3D thermal modeling of the South Chukchi Basin was performed. The model was calibrated using temperature data and vitrinite reflectance from wells on the Alaska shelf. The distribution of temperatures and the maturity of organic matter over time were obtained and the likelihood of hydrocarbon generation and accumulation was assessed.

The results of a comprehensive study of the Lower–Upper Ypresian boundary sediments at the Suvlu-Kaya Mountain section (Bakhchisarai region, Southwestern Crimea) are presented. The evolution of formation of the nummulitic bank in the study area is shown based on a detailed microfacies analysis. The sediments were formed on a deep shelf with depths at and below the storm wave base. It has been established that the Ypresian transgression was developed gradually, in three stages, with the maximum at the end of the Early Ypresian, and then was followed by a long-term regression. The data are confirmed by the results of the micropaleontological analysis of the morphogroups.

Fracture modeling in the section of Suvlu-Kaya Mountain, South-West Crimea, was made. A three-dimensional geological model of the section was constructed, geomechanical modeling was performed, and stress fields in the three-dimensional geological structure were calculated. The modeling results were compared with real measurements of fracturing in the field.

The Cheka pluton, located in the Magnitogorsk megazone of the Southern Urals and composed of the alkaline granitoids, is meridionally elongated (1–2)×6.5 km. The fracture and anisotropy of magnetic susceptibility analysis was made on the Cheka pluton for the first time. The study materials comprise fracture measurements and core samples. The fractures were classified as either prototectonic or tectonic using the Stereonet software. The prototectonic fractures were classified into three standard types in accordance with the system proposed by G. Cloos: S, Q, and L. The system of tectonic fractures corresponds to the Riedel model, which confirms the formation of the pluton as a dextral magmatic strike-slip duplex supposed earlier. The analysis of magnetic mineralogy revealed that the most prevalent magnetic mineral is fine- to-medium-grained magnetite. Using the anisotropy of magnetic susceptibility data, the main direction of melt flow during the formation of the pluton was determined to be 36° NE. This direction is in close alignment with the orientation of one of the prototectonic fracture systems, which has a strike azimuth of 39° NE. The constructed complex structure model indicates that the Triassic granitoid plutons of the Magnitogorsk megazone were most likely formed during the right-lateral transpression, associated with a local strike-slip fault-related extension zone. These conditions are generally consistent with the Triassic rifting and dextral strike-slip motion in the Southern Urals.

Retrogressive thaw slumps (RTS) are an important landform of rapid permafrost degradation in regions with very high ground ice contents. RTS mobilize significant amounts of sediment, meltwater and organic carbon and impact downstream hydrological systems by directly affecting topography and water quality. The term megaslump has previously been coined for RTS exceeding 20 ha in size. The Batagay megaslump in the Yana highlands of NE Siberia with an area of 87.6 ha (in 2023, including the bowl-shaped part and the erosional outlet) has been identified as the largest megaslump on Earth. We use very high resolution remote sensing from satellite data and drones, geological structure modeling, and field data to assess how much and what material is thawed and mobilized in the Batagay megaslump. The total volume of permafrost thaw and material loss from the Batagay RTS amounts to about 1 million m 3 per year. The material is by one third composed of thawed sediments and by two thirds of melted ground ice. About 4000 to 5000 tons of previously permafrost-locked organic carbon is released every year. Organic carbon content has been measured as Total Organic Carbon (TOC) of sediments and as Dissolved Organic Carbon (DOC) of ground ice. From its formation in the 1970s until 2023, the Batagay RTS - due to thermal denudation and headwalls retreat - mobilized a total volume of about 34.7 million m 3 of which 23.4 million m 3 were melted ground ice and 11.3 million m 3 were thawed deposits including a total of about 169,500 t organic carbon. With these rates of sediment and carbon mobilization, the Batagay megaslump is not only a prominent local feature of rapid permafrost thaw, but offers excellent conditions to study rates and mechanisms of rapid permafrost degradations and to calculate the stock and release of, e.g., organic matter.

