sites) with corresponding palaeopole at 4°S, 143°E; (3) in southern Sweden, all Lower Ordovician “Orthoceras limestones” sampled in Dalarna, Närke, Scania, Väster-götland, Öland and Öster-götland display the same well-defined southeast steep high-temperature components, leading to a palaeopole at 30° N, 55° E, A95 = 9°; in areas with late Palaeozoic volcanic activity (Scania, Väster-götland), this high-temperature component is partially overprinted by a remagnetization associated with dolerite emplacement. According to these results, Baltica was situated in temperate southern latitudes (50° S) in the Early Ordovician and has drifted northwards at a latitudinal drift rate of 4 to 7 cm/year to reach tropical latitudes (20°S) by the Late Ordovician to Late Silurian. This interpretation is in agreement with previous models based on the occurrence of Ordovician reef structures, and sheds new light on the early Palaeozoic position of Baltica relative to other continental blocks.