COSC-1 – drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia

Abstract

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 lowergrade 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.Published by Copernicus Publications on behalf of the IODP and the ICDP.

A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

“…The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific continental drilling project was designed to focus on Mid-Palaeozoic mountain building processes in western Scandinavia (Lorenz et al 2011(Lorenz et al , 2015a. With two fully cored boreholes (COSC-1, drilled in 2014 May-August; COSC-2, in the planning phase), each to ∼2.5 km depth, the COSC project aims to investigate both the Caledonian nappes and the underlying basement to further our current understanding of orogenic processes (Gee et al 2010).…”

Utilizing the International Geo Sample Number Concept in Continental Scientific Drilling During ICDP Expedition COSC-1

“…Different icons indicate the type of sample (Hole, Core, Core Section, Core Sample). The map below shows the geographical location, whereas the lower right part highlights publications that are related with the sample (here the initial scientific publication in Scientific Drilling Journal 6 and the Operational Report of COSC-1, Lorenz et al 2015a and2015b, and the master thesis and the data publication of Hierold 2016 andHierold et al 2016).…”

Report on the ICDP workshop DIVE (Drilling the Ivrea–Verbano zonE)

“…In addition, once the borehole is considered complete, vertical seismic profiling (i.e. a technique devoid of blasting, employed in the recent ICDP-COSC project in Sweden; Lorenz et al, 2015) will provide an outstanding controlled-source complement to the dataset.…”