The first broad Russian experiment aimed at the study of the deep structure of Earth's crust and upper mantle by the common midpoint (CMP) method along the 1-EU geotraverse and cross-traverse 4B was realized in 1995–2008 in the territory of the East European Platform under the Russian Federal Program on Development of the State Geotraverse Network and Deep and Superdeep Boreholes. At the same time, the EGGI profile, geotraverses TATSEIS, ESRU 2003–2005, and DOBRE in Ukraine, as well as the system of profiles under the FIRE project in the adjacent territory of Finland were acquired. Integration of the existing geological maps and available geological (in the widest sense: structural, geochemical, geochronological, and so forth) data with results of geological interpretation of seismic images of the crust and upper mantle have led to a three-dimensional (3D) model of the deep crustal structure of the East European craton and a significant revision of previous ideas on the deep structure and Early Precambrian evolution of the region. In the geological interpretation of seismic data, we attached particular significance to the direct tracing of geological boundaries and fault zones recognized on the seismic-reflection pattern and the section of effective acoustic impedance toward the present-day surface and to their correlation with mapped geological and tectonic units. Comparison of the seismic image geometry with the geology of the eastern Fennoscandian Shield at the present-day erosion level shows that the reflection pattern matches the general trends of compositional layering, gneissic banding, and schistosity. The roughly homogeneous structural domains of the crust correspond to relatively large tectonic sheets, 3–5 km thick. Their inner structure commonly is not discernible in reflection patterns. The 3D model of deep structure in the Kola-Lapland region is based on correlation of tectonostratigraphic complexes depicted in the geological-tectonic map with structural subdivisions recognized as a result of interpretation of seismic crust images and their tracing to depth. In addition to the geological section along the 1-EU geotraverse, the model includes the section along the FIRE-4–4a profile that crosses the western part of the region studied in Finland. The 3D model shows the Paleoproterozoic tectonic structures (Lapland granulite-gneiss belt and its structural and evolutionary relationships with lower-crustal granulites, the structure and tectonic position of the Tana belt) and Archean tectonic structural units (the Central Kola granulite-gneiss belt, the Inari-Kola granite-greenstone domain, and the boundary zone between the Kola craton and the Belomorian orogen). The detailed 3D model of the crust and uppermost mantle in the Karelian-Belomorian region is also based on correlation between the exposed geological structure and geological interpretation of seismic images along the 1-EU geotraverse and cross-traverse 4B. The geological interpretation of the seismic crust image along the FIRE-1 profile serves as the additional basis for the Svecofennian accretionary orogen and its boundary with the Kola-Karelia continent. The model of the crust in the Karelian-Belomorian region contains the Paleoproterozoic tectonic structures (East Karelian imbricate thrust belt, Svecofennian accretionary orogen, and Onega Depression) and also Archean tectonic structures (Kuhmo-Segozero and Kovdozero microcontinents, and Chupa granulite-gneiss belt). The deep structure of the platform basement beneath the Moscow syneclise is an immediate extension of the Fennoscandian Shield. The basement structure in this area was controlled by Paleoproterozoic processes resulting in formation of the Lapland–Mid-Russia–South Baltia intracontinental orogen. The 3D model shows the marginal Totma and Aprelevka volcanic-sedimentary belts, and a synformal structure for the upper crust in the central domain of the orogen. The rock complexes of the Zubtsov-Diakonovo granulite-gneiss belt in the northwest and the Dmitrov-Galich belt in the southeast make up a distinctly outlined stage in the synform section. These complexes are underlain by gneiss-migmatite-amphibolite associations of the Bologoevo and Ivanovo-Sharya belts and are overlain by similar rocks of the Tver and Bukalovo belts. Lastly, the Kashin synformal granulite-gneiss belt is localized in the upper part of the section. The alternation of rocks of differing metamorphic grade clearly indicates the tectonic or tectonized stratigraphic character of the section in the Nelidovo synform. The crust sandwiched between the southward-plunging Totma and Aprelevka belts is characterized by rough layering. The reflections and boundaries of crustal sheets outlined in agreement with this pattern plunge southward beneath the Archean Sarmatia and Volgo-Uralia continental blocks. The deep crustal structure of the Voronezh Crystalline Massif is determined by a succession of geodynamic settings and Archean and Paleoproterozoic tectonic events that resulted in the formation of the Archean crust in the Kursk granite-greenstone domain and probably in the Khopior microcontinent, the Middle Paleoproterozoic East Voronezh orogen, and the Late Paleoproterozoic North Voronezh orogen. The 3D model applies particularly well to the Middle Paleoproterozoic East Voronezh orogen. The orogen is localized in the area of collision of the Kursk and Khopior microcontinents, which differ markedly in crustal structure and composition. The crocodile-type