Solid Earth, 7, 769–787, 2016 www.solid-earth.net/7/769/2016/ doi:10.5194/se-7-769-2016 © Author(s) 2016. CC Attribution 3.0 License. Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden Christopher Juhlin, Peter Hedin, David G. Gee, Henning Lorenz, Thomas Kalscheuer, and Ping Yan Department of Earth Sciences, Uppsala University, Uppsala, Sweden Correspondence to: Peter Hedin ([email protected]) Received: 1 December 2015 – Published in Solid Earth Discuss.: 15 January 2016 Revised: 13 April 2016 – Accepted: 28 April 2016 – Published: 18 May 2016 Abstract. The Collisional Orogeny in the Scandinavian A series of high-resolution seismic profiles have been ac- Caledonides (COSC) project, a contribution to the Interna- quired along a composite ca. 55 km long profile to help lo- tional Continental Scientific Drilling Program (ICDP), aims cate the COSC drill holes. We present here the results from to provide a deeper understanding of mountain belt dynam- this COSC-related composite seismic profile (CSP), includ- ics. Scientific investigations include a range of topics, from ing new interpretations based on previously unpublished data subduction-related tectonics to the present-day hydrologi- acquired between 2011 and 2014. These seismic data, along cal cycle. COSC investigations and drilling activities are fo- with shallow drill holes in the Caledonian thrust front and cused in central Scandinavia, where rocks from the middle previously acquired seismic, magnetotelluric, and magnetic to lower crust of the orogen are exposed near the Swedish– data, are used to identify two potential drill sites for the Norwegian border. Here, rock units of particular interest oc- COSC-2 borehole. cur in the Seve Nappe Complex (SNC) of the so-called Mid- dle Allochthon and include granulite facies migmatites (lo- cally with evidence of ultra-high pressures) and amphibo- lite facies gneisses and mafic rocks. This complex overlies 1 Introduction greenschist facies metasedimentary rocks of the dolerite- intruded Särv Nappes and underlying, lower grade Jämt- Following the Ordovician closure of the Iapetus Ocean, ma- landian Nappes (Lower Allochthon). Reflection seismic pro- jor Caledonian orogeny involved continent collision and files have been an important component in the activities to underthrusting of Baltica beneath Laurentia. Subduction- image the subsurface structure in the area. Subhorizontal re- related metamorphism along the Baltica margin already took flections in the upper 1–2 km are underlain and interlayered place in the early to middle Ordovician (Gee et al., 2012; Ma- with strong west- to northwest-dipping reflections, suggest- jka et al., 2012) and the initial stages of continent–continent ing significant east-vergent thrusting. Two 2.5 km deep fully collision are believed to have occurred around 445 Ma (e.g., cored boreholes are a major component of the project, which Ladenberger et al., 2012, 2014). Thrust tectonics, which will improve our understanding of the subsurface structure dominated throughout the collision, resulted in the emplace- and tectonic history of the area. Borehole COSC-1 (IGSN: ment of allochthonous units both westwards onto the Lauren- http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in tian platform of Greenland (Higgins and Leslie, 2000), with the summer of 2014, targeted the subduction-related Seve displacements of the higher allochthons at least 200 km, and Nappe Complex and the contact with the underlying al- eastwards onto the Baltoscandian platform, with displace- lochthon. The COSC-2 borehole will be located further east ments of more than 500 km (Gee, 1978). and will investigate the lower grade, mainly Cambro-Silurian Towards the end of Caledonian orogeny, in the early Devo- rocks of the Lower Allochthon, the Jämtlandian décollement, nian, the mountain belt was in many aspects comparable to and penetrate into the crystalline basement rocks to iden- the presently active Himalaya-Tibet orogen (Dewey, 1969; tify the source of some of the northwest-dipping reflections. Gee et al., 2010; Labrousse et al., 2010). Following post- orogenic collapse, extension, and deep erosion, the surface of Published by Copernicus Publications on behalf of the European Geosciences Union. 