Abstract
Abstract. The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to characterise the structure and orogenic processes involved in a major collisional mountain belt by multidisciplinary geoscientific research. Located in western central Sweden, the project has drilled two fully cored deep boreholes into the bedrock of the deeply eroded Early Paleozoic Caledonide Orogen. COSC-1 (2014) drilled a subduction-related allochthon and the associated thrust zone. COSC-2 (2020, this paper) extends this section deeper through the underlying nappes (Lower Allochthon), the main Caledonian décollement, and the upper kilometre of basement rocks. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early Paleozoic palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, and the characterisation of the microbial community in the present hard rock environment of the relict mountain belt. COSC-2 successfully, and within budget, recovered a continuous drill core to 2276 m depth. The retrieved geological section is partially different from the expected geological section with respect to the depth to the main décollement and the expected rock types. Although the intensity of synsedimentary deformation in the rocks in the upper part of the drill core might impede the analysis of the Early Paleozoic palaeoenvironment, the superb quality of the drill core and the borehole will facilitate research on the remaining targets and beyond. Protocols for sampling in the hard rock environment and subsequent sample preservation were established for geomicrobiological research and rock mechanical testing. For the former, a sparse sample series along the entire drill core was taken, while the target of the latter was the décollement. COSC-2 was surveyed by a comprehensive post-drilling downhole logging campaign and a combined borehole/land seismic survey in autumn 2021. This paper provides an overview of the COSC-2 (International Continental Scientific Drilling Project – ICDP 5054_2_A and 5054_2_B boreholes) operations and preliminary results. It will be complemented by a detailed operational report and data publication.
Highlights
The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to characterise the structure and orogenic processes involved in a major collisional mountain belt by multidisciplinary geoscientific research
This paper provides an overview of the COSC-2 (International Continental Scientific Drilling Project – ICDP 5054_2_A and 5054_2_B boreholes) operations and preliminary results
The primary target of the Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project (Fig. 1) is to study orogenic processes at various scales along a continuous section through the interior of this deeply eroded mountain belt, from the high-grade metamorphic, subduction-related allochthons derived from the distal margin of Baltica, known as the Middle Allochthon, through the low-grade metamorphic continental margin sediments of the Lower Allochthon, the main Caledonian décollement and into the underlying sedimentary basement cover, and, at the deepest level, the crystalline basement of palaeocontinent Baltica
Summary
The Caledonides are a mid-Paleozoic collisional orogen of Alpine–Himalayan dimension. The primary target of the Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project (Fig. 1) is to study orogenic processes at various scales along a continuous section through the interior of this deeply eroded mountain belt, from the high-grade metamorphic, subduction-related allochthons derived from the distal margin of Baltica, known as the Middle Allochthon, through the low-grade metamorphic continental margin sediments of the Lower Allochthon, the main Caledonian décollement and into the underlying sedimentary basement cover, and, at the deepest level, the crystalline basement of palaeocontinent Baltica (i.e. the Baltoscandian basement of the Fennoscandian Shield). COSC-1 is located close to the timberline on the eastern slopes of the Åreskutan mountain, with subductionrelated ultra high pressure metamorphic rocks of the Middle Seve Nappe exposed on its summit and upper slopes (Klonowska et al, 2017) This drill core samples the underlying gneisses of the Lower Seve Nappe and thrust zone (see Lorenz et al, 2015a for science report and more extensive project background). Ongoing research is testing different hypotheses of nappe emplacement through an investigation of the kinematics and thermochronological development of the different allochthons and their interaction with the basement of the underthrusting plate in this major collisional orogen
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