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Seismic imaging in the eastern Scandinavian Caledonides: Siting the 2.5 km deep COSC-2 borehole, central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. (Applied Geophysics)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. (Applied Geophysics)ORCID iD: 0000-0003-1963-2588
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
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2016 (English)In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 7, no 3, 769-787 p.Article in journal (Refereed) Published
Abstract [en]

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project, a contribution to the International Continental Scientific Drilling Program (ICDP), aims to provide a deeper understanding of mountain belt dynamics. Scientific investigations include a range of topics, from subduction-related tectonics to the present-day hydrological cycle. COSC investigations and drilling activities are focused in central Scandinavia where rocks from the mid to lower crust of the orogen are exposed near the Swedish-Norwegian border. Here, rock units of particular interest occur in the Seve Nappe Complex (SNC) of the so-called Middle Allochthon and include granulite facies migmatites (locally with evidence of ultra-high pressures) and amphibolite facies gneisses and mafic rocks. This complex overlies greenschist facies metasedimentary rocks of the dolerite-intruded Särv Nappes and underlying, lower grade Jämtlandian Nappes (Lower Allochthon). Reflection seismic profiles have been an important component in the activities to image the sub-surface structure in the area. Sub-horizontal reflections in the upper 1-2 km are underlain and interlayered with strong west- to northwest-dipping reflections, suggesting significant east-vergent thrusting. Two 2.5 km deep fully cored boreholes are a major component of the project which will improve our understanding of the sub-surface structure and tectonic history of the area. Borehole COSC-1 (IGSN: http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in the summer of 2014, targeted the subduction-related Seve Nappe Complex and the contact with the underlying allochthon. The COSC-2 borehole will be located further east and investigate the lower grade, mainly Cambro-Silurian rocks of the Lower Allochthon, the Jämtlandian décollement and penetrate into the crystalline basement rocks to identify the source of some of the northwest-dipping reflections. A series of high resolution seismic profiles have been acquired along a composite c. 55 km long profile to help locate the COSC drill holes. We present here the results from this COSC-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014. These seismic data, along with shallow drill holes in the Caledonian thrust front and previously acquired seismic, magnetotelluric, and magnetic data, are used to identify two potential drill sites for the COSC-2 borehole.

Place, publisher, year, edition, pages
2016. Vol. 7, no 3, 769-787 p.
Keyword [en]
reflection seismic, collisional orogeny, Scandinavian Caledonides, COSC, scientific drilling
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-261094DOI: 10.5194/se-7-769-2016ISI: 000379431700004OAI: oai:DiVA.org:uu-261094DiVA: diva2:849647
Projects
COSC
Funder
Swedish Research Council, 2013-5780
Available from: 2015-08-30 Created: 2015-08-30 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Geophysical studies of the upper crust of the central Swedish Caledonides in relation to the COSC scientific drilling project
Open this publication in new window or tab >>Geophysical studies of the upper crust of the central Swedish Caledonides in relation to the COSC scientific drilling project
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project aims to provide a deeper understanding of mountain belt dynamics through scientific deep drilling in the central parts of the mountain belt of western Sweden. The main targets include a subduction related allochthon, the basal orogenic detachment and the underlying partially subducted Precambrian basement. Research covered by this thesis, focusing primarily on reflection seismic data, was done within the framework of the COSC project.

The 55 km long composite COSC Seismic Profile (CSP) images the upper crust in high resolution and established the basis for the selection of the optimum location for the two 2.5 km deep COSC boreholes. Together with potential field and magnetotelluric data, these profiles allowed the construction of a constrained regional interpretation of the major tectonic units. Non-conventional pseudo 3D processing techniques were applied to the 2D data prior to the drilling of the first borehole, COSC-1, to provide predictions about the 3D geometry of subsurface structures and potential zones of interest for the sampling programs.

COSC-1 was drilled in 2014 and reached the targeted depth with nearly complete core recovery. A continuous geological section and a wealth of information from on-site and off-site scientific investigations were obtained. A major post-drilling seismic survey was conducted in and around the borehole and included a 3D reflection seismic experiment. The structurally and lithologically complex Lower Seve Nappe proved difficult to image in detail using standard processing techniques, but its basal mylonite zone and underlying structures are well resolved. The 3D data, from the surface down to the total drilled depth, show good correlation with the initial mapping of the COSC-1 core as well as with preliminary results from on-core and downhole logging.  

Good correlation is also observed between the 2D and 3D reflection seismic datasets. These will provide a strong link between the two boreholes and a means to extrapolate the results from the cores and boreholes into the surrounding rock. Ultimately, they will contribute to the deeper understanding of the tectonic evolution of the region, the Scandinavian Caledonides and the formation of major orogens. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 87 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1281
Keyword
reflection seismic, collisional orogeny, Scandinavian Caledonides, COSC, scientific drilling, geophysical logging, gravity, magnetics
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-261112 (URN)978-91-554-9320-2 (ISBN)
Public defence
2015-10-16, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Projects
Collisional Orogeny in the Scandinavian Caledonides
Funder
Swedish Research Council, 2009-4413Swedish Research Council, 2013-5780
Available from: 2015-09-25 Created: 2015-08-30 Last updated: 2016-08-16Bibliographically approved

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Juhlin, ChristopherHedin, PeterGee, DavidLorenz, HenningKalscheuer, ThomasYan, Ping

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