The article discusses deep Earth crustal structure in the Baikal region, neighboring territory and Mongolia. The EIGEN-6C4 satellite gravity model-based Moho depth map was drawn for the territory with coordinates 43 to 61° N, 88 to 120° E. Gravity data interpretation was performed using seismic results obtained. The Bouguer EIGEN-6C4 model data interpretation was performed within the two-layer crust – mantle model with density diference 0.5·103 kg/m³. According to interpretation, Moho depth of the West Siberian Plate and Siberian Platform was estimated at 40–45 km with slight lateral variations, The depth decreases to 34–38 km near the Baikal basins. Strong lateral variations in Moho depth were obtained in mountainous areas south of the boundary going along the Main Sayan fault. The crustal thickness reaches 55–60 km in the area of active orogens (Mongolian Altai, Gobi Altai, Hangai) and decreases to 45 km in the Great Lakes Basin. The Stanovoi Range, Gobi Desert and Big Hingan have Moho depths ranging from 45 to 50 km. The southwesternmost part of the territory near Dzungaria, Tien Shan and Tuffan basin show the maximum drop of Moho from a depth of 40 km to 70 km. The Moho depth estimation error is from 2 to 4 km. The depths obtained for the Central Siberian Plateau and the Baikal basins generally correspond to the seismic data. The ground-based gravity data estimations do not contradict our results. The height anomalies of the quasigeoid model for the EIGEN-6C4 geopotential model relative to the WGS84 ellipsoid are found on the northwestern boundary of the Amur plate. The northern boundary of the Amur plate is clearly defined in the lateral distribution of the Bouguer anomalies and crustal thickness.

The areal gravity survey conducted in Eastern Mongolia to investigate morphology and deep structure of the model Mesozoic granite massifs served as the basis for the field measurements using high-precision Scintrex Autograv CG-5 gravimeters in strict accordance with accepted methods for regional gravity surveys. The elevations of gravity stations were determined using Trimble 5700 GPS instrument, thus ensuring high accuracy of the vertical positioning and minimizing the errors in gravity anomaly calculations. The accuracy of the derived Bouguer gravity anomalies, together with the density of the observation network, fully meet the technical requirements for 1:100000 surveys, which ensures data reliability and validity. A detailed Bouguer anomaly map was prepared based on the processed data. Gravity field inversion enabled three-dimensional modeling of the granite massifs. Based on the derived geometry and dimensions of the bodies, as well as considerations of isostatic uplift, it is inferred that the development of positive relief landforms is associated with the upward displacement of granite bodies along the faults whose existence and activity in Eastern Mongolia have been theoretically and experimentally proved.

The modern geodynamics and seismicity of the Far Eastern region are largely determined by the interaction of the Eurasian, Pacific and North American plates, as well as by that of smaller Amurian and Okhotsk plates. Wellknown studies of the configuration and position of boundaries using geological methods are currently supplemented by the results of potential geophysical fields, seismology and space geodesy data interpretation. But despite the abundance of available information, there is ambiguity in location of interplate junction boundaries, particularly of the Amurian and Okhotsk plates, which makes it necessary to use new additional information about plate junction zones. The-past-twodecade deep seismic surveys along the reference geological and geophysical profiles make it possible to clarify the relationship between plate displacements and the deep structure of the Far East region. The comparison was made between the results of seismological and deep seismic sounding (DSS) (on the 1-SB and 3-DV reference geophysical profiles) and the existing geodynamic models in the Eurasian and Amurian lithospheric plates junction area. The confirmation has been provided for the ealier obtained seismic criteria of the boundaries in the Eurasian, Okhotsk and North American plates junction area, which involve a small crustal thickness (37–42 km), low boundary velocity along the Moho (7.85 to 8.0 km/s) and a reduced average (effective) velocity of longitudinal waves in the earth's crust (~6.3 km/s). The Eurasian and Amurian plates junction area along the 1-SB and 8-DV reference profiles is a wide stress zone with a maximum number of earthquakes, a maximum total released energy indicator and smaller hypocentral depths. The CDPM-based deep seismotomographic sections in this zone show an extremely heterogeneous middle crust, as well as poor reflections in the lower crust and in the Moho section.

The present paper deals with the use of a probabilistic spectral analysis in the study of statistical properties of the variations in Z-component of electromagnetic field during 14 days prior to the July 29, 2025, M 8.8 earthquake which struck off Kamchatka. Additional analysis has been made on similar data for 9 days after the event. The analysis showed persistent differences in the statistical structure of geomagnetic variations at four spatially spaced magnitovariational stations. The persistence of differences is evidenced by the fact that they re-arise during the variation of processing parameters (N₁, M, h). The results obtained are consistent with the model of directed (anisotropic) effective current structure forming in the earthquake preparation zone. A simple geometric assessment based on the Biot – Savart – Laplace Law yields angular orientations comparable with an axial direction of seismic zone. The revealed differences do not correlate with the level of moderate geomagnetic activity of the period considered and may reflect large-scale anisotropy of magnetic response in the pre-seismic interval. The proposed approach is oriented at the analysis of statistical structure of geomagnetic variations and can be used to identify spatial features of strong earthquake preparation process. Further testing of the method on independent events is required to assess its practical applicability.

This paper presents the results of a comprehensive study of a catastrophic karst-suffosion sinkhole that occurred on September 19, 2024, in the Khadakhan-Melkhituy karst massif (Southern Angara region) and resulted in a human fatality. Using UAV‑derived data, the morphometric parameters of the sinkhole were determined (depth 9.7 m, volume 412 m³), and a comparison was made with historical monitoring data from the 1980s. Laboratory tests performed on samples collected from the sinkhole walls revealed that the 9.7‑m thick cover sequence consists of structurally unstable loess‑like loams and sandy loams, with anomalously high silt content (up to 84 %) and porosity (up to 64 %). The interaction between karst and suffosion processes plays a key role in the sinkhole formation. The long‑term (57‑year) backwater effect of the Bratsk Reservoir and its cyclic level fluctuations caused leaching of sulfate‑carbonate rocks, promoting mechanical removal of silt‑sized particles from the cover sequence by percolating water. A two-stage mechanism of karst-suffosion sinkhole development was identified: from the formation of a loosening zone above an ancient karst cavity to the instantaneous (brittle) roof collapse triggered by a technogenic factor – the weight of heavy agricultural machinery. The study of the reactivation of karst‑suffosion processes in the backwater zone of the Bratsk Reservoir contributes to the understanding of modern geodynamic processes and ground surface deformation mechanisms, including those induced by technogenesis.

The key role of recent tectonics in the redistribution of the fresh groundwater resource base in the intensive water exchange zone is revealed. New approaches to the study of the conditions of formation and accumulation of groundwater in the upper hydrodynamic zone, as well as the assessment of their reserves and formation sources, were implemented through the integration of geomorphological and structural-hydrogeological criteria of water cut. A methodology is scientifically substantiated and hydrogeological criteria for searching for areas with localized fresh groundwater reserves in large depressions and elevations of the relief are developed. Based on a comprehensive hydrogeological and morphostructural analysis of the Irkutsk artesian basin of the second order (taking into account explored and evaluated deposits of fresh groundwater), the territory is detailed according to flow conditions, and high-performance collectors of the enclosing reservoirs are mapped (high filtration and transmissibility coefficients, renewable fresh groundwater reserves due to attracted resources – overflow from surface watercourses and reservoirs and natural resources – infiltration of atmospheric precipitation). The author outlines typical schemes of the formation of fresh groundwater deposits in the study area. Each scheme corresponds to deposits based on quantitative and qualitative characteristics, lithological and petrographic composition of water-bearing rocks, reservoir type and filtration indicators of production horizons, as well as the conditions of fresh groundwater formation. Deposits formed due to natural resources in positive direct and positive inverted morphostructures are distinguished. Based on the resources involved, deposits formed in positive direct, negative direct, and negative inverted morphostructures are distinguished. Within negative morphostructures (both direct and inverted), the production aquifers of fresh groundwater deposits are represented by pore-formation waters of loose Quaternary sediments. Within positive, straight morphostructures, fissure-formation and fissure-karst waters (outcrops of bedrock in which karst develops along fracture zones, forming a filtration field due to heterogeneous fracturing) of Jurassic and Paleozoic deposits are formed. Within positive, inverted morphostructures, fissure-formation waters of Jurassic deposits are formed.

