This study addresses some fundamental problems of tectonics and geodynamics of the Central Asian folded belt, the largest tectonic structure in Eurasia. The article presents results of long-term researches conducted by scientists and geologists specialized in various disciplines which contribute to the knowledge of the origin and breakup of Rodinia and formation of the Paleo-Asian Ocean. It considers problems of development of the Central Asian folded belt that was formed in the area of the Paleo-Asian Ocean and describes its subduction magmatism and marginal-marine sedimentation, formation of island arcs, processes of exhumation of the oceanic crust and formation of high-pressure blueschist and eclogite-blueschist complexes. Outstanding fundamental issues of the geodynamic evolution of the Central Asian folded belt are noted. 

The Early Paleozoic collisional system located in the Olkhon region at the western shores of Lake Baikal resulted from collision of the Siberian paleocontinent and a complex aggregate composed by fragments of a microcontinent, island arcs, back-arc structures and accretionary prisms. The main events were associated with complete manifestation of shear tectogenesis initiated by oblique collision. The current structure includes tectonically displaced components of ancient geodynamic systems that used to have been located dozens and hundreds of kilometres apart. Horizontal amplitudes of tectonic displacement seem to have been quite high; however, numerical data are still lacking to support this conclusion. Information about the structure of the upper crust in the Paleozoic is also lacking as only deep metamorphic rocks (varying from epidote-amphibolite to granulite facies) are currently outcropped. Formations comprising the collisional collage are significantly different in composition and protoliths, and combinations of numerous shifted beds give evidence of a 'bulldozer' effect caused by the collisional shock followed by movements of crushed components of the ocean-continent zone along the margin of the Siberian paleocontinent. As evidenced by the recent cross-section, deep horizons of the Early Paleozoic crust comprise the collisional system between the Siberian craton and the Olkhon composite terrain.

 A permanent inclusion in the collisional combinations of rocks are unusual synmetamorphic injected bodies of carbonate rocks. Such rocks comprise two groups, marble melanges and crustal carbonate melted rocks. Obviously, carbonate rocks (that composed the original layers and horizons of stratified beds) can become less viscous to a certain degree at some locations during the process of oblique collision and acquire unusual properties and can thus intrude into the surrounding rocks of silicate composition. Such carbonate rocks behave as protrusions or intrusions and contain inclusions of silicate rocks. Formation of marble melanges is a multi-staged process: they occur as early tectonic covers and, more often, accompany shear zones of large lengths, comprise late push-out nappes initiated by shear faults, participate in construction of ring and vortex structures that are generated by shearing in the geological medium of inhomogeneous rheology. In general, the available data give evidence of the fact that formation of synmetamorphic marble melanges is a direct consequence of the oblique collision geodynamics and a sensitive indicator of such a regime. A pure guesswork may suggest that the occurrence of the marble melanges can be associated with a catastrophic loss of viscosity of the carbonate rocks due to a sharp increase of velocities of shear deformations that accompanied the oblique collision.

 

The paper provides a review of early stages of development the Solar System and the geological history of Earth with reference to the latest data on the origin of the Solar System and the formation of the first continental rocks and results of studies of zircon, the oldest mineral so far dated on Earth. The formation of the Solar System from a gas-and-dust nebula is estimated to have begun 4.568 billion years ago. Ice was formed 1.5 million years later; it concentrated at the periphery of the system and served as the material for the largest planets, Jupiter and Saturn. In the central areas of the system, asteroids with diameters of about 10 km were formed. Their small bodies were composed of the basic material of the solar nebula, as evidenced by carbonaceous chondrite, CI, which composition is similar to the composition of the Sun, with the exception of hydrogen, helium, and volatile components that served as the main material for peripheral planets of the Solar System. Due to collision and partial merger of such small bodies, the formation of embryos of the terrestrial planets was initiated. Gravity made such embryos to cluster into larger bodies. After 7 million years, large asteroids and planet Mars were formed. It took 11 million years to form Planet Earth with a mass of 63 %, and 30 million years to form 93 % of its mass. Almost from the beginning of the formation of the Earth, short-lived radionuclides, 26Al and 60Fe, caused warming up of the small planetary bodies which led to the formation of their cores. During the initial stages, small magma reservoirs were formed, and molten iron particles gathered in the centres of the planetary bodies. As suggested by the ratio of 182W/184W, the major part of the core was formed within 20 million years, while its full mass accumulated completely within the next 50 million years. In 30–40 million years after the creation of the Solar System, the Earth collided with a cosmic body which mass was close to the mass of Mars, and it was the beginning of the formation of its satellite, Moon. When the Earth – Moon system was subject to the major meteorite bombardment 4.5–4.1 billion years ago, zircons formed due to impact activity on the Moon, and on Earth it caused major eruption of basaltic magma, the differentiation of which led to the formation of acid magmas in small amounts which caused the formation of zircons, the only remnants of the first continental rocks of Earth. Later on, meteorite bombardment continued and contributed to the burial of the first continental acid and basic rocks in the mantle, wherein such rocks eventually became part of the mantle which became the progenitor of granite-greenstone rock associations. Grey gneisses contain traces of the first continental formations of Earth as evidenced by ancient zircons that are ubiquitous in the crust of Earth. The above is confirmed by results of detailed studies of regional zircons with the use of modern analytical equipment providing for high accuracy of local analyses.

