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COSC-1 – Drilling of a Subduction-Related Allochthon in the Palaeozoic Caledonide Orogen of Scandinavia

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COSC-1 – Drilling of a Subduction-Related Allochthon in the Palaeozoic Caledonide Orogen of Scandinavia Science Reports Sci. Dril., 19, 1–11, 2015 www.sci-dril.net/19/1/2015/ doi:10.5194/sd-19-1-2015 © Author(s) 2015. CC Attribution 3.0 License. COSC-1 – drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia H. Lorenz1, J.-E. Rosberg2, C. Juhlin1, L. Bjelm2, B. S. G. Almqvist1, T. Berthet1, R. Conze3, D. G. Gee1, I. Klonowska1, C. Pascal4, K. Pedersen5, N. M. W. Roberts6, and C.-F. Tsang1,7 1Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden 2Engineering Geology, Lund University, John Ericssons väg 1, 221 00 Lund, Sweden 3GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 4Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universitätsstraße 150, 44780 Bochum, Germany 5Department of Civil and Environmental Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden 6NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham, NG12 5GG, UK 7Lawrence Berkely National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS74R316C, Berkeley, CA 94720, USA Correspondence to: H. Lorenz ([email protected]) Received: 13 February 2015 – Revised: 24 April 2015 – Accepted: 5 May 2015 – Published: 29 May 2015 Abstract. The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Palaeozoic orogen in western Scandinavia and its comparison with modern analogues. The project investigates the subduction-generated Seve Nape Complex. These in part un- der ultra-high-pressure conditions metamorphosed outer continental margin and continent–ocean transition zone assemblages were emplaced onto the Baltoscandian platform and there influenced the underlying allochthons and the basement. COSC-1 is the first of two ca. 2.5 km deep, fully cored drill holes located in the vicinity of the abandoned Fröå mine, close to the town of Åre in Jämtland, central Sweden. It sampled a thick section of the lower part of the Seve Complex and was planned to penetrate its basal thrust zone into the underlying lower- grade metamorphosed allochthon. The drill hole reached a depth of 2495.8 m and nearly 100 % core recovery was achieved. Although planning was based on existing geological mapping and new high-resolution seismic surveys, the drilling resulted in some surprises: the Lower Seve Nappe proved to be composed of rather homoge- nous gneisses, with only subordinate mafic bodies, and its basal thrust zone was unexpectedly thick (> 800 m). The drill hole did not penetrate the bottom of the thrust zone. However, lower-grade metasedimentary rocks were encountered in the lowermost part of the drill hole together with garnetiferous mylonites tens of metres thick. The tectonostratigraphic position is still unclear, and geological and geophysical interpretations are under revi- sion. The compact gneisses host only eight fluid conducting zones of limited transmissivity between 300 m and total depth. Downhole measurements suggest an uncorrected average geothermal gradient of ∼ 20 ◦C km−1. This paper summarizes the operations and preliminary results from COSC-1 (ICDP 5054-1-A), drilled from early May to late August 2014, and is complemented by a detailed operational report and the data repository. Published by Copernicus Publications on behalf of the IODP and the ICDP. 2 H. Lorenz et al.: COSC-1 – drilling of a subduction-related allochthon 10°E 20°E a NZ v ES ID LEGEND so N o A 70°N n F M o I T A Younger orogenic belts m o C L Svalbard TI Caledonide Orogen L BA Timanide Orogen Lofoten 68°N COSC d Older cratons es n and c S Norrbotten Franz Joseph Land eenla F r NZ Novaya Zemlya G Outer margin of Baltica 66°N (Lower part of Middle Allochthon) A Inner margin of Baltica British (Lower Allochthon) Västerbotten TI Isles Precambrian in windows N NW E 64°N UR A Jämt- røndelag ÅRE North L T land SE Atlantic 62°N Caledonides BERGEN Siljan Ring Figure 1. The Caledonides prior to opening of the North Atlantic 60°N Ocean. Modified from Lorenz et al. (2012). STOCKHOLM Baltic Sea 10°E 20°E 1 The Caledonides – the world’s major NW SE mid-Palaeozoic mountain belt NORWAY SWEDEN TRONDHEIM ÖSTERSUND Åre The Caledonides of western Scandinavia and eastern Green- . .. land (Fig. 1) have long been recognized to have been part 0 . 