Three-component borehole magnetics in the COSC-2 drill hole

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About 400 million years ago, in the Silurian and lower Devonian geological time periods, a collision of two continents took place in the region of what is now Scandinavia. The paleo-continent Laurentia was underthrusted by Baltica, and the Caldonian montains were formed. The mountain-building processes are of great interest for geosciences due to several similarities to the processes currently taking place in the Himalayan region. In order to study the mountain building processes, two boreholes, each approx. 2,5 km deep, were planned to be drilled within the COSC (Collisional Orogeny in the Scandinavian Caledonides) project into the suture zone between the former continents. COSC-1 was drilled in 2014, COSC-2 is scheduled for summer 2020. COSC-2 will be drilled through several units of the lower allochthon down to the autochthonous Fennoscandian basement. One of the main objectives of the COSC-2 project is to define the character and age of deformation of the greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement. In the proposed study, we will carry out three-component magnetometer measurements in the COSC-2 borehole. The data will be processed and reoriented to a geographical reference frame. The resulting magnetic field anomalies will be interpreted in combination with aeromagnetic and seismic data (both surface and wire-line data) in terms of a large-scale 3-D model. Through three-dimensional modelling, we will significantly reduce the non-uniqueness in present models. The results are incorporated into the existing models and thus increase accuracy of the regional structural model.The main focus of interpretation will be on the contact zone between the Caledonide décollement, hosted by Alum shale, and the Fennoscandian basement. Using the vector information of the natural remanent magnetization derived from the three-component borehole magnetic data we will be able the test several hypotheses about the geological setting in the vicinity (~ 200 m) of the drill hole. In regions of significant remanent magnetization, for example within the Rätan granite forming the Fennoscandian basement, the natural remanent magnetization will be used to derive important constraints for the reconstruction of deformation processes and the tectonic history of the Scandinavian Caledonides. Therefore, we will contribute to an improved understanding of mountain building processes, one of the major goals of the COSC project.