Baikal Rift System Research Articles

AMBIENT SEISMIC NOISE VARIATIONS BEFORE EARTHQUAKES IN THE BAIKAL RIFT SYSTEM

This paper proposes a method to observe the pre-earthquake features based on the variations in ambient noises using local moderate and strong seismic events as an example. Eight earthquakes in the Baikal rift system with energy classes K=10.1–15.9 were associated to a significant decrease in the level of ambient noises at epicentral distances from 3 to 81 km a few hours before the shock. The observed decrease in the ambient noise level can be classified as a short-term precursor of the medium consolidation 4–5 hours before the earthquake.

Earthquake Depth Frequency Distribution in the Baikal Rift System

Earthquake Depth Frequency Distribution in the Baikal Rift System

Fractal dimension and area of seismicity in the Baikal Rift System: Implications for modern geodynamics

The fractal geometry and extent of seismicity in the Baikal Rift System (BRS) are estimated from data on 52,700 instrumental events of MLH ≥ 2.5 magnitudes for fifty years (1964–2013). The seismic pattern is characterized by the box-counting Hausdorff dimension D0, multifractal spectra f(α), and surface area S of seismicity at three scales: the rift system as a whole, its three zones, and six subzones. The multifractal spectra record a self-similar hierarchical structure of the BRS seismicity pattern. The space and time variations in the fractal dimension (D0) and area of seismicity (S), which are mapped and plotted as a function of time, show good correlation. The two parameters depend on three related factors: progressive increase in the amount of instrumental data (dataset size), structure of seismogenic fault network, and geodynamic activity. They increase as ever more data appear with time and acquire high local values at increasing extent and density of quakes. Moreover, the obtained D0 estimates reflect statistical self-similarity of earthquake patterns being in the range ≈ 1.45–1.55 over most of BRS, except one zone and one subzone in the rift flanks. They are the highest in the southwest and the lowest in the northeast of the rift system (D0 ≈ 1.60 ± 0.02 and D0 ≈ 1.37 ± 0.02 respectively). This dissimilarity indicates that seismogenic faulting occurs by different mechanisms: distributed failure as a result of superposed global-scale collisional compression and regional rifting in the SW flank and quasi-linear rift propagation in the NE flank. In general, D0 decreases toward the northeastern part of the BRS, where the pattern of earthquakes becomes localized along lineaments instead of being distributed over an area. The space and time variations of D0 and S revealed in the earthquake data are consistent with the location and activity pulses of rifting attractors and provide a realistic explanation of BRS geodynamics and tectonophysics. The global lithospheric compression and the regional pulse-like activity of rifting attractors control the network of seismogenic faults which, in turn, govern the fractal geometry and 2D structure of seismicity in the region. The obtained results confirm the oscillatory dynamics of the regional seismicity at a decadal period correlated with activity pulses of rifting attractors. The oscillations stand out against the background of decreasing global low-frequency secular cycle of the BRS seismicity. The BRS lithospheric geodynamics fits the model of a nonlinear oscillator with dissipation. The suggested analysis of the fractal geometry and extent of seismicity as proxies of the faulting evolution provides insights into modern geodynamics of the Baikal Rift System and its constituents.

Lithotypes, Facies, and Sources of Sedimentary Rocks in the Barguzin Valley of the Baikal Rift System

Lithological, petrographic, and lithogeochemical data are given for sedimentary rocks of alluvial, lacustrine-marsh, and lacustrine facies in the Barguzin valley, uncovered by new boreholes. During petrographic studies of thin sections, the main lithotypes of stratons of rift basins were identified. According to the mineral composition of the rocks, petrochemical and microelement characteristics, a conclusion was made about the origin of sedimentary deposits as a result of erosion of the granite basement rocks.

207Pb–206Pb возраст источников позднекайнозойских вулканических пород коро-мантийного перехода в соотношении с возрастом офиолитов и древних блоков, экспонированных на поверхности коры: трансект Китой–Байдраг Байкало-Монгольского региона

