Abstract
Abstract. The Collisional Orogeny in the Scandinavian Caledonides (COSC) project, a contribution to the International Continental Scientific Drilling Program (ICDP), aims to provide a deeper understanding of mountain belt dynamics. Scientific investigations include a range of topics, from subduction-related tectonics to the present-day hydrological cycle. COSC investigations and drilling activities are focused in central Scandinavia, where rocks from the middle to lower crust of the orogen are exposed near the Swedish–Norwegian border. Here, rock units of particular interest occur in the Seve Nappe Complex (SNC) of the so-called Middle Allochthon and include granulite facies migmatites (locally with evidence of ultra-high pressures) and amphibolite facies gneisses and mafic rocks. This complex overlies greenschist facies metasedimentary rocks of the dolerite-intruded Särv Nappes and underlying, lower grade Jämtlandian Nappes (Lower Allochthon). Reflection seismic profiles have been an important component in the activities to image the subsurface structure in the area. Subhorizontal reflections in the upper 1–2 km are underlain and interlayered with strong west- to northwest-dipping reflections, suggesting significant east-vergent thrusting. Two 2.5 km deep fully cored boreholes are a major component of the project, which will improve our understanding of the subsurface structure and tectonic history of the area. Borehole COSC-1 (IGSN: http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in the summer of 2014, targeted the subduction-related Seve Nappe Complex and the contact with the underlying allochthon. The COSC-2 borehole will be located further east and will investigate the lower grade, mainly Cambro-Silurian rocks of the Lower Allochthon, the Jämtlandian décollement, and penetrate into the crystalline basement rocks to identify the source of some of the northwest-dipping reflections. A series of high-resolution seismic profiles have been acquired along a composite ca. 55 km long profile to help locate the COSC drill holes. We present here the results from this COSC-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014. These seismic data, along with shallow drill holes in the Caledonian thrust front and previously acquired seismic, magnetotelluric, and magnetic data, are used to identify two potential drill sites for the COSC-2 borehole.
Highlights
Following the Ordovician closure of the Iapetus Ocean, major Caledonian orogeny involved continent collision and underthrusting of Baltica beneath Laurentia
We present here the results from this Collisional Orogeny in the Scandinavian Caledonides (COSC)-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014
The first phase of the project, COSC-1, targeted the lower units of the high-grade Seve Nappe Complex (SNC). These rocks that originated along the rifted outer margin of continent Baltica, including the continent–ocean transition (COT) zone (Andreasson, 1994), were partially subducted during the Ordovician and emplaced hot onto underlying allochthons
Summary
Following the Ordovician closure of the Iapetus Ocean, major Caledonian orogeny involved continent collision and underthrusting of Baltica beneath Laurentia. The sole thrust corresponds to the lower limit of Caledonian deformation, i.e., involving both the long-transported allochthons and the underlying crystalline basement in and below the antiformal windows. The first phase of the project, COSC-1, targeted the lower units of the high-grade Seve Nappe Complex (SNC) These rocks that originated along the rifted outer margin of continent Baltica, including the continent–ocean transition (COT) zone (Andreasson, 1994), were partially subducted during the Ordovician and emplaced hot onto underlying allochthons. The focus of COSC-2 lies in understanding the thin-skinned thrusting over this detachment horizon, the character of the deformation in the underlying crystalline Fennoscandian basement, and how this foreland deformation relates to the partial subduction of the Baltica margin in the hinterland (e.g., the Western Gneiss Region of southwestern Norway) in the early Devonian (Robinson et al, 2014). Based on our interpretations of the CSP data and the goals of the COSC scientific deep drilling project, we propose two candidate locations for the second borehole, COSC-2
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