The paper reviews goals and objectives, stages and the content of seismotectonic studies conducted in Eastern Siberia. Such studies are based on a comprehensive analysis of geological and geophysical data and provide for establishing whether the local earthquakes are of tectonic origin and revealing their relationships with recent geodynamic processes in the area under study. Seismic hazard assessment and evaluation of tectonic processes are the two major, closely interrelated aspects of seismotectonic studies. The latter are generally conducted in combination with seismic studies prior to the stage of detailed seismic zonation (DSZ) which is followed by seismic micro-zonation (SMZ). In three stages of seismotectonic studies, we analyse specific geological structures, reveal the regional dynamics of seismotectonic processes, clarify details of potential seismic hazard locations and identify sites of the potential instantaneous deformation of the crust which may take place due to active faulting. Based on results of the long-term studies conducted by the authors, a seismotectonic map of Eastern Siberia is compiled. The paper briefly reviews the methods of mapping and refers to data on active faults and neotectonic structures revealed in the area under study, which are closely related to regional earthquake sources.

The geomedium block concept envisages that stresses in the medium composed of rotating blocks have torque and thus predetermine the medium's energy capacity (in terms of [Ponomarev, 2008]). The present paper describes the wave nature of the global geodynamic process taking place in the medium characterized by the existence of slow and fast rotation strain waves that are classified as a new type of waves. Movements may also occur as rheid, superplastic and/or superfluid motions and facilitate the formation of vortex geological structures in the geomedium.

Our analysis of data on almost 800 strong volcanic eruptions shows that the magma chamber’s thickness is generally small, about 0.5 km, and this value is constant, independent of the volcanic process and predetermined by properties of the crust. A new magma chamber model is based on the idea of 'thermal explosion’/‘self-acceleration' manifested by intensive plastic movements along boundaries between the blocks in conditions of the low thermal conductivity of the geomedium. It is shown that if the solid rock in the magma chamber is overheated above its melting point, high stresses may occur in the surrounding area, and their elastic energy may amount to 1015 joules per 1 km3 of the overheated solid rock. In view of such stresses, it is possible to consider the interaction between volcano’s magma chambers as the migration of volcanic activity along the volcanic arc and provide an explanation of the interaction between volcanic activity and seismicity within the adjacent parallel arcs.

The thin overheated interlayer/magma chamber concept may be valid for the entire Earth's crust. In our hypothesis, properties of the Moho are determined by the phase transition from the block structure of the crust to the nonblock structure of the upper mantle.

For purposes of seismicity microzonation of the Ulan‐Ude city territory, engineering geophysical studies are conducted to reveal which types of rocks and soils are dominant in the study area and to classify them by site‐specific velocities of P‐ and S‐waves and amplitude‐frequency characteristics. The article describes a technique for establishing the baseline seismic signal corresponding to parameters of relatively strong earthquakes in potential earthquake foci (PEF) zones. It is shown that the established baseline signal is applicable. Presented are results of theoretical calculations based on seismicity‐soil models providing reference parameters of bedrock, medium and water‐saturated soils (soil categories 1, 2 and 3, respectively).

Seismic impacts are assessed for the zone with the baseline seismic intensity of 8 points, as per MSK‐64 seismic intensity scale. The reference model is used to identify zones with seismic intensity from 7 to 9 points in the city territory, and it is established that such zones differ in thickness of water‐saturated and non‐water‐saturated soil layers. As a result, a schematic map showing the main parameters of seismic impacts is constructed in the first approximation. The obtained data are useful for the development of recommendations concerning further engineering seismological studies and activities for the appropriate revision and upgrading of the seismic microzonation technique in order to complete seismic microzonation of the Ulan‐Ude city territory.

Dykes composed of basic rocks and granite are formed due to interactions between melts in a wide range of conditions, from contrasting compositions and fluid saturation rates to various tectonic settings and processes at different depths. Textures and petrochemical characteristics of the dykes are thus widely variable. This paper is focused on composite dykes observed in the West Sangilen region in South-East Tuva, Russia.

The Sangilen wedge is a fragment of the Early Caledonian orogenic structure of the Tuva-Mongolia Massif which evolved in a succession of geodynamic settings, from collision (transpression, 570–480 Ma) to transform faulting (transtension, 480–430 Ma). Intensive tectonic deformation facilitated massive basic-rock and granite magmatism at various layers of the crust and associated heating and metamorphism of the rocks (510–460 Ma). Basic-rock–granite composite dykes were formed in the above-mentioned period in various tectonic settings that controlled conditions of dyke intrusions and their compositions.