The results of determinations of Pb isotope ratios in Late Cenozoic volcanic rocks of the southwestern part of the Baikal rift system along the Kitoi-Baidrag transect are presented. The obtained 207Pb–206Pb estimates of the restite material incubation time in the springs are in good agreement with the Precambrian dating of ensembles of ancient continental blocks and ophiolite belts exposed on the Earth's surface. The sources are distinguished along the transect: the ancient Gargan block of the Tuva-Mongolian microcontinent (massif) with the Hadean protolith and Archean-Neoproterozoic events; Khangai and, possibly, Dzhida-Khangai slabs, and the ancient Baidrag block, a fragment of the Tarbagatai-Dzabkhan microcontinent. According by the dating of the sources, the tectonosphere of the early and middle mantle geodynamic epochs in the Baikal-Mongolian region included 3 layers: lower (proto-mantle), middle (mantle evolved), and upper (crust-mantle transition, CMP). In the Khangai continental domain, the protoliths of a wide Precambrian age range of sources were generally modified about 660 Ma ago. At the end of the Neoproterozoic and in the Early-Middle Phanerozoic, the mantle stabilized against the background of transformations of the ILC zone. In the tectonosphere of the latest geodynamic stage, the processes of transformation of three layers became active again: the proto-mantle, the evolved mantle, and the IMF.

COMPARISON OF MODERN GEODYNAMIC MODES OF FRAMING OF THE EUROASIAN LITHOSPHERIC PLATE

The purpose of the work is to analyze the modern geodynamic mode of framing of the Eurasian lithospheric plate to determine the types of deformations in the structures of spreading, subduction and rift genesis and favorable conditions for oil and gas formation at certain stages of structure development. The authors have analyzed the Arctic spreading zone, the Kamchatka subduction zone, the southwestern part of Eurasia and the Baikal rift system. The author's development of calculation of slip vectors of rock masses in foci of earthquakes and methods of tensor analysis of mechanics of fractured media were used in the work. Differences and similarities between the Arctic spreading zone, the Kamchatka subduction zone, the southwestern part of Eurasia and the Baikal rift system are shown. All of the above structures have a complex layered-block structure. That is, in a layered-block medium in layers, and in individual blocks and in time, the stress-strain state of the lithosphere changes, which directly depends on the direction of the axes of compressive stresses

The propagation velocity of seismic activity migrating along the directions of the geodynamic forces prevailing in the northeastern Baikal rift system, Russia

The recent tectonic stress field in the northeastern Baikal rift system (BRS) corresponds to the crustal deformation field. The stress-strain state of the Earth’s crust determines the fault network geometry and spatiotemporal structure of the epicentral field characterized by many earthquake swarms and earthquake migrations in the study area. In order to study the seismic process dynamics in different directions of the crustal deformation, the spatiotemporal analysis of earthquake time series has been made over the 1964–2015 instrumental period. To determine the relationship between crustal stress and spatiotemporal features of the epicentral field the seismic data were projected along horizontal stress tensor axes σ3 and σ2, consistent with major directions of the crustal deformation, a strike of major rifting structures, and a general azimuth of active fault groups. The NE-SW direction along the intermediate horizontal stress axes and main faulted arears exhibits slow earthquake migrations up to 60 km long, propagating with a modal velocity of about 30 kilometers per year. The NW-SE direction along the principal horizontal stress axes, orthogonal to the main faulted areas, is characterized by shorter migration sequences of less duration, propagating with a higher velocity than sequences registered in the NE-SW. The difference between the migration dynamics in mutually orthogonal directions can be attributed to the fault network configuration and the differences in the deformation process.

Hydrocarbon status of soils in zones of active tectonic faults of Baikal rift

Some parameters of the hydrocarbon status of two soils of the Ulyunkhan depression (North-Western part of the Barguzin depression, Western Transbaikalia) within the Baikal rift system formed under the endogenous effect in the zone of hydrothermal waters active discharge, were studied. Soil samples were analyzed for the organic carbon, total content of hydrocarbons, concentrations of polyсyclic aromatic hydrocarbon compounds. The preliminary results of the study showed that total content of hydrocarbons in soils varied from 15 to 115 mg/kg and had no common profile distribution and evident correlation with other studied soil properties. Polycyclic aromatic hydrocarbons (PAHs) accumulation was marked within humus horizons and lower impregnated layers with high content of easily soluble salts that do not correspond to the total hydrocarbons amount. Total PAHs concentrations positively tightly correlated with the soil texture parameters (r = 0.89). Several typical features characterizing the HC state in the zone of active discharge of gas-hydrothermal fluids were revealed: increased HC content, the predominance of phenanthrene in the total PAHs, and the presence of a wide range of PAHs, including heavy ones, consisting of 4–6 rings as well as the high benz(a)pyrene concentration that was revealed in the gley alluvial soil.

