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Abstract:
The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid‐Paleozoic orogen in western Scandinavia and its comparison with modern analogues. The project investigates a subduction‐generated complex (Seve Nappes) and how these in part under ultra‐high pressure conditions metamorphosed outer continental margin and continent‐ocean transition zones (COT) assemblages were emplaced onto the Baltoscandian platform and there influenced the underlying allochthons and the basement in a section provided by two fully cored 2.5 km deep drill holes. This operational report concerns the first drill hole, COSC‐1 (ICDP 5054‐1‐A), drilled from early May to late August 2014.
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 8 fluid conducting zones of limited transmissivity between 300 m and total depth. Downhole measurements suggest an uncorrected average geothermal gradient of ~20°C/km.
The drill core was documented on‐site and XRF scanned off site. During various stages of the drilling, the borehole was documented by comprehensive downhole logging. This operational report provides an overview over the COSC‐1 operations from drilling preparations to the sampling party and describes the available datasets and sample material.
