The continent-ocean (Seve-Koli) boundary in the Sarek-Padjelanta Mts. revisited: Swedish Caledonides

摘要

In the Swedish Caledonides, the continent-ocean boundary is represented by the basal thrust of the Iapetus-derived Koli Nappe Complex (KNC) which overlies the Seve Nappe Complex (SNC) derived from the outer rifted margin of Baltica and the continent-ocean transition (COT). Current maps and literature subdivide the Seve teconostratigraphy in northern Sweden (Sarek-Padjelanta region) into three mega-lenses, the lowest of which (Vaimok lens) is composed of quartz-feldspathic gneisses and subordinate marble, all hosting eclogites. It is overlain by the Sarek lens composed of a COT dyke complex and, along its western margin, a narrow belt of phyllites, schists, quartzite, marbles and greenschists. Overlying this belt is Tsakkok lens of the same composition as the Vaimok lens. The situation with an intermediate-pressure and often excellently preserved sheeted-dyke complex intercalated between two lenses of complexly deformed high-pressure rocks invited to intricate models of accretion, including out-of-sequence stacking of slices of the continental margin during early Ordovician subduction. Based on a detailed study of a select area and examination of the evolution of previous concepts, this study reinterprets the narrow belt along the western margin of the Sarek Lens as typical of the lower KNC. The favoured explanation of the presence of intermediate-pressure Koli rocks below high-pressure Seve rocks is recumbent folding and/or mega-sheath folding of the Seve-Koli nappe sequence during Scandian (Siluro-Devonian) accretion, by analogy with the situation in places in the central Swedish Caledonides. Previous models of early out-of-sequence stacking during subduction of the continental margin need reconsideration.