The records of distant strong earthquakes,obtained by way of a dense linear network of seismic stations in the Severomuysk segment of the Baikal rift system,revealed a complex layered-block structure of the Earth's crust and subcrustal mantle using the longitudinal receiving function.The distribution of cross-wave velocities indicates that the properties of the blocks that make up the Severomuysk Earth's crust differ.The western vergence of these blocks and the stratification of the lower part of the Earth's crust confirm the accretion-collision origin of the uplift.The intensity of the collision effect on the Earth's crust of the region is explained by the location of the Severomuysk segment on the thinned inclined edge of the Siberian Craton.A convincing correlation was found between the focal depths of earthquakes in 2015 and contrasting velocity heterogeneities in the upper part of the Earth's crust of the Muyakan depression.

Transects are vertical sections of the Earth's crust,which reveal the nature of tectonic zones,as well as their spatial relationships through a combined analysis of their geology and geophysics.Transect documents contain a geological map for a strip of land 100 km wide,a geological section of the upper crust,gravity and magnetic maps (and/or corresponding profiles along the transect),and a geophysical profile of the crust,differentiated by seismic velocities,densities and other geophysical properties.These data are used to compose a combined cross-section (the resulting section),which shows a set of rocks typical of various geodynamic conditions (rifts,oceans,collision zones,orogenic basins,continental platforms and magmatic arcs,including Andean island arcs,active continental outskirts,trenches,basins of front and rear arcs).The objective of this project was to build deep sections according to unified legends based on the interpretation of all available geological and geophysical data in order to determine the spatial relationship of terranes and their geodynamic nature in terms of plate tectonics.A number of terranes have been discriminated in the territory of the southern part of Eastern Siberia and the territory of Mongolia,and their geodynamic nature and space-time relations were analysed.The terranes were found out to be Vendian-Early Paleozoic,Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic island arcs and microcontinents.Moreover,Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic Andean-type active continental margins and Late Paleozoic-Early Mesozoic passive margins and Early Cretaceous rifts were identified and studied.The rock complexes related to the island arcs and Andean-type active continental margins are thrust over the bordering continents and microcontinents,the width of the respective tectonic nappes attaining 150 km.Schematic paleogeodynamic reconstructions for the area of the Mongolia-Okhotsk ocean have been performed,spanning the period from Devonian to Late Jurassic."Non-geosyncline" granitoid magmatism finds straightforward and sound explanation in terms of plate tectonics where provinces of Devonian-Carboniferous and Permian-Triassic magmatism correspond to Andean-type active continental margins and Middle-Late Jurassic magmatism is associated with Siberia/Mongolia-China collision.The presence of a subalkaline (mantle) element in collisional magmatism and the great extent of the area it occupies can be explained by suggesting that an oceanic rift (a mantle hotspot) was buried under thick continental lithosphere after closure of the Mongolia-Okhotsk ocean.In the Early Cretaceous,the setting of collision gave way to that of continental rifting.The existence of an Andean-type active margin over the great extent of the southern border of Siberia is likewise responsible for minor abundance of ophiolites along the Mongolia-Okhotsk suture.When one colliding continent has an Andean-type active margin and the other has a passive margin,the continental crust of the former thrusts over the latter,and no conditions arise for ophiolites to expose.Blocks of dismembered ophiolites,that are remnants of truncated seamounts,can be part of chaotic complexes building accretion-subduction wedges.However,accumulation of such wedges in the Late Permian-Early Jurassic was not typical of the active margin of Siberia because of rapid subduction.An analysis of geological and geophysical data on transects shows that the Asian continent was formed inthe Phanerozoic as a result of accretion of terranes,some of which were microcontinents with a Precambrian foundation.Precambrian blocks are separated by deformed and strongly eroded Phanerozoic igneous arcs of various widths,also classified as specific terranes.

