Development and application of the magnetotelluric method to study the crustal structure of central Finnish Lapland

• Main Author: Autio, U. (Uula)
• Other Authors: Smirnov, M. (Maxim), Kozlovskaya, E. (Elena)
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Oulun yliopisto 2021
• Subjects: Fennoscandian shield; Finnish Lapland; crust; electrical conductivity; extreme data; magnetotelluric; Fennoskandia; Lappi; maankuori; magnetotelluurinen; sähkönjohtavuus; äärivaste; Kuusamo; Fennoscandian; Lapland
• Online Access: http://urn.fi/urn:isbn:9789526231709

Abstract In this thesis, the magnetotelluric (MT) method has been applied to study the crustal geoelectric structure of central Finnish Lapland and new advances in MT data analysis have been realized. MT data acquired in 2014 display pronounced 3D effects, reflecting the complex tectonic history of the bedrock in the area. Particularly anomalous, so-called extreme data are observed in the northern part of the study area. A definition for extreme MT data is given by the condition where the determinant of the real and/or imaginary part of the impedance tensor becomes negative. Physically, such data are associated with reversal of, for example, the electric field as compared to its direction for a one-dimensional resistivity structure. The behaviour of the phase tensor and the determinant average of the impedance tensor, for instance, have been analysed in the case of extreme data. 3D conductivity models derived using the ModEM code display high crustal conductance (thousands of siemens) in the vicinity of the Central Lapland Greenstone Belt, the Peräpohja Belt and the Kuusamo Belt. A remarkable feature is an arc-shaped conductor inside the northern part of the Central Lapland Granitoid Complex, which continues into the Central Lapland Greenstone Belt in the north. The conductive structures in the models could represent deeply buried graphite and sulphide bearing metasedimentary rocks or reactivated Archaean shear zones. The conductors in the northern and southern parts of the study area are separated by a resistor coinciding with the Central Lapland Granitoid Complex. A possible explanation for the observed pervasive E–W principal direction of the phase tensor data in the study area could be the failed rift suggested in other studies. In addition to the magnetotelluric studies in central Lapland, as a methodological development, an express approach where time-domain electromagnetic (TEM) data are transformed and subsequently used in 2D MT determinant inversion is presented. The methodology is applied in a simple ...