The Baltica-Iapetus boundary in the Scandinavian Caledonides and a revision of the Middle and Upper Allochthons

Stratigraphical and sedimentological studies of the Middle and Lower Allochthons in the Scandes have allowed the reconstruction of the Cryogenian to Silurian basin evolution of the Baltoscandian margin and correlation with sequences preserved in the Autochthon/Parautochthon of the foreland. The Särv Nappes (Middle Allochthon) include sandstones, subordinate dolomite, limestone, shale and, locally, Varangerian tillite; all cut by rift-facies dolerite dyke swarms. These lithologies reappear in the overlying, 1000 km long Seve Nappe Complex (SNC), where they are more penetratively deformed and at higher and varying metamorphic grade. This complex also includes the outermost continental margin and continent-ocean transition zone (COT) with sheeted dolerite dykes, spilitized amphibolites and older plinths of Precambrian basement.
In northern Sweden, exposures of the SNC in the Sarek Mts include dyke complexes (locally 100% of bedrock) intruded into sandstones and evaporites at c. 605Ma. In the Kebnekaise Mts, 50 km to the north, the SNC apparently preserves the oceanic part of the COT, with dyke-intruded gabbro, subordinate ultramafic rocks and dykes of plagiogranites. The rocks suffered a pressure increase, possibly reflecting partial subduction before detachment and emplacement onto psammitic gneisses hosting boudins of coronitic dolerites. This occurred at c.487Ma, as indicated by U-Pb dating of titanite which grew along the fabric of a granitic dyke near the base of the oceanic assemblage.
Current plate reconstructions for the Early Ordovician feature the Baltoscandian Margin subducting beneath an island arc in the Iapetus Ocean. The evidence for subduction is provided by Seve eclogites formed from rift-facies dolerites, previously dated at c.505-475Ma and, recently, more precisely defined at c.482Ma (Root, 2007). Initiation of subduction apparently coincided with the end of mid Cambrian to mid Tremadocian black (alum) shale deposition on the Baltoscandian platform. The related island arc is inferred to be preserved in the Virisen terrane of the overlying Lower Köli Nappe which includes ensimatic arc volcanics and marine sediments preserving Ashgillian faunas of Baltoscandian affinity; trondhjemites in this arc have yielded an age of c.488Ma. These three lines of evidence favour subduction along the Baltoscandian margin in the Early Ordovician.
The established tectonostratigraphic subdivision of the Scandes into allochthons has proved convenient for mapping and belt-long correlation. However, developed from a classification erected before plate tectonic and terrane analytical aspects had been introduced, the subdivision has become inadequate with respect to the Upper Allochthon, which includes both Seve and Köli Nappes, i.e. the boundary between the Baltoscandian COT and Iapetus Ocean. We propose that the SNC should be included in the Middle Allochthon and that the Upper Allochthon be restricted to the outboard oceanic terranes of the Köli Nappes.