Establishing relationshipsbetween the regional seismicity andthe depth geometry and compositions of large-size density inhomogeneities of the upper crust is one of the ways to determine the parameters of mean long-term seismicity. Theoriginof geological bodies and mechanisms of penetration of intrusive bodiesinto the upper crust aredetectable from the shapes and sizes of such bodies.Knowledge on the shapes and sizes of large intrusive bodies is important for prospecting and exploration of mineral resources. Based on the medium-scale areal gravimetric survey data, a regional map of Bouguer gravity anomalies was constructed for theUlaanbaatar region, Central Mongolia.Interpreted gravimetric data from the solutions of the inverse problem of gravimetry based on the body-shape selection were used to developthe 3D models showing the largest crustal density inhomogeneities of the study area – the Tolskaya (Ulaanbaatar) basin, Nalaiha depression and Bogdoulin granite massif. It is confirmed that the basement of these depressionshas a block structure. The basement of the Tolskaya basinincludes two depressions, the western and eastern ones.In the western depression, the sediment thickness is 150 m. It amounts to 400 m in the eastern depression at the intersection of the Selbin and Tolskaya deep fault systems. The information on the shapes and thicknesses of the sediment bodiesisused in the seismic studies aimed at obtaining a more precise assessment of seismic hazard of the Ulaanbaatar city and the region. In the Nalaiha depression, the sediments are 800 m thick.The morphology of the basement and the thickness of sediments in this depression can be used to update the mining prospects of the Nalaiha coal deposit. The maximum thickness of granitoids in the Bogdoulin massif is 9.0 km. The area of its projection to the ground surface is twice as big as the area of the outcrops. Our research results can be useful for assessing the metallogenic specialization of granitoids, which indicators in relation to some minerals are related to the specific features of the Bogdoulinsky massif. Such indicators have beendiscovered by the geological, geophysical and geochemical methods. Our study shows that the geophysical methods (in particular, gravimetry) can considerably expand the knowledge of the morphology of geological objects at depth and facilitate reaching a new level in structural analysis, which is critical for developingnew ideas to clarify the geological history and tectonic conditions of the formation of the Mongolia-Siberian region.

The models of the velocity structure of the southern margin of the ancient Siberian Craton are constructed for depths down to 80 km from teleseismic records by the method of longitudinal receiver function (P-to-S). The relationship between depth and surface structures revealed using the modeling indicates a submeridional convergence of the southwestern margin of the Siberian craton and the Central Asian mobile belt. This convergence caused sublatitudinal extension of the crust in the contact zone, and this extension could in turn have triggered the Baikal rifting in the southeastern margin of the craton.

Summary As a result of gravimetric studies, characteristics of the mesozoic structures and data on the morphological features of the submerged part of the crystalline fundament were obtained. According to the interpretation of gravity profile, Nalaikh basin has a complex block structure and it is divided into two main parts. The thickness of the Mesozoic deposits in the center of the basin is about 600 m.

Summary The performed interpretation of seismic-gravimetric data in the North-Muysky area of the Baikal rift zone makes it possible to estimate the thickness, extent and direction of sinking of the sole of large geological bodies of acidic composition and to isolate in the earth’s crust large-amplitude thrusts with a length of hundreds of kilometers.

The medium-scale gravity survey over the Ulaanbaatar area (Central Mongolia) has yielded the Bouguer gravity anomaly map. Based on interpretation of these data, 3D models have been built for large density inhomogeneities in the Earth’s crust - Ulaanbaatar (Tola) depression and Bogdo granite massif. It has been identified that the basement of the depression has a block structure and complicated by two basins: western and central (Selbe) The sediment thickness averages about 150 m in the western basin and may reach 300 m in the Selbe. The thickness of granitoids in the Bogdo massif reaches 4.5 km, the area of its projection on the earth's surface is twice the area of the exits. The data on the form and dimensions of these density inhomogeneities may be used in engineering seismology to update seismic hazard estimates for the town of Ulaanbaatar and to conclude about the prospects for metallogenic specialization of granitoids whose features concerning some minerals are related to peculiar massif form and identified by geological-geophysical and geochemical studies.

The deep structure and paleogeodynamics of the Siberian craton and the Central Asian folded belt are studied by two specialized laboratories of the Institute of the Earth’s Crust, SB RAS – the Laboratory of Complex Geophysics and the Laboratory of Paleogeodynamics. A variety of modern geophysical methods is applied. Surface wave tomography is focused on determination of 3D velocity structure of the upper mantle. Seismic, gravimetric and electrical surveys are aimed at stu­dies of structures of the crust and the upper mantle. Geothermic parameters of the lithosphere in Central Asia are measured. In search for mineral resources, new geophysical prospecting technologies are developed. Quality paleogeodynamics reconstructions require a proper understanding of the structural evolution of the Siberian craton and the Central Asian folded belt. Researches in this scientific field are conducted by the Laboratory of Paleogeodynamics. Besides, the Laboratory studies new minerals and conditions of their formation. Some of the scientific research projects are jointly implemented by the two laboratories, and research results are regularly published in Russia and abroad.

It has been found that large density inhomogeneities in the basite crust of the Baikal mountain system whose shape and size were determined from the gravimetric data interpretation are of minimal seismic activity as compared to granitized blocks.