0 of a collisional orogen of Alpine–Himalayan dimensions, es- BAL TIC A BASEM E N T sentially the result of the closure of the Iapetus Ocean dur- HIGHER ALLOCHTHONS LOWER ALLOCHTHON, PARAUTOCHTHON and AUTOCHTHON ing the Ordovician and subsequent underthrusting of con- Köli Nappes (Laurentian fauna in uppermost nappe) Baltoscandian foreland basin & platform Seve Nappe Complex (high grade) Baltica Baltica basement (allochthonous & tinent Laurentia by Baltica in the Silurian and Early De- Särv Nappes (dike swarms) outer autochthonous) 0 100 200 .... Mylonitic granites & psammites margin vonian during Scandian collisional orogeny. Several hun- Horizontal scale in km dreds of kilometres of thrust emplacement of allochthons have been demonstrated, E-directed in the Scandes and W- Figure 2. Tectonostratigraphic map of the Scandinavian Cale- donides and sketch section along the geotraverse from Östersund directed in Greenland. In both the Scandinavian and Green- to the Norwegian coast. Modified from Gee et al. (2010). land Caledonides, the allochthons that originated from the outer parts of the continental margins were subjected to high- grade metamorphism and emplaced, apparently hot, onto the adjacent platforms (cf. Gee et al., 2008). Baltica, as the bidites in the Ordovician and Silurian. Only minor basement- smaller of the two palaeo-continents involved in the colli- derived units are incorporated in eastern parts of this al- sion (referred to as Scandian orogeny in the North Atlantic lochthon (Greiling et al., 1998), but towards the west, seis- region), played a similar role to that of India in the present- mic profiling suggests that the basal decollement passes be- day Himalaya–Tibet context. The Scandinavian Caledonides neath the crystalline basement exposed in windows (Palm et comprise thrust sheets transported onto the Palaeozoic plat- al., 1991; Juhojuntti et al., 2001). The Middle Allochthon form successions of the Baltoscandian margin of Baltica is of higher metamorphic grade than the underlying units (Fig. 2). The Caledonian front is marked by a sole thrust (Andréasson and Gorbatschev, 1980). In most areas it con- that dips 1–2◦ westwards beneath the orogen, underlain tains a basal basement-derived thrust sheet (e.g. the Tännes by a thin veneer of Cambrian (locally Ediacaran) sedi- Augen Gneiss Nappe), overlain by greenschist facies Of- mentary rocks that unconformably overlie Proterozoic crys- ferdal Nappe metasandstones, composing the footwall for talline basement. The thrust sheets are subdivided into the the remarkable Särv Nappe (Strömberg, 1961) with its abun- Lower, Middle, Upper and Uppermost allochthons (Gee et dant ca. 600 Ma dolerite dyke swarms. The sedimentary host al., 1985). The Lower Allochthon (Jämtlandian Nappes) is rocks are composed of Neoproterozoic sandstones with sub- dominated by a sedimentary succession of Neoproterozoic ordinate carbonates and tillites (Kumpulainen, 1980). The and Cambro-Silurian strata, featuring westerly-derived tur- uppermost tectonic unit in the Middle Allochthon (Andréas- Sci. Dril., 19, 1–11, 2015 www.sci-dril.net/19/1/2015/ H. Lorenz et al.: COSC-1 – drilling of a subduction-related allochthon 3 son and Gee, 2008; Gee et al., 2008), previously included Topical working groups developed the scientific objectives in the Upper Allochthon, is the Seve Nappe Complex, com- of the COSC scientific drilling project. Major targets are the posed in most areas of three units (Sjöström, 1983; van following: Roermund, 1985; Bergman and Sjöström, 1997): a lower part of similar protolith to the Särv Nappe, but ductilely – to establish a coherent model of mid-Palaeozoic (Scan- deformed in amphibolite (locally eclogite) facies; a cen- dian) mountain building and to apply these new insights tral part (e.g. Åreskutan Nappe) of migmatites and parag- to the interpretation of modern analogues, in particular neisses (Arnbom, 1980) with a previous ultra-high-pressure the Himalaya–Tibet mountain belt; metamorphic history (Klonowska et al., 2015); and an up- per, amphibolite-dominated unit with micaschists and psam- – to determine the origin of observed seismic reflections mites. The high-grade metamorphism and leucogranite in- and constrain geophysical interpretations in order to trusions of the Seve Nappes have yielded Early Silurian use this information to further our understanding of ages (Gromet et al., 1996; Williams and Claesson, 1987; the geological structure of the mountain belt and the Ladenberger et al., 2014). The entire Middle Allochthon Fennoscandian basement; was derived from west of the Norwegian coast and the up- – to refine knowledge on climate change at high latitudes, per units transported at least 400 km eastwards during Scan- including historical global changes, recent palaeo- dian orogeny (Gee, 1978). The tectonostratigraphically high- climate development (since last ice age); est rocks in Jämtland are the Köli Nappes in the Upper Al- lochthon. This low to high greenschist facies unit contains – to understand the hydrological characteristics of the ge- basal slices of ophiolite (e.g. at Handöl) and is dominated by ological units and research present groundwater circu- sedimentary rocks of Early Palaeozoic age (Kulling, 1933). lation patterns of the mountain belt; The Uppermost Allochthon, metasedimentary and carbonate rocks inferred to be derived from the Laurentian margin are – to analyse the extent, functions and diversity of microor- only exposed farther to the northwest in the mountain belt. ganisms in the drill hole as a function of the different penetrated geological strata and their depth. 3 Strategy 2 Scientific objectives Two wireline fully cored drill holes, each to ca. 2.5 km depth, can penetrate through the tectonic stack from the high-grade The COSC project aims to study mountain building pro- Lower Seve Nappe and well into the Baltican basement.
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