Home – Home – Jornals – Russian Geology and Geophysics 2022 number 11

The Kamchatka Peninsula is one of the most tectonically active regions in the world, where intensive and diverse modern volcanic activity takes place. In the recent decade, a substantial progress in the investigation of deep structures beneath Kamchatka has been achieved owing to numerous tomography studies based on seismological data provided by permanent stations and temporary networks deployed in some key areas. The goal of this review is summarizing and systematizing dozens of separate multiscale geophysical studies in Kamchatka and constructing an integral model of volcano-feeding systems. An important part of this review contains the description of results of various seismic studies related to the Klyuchevskoy group volcanoes, which can now be considered one of the best studied volcanic areas in the world. The results of the regional-scale seismic tomography reveal the existence of the Pacific slab window, which determines the particular activity of the Klyuchevskoy group volcanoes. Middle-scale tomography studies have found traces of an ascending hot mantle flow that passes through the slab window, reaches the bottom of the crust below Shiveluch Volcano and then propagates laterally toward the Klyuchevskoy group. Seismic models of the entire crust in the area of the Klyuchevskoy group were used to identify different mechanisms of magmatic feeding of three most active volcanoes: Klyuchevskoy, Bezymianny, and Tolbachik. The data of local networks deployed on several volcanoes of Kamchatka were used to image the magma sources in the upper crust, which are directly responsible for the current eruption activity. The comparison of the results for the Kamchatkan volcanoes with tomography models of several other volcanoes of the world allowed determining some common features and differences in feeding active magmatic systems.

Along the submeridional seismic and magnetotelluric profiles, crossing the junction zone of the Kyrgyz Range and the Chu basin of the Northern Tien Shan, the models of density, Poisson’s ratio, elastic modulus, silica content, temperature, and porosity have been constructed. Based on the complex analysis of the constructed profiles of physicomechanical rock properties, a geological-geophysical lens model has been constructed. This model addresses the sources of geophysical anomalies (found at the northern border of the lens) and possible mechanisms of the lens formation.

Detection of common quasiperiodicities in the paleointensity behavior and lengths of polarity intervals of the Earth’s magnetic field was carried out. The paleointensity data were analyzed in the 170 Ma-present day interval. Behavior of the lengths of geomagnetic polarity intervals was investigated within the interval spanning the entire Phanerozoic (540 Ma-present age). It was found that the spectrum of the main paleointensity variations and polarity interval lengths is discrete and includes quasiperiodic variations with characteristic times of 15, 8, 5, and 3 Ma. The characteristic times of these quasiperiodic variations in the geomagnetic field at the beginning and end of the Phanerozoic differed not more than 10%. The spectral density of quasiperiodic changes in the geomagnetic field changed cyclically over geological time. The connection between the behavior of the amplitudes of paleointensity variations, the lengths of geomagnetic polarity intervals, and their spectral density is shown. The spectral density of quasiperiodic paleointensity variations (geomagnetic activity) was relatively high in the 150-40 Ma interval (Cretaceous-early Paleogene). At this time, the amplitudes of paleointensity variations and the lengths of geomagnetic polarity intervals increased. Within the intervals spanning 170-150 Ma and 30 Ma-present age, the quasiperiodic variations of paleointensity were barely expressed against its background noise variations, while the amplitudes of paleointensity variations and the lengths of polarity intervals were decreasing. Alternations of the time intervals in which paleointensity variations acquired either a quasiperiodic or noise character took place during the evolution of the geomagnetic field.

The self-excited oscillator describing high-frequency excitations of a drillstring with high amplitudes of torsional oscillations has been presented. The self-excited oscillator is characterized by discrete degrees of freedom. Fundamentally, the work of the self-excited oscillator is based on the idea of the maximum cutting force during the destruction of rock by PDC bits and the results of laboratory studies by M.I. Vorozhtsov, K.I. Borisov, G.P. Cherepanov. The differential equations of the non-autonomous self-excited oscillator are based on the results of laboratory studies. A typical dynamic mode of the self-excited oscillator operation has been presented. The results have been compared with experimental data.

