Depth structure of the Transcarpathian Depression (Ukrainian part) according to density modeling data

The presented research is devoted to the construction and calculations of the density model along the regional CMRV-DSS profile RP-17 (Chop–Velykyy Bychkiv), running along the Transcarpathian depression. Based on the results of density modeling, the distribution of density in the Earth’s crust was obtained in accordance with its seismic structure and gravity field as well as the density structure of individual layers. A tectonic interpretation of the obtained results was provided. The Mukachevo and Solotvyno parts of Transcarpathian depression have their own structural features, autonomous geological development and are distinguished by Neogene geodynamics. The analysis of density properties showed that in the Neogene sedimentary layer of both depressions there is a change in the density of various rocks in the depth intervals of 200—950; 950—1450; 1450—2050 m. In the Solotvyno depression in the depth intervals of 200—950 and 1450—2050 m, the density is greater than in Mukachevo, due to the presence of sandstones, tuffs, mudstones, and siltstones. In the interval 950—1450 m of Solotvyno depresson, on the contrary, the density is lower than in Mukachevo one, due to the presence of salt and clay. The Earth’s crust of Mukachevo depression is more compacted, as it contains a «basalt» layer. The Solotvyno depression consists of two parts and its average density corresponds to a diorite composition. The north-western block is of higher density and more homogeneous. The south-eastern one is of lower density and composed of a large number blocks of different densities separated by faults. The boundary of the lower density zone (PK 105—110) runs along the south-eastern branch of the Stryi-Latorytsia shear zone. This zone appears fragmentary on the density section, being similar to a mantle fault, with a lateral differentiation of density values, as well as the largest concentration of earthquakes, especially in the upper part of the Earth’s crust. The low-density area is probably associated with the transition from the Solotvyno depression to the structures located to the south-east of it. Thus, the block with the lowest density (2,38 g/cm3) of the Mesozoic-Paleozoic folded basement can be attributed to the Fore-Alkapa suture zone, represented by the Pieniny Klippen Belt and the Monastyrets nappe, which turns in the meridional direction in the zone of junction with the Tisza-Dacia terrane. The block located below with a density of 2,64 g/cm3 can be connected with the Marmarosh massif, or with the Rakhiv nappe. It was established that the crust of the Alkapa terrain along the profile is represented by three large blocks with a smaller block structure inside each one. The two more density blocks with different crust structure correspond to the Mukachevo depression. The lowest density third block belongs to the north-eastern part of the Solotvyno depression, the eastern border of which coincides with the area of clustering of earthquake hypocenters. The south-eastern part of the Solotvyno depression probably represents a transition zone between the Alkapa and Tisza–Dacia terrains. Two lithospheric fault zones are distinguished. The first one separates Mukachevo and Solotvyno depressions. The second zone is located between the Khust and Tyachiv faults in the low-density zone along the entire section of the Earth’s crust. These two zones are assumed to be connected by the Vynogradiv fault. It is characterized by a large cluster of earthquakes and located in the transition from extension and subsidence to compression and uplift. Low velocity (density) zones of the Transcarpathian depression can be associated with lithospheric fault zones. There are the most active horizons of modern geological and geophysical transformations of the mineral environment of the Earth’s crust and can be a potential source of deep oil and gas.