The Collisional Orogeny in the Scandinavian Caledonides (COSC) Project: Investigating Exposed Middle Crust Through Geological Mapping, Drilling and Geophysics

Abstract:

The COSC project aims to provide a deeper understanding of mountain belt dynamics in the Scandinavian Caledonides. Scientific investigations include a range of topics, from understanding the ancient orogeny to the present-day hydrological cycle. Main objectives of the project, from a tectonic viewpoint, are to obtain (i) better understanding of the exhumation and emplacement of the hot middle allochthon, which may enable comparison with exhumation processes in the Himalaya-Tibet orogen, (ii) a broad understanding of orogeny and deformation in the middle to deep crust and upper mantle of mountain belts, and (iii) constraints on the abundant geophysical data that have been acquired in the area. COSC investigations and drilling activities are focused in central Scandinavia, near Åre (Sweden), where rocks from the mid to lower crust of the orogen are exposed. Rock units of interest include granulite facies migmatites (locally ultra-high pressure), gneisses and amphibolites in the middle allochthon (Seve nappe) that overlie greenschist facies metasedimentary rocks in the lower allochthons (Särv and Jämtlandian nappes). The base of the lower allochthon marks the contact with the autochthonous Precambrian basement. To investigate the high grade Seve nappe the COSC-1 borehole was drilled to 2496 m, with almost 100 % core recovery, during summer 2014. The top 1800 m consists mostly of sub-horizontal and shallowly dipping intermittent layers of gneiss and amphibolite, with lesser amounts of calc-silicates, metagabbro, marble and lenses of pegmatite. The first signs of increasing strain appear shortly below 1700 m in the form of narrow deformation bands and thin mylonites. Below c. 2100 m, mylonites dominate and garnets become common. A transition from gneiss into lower-grade metasedimentary rocks occurs between 2345 and 2360 m. The lower part of the drill core to TD is dominated by quartzites and metasandstones of unclear tectonostratigraphic position that are mylonitized to a varying degree. An integrated structural interpretation based on surface geological mapping, results from the COSC-1 borehole and high quality geophysics will be presented.