Home – Home – Jornals – Russian Geology and Geophysics 2022 number 8

A new variety of Ti-clinohumite garnetites (‘garnet peridotites’) of the diamond-bearing Kumdy Kol terrane in the Kokchetav Massif has been studied. An ultrahigh-pressure phase, Ti-chondrodite, orthopyroxene and contrast-zoned garnets of retrograde zoning have been found in these rocks for the first time. Ti-chondrodite and orthopyroxene were not described in these rocks earlier, and garnet composition was supposed to be homogeneous. Based on the character of chemical zoning of garnet and orthopyroxene, the peak and three retrograde parageneses for these minerals have been distinguished. Using the garnet-orthopyroxene geothermobarometry, for the Kumdy Kol terrane, based on the quantitative P-T estimates of Ti-clinohumite garnetites, a detailed retrograde P-T trend has been constructed for the first time; this trend includes the UHP peak stage (55-57 kbar, 1075-1120 °С), and three stages of retrograde metamorphism under UHP (32 kbar, 830 °С), HP (21 kbar, 750 °С), and MP (12 kbar, 740 °С) conditions. For the Kumdy Kol terrane this is the most detailed Р-Т trend indicating subisothermal decompression during crustal rock exhumation from the mantle depths (~ 170 km) to the lower-crust conditions (~ 36 km). The above-mentioned mineralogical findings allowed reconstructing all the described metamorphism stages, using the same rock and various generations of the same minerals (garnet and orthopyroxene) formed at different stages of tectonometamorphic evolution of the Kumdy Kol terrane. Preservation of the relics for mineral zoning of the early metamorphic stages seemed to be due to high uplift velocities resulted in rapid decrease in P-T parameters and short duration of regressive metamorphism. The virtually complete identity of the obtained P-T trend for the Kumdy Kol terrane and the P-T trend for the Barchi terrane supports the idea of the common tectonometamorphic evolution of the whole western (diamond-bearing) part of the Kokchetav massif.

Pyrolysis-free gas chromatography-mass spectrometry was used to determine the gas phase composition of inclusions in phenocrysts from basalts and rhyolites of the Men’shii Brat Volcano (Medvezh’ya caldera, Iturup Island). Among more than 300 compounds detected in the inclusions, hydrocarbons are predominant (52-92 rel.%). These hydrocarbons (С₁-С₁₇) are alkanes, alkenes, alcohols, polycyclic aromatic hydrocarbons, ethers and esters, aldehydes, ketones, and carboxylic acids as well as sulfonated, nitrogenated, and halogenated organic compounds. Inorganic substances, which are predominantly CO₂, H₂O, SO₂, and N₂, are present in subordinate amounts. The organic compounds are interpreted as products of abiogenic synthesis of hydrocarbons in magmatic gases. This fundamentally new information about the composition of magmatic fluid suggests that mantle and crustal magmas can transport hydrocarbon substance.

We present results of Pb-Pb geochronology and Sr- and C- chemostratigraphic study of limestones and dolomites of the Ara-Oshei Formation of the Tunka ridge in East Sayan. The δ¹³C value of the limestones varies from -0.6 to 2.3‰ PDB, the ⁸⁷Sr/⁸⁶Sr ratio is within 0.70851-0.70864, and the Pb-Pb age is 521 ± 21 Ma (MSWD = 0.9). The δ¹³C value of the dolomites varies from -3.9 to 0.5‰ PDB, the ⁸⁷Sr/⁸⁶Sr ratio is 0.70844-0.70882, and the Pb-Pb age is 417 ± 8 Ma (MSWD = 2.5). The U-Pb isotope system of the dolomites was disturbed during the geologic evolution of the region, which led to the rejuvenation of their Pb-Pb age in the Early Devonian. The obtained Sr-chemostratigraphic and Pb-Pb data show that the limestones of the Ara-Oshei Formation accumulated in the early Cambrian. This provides a reliable proof that the limestones are part of the Vendian-Cambrian sedimentary cover of the Tuva-Mongolian massif.

Coal fragments in basaltic melt were subjected to degassing, graphitization, and disintegration. As a result, the partial pressure of hydrocarbon (HC) fluids near xenoliths of graphitized coal in the melt increased, which provoked the sialic-mafic segregation of the basaltic melt, the formation of anorthosite-hortonolite association, and the separation of bitumens and ore mineral phases. The fluid-magmatic differentiation of the melt and the formation of ore phases were caused mostly by the reducing properties of HC fluids, their high affinity for metals, and their unique collective capability to adsorb microimpurities from the melt and rocks during migration and to form anomalous mineral and geochemical clusters at geochemical barriers. The diversity of ore mineral phases is due to the extraction of trace elements Fe, As, Ge, Sb, Sn, and S from coal and of Fe, Mg, Cu, Ni, Co, Pt, Pd, Rh, Au, and Ag microimpurities from the basaltic melt.