 

The terrain analysis concept envisages primarily a possibility of approximation of fragments / terrains of various geodynamic settings which belong to different plates. The terrain analysis can supplement the theory of plate tectonics in solving problems of geodynamics and tectonics of regions of the crust with complex structures. The Central Asian belt is among such complicated regions. Terrain structures occurred as a result of combined movements in the system of 'frontal' and/or oblique subduction – collision. In studies of geological objects, it is required first of all to prove their (vertical and horizontal) autochthony in relations to each other and then proceed to paleogeodynamic, paleotectonic and paleogeographic reconstructions. Obviously, such a complex approach needs data to be obtained by a variety of research methods, including those applied to study geologic structures, stratigraphy, paleontology, paleogeography, lithothlogy, geochemistry, geochronology, paleomagnetism etc. Only by correlating such data collected from inter-disciplinary studies of the regions, it is possible to establish reliable characteristics of the geological settings and avoid mistakes and misinterpretations that may be associated with the 'stratigraphic' approach to solutions of both regional and global problems of geodynamics and tectonics of folded areas. The terrain analysis of the Central Asian folded belt suggests that its tectonic structure combines marginal continental rock complexes that were formed by the evolution of two major oceanic plates. One of them is the plate of the Paleo-Asian Ocean. As the analogue of the current Indo-Atlantic segment of Earth, it is characterised by the presence of continental blocks in the composition of the oceanic crust and the formation of oceanic basins resulting from the breakup of Rodinia and Gondvana. In the course of its evolution, super-continents disintegrated, and the blocks were reunited into the Kazakhstan-Baikal continent. The base of the Kazakhstan-Baikal continent was formed in the Vend-Cambrian due to subduction of the oceanic crust of the Paleo-Asian Ocean, including the Precambrian microcontinents and terrains of the Gondvana group, underneath the south-eastern margin of the Siberian continent (in the current coordinates). Due to subduction followed by collision of the microcontinents with the Kazakhstan-Tuva-Mongolia island arc, the crust had consolidated, and a complex continent was formed. Another major plate is the plate of the Paleo-Pacific Ocean. It is characterized by the long-term tectono-magmatic evolution without any involvement of the continental crust and by complex processes of the formation of the continental margins. Its evolution resulted in the formation of the Vend-Paleozoic continental margin complexes of the western segment of the Siberian continent which comprise the Vend-Cambrian Kuznetsk-Altai island arc and a complex of rocks of the Ordovic-Early Devonian passive margin and the Devon-Early Carbonic active margin. In the accretional wedges of the Kuznetsk-Altai island arc, abundant are only fragments of the Vend-Early Cambrian oceanic crust including ophiolites and paleo-oceanic uplifts. The contemporary analogue of the Central Asian folded belt is the south-eastern margin of Asia, represented by the junction area of the Indo-Australian and Pacific plates.

 

 

The article describes tectonics and the deep structure of the lithosphere in the eastern fragment of the Mongol-Okhotsk orogenic belt according to results of analysis of the geological and geophysical database. With account of palinspastic reconstructions, the model is developed to show the most probable paleogeodynamics of the lithosphere in the Mongol-Okhotsk orogenic belt and its heterochronous terrains.

