The velocity structure of the mantle under the Baikal Rift Zone (BRZ) is investigated with the P‑wave receiver functions (PRFs) for a group of 10 seismograph stations. The BRZ presents one of the world’s most active continental rift zones. The peculiarities of the BRZ include Cenozoic magmatism in the upper mantle of the southwestern part of the BRZ, which disappears in the central and northeastern parts. The analysis of seismic data reveals other indications of the significant lateral heterogeneity of the mantle beneath the BRZ. At half of the stations there is evidence of a sharp rise of the S velocity with depth at a depth of around 330 km, similar to the descriptions for the “X” or the “300-km” discontinuity. In the central and northeastern regions at depths from about 350 to 410 km there is a well pronounced low S velocity layer, which is practically missing in the southwestern part. The origin of this layer is apparently related to the upwelling and dehydration of wadsleyite in the transition zone. At depths from 500–600 to 660 km in the central and northeastern regions there is another low velocity layer that may be explained by the accumulation of garnetite in the process of subduction of the lithosphere of the Pacific. This layer is poorly pronounced in the southwestern region. The difference between the travel times of the P410s and P660s seismic phases (differential time) in the southwestern region (23.5 s) is close to the data for the standard model (Kennett, Engdahl, 1991). In the central and northeastern regions, the observed differential time is larger than the nominal time by 1.0 s. The rise of the differential time may be related to the cooling and/or hydration of the transition zone by the slabs of the subducted oceanic lithosphere. The obtained seismic data suggest a large role of processes of hydration and dehydration in the central and northeastern regions, however, this role is comparatively small in the southwestern region.

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.

Summary Models of the deep structure, made by the authors by the method of longitudinal receiver function based on the data of two mutually intersecting profiles of seismic stations, indicate anisotropy of the deep structure in the area of the Klyuchevsky group of volcanoes. A denser layer in the depth interval of 15–20 km, revealed on the northeastern profile, can be an obstacle to the direct ascent of more heated masses and the reason for the periodicity of eruptions of western volcanoes.

The velocity structure of the southern edge of the ancient Siberian craton has been modeled to an 80-km depth based on teleseismic records of a P-receiver function method (P–S). The correlation between the deep and the surface structure determined through modeling is indicative of the submeridional convergence of the south-western edge of the Siberian craton with the Central Asian mobile belt. A sublatitudinal crust extension in the contact zone caused by such convergence may initiate Baikal rifting at the craton’s southeastern edge.

The article discusses a comparison of theoretical seismograms for two velocity models of the Earth's crust and P-wave arrival times estimated from experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. The theoretical seismograms were obtained by mathematical simulation of wave fields using the Earth's crust velocity models based on the data of the BEST and PASSCAL experiments. Vibration seismograms were obtained by measuring the wave field of a CVO-100 vibrator in the SB RAS Southern Baikal polygon. In the experiments, the vibration seismograms show that arrival times in the P-wave group correspond to the values for waves of large amplitudes in the theoretical seismograms. The P-wave arrival times in the theoretical seismograms of the BEST experiment are compared to the values in the experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. This comparison shows that the arrival times of maximum amplitude waves correspond to the theoretical hodographs of waves with velocities of 6.25–6.80 km/sec in the BEST experiment velocity model. At the same time, the experimental data set does not contain arrival times corresponding to longitudinal waves with Vp=7.25 km/sec, which are related to an assumed layer (more than 10 km thick) in the lower crust for the BEST experiment velocity model. In the experiments, the P-wave arrival times in the vibration seismograms correspond to the P-wave arrival times in the theoretical seismograms of the PASSCAL experiment throughout the entire 400-km long section of the Baikal–Ulaanbaatar profile. It is thus confirmed that the average values of the wave velocities in the PASSCAL velocity model have been reliably estimated. It should be noted that the experimental values of the arrival times of the first wave in the P-wave group are in agreement with the first arrival times in the hodographs of the theoretical seismograms for the velocity model in the PASSCAL experiment considering the distances from the source in a range of 65–380 km.

The article discusses a comparison of theoretical seismograms for two velocity models of the Earth's crust and P-wave arrival times estimated from experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. The theoretical seismograms were obtained by mathematical simulation of wave fields using the Earth's crust velocity models based on the data of the BEST and PASSCAL experiments. Vibration seismograms were obtained by measuring the wave field of a CVO-100 vibrator in the SB RAS Southern Baikal polygon. In the experiments, the vibration seismograms show that arrival times in the P-wave group correspond to the values for waves of large amplitudes in the theoretical seismograms. The P-wave arrival times in the theoretical seismograms of the BEST experiment are compared to the values in the experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. This comparison shows that the arrival times of maximum amplitude waves correspond to the theoretical hodographs of waves with velocities of 6.25–6.80 km/sec in the BEST experiment velocity model. At the same time, the experimental data set does not contain arrival times corresponding to longitudinal waves with Vp=7.25 km/sec, which are related to an assumed layer (more than 10 km thick) in the lower crust for the BEST experiment velocity model. In the experiments, the P-wave arrival times in the vibration seismograms correspond to the P-wave arrival times in the theoretical seismograms of the PASSCAL experiment throughout the entire 400-km long section of the Baikal–Ulaanbaatar profile. It is thus confirmed that the average values of the wave velocities in the PASSCAL velocity model have been reliably estimated. It should be noted that the experimental values of the arrival times of the first wave in the P-wave group are in agreement with the first arrival times in the hodographs of the theoretical seismograms for the velocity model in the PASSCAL experiment considering the distances from the source in a range of 65–380 km.

