Collisional Orogeny in the Scandinavian Caledonides (COSC)

Principal Investigators (science):
    David G. Gee, Uppsala University, Sweden – Geology.
    Christopher Juhlin, Uppsala University, Sweden – Geophysics.
    Christophe Pascal, Geological Survey of Norway – Geothermics.
    Chin-Fu Tsang, Lawrence Berkeley National Laboratory, US – Hydrogeology.
    Karsten Pedersen, University of Gothenburg, Sweden – Microbiology.
Principle Technical Investigators:
    Leif Bjelm, Lund University, Sweden – technology development.
    Jan-Erik Rosberg, Lund University, Sweden – drilling, testing, well & aquifer analyses.

The Caledonides of western Scandinavia (Fig.1) and eastern Greenland have long been recognised to have been part of a collisional orogen of Alpine-Himalayan dimensions, essentially the result of the closure of the Iapetus Ocean during the Ordovician with development of island-arc systems and subsequent underthrusting of continent Laurentia by Baltica in the Silurian and Early Devonian during Scandian collisional orogeny. Several hundreds of kilometres of thrust emplacement of allochthons have been demonstrated, E-directed in the Scandes and W-directed in Greenland.
In Scandinavia, major allochthons were derived from Baltica’s outer shelf, dyke-intruded COT (continent-ocean transition zone), Iapetus oceanic domains and, uppermost, from the Laurentian margin. On the western side of the North Atlantic, exposed along the eastern edge of the Greenland ice-cap, there are major thrust sheets, all derived from the Laurentian continental margin and transported at least two hundred kilometres westwards onto the platform. In both the Scandinavian and Greenland Caledonides, the major allochthons that were derived from the outer parts of the continent margins have been subject to high-grade metamorphism and, apparently, emplaced hot onto the adjacent platforms.
There are not many places in the world where a subduction-generated HP (perhaps UHP) complex can be traced across an orogen for 300 km from hinterland to foreland, and at least 1000 km (perhaps nearly 2000 km) along the length of the mountain belt. The Scandinavian Caledonides are one of the best places on the planet to study the emplacement not only of a highly ductile outer continental margin subduction complex, but also associated hot (granulite facies paragneisses with leucogranites) extruding nappes that are the potential heat source for the metamorphism of both the underlying and overlying long-transported allochthons.

Mountain Belt Dynamics
Comparison of the North Atlantic Caledonides and the Himalaya-Tibet orogen is stimulating much new research. Very different interpretations of both orogens (Soper et al., 1992; Gee et al., 2010; Streule et al., 2010) are being tested. The Caledonides in Scandinavia provide special opportunities for understanding Himalayan-type orogeny and the Himalayan Orogen itself. The last two decades have seen a growing appreciation of the importance of ductile emplacement of long-transported allochthons in the Scandes and elsewhere. In particular, in the Himalayas vast lateral transport of ductile allochthons over distances exceeding those in the Scandes has been demonstrated. The Izu-Bonin-Mariana arc system, target of the ODP leg 125 (c.f. Fryer et al., 1990), takes the comparison of collisional systems a step further – to a fore-arc system where subduction, collision, thrusting, and related igneous activity and exhumation have been studied in a currently existing smaller framework.

COSC project rationale
The COSC project focuses on the transport and emplacement of subduction-related high-grade COT complexes onto the Baltoscandian platform and their influence on the underlying allochthons and basement. Research will be performed by an international working group with experience from fossil and active mountain belts, investigating orogenic processes and their development over time by scientific drilling in the deeply eroded (mid-crustal levels) Caledonides and comparing the results from this unique locality with a modern analogue of similar size, the Himalaya-Tibet mountain belt, and the arc collisional systems between the Eurasian and Pacific plates.

An ICDP supported science workshop with 58 participants was held from the 21st to the 25th June 2010 in Åre, southern Jämtland, Sweden, close to the potential drill site locations (Fig. 2).

COSC drilling programme
The geology on the mountain Åreskutan, with the highest grade metamorphic rocks cropping out on its peak (Figs. 1 & 3), has been studied in great detail through conventional geological fieldwork. By starting drilling lower down in the well-exposed rocks that make up the upper part of the mountain, COSC will extend the already studied section into and through the tectonostratigraphic successions that are not continuously exposed along the slopes of Åreskutan and in the Järpen region deep into the underlying autochthonous Fennoscandian basement.
The high spatial resolution provided by continuous drill core will allow a detailed study of the metamorphism and its changes through and across tectonic contacts, from the hot Åreskutan Nappe, at the top of the mountain, into the underlying, less metamorphosed nappes and basement. Oriented drill core will serve as a basis for understanding deformation and thrust emplacement, and heat transport and fluid migration during metamorphism, in time and space. COSC drilling will then penetrate the lower allochthonous units and the basal décollement, most likely in Cambrian alum shale, to enter into Precambrian crystalline basement. Prominent basement seismic reflectors will be drilled through and studied in detail. Investigation of the apparent deformation pattern in the autochthonous basement that is observed in the seismic data will be achieved by drill core studies and in-hole measurements. In-situ and drill core investigations are also necessary to study the amount of Caledonian and older basement deformation and metamorphism.

To make the COSC project feasible, two holes will be drilled to c. 2.5 km instead of one deep hole. The second drill hole will be located further towards the foreland of the Caledonides, starting in the tectonostratigraphy just above the base of the first hole (Figs. 3 & 4). These holes will be drilled with a diamond coring drill rig, maximising core recovery and minimising costs. Both boreholes will investigate regional heat flow and water circulation patterns, the deep biosphere, the mineral potential of the area, and allow calibration of high quality surface geophysical data at depths which are not normally accessible to drilling in this tectonic environment.