<p><strong>Introduction.</strong> This work is devoted to the study of the sources of   drift material during the formation of Late Paleozoic deposits of the southern part of the Pre-Ural trough. Sample for the study was taken in a quarry near the Urgala region, Bashkortostan area. The section is represented by conglomerates with a sand matrix. These deposits belong to Ural forland basin. The age of this conglomerate formation – Moscovian (Middle Carboniferous).</p> <p><strong>Materials and methods.</strong> The most reliable determination of sources is possible due to U-Pb zircon dating. We also analyzed some thin sections for detailed studying of sandstone composition.</p> <p><strong>Results and discussion</strong>. Zircon grains vary greatly in shape and size. In some grains, the core and edges are clearly visible; others are full of inclusions, cracks, and zones of metamict decay. The size of the crystals varies from 60 to 400 microns. Most of the ages obtained fall in the interval from the Ordovician to the Devonian, less on the Lower and Middle Riphean. Single grains are of Cambrian, Vendian and Late Riphean age. Early Proterozoic and Archean grains are absent in the sample.</p> <p>The most difficult interval is from the Cambrian to the Devonian, it accounts for the majority of the ages (410-430 Ma). Within the studied territory, the volcanic rocks closest to the sampling site are located in Nyazepetrovsk and Bardym allochthons, as well as in the Tagil arc. In addition, Devonian granitoids are found within the Ufalei anticlinorium. The largest number of Precambrian dates falls on the Middle Riphean. The source of zircons during the middle Riphean could be the Mashak formation, whose age is 1350-1346 Ma, however, there are no grains with the age of the Mashak formation in the sample.</p> <p>A relatively large number of grains have the early Riphean age of 1650-1500 Ma, which correlates perfectly with the age of the Ai formation. However, almost all Riphean formations, including the Ai formation, contain zircons with the peak at 2050 Ma (the age of migmatization in the Taratash block),but  the studied sample contains no zircons of 2050 million years age or older. This means that the Taratash block and the surrounding Riphean formations were not exposed at that time.</p> <p>Also, the largest number of lithoclasts in the studied sandstones are represented by siliceous rocks. The similar rocks compose the Ordovician-Devonian section of the Mayaktau Allochthon, which is located closely to the sampling site. Also, the thickness Aziam  formation   increases towards Mayaktau Allochthon. In addition to the sources described above, there is jne more source – Asha series (Vendian), because there are quite a large number of Middle-Riphean dates in the sample, which are typical for the rocks of the Asha series.</p> <p><strong>Financial support</strong>. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 Czechia_a</p>

In this article we present the first data on the anisotropy of magnetic susceptibility (AMS) for the rocks of the Early Carboniferous Nepluyevka polyphase pluton, which is situated in the southern part of the East Urals megazone. We consider the properties of magnetic mineralogy of all phases of the batholith, and show that the leading role in determining the magnetic properties of the rocks belongs to an isotropic grain population of MD magnetite and multiple populations of SD magnetite, which are the chief contributors to AMS. We consider the characteristic parameters of AMS for each phase, and show that the AMS is magmatic (non-deformational) in nature. An emplacement mechanism of “magmatic duplex” developing in a kinematic setting of sinistral transtension is proposed for the batholith, based on the analysis of the characteristic magnetic fabrics.

Early Carboniferous (359.3–323.4 Ma) volcanic complexes are widespread in the Southern Ural tectonic province, a fragment of the western (in present-day coordinates) segment of the Central Asian Orogenic Belt. Here, the Lower Carboniferous sequences crop out within the following N–S-trending tectonic zones (from west to east): Magnitogorsk, Ui River, East Ural, Transural, and Valeryanovka. We describe and discuss the geology and geochemistry of the Early Carboniferous volcanic complexes on the basis of published and newly obtained data, with implications for paleo-tectonic models. The western zones are dominated by bimodal rhyolite-basalt series, with the basalts relatively enriched in Ti and Zr but depleted in Nb. The volcanics of the Valeryanovka zone belong to the typically evolved calc-alkaline series, with the derivatives depleted in Ti and Nb. Almost all of the selected groups of volcanics bear geochemical signatures transitional between those of subduction-related and intraplate igneous rocks. The relative enrichment of the volcanics of the East Ural and Transural Zones may be interpreted as a result of a contribution from asthenospheric mantle and/or from subcontinental lithospheric mantle. The volcanics of the Valeryanovka zone reveal features common to subduction-related series of the Andean type. The data obtained allow us to compare the Early Carboniferous geodynamic settings in the western zones with the modern setting of the Northeastern Pacific, whereas the geodynamic setting of the Valeryanovka zone resembles that at the western margin of South America.