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 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.

We have obtained P-wave and S-wave receiver functions for 10 broadband seismograph stations in the Baikal rift zone (BRZ) and inverted them for seismic velocity models of the crust and upper mantle. The thinnest crust (30–35 km) is found in the Baikal basin, the thickest in the East Sayan uplift (45–50 km). Intermediate values (40 km) are found in the BRZ at distances around 100 km from Lake Baikal. A high (at least 1.8) Vp/Vs ratio is observed in the middle and lower crust. It exceeds 2.0 at some stations. In our opinion, the highest Vp/Vs ratios are due to fluid-filled porosity with a high pore pressure. The seismic lithosphere – asthenosphere boundary (LAB) is manifested by a shear velocity drop from 4.5 km/s to 4.0–4.2 km/s. Beneath the Baikal basin, the LAB is located at a depth not more than 50 km, and the S velocity drop is maximal (10 %). A similar structure is found outside the basin, underneath a segment of the East Sayan uplift. At other locations in the BRZ, a typical depth of the LAB varies from 80 to 90 km. Having considered changes in the depth of the 410 km seismic discontinuity, we cannot find any evidence of an elevated temperature of a hypothetical thermal plume beneath the BRZ.

Deep velocity sections of the transition zone from the Siberian platform to the Central Asian mobile belt are constructed by teleseismic tomography and P-receiver function techniques. An array of the dense ancient Siberian craton is identified in the velocity sections with areas of high seismic velocity. In the SSW section MOBAL_2003, the surface boundary of the craton corresponds to the southern margin of the Siberian platform and is nearly vertical to a depth of 120 km. At larger depths, the craton slides almost horizontally underneath the Tunka rift area. At depths from 150 to 250 km, it is in contact with the area under the Khamar-Daban mountain range. In the southeast, according to the SE velocity section PASSCAL_1992 across the South Baikal basin and the Khamar-Daban mountain range, the Siberian craton thickness is reduced from 270 to 150 km at the contact of the Siberian platform with the Baikal folded area. In this contact zone, the upper part of the craton is wedge-shaped and has an angle of about 45° with the ground surface; it completely tapers off at a depth of 150 km to the east of Lake Baikal. The vertical configuration of the southern segment of the Siberian craton, which evolved with time, may determine the nature of the Baikal rifting in the Cenozoic.

Summary The authors determined the velocity structure from the Earth’s surface to a depth of 270 km around each point of observation in southern Siberia according to the P-receiver functions of broadband seismic stations. An example of the station Ulan-Ude (UUD) in East Baikal indicates that the velocity deep structure around the station can be graphically represented in the form of a specially designed circular model.

Summary Results of the distant earthquakes records analysis and P-receiver function inversion provide valuable information on the distribution of S-wave velocity in the crust and upper mantle and the geometry of seismic boundaries. The developed approach with a sufficiently dense arrangement of seismic stations is useful for solving fundamental problems of studying the structure and formation of the Earth and in identifying promising areas in the search mineral deposits.

About 3D structure of the Baikal rift upper crust from DSS, receiver function and local earthquake data V.D. Suvorov (IPGG SB RAS, Novosibirsk), V.V. Mordvinova (IEC SB RAS, Irkutsk) G.I. Tat’kov, S.A. Tubanov (GIN SB RAS, Ulan-Ude) Connection between the crust structure and distribution of earthquake sources remains not clear yet. We show distribution of P- and S- wave velocities in the upper crust beneath central part of Baikal Lake according to the three methods together with earthquake focal depth distribution. The one-dimensional model of P-wave velocity from DSS data is initial. A seismic active layer with earthquake sources on the depth of 8-22 km is mapped and change trends of focal depth earthquakes are defined. Variations of P- and S-wave velocities are found out lengthways seismically active layer. It is revealed that S-wave velocity according to DSS and earthquakes data in the upper crust considerably is above, than from receiver function observations. At the same time that velocity is close to DSS data in the lower crust. Probably P-to-S converted waves in upper crust are formed near to surface only. This research is supported by the Integration Project of Russian AcadeУmy of Sciences № 6.17.

Low-velocity layers in the Earth’s crust, related to large-scale thrusts corresponding to the suture zones, can be as the sign of perspective of this territory on the deposits of minerals and recommendation to more detailed researches

The three-dimensional shear velocity lithospheric structure at depths from 0 to 70 km beneath the southern Baikal rift system and its surroundings has been imaged by inversion of P-to-SV receiver functions from 46 digital stations operated in two teleseismic international projects in southern Siberia and Mongolia. The receiver functions were determined from teleseismic P waveforms and inverted to obtain depth dependences of S velocities at each station which were related to tectonic structures. The computed vertical and horizontal sections of the 3D shear velocity model imaged a transition from relatively thin crust of the southern Siberian craton to thicker crust in the folded area south and southeast of Lake Baikal, with a local zone of thin crust right underneath the South Baikal basin. The velocity structure beneath the Baikal rift, the mountains of Transbaikalia, Mongolia, and the southern craton margin includes several low-velocity zones at different depths in the crust. Some of these zones may record seismic anisotropy associated with mylonite alignment along large thrusts.