Pub Date : 2021-10-19DOI: 10.5800/gt-2021-12-3s-0549
I. Buddo, I. Shelokhov, N. V. Misyurkeeva, Y. Agafonov
{"title":"TRANSIENT ELECTROMAGNETIC SOUNDING IN 2D, 3D, AND 4D MODES: SEQUENCE OF GEOLOGICAL EXPLORATION ACTIVITIES","authors":"I. Buddo, I. Shelokhov, N. V. Misyurkeeva, Y. Agafonov","doi":"10.5800/gt-2021-12-3s-0549","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3s-0549","url":null,"abstract":"","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80795892","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0543
А. Pospeev
The article is focused on the role of natural hydrogen in the Earth geodynamics and energy potential. With a proper consideration of the physical parameters of the Earth’s core and mantle, we discuss the aspects of the Hydridic Earth (or Primordially Hydrogen-Rich Planet) theory, which is currently used as a fundamental hypothesis in modern projects aimed at hydrogen energetics.A probability of finding natural hydrogen deposits in sedimentary traps is estimated. It is shown that the volume of deep degassing of hydrogen can be calculated from various cosmological, petrophysical and geochemical data, and an average volume is two orders of magnitude less than the amount predicted by the Hydridic Earth hypothesis. This hypothesis gives grounds to conclude that the major part of Earth’s mantle is a metal sphere; however, this conclusion is not supported by the geological and geophysical data.
{"title":"EARTH AS PRIMORDIALLY HYDROGEN-RICH PLANET: HYPOTHESIS AND REALITY","authors":"А. Pospeev","doi":"10.5800/gt-2021-12-3-0543","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0543","url":null,"abstract":"The article is focused on the role of natural hydrogen in the Earth geodynamics and energy potential. With a proper consideration of the physical parameters of the Earth’s core and mantle, we discuss the aspects of the Hydridic Earth (or Primordially Hydrogen-Rich Planet) theory, which is currently used as a fundamental hypothesis in modern projects aimed at hydrogen energetics.A probability of finding natural hydrogen deposits in sedimentary traps is estimated. It is shown that the volume of deep degassing of hydrogen can be calculated from various cosmological, petrophysical and geochemical data, and an average volume is two orders of magnitude less than the amount predicted by the Hydridic Earth hypothesis. This hypothesis gives grounds to conclude that the major part of Earth’s mantle is a metal sphere; however, this conclusion is not supported by the geological and geophysical data.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79581435","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0536
A. Lermontova
Using the analytical approximation method, we calculated stress field parameters for cases with different relative positions of Riedel shears and loads required for shearing. Considering an internal friction angle of 30°, and the distance between adjacent shears exceeding 0.7 of the characteristic shear length, we estimated the Coulomb stress that can lead to fracturing. In the areas between the shears, it is below the shear strength value. This means that if an increase in the external load is lacking, there are no prerequisites for the formation of new fractures that may connect adjacent shears. If the shears are spaced closer to each other (i.e. at distances less than 0.7 of the shear length), the shear strength is exceeded in the areas between them, and new shears can occur there and connect the Riedel shears to each other. Therefore, in observations of a natural system of Riedel shears, it becomes possible to assess whether this system is sufficiently stable in its current status, or, in case of a critical increase in the Coulomb stress in the areas between adjacent shears, the equilibrium can be easily disturbed, and there is a possibility that the main fault forms in the strike-slip zone under study.
{"title":"STRESS FIELD IN A SHEAR ZONE, AND FORMATION OF THE MAIN FAULT","authors":"A. Lermontova","doi":"10.5800/gt-2021-12-3-0536","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0536","url":null,"abstract":"Using the analytical approximation method, we calculated stress field parameters for cases with different relative positions of Riedel shears and loads required for shearing. Considering an internal friction angle of 30°, and the distance between adjacent shears exceeding 0.7 of the characteristic shear length, we estimated the Coulomb stress that can lead to fracturing. In the areas between the shears, it is below the shear strength value. This means that if an increase in the external load is lacking, there are no prerequisites for the formation of new fractures that may connect adjacent shears. If the shears are spaced closer to each other (i.e. at distances less than 0.7 of the shear length), the shear strength is exceeded in the areas between them, and new shears can occur there and connect the Riedel shears to each other. Therefore, in observations of a natural system of Riedel shears, it becomes possible to assess whether this system is sufficiently stable in its current status, or, in case of a critical increase in the Coulomb stress in the areas between adjacent shears, the equilibrium can be easily disturbed, and there is a possibility that the main fault forms in the strike-slip zone under study.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90687700","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0537
A. Rybin, E. Bataleva, K. Nepeina, V. E. Matyukov
This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relati
{"title":"VOLUMETRIC AND SPATIAL SEGMENTATION OF THE TIEN SHAN LITHOSPHERE ACCORDING TO GEOPHYSICAL DATA","authors":"A. Rybin, E. Bataleva, K. Nepeina, V. E. Matyukov","doi":"10.5800/gt-2021-12-3-0537","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0537","url":null,"abstract":"This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relati","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85487587","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0540
N. Kuznetsov, A. Baluev, E. N. Terekhov, S. Kolodyazhnyi, E. S. Przhiyalgovskii, T. Romanyuk, A. S. Dubensky, V. Sheshukov, S. Lyapunov, T. Bayanova, P. Serov
Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations.