— The World experience of site investigation and construction under different engineering-geological conditions educational geoinformation system is considered. The purpose of its compilation is to collect and study the most diverse experience of surveys: for different types of engineering structures, in different natural conditions and countries. GIS database contains 46 layers divided into four blocks: (1) base layers; (2) geological maps; (3) map of engineering-geological structures of the Earth; and (4) actual data on engineering-geological surveys and special features of the construction. Attribute tables are described and an object classifier of the Engineering-points and Engineering-lines layers is given.

<p><strong>Introduction.</strong> The Early Carboniferous Nepluyevka polyphase granitic batholith is situated in the East Ural zone. Its emplacement happened during the Sudetian orogeny, which initially shaped the structure of the southwestern segment of the Ural-Mongolian fold belt. As such, the pluton is a repository of information on tectonic evolution and geodynamics of said orogen, which can be used to enhance our understanding of interactions between Laurussia and the microcontinent of Kazakhstania during the Early Carboniferous.</p> <p><strong>Methods and materials. </strong>We have investigated the existing data on the petrology, petrochemistry, isotope systems, and U-Pb geochronology of Nepluyevka batholith, and performed our own analysis of the trace element distribution of the constituting rocks using ICP-MS method. The mechanism of emplacement and its kinematic setting were investigated through an analysis of oriented fabrics and anisotropy of magnetic susceptibility (AMS) for each phase. Paleomagnetic methods were employed for establishing the position of pluton’s host terrain during its emplacement. A total of five specimen, characterizing all of the phases of the batholith, were chosen for petrochemical analyzes, and 186 oriented specimen from 16 sites were used for rock- and paleomagnetic studies.</p> <p><strong>Results. </strong>Combinations of <sup>87</sup>Sr/<sup>86</sup>Sr (0,70491–0,70504) and εNd (-0,29-0,5) ratios for different phases indicate that both depleted mantle and crustal sources were involved in petrogenesis. Trace element distribution is characteristic of subduction settings. AMS parameters’ spatial distribution and observed fabric features show that the batholith was emplaced in a kinematic setting of sinistral transtension. Virtual geomagnetic poles (VGPs) obtained from ChRM components of remanent magnetization do not fall anywhere on the Carboniferous-Quaternary sections of apparent polar wander paths (AWP) for Eastern Europe or Siberia.</p> <p><strong>Discussion. </strong>Combined data on geological structure of the pluton, isotope systems, petrochemistry, and rock magnetic properties of rocks lead us to the conclusion that the batholith had developed as a part of a magmatic system associated with an oblique subduction setting. Paleotectonic reconstructions of pluton’s host terrane Visean location derived from our paleomagnetic data contradict the traditional models for the region. We suggest a model featuring rotation of the host terrane in a strike-slip displacement zone to deal with the contradiction. A paleotectonic reconstruction corrected for such a rotation puts the host terrane into the Visean paleo-position of Kazakhstanian microcontinent. This reconstruction agrees well with the the model proposed by Sengor, Natalin and Burtman in [Sengor et al., 1993], featuring a single subduction system (“Kipchak arc”) stretching from Laurussia to Siberia, which existed through much of the Paleozoic and controlled the crustal growth and development of what is now known as Ural-Mongolian fold belt.</p> <p><strong>Financial support</strong>. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 with the use of materials of the "Geoportal" Center of the Lomonosov Moscow State University.</p>

