by Christopher Juhlin, David G. Gee, Randall R. Parrish, Christophe Pascal, Chinfu Tsang, Karsten Pedersen

The COSC project proposes two scientific core holes, both to c. 2.5 km depth, in the Swedish Caledonides, near the towns of Åre and Järpen. Together they will provide a c. 5 km deep high-resolution mid-crustal section through this major mid-Palaeozoic orogen. The Caledonide Orogen in Scandinavia is comparable in size and many other respects to today's Himalayan mountain belt. It was formed in the Silurian by collision of the palaeocontinents Baltica and Laurentia and underthrusting of the latter by the former. Major allochthons in the Scandinavian Caledonides were derived from the outer margin of Baltica, the Iapetus Ocean and, uppermost, Laurentia with transport distances of many hundreds of kilometres onto the Baltoscandian Platform. Part of this nappe stack, and a key to studying mountain building processes in general, is the Seve Nappe Complex (SNC), comprising high-grade metamorphic rocks and migmatites which were generated during continental margin subduction and emplaced ductilely at mid-crustal levels onto the platform. The COSC project focuses on the process of emplacement of the SNC and its influence on the underlying lower grade metasediments and Precambrian basement. COSC drilling and integrated geoscientific studies will provide detailed insight into mid-Palaeozoic mountain building processes and further our understanding of past and present orogen dynamics, e.g. continent-continent collision in the Himalaya-Tibet mountain belt and arc-arc collision in the Izu-Bonin-Mariana Arc System. Located in a key-area for Caledonian geology, it is close to a major geophysical transect across the mountain belt. A pattern of prominent seismic reflections in the Fennoscandian basement provides critical evidence of crustal shortening that may be of Caledonian or Precambrian age – a question of fundamental importance for understanding the orogen that can only be addressed by drilling. Major COSC targets include also the study of the geothermal gradient and its response to palaeoclimatic influence, the hydrogeological-hydrochemical state of the mountain belt and the deep biosphere in the metamorphic rocks and crystalline basement. The COSC research programme is being developed by five working groups, geology, geophysics, geothermics, hydrogeology and microbiology. It is dominated by basic science addressing six of ICDP's eight themes, Collision Zones and Convergent Margins, Active Faulting, Geobiosphere and Early Life, Natural Resources, Thermal Regimes, and Climate Dynamics & Global Environment. It has direct relevance for society by improving our understanding of mountain building processes, hydrological-hydrochemical regimes in mountain areas and Precambrian shields, deep subsurface conditions for underground engineering and assessment of geothermal potential.

• COSC principal scientific objectives are a) to study mountain building and nappe emplacement processes in a deeply eroded mid-Palaeozoic orogen and apply this new knowledge to the interpretation of modern analogues, e.g. the Himalaya-Tibet orogen (geology working group). b) to calibrate regional geophysical studies with borehole data along one of the best studied transects across a mid- Palaeozoic mountain belt, and complement existing high-quality surface reflection seismic studies with borehole seismic experiments (geophysics working group). c) to generate basic knowledge about the thermal regime of ancient shield areas and in the overlying allochthonous units of the Scandinavian mountain belt, and improve the knowledge about palaeoclimatic variations in Scandinavia by inverse modelling of measured borehole temperature profiles (geothermics working group). d) to understand the hydrological-hydrochemical characteristics of the diverse geological units and develop a largescale hydrogeological model of the Scandinavian mountain belt and Fennoscandian basement (hydrogeology working group). e) to analyse the extent, functions and diversity of microorganisms as a function of depth and character of the different penetrated geological units, metamorphic and crystalline, and to study the role of deep-seated crustal structures for the evolution of the deep biosphere (microbiology working group).

Caledonides, Deep Hydrosphere, Dynamics, Europe, Heat Flow, Himalaya, Microbiology, Norway, Orogen, Scandes, Scandinavia, Seismic, Sweden

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