Articles | Volume 19
Sci. Dril., 19, 1–11, 2015
Sci. Dril., 19, 1–11, 2015
Science report
29 May 2015
Science report | 29 May 2015

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 H. Lorenz et al.
  • 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

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 under 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 homogenous 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 revision. 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.

Short summary
The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project successfully drilled a 2.5km fully cored borehole (COSC-1) through allochthonous subduction-related high-grade metamorphic gneisses and into the underlying thrust zone. This paper summarizes the scientific rationale of the project and presents first preliminary results.