We distinguish two groups of composite dykes observed on two sites, in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river (Strelka and Erzin Sites, respectively) (Fig. 1). The dykes in both groups originated from one and the same basic-rock melt source. However, mingling of the contrasting melts was carried out by different mechanisms as suggested by the proposed intrusion models.

In the area between the Erzin and Naryn rivers (Strelka Site), the host rock of the composite dykes is granite of the Nizhneerzin massif. The mingling dykes are composed of amphibole gabbro and monzogabbro, granosyenite and twofeldspar granite. Contacts between basic and felsic rocks vary from smooth contrasting to complex ‘lacerated’ flameshaped, and gradual transition zones are present (Fig. 6).

The dykes were formed at mesoabyssal or abyssal depths, and the subliquidus heat regime was thus maintained for a long time, and even the smallest portions of the basic-rock melt were consolidated through quite a long period of time. As a consequence, indicators of deformation are lacking in the composite dykes, while transition zones and hybridization are present.

On the right bank of the Erzin river (Ersin Site), the dykes cut through migmatite-granite of the Erzin formation in the same-name tectonic zone. Contacts with host rocks are transverse. Melanocratic rocks are represented by smallgrained diorite and quartz diorite, and the felsic composite dykes are composed of medium- and small-grained twofeldspar granite and leukogranite. Transition zones, hornfelsing and contact alterations are absent at contacts of all the types (Fig. 8).

The composite dykes of this type intruded and emplaced when the shear zone was subject to extension and fragmentation, which predetermined active intrusion of basic and, possibly, felsic melts through conjugated faults. Crystallization of the melts was rapid, and their potential heat impact on the adjoining rocks was thus excluded, as evidenced by the presence of oxygonal chips of igneous and host metamorphic rocks, vein pegmatoid intrusions, and composite dykes of the reticulate-cuspate texture with the dominant basic-rock component.

The mingling dykes classified in the first group intruded when the Erzin and Kokmolgarga shear zones were formed at the early stage of the tectonic-magmatic evolution of the Sangilen orogen (510–490 Ma). Intrusions of the basic-rock melts were accompanied by the formation of relatively large massifs of the basic composition, i.e. the Erzin and Bayankol gabbro-monzodiorite massifs, as well as by the occurrence of composite dykes that are abundant in the area between the Erzin and Naryn rivers. In the second stage (460–430 Ma), the composite dykes occurred when the orogen was subject to extension along the system of tectonic zones, the Bashkymugur gabbro-monzodiorite massif was emplaced, and fracture-vein structures, including the dykes, were formed.

The area of studies covers the north-eastern part of the Siberian craton (the Birekte terrain), Russia. The influence of metasomatic and magmatic processes on the mantle lithosphere is studied based on results of analyses of phlogopite- and phlogopite-amphibole-containing deep-seated xenoliths from kimberlites of the Kuoika field. In the kimberlitic pipes, deep-seated xenoliths with mantle phlogopite- and phlogopite-amphibole mineralization are developed in two genetically different rock series: magnesian (Mg) pyroxenite-peridotite series (with magnesian composition of rocks and minerals) and phlogopite-ilmenite (Phl-Ilm) hyperbasite series (with ferrous types of rocks and minerals). This paper is focused on issues of petrography and mineralogy of the xenoliths and describes the evidence of metasomatic / magmatic genesis of phlogopite and amphibole. We report here the first data set of 40Ar/39Ar age determinations for phlogopite from the rocks of the magnesian pyroxenite-peridotite series and the ferrous Phl-Ilm hyperbasite series.

