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Electrical Conductivity Structure of the Lithosphere in Western Fennoscandia from Three-Dimensional Magnetotelluric Data

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Electrical Conductivity Structure of the Lithosphere in Western Fennoscandia from Three-Dimensional Magnetotelluric Data A 647 OULU 2014 A 647 UNIVERSITY OF OULU P.O.BR[ 00 FI-90014 UNIVERSITY OF OULU FINLAND ACTA UNIVERSITATISUNIVERSITATIS OULUENSISOULUENSIS ACTA UNIVERSITATIS OULUENSIS ACTAACTA SERIES EDITORS SCIENTIAESCIENTIAEA A RERUMRERUM Maria Cherevatova NATURALIUMNATURALIUM ASCIENTIAE RERUM NATURALIUM Maria Cherevatova Professor Esa Hohtola ELECTRICAL CONDUCTIVITY BHUMANIORA STRUCTURE OF University Lecturer Santeri Palviainen CTECHNICA THE LITHOSPHERE IN Postdoctoral research fellow Sanna Taskila WESTERN FENNOSCANDIA DMEDICA Professor Olli Vuolteenaho FROM THREE-DIMENSIONAL ESCIENTIAE RERUM SOCIALIUM MAGNETOTELLURIC DATA University Lecturer Veli-Matti Ulvinen FSCRIPTA ACADEMICA Director Sinikka Eskelinen GOECONOMICA Professor Jari Juga EDITOR IN CHIEF Professor Olli Vuolteenaho PUBLICATIONS EDITOR Publications Editor Kirsti Nurkkala UNIVERSITY OF OULU GRADUATE SCHOOL; UNIVERSITY OF OULU, OULU MINING SCHOOL ISBN 978-952-62-0709-4 (Paperback) ISBN 978-952-62-0710-0 (PDF) ISSN 0355-3191 (Print) ISSN 1796-220X (Online) ACTA UNIVERSITATIS OULUENSIS A Scientiae Rerum Naturalium 647 MARIA CHEREVATOVA ELECTRICAL CONDUCTIVITY STRUCTURE OF THE LITHOSPHERE IN WESTERN FENNOSCANDIA FROM THREE-DIMENSIONAL MAGNETOTELLURIC DATA Academic dissertation to be presented with the assent of the Doctoral Training Committee of Technology and Natural Sciences of the University of Oulu for public defence in Kuusamonsali (YB210), Linnanmaa, on 12 December 2014, at 12 noon UNIVERSITY OF OULU, OULU 2014 Copyright © 2014 Acta Univ. Oul. A 647, 2014 Supervised by Doctor Maxim Smirnov Doctor Toivo Korja Professor Pertti Kaikkonen Reviewed by Doctor Heinrich Brasse Doctor Josef Pek Opponent Professor Thorkild Rasmussen ISBN 978-952-62-0709-4 (Paperback) ISBN 978-952-62-0710-0 (PDF) ISSN 0355-3191 (Printed) ISSN 1796-220X (Online) Cover Design Raimo Ahonen JUVENES PRINT TAMPERE 2014 Cherevatova, Maria, Electrical conductivity structure of the lithosphere in western Fennoscandia from three-dimensional magnetotelluric data. University of Oulu Graduate School; University of Oulu, Oulu Mining School Acta Univ. Oul. A 647, 2014 University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland Abstract The lithospheric conductivity in the westernmost Fennoscandia has been studied using magnetotelluric (MT) data. The western margin of Fennoscandia was significantly affected in Paleozoic by the Caledonian orogeny and later by the rifting of Laurentia and the opening of the Atlantic Ocean c. 80 Ma ago. Magnetotelluric studies have been carried out in two target areas in southern Norway and in western Fennoscandia. The first study resulted in 2-D geoelectric models of two profiles stretching from Oslo to the Norwegian coast. The interpretation suggests that the basement is in general very resistive with a few conductive upper crustal layers, representing the alum shales, and middle crustal conductors possibly imaging the remnants of the closed ocean basins. A more extensive MT study was performed within the project "Magnetotellurics in the Scandes". Measurements were carried in summers of 2011 to 2013, resulting in an array of 279 MT sites. The data allowed us to derive 2-D geoelectric models for the crust and upper mantle as well as 3-D models for the crust. The inversions revealed a resistive upper crust and a conductive lower crust, two upper crustal conductors in the Skellefteå and Kittilä districts, highly conducting alum shales in the Caledonides and a conductive upper crust beneath the Lofoten peninsula. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west. The Palaeoproterozoic lithosphere is the thickest, not the Archaean, on contrary to a generally accepted hypothesis. A better image of the lithosphere will help to evaluate the proposed mechanisms of the exhumation of the Scandinavian Mountains. The theoretical part of this study is the development of a new multi-resolution approach to 3- D electromagnetic (EM) modelling. Three-dimensional modelling of MT data requires enormous computational resources because of the huge number of data and model parameters. The development of the multi-resolution forward solver is based on the fact that a finer grid resolution is often required near the surface. On the other hand, the EM fields propagate in a diffusive manner and can be sufficiently well described on a grid that becomes coarser with depth. Tests showed that the total run time can be reduced by five times and the memory requirements by three times compared with the standard staggered grid forward solver. Keywords: Caledonides, electomagnetic forward modelling and inversion, electrical conductivity, Fennoscandia, lithosphere-asthenosphere boundary, magnetotellurics Cherevatova, Maria, Maankamaran sähkönjohtavuus Fennoskandian länsiosassa magnetotelluuristen tutkimusten valossa. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Kaivannaisalan tiedekunta Acta Univ. Oul. A 647, 2014 Oulun yliopisto, PL 8000, 90014 Oulun yliopisto Tiivistelmä Olemme tutkineet litosfäärin sähkönjohtavuutta Fennoskandian länsiosassa magnetotelluurisen (MT) menetelmän avulla. Fennoskandian länsireuna muokkautui merkittävästi paleotsooisena aikana Kaledonidien vuorijonopoimutuksessa sekä myöhemmin mesotsooisena aikana Lauren- tia-mantereen repeytyessä ja Atlantin valtameren syntyessä noin 80 miljoonaa vuotta sitten. MT- tutkimukset tehtiin Etelä-Norjassa ja Fennoskandian luoteisosassa. Ensimmäisessä tutkimukses- sa kallioperän sähkönjohtavuutta kuvattiin kaksiulotteisilla (2-D) johtavuusmalleilla, jotka ulot- tuvat Oslosta Norjan rannikolle. Mallien tulkinta viittaa siihen, että maan kuori on pääosin hyvin eristävä lukuun ottamatta muutamaa kuoren ylä- ja keskiosassa olevaa johdekerrosta. Yläkuoren johteet edustavat alunaliuskeita ja keskikuoren johteet todennäköisesti suljetuissa merialtaissa syntyneitä hiilipitoisia sedimenttikerrostumia. Laajempi MT-tutkimus tehtiin ”Magnetotellurics in the Scandes” -hankkeessa. Mittauksia tehtiin 279 mittauspisteessä kesinä 2011–2013. Saadun aineiston avulla voitiin laatia 2-D inversiomallit kuoresta ja ylävaipasta sekä 3-D inversiomalli kuoresta. Tulosten mukaan täällä kuoren yläosa on eristävä kun taas kuoren alaosa on sähköä hyvin johtava. Edellisen lisäksi malleissa näkyy yläkuoren johtavat muodostumat Skellefteån ja Kittilän alueilla, korkean johtavuuden alunaliuskeet Kaledonidien alueella sekä johde Lofoot- tien alla. Litosfäärin paksuus on noin 200 km mittausverkon pohjoisosassa ja noin 300 km lou- naassa. Tämän mukaan litosfääri on paksuin varhaisproterotsooisen litosfäärin alueella, ei arkeeisen litosfäärin alueella vastoin yleistä hypoteesia. Tutkimuksen teoreettisessa osassa kehitettiin sähkömagneettiseen mallinnukseen uusi moni- tasoiseen diskretisointiin perustuva menetelmä. MT-aineiston 3-D käänteisongelman ratkaisu ja siihen liittyvä suora mallintaminen vaativat suuren laskennallisen kapasiteetin, koska havainto- jen ja mallin kuvaamiseen tarvittavien parametrien määrä on erittäin suuri. Moniresoluutio-algo- ritmi perustuu siihen, että mallin hienojakoisempaa diskretisointia tarvitaan yleensä lähellä maan pintaa kun taas syvemmälle edettäessä, sähkömagneettisen aallon diffuusin etenemisen vuoksi, malli voi olla karkeampi. Tietokonesimulaatioiden mukaan suoritusaika on viidennes ja muisti- tarve kolmannes verrattuna tavanomaiseen suoran laskennan ”staggered grid” -diskretisointiin. Asiasanat: Fennoskandia, Kaledonidit, litosfääri-astenosfääri -raja, magnetotelluriikka, sähkömagneettinen mallinnus ja inversio, sähkönjohtavuus To my parents, Alvina and Igor 8 Acknowledgements I would like to express my greatest gratitude to the people who have helped and sup- ported me throughout my dissertation. I am grateful to my main supervisor Dr. Maxim Smirnov for his extraordinary support in this thesis process. He has always been patient in countless discussions, which helped me to better understand the magnetotelluric method and moved me up to a new level. He shared with me his huge experience in managing and carrying out magnetotelluric field campaigns providing me an opportunity to enjoy the field works in Norway and Sweden. He guided and supported me in my theoretical exercises and codes development, teaching me to be details oriented. I would like to express my deep regards to my supervisor Dr. Toivo Korja for guiding me in geological interpretation of my models. He has always been open minded providing many interesting discussions about various social aspects. This PhD research would have been impossible without my first supervisor from Saint-Petersburg, Dr. Stanislav Vagin. I would like to thank him for attracting my interest to geophysics. I would like to thank Prof. Pertti Kaikkonen for his support with various administrative issues. All other colleagues and PhD students in our department for thorough discussions during seminars and in person. I would like to thank the Academy of Finland and the University of Oulu Graduate School for opportunity to study at University of Oulu and participate in national and international conferences where I met many famous scientists. Owing to Dr. Smirnov and financial supports from the Academy of Finland I was able to visit Prof. Gary Egbert at Oregon State University (Corvallis, Oregon, USA) for several times. I greatly acknowledge his lessons on 3-D modelling and inversion, which sometimes were smoothly moved to “Block 15”. I would like to thank him for amazing excursion in San Francisco. I would also like to thank Dr. Anna Kelbert for many useful discussions during
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