770 C. Juhlin et al.: Seismic imaging in the eastern Scandinavian Caledonides the present-day Caledonides cuts through the internal archi- 10° E 20° E tecture of the paleo-orogen, revealing the nappe structure at Oslo Permian Rift mid-crustal depths. The Scandinavian mountains, the Scan- Devonian - Old Red Sandstones 72° N des, have long been recognized as an excellent environment Laurentian margin to study thrust tectonics (Törnebohm, 1888) and the pro- (uppermost allochthon) cesses involved in continent–continent collision (Gee, 1975; Outboard Iapetus-derived terranes Hossack and Cooper, 1986). Köli Nappe Complex 70° N Investigations of the Scandinavian Caledonides (Fig. 1) (upper allochthon) Outermost Baltica margin were intensified in the 1970s (Gee and Sturt, 1985) and our - Seve & related nappes understanding has improved since then through continued (upper part of middle allochthon) 68° N geological (e.g., the many contributions in Corfu et al., 2014) Outer margin of Baltica and numerous geophysical (e.g., Dyrelius, 1980, 1986; Elm- (lower part of middle allochthon) Inner margin of Baltica ing, 1988; Hurich et al., 1989; Palm et al., 1991; Hurich, (lower allochthon) 66° N 1996; Juhojuntti et al., 2001; Pascal et al., 2007; Korja et al., Precambrian in windows Laisvall 2008; England and Ebbing, 2012) studies. One key area of (lower allochthon) investigation (Dyrelius et al., 1980) has been along a profile Autochthon Sedimetary NW crossing the mountain belt through the provinces of Jämt- cover 64° N land (Sweden) and Tröndelag (Norway). Reflection seismic Precambrian Åre basement Trondheim Östersund surveys were conducted along the Central Caledonian Tran- Jämt- land röndelag SE sect (CCT), which stretches from the east of the Caledo- T 62° N nian thrust front in central Jämtland to the Atlantic coast in Vassbo western Tröndelag (Hurich et al., 1989; Palm et al., 1991; Siljan Ring Hurich, 1996; Juhojuntti et al., 2001). The highly reflective N upper crust shows a reflectivity pattern of crustal shorten- Oslo Stockholm 60° N ing consistent with surface observations, i.e., imbrication of Baltic Sea allochthonous units and folding by major N–S to NE–SW- 100 km 10° E 20° E trending antiforms and synforms. At the thrust front in central Sweden, Cambrian alum NW Norway Sweden SE shales, deposited unconformably on the autochthonous crys- Trondheim Östersund Åre talline basement, are separated from the overlying Caledo- . nian allochthons by a major décollement (Gee et al., 1978). 0 . 0 Comprehensive drilling programs targeting the metalliferous Baltica Basement organic-rich alum shales (Gee et al., 1982) in the thrust front Higher allochthons Lower allochthon, parautochthon south of lake Storsjön reached about 30 km to the northwest, Köli Nappes (Laurentian and autochthon ◦ fauna in uppermost nappe) Baltoscandian foreland basin establishing a 1–2 westwards dip of the décollement. At Seve Nappe Complex & platform (high grade) Baltica Baltica basement (allochthonous the Caledonian front in central Jämtland, this major detach- Särv Nappes (dike swarms) outer & autochthonous) margin ment coincides with the Caledonian sole thrust (see profile . Mylonitic granites . & psammites 0 100 200 in Fig. 1). We define the main décollement (in Jämtland – Horizontal scale in km the Jämtlandian décollement, at the base of the Jämtlandian Nappes) as the thrust zone that separates all the overlying Figure 1. (a) Provenance interpretation of the tectonostratigraphic long-transported allochthons from the underlying less de- map of the Scandinavian Caledonides, modified from Gee et formed basement. The sole thrust corresponds to the lower al. (1985). The star marks the location of the COSC-1 borehole. limit of Caledonian deformation, i.e., involving both the (b) Schematic cross section (vertical exaggeration x10) along the long-transported allochthons and the underlying crystalline NW–SE profile in (a), from Gee et al. (2010). The autochthonous basement in and below the antiformal windows. Along the basement (light gray) is separated from the Caledonian deformed CCT reflection seismic profile, the sole thrust in the west- basement (dark gray) by the Scandian sole thrust. ern part (Palm et al., 1991) was inferred to ramp up east- wards and pass into the Jämtlandian décollement, as defined tation is in agreement with previous modeling of refraction in areas north of Storsjön (Juhojuntti et al., 2001). The sole seismic (Palm, 1984), aeromagnetic (Dyrelius, 1980), and thrust defined by Palm et al. (1991) beneath the Åre Syn- gravity data (Dyrelius, 1985; Elming, 1988). Magnetotelluric form and Mullfjället Antiform was inferred to continue west- measurements along the Swedish section of the CCT pro- wards to the Swedish–Norwegian border, where it appears file (Korja et al., 2008), targeting the highly conductive alum to reach a depth of ca. 7 km (Hurich et al., 1989), perhaps shales, further support this interpretation. deeper (Gee, 1988, Hurich, 1996), beneath the imbricated crystalline basement of the Skardöra Antiform. This interpre- Solid Earth, 7, 769–787, 2016 www.solid-earth.net/7/769/2016/ C. Juhlin et al.: Seismic imaging in the eastern Scandinavian Caledonides 771 A transition from thin-skinned (where deformation is basement