Working groups
Working groups for tectonics, geophysics, geothermal, hydrogeology and the deep biosphere have been established. Technical operations and research will be performed by the drilling management and technology working group, utilising the up-coming Swedish scientific drilling infrastructure, a diamond core drill rig with a depth capacity of at least 2500 m.

The target of the tectonics working group is to establish a coherent model of mid Palaeozoic (Scandian) mountain building in the regional context of the North Atlantic Caledonides. Orogenic processes will be compared and contrasted in on-going and older orogens, with underthrusting of continents, doubling (even trebling) of continental thicknesses, elevation of high plateau, partial melting of hinterland regions and ductile extrusion of allochthons many hundreds of kilometres onto adjacent platforms. The new insights will be applied to the interpretation of modern analogues, in particular the Himalaya-Tibet mountain belt, to better understand how the collision of continents and resulting orogeny control the environment on Earth and the evolution of the biosphere.

The geophysics working group will map the large scale geological structure around the boreholes and relate it to surface measurements. Cores and in-hole observations will allow the origin of the observed seismic reflections during site investigations and regional seismic profiling across the mountain belt (Fig. 4) to be determined. This information will be used to further our understanding of the geological structure of the mountain belt and the Fennoscandian basement. Other research will address methodology development for seismic in-hole experiments and vertical seismic profiling, and seismic long-term monitoring with down-hole seismometers.

The geothermics working group focuses on assessing the potential for deep geothermal energy in crystalline bedrock, in the case of COSC the Fennoscandian Shield basement and the allochthons above. In this context it is planned to investigate shallow boreholes (300-400 m) that have been drilled for mineral exploration purposes several decades ago, looking for a method to correct temperature gradients for disturbances by surface processes, i.e. extrapolate the temperature data gathered at relatively shallow depths to deeper levels and, vice versa, correct estimates based on shallow measurements with the knowledge from the deeper COSC holes. Furthermore, temperature disturbances can be used to obtain information on variations of Holocene and late Pleistocene palaeotemperature.

Our understanding of the deep hydrosphere in the crystalline bedrock of the Baltic Shield is still rudimentary, mainly because of the lack of information about the state of the subsurface below 1 km depth. The hydrogeology working group aims to model the large-scale groundwater circulation patterns in the mountain belt and their influence on the hydrosphere of the Fennoscandian Shield basement beneath other parts of Sweden. For this purpose it is necessary to build a regional geological model which is an important but yet unadressed problem in shield areas, with importance beyond the borders of Scandinavia.

Most other ICDP projects with a deep biosphere interest target sedimentary rocks. The microbiology working group will investigate microbial life in highly metamorphosed sedimentary and crystalline bedrock, which will introduce challenges different from those dealt with in sediments. The most obvious difference is that we anticipate life will only be present in fractures with water, while they commonly occur evenly distributed in sedimentary rocks. A second important difference is that sedimentary rocks basically offer surface generated energy and nutrient sources to the deep biosphere, while in crystalline rock, microbial life must utilise inorganic, geological sources such as energy-rich gases.

The availability of the diamond core drill rig that was funded by the Swedish Research Council will allow the drilling management and technology working group to develop and test new drilling technologies and tools. The emphasis will be on enhanced sampling methods in fractured formations, data transmission while drilling, measurement while drilling, integration of true-gyro measurements while drilling, core orientation tools, and hydraulic conductivity tools. Exactly what technologies will be tested will be decided during advanced project planning.


Relevance of COSC Science
Scientific and societal benefits go hand in hand in COSC. The research programme is driven by basic scientific curiosity. However, the majority of problems being addressed have a direct or indirect importance for society.
Geological processes along active continental margins, followed by collisional tectonics and mountain building have a profound influence on human society. Massive mountain belts like the Himalayas influence climate and weather, natural disasters are common for settlements on its steep slopes and narrow valleys. Younger collisional systems are infamous for treating inhabitants with earthquakes. COSC takes a comprehensive approach to mountain building processes and their development through geological time by integrating the drilling project in the fossil orogen of the Scandinavian Caledonides with research on the Himalayas and the Izu-Bonin-Mariana arc collisional systems (ICDP themes Collision Zones and Convergent Margins; Active Faulting; Global Environment).
After COSC drilling, the Jämtland transect across the Scandinavian Caledonides will be one of the best investigated profiles across a Palaeozoic mountain belt. Calibrated geophysical investigations will give insight into the structure of the shield basement and the overlying allochthons. Detailed geological studies will cover the section from the basement and into the Upper Allochthon (Fig. 1), including ore-bearing horizons. Intra-terrestrial life, its activity, nature and origin are much less studied in crystalline bedrock than in sedimentary environments. This is also true for the hydrogeological conditions. An integrated geological-geophysical-hydrogeological model is envisaged based on new knowledge concerning the structure of the thrust sheets and the underlying basement.
Results will be of importance for all kinds of underground infrastructure projects, in particular when very long-term resistance to the underground environment is central, like for waste storage. Heat flow studies will increase our knowledge about the thermal regime in the allochthons and the crystalline basement of the Fennoscandian Shield and, together with the hydrogeological results assess the potential for energy extraction in the Åre-Järpen region. For a more far-reaching approach the evaluation and development of methodology to more reliably predict the geothermal gradient from shallow drill holes is of importance. Inversion of heat flow data will also provide valuable information about palaeotemperature. (ICDP themes Geobiosphere and Early Life, Natural Resources, Thermal Regimes, Climate Dynamics & Global Environment).