{"title":"TIME CONSTRAINTS ON THE FORMATION OF THE KANDALAKSHA AND KERETSK GRABENS OF THE WHITE SEA PALEO-RIFT SYSTEM FROM NEW ISOTOPIC GEOCHRONOLOGICAL DATA","authors":"N. Kuznetsov, A. Baluev, E. N. Terekhov, S. Kolodyazhnyi, E. S. Przhiyalgovskii, T. Romanyuk, A. S. Dubensky, V. Sheshukov, S. Lyapunov, T. Bayanova, P. Serov","doi":"10.5800/gt-2021-12-3-0540","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0540","url":null,"abstract":"Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90062663","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0532
T. L. Ibragimova, R. Ibragimov, M. Mirzaev, Y. Rebetsky
The current stress of Earth's crust in the territory of Uzbekistan has been studied using a focal earthquake mechanisms catalogue that includes the data provided by many authors. Stress reconstructions are based on the cataclastic analysis of displacements along fractures. For reconstructing the stress state at different depths of the crust in several seismically active regions of the study area, we consider a minimum number of earthquakes in a homogeneous sample equal to 6 and an averaging radius of 10 to 30 km within a single domain. The azimuths and dip angles of the principal stress axes, Lode – Nadai coefficients, geodynamic types of stress modes, relative (normalized to rock strength) values of maximum shear stresses, and effective pressure values are determined. Maps showing the spatial distribution of the studied parameters are constructed for both the entire seismically active layer and the depth layers. Stress fields are reconstructed and compared at two hierarchical levels based on the parameters of focal mechanisms of weak and moderate earthquakes (М≤4.5) and those of strong (М≥5.0) earthquakes. "Tectonic Stresses of Eurasia", the Internet resource created by IPE RAS, is used to visualize the stress field reconstructed from the data on strong (М≥5.0) earthquakes.
{"title":"THE CURRENT STRESS OF EARTH'S CRUST IN THE TERRITORY OF UZBEKISTAN ACCORDING TO FOCAL EARTHQUAKE MECHANISMS","authors":"T. L. Ibragimova, R. Ibragimov, M. Mirzaev, Y. Rebetsky","doi":"10.5800/gt-2021-12-3-0532","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0532","url":null,"abstract":"The current stress of Earth's crust in the territory of Uzbekistan has been studied using a focal earthquake mechanisms catalogue that includes the data provided by many authors. Stress reconstructions are based on the cataclastic analysis of displacements along fractures. For reconstructing the stress state at different depths of the crust in several seismically active regions of the study area, we consider a minimum number of earthquakes in a homogeneous sample equal to 6 and an averaging radius of 10 to 30 km within a single domain. The azimuths and dip angles of the principal stress axes, Lode – Nadai coefficients, geodynamic types of stress modes, relative (normalized to rock strength) values of maximum shear stresses, and effective pressure values are determined. Maps showing the spatial distribution of the studied parameters are constructed for both the entire seismically active layer and the depth layers. Stress fields are reconstructed and compared at two hierarchical levels based on the parameters of focal mechanisms of weak and moderate earthquakes (М≤4.5) and those of strong (М≥5.0) earthquakes. \"Tectonic Stresses of Eurasia\", the Internet resource created by IPE RAS, is used to visualize the stress field reconstructed from the data on strong (М≥5.0) earthquakes.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76671496","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0535
B. V. Lunev, V. Lapkovsky
We propose a model of decompression melting, separation, migration and freezing of the melt in the upper mantle during the convective instability process. The model takes into account differences between phase diagrams of the melt and the matrix and the resultant features of the melt’s behavior, without calculating reaction rates in a multicomponent medium. It is constructed under an explicit concept of the local thermodynamic equilibrium of the existing phases. Therefore, we further develop the first approximation of the descriptions of convection in the upper mantle and the formation of large epicontinental sedimentary basins, which have been presented in earlier publications. Our computational experiments show that primary melting of the upper mantle’s fertile material occurs intensively in a narrow frontal part of the ascending hot material flow. Then, the depleted and partially melted material rises farther upward from the front of primary melting. Melting