RomUkrSeis profile: a model of the deep structure of the lithosphere and its geological and geophysical interpretation. P. I. Density heterogeneity and electrical conductivity anomalies

For the first time, a 2D gravity model was calculated for the RomUkrSeis profile, and the lithosphere density heterogeneity was analyzed. The synthetic model of the geoelectric heterogeneities of the Earth’s crust and upper mantle was created. It was obtained from 2D―3D modeling of the Earth’s electromagnetic field. An overview of modern ideas about the geological structure of tectonic units along the profile is presented. We propose a deep position of crustal and crustal-mantle faults dividing the Earth’s crust and upper mantle into separate blocks according to the gravity model, taking into account the seismic model and geological-geophysical data. The southwest part of the profile is mainly characterized by a high fragmentation into blocks, while the northeast, by horizontal stratification. The lower densities (2.36―2.76 g/cm3) of the whole Earth’s crust up to 36 km depth in relation to the surrounding structures is confirmed in the Outer Carpathians. According to the gravity model, a low-density zone (2.55―2.60 g/cm3) in the upper crust, which covers the northeastern part of the Apuseni Mountains and partially the Transylvanian basin, is revealed. From the northeast, this zone is limited by the Bistrica-Pryde fault. We identified compaction zones in the lower crust and in two upper mantle blocks in the narrow keel of the Moho boundary; the blocks with the highest densities of 3.41 g/cm3 (to the southwest) and 3.42 g/cm3 (to the northeast) are separated by the Pre-Carpathian fault. The synthetic model of the resistivity distribution along the RomUkrSeis profile is a generalization of the results of geoelectrical models of various degrees of approximation of the geological environment, which were obtained from the experimental data of magnetotelluric sounding and magnetovariational profiling. Anomalies of high electrical conductivity in the Earth’s crust and upper mantle have a complex configuration, different intensity, and depth. They do not always correspond to surface geology. Electrical conductive objects are located at depths from 15 to 25―30 km and are characterized by anomalous resistivity from the first units of Ohm∙m in the Carpathian-Pannonian region to 10―20 Ohm∙m under the Volyn-Podilsk monocline and the western part of the Ukrainian Shield. As a result of the review of available geoelectrical data in the Pannonian-Carpathian region, the lithosphere-asthenosphere boundary depth is assumed to be 70―90 km with a total longitudinal conductivity of up to 6 kS. The descending of the asthenosphere top from 70 to 100 km in the transition zone between the Inner and Outer Eastern Carpathians and its rise to 70 km under the Carpathian Foredeep were revealed. Under the Volyn-Podilsk monocline and the western part of the Ukrainian shield, the total longitudinal conducti­vity of the asthenosphere does not exceed 1―2 kS.