by Christopher Juhlin, Mark W. Anderson, Nick Roberts, Sandra Piazolo, Christophe Pascal, Chinfu Tsang, Mark Geoffrey Dopson

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project is investigating mountain building processes at mid to lower crustal levels in a deeply eroded Paleozoic collisional orogen of Himalayan dimensions. Two fully cored COSC boreholes will provide a unique circa 5 km deep composite section from a subduction related allochthon through the underlying continental margin, mostly cover-derived nappes, and the basal décollement into Precambrian crystalline basement. Utilising the Caledonides as an analogue to comparable tectonic settings, such as the Himalaya, will advance our understanding of orogenic systems and how they affect the stability of the living environment. COSC-1, drilled in 2014, investigated the emplacement of the high-grade (UHP) metamorphic allochthons. COSC-2 (this proposal) will define the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement. The upper part of the borehole will sample a Cambro-Silurian succession, including both distal shelf sedimentary formations typical of the Baltoscandian platform and also foreland basin turbidites of both mid Ordovician and Early Silurian age, separated by significant environmental changes at the Ordovician-Silurian boundary. After drilling through the main décollement, COSC-2 will penetrate 1-1.5 km into the basement of the Fennoscandian Shield, where prominent seismic reflections indicate the presence of significant crustal shortening of Caledonian or Sveconorwegian age. Study of this internal basement deformation is particularly important for understanding the passage from thin-skinned tectonics in the foreland of the orogen to thick-skinned tectonics in the hinterland, where the underriding Baltica plate experienced deformation and (U)HP metamorphism. ICDP drilling will result in a fully cored borehole that will be further characterised by downhole logging, as well as complemented by VSP and electromagnetic surveys in separate projects. COSC-2 research will span several geoscientific disciplines – geology, geophysics, geochronology, hydrogeology, geothermics and microbiology – performed by various constellations of the COSC science team who will seek funds from national and international funding agencies. Tasks such as Modern analogues – incipient and active orogens, Foreland – hinterland – off-shore connection and others will coordinate the multidisciplinary expertise of the international science team to effectively address the scientific targets. The COSC-2 scientific targets address four out of the five themes identified as the way forward in the ICDP Science Plan 2014: Active Faults and Earthquakes, Global Cycles and Environmental Change, Heat and Mass Transfer and The Ubiquitous Hidden Biosphere.

• Paleozoic mountain building and modern analogues on a regional to global scale Understand the main décollement in the foreland of one of the Earth’s largest orogens and the character of the deformation in the basement below.
• Research foreland-hinterland connections during continental collision and passive margin development after continental break-up.
• Utilise the new knowledge to better understand the nature and occurrence of natural hazards in modern orogenic environments. Early Paleozoic evolution of Baltica and the Ordovician-Silurian boundary Investigate the development of the Early Paleozoic basin.
• Investigate the environmental conditions during this period of Gondwana-Baltica-Laurentia juxtaposition with major episodes of change such as extensive glaciations on Gondwana and mass extinction at the Ordovician-Silurian boundary. Holocene climate evolution Reconstruct the ground surface temperature history to gather new knowledge about the Weichselian glaciation and climate evolution in northern Europe during the Holocene. Hydrogeological and geothermic characteristics of (old) mountain belts Understand the origin and genesis of fluids and gases and their impact on seismicity and rheology.
• Explore the means of heat transfer in the basement, across thrust zones and in the allochthons. The composition of deep microbial communities and energy sources Assess the change of populations and activities with depth and the concomitant transition between the energy sources organic carbon from surface and the geogases carbon dioxide and hydrogen.
• Investigate the transition with depth from microbial communities dominated by bacteria to those primarily composed by archaea in crystalline bedrock fracture systems.
• Investigate the traces of Paleozoic to Quaternary microbial activity in secondary minerals.

Basement Deformation, Europe, Foreland, Natural Hazards, Paleoenvironment, Paleozoic, Post-Weichselian Temperature Development, Sweden

Latitude: 63.3124, Longitude: 13.5259