The Mg series is represented by a continuous transition of rocks from Sp, Sp-Grt, Grt clinopyroxenite and ortopyroxenite to websterite and lherzolite. Many researchers consider it as a layered intrusion in the mantle [Ukhanov et al., 1988; Solov’eva et al., 1994]. The mantle metasomatic phlogopite and amphibole are revealed in all petrographic types of the rocks in this series and compose transverse veins and irregular patchs at grain boundaries of primary minerals. At contacts of xenolith and its host kimberlite, grains of phlogopite and amphibole are often cut off, which gives an evidence of the development of metasomatic phlogopite-amphibole mineralization in the rocks before its’ entraiment into the kimberlite. In the xenoliths with exsolution pyroxene megacrystalls, comprising parallel plates of clino- and orthopyroxene ± garnet ± spinel (former high-temperature pigeonite [Solov’eva et al., 1994]), the metasomatic phlogopite-amphibole aggregate mainly replace laminar intergrowths of one of pyroxenes and garnet and also develops in the re-crystallized fine-grained rock matrix. This suggests a considerable period of time between the crystallization of rocks of the pyroxenite-peridotite series and the development of phlogopite-amphibole metasomatism.

The Phl-Ilm hyperbasites comprise a complex association of parageneses represented by garnet- and garnetless pyroxenites, websterites, olivine websterites, orthopyroxenites, lherzolites and olivinites. A specific feature of this series is high contents of K, Ti and Fe in the rocks and minerals. The content of phlogopite is widely variable, from a few percent to 40–80 %. The content of ilmenite ranges from a few percent to 15 %, rarely to 30–40 %. Mica and ilmenite contents sharply decrease in garnetized xenolithes, where these two minerals, as soon as olivine and pyroxenes are replaced by garnet.

Euhedral, subhedral, sideronitic and porphyraceous structures in garnetless xenoliths suggest the primary magmatic genesis of the rocks. In the series of Phl-Ilm hyperbasites, a special type of parageneses is represented by strongly deformed phlogopite-amphibole rocks with newly-formed chromite and relict resorbed ilmenite and clinopyroxene. Phl-Ilm rock series is also characterized by a variety of autometasomatic and metasomatic reaction structures. Garnet and phlogopite develop nearly simultaneously at the sub-solidus stage: garnet develops due to cooling of the primary magmatic rocks, and phlogopite develops under the influence of residual rich in potassium and volatiles fluids – melts. Phlogopite in the rocks of the Phl-Ilm series form porphyraceous plates, late intergranular xenomorphic grains, porphyroblasts of the solidus stage and strongly deformed irregular plates in the phlogopite-amphibole rocks. Amphibole occurs in garnetless parageneses and deformed phlogopite-amphibole rocks in amounts of a few percent and up to 40–50%, respectively. Petrographically, the differentiated series of phlogopite-ilmenite hyperbasites belongs to mantle magmatites, except for younger deformed phlogopite-amphibole rocks from zones of deep faults.

Unlike corresponding minerals in the Mg pyroxenite-peridotite series, minerals from the Phl-Ilm hyperbasites are characterized by lower magnesium index (Mg#), considerably higher contents of TiO2 and FeO, and lower contents of Cr2O3 (Table). In diagrams Mg# – TiO2 and Mg# – Cr2O3, metasomatic phlogopite points from Mg series rocks are significantly distant from points of mica from the phlogopite-ilmenite parageneses (Fig. 24). In the parageneses of the Mg pyroxenite-peridotite series, phlogopite plates have homogenous compositions in contrast to zonal phlogopite in the Phl-Ilm hyperbasites. In Phl-Amph metasomatites of the Mg series, amphibole is represented by typical pargasite, and its chemical composition is sharply different from that of K-richterite from the deformed phlogopite-amphibole rocks of the series of the Phl-Ilm hyperbasites (Table).

The 40Ar/39Ar age in the range from 1640 to 1800 Ma (Fig. 25) is determined for phlogopite from the metasomatic phlogopite-amphibole veinlets and intergranular reaction patches in the garnet olivine websterite of the Mg series. For mica from the garnetless Phl-Ilm websterites, ages are 869 and 851 Ma (Fig. 25). Mica from the garnet-containing Phl-Ilm lherzolites is much younger (608 and 495 Ma). The age of mica from the deformed phlogopite-amphibole rock is 167 Ma, which is close to the age of kimberlites of the Kuoika field.

