Tectonics and deep structure of the southwestern part of the East European Platform within Ukraine. PART I.

Abstract

The paper consists of two parts. The first part, which is presented here, briefly reviews the history of geological and geophysical study of the platform part of the territory of Ukraine, which is located within the southwestern margin of the East European craton and occupies a significant area of the Sarmatian lithospheric mini-plate. The main attention is focused on the studies of the XXI century, which made a decisive contribution to the revision of some issues related to geodynamic processes and clarification of the tectonics and deep structure of the studied region. The main geophysical method is deep seismic sounding of the Earth's crust and mantle in the WARR (wide-angle reflection/refraction) modification, which was carried out in Ukraine by an international team of geophysicists and geologists at the Institute of Geophysics of the National Academy of Sciences of Ukraine with the participation of specialists from Ukraine, Poland, Sweden, Germany, the Netherlands, Denmark, Scotland, Finland, Romania and some other countries. The deep seismic sounding was accompanied by the construction of density, geomagnetic, geothermal, geoelectric, tectonophysical, seismic and tomographic models of the crust and mantle, and was supplemented by geological data and rock isotope age determination. The section on the structure of the crust of the Ukrainian Shield (USh) considers the fault domain structure of the shield and shows that its megablocks differ in many geological and geophysical features: age, composition and type of magmatism, nature of geophysical fields, and crustal thickness. According to the average velocity characteristics, the crust of the Ush is divided into three layers: upper (5.8-6.3 km/s), middle (6.3-6.7 km/s), and lower (6.7-7.4 km/s). The average thickness of the crust is 40-45 km. Geotraverses of the GSZ and WARR profiles revealed lithic fault zones that, when extended, sink into the mantle or are observed directly in the mantle. The tectonophysical studies revealed subhorizontal movements along the faults, as well as horizontal stress fields covering the crust of the Ush. These data indicate that, since the Neoarchean, the shield has developed by the mechanisms of plate-plume tectonics. The section "Dnipro-Donetsk Basin (DDB)" shows that the formation of the DDB rift-syneclise was mainly caused by horizontal movements of the lithosphere: stretching and compression and right-lateral shear. The WARR Dobre-1999-2001 and Georift-13 profiles finally established that the Devonian sediments of the DDZ axial rift are underlain by oceanic-ultrabasic rocks, not by reefal sediments, as previously thought. The Dobre-1999-2001 profile revealed lithic faults that delimit the Donetsk Folded Structure (DFS) and indicate, in the opinion of the authors of this article, the subduction processes that led to its formation. It is concluded that the most correct hypothesis of the formation of the DSS may be the rift-generating geodynamic hypothesis, which is based on the existence of the Paleotethys Ocean and the unification of the continents of Laurentia and Gondwana in the Carboniferous-Triassic by subduction-collision. In the second part of the article, which will be published in the next issue of the Geophysical Journal, the reader will get acquainted with the sections "Volno-Podilsky plate", "Geothermal and density models", "Moho surface", "Mantle", and "Conclusions".