Chapter 19 Regional Context and Tectonostratigraphic Framework of the Early–middle Paleozoic Caledonide Orogen, Northwestern Sweden

The Scandian mountains in northwestern Sweden are dominated by the eastern part of the Scandinavian Caledonides, an orogen that terminated during the middle Paleozoic with Himalayan-style collision of the ancient continents of Baltica and Laurentia. In this foreland region, far-transported higher allochthons from an exotic continental margin (Rodingsfjallet Nappe Complex) and underlying mostly oceanic-arc basin character (Koli Nappe Complex) were emplaced at least 700 km onto the Baltoscandian margin of Baltica. The thrust sheets below the Iapetus Ocean terranes were derived from the transition zone to Baltica (Seve Nappe Complex), comprising mainly siliciclastic meta-sedimentary rocks, hosting abundant metamorphosed c. 600 Ma mafic intrusions. They preserve evidence of subduction (eclogites, garnet peridotites and microdiamonds in host paragneisses), starting in the late Cambrian; exhumation continued through the Ordovician. Underlying allochthons derived from the outer margin of Baltica are less-metamorphosed Neoproterozoic sandstone-dominated successions, also intruded by Ediacaran dolerite dykes (Sarv Nappes); they are located tectonically above similar-aged metasandstone and basement slices, devoid of dykes (Offerdal and Tannas Augen Gneiss nappes and equivalents). Lowermost allochthons (Jamtlandian Nappes and equivalents), from the inner Baltoscandian margin, provide evidence of Cryogenian rifting, Ediacaran-Cambrian drifting and platformal sedimentation, followed by foreland basin development in the Ordovician and Silurian.