tectonic structure of the East Voronezh orogen is clear evidence for collisional compression. The countermotion of microcontinents resulted in the wedge-shaped structure of the Kursk microcontinent extending for 150 km, delamination of crust in the Khopior microcontinent, and counterdisplacement of tectonic sheets coherently thrust over and under the Kursk microcontinent. The tectonic structure of the central and western Volgo-Uralia continent to a depth of 15–20 km is characterized by sections of 3D models of effective density and magnetization. The second block of information on the Volgo-Uralia continent deep structure comes from results of seismic profiling along the TATSEIS, ESRU 2003–2005, and URSEIS geotraverses. The TATSEIS geotraverse crosses a significant part of the Volgo-Uralia continent from southeast to northwest. The seismic crust images along this geotraverse not only create the basis for interpretation of regional deep structure, but also robustly link the crustal models of the western and southeastern parts of the Volgo-Uralia continent. The data along the ESRU 2003–2005 geotraverse played an important role in ascertaining the deep structure of the Krasnoufimsk ovoid, which is overlapped by sedimentary fill of the Ural foredeep. Additional evidence was provided by the URSEIS geotraverse. The Archean crust, slightly modified in the Paleoproterozoic, which forms the East European Platform basement in the Volgo-Uralia continent, is made up of mafic granulites, khondalite, mafic-ultramafic intrusions, and granitoid plutons. The 3D model of the crust based on the TATSEIS geotraverse demonstrates the deep structure of the Vetluga synform in the Tokmov ovoid and of the interovoid domain. Ovoids play a crucial role in the Volgo-Uralia continent structure and occupy no less than 60% of the crust. In 3D representation, they are bowl-shaped blocks, round or oval in outline, and 300–600 km in diameter at the basement surface, and with a base at the level of the crust-mantle interface, i.e., at a depth of 60 km. The thickness of crust of the interovoid domain does not commonly exceed 50 km. Two types of elongated oval synforms are distinguished: the interovoid ovals (Verkhnevyatka, North Tatar, Almetevsk), up to 200–300 m long, with aspect ratio of 2:1–3:1, and the interovoid belts (Usovo, Vyatka, Kilmez, Elabuga-Bondyuga, Tuma-Penza, as well as Zhiguli-Pugachev homocline), 300–400 km in extent, with aspect ratio of 4:1–5:1. The bottom of the largest interovoid oval crossed by the TATSEIS profile reaches 25 km in depth. In crustal section, the structural elements of the interovoid domain are underlain or partly crosscut by acoustically transparent layers composed of the Bakaly-type granitoids. The lower crust of the interovoid domain is ~35 km in thickness and is composed of tectonic sheets plunging toward the northwestern end of the geotraverse and penetrating into the mantle.

The major tectonic units of the Neoarchean Volgo-Uralia continent, which is ~600,000 km 2 in area, are contrastingly expressed in regional gravity and magnetic maps. Interpretations of seismic images of the crust along the TATSEIS geotraverse in combination with 3D density and magnetic crust models provide insights into the volumetric representation of tectonic structures of various ranks. Granulite-gneiss crust of Volgo-Uralia is characterized by elevated thickness (~60 km and locally up to 65–70 km). The deep structure of Volgo-Uralia assumes that the entire crustal section, including the lower crust, is composed of high-density granulite metamorphic facies rocks. Specific structural units called ovoids play the main role in the structure of this continent. The ovoids are bowl-shaped crustal blocks, round or oval in plan view, 300–600 km across, and with the base reaching a level of crust-mantle interface at ~60 km. Ovoids are bounded by conic surfaces of reverse (thrust)-faults, along which their outer parts are thrust over the framework. The Tokmov, Buzuluk, Verkhnekamsk, Krasnoufimsk, and Orenburg ovoids, which generally are not in contact with one another, are dominated by mafic granulites, gabbroic rocks, gabbroanorthosites, and ultramafic rocks. A significant contribution of deep-seated intrusive rocks suggests that metamorphism developed in the lower and middle crust at high PT parameters that exceed the maximum estimates (940–950 °C, 9.5 kbar) recorded in samples of borehole cores. The interovoidal space is occupied by elongated oval synforms up to 200–400 km long. This space is considered to be an interovoidal domain, which includes three relatively narrow, compressed synforms (Yelabuga-Bondyug, Kilmez, Chusovaya) and four oval synforms (Srednevyatka, Verkhnevyatka, North-Tatar, Almetevsk). The Tokmov ovoid is framed in the southeast by the Tuma and Penza belts. Synforms are filled with metasedimentary granulites and mafic metaigneous rocks. The protoliths were formed over the time interval from 3.4–3.2 to 3.1–3.0 Ga. The internal zoning of the Volgo-Uralia crust is related to numerous local centers within ovoids and interovoidal region. At least two high-temperature metamorphic events were followed by periods of retrogression: 2.74–2.70 and 2.62–2.59 Ga. The areal and especially high-temperature character of tectonothermal processes during formation of the Neoarchean crust of the Volgo-Uralia Craton, and distinct geometrization of space with recognition of several concentric domains, finds a universal explanation in ascent of multiple plumes.