In the present paper we demonstrate that most sulfides of the studied deposits of the Archean Sumozero-Kenozero greenstone belt within the Karelian Craton on the Fennoscandian Shield have nonzero Δ³³S values. This indicates that proportions of seawater sulfate and elemental sulfur in Mecoarchean included into the ores and resulting from UV photolysis, are different. Our results show that systematics of sulfur isotopes of sulfides generally reflects the mixing of mass-independently fractionated sulfur reservoirs with positive and negative Δ³³S values. Pyrite is depleted in ³⁴S isotope, which was interpreted as evidence for microbial sulfate reduction. Variations in the positive Δ³³S anomalies of the Leksa deposit and the general tendency for Δ³³S sulfide content to increase with stratigraphic levels in certain boreholes most likely reflect the change in temperature and the fluid mixing throughout the life of the hydrothermal system. The presence of sulfides with strongly negative Δ³³S anomalies suggests that atmospheric sulfur and seawater sulfate, rather than volcanic sulfur, were the prevailing source for mineral systems of the studied deposits. The presented data require the Mesoarchean seawater to contain sulfates at least locally.

Analysis shows that the occurrences of ore gold of different formational types and ages widespread in the Republic of Tyva are related to the intricate and long geologic history of this region. The Tyva region is of polycyclic development; in the period from the Precambrian to the Cenozoic, there were stages of subduction, accretion-collision, and rifting processes accompanied by volcanism of different types and a wide manifestation of granitoid magmatism. Gold metallogeny was discretely developed throughout the geologic history of the region and was most widely manifested at the island arc stage (570-510 Ma), in connection with the formation of gold-bearing pyrite-polymetallic deposits. The late Cambrian-Ordovician period of granitoid magmatism was also productive for gold mineralization: It was marked by the formation of porphyry Au-Cu-Mo deposits, proper gold deposits, and gold-skarn, gold-quartz, and gold-beresite ore occurrences. Several gold-bearing clusters confined to different structure-formational zones are distinguished in the Tuva area. To date, gold contents of commercial interest have been identified in the Tardanskoe gold deposit with Au reserves of 7371.8 kg, in the gold-bearing Kyzyl-Tashtyg pyrite-polymetallic deposit with estimated Au reserves of 15.08 tons and Ag reserves of ~700 tons, and in the gold-bearing Ak-Sug porphyry Cu-Mo deposit with C₂ gold reserves of 83 tons and silver reserves of 368 tons. In the course of prospecting and revision performed at many gold ore clusters in recent years, additional information about the high endogenous-gold potential of the Republic of Tyva has been obtained. There are a number of objects promising for commercial reserves of endogenous gold, such as The Kara-Bel’dir gold ore cluster with C₁ and C₂ gold reserves of 22.8 tons, the Aldan-Maadyr gold ore cluster with predicted P₂ gold resources of 38 tons and Р₃ gold resources of 80 tons, the Eiligkhem ore cluster with predicted P₃ gold resources of 36 tons, the Kyzyk-Chadr ore cluster with predicted Р₁ + Р₂ gold resources of 39.7 tons and silver resources of 198 tons at the Kyzyk-Chadr deposit, and the East Tannu-Ola ore district with Au resources of 68.3 tons. Thus, the intricate geologic development of the Tyva area and various geologic events, including large-scale magmatic processes, led to the formation of both gold and gold-bearing deposits and ore occurrences, which give reason to anticipate revealing of large commercial reserves of endogenous gold and the development of gold mining industry in the republic.

This study describes a procedure for estimating the distribution of hydrocarbon resources by fields with different amounts of reserves and by the number of pools in each field. Such estimation is important for predicting the exploration efficiency. The procedure first uses a simulation model for generating a population of hydrocarbon pool sizes. Then, the spatial distribution of pools over a petroleum basin is modeled as a nonstationary Poisson point field. When generating a field population in the basin, each field is considered a geometric object consisting of several pools according to the principle of their mutual geometric location. The proposed model was used to estimate the two-dimensional distribution of the number of fields and the amount of resources in them by the reserve sizes and by the number of pools in each field. The proposed assessment was made for oil resources in the West Siberian petroleum province.

From the viewpoint of modern knowledge, the geological structure of the Upper Jurassic-Neocomian clinoform complex, its regional seismogeological model has been constructed, and the criteria to locate oil accumulation zones (OAZ) and specific oil-promising prospects have been identified. All these factors made it possible to provide a significant increase in oil resources.