Introduction. Determination of the age of igneous roc Comparative analysis of U-Pb dating of zircons from Early Carboniferous volcanites and Middle Triassic alkaline granitoids of the Magnitogorsk zone (Southern Urals)ks by the U-Pb isotope method using zircons is currently one of the main dating methods. Here we present new isotopic data of zircons from alkaline granitoids of the Cheka massif and zircons from acidic volcanites of the lower Carboniferous of the Magnitogorsk zone (Southern Urals).Materials and methods. The Middle Triassic isotopic age of the Cheka massif was determined by the Rb-Sr isochron method. Currently, we obtained new seven U-Pb dates based on zircons isolated from various phases of the massif. Early Carboniferous volcanites are represented by a contrast moderately alkaline series. Volcanites have been sampled at two points. The U-Pb dating was performed at the All-Russian Geological Research Institute using SHRIMP-II.Results. At least two zircon populations of early Carboniferous isotopic age have been identified in acid volcanites. The first population is represented by full crystals and their fragments 100-200 microns in size. They have a short-prismatic habit and a clear oscillatory zonation. This population is predominant in all samples. Zircons have a moderate content of U and Th. The population is homogeneous with average concordant age is 348.5 ± 3.1 Ma.Zircons of the second population were found in all samples. They are small (about 50 microns), perfectly faceted crystals with an increased content of U and Th. Their isotopic ages (344 and 351 Ma) are entirely fit the age range of the first population. Thus, completely different in morphology and composition, zircons have the same isotopic age.Two most representative samples of alkaline granitoids, provide zircons 150-250 microns in size. They are light in the cathodoluminescent image, with a clear fine oscillatory zonation and weakly expressed sectorial. The range of isotopic ages of these zircons in is 342.6–376.6 Ma, and their average concordant age is almost the same: 353.9±4.0 and 352.7±3.9 Ma.Discussion. U-Pb dating of zircons from acidic volcanites confirmed their Tournaisian age. The morphology and composition of zircons turned out to be an important key to understanding the age of volcanites intruded by the alkaline granitoids.Inherent zircons in alkaline granitoids may not be crystallized at all, since all zirconium should be concentrated in alkaline dark-colored minerals. In this case, only the inherited zircon will remain in the rock. In addition, the dissolution of inherited zircons can also occur in alkaline melts.Early Carboniferous zircon grains in all samples of alkaline granitoids are similar to those from volcanites. They have a typically magmatic appearance and zonation and the concentration and ratio of uranium and thorium are also typical. At the same time, alkali-rich fluid-saturated magmatites are usually characterized by a Th/U ratio close to or significantly higher than 1. Uranium and thorium concentrations are usually very high. The described features most likely indicate the xenogenic nature of Early Carboniferous zircons in relation to granitoids.Financial support. The research has been funded by RFBR (research project № 19-55-26009).

A kinematic model has been built for the seismic profile traversing the Pegtymel inverted rift (Chukchi Sea). Three main stages of rift formation have been identified: 125–34, 34–20, and 20–0 Ma. The identified stages are compared with the geological evolution of the Eastern Arctic.

The first results of U–Pb (LA-ICP-MS) dating of detrital zircons from the Upper Visean sandstones of the Solnechnaya Formation of the Southern Urals are presented. The studied zircons are predominantly of Ordovician and Cambrian ages. The main peak falls at the beginning of the Ordovician period (about 480 Ma), and the local peaks at the beginning of the Late Ordovician, the middle and the beginning of the Cambrian. Comparison of the obtained age distributions of detrital zircons from sandstones of the Solnechnaya Formation with the nearby formations of the Western and Eastern Urals showed that the source of detrital material could be the Early Paleozoic metamorphic, igneous and terrigenous rocks of the East Uralian Megazone.

