Home – Home – Jornals – Russian Geology and Geophysics 2024 number 7

The Qilian Shan (or Qilian Mountains), located on the northeastern margin of the Tibetan Plateau, is an actively growing orogenic belt resulting from the far-field impact of the India-Eurasia collision. The northward penetration of the Indian Plate is responsible for intense crustal shortening in the Qilian Shan. However, the tectonic deformation pattern in response to the crustal shortening remains unclear. In this study, we present the regional seismicity, fault activity, and GPS crustal movement velocity field to characterize the active tectonic deformation of the Qilian Shan based on historical data over the past two decades. The results suggest that the western Qilian Shan is characterized by distributed north-south crustal shortening, while the eastern Qilian Shan is dominated by blocklike eastward extrusion of crust along major strike-slip faults coupled with clockwise rotation. North-south crustal shortening and east-west lateral extrusion, two deformation modes responding to the India-Eurasia convergence, match the crustal deformation in the Qilian Shan. The tectonic deformation of the western Qilian Shan is largely in agreement with the former, while the eastern Qilian Shan corresponds closely to the latter. Lower crustal flow beneath the central Tibetan Plateau provides the potential driving force to induce the eastward extrusion of crustal material out of the plateau and the growth of some boundary mountain ranges, such as the Qilian Shan.

Polymigmatites in the Early Proterozoic metamorphic complex of the Northern Ladoga area trace the evolution of thermodynamic conditions during anatexis. The P-T conditions inferred for anatectic leucosome correspond to the onset of partial melting under granulite facies conditions of 5.5-6.2 kbar and 720-810 °C. After the peak of metamorphism, pressure and temperature show a coupled decrease to 4 kbar and ~550 °C, respectively. The latest granitic veins were intruded into metamorphic rocks during an event of brittle deformation. The P-T trend correlates with changes in migmatitic mineral assemblages and in chemistry of minerals. Newly formed leucocratic material changed from plagiogranitic to granitic composition when melting involved plagiogneiss but melt derived from Al-rich metapelite remained granitic. The analyzed leucosomes and granitic veins originated by multistage melting, under P-T conditions changing from granulite to amphibolite facies, between 1875 and 1865 Ma. Judging by their ages, the leucosome and granitic vein bodies from the Lakhdenpokhia and Priozersky zones of the area differed in the total duration of crystallization. Therefore, a single thermal event involving different lithologies can produce intrusions of different ages.

We report study results of basic intrusive bodies in the middle Paleozoic Vilyui paleorift (eastern Siberian Platform). Geochemical data for basic sills penetrated by boreholes in the rift’s dike swarms are presented, as well as our data on the time of formation of sills and dikes. We also studied mineral-hosted melt inclusions from a dolerite dike of the Vilyui-Markha dike swarm on the northwestern flank of the Vilyui paleorift. Data on the compositions of homogenous glasses of mineral-hosted melt inclusions yielded the P-T parameters of mantle sources of basic melts responsible for the formation of the Vilyui paleorift gabbro-dolerites. Two depth levels of basic melt generation have been established: 95-65 km at 1480-1400 °C and 55-45 km at 1360-1320 °C. Crystallization of the melts occurred at a shallow depth of 12-4 km with a decrease in temperature from 1185 to 1125 °С. This occurrence of basic melt sources at two different depths, as well as the heterogeneity of the chemical composition of melt inclusions and their host minerals explains the presence of two pulses of Devonian basic magmatism. Our new numerical thermomechanical model of magma rise during melting of the lithospheric mantle above a mantle plume supports the existence of two chambers at the spinel-garnet peridotite boundary and under the base of the crust, as well as the two-stage nature of Devonian magmatism.

The study of minerals, melt inclusions, as well as natural glasses showed that two different melts contributed to the formation of ignimbrites of the Khangar Volcano. The first, providing the information on melt inclusions in plagioclase and quartz phenocrysts, represents the state of magma in a deep source. The other type of melt is responsible for the formation of glasses and microcrystals of feldspars in fiamme. Experimental and analytical studies of melt inclusions showed that crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano occurred at temperatures of 840-960 °C and pressures up to 1.1 kbar, from the melt with water contents up to 3.23 wt.%, under the conditions of magma chamber. The presence of syngenetic primary melt and fluid inclusions in plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano indicates phase separation (“boiling”) of the melt with mass formation of СО₂ microbubbles in magma. The other type of melt is secondary relative to the magmatic systems of the Khangar Volcano and is formed by sintering and melting of tuffogenic volcanoclastic material. This melt contributed to the formation of fiamme in the ignimbrites under consideration. Based on the study of glasses and microcrystals of feldspars in fiamme, it was found that crystallization of oligoclase occurred at temperatures of 770-840 °C in the melt between the spherules (with water content up to 2.91 wt.%). Sanidine crystals grew over spherules at lower temperatures, 680-760 °C.

Carbonaceous (shungite) rocks have high contents of trace elements, which can get to the environment through natural weathering. Shungite rocks are a group of Precambrian carbonaceous rocks of volcanic and sedimentary genesis in Karelia. In this work we present results of studying the mineral and geochemical compositions of shungite rocks at their outcrops on the shoreline of Lake Onega. The interaction of the Onega waters with shungite rocks led to: (1) the removal of most elements, except for K, Mn, Ba, and Mg, whose contents in the rocks increased; (2) the formation of an assemblage of secondary minerals, such as hematite, jarosite, goethite, chalcocite, anglesite, brookite, and Mn hydroxides. Based on the results obtained, we propose a model of the transformation of high-carbon (shungite) rocks by the Onega waters.

The objects of study were bitumens of sedimentary rocks and paraffin oils of the Middle Devonian-lower Frasnian terrigenous complex of the Timan-Pechora basin. We studied the composition of biomarker hydrocarbons and the carbon isotope composition of individual n-alkanes of bitumens from the cores of the wells of the Omra-Lyzha saddle. The results were compared with similar data on the composition of paraffin oils from the south of the Pechora-Kozhva megaswell. The generation potential and thermal maturity of organic matter were studied by the Rock-Eval pyrolysis method. The maturity of organic matter in the rocks of the studied complex in the Omra-Lyzha saddle and the Pechora-Kozhva megaswell corresponds to the oil window. Data on the carbon isotopy of n-alkanes and on the composition of biomarker hydrocarbons do not contradict the possible genetic relationship between paraffin oils and the organic matter of the host sedimentary complex. Both the studied paraffin oils and the rock bitumens contain early eluting rearranged hopanes atypical of oils of other petroliferous complexes and show high contents of n-alkanes relative to iso-alkanes and polycyclic biomarkers. The content of ¹³C tends to decrease with an increase in the number of carbon atoms in the n-alkane molecule in both oils and bitumen rocks.

This article presents a novel approach to addressing the challenges in permafrost monitoring through the integration of deep-learning techniques with conventional electromagnetic sounding methods. Our methodology comprises a forward finite element method (FEM) solver, augmented with the Sumudu transform, and an artificial neural network (ANN) solver trained on FEM-generated data. Remarkably, the ANN solver demonstrates similar accuracy to the FEM solver but operates at a superior speed that is nearly 10,000 times faster. Furthermore, we introduce an inverse problem solution drawing on the PARS algorithm. In addition, we present an ANN-based inverse solver, where the input and output roles are inverted. The ANN inverse solver is trained on the same data, thereby offering an alternative approach to solving the inverse problem. In a computational experiment, we compare the numerical inversion results using the PARS algorithm with those obtained from the ANN forward solver, ANN inversion, and a linear combination of these solutions. This comprehensive analysis sheds light on the effectiveness of our deep-learning-based approach in permafrost monitoring, providing insights for future applications in geophysics and environmental science.

We present the results of the study of the effect of induced magnetic polarization of clay beds under the influence of an external harmonic electromagnetic field (frequencies 70 and 875 kHz). A two-stage numerical modeling procedure is proposed. At the first stage we determine the effective relative magnetic permeability of a synthetic sample with inclusions of clay particles. In this case a 3D heterogeneous mesh sample is generated. Then we numerically model a spatial distribution of an electric field. The electromotive force (EMF) induced in the measuring coil is calculated from this distribution. Relative magnetic permeability is determined by comparison with EMF of homogeneous samples with different values of magnetic permeability. It has been found that during the electric field excitation by an alternating current coil, the effect of induced magnetic polarization appears in the sample with clay particles. Its manifestation is that the effective magnetic permeability becomes complex. At the second stage we calculate the EMF diagram of the three-coil logging probe in the macro-model ‘clay cap - reservoir’. Magnetic permeability of the clay cap is given by a complex value. In the generated logs, extremes appear in the vicinity of the bottom of the clay cap; they do not correspond to the distribution of electrical conductivity and magnetic permeability in the given model. They can be incorrectly interpreted when analyzing real logs into individual formations. Numerical modeling at all stages is performed by the Vector Finite Element Method on a consistent adaptive tetrahedral partition and the first-order Webb vector basis.

This work is devoted to the development of the engineering seismic monitoring method created in Geophysical Survey of the Russian Academy of Sciences (GS RAS). In previous years, the “method of standing waves” was created and put into practice. It helps to separate natural oscillation modes of buildings and other engineering structures. The natural oscillations of hundreds of various objects (buildings, bridges, dams, etc.) had been studied and identified. We assumed that the physical condition of studied constructions could be controlled during exploitation by measuring the changes of natural oscillation frequencies. That would help to identify the appearance of defects in constructions, to prevent the risk of their destruction. However, it turned out that not everything is that simple: changes in frequency values are logically affected by changes in the environment around the studied objects. This article provides examples of these relations, influence of changes in environmental temperature, mass of objects and precipitation on the frequencies of natural oscillations.