Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1281 Geophysical studies of the upper crust of the central Swedish Caledonides in relation to the COSC scientific drilling project PETER HEDIN ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-9320-2 UPPSALA urn:nbn:se:uu:diva-261112 2015 Dissertation presented at Uppsala University to be publicly examined in Hambergsalen, Geocentrum, Villavägen 16, Uppsala, Friday, 16 October 2015 at 10:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Associate Professor Charles Hurich (Department of Earth Sciences, Memorial University of Newfoundland, St. Johns, Canada). Abstract Hedin, P. 2015. Geophysical studies of the upper crust of the central Swedish Caledonides in relation to the COSC scientific drilling project. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1281. 87 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9320-2. 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. Keywords: reflection seismic, collisional orogeny, Scandinavian Caledonides, COSC, scientific drilling, geophysical logging, gravity, magnetics Peter Hedin, Department of Earth Sciences, Geophysics, Villav. 16, Uppsala University, SE-75236 Uppsala, Sweden. © Peter Hedin 2015 ISSN 1651-6214 ISBN 978-91-554-9320-2 urn:nbn:se:uu:diva-261112 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-261112) Dedicated to my beloved Marilyn and wonderful son Samuel Supervisors & Committee Supervisor Professor Christopher Juhlin Department of Earth Sciences – Geophysics, Uppsala University, Uppsala, Sweden Assistant Supervisor Associate Professor Alireza Malehmir Department of Earth Sciences – Geophysics, Uppsala University, Uppsala, Sweden Dr. Bjarne Almqvist Department of Earth Sciences – Geophysics, Uppsala University, Uppsala, Sweden Faculty Opponent Associate Professor Charles Hurich Department of Earth Sciences, Memorial University of Newfoundland, St. Johns, Canada Examination Committee Dr. Joaquina Álvarez Marrón Department of Earth’s Structure and Dynamics and Crystallography – Earth’s structure and Dynamics, Institute of Earth Sciences Jaume Almera, Barcelona, Spain Dr. Cédric Schmelzbach Department of Earth Sciences – Geophysics, ETH Zürich, Zürich, Switzer- land Dr. Ari Tryggvason Department of Earth Sciences – Geophysics, Uppsala University, Uppsala, Sweden List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Hedin, P., Juhlin, C., Gee, D. G. (2012) Seismic imaging of the Scandinavian Caledonides to define ICDP drilling sites. Tectonophysics, 554-557:30–41 II Hedin, P., Malehmir, A., Gee, D. G., Juhlin, C., Dyrelius, D. (2013) 3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, cen- tral Swedish Caledonides. Geological Society, London, Special Publications, 390:301–319 III Hedin, P., Almqvist, B., Berthet, T., Juhlin, C., Buske, S., Si- mon, H., Giese, R., Krauß, F., Rosberg, J-E., Alm, P-G. (2015) 3D reflection seismic imaging at the 2.5 km deep COSC-1 sci- entific borehole, central Scandinavian Caledonides. Tectonophysics, manuscript under review IV Juhlin, C., Hedin, P., Gee, D. G. (2015) Seismic imaging of the eastern Scandinavian Caledonides: Siting the 2.5 km deep COSC-2 borehole. Solid Earth, manuscript to be submitted Reprints were made with permission from the respective publishers. Additional contributions to conference proceeding written during my Ph. D. which are not included in this thesis: Hedin, P., Malehmir, A., Gee, D. G., Juhlin, C., Dyrelius, D. (2013) COSC geophysical and geological site investigations. 75th EAGE Con- ference & Exhibition, London, UK, Extended Abstract 16529, TuSP1- 07, doi: 10.3997/2214-4609.20131097 Ahmadi, O., Hedin, P., Malehmir, A., Juhlin. C. (2013) 3D Seismic In- terpretation and Forward Modeling – an approach to providing reliable results from 2D seismic data. In: Johnson, E. (Ed.), Mineral Deposit Re- search for a High-Tech World, vols 1-4, p 50-53, 12th SGA Biennial Meeting, Uppsala, Sweden. Juhlin, C., Hedin, P. (2014) 3D Seismic Processing of Crooked Line 2D Data in the Vicinity of the COSC 2.5 Km Deep Scientific borehole. 76th EAGE Conference & Exhibition, Amsterdam, The Netherlands, Extend- ed Abstract 21889, WS5-P11, doi: 10.3997/2214-4609.20140522 Contributions The papers included in this thesis are the result of collaboration with several authors. The individual contributions of the author of this thesis are summa- rized below. I I participated in the seismic acquisition and performed the data decoding, processing and analysis. I participated in the discus- sion and interpretation and then wrote the first draft, with input from co-authors on geology. Worked with the co-authors to re- fine the manuscript. II I constructed the 3D geological model. Participated in the dis- cussion and interpretation and wrote a draft of the manuscript, with input from co-authors on the inverse modeling. I then made improvements to the manuscript based on feedback and guidance from the co-authors. III I participated in the acquisition, processing and analysis of core logging data as well as the processing and analysis of downhole logging data. I participated in the seismic acquisi- tion and performed the decoding, processing and analysis. I wrote the first draft, with input from co-authors on geology, and then improved the manuscript after helpful feedback. IV I participated in the seismic acquisition in 2010 and 2011 and processed the data from these two surveys. Participated in the discussion and interpretation of the full seismic profile and contributed with some parts of the draft as well as refinement of the final manuscript. Contents 1 Introduction ......................................................................................... 13 2 Collisional Orogeny in the Scandinavian Caledonides ........................ 16 2.1 The Caledonian geology of central Sweden .................................... 17 2.2 The Seve Nappe Complex ............................................................... 21 2.3 Geophysical Background ................................................................ 23 2.4 Scientific Drilling in the Scandinavian Caledonides ....................... 26 3 Methods ............................................................................................... 29 3.1 The reflection seismic method and data acquisition ....................... 29 3.1.1 2D crooked line acquisition ................................................... 30 3.1.2 Limited 3D acquisition .......................................................... 31 3.2 Reflection seismic processing ......................................................... 32 3.2.1 Crossdip analysis ................................................................... 33 3.2.2 Swath 3D imaging ................................................................. 37 3.3 Drilling of COSC-1 scientific borehole and logging of geophysical rock parameters .................................................................... 42 4 Summary of Papers .............................................................................