Metasomatic phlogopite (1640–1800 Ma) originated somewhat later than the Birekte terrain accretion to the Siberian craton (1.8–1.9 Ga) [Rosen, 2003], and its age determination may be explained by a partial loss of 40Ar in the analysed medium. This age is also close to the late episode when the crust was formed in the Birekte block 1.8–2.1 Ga ago [Nasdala et al., 2014], and corresponds to the time when radiogenic osmium was supplied into the mantle lithosphere from the subduction zone (1.7–2.2 Ga, according to [Pernet et al., 2015]). In analyses of minerals in the pyroxenite-peridotite series from the Obnazhennaya pipe, data on the oxygen isotope geochemistry give evidence of an ancient subduction component (Fig. 26). It can be thus assumed that in the mantle lithosphere of the Birekte terrain, phlogopite-amphibole metasomatism took place due to fluids-melts ascending from the subduction zone about 1.8 Ga ago and correlates to the accretion of this block to the Siberian craton. The complex magmatic series of Phl-Ilm rocks formed later than the Mg pyroxenite-peridotite series. The more ancient ages of phlogopite (869–851 Ma) from PhlIlm hyperbasites are somewhat higher than the most ancient dating of alkaline ultrabasic-carbonatite Tomtor massif (800 Ga, according to [Entin et al., 1990]) and the time when the breakup of Rodinia began (825 Ga, according to [Li et al., 2008]). The difference may be explained by an advance occurrence of high-potassium, titanian, ferrous magmatites in the mantle lithosphere of the Birekte block as compared to their appearance on the surface. Phlogopite from xenoliths with subsolidus garnetization is significantly younger in age (500–600 Ma), may be, due to a loss of radiogenic argon caused by mica replacement. H2O, K, Ba, F and Cl were abundantly released during the replacement and supplied into the upper layers of the crust and mantle. The mantle high-potassium and high titanian Phl-Ilm series seems comagmatic with the surficial potassium ultramafites and mafites of the Siberian Platform and associated with the earlier episode of the Rodinia breakup.

The paper presents the first tectonophysical reconstruction of initial divisibility of the protolithosphere as a result of convection in the cooling primitive mantle. Initial division of the protolithosphere into separate masses, i.e. prototypes of the blocks, and their size are predetermined by the emerging Rayleigh-Benard convection cells. In studies of geology and geodynamics, the Rayleigh-Benard convection cells were first referred to as a factor to explain the formation of initial continental cores. Considering the Rayleigh-Benard cells and their structural relics can help clarify initial divisibility of the protolithosphere and the origin of the major lithospheric plates, i.e. prototypes of continents. In our opinion, the initial mega-scale block structure of the protolithosphere and the emerging lithosphere were predetermined by the Rayleigh-Benard cells as they were preserved in the emerging lithosphere and their lower boundaries corresponded to the core-mantle boundary, i.e. one of the major discontinuities of the planet. Our theoretical estimations are in good agreement with the number and sizes of the Earth's theorized first supercontinents, Vaalbara and Ur.

In our tectonophysical discussion of the formation of the lithospheric block structure, we analyze in detail the map of modern lithospheric plates [Bird, 2003] in combination with the materials from [Sherman et al., 2000]. In the hierarchy of the blocks comprising the contemporary lithosphere, which sizes are widely variable, two groups of blocks are clearly distinguished. The first group includes megablocks with the average geometric size above 6500 km. Their formation is related to convection in the Earth mantle at the present stage of the geodynamic evolution of the Earth, as well as at all the previous stages, including the earliest one, when the protolithosphere emerged. The second group includes medium-sized blocks with the average geometric size of less than 4500 km and those with minimum sizes, such as rock lumps. They reflect primarily the degradation of megablocks as a result of their destruction due to high stresses in excess of the tensile strength of the medium. This group may also include blocks which formation is related to convection in the upper mantle layer, asthenosphere. There are grounds to assume that through the vast intermediate interval of geologic time, including supercycles of Kenorlend, Rodin, and and partically Pangea, the formation of the large lithospheric blocks was controlled by convection, and later on, they were 'fragmented' under the physical laws of destruction of solid bodies. However, it is difficult to clearly distinguish between the processes that predetermine the hierarchy of formation of the block structures of various origins – sizes of ancient lithospheric blocks cannot be estimated unambiguously.

Thus, mantle convection is a genetic endogenous source of initial divisibility of the cooling upper cover of the Earth and megablock divisibility of the lithosphere in the subsequent and recent geodynamic development stages. At the present stage, regular patterns of the lithospheric block divisibility of various scales are observed at all the hierarchic levels. The areas of the lithospheric megaplates result from regular changes of convective processes in the mantle, which influenced the formation of plates and plate kinematics. Fragmentation of the megaplates into smaller ones is a result of destruction of the solid lithosphere under the physical laws of destruction of solid bodies under the impact of high stresses.