The nature of Cretaceous-Eocene boundary is one of the outstanding questions of Crimea Geology. The new data are presented to show that the Cretaceous-Eocene boundary can be established in the Central Crimea very accurately by using the method of quantitative genetic analyses including the Isotope Geochemistry. Integrated lithostratigraphic investigations and Isotope composition of Carbon/Oxygen were conducted on the Cretaceous -Eocene section of the western slope of Ak-Kaya mount (Belogorsk, Crimea). Four layers of different types of rocks were investigated, where the layer 1 and 2 belong to the Maastrichtian, 3 and 4 to the Eocene.The top of the Maastrichtian layer is characterized by a differently oriented fracture system, including large paleoseismic dislocations or a seismogenic trench. The fracture networks are connected and filled with material similar to the Eocene basal horizon including fragments of various sizes of Maastrichtian rocks.Five microfacial types of the collected rock samples were distinguished as a result of microscopic examination. Also X-ray phase analysis, δ13С and δ18О isotopic analysis and X-ray fluorescence analysis were made to specify and compare the mineral composition of Maastrichtian and Eocene rocks. These analyzes allowed to specify paleogeographic conditions. In addition, measurements of fractures in the Cretaceous–Eocene boundary deposits were made to determine the stages of deformation of the whole structure.As a result of the research, it was obtained:1) throughout the entire studied geological interval, sedimentation occurred in a shallow sea of normal salinity. However, conditions were probably more humid in the Eocene, based on lower salinity values.2) Three major stages of deformation were identified: pre-Eocene, Eocene, and post-Eocene.3) The average temperature of the formation of Maastrichtian rocks is 19-22°C, and Eocene rocks is 24-27°C. The increase in temperature up to 38°C during the formation of the Eocene basal horizon may be associated with the global climatic event EECO (Early Eocene Climate Optimum). The synchronicity of the formation of steep submeridional fractures and the basal horizon of the Eocene has been proved. It is shown that the Eocene deformation stage corresponds to the formation of paleoseismic dislocations during the main phase of tectonic activity in the Pontids (Eastern Turkey).

Introduction. This study investigates the Cheka block (pluton) of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to further investigate its existing deformation model after previous studies though the methods of fracture analysis, petromagnetic studies and geochemical analysis.The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone.During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones. These changes were followed by low-grade metamorphism.This study can be split into two sections: structural analysis and geochemical/isotopes description. The first part was partially conducted previously and presented in 2022.Materials and methods. To reconstruct structural evolution of the pluton a combination of petromagnetic studies, magnetic mineralogy and fracture analysis were used as well as supporting aerial and satellite imagery. 62 core samples and over 180 fracture measurements from 7 locations were used for each method respectively. Petromagnetic data was collected by drilling procedures, processed using MFK-1 kappabridge at room temperature and after heating to 470 °C and analyzed in Anisoft 5.1.08 software. Magnetic mineralogy lab analyses were performed with interpretation using Max UnMix software. Fracture analysis was conducted in Stereonet v.11.3.0.As the second part of the study geochemical analyses were conducted – silicate geochemistry and ICP-MS at 6 locations.Results and discussion. Petromagetic studies showed the magma flow to have an orientation of 036°. Analysis of tectonic fractures points to the Riedel fracture model with main fracture zone orientation (compression) of 039°. Since the magma flow and compression orientation match a deformation model can be constructed. Also based on the magma flow orientation, types of protectonic fractures were identified (S, Q, L).Geochemical analyses showed that the elemental signature of the pluton matches the upper crust the best and shows signs of subduction. Silicate geochemistry shows a clear trend in Na2O concentration, while K2O concentrations do not. This pattern is interpreted as a sign of low-grade metamorphism (prehnite-pumpellyite facies).A full deformation model was created based on two methods with additional supporting data providing strong evidence for the Riedel based deformation model, which corresponds to previous structural and geochemical findings. The model suggests that the Cheka pluton was formed in a general right-lateral transpression setting with following tectonic developments and related low-grade metamorphism.Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