of the depleted material continues at lower pressures in a rather wide range of depths (120–77 km). Further, the migrating melt is supplied by two sources, i.e. a deep-seated one, wherein the fertile material melts, and the medium-depth one, wherein melting of the depleted material takes place. Once the temperature and pressure rates of the melt reach the values corresponding to those of its solidus, a narrow freezing front is formed. Its width is almost similar to the primary melting front. As the ascending convective flow develops, the freezing front shifts upward. As a result, a quite thick (around 40–50 km) basalt-saturated layer occurs above the freezing front. An important observation in our modeling experiments is that, despite a considerably large total volume of the melted material, a one-time melt content in the mantle does not exceed tenths of one percent, when we consider averaging to volumes with a linear size of about 1.0 km. The basalt melt extraction depletes iron in the mantle and significantly reduces the mantle density. Considering the calculated basalt-depletion values for the matrix at 0.1–0.2, the density deficit doubles in comparison to the thermal expansion of the material. Logically, both the Rayleigh number and the intensity of convection also double (and this is confirmed by the calculations), which means that convection is enhanced after the melting start.Testing of the model shows that it gives a reasonable picture that is consistent with the available geological and geophysical data on the structure of the lithosphere underneath the currently developing epicontinental sedimentary basins. Furthermore, within the limits of its detail, this model is consistent with the results of modeling experiments focused on melting and melting dynamics, which are based on calculations of reactions between components of the mantle material.
{"title":"MODEL OF DECOMPRESSION MELTING MECHANISM IN CONVECTIVE-UNSTABLE THERMAL LITHOSPHERE (FIRST APPROXIMATION)","authors":"B. V. Lunev, V. Lapkovsky","doi":"10.5800/gt-2021-12-3-0535","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0535","url":null,"abstract":"We propose a model of decompression melting, separation, migration and freezing of the melt in the upper mantle during the convective instability process. The model takes into account differences between phase diagrams of the melt and the matrix and the resultant features of the melt’s behavior, without calculating reaction rates in a multicomponent medium. It is constructed under an explicit concept of the local thermodynamic equilibrium of the existing phases. Therefore, we further develop the first approximation of the descriptions of convection in the upper mantle and the formation of large epicontinental sedimentary basins, which have been presented in earlier publications. Our computational experiments show that primary melting of the upper mantle’s fertile material occurs intensively in a narrow frontal part of the ascending hot material flow. Then, the depleted and partially melted material rises farther upward from the front of primary melting. Melting of the depleted material continues at lower pressures in a rather wide range of depths (120–77 km). Further, the migrating melt is supplied by two sources, i.e. a deep-seated one, wherein the fertile material melts, and the medium-depth one, wherein melting of the depleted material takes place. Once the temperature and pressure rates of the melt reach the values corresponding to those of its solidus, a narrow freezing front is formed. Its width is almost similar to the primary melting front. As the ascending convective flow develops, the freezing front shifts upward. As a result, a quite thick (around 40–50 km) basalt-saturated layer occurs above the freezing front. An important observation in our modeling experiments is that, despite a considerably large total volume of the melted material, a one-time melt content in the mantle does not exceed tenths of one percent, when we consider averaging to volumes with a linear size of about 1.0 km. The basalt melt extraction depletes iron in the mantle and significantly reduces the mantle density. Considering the calculated basalt-depletion values for the matrix at 0.1–0.2, the density deficit doubles in comparison to the thermal expansion of the material. Logically, both the Rayleigh number and the intensity of convection also double (and this is confirmed by the calculations), which means that convection is enhanced after the melting start.Testing of the model shows that it gives a reasonable picture that is consistent with the available geological and geophysical data on the structure of the lithosphere underneath the currently developing epicontinental sedimentary basins. Furthermore, within the limits of its detail, this model is consistent with the results of modeling experiments focused on melting and melting dynamics, which are based on calculations of reactions between components of the mantle material.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79856746","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0542
Z. L. Motova, A. Plyusnin, E. V. Nikulin
We have studied terrigenous-carbonate rocks in the area near the Sayan mountains in the Irkutsk Region (Russia), specifically at the Shaman Cliff, being the stratotype area of rocks that belong to the Moty group. The cliff’s lower part is composed of sandstones, which fragments gradually decrease in size upward the cross-section. The middle and upper parts are composed of sandy dolomites and dolomites, respectively. In terms of material characteristics, the terrigenous rocks correspond to arkoses. According to the genetic typification, the arkoses are composed of destructed primary igneous rocks. The terrigenous-carbonate rocks contain a carbonate component that gradually increases in the upper part of the cross-section. In the Shaman Cliff cross-section, we distinguish 32 lithological units and eight lithologicalgenetic types of deposits. Paleogeodynamic conditions are reconstructed for the formation of the sedimentation basin. Our study of the Shaman formation reveals specific features of the lithological facies, which suggest that these rocks accumulated in a coastal environment affected by tides. In the study area, clastic materials were mainly removed from an orogen that formed due to the Vendian accretion-collision events in the southern folded frame of the Siberian platform. Dolomites composing the upper part of the cliff are attributed to the Irkutsk formation of the Moty group. Their lithological features give evidence of shallow-marine conditions of their formation, without any supply of clastic material, which contributed to mass dispersal of the Cambrian biota described in [Marusin et al., 2021]. It is our first initiative to draw a boundary between the Shaman and Irkutsk formations of the Moty Group along the base of the carbonate eluvial breccia unit that marks the stratigraphic break. In the cross-section, this boundary represents the border between the Upper Vendian and Lower Cambrian.Our conclusions are generally consistent with the ideas of most researchers about the Late Vendian evolution of the southern margin of the Siberian platform. The results of our study can be used in further investigation of this area and provide a basis for correlating the studied strata with the same-age reference cross-sections of other regions in Siberia.
我们研究了俄罗斯伊尔库茨克地区萨彦山附近的陆源碳酸盐岩，特别是在萨满悬崖，这是属于Moty群的岩石的层型区域。崖体下部为砂岩，沿断面向上碎块逐渐变小。中部和上部分别由砂质白云岩和白云岩组成。从物质特征上看，陆源岩与长石相对应。根据成因类型，该岩系由破坏的原生火成岩组成。陆源-碳酸盐岩中含有碳酸盐岩成分，在剖面上部逐渐增加。在萨满崖剖面上，划分出32个岩性单元和8种岩性成因类型。重建了沉积盆地形成的古地球动力学条件。我们对萨满组的研究揭示了岩石相的特定特征，表明这些岩石是在受潮汐影响的沿海环境中积累的。研究区碎屑物质主要来自于西伯利亚地台南褶皱架文迪期吸积碰撞事件形成的造山带。构成悬崖上部的白云岩属于莫蒂群的伊尔库茨克地层。它们的岩性特征证明了它们形成时的浅海条件，没有任何碎屑物质的供应，这导致了[Marusin et al.， 2021]中描述的寒武纪生物群的大量扩散。这是我们第一次沿着碳酸盐岩残积角砾岩单元的底部，在Moty群的萨满和伊尔库茨克组之间绘制边界，标志着地层断裂。在剖面上，这一界线代表了上文帝统与下寒武统的界线。我们的结论与大多数研究者关于西伯利亚地台南缘晚旺代演化的观点基本一致。研究结果可为该地区的进一步勘探提供依据，并为研究地层与西伯利亚其他地区同年龄参考剖面的对比提供依据。
{"title":"LITHOLOGICAL AND FACIAL FEATURES, COMPOSITION, AND SEDIMENTATION CONDITIONS OF TERRIGENOUS-CARBONATE ROCKS OF THE MOTY GROUP (SHAMAN CLIFF, IRKUTSK REGION)","authors":"Z. L. Motova, A. Plyusnin, E. V. Nikulin","doi":"10.5800/gt-2021-12-3-0542","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0542","url":null,"abstract":"We have studied terrigenous-carbonate rocks in the area near the Sayan mountains in the Irkutsk Region (Russia), specifically at the Shaman Cliff, being the stratotype area of rocks that belong to the Moty group. The cliff’s lower part is composed of sandstones, which fragments gradually decrease in size upward the cross-section. The middle and upper parts are composed of sandy dolomites and dolomites, respectively. In terms of material characteristics, the terrigenous rocks correspond to arkoses. According to the genetic typification, the arkoses are composed of destructed primary igneous rocks. The terrigenous-carbonate rocks contain a carbonate component that gradually increases in the upper part of the cross-section. In the Shaman Cliff cross-section, we distinguish 32 lithological units and eight lithologicalgenetic types of deposits. Paleogeodynamic conditions are reconstructed for the formation of