Revised tectonostratigraphy and structural evolution of the Köli Nappe Complex, Central Caledonides in Nordland, Norway

The nappe stack in the Røssvatnet-Hattfjelldal region in the Central Norwegian Caledonides consists of seven nappes formed at the boundary between tectonostratigraphically upper and uppermost Caledonian levels. The rocks of all nappes share a polyphase tectono-metamorphic evolution that is younger than the 491±10 Ma depositional (volcanic) age of parts of the succession. Early deformation stages characterized by centimetre- to kilometre-scale folding and intense shearing accompanied by greenschist- to amphibolite-facies peak metamorphism are correlated with the Early Ordovician Taconian accretionary orogeny along the Laurentian margin. The Taconian structures are cut by the Krutfjellet gabbro and diorite, which yield U-Pb zircon ages of 446±5 and 444±4 Ma, respectively. Large scale nappe stacking and folding postdating emplacement of the gabbro was related to collision of Laurentia with Baltica (Scandian orogeny) and followed by late-/postorogenic extension. The revised tectonostratigraphy assigns the structurally higher nappes to the Uppermost Allochthon while the lower nappes are correlated with the Middle Köli Nappe Complex (Upper Allochthon). The boundary between them is marked by an imbricate zone. Taconian deformation was probably much more penetrative and widespread than hitherto thought; thus, parts of the nappe stack were likely assembled prior to Scandian collision.