Collisional Orogeny in the Scandinavian Calodenides (COSC)

This SDDP project seeks to better understand collisional orogeny – the processes involved in the partial subduction of continental crust and its subsequent exhumation. Doubling, even trebling the thickness of continental crust is known from several orogens, with the record of crustal thickening preserved in the high pressure mineralogy of the metamorphic rocks (e.g. eclogites with coesite and diamonds). The Scandinavian Caledonides provide a particularly good example of Paleozoic plate collision, with the underthrusting of continent Baltica beneath Laurentia and thickening of the continental crust to at least 100 km. Lateral transport of material over distances of several hundreds of kilometers in the Scandinavian Caledonides, by a combination of thrusting and ductile extrusion, is comparable to that recognized in recent years in the Himalayas. The Caledonides in Scandinavia provide unique opportunities for understanding Himalayan-type orogeny, thanks to the deep level of erosion (mid to lower crustal) and the paucity of superimposed post-Paleozoic deformation. Whereas in the Himalayas, an understanding of the on-going collisional processes in the middle and lower crust is largely dependent on interpretation of geophysical data – seismic, potential field and geothermal, in the Scandes it is possible to directly observe the ‘fossilized’ collisional process at a former mid-crustal level; geophysical data provide control of the geometry of the structure and the target for drilling. In the Scandinavian Caledonides, granulite facies paragneisses and garnet-pyroxene bearing migmatites of the Seve Nappe Complex overlie amphibolite facies metasedimentary units; metamorphic grade decreasing downwards into the underlying allochthons. Two drillholes to about 2-3 km depth will provide a section from the lower Seve Nappe, through underlying Särv and Offerdal Nappes, lower grade Cambro-Silurian sedimentary rocks of the Jämtlandian Nappes, and the basal Caledonian décollement, with underlying Cambrian Alum Shales resting on Precambrian basement of unknown lithology (possibly Dala sandstones or older granites, intruded by sheets of mafic rocks). Whereas the highly bituminous Alum Shales were an ideal medium for the basal Caledonian detachment, facilitating underthrusting of Baltica basement, the high ductility with melting of the central Seve Nappe may provide an explanation for the vast (over 300 km) displacement of ocean-derived allochthons onto the craton, as in the Himalayas. These drillholes will also provide unique information about the present temperature gradient in the Caledonides, the porosity and permeability of the rock formations, and the potential for mineral resources, geological storage of CO2 and geothermal energy in western Jämtland.

Principal investigators: David Gee & Christopher Juhlin, Uppsala University; Christophe Pascal & Peter Robinson, Geological Survey of Norway (NGU)

Link to project page at ICDP (Internatinal Continental Scientific Drilling Program)

Information about the ICDP Science Workshop is available at http://www.sddp.se/COSC-workshop.

Download documents about the COSC project: