Home – Home – Jornals – Russian Geology and Geophysics 2025 number 6

Caldera-forming explosive volcanism is the most dangerous natural hazard, which has catastrophic consequences to the life, humans and their economic activities. The paper presents a summary of published and original data on the late Pleistocene-Holocene caldera-forming volcanism within the Great Kuril Arc (GKA) available to the recent times. The published data reveal that formation of explosive calderas occurred throughout all GKA segments in the late Pleistocene and Holocene. Most frequent it was in the Southern and Central segments of GKA, where it meets the back arc Kuril Basin. The majority of the studied calderas appeared in the late Pleistocene 50-12 Ka and early Holocene 8-6 Ka. Intensive caldera-forming volcanism in GKA could be contemporaneous to similar events in the East-Kamchatka Volcanic Belt and Southern Kamchatka. Caldera eruptions of GKA in the late Pleistocene and early Holocene were linked to evolution of large reservoirs of predominantly dacitic magmas, which were generated by partial melting of metabasitic protholiths in the shallow crust (3-12 km) at 810-930°C. Rhyolitic melts of these magmas were saturated with H₂O, CO₂, sulfur compounds, and probably other gaseous species. This caused shallow degassing at the pre-eruptive stages of the magma reservoir evolution. The study rises problems, which solution would provide a basis for effective prediction of catastrophic volcano explosions and monitoring of active GKA caldera volcanoes.

The junction zones of mountain ranges with adjacent intramontane basins and foreland basins, developing under regional compression and transpression, are concentrators of key seismogenic faults. In this case, two counter-dipping systems of reverse faults and thrusts develop, which leads to the formation of positive (forebergs, pop-up structures, fault and tectonic scarps, and fault-related folds) and negative morphostructures (pop-down structures) in the marginal parts of sedimentary basins. As a result, the marginal parts of the basins are involved in the uplift. This results in the gradual growth and expansion of mountain ranges, accompanied by a corresponding reduction in the size of intramontane basins, indicating that the upper portion of the Earth’s crust experiences shortening. However, the mechanisms of occurrence of conjugate fault systems remain not fully understood. The mechanisms of such deformations in the upper crust are investigated under lateral compression of the rock mass using two-dimensional numerical modeling. The problem is solved in the elastic-plastic approximation using the Drucker-Prager-Nikolaevsky model with a non-associated flow law. In all models, regardless of the number of layers, reverse faults and thrusts with direct and reverse dips relative to the direction of horizontal compression are formed. As a result, positive and negative structures are formed in the model’s top surface relief, which are analogs of the corresponding natural morphostructures. The resulting data show that the development and configuration of localized shear bands corresponding to reverse faults and thrusts are affected by elastic-strength parameters, friction at the base of the model, and conditions on its lateral boundaries. It is revealed that, in the case of a multilayer medium, a single stage of deformations may result in a multitiered system of localized shear bands, characterized by different slopes and limited only by a specific layer. Special attention is paid to models that exhibit interlayer slipping, driven by varying relative movement rates of the layers due to differences in the elastic and strength properties of the rocks, thereby leading to the development of backthrusts in the upper part of the section that are not associated with the base of the model. Backthrusts are most often observed in the upper part of the model. Block inclusions at the base of the models, regardless of their strength properties, can affect the spatial localization of multidirectional localized shear bands that arise at their boundaries. The numerical modeling data allow for a better understanding of the relationship between the mechanical properties of rocks and sediments with the features of the development of faults, thrusts, and backtrusts.

The study focuses on determining the position of the southern boundary of the Okhotsk Plate based on the analysis of seismicity distribution in the Hokkaido and Honshu regions as well as adjacent territories according to the Japan Meteorological Agency (JMA) data for a period of 1998-2022. The seismicity distribution data are compared with regional seismic tomography models and the distributions of the directions of principal seismotectonic deformation axes according to data on the focal mechanisms of strong ( M_w > 4.7) earthquakes using the International Seismological Center (ISC) data for a period of 1976-2022 and other recent geological-geophysical characteristics, such as gravity field heterogeneities, crustal thickness, volcanic manifestations, etc. It is revealed that the southern boundary of the Okhotsk Plate actually passes along the southern tip of Hokkaido Island (through the Oshima Peninsula and Uchiura Bay) rather than along the Hidaka Ridge or through Honshu Island, as previously assumed by other authors.

We discuss possibilities of application of radon activity concentration variations in investigating changes in the stress-strain state of a rock massif. Based on the long-term radon monitoring at the South Kuril geodynamic polygon, the methodology for interpretation of soil radon activity concentration anomalies has been developed. A causal relationship between earthquakes and radon anomalies has been established. It is shown that tectonic events occur after the radon anomaly passes over the maximum level, and the reflection time depends on the distance between the observation point and the epicenter of the event. The mechanisms of formation of radon anomalies in the compression/extension zones are described.

The stored waste from processing Ni-Co arsenide ores of the Khovu-Aksy deposit (Republic of Tyva, Russia) is a unique geochemical system, in which the joint behavior of As and metals (Fe, Co, Ni, Cu, Zn, and Pb) under exogenous conditions can be directly traced. We have studied the mineralogical and geochemical specifics of the distribution of arsenic (from primary arsenides to newly formed phases) and associated metals in waste with a high As content (up to 4%) throughout the section of trench burial No. 3. These strata are characterized by a slightly alkaline environment with pH = 7.7 and Eh = 486 mV. Four horizons are distinguished in the section. According to elemental analysis (XRF-SR), As, Mo, Pb, Sb, Co, and Cu accumulate in horizon 2 (80 cm), whereas Cd, Zn, and Ni, in horizon 3 (110 cm). In the processed ores, nonmetallic (rock-forming) minerals are represented by quartz, calcite, dolomite, garnet, amphibole-chlorite aggregates, single grains of K-feldspar (Kfs), apatite, barite, and muscovite. Arsenic minerals are distributed extremely unevenly throughout the section and are absent from the soil horizon (horizon 4). Arsenic is found in the section as: (1) arsenic minerals, namely, safflorite with hovuaksite, conichalcite, scorodite, arseniosiderite, sarmientite, hörnesite, annabergite, and picropharmacolite; (2) isomorphic impurity in secondary products (iron hydroxides developed after pyrite, amorphous silica, and chlorite). The presence of carbonate minerals in primary ores and the applied technological scheme of ore dressing with purification of solutions from arsenic affect directly the secondary assemblage of arsenic minerals.

Experimental studies on modeling the diamond resorption processes during mantle metasomatism accompanied by oxidation process in solid-phase matrix in the presence of intergranular fluid have been carried out. The reaction conducted between diamond and periclase to form magnesite can be considered as prototype EMOD (enstatite-magnesite/olivine-diamond) or DCDD (dolomite-coesite/diopside-diamond) reactions. The experimental studies were conducted at a pressure of 6.3 GPa in the 1100-1400 °C temperature range under redox conditions corresponding to the WM (wüstite/magnesite) buffer. It was found that the reaction between diamond and periclase to form magnesite occurs only in the presence of 0.5-0.8 wt.% water at temperatures above 1200 °C. The morphology of diamond crystals partially dissolved by intergranular H₂O-fluid at f _O2 at the WM buffer level represents a typomorphic feature of diamond dissolution/resorption in water-containing carbonate and carbonate-silicate melts. The main microrelief elements of diamond dissolution forms are negatively orientated triangular etch pits on relict octahedral faces, shield-shaped or ditrigonal dissolution layers, and drop-shape hillocks. The obtained resorption rates at these P-T- f _O2 parameters indicate that the absence of diamond in kimberlites or low-grade potentially diamondiferous kimberlite pipes might be caused by oxidizing metasomatic events in the lithospheric mantle in the regions of kimberlite emplacement.

A synthetic analog of mourite (SM), (UO₂)Mo₅O₁₄(OH)₄(H₂O)₂, has been hydrothermally synthesized at 220 ºС and characterized using single-crystal X-ray diffraction, single-crystal and powder X-ray diffraction studies at non-ambient temperatures, X-ray photoelectron spectroscopy, infrared spectroscopy, thermal, and chemical analyses. SM is monoclinic, P 2/ c , a = 9.9063(6), b = 7.1756(4), c = 12.2105(7) Å, β = 102.496(6)°, V = 847.41(9) ų; the crystal structure has been refined to R ₁ = 0.043. The chemical composition of the SM is (the Mo₂O₅:MoO₃ ratio obtained from X-ray photoelectron spectroscopy, H₂O by stoichiometry; wt.%): Mo₂O₅ = 4.61, MoO₃ = 61.06, UO₃ = 26.95, H₂O = 6.76, total 99.38. The empirical formula calculated on the basis of 22 oxygen atoms per formula unit with Mo^V + Mo^VI = 5 is (U^VI_1.03O₂)[(Mo^VI_4.63Mo^V_0.37)_Σ5.00O_13.81(OH)_0.19](OH)₄(H₂O)₂. The crystal structure of SM contains UO₈, Mo1O₆, Mo2O₅(H₂O), and Mo3O₄(OH)₂ polyhedra that share vertices and edges to form layers linked by hydrogen bonds only. SM is stable up to 250 ± 10 ºС. Upon heating, continuous dehydration occurs between 160-250 ºС until the formation of amorphous products; crystallization above 450 ºС produces UO₂MoO₄, MoO₃, and UMo₁₀O₃₂. Below 250 ºС, thermal expansion of the compound is strongly anisotropic, with the maximal direction perpendicular to the plane of the layers.

Studying the spatial distribution of specific groups of marine invertebrates in the geological past and analyzing their geographic differentiation over time are crucial for understanding both their evolutionary patterns and the history of marine basin development. This study, based on modern paleontological and biostratigraphic data of the Boreal Triassic, refines the taxonomic composition and distribution of Ladinian ammonoids across various regions of the Boreal Realm. Zonal correlation on ammonoids of Ladinian deposits has been carried out for Northeast Asia, British Columbia, the Canadian Arctic Archipelago, northern Greenland, Svalbard, and Franz Josef Land, providing a chronological basis for comparative analysis of coeval ammonoid faunas. The qualitative and quantitative comparative analysis of ammonoid assemblages for different phases of the Ladinian Age has established that Northeast Asia consistently belonged to the Siberian Province of the Boreal Realm. The separation of the Canadian Province of the Boreal Realm occurred at the end of the constantis Phase due to the penetration of trachyceratids (genus Protrachyceras ) into the paleobasins of the Canadian Arctic Archipelago. Subsequently, starting from the maclearni Phase, the area of the Canadian province expanded due to the migration of Tethyan forms into the paleobasins of Svalbard. The migrations of trachyceratids, gymnitids, and lobitids into boreal paleobasins from the Tethys Ocean, as well as the dispersal of Boreal tsvetkovitids and nathorstitids into ecotonal and Tethyan paleowaters of British Columbia, were influenced not only by circumpolar currents but likely, by the lifestyle of ammonoids and their adaptation to a broader or narrower range of marine basin depths.

The study compares in detail the signals of an electromagnetic tool with toroidal coils which were measured in an electrolytic tank with a borehole and numerically calculated in its three-dimensional geoelectric model. For each electrical resistivity of the electrolyte, we performed the profiling of the air-tank and tank-borehole boundaries during round-trips of the tool. The values of the coupling coefficient of the measured and modeled signals have been determined for the entire set of frequencies and positions of the measuring coils in the summary and differential operating regimes. We have identified a pair of signals with a virtually constant coupling coefficient at varying electrolyte mineralization. Drawing on this pair, transformations of the tool signals into the apparent electrical resistivities of the geoenvironment have been constructed. The resulting transformation graphs allow a reliable recalculation of the measured signals of the toroidal tool into the apparent electrical resistivity distribution in the near-wellbore space, which is necessary for the petrophysical interpretation of the field log data.

The 29.07.2022 (UTC time 13:01:10.1) Tsagan-Shibetu earthquake with M_L = 6.2, M_W = 5.5 occurred in the eponymous mountain range in the eastern Altai Mountains (Gorny Altai), in proximity to the Tuva Basin (50.51º N, 90.69º E). The Tsagan-Shibetu Range was seismically inactive in the twentieth century, while neighboring with seismically active ones: the focal area of the 1970 Ureg-Nur earthquake with M_S = 7.0 and the seismically active Shapshal Range where earthquakes with magnitude of up to five occurred quite frequently, however, no large earthquakes have hitherto been reported. Significant alterations in the structure of seismicity of Gorny Altai occurred after the 2003 Chuya earthquake with M_S = 7.3: the period of quiescence was followed by emergence of new zones of enhanced seismic activity, with the Tsagan-Shibetu Range representing one of them. The internal structure of the mountain range is characterized by the formation of a triple-planed focal area with the pulse-like seismic process developing over time. The evolution of high seismicity in the considered mountain range occurred in the presence of foreshocks.