Geoelectric model of the Earth’s crust and upper mantle of the Dniester-Bug megablock of the Ukrainian Shield

Abstract

A network of long-period magnetotelluric and magnetovariational data (124 sites) in the period range of 9—16 to 2500—6400 s made it possible to explore the geoelectric structure of the Erth’s crust and upper mantle of most of the Dniester-Bug and adjacent megablocks of the Ukrainian shield. Based on the resistivity cross-sections along the profiles (with 2D inversion), a three-dimensional matrix was created for the territory limited by coordinates 27.7—30.4° E and 47.7—49.4° N, which included the spatial coordinates of each grid node on each profile, the power of the model cells, and the resistance value in the cell. As a result, geoelectrical anomalous structures were identified at different depths from 3 to 100 km. The entire block of rocks 200×200 km down to a depth of 100 km is characterized by high resistivity, against which objects of reduced resistivity (LRO) are identified. The resulting distribution of high-resistivity rocks over the entire depth of the model is in good agreement with the laboratory dependencies obtained both for the rocks of the Ukrainian Shield and other data. Model data show a significant difference in resistivities in the upper 14—16 km (above 105 ohm), lower crust (about 104 ohm), and upper mantle (103 ohm). Against a general decrease in resistance with depth in the Earth’s crust, three regions were identified in which anomalously high (for a given depth) resistances extend to the entire thickness of the crust. These high-resistivity objects are consistent with positive Bouguer anomalies. Against the background of high-resistivity rocks, LROs stand out, the resistance of which does not exceed 120 ohm·m. The spatial dimensions of the LRO zones indicate their locality and do not form a continuous layer. An analysis of the distribution of LROs in space and depth suggests a genetic relationship between mantle LROs and crustal LROs. Comparison of mantle LROs with the Beltska zone of modern activation on the territory of Ukraine shows their good agreement both vertically and horizontally. To explain the lower LRO resistivity in the upper mantle, overheating of the rocks to solidus and 2—3 % melting and/or the presence of fluids is necessary [Gordienko, 2017]. In recent studies discussing the influence of thermobaric conditions and the fluid content necessary to explain the presence of increased conductivity in the upper mantle, the authors of [Blatter et al., 2022] concluded that an anomalously large amount of volatiles is needed with small amounts of melt. The assumption that mantle LROs are related to crustal LROs has been tested by comparing LROs with fault zones. The presence of LROs in the mantle, their vertical extent, and their connection with rejuvenated fault systems can serve as a basis for the deep migration of fluids enriched in volatiles.