Koli nappes in the north-central Swedish Caledonides - new views on stratigraphy and structural evolution

摘要

The Koli Nappes of the Scandinavian Caledonides are composed of oceanic and island-arc terranes of Cambro-Silurian age and represent the upper part of the major Caledonian Upper Allochthon. Field, structural, microscopic, geophysical and geochemical data, together with a review of published information show new aspects on stratigraphy, structural evolution, nappe geometry and late Ordovician, greywacke provenance in the Koli Nappes of southwestern Vasterbotten and northern Jamtland counties, Sweden. It is proposed here to divide the lower part of the Koli succession of this area, i.e. the correlative of the Seima Formation, into three formations, the Kvemoen Mica Schist, the Ankarede Volcanite, and the Gilliks Formations. Trace element geochemistry of Ordovician Gilliks Formation greywackes indicates a provenance from a continental magmatic arc. New data also show that the overlying quartzite and limestone succession interpreted as the Vojtja and Slatdal Formations here is representing the top of the Stikke Nappe succession, which thus now reaches up into the highest Ordovician. The early repetition and inversion of the stratigraphy is an effect of D1-D2 deformation. The subsequent D3 episode comprises thrusting of, for example, the Middle Koli Nappe over the Lower Koli Nappe. A new aspect is that fault bends in D3 thrusts may give rise to fault-bend folds in overlying units, which are also D3 structures. The D3 episode also includes movement of the Koli units relative to the underlying Seve Nappes with top-to-the-west-directed shearing at the Seve-Koli boundary. It is succeeded by D4 regional-scale folds, related to nappe stacking in the underlying allochthons. The east-dipping limbs of these folds are thus locked for later, top-to-the-west shear deformation. Therefore, at least these parts of the Seve-Koli boundary cannot be regarded as re-activated during late (D5) and post-Caledonian gravitational deformation. Further work will have to show, whether the observed latest stage deformation represents late Caledonian tectonics or is related to subsequent extensional tectonics.