COSC-1 - drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia
Journal article, 2015

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 lowergrade metamorphosed allochthon. The drill hole reached a depth of 2495.8m 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 300m 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.

Author

H. Lorenz

Uppsala University

J. E. Rosberg

Lund University

C. Juhlin

Uppsala University

L. Bjelm

Lund University

B. S G Almqvist

Uppsala University

T. Berthet

Uppsala University

R. Conze

German Research Centre for Geosciences (GFZ)

D. G. Gee

Uppsala University

I. Klonowska

Uppsala University

C. Pascal

Ruhr-Universität Bochum

Karsten Pedersen

Chalmers, Civil and Environmental Engineering, GeoEngineering

N. M W Roberts

British Geological Survey (BGS)

C.-F. Tsang

Lawrence Berkeley National Laboratory

Uppsala University

Scientific Drilling

1816-8957 (ISSN) 1816-3459 (eISSN)

Vol. 19 1-

Subject Categories

Geophysical Engineering

Earth and Related Environmental Sciences

DOI

10.5194/sd-19-1-2015

More information

Latest update

6/8/2018 8