This article reviews data on ages of rocks in the footwall of the Butuliyn-Nur and Zagan metamorphic core complexes (MCC) and provides new data on the geochemistry of the rock complexes. It is noted that the oldest rocks are mylonitized gneisses on rhyolites (554 Ma) in the footwall of the Butuliyn-Nur MCC. The Late Permian – Triassic (249–211 Ma) igneous rocks are ubiquitous in the footwall of the Butuliyn-Nur and Zagan MCC. The youngest rocks in the studied MCC are the Jurassic granitoids (178–152 Ma) of the Naushki and Verhnemangirtui massifs. In the footwall of the Butuliyn-Nur and Zagan MCC, the most common are granitoids and felsic volcanic rocks (249–211 Ma) with many similar geochemical characteristics, such as high alkalinity, high contents of Sr and Ba, moderate and low concentrations of Nb and Y. Considering the contents of trace elements and REE, the granitoids and the felsic volcanic rocks are similar to I-type granites. Specific compositions of these rocks suggest that they might have formed in conditions of the active continental margin of the Siberian continent over the subducting oceanic plate of the Mongol-Okhotsk Ocean. The granitoids of the Naushki and Verhnemangirtui massifs, which are the youngest of the studied rocks (178–152 Ma), also have similar geochemical characteristics. In both massif, granitoids are ferriferous, mostly alkaline rocks. By contents of both major and trace elements, they are comparable to A-type granites. Such granitoids formed in conditions of intracontinental extension while subduction was replaced by collision. Based on ages and geochemical characteristics of the rocks in the footwall of the Butuliyn-Nur and Zagan MCC, a good correlation is revealed between the studied rocks  and the rock complexes of the Transbaikalian and North-Mongolian segments of the Central Asian fold belt (CAFB), and it can thus be suggested that the regions under study may have a common evolutionary history.

Many mobile belts contain narrow (a few kilometres wide) and elongated (several hundreds and thousands kilometres long) zones characterized by complicated tectonic interiors and intensive rock metamorphism. Such zones are important components of the crustal structure; they determine the internal divisibility of the crust and act as accommodation areas or buffers between fragments of the crust which differ in origin and have specific evolution features. The article describes the paleo-evolution and characteristics of the Pieniny klippen belt, a major tectonic structure of the Carpathians and one of the largest linear tectonic zones in the Alpine-Himalayan folded belt.

The article presents new data on the deep crustal structure, origin and evolution of the Bryansk-Kursk-Voronezh orogen in the south-eastern segment of the East European craton; it is composed of the Paleoproterozoic formations and areas of reworked Archean crust. The purpose of this paper is the development and improvement of ideas on intra-continental orogens. The deep structure of the orogen is presented by the 3D model based on results of geological mapping of the Precambrian basement and interpretations of geophysical fields and seismic images of the crust along geotraverse 1-EB and profile DOBRE. It is established that the orogen originated with riftogenic extension of the crust at 2.6–2.5 Ga, that was repeated at 2.2–2.1 Ga, and formation of wide depressions that were efficiently filled in with volcanosedimentary layers including BIF, which accumulation was followed by high-temperature up to granulite facies metamorphism. Suprasubduction magmatism took place at 2.10–2.08 Ga and resulted in formation of the Lipetsk-Losevka volcano-plutonic complex. The active margin was completely formed at about 2.05 Ga. The short duration of subduction-related magmatic activity and the lack of relics of the oceanic lithosphere suggest short-term and spatially limited developing of the oceanic structure. The tectonothermal activity of collisional and postcollision stages was expressed in emplacement of alkaline ultramafic (2.1–2.0 Ga) and gabbro-syenite (1.8–1.7 Ga) complexes. It is difficult or impossible to explain specific features of the structure and evolution of the orogen in framework of the model of the Cordilleras type  accretionary orogen. Mafic-ultramafic magmatism and indications of intensive heating of the crust suggest a special role of plume type processes that provided for influx of mantle heat and juvenile mantle derived matter.

 

Metamorphosed volcanic rocks of the Ushmukan suite were studied in the Mukodek gold-ore field located in the Baikal-Muya belt in the Northern Baikal area, Russia. The Ushmukan suite shows interleaving of ortoschists which compositions are widely variable. Basalt-andesite-dacite series of normal alkalinity are the substrate of the studied metavolcanic rocks. Based on the set of geochemical characteristics, it is concluded that the rocks were formed in suprasubduction geodynamic conditions corresponding to a mature island arc. The proximity of the geological locations and the similarity of the geochemical characteristics of the volcanic rocks of the Ushmukan suite and rocks of the Kelyan suite (Neoproterozoic, 823 Ma), which have similar compositions, give grounds to consider these two rock suites as age peers. Specific features of gold distribution through the Mukodek gold-ore field are analyzed. Industrial gold contents are recorded only in berezite-listvenite metasomatic rocks of the gold-quartz-sulfide formation which were formed on metavolcanic rocks of the Ushmukan suite. It is concluded that the volcanic rocks, which are specific of the island-arc setting, could be a source of gold for deposits in the Mukodek gold-ore field.