Introduction. Two structural zones are traditionally distinguished in the Eastern Urals. They are the Magnitogorsk zone and the East Ural zone, which are divided by a narrow suture. The Early Sudetian (Visean) orogenic phase is marked by a structural unconformity in the base of Upper Visean terrigenous-carbonate sequence both in the suture zone and the East Ural zone. According to drilling data granitic-pebble-bearing conglomerates are present at the base of this sequence. The Sudetian rearrangement is implied in the Magnitogorsk zone by the end of rifting and the initiation of carbonate deposition. The Pre-Sudetian basement of the East Ural zone is comprised of the Lower Paleozoic deformed metamorphic rocks (gneisses, schists and carbonaceous quartzites), clastic deposits of the Ordovician (wackes and meta-arkoses) and the Lower Carboniferous (greywackes), as well as the volcanic rocks of the Tournasian-Early Visean and the granitoids of the Neplyuevka complex, dated 355-340 Ma. The Pre-Sudetian basement of the Magnitogorsk zone is represented by igneous complexes of the Devonian – Early Carboniferous age. This research aims to determine the source areas and migration paths of the sediments for local basins associated with the Sudetian orogenic phase. The basins at the conjunction of the two megazones could derive clastic material from both. Materials and methods. The specimens were collected from quarries near the Novinka village where sandstones of the terrigenous-carbonate succession are exposed. The sandstones are commonly cross-bedded, medium to coarse-grained, have sub-arkose, arkose, greywacke or, sometimes, quartz-arenitic composition. 210 zircon grains were extracted from two samples. 99 zircon grains characterized by discordancy no more than 5% have been chosen for the evaluation of age distribution parameters. Results and discussion. The dating results appeared to be unexpected. Firstly, no single analysis yielded a Devonian isotope age, and only a single grain yielded the Tournaisian isotope age. Secondly, the vast majority of the zircon grains appeared to have the Cambrian and Ordovician isotope ages, with the main peak corresponding to the beginning of the Ordovician (480 Ma) and secondary ones corresponding to the beginning of the Late Ordovician (450-460 Ma), the Middle Cambrian (510-520 Ma) and the Early Cambrian (530-540 Ma). So, the Magnitogorsk zone could not house the zircone source area for the local basin associated with the Sudetian orogenic phase. The clastic material could only be derived from the East Ural zone. However, the study area does not contain any known igneous complexes with suitable ages. The local source areas of detrital zircons are, in fact, associated with the scarps of metamorphic complexes of the East Ural zone, which host the zircons with the isotope ages of 478±5 Ma and 529±6 Ma. Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009. The U-Pb dating of the zircon is executed as a part of the research project № АААА-А18-118053090045-8 of State task of IGG UB RAS. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

Introduction. The Neplyuevka pluton is situated in the Chelyabinsk region of the Southern Urals, in the western part of the Eastern Uralian megazone. The area of pluton is 20×14 km. The Neplyuevka intrusion is comprised of 4 phases: 1) gabbro and diorites, 2) quartz diorites and granodiorites, 3) adamellites, 4) leukogranites. The granitoids intrude the terrigenous rocks of the Lower Ordovician. The overlying terrigenous deposits of the Upper Visean contain granitic lithoclasts. The isotope Rb-Sr ages of the rocks comprising the Neplyuevka pluton lie in the range of 346-340 Ma. The variations in the isotopic ages are consistent with the order of individual phases: 346 Ma for the 2nd phase, 342 for the 3rd phase and 340 for the 4th phase. So, according to the isotope ages, the evolution of the pluton took at least 6 Ma. The pluton also contains Cisuralian leucogranites (278 Ma). Materials and methods. We have studied the zircon grains extracted from 4 specimens: 1 – granodiorites of the 2nd phase, 2 and 3 – adamellites from the 3rd phase, 4th – leucogranites of the 4th phase. The specimens themselves have been collected from the exact locations and the same rock types as previous sampling for Rb-Sr isotope dating. We have studied the morphology of the grains and their internal structure using cathodoluminescence imaging. The U-Pb dating was performed at the Russian Geological Research Institute (VSEGEI) using SHRIMP-II. Results and discussion. The zircon grains from the granodiorites and adamellites are represented by transparent and light-yellow, idiomorphic, bipyramidal-prismatic crystals, 200-600 μm in length. The crystals are characterized by medium intensity of luminosity with apparent medium-contrast coarse and fine oscillatory zonation. By their obtained isotopic ages the zircons can be divided into 2 populations 1) 334-342 Ma and 2) 354-356 Ma. The isotopic ages of the 1st population are close to those obtained by Rb-Sr dating. The formation of the Neplyuevka complex in the beginning of the Carboniferous signifies an important event in the geodynamic evolution of the Southern Urals – the fast transition from island arc magmatism, which continued during the whole Devonian, to the rift magmatism, which ceased only in the middle of the Visean. The termination of island arc magmatism is usually explained by the slab delamination at the Devonian-Carboniferous boundary, while the initiation of the rifting is attributed to the oblique collision between the Paleo-Ural island arc with the Laurussia in the middle of the Tournaisian. The obtained data allows us to reevaluate the age of the rifting initiation, assigning it to the Devonian-Carboniferous boundary. Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009. Interpretation of U-Pb data was carried out within the framework of the government assignment of IGEM RAS.

