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

摘要

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientificdrilling project focuses on mountain building processes in a majormid-Palaeozoic orogen in western Scandinavia and its comparison with modernanalogues. The project investigates the subduction-generated Seve NapeComplex. These in part under ultra-high-pressure conditions metamorphosedouter continental margin and continent–ocean transition zone assemblageswere emplaced onto the Baltoscandian platform and there influenced theunderlying allochthons and the basement. COSC-1 is the firstof two ca. 2.5 km deep, fully cored drill holes located in the vicinity of theabandoned Fr?? mine, close to the town of ?re in J?mtland,central Sweden. It sampled a thick section of the lower part of the SeveComplex and was planned to penetrate its basal thrust zone into theunderlying lower-grade metamorphosed allochthon. The drill hole reached adepth of 2495.8 m and nearly 100 % core recovery was achieved. Althoughplanning was based on existing geological mapping and new high-resolutionseismic surveys, the drilling resulted in some surprises: the Lower SeveNappe proved to be composed of rather homogenous gneisses, with onlysubordinate mafic bodies, and its basal thrust zone was unexpectedly thick(> 800 m). The drill hole did not penetrate the bottom of thethrust zone. However, lower-grade metasedimentary rocks were encountered inthe lowermost part of the drill hole together with garnetiferous mylonitestens of metres thick. The tectonostratigraphic position is still unclear, andgeological and geophysical interpretations are under revision. The compactgneisses host only eight fluid conducting zones of limited transmissivitybetween 300 m and total depth. Downhole measurements suggest an uncorrectedaverage geothermal gradient of ~ 20 °C km?1. Thispaper summarizes the operations and preliminary results from COSC-1 (ICDP5054-1-A), drilled from early May to late August 2014, and is complemented bya detailed operational report and the data repository.