 

The article presents results of specialized processing of the deep seismic profile along a part of Reference Profile 3-DV which crosses the Aldan-Stanovoi shield in the meridian direction and goes across its buried northern slope. The study is aimed at determining frequency-energy characteristics of the seismic wave field which are related to physical conditions of geological features of the crust. Based on analysis and interpretation of the dynamic profiles, it is possible to reveal and contour the Archean cores of consolidation of the Aldan shield and its buried continuation that is covered by sediments of the Middle Lena monocline and to input new facts in the proposed geodynamic model showing formation of the crust in the south-eastern segment of the North Asian craton.

The Baikal rift is characterized by high heat flow, seismic activity and large thickness of sediments through which gas and hydrothermal water are intensely released into the lake water. In the area of the southern Baikal at the beginning of the 20th century, 'water pillars' were observed to reach almost a dozen metres height when earthquakes took place. This suggests potential uplift of significant amounts of gas from the lake's bottom into the atmosphere and confirms a relationship between seismicity and methane emissions. Moreover, strong emissions of gas take place in many regions of Lake Baikal, and when the lake is covered by ice, such phenomena can cause the occurrence of spots with melted ice wherein an effect of water 'boiling' is observed. In recent international space studies of the surface of ice covering Lake Baikal in spring periods, mysterious rings of 5 to 7 km in diameter were discovered. Causes and mechanisms of their occurrence have not been studied in detail yet. It is established that a ring-shaped structure results from an uplift of deep water which causes clock-wise vortex flows. Uplifting of deep water can accompany emissions of significant amounts of methane from sediments, activation of thermal vents or gas-water-mud volcanoes at the bottom of Lake Baikal. In order to reveal causes and conditions of the above-described phenomena, the author designed a 3D model of heat-and-mass transfer in viscous medium and used it for numerical simulations. Based on the obtained results, it is established that a ring-shaped structure is formed on ice of the lake by a toroid-shaped ascending convective flow that occurs due to horizontal and vertical gradients of density, and the central part of such a flow rotates in the counter-clockwise direction (i.e. cyclonic vortex), while its periphery parts rotate in the clockwise direction (i.e. anticyclonic vortex).  Both hydrothermal vents and gas emissions can produce such ascending flows. Spots with melted ice can be formed when the temperature of hydrothermal vents amounts to 30–50 °С; such a melted-ice spot can stay open as long as the hydrothermal vent is active. With an assumption of 100 % concentration of gas in the source, the numerical simulation shows that during gas release into the atmosphere, a gas pillar can reach a height of 15 metres if the source of gas is active for a period no shorter than the time required for the gas flow to ascend through the water layer and to release into the air above the water surface. An area, wherein gas is released in bubbles, can be formed in case of lasting activity of a gas source wherein the volume of gas varies from 1 % to 20 % (i.e. gas-water mix).

 

On 28 June 2014, Arshan village and its heath resort facilities suffered from a shock descent of mudflows from Tunka Goltsy in the Republic of Buryatia, Russia. In this region, the last mudflow descent was recorded in 1971. The article provides an overview of the local natural environment of the site where the mudflows descended and the Kyngarga river flooding took place. Preliminary recommendations are given to ensure protection of the local population and regional infrastructure facilities from natural hazardous phenomena that are typical of sides of riftogenic basins located in the Baikal region.

 

The 2008 Wenchuan MS8.0 earthquake occurred on Longmenshan fault zone (LMSF), which is at the eastern margin of the Tibetan plateau. The epicenter is near the Shuimogou earthquake swarm, which was thought to be triggered by the Zipingpu reservoir after its impounding in 2004. People have speculated that the large earthquake was triggered by the water filling of the reservoir. To figure out the role of the Zipingpu reservoir on the earthquake, the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008 were analyzed in detail. The distribution of hypocenters showed that most earthquakes occurred on Yingxiu-Beichuan fault (YBF) in the reservoir area with hypocenters depth less than 10 km, which is a major source fault of the Wenchuan earthquake. Useful information on fault geometry in the depth was also obtained. The spatial-temporal distribution of hypocenters demonstrated clear migration pattern that indicated pore-pressure diffusion, it also showed a hydraulic diffusivity (D) of 0.7 m²/s. Previous experiments show the existence of the synergism process of the fault under a meta-instability state before fault sliding. It enhances the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. According to this view, the pore pressure diffusion by water filling of Zipingpu reservoir increased the total length of weak portions and enhanced the stress at the focal.

The article is devoted to the 60th Jubilee of Evgeny V. Sklyarov, a recognized expert in a wide variety of igneous and metamorphic processes and geodynamics. He is a Corresponding Member of the Russian Academy of Sciences, Professor, Doctor of Geology and Mineralogy, Chief Researcher of Laboratory of Paleogeodynamics in IEC SB RAS, and Chief Editor of Geodynamics and Tectonophysics.