T. Korja

Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian Shield implications for Precambrian crustal evolution

Extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles, normal-incidence reflection profiles, several hundred magnetotelluric soundings, and several magnetovariational array observations. Results are compiled into four maps: (1) Moho depth; (2) thickness of the lower crustal high-velocity layer (Vp > 7 km/s); (3) depth to the upper interface of the high-velocity layer; and (4) distribution of upper and middle crustal electrical conductors. The Moho depth is approximately 40 km in the Archaean Domain and varies between 40 and 65 km in the Svecofennian Domain. Most of the thickness variation (0–24 km) is concentrated in the lower crustal high-velocity layer. Electrically, the crust is formed of several rather homogeneous, either resistive or conductive, blocks that are bounded by upper and middle crustal conductive zones. The cratonized Archaean crust experienced an extensional phase during which a thin, mafic, lower crustal high-velocity layer was produced in association with mafic dykes (2.2–2.1 Ga) and well-conducting volcanogenic and metasedimentary belts (2.5–2.0 Ga). The Moho of the Archaean Domain was, consequently, reformed in the Early Proterozoic. Major thickening of the crust (50–65 km) occurred when a Svecofennian island arc and the Archaean craton collided. The Svecofennian orogeny (1.76–1.9 Ga) resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc. The internal terrane boundaries are seen as good reflectors and/or inclined or vertical conductors that extend at least to the middle crust. The thick high-velocity (> 7 km/s) lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of Late Svecofennian subduction. The collisional boundary is also preserved as a discontinuous conductive zone between the Archaean and Svecofennian Domains. The thickened crust was subsequently thinned to 42 km along E—W-striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths, coeval mafic dykes, and gabbro-anorthosite bodies (1.6–1.5 Ga). The crust of the central part of the shield has since remained virtually intact.

Electrical conductivity distribution of the lithosphere in the central Fennoscandian Shield

Abstract Geoelectric information on the crustal structures of the central Fennoscandian Shield consists of magnetovariational (MV) and magnetotelluric (MT) data. MV-data are obtained from magnetometer arrays devised to map lateral variations of subsurface resistivity; MT-data are obtained from soundings yielding knowledge on vertical resistivity distribution in the form of cross-sections. The electromagnetic data base for Fennoscandia includes data from about 350 magnetometer sites and about 600 magnetotelluric soundings. The results show that the crustal resistivity of the Archaean and Palaeoproterozoic crust varies greatly both laterally and vertically. The model resistivities range from several tens of thousands of Ωm to below 1 Ωm, which are extreme values amongst those normally observed in crustal rocks. Several large elongated and inclined crustal conductors, which mark fossil tectonic boundaries, have been discovered in the Shield. These include: (1) the Lake Ladoga, Outokumpu, Kainuu Schist Belt, and Oulu conductors that form a discontinuous conductive boundary between the domains of Archaean and Palaeoproterozoic crust; (2) the Southern-Finland conductor caused partly by nearly vertical, several kilometres deep graphite-bearing metasedimentary layers that were tilted and deeply buried in a Svecofennian collision; (3) the Kokkola, Bothnian, Skelleftea, Storavan and Oulu conductors representing a collisional belt in a northwards accreted Palaeoproterozoic terrain; and (4) the Lapland Granulite Belt conductors imaging a sequence of sheared thrust sheets that developed as the granulites were upthrusted southwestwards onto the Archaean craton. Zones of enhanced electrical conductivity delimit more resistive and homogeneous crustal blocks including the Archaean Kuhmo block in eastern Finland and the Svecofennian Central Finland Granitoid Complex (CFGC). Highly resistive upper crust ( > 30,000 Ωm ) in the CFGC coincides with the granitoid intrusions. Groundwater, which fills the fractures and pores of the granitoids, is non-conductive (100 Ωm) and is thus not able to enhance the bulk conductivity. Therefore, the resistivity of the upper crust approaches that of dry rocks. At depths of 10 to 14 km, a decrease in resistivity by one order of magnitude coincides with a seismic low velocity (vP) zone that appears related to a porous fractured zone filled with saline water. A lower crustal conductor beneath the CFGC at depths of 30 to 40 km has a thickness of 10 to 15 km and a resistivity of 20 to 100 Ωm. It is most probably caused by graphite that can have been precipitated from CO2-rich fluids during the peak metamorphism of the Svecofennian orogeny and the subsequent cooling period. Free fluids trapped in the lower crust appear less plausible. Magnetotelluric soundings in the resistive CFGC in the central Fennoscandian Shield have not provided indications of a well-developed electrical asthenosphere within the uppermost 300 km.