Tectonic Deformations and Subsequent Seismic Events in the Southwestern Flank of the Baikal Rift System Based on GPS Data

Tectonic Deformations and Subsequent Seismic Events in the Southwestern Flank of the Baikal Rift System Based on GPS Data

Features of ground inflow testing in the areas of double porosity aquifer development

The purpose of the study is optimization of geological exploration at the groundwater deposits of the upper hydrodynamic zone formed in the fault-block structure conditions in Eastern Siberia. The authors analyze the structural and tectonic features of the areas under investigation, the results of areal geophysics studies, exploration drilling and the use of the author's methodology in carrying out of the ground inflow testing. The objects of research are groundwater deposits located in various tectonic structures of Eastern Siberia. The analysis of field study results involving researches of groundwater deposits located in various tectonic structures of Eastern Siberia made it possible to identify the double porosity in the aquifers of free water exchange in the region, which is due to the stresses arising during the inland Baikal rift system formation. As a result, the optimization of geological exploration work, methodology and processing methods of ground inflow testing in the areas of fault-block structures development were introduced on the basis of the features of structural-tectonic and hydrogeological conditions.

Geoinformational criteria for distinguishing the quasi-linear chains of earthquake migration in epicentral fields of complex geometry and variable density of shocks distribution

Within the framework of the basic model of migrating seismicity, the main geoinformation criteria for identifying quasi-linear chains of earthquakes in epicentral fields of complex geometry and variable density of shock distribution have been determined by numerical methods. The developed model is used to study the migrations of earthquake foci and identify zones of hidden seismically active faults: it provides statistical criteria for the presence of quasi-linear chains of seismicity migration
 and zones of hidden seismic faults in arrays of various volumes of seismological data as an excess at a given level of significance
 of the average numbers of chains of earthquakes, spatially temporal distribution of shocks. The established dependences of the average number of selected chains of random events on the sample size and site shape make it possible to reveal the presence of seismicity migration and zones of hidden faults under the given criteria of “significance”, “representativeness” and ISMA. The developed methods, implemented programs and patents lay the theoretical and practical basis for GIS technology for identifying hidden faults and studying the migrations of earthquake foci in the lithosphere of the Baikal rift system. Since the main laws governing the formation of the internal structure of continental fault zones and seismicity are determined by the fundamental properties of the progressive deformation of transgressive shearing realized within them and are uniform, the developed GIS technologies can be applied in other seismically active regions.

Tectonic Generation of Pseudotachylytes and Volcanic Rocks: Deep-Seated Magma Sources of Crust-Mantle Transition in the Baikal Rift System, Southern Siberia

Volcanic rocks from deep-seated sources of the crust-mantle transition (CMT) are geochemically distinguished from those of ocean island basalts (OIB). Here, we report geochemical data on tectonic pseudotachylytes from the Main Sayan Fault (MSF) and volcanic rocks from the Kamar-Stanovoy Zone of Hot Transtension (KSZHT) that represent the deep-seated CMT magmatic sources in the central part of the Baikal Rift System (BRS). The tectonic generation of the KSZHT magmas between 18.1 and 11.7 Ma is compared with present-day seismogenic deformations in the middle-upper crust of the South Baikal Basin and adjacent Tunka Valley, where strong earthquakes are distributed along the Main Sayan and Primorye sutures of the Siberian paleocontinent. From a detail seismic tomography model and geological evidence, we infer that the KSZHT crust–mantle magmatic processes were due to delamination and lamination of a thickened root part of the South Baikal Orogen existed in the Late Cretaceous and Paleogene. In addition, we identify similar deep-seated CMT sources for melts erupted in the past 17 Ma from a delaminated root part of the East Hangay Orogen and adjacent Orkhon-Selenga Saddle in the southwestern BRS. We suggest that both in the central and in the southwestern BRS, the deep-seated CMT magma sources designate cooperative pull-to-axis and convergent effects created in the Japan-Baikal Geodynamic Corridor and in the Indo-Asian interactional region, respectively.

Sedimentation and environmental dynamics of the Tunka rift valley (Baikal region) in the Late Pleistocene-Holocene based on the analysis of lithological and rock magnetic properties of the deposits from Upper Paleolithic sites