Early Carboniferous igneous rocks are widespread in the Southern Urals. We have obtained new stratigraphic and isotopic data on plutonic and volcanic complexes, allowing us to determine correlation of their age and to construct a new geodynamic model. The prevailing tectonic setting in the Southern Urals during the Early Carboniferous was sinistral transtension. Volcanic and plutonic complexes in transtensional zones were synchronously formed along large submeridional orogen-parallel strike-slip faults, but are particularly abundant within two N–S-trending zones: Magnitogorsk and East Ural. The upper Tournaisian–lower Visean sequence in the Magnitogorsk zone consists mainly of moderately alkaline volcanic rocks, basalt and rhyolite are predominant, but pyroclastic, volcano-sedimentary, terrigenous, and carbonate rocks are also widespread. The middle Visean sequence consists of moderately alkaline basalt, andesite, dacite including lavas, tuffs and tuffites. The thickness of the Lower Carboniferous volcanic group varies from 1200 to 5500 m. The age of the volcanic rocks has been proved by findings of foraminifera in limestone interbeds. The oldest volcanic rocks appear in upper Tournaisian, while the youngest are found in the middle upper Visean. New U–Pb zircon dating using SHRIMP is now in progress. Volcanic rocks in the East Ural zone occur within a few tectonic sheets. The sequence consists of lavas and tuffs of basalt, basaltic andesite, andesite and rhyolite. The total thickness of the sequence varies from 800 to 1500 m. The age of the sequence is determined by findings of fossil plants as middle Visean. We studied eight plutons in the Magnitogorsk and six in the East Ural zones. Most of them record several intrusive phases. The composition of the rocks varies from gabbro to granodiorites and granites from normal to moderately alkaline series. We combined our new isotopic data on zircons (SHRIMP) with published ages and came to the following conclusions. * Two main stages of Early Carboniferous plutonism can be distinguished in the Southern Ural. The first began simultaneously in both zones at the Devonian/Carboniferous boundary (ca. 356–357 Ma) and then changed to volcanic activity at around 346 Ma in the Magnitogosk zone and at around 340 Ma in the East Ural zone, respectively. The second stage began after the termination of volcanic activity and corresponds to 334–327 Ma interval in both zones. So, stages of active volcanism and plutonism alternate in time. * Early Carboniferous rifting began with intrusion of plutons, usually associated with transtensional zones under oblique collision. The subsequent volcanic stage corresponds to local extension. The next stage of plutonism began just after volcanism termination and marked a cessation of tectonic activity. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