The geoelectric model of the POLAR Profile, Northern Finland

Abstract Electromagnetic soundings have been made in order to construct a geoelectrical (conductivity) model of the crust along the European Geotraverse (EGT) POLAR Profile. Forty magnetotelluric (MT) soundings, eighteen audiomagnetotelluric (AMT) soundings and ten magnetohydrodynamic (MHD) soundings were made on the main POLAR Profile (POLAR I) and ten more MT soundings on a parallel profile (POLAR II), 40 km to the southeast of the main profile. Analysis of simultaneous recordings by the EISCAT magnetometer chain, and thin-sheet modelling of the effect of the Barents Sea, indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area. The magnetotelluric data have been modelled with two-dimensional models representing the regional structure along the profiles. In addition to the regional structure, a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions. Although details of the surface electrical structures are poorly resolved, the gross features of the geoelectrical cross section are considered to be reliable. The results divide the POLAR Profile into three different blocks. The better conducting Karasjok-Kittila Greenstone Belt in the south has an average resistivity of less than 10 Ωm. The more resistant Lapland Granulite Belt, with a resistivity between 100 and 200 Ωm, is underlain by conductive ( The geoelectric cross section agrees, in gross detail, with the corresponding gravity, refraction seismic and reflection seismic cross sections of the POLAR Profile. All methods indicated a similar shape for the southwestern part of the Lapland Granulite Belt i.e., granulites have a gently NE-dipping boundary against the underlying Karelian Province. In the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep S-dipping boundary against the Inari Terrain northeast of the granulite belt.

Probing electrical conductivity of the Trans-European Suture Zone

The Trans-European Suture Zone (TESZ) is the largest tectonic boundary in Europe, crossing northwest-southeast through central Europe from the North Sea to the Black Sea. More than 2000 kilometers long, it constitutes a complex transition between the thick and cold East European Craton (EEQ/Baltic Shield, created more than 650 million years ago (Ma) during the Precambrian, and the warmer, younger Paleozoic (650 to 250 Ma) central European mobile belts.

Deep electromagnetic sounding of the lithosphere in the eastern Baltic (fennoscandian) shield with high-power controlled sources and industrial power transmission lines (FENICS experiment)

The paper addresses the technique and the first results of a unique experiment on the deep tensor frequency electromagnetic sounding, the Fennoscandian Electrical conductivity from results of sounding with Natural and Controlled Sources (FENICS). In the experiment, Energy-1 and Energy-2 generators with power of up to 200 kW and two mutually orthogonal industrial 109- and 120-km-long power transmission lines were used. The sounding frequency range was 0.1–200 Hz. The signals were measured in the Kola-Karelian region, in Finland, on Svalbard, and in Ukraine at distances up to 2150 km from the source. The parameters of electric conductivity in the lithosphere are studied down to depths on the order of 50–70 km. A strong lateral homogeneity (the one-dimensionality) of a geoelectric section of the Earth’s crust is revealed below depths of 10–15 km. At the same time, a region with reduced transverse crustal resistivity spread over about 80 000 square kilometers is identified within the depth interval from 20 to 40 km. On the southeast the contour of the anomaly borders the zone of deepening of the Moho boundary down to 60 km in Central Finland. The results are compared with the AMT-MT sounding data and a geodynamic interpretation of the obtained information is carried out.

