TEMPERATURE AND RHEOLOGICAL PARAMETERS
OF THE BALTIC SHIELD LITHOSPHERE
FROM ELECTROMAGNETIC SOUNDING RESULTS

 2021 A.N. Shevtsov^{1, 2} *, A.A. Zhamaletdinov ^1–4

¹ Geological Institute, Kola Scientific Center of the Russian Academy of Sciences,
Apatity, Murmansk region, Russia

² Murmansk Arctic State University (Apatity Branch), Apatity, Murmansk region, Russia

³ St. Petersburg Branch of the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, St. Petersburg, Russia

⁴ Center of Physical-Technical Problems of Power Engineering of the North, Kola Scientific Center of the Russian Academy of Sciences, Apatity, Murmansk region, Russia

*e-mail: anshev2019-01@mail.ru

Abstract. We consider methods for calculating of the temperature and rheological parameters (differential stresses) distribution in the lithosphere with the use of the deep electromagnetic sounding results in the Russian Arctic zone, on the territory of the Kola-Karelian region. Temperature modeling was carried out on the assumption that within the upper part of section (at depths less than 20–30 km), changes in resistivity with depth are due to the influence of porosity, permeability, moisture and salinity of pore solutions. For depths of more than 20–30 km, it is assumed that the resistivity of the lower part of the Earth's crust and upper mantle is determined, first of all, by the mineral composition of «dry» rocks, temperature and pressure. Estimates of temperature at depth based on geoelectric sections were made by calculating the heat balance equation taking into account the assumptions about the distribution of energy sources in the Earth, about the thermophysical properties of its individual layers and about their petrographic composition. The change in the mineral composition of the lithosphere with depth was taken on the basis of a priori geological estimates and superdeep drilling data. The dependence of temperature on depth is calculated by minimizing the discrepancy between the conductivity profile obtained on the basis of theoretical calculations and the conductivity profile based on a priori data from petrophysical studies. Taking into account the developed foundations of geothermal interpretation, the modeling of geoelectric sections obtained in the FENICS experiment on deep sounding of the lithosphere using industrial power lines was carried out. Rheological studies were carried out with the aim of studying the possible existence in the Earth’s crust of the Baltic Shield of the boundary of the transition of matter from a brittle (brittle) to a plastic (ductile) state in accordance with the theoretical concepts of V.N. Nikolaevsky. This hypothetical boundary in the scientific literature is called the «Brittle-Ductile Transition zone» (BDT zone). Theoretical calculations of the rheological parameters of the lithosphere are performed using nonlinear equations connecting the components of the stress tensor and the strain rate tensor with the components of stress tensors and their time derivatives. The calculation results showed that the boundaries of uncertainty in the estimates of the position of the BDT boundary are in the range from 5–10 to 3–40 km. More reliable estimates make it possible to obtain soundings with controlled sources. For this purpose, the paper presents the results of the Murman-2018 experiment on deep remote sounding in the Murmansk block with a linear (uniform) step of changing the separation between the source and receiver up to 56 km. A three-layer gradient-stepped section of the A type was revealed. The upper layer with a resistivity of 210⁴ Ohmm and a thickness of 10 km is characterized by increased fragility due to the existence of sub-vertical, flattening with depth zones of fractures and faults. Deeper than 10 km, the border of a sharp increase in resistivity to 10⁶–10⁷ Ohmm is. It is conventionally defined as the «impenetrability» boundary. On the basis of this result and taking into account a large number of indirect geophysical factors, we concluded that the «impermeability» boundary corresponds to the BDT boundary. The results of this work show that the use of electrical sounding allows one to obtain quantitative estimates of the temperature and rheological parameters of the lithosphere.

Keywords: electrical conductivity, frequency sounding, magnetotelluric sounding, apparent resistivity, static shear, temperature, rheology, lithosphere, Fennoscandian shield, Arctic.

About the authors

SHEVTSOV Alexander Nikolaevich – Geological Institute, Kola Scientific Center of the Russian Academy of Sciences. 184209, Apatity, Murmansk region, Fersman st., 14; Murmansk Arctic State University (Apatity Branch). Russia, 184209, Murmansk region, Apatity, Lesnaya st., 29. E-mail: anshev2019-01@mail.ru

ZHAMALETDINOV Abdulkhay Azimovich – Geological Institute, Kola Scientific Center of the Russian Academy of Sciences. 184209, Apatity, Murmansk region, Fersman st., 14; Murmansk Arctic State University (Apatity Branch). Russia, 184209, Murmansk region, Apatity, Lesnaya st., 29; St. Petersburg Branch of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences. Russia, 199034, St. Petersburg, Universitetskaya emb., 5, lit. B; Center of Physical-Technical Problems of Power Engineering of the North, Kola Scientific Center of the Russian Academy of Sciences. Russia, 184209, Murmansk region, Apatity, Academgorodok, 21a. E-mail: abd.zham@mail.ru

Cite this article as: Shevtsov A.N., Zhamaletdinov A.A. Temperature and rheological parameters of the Baltic Shield lithosphere from electromagnetic sounding results, Geofizicheskie Protsessy i Biosfera (Geophysical Processes and Biosphere), 2021, vol. 20, no. 1, pp. 33–49 (in Russian). https://doi.org/10.21455/GPB2021.1-4

English version: Izvestiya, Atmospheric and Oceanic Physics, 2021, vol. 57, iss. 7. ISSN: 0001-4338 (Print), 1555-628X (Online). https://link.springer.com/journal/volumesAndIssues/11485