The Tunka depression is a valley in the Baikal rift system. It contains several Paleolithic archaeological sites, however, the number of finds varies sharply across the different sections of the valley. The purpose of this study is to reconstruct the natural environment and sedimentation conditions of Upper Paleolithic deposits in the Tunka valley and to explore the effects of these conditions on the prehistoric humans that peopled the landscape. A total of 923 samples from 6 section sites (with different geological structure, thickness of layers, and geomorphological position) were evaluated for grain size and rock magnetic properties. The age of all sections is supported by 14C and TL dates. Sedimentation at Belyi Yar 1 and Belyi Yar 2 is characterized by high deposition rates and a large influx of loose sedimentary material due to catastrophic floods that prevented soil formation and resulted in unsuitable conditions for the habitation of prehistoric humans. Sedimentation rates at the Shimki site (which has not yet yielded archaeological finds) were also high but the presence of paleosol horizons indicates milder environmental conditions. The numerous archaeological finds at the Tuyana site, located at the highest geographical altitude, are explained by the features of the sedimentary environment: low sedimentary input, well-developed buried soils and a lack of signs indicating catastrophic flood. The Zaktui site, with its lower altitude, is characterized by less suitable conditions for human habitation which also explains the restricted number of archaeological finds in the section. An intermediate position is occupied by the Slavin Yar section, where sedimentation rates were higher than at Zaktui or Tuyana and paleosol horizons are quite well developed, but poor archaeological finds are explained by the influence of catastrophic floods. The integration of the studied parameters helps to reconstruct different sedimentary conditions in the explored sites of the Tunka depression and to estimate their suitability for prehistoric human habitation.

Новейшая активизация шовной зоны Сибирского кратона под Южным Байкалом: от мел-палеогенового орогена к неоген-четвертичному рифту

From the analysis of spatial-temporal distribution of sediments in the south of East Siberia, we present evidence on two stages in the evolution of neotectonic structure: Cretaceous-Paleogene and Neogene-Quaternary. The former was exhibited by formation of the South Baikal orogen accompanied by flank development of the Predbaikal and Selenga-Vitim foredeeps, the latter by the subsidence of the South Baikal basin, its filling of sediments and water simultaneously with the development of the whole Baikal Rift System.

Структурное развитие центральной части Байкальской рифтовой системы: сходство и отличие Баргузинской и Тункинской долин

The central part of the Baikal Rift System is exhibited by the lacustrine South Baikal basin and land Barguzin and Tunka valleys. In the Late Cenozoic, the former valley developed consistently with the South Baikal basin as its distal extensional pattern, the latter underwent both extension and compression as an individual structure of the Hangay-Belaya orogenic zone.

THE CENOZOIC CRUSTAL STRESS STATE OF MONGOLIA ACCORDING TO GEOLOGICAL AND STRUCTURAL DATA (REVIEW)

This article gives a chronological review of the main published research results concerning the Cenozoic crustal stress-strain state in Mongolia and adjacent territories. The studies commenced in the southern Baikal rift zone in the 1970s and were extended further southwards to cover mobile regions neighbouring the Siberian platform. Geological, structural and morphostructural data were collected and analysed to define the crustal stress types and spatial characteristics. The authors have consolidated their reconstructions of the crustal stress-strain state of Mongolia, which were based on tectonic fracturing data and displacements along fractures in fault zones active in the Cenozoic. We consolidated a database of reconstructed stress tensors, which now contains more than 750+ solutions. The Late Cenozoic stress field was mapped. The map shows domains differing in types of the paleostress state of the crust. The reconstructions were compared to our calculations of the present-day crustal stress state, which were based on earthquake focal mechanisms, and to calculations by other authors. At the Late Cenozoic and current stages, the maximum horizontal compression axis (S Hmax ) has varying orientations, from submeridional (Western Mongolia) to NE and ENE (Eastern Mongolia). The role of compression increases from the northern domains, where the reconstructions show shear and transtension, to the southern domains with prevailing transpression and compression. Regular changes occur in the stress state and rupture parageneses along the largest latitudinal faults, North Khangai and Dolinoozersky; such changes are related to left-lateral strike-slip faulting. We analysed the sequence of the occurrence of stress fields differing in types and spatial characteristics, and revealed the main regularities in the evolution of the crustal stress-strain state in time. In the Cenozoic history of crust deformation in Mongolia, we can distinguish several episodes that differ in the dominant impacts of various tectonic force sources or combinations of such impacts. At the beginning of the Cenozoic, tectonic structures developed mainly under the influence of the interaction of East Asia and the Pacific Plate, which was manifested in the southeastern domains of the study area. The long-term SE-trending asthenospheric flow caused crustal stretching, which initiated the formation of tectonic structures comprising the Baikal rift system. Starting from the Pliocene, crustal stretching took place in combination with NNE compression caused by the India–Eurasia convergence. As a result, shearing occurred along the large faults. At this background, the Khangai and Khentei uplifts (including crust extension zones at their crests) are large structures that developed due to the dynamic effect of local mantle anomalies.