Preface to the Special Issue on “The 20th Electromagnetic Induction Workshop”

Electromagnetic (EM) induction methods are used and continue to be developed for a wide range of applications, ranging from exploration near the Earth’s surface to investigations of the deep mantle. In this research, important scientific and societal challenges, such as to search for hydrocarbons and other Earth resources, to probe the structure and dynamics of the lithosphere, to study environmental issues and to monitor and mitigate natural hazards, are addressed. The Working Group I-2 of the International Association of Geomagnetism and Aeronomy on ‘‘Electromagnetic Induction in the Earth’’ has held, since the Edinburgh, United Kingdom, Workshop of 1972, biennial workshops. Here, selected topics are extensively explored by the participants, in the form of oral and poster presentations and discussion sessions. An essential and important part of the EM Induction Workshops (EMIWs) has been the invited review presentations on themes selected by the program committee. These themes vary from workshop to workshop; usually, they highlight recent advances in the rapidly evolving fields of electromagnetic induction and introduce important new directions of research as well as highlight and review results focusing on certain geological targets. The review papers presented at the workshops have traditionally been published as Special Issues of Surveys in Geophysics/Geophysical Surveys since the 1978 workshop in Murnau, Germany. This Special Issue of Surveys in Geophysics contains expanded articles from six review papers presented at the 20th Workshop on Electromagnetic Induction in the Earth (http://mtnet.dias.ie/workshops/2010_Giza/website/index.html). The Workshop was held

Preface to the Special Issue on ''The 21st Electromagnetic Induction Workshop''

Electromagnetic (EM) induction methods are used and continue to be developed for a wide range of applications, ranging from exploration near the Earth’s surface to the investigations of the deep mantle. In this research, important scientific and societal challenges, such as to search for hydrocarbons and other Earth resources, to probe the structure and dynamics of the lithosphere, to study environmental issues and to monitor and mitigate natural hazards, are addressed. The Working Group I-2 of the International Association of Geomagnetism and Aeronomy on ‘‘Electromagnetic Induction in the Earth’’ has held biennial workshops since the Edinburgh, United Kingdom, Workshop of 1972. Here, selected topics are extensively explored by the participants, in the form of oral and poster presentations and discussion sessions. An essential and important part of the EM Induction Workshops (EMIWs) has been invited review presentations on themes selected by the program committee. These themes vary from workshop to workshop; usually, they highlight recent advances in the rapidly evolving fields of electromagnetic induction and introduce the important new directions of research as well as highlight and review results focusing on certain geological targets. The review papers presented at the workshops have traditionally been published as Special Issues of Surveys in Geophysics/Geophysical Surveys since the 1978 workshop in Murnau, Germany. This Special Issue of Surveys in Geophysics contains nine expanded articles from review papers presented at the 21st Workshop on Electromagnetic Induction in the Earth. The workshop was held between 25 and 31 July, 2012, in Darwin, Australia. It was organised by the Working Group I-2 of the International Association of Geomagnetism and

International FENICS Experiment on the Tensor Frequency Electromagnetic Sounding of the Lithosphere in the Eastern Baltic (Fennoscandian) Shield

ISSN 1028-334X, Doklady Earth Sciences, 2009, Vol. 427A, No. 6, pp. 979–984.

Study of interaction of ELF–ULF range (0.1–200 Hz) electromagnetic waves with the earth’s crust and the ionosphere in the field of industrial power transmission lines (FENICS experiment)

This article is devoted to describing the theory, technique, and first experimental results of a control source electromagnetic (CSEM) study of the Earth’s crust and ionosphere with the use of two mutually orthogonal industrial transmission lines 109 and 120 km in length in the frame of FENICS (Fennoscandian Electrical Conductivity from Natural and Induction Control Source Soundings) experiment. The main part of the measurements is executed on the territory of the Fennoscandian shield at distances from the first hundreds kilometers up to 856 km from the source with the purpose of the deep electromagnetic sounding of the Earth’s crust and upper mantle. According to the results of these studies clarifying the parameters of “normal” (standard) geoelectric section of the lithosphere to a depth of 60–70 km, the anisotropy parameters are evaluated and a geothermal and rheological interpretation in conjunction with the analysis of the seismic data is executed. Furthermore, to study the propagation of ELF–LLF waves (0.1–200 Hz) in an “Earth–Ionosphere” waveguide, the measurements are carried out apart from Fennoscandian shield at distances up to 5600 km from the source (in Ukraine, Spitsbergen, Poland, Kamchatka, and other areas). According to the results of these studies, the experimental estimates of the influence of the ionosphere and of the displacement currents on the propagation of ELF–ULF waves in the upper half-space at the different azimuths generation of the primary field are obtained.