Investigating the origin and nature of the Precambrian seismic reflectivity at the deep COSC-2 borehole

Description

Reflection seismic data were acquired at the International Scientific Continental Drilling Program (ICDP) COSC-2 site in Jämtland in 2021. COSC-2 was drilled to 2275 m in 2021 to study the structure of the Scandinavian Caledonides in central Sweden as a follow-up to the COSC-1 borehole that was drilled in 2014 to 2500 m in the Åre area in Jämtland. The seismic data are currently being looked into in more detail and a number of papers are being prepared. The seismic data set is extensive with several components, including sparse 3D, long offset 2D and borehole data being acquired within the project framework. The bedrock in the area is highly reflective and the low noise conditions during acquisition allowed high quality data to be acquired. The COSC boreholes were drilled with the Swedish national research infrastructure for scientific drilling, Riksriggen (www.riksriggen.se).

Extensive mountain ranges, like the Himalayas, have a significant impact on the Earth's environment. The development of mountain ranges is closely linked to important issues such as natural disasters (earthquakes, floods), weather (monsoon), climate (Tibet versus India, Pakistan, etc.), and natural resources (e.g. ore formation). The Caledonian mountain range was a 'Paleozoic Himalaya,' comparable in size and developed in a similar tectonic environment. 400 million years ago, it stretched in a belt from what are now the Appalachians in North America to Svalbard in the Arctic Ocean. The Scandinavian mountain range is part of the Caledonides. Up to 20 km of rock has eroded since the mountain range was formed, which provides excellent conditions for geoscientific basic research into the geological processes that occur at depth when the mountain range was formed and how these can be compared with the development on the surface in a modern mountain range. In addition, it is possible to analyze past processes in a deeply eroded, ancient mountain range and use the newfound knowledge in applied research tasks such as natural disasters, mineral exploration, underground construction, and geothermal energy.

The COSC project (Collisional Orogeny in the Scandinavian Caledonides) provides researchers with the opportunity to study and observe an area of the Caledonides where knowledge has been gathered for more than 100 years. The focus is on a profile from Trondheim in the west to Östersund in the east. The project includes two 2–2.5 km deep boreholes with core samples, as well as associated data and metadata from all surveys in the boreholes and on the core samples. Additionally, there are extensive geophysical measurements that have been collected since the 1970s and geological observations since the 1880s. Research within the project is multidisciplinary and includes geophysics, geology, hydrogeology, geothermics, rock physics, geochemistry, microbiology, and engineering.

The first borehole (COSC-1) was successfully drilled in 2014 at Fröå Gruva near Åre. The goal of COSC-1 was to investigate highly metamorphosed rocks (nappes) that had been transported several hundred kilometers due to tectonic forces during mountain building. The next step (the COSC-2 borehole) was to characterize and determine the age of the deformation in the underlying Fennoscandian shield and how it differs from the deformation in the overlying nappes, which were transported over the shield during ongoing mountain building. The thrust zone itself is studied in detail to better understand how this layer acted to separate the nappes from the underlying shield. One hypothesis that is tested is whether large earthquakes occurred in association with the thrust zone, which can be compared to the destructive earthquakes indirectly observed with geophysical methods at depth in the Himalayas. The borehole also provides, for the first time, information about which geological structures cause the strong, clear reflections of seismic waves in the local (tens of kilometers) and regional (hundreds of kilometers) long seismic profiles. The COSC project aims to, through a combination of basic and applied research, increase the understanding of mountain building, which forms a cornerstone of the plate tectonic paradigm. In addition, the project will result in new strategies for ore exploration, critical data for reconstructing the climate in the Paleozoic, and new information on how the bedrock can be used in energy systems.