Upper and uppermost thrust sheets in the Caledonide Orogen, Sweden; outboard oceanic and exotic continental terranes
Upper and uppermost thrust sheets in the Caledonide Orogen, Sweden; outboard oceanic and exotic continental terranes (in Sweden; lithotectonic framework, tectonic evolution and mineral resources, M. B. Stephens (editor) and J. Bergman Weihed (editor))
Memoirs of the Geological Society of London (2020) 50: 549-575
Three separate stacks of thrust sheets (Koli Nappe Complex) constitute the Upper Allochthon in the Caledonide orogen, Sweden. This thrust complex is dominated by late Cambrian-Ordovician successions deposited in subduction-related, marginal oceanic basins. Magmatic activity at c. 488 Ma (Lower Koli) and c. 492-476 Ma (Middle Koli) is linked to rifted volcanic arcs and Zn-Cu-Fe-(Pb-Au-Ag) sulphide mineralization; serpentinite bodies with talc deposits are also conspicuous. Renewed magmatic activity, both plutonic (Upper and Middle Koli) and mafic volcanic (Middle and Lower Koli), occurred at c. 440-434 Ma during crustal extension. Late Ordovician shallow-marine sedimentation, deepening upwards into an early Silurian succession also prevailed (Lower Koli). Silurian (c. 430 Ma and later) folding, eastwards-vergent thrusting and greenschist or lower amphibolite facies metamorphism preceded upright, orogen-parallel and orogen-transverse open folding. Juxtaposition of an arc-related terrane to an ancient continental margin, comprising slices of gneiss and marble, in the Middle Koli occurred prior to c. 437 Ma and the eastwards-vergent thrusting; remnants of an Ordovician amphibolite facies tectonothermal event are also preserved in the Upper Koli. The tectonic roof to the Koli complex contains amphibolite facies mica schist, gneiss and marble, derived from the Laurentian continental margin, and a major gabbroic pluton (Rodingsfjallet Nappe Complex, Uppermost Allochthon).