This study presents U-Pb dating results for zircon collected from the beaches of recreation zone "Peschanaya" (Sandy) on Lake Baikal. These beaches are formed exclusively by the erosion of Early Proterozoic rapakivi granites from the Primorsky Complex. Zircon from the granites yields a U-Pb age (ID-TIMS) of 1859±16 Ma, with an average U concentration of ~120 μg/g. Analysis of detrital zircon from the beaches of recreation zone "Peschanaya" was conducted using the ID-TIMS method on three aliquots and two in-situ methods – LA-ICP-MS and SHRIMP – on multiple (>600) individual grains. The new ID-TIMS and SHRIMP data are in complete agreement with each other, yielding ages of 1853.6±6.5 Ma and 1853.0±3.3 Ma, respectively. The LA-ICP-MS data, obtained in 7 Russian and 1 Chinese laboratory, are in general consistence with these results. Zircon from the beaches of the recreational zone "Peschanyа" is recommended as a secondary standard for U-Pb dating of early Precambrian samples.

The Dzhida zone of the southern Baikal region comprises andesite-basalt and sedimentary-volcanic rocks of the Khokhyurt formation and the sandy-shale flyschoid Dzhida formation. This paper presents the results of lithogeochemical study of terrigenous deposits of the Khokhyurt and Dzhida formations, which demonstrate the evolution of terrigenous sedimentation during the geological development of the Dzhida guyot. Thin sandstone lenses of the Khokhyurt formation and turbidite sequences from the Dzhida formation contain mafic or intermediate clasts and fragments of carbonate rocks that composed the guyot caps. New geochemical data indicate that the flyschoid sequence from the Dzhida formation is similar in characteristics to the deposits of active rather than passive margins. Geochemical characteristics and highly fractionated REE patterns of sandstones suggest erosion of island-arc complexes that were contaminated with ancient crustal material during the early stages of their emplacement. Carbonate clasts of the Dzhida formation contain fragments of the Late Riphean and Early Cambrian carbonate formations, but lack the fragments of the Vendian formations that form the closely located margin of the Siberian craton. Nothing in the geochemical data suggests a direct supply of detrital material from the craton. The terrigenous strata of the Khokhyurt and Dzhida formations formed in deep-water fore-arc/back-arc basin settings on the active margin of the Paleo-Asian Ocean. The obtained data indicate that the Dzhida paleobasin was located at a considerable distance from the Siberian craton in the Late Precambrian and Early Paleozoic.

The large Malo-Oinogor W-Mo deposit is located in the Zakamensk ore cluster of the Dzida ore district of the Western Transbaikalia. The W-Mo mineralization area is located within the Malo-Oinogor granite porphyry stock and surrounding metamorphic rocks of the Khasurta formation. The features of geological relationships with the surrounding rocks, as well as mineralogical-petrographic and geochemical features of the granite porphyries, indicate that the stock relates to the pre-ore stage and that the development of ore-bearing greisens occurred much later than the stock formation. The U-Pb (LA-ICP-MS) dating of zircons from granite porphyries of the Malo-Oinogor stock yielded an isotopic age of 310±2.5 Ma which does not correlate with the Mesozoic age of the granite porphyries from the Gudzhir intrusive complex, genetically associated with the W-Mo mineralization of the Dzhida ore district and, in particular, the Zakamensk ore cluster. The widespread occurrence of zircons, prone to lead loss, in the granite porphyry stock made it possibe to assess the probable threshold time of development of high-temperature hydrothermal metasomatism (132±6 Ma ago) in which U-Pb isotopic re-equilibration could take place. Within the margin of error this threshold time is comparable to the formation time of the Gudzhir ore-magmatic system (133–123 Ma). The formation of the Malo-Oinogor granite porphyry stock 310 Ma ago is due to the beginning of the formation of the Mongolian-Transbaikalian volcanoplutonic belt (PZ₃–MZ₁) during Mongol-Okhotsk orogeny which, in its turn, was related to the subduction of the Mongolian-Okhotsk paleooceanic plate beneath the southern margin of the Siberian continent. It is believed that W-Mo mineralization is related to hydrothermal activity, the source of which was the Early Cretaceous ore-bearing Gudzhir intrusive. The Gudzhir ore-magmatic system of the Zakamensk ore cluster was formed under tectonic extension during the Early Cretaceous Mongol-Okhotsk orogenic collapse. The arguments are presented for the fact that the Zakamensk ore cluster, to which the Malo-Oinogor W-Mo deposit is confied, is a promising area for the discovery of new deep rare metal reserves.