Cryoturbation versus tectonic deformation along the southern edge of the Tunka Basin (Baikal Rift System), Siberia: New insights from an integrated morphotectonic and stratigraphic study

The Tunka Basin is a broad, emerging basin situated between the Baikal Lake to the east and the the Hovsgol Lake to the west. The basin is bounded to the north and to the south by the Tunka and the Khamar-Daban mountain ranges, respectively. The Tunka normal fault, located at the southern foothills of the Tunka mountain range, is the main structure that controlled the development of the Tunka Basin during the Neogene. Paleoearthquake-surface ruptures attest of its present activity; and show that its western and eastern terminations are undergoing a tectonic inversion characterized by left-lateral-reverse deformations. The southern edge of the Tunka Basin is classically interpreted as being tectonically controlled. In this paper, we present the results of a geomorphological and stratigraphic analysis within its southwestern and southeastern parts suggesting that there is no active fault affecting the foothills of the Khamar-Daban mountain range. The different features observed in the Quaternary deposits are interpreted to be the result of periglacial processes induced by alternating episodes of permafrost aggradation and degradation during the Holocene. Our study concludes that the Khamar-Daban Range and the Tunka Basin are uplifting together, and that the Tunka and Mondy faults are the two main triggers of regional earthquakes.

Methodical approach to isolation of seismic activity migration episodes of the northeastern Baikal rift system (Russia)

Methodical approach to isolation of seismic activity migration episodes of the northeastern Baikal rift system (Russia)

Thermochronology of the Angara–Vitim Granitoid Batholith, Transbaikalia, Russia

We reconstructed the history of the Angara–Vitim batholith (AVB), characterized by the formation of huge volumes of granitic magma using thermochronological analysis (40Ar/39Ar, U/Pb) and math testing selected age milestones on the basis of the model crystallization differentiation and dynamics of heat and mass transfer in a magmatic chamber. The consolidation time of the granite melt is estimated to be in the range of 320–290 Ma. Events with age are distinguished based on the presence of isotope dating clusters T1 = 245 ± 1 Ma; T2 = 212 ± 1 Ma; T3 = 156 ± 1 Ma; T4 = 125 ± 2 Ma. The thermal history of samples corresponding to the modern erosion level immediately imposes strict restrictions on the history of consolidation and cooling for rocks corresponding to the deep (>20 km) levels of the magmatic chamber. Calculations show that the lifetime of the residual melt at the deep levels of the magmatic chamber of the AVB can reach 100 Ma. Events with age 245 ± 1, 212 ± 1, 156 ± 1, 125 ± 2 Ma reflect the gradual transformation of the “semi-frozen granite layer” and the discrete nature of its tectonic exposure to the upper level of the Earth’s crust under the influence of tangential elastic deformations caused by the pulsating manifestation of intraplate mantle magmatism within the Siberian platform and its folded frame. The final stage of AVB tectonic exposure to the Earth’s surface (as a solid body) occurred from 60 Ma to the present time, reflecting the process of origin and development of the Baikal rift system.

Условия формирования основных экзогенных процессов в юго-западной части Байкальской рифтовой системы (Россия) и восточной части Южно-Китайского блока (Китай)

The aim of the study is to analyze and compare the causes and conditions for the formation of the exogenous processes in the southwestern part of the Baikal rift system (Russia) and in the eastern part of the South China block (China) with the emphasis on the weathering, gravitational and surface-water processes. The Tunka rift zone is taken as the main site. It is compared with the eastern part of the South China block where the main observation points are associated with the Yangtze River delta and adjacent territories. The main research methods include field routes with visual examination of the observation points and measurement of certain characteristics and parameters, as well as selection and analysis of the published and archive literary sources. The comparison of the geodynamic processes emerging in the study territories has concluded that the dominant processes are weathering and gravitational processes, namely, landslide formation. The intensity of the processes is primarily determined by the sharply broken relief. The differences between the two study regions are determined by the climatic peculiarities including the precipitation level and temperature change, as well as by the seismicity, the presence of the glacier forms, and the landscape features. The combination of the above factors predetermines the wet landslide activity of the Tunka branch of the rift depressions vs. almost none of the kind in the Yangtze River delta. The analysis of the defined factors makes it possible to predict exogenous processes such as landslide and mudslide, and take timely measures on minimizing the negative impact of the above. The lack of the large scale maps showing the danger zones and adequate control of identifying the land areas that are knowingly in need of protection from the geodynamic processes’ impact, are increasingly leading to the extension of the danger zones and associated risk both in the eastern part of China and throughout the Baikal region.