the sedimentation basin. Our study of the Shaman formation reveals specific features of the lithological facies, which suggest that these rocks accumulated in a coastal environment affected by tides. In the study area, clastic materials were mainly removed from an orogen that formed due to the Vendian accretion-collision events in the southern folded frame of the Siberian platform. Dolomites composing the upper part of the cliff are attributed to the Irkutsk formation of the Moty group. Their lithological features give evidence of shallow-marine conditions of their formation, without any supply of clastic material, which contributed to mass dispersal of the Cambrian biota described in [Marusin et al., 2021]. It is our first initiative to draw a boundary between the Shaman and Irkutsk formations of the Moty Group along the base of the carbonate eluvial breccia unit that marks the stratigraphic break. In the cross-section, this boundary represents the border between the Upper Vendian and Lower Cambrian.Our conclusions are generally consistent with the ideas of most researchers about the Late Vendian evolution of the southern margin of the Siberian platform. The results of our study can be used in further investigation of this area and provide a basis for correlating the studied strata with the same-age reference cross-sections of other regions in Siberia.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88147283","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0538
E. Vologina, M. Sturm, Y. Radziminovich
Sedimentation in Lake Baikal is significantly affected by continuous seismic activity in the Baikal Rift Zone. Our study shows that historical earthquakes, as well as recent seismic events, considerably influenced sedimentation in this deep tectonic basin. Here we present some of the results of extensive international research activities during the period of 1996–2019. To identify traces of seismic events in the uppermost sediments (<1.5 m), short cores were recovered from many coring stations throughout the entire lake. Based on lithological descriptions, measurements of magnetic susceptibility, and concentration of inorganic and organic components, we identified earthquake indicators in the sediment cores. Impacts of historical earthquakes were traced within South Baikal (near the Sharyzhalgai Station and the Station 106-km of the Circum-Baikal railway, hereafter CBR) and Proval Bay (near the Selenga River delta).
{"title":"TRACES OF HIGH SEISMIC ACTIVITY IN THE UPPERMOST SEDIMENTS OF LAKE BAIKAL, SIBERIA","authors":"E. Vologina, M. Sturm, Y. Radziminovich","doi":"10.5800/gt-2021-12-3-0538","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0538","url":null,"abstract":"Sedimentation in Lake Baikal is significantly affected by continuous seismic activity in the Baikal Rift Zone. Our study shows that historical earthquakes, as well as recent seismic events, considerably influenced sedimentation in this deep tectonic basin. Here we present some of the results of extensive international research activities during the period of 1996–2019. To identify traces of seismic events in the uppermost sediments (<1.5 m), short cores were recovered from many coring stations throughout the entire lake. Based on lithological descriptions, measurements of magnetic susceptibility, and concentration of inorganic and organic components, we identified earthquake indicators in the sediment cores. Impacts of historical earthquakes were traced within South Baikal (near the Sharyzhalgai Station and the Station 106-km of the Circum-Baikal railway, hereafter CBR) and Proval Bay (near the Selenga River delta).","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85452796","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-09-17DOI: 10.5800/gt-2021-12-3-0544
S. Skublov, A. Berezin, L. I. Salimgaraeva
The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.
{"title":"COMMENTS ON THE ARTICLE AUTHORED BY M.V. MINTS AND K.A. DOKUKINA – THE BELOMORIAN ECLOGITE PROVINCE (EASTERN FENNOSCANDIAN SHIELD, RUSSIA): MESO-NEOARCHEAN OR LATE PALEOPROTEROZOIC?","authors":"S. Skublov, A. Berezin, L. I. Salimgaraeva","doi":"10.5800/gt-2021-12-3-0544","DOIUrl":"https://doi.org/10.5800/gt-2021-12-3-0544","url":null,"abstract":"The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.","PeriodicalId":44925,"journal":{"name":"Geodynamics & Tectonophysics","volume":null,"pages":null},"PeriodicalIF":0.7,"publicationDate":"2021-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79412645","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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