Chapter 22 Upper and uppermost thrust sheets in the Caledonide orogen, Sweden: outboard oceanic and exotic continental terranes

Abstract

AbstractThree separate stacks of thrust sheets (Köli 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 Köli) and c. 492–476 Ma (Middle Köli) 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 Köli) and mafic volcanic (Middle and Lower Köli), occurred at c. 440–434 Ma during crustal extension. Late Ordovician shallow-marine sedimentation, deepening upwards into an early Silurian succession also prevailed (Lower Köli). 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 Köli 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 Köli. The tectonic roof to the Köli complex contains amphibolite facies mica schist, gneiss and marble, derived from the Laurentian continental margin, and a major gabbroic pluton (Rödingsfjället Nappe Complex, Uppermost Allochthon).