Introduction. Fracture analysis of rock formations allows us to reconstruct formation history and structural development of magmatic blocks. This study investigates the Cheka block of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to evaluate the main deformation characteristics of the sincollisional Cheka block (pluton). For this purpose, stress fields were reconstructed. The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone. During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones. Materials and methods. Space images show several fracture systems with approximate strike lengths: -20° and 310°. During the field work more than 180 fracture orientations were measured Samples were taken for petro- and paleomagnetic, geochemical investigations, and isotope dating at five locations. The Stereonet v.11.3.0 software was used to analyse the fractures. Schematics (Mohr circles) with fracture poles were created for each location. From these, five swarms of poles with Kamb contours were extracted, showing the statistical concentration of the poles. At locations 701 and 702, three swarms of sub-perpendicular poles were most clearly observed and interpreted as a system of tectonic fractures. The S, Q, and L fractures were identified among the prototectonic fractures based on the relation to linearity and pluton contacts. All the poles that fell within these three swarms were treated as prototectonic, while remaining locations outside these zones were treated as a system of fractures of tectonic origin. Numerous fractures, which are not part of the described systems, are most likely random and require more detailed research. Results and discussion. A series of vertical fractures, arranged in a pattern relative to each other, were considered. Based on these swarms, a deformation model was built, and the directions of tension and compression were determined. Sub-horizontal compression was oriented northeastward, resulting in the formation of sub-meridional right-lateral shear and a general right-lateral transpression setting. The predominant fractures were synthetic P (according to Riedel), and they are also the most pronounced geomorphologically and on satellite images. Less pronounced are synthetic R and antithetic R' fractures. This study of the Cheka pluton made it has possible to separate two fracture systems. These systems point to right-lateral transpression, which confirms the model of the massif formation as a shear magmatic duplex. Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

Introduction. The Nepluyevka pluton is the Early Carboniferous polyphase batholith situated in the East Ural zone. The batholith is subdivided into 4 phases ranging from basic to felsic in composition. The pluton formed during the Early Sudetian orogenic phase of the East Ural zone, which was characterized by a complex alternation of pure and sub-simple shear kinematic settings over its’ duration. Evidently, this alternation was connected with changes in the kinematics of the subduction zones preceding the Late Visean collision of Laurussia and Kazakhstania. These transformations had defined the characteristic features of the tectono-magmatic evolution of the southern part of the East Ural zone. Thus, their investigation is crucial for improving our understanding of the geological history of the Southern Urals. Methods and materials. We have investigated anisotropy of magnetic susceptibility (AMS) and magnetic mineralogy of the rocks of the Nepluyevka batholith to gain insights into the circumstances of its’ formation and its’ deformation history. Totally 186 oriented specimens from 16 sites spread over all the phases of the pluton were collected. MFK-1A kappa-bridge was used to measure MS and AMS, temperature dependencies of induced magnetization were studied with Curie balance, magnetic hysteresis loops were obtained on J_meter coercivity spectrometer. Results. The specimen appeared to contain PSD high-Ti magnetite (magmatic), MD low-Ti magnetite (hydrothermal), as well as the minerals of goethite and maghemite-hematite series. The AMS data tells the history of the formation and structural evolution of the batholith. Gabbro (1st phase) and granodiorites (2nd phase) in the center of the pluton are characterized by prolate magnetic fabrics. Lineation there is steep to sub-vertical and marks the flow direction near the feeder shared by both phases. Granodiorites (2nd phase) in the north and adamellites (3rd phase) in the north and the south of the pluton are characterized by predominantly oblate, flat-dipping fabrics, corresponding to lateral spreading of the melt. The magnetic fabrics of the adamellites (3rd phase) near the pluton’s southern boundary are oblate and dip steeply in the SW direction, marking the melt flowing parallel to the contact. The magnetic fabrics of the adamellites in the NE part of the batholith are similarly oblate and subparallel to the contact. Discussion. We propose the model of “magmatic duplex” for the formation of the pluton. The upper-crust transtensional structure associated with a sinistral strike-slip fault was draining the lower-crust magma chamber. Due to the fractionation and assimilation of the chamber’s wall material, it was supplying increasingly felsic melt. Formation of the first two phases was controlled mainly by the central feeder. The 3rd phase adamellites intruded two weakened contact zones of the pluton as the transtensional structure continued to grow sub-longitudinally. The pluton has experienced secondary heating and some metasomatic alteration, but no significant deformations occurred. Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 with the use of materials of the "Geoportal" Center of the Lomonosov Moscow State University.