3D reflection seismic imaging at the 2.5 km deep COSC-1 scientific borehole, central Scandinavian Caledonides

Research output: Contribution to journalArticle

Abstract

The 2.5 km deep scientific COSC-1 borehole (ICDP 5054-1-A) was successfully drilled with nearly complete core recovery during spring and summer of 2014. Downhole and on-core measurements through the targeted Lower Seve Nappe provide a comprehensive data set. An observed gradual increase in strain below 1700 m, with mica schists and intermittent mylonites increasing in frequency and thickness, is here interpreted as the basal thrust zone of the Lower Seve Nappe. This high strain zone was not fully penetrated at the total drilled depth and is thus greater than 800 m in thickness. To allow extrapolation of the results from downhole logging, core analysis and other experiments into the surrounding rock and to link these with the regional tectonic setting and evolution, three post-drilling high-resolution seismic experiments were conducted in and around the borehole. One of these, the first 3D seismic reflection land survey to target the nappe structures of the Scandinavian Caledonides, is presented here. It provides new information on the 3D geometry of structures both within the drilled Lower Seve Nappe and underlying rocks down to at least 9 km. The observed reflectivity correlates well with results from the core analysis and downhole logging, despite challenges in processing. Reflections from the uppermost part of the Lower Seve Nappe have limited lateral extent and varying dips, possibly related to mafic lenses or boudins of variable character within felsic rock. Reflections occurring within the high strain zone, however, are laterally continuous over distances of a kilometer or more and dip 10–15° towards the southeast. Reflections from structures beneath the high strain unit and the COSC-1 borehole can be followed through most of the seismic volume down to at least 9 km and have dips of varying degree, mainly in the east–west thrust direction of the orogen.

Details

Authors
  • Peter Hedin
  • Bjarne Almqvist
  • Théo Berthet
  • Christopher Juhlin
  • Stefan Buske
  • Helge Simon
  • Rüdiger Giese
  • Felix Krauß
  • Jan Erik Rosberg
  • Per Gunnar Alm
Organisations
External organisations
  • Uppsala University
  • Freiberg University of Mining and Technology
  • GFZ German Research Centre for Geosciences
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Geophysics

Keywords

  • 3D reflection seismic, Borehole geophysics, Collisional orogeny in the Scandinavian Caledonides, Continental scientific deep drilling, Seve Nappe Complex, Shear zones
Original languageEnglish
Pages (from-to)40-55
Number of pages16
JournalTectonophysics
Volume689
Publication statusPublished - 2016 Oct 15
Publication categoryResearch
Peer-reviewedYes