Complex analysis of jointing in the Morskoi fault zone rocks (Cisbaikalia) has been carried out to compare two principally different methods of tectonophysical reconstruction of paleostress tectonic field, by geologic-structural data. A new approach to the paragenetic analysis of statistic measurements for ‘mute’ joints has been used; this approach makes it possible to reconstruct the stress state in the local rock exposure and to understand whether it belongs to the fault zone of a certain morphogenetic type and orientation. The second approach to reconstruct the stress filed is the Angelier-Delvaux kinematic analysis, which allows us to calculate the stress tensor and determine the stress regime, based on the analysis of strike-slip vectors on joints. Using the two methods we obtained the stress state solutions of local and regional levels for the study area. The paragenetic analysis gave twice as many local solutions for the same number of observation points. This is due to different environments of the formation of jointing sets and slickenlines in time (stages of fault zone evolution) and space (closeness to the fault plane). Most of the local solutions of the kinematic analysis coincide with the identical solutions of the paragenetic method on the stress state of the first or second orders. We obtained by an order of magnitude more ‘new’ (not repeated in the other method) paragenetic solutions than kinematic ones. At the next hierarchic level, the paragenetic analysis made it possible to reconstruct the stress field and fault zones of a higher rank. The results of both methods involve several stages of tectonic evolution of the rock massif. The identical regional stress fields reconstructed by different methods seem to belong to one stage. The studied fragment of the Morskoi fault, according to both methods, was activated in the three most intensively pronounced settings: compression, left-lateral strike-slip, and extension. Strike-slip stresses are concentrated closer to the fault plane. Moreover, we also revealed submeridional extension and NW compression. The results of the stress field reconstruction using the two methods are compatible and, in general, successfully complement and justify each other; however, the paragenetic method gives more numerous and variable solutions, resulting in the transition to the regional level and construction of the map for the study area fault zones. Complex application of both methods is recommended.

The purpose of this study is to determine a morphologically pronounced fault-block structure identified with the neotectonic stage and compare it with sites with manifested exogenous processes, as well as modern and historical paleoseismicity in order to establish faults activated in the postglacial time. Based on the analysis of space images (Landsat-ETM+) and a digital elevation model (GTOPO-30), the territory of the Kola Peninsula and the adjacent part of North Karelia is subjected to morphostructural interpretation with identifying morpholineaments and an elementary block structure. It is shown by the analyzing the directions and extent of elementary, single (simple), and complex (echeloned, parallel conjugated, and imbricated) linear structures and their zones that both linear (fault) and areal (block) structures are characterized by a predominance of a single system of northwestern and northeastern differences with a clear dominance of the former and unimodal distribution of the extent of faults and the area of blocks, depending on their number. This indicates a single (recent) stage in the formation of the morphotectonic appearance of the territory and no discrete hierarchy the morphostructures. The degree of fragmentation of the territory at different depths is calculated depending on the number and extent of morpholineaments. It is determined that morpholineaments have a high degree of inheritance from Archean-Proterozoic structures (≈50%). Elementary morphotectonic blocks are grouped into composite blocks bounded by linear zones of great extent (100-600 km) having individual physiognomic features determined by fault patterns, which indicates the nature of the neotectonic dynamics and the degree of inheritance or reformation of the structural plan. The localization of manifestations of exogenous processes, epicenters of paleo-, historical, and modern earthquakes is determined on the basis of the analysis of topographic maps scaled at 1:100,000, catalogs of historical and paleoearthquakes, and the consolidated literature (including the data obtained by the authors of this study) on paleoseismic deformations. A geoinformation base is compiled, which is used to simulate the spatial distribution of endo- and exogenous signs of tectonic activity and compare it with the neotectonic fault-block structure. The spatial similarity of endogenous and exogenous activation zones and their confinement to faults, defined as activated in the postglacial time, are revealed. It is revealed that the following elements are most active in the postglacial-Holocene. First, flank elements on the Kola Peninsula along the Barents Sea coast, the Kandalaksha Bay shores, and the Gorlo Strait of the White Sea. Second, the central (nodal) part with the Khibiny and Lovozero massifs. Third, submeridional (transverse) secant structures separating the eastern part of the peninsula from the western part (Khibiny-Kola and Khibiny-Niva). The spatial parameters of the activated zones indicate a range of earthquake magnitudes M ≈ 6.5-7.5 generated by these structures both in the postglacial period and in the Neopleistocene as a whole.

The paper presents some results of the development of a digital permanently operating structural and geological model (implemented at the Autonomous Institution of the Khanty-Mansiysk Autonomous District-Yugra V.I. Shpilman Research and Analytical Center for the Rational Use of the Subsoil) for the area of central West Siberia and important aspects that need to be considered in implementation practice. The methodological challenges of the variational-grid mapping method and its application include problems such as finding an optimal computational grid and using heterogeneous input information for mapping key seismic horizons. An effective method is considered for using seismic data with uncorrected misties in geological mapping based on corrections for derivatives of the desired function along the seismic profile direction (i.e., seismic surface shape). Special attention is paid to a priori information when mapping stratigraphic boundaries with poorly correlated seismic sections. The conformal thickness model linking geometry of the mapped surface with the structural framework of the two reference boundaries has been tested on the construction of regional structural and geological models for the Jurassic interval of the cross section of West Siberia. The model modification can also be applied to mapping clinoform formations. Given the scale and non trivial tasks of regional mapping, the issues related to setting up the computing workflow are considered in detail: algorithmization, programming protocols, and automation. In the light of the accentuated relevance of the development of specialized software, the GST (Geo-Spline Technology) software product is considered, with the digital structural framework of the region implemented therein. This resulted in a permanently operating model, since the hierarchical object-oriented approach implemented in GST ensures a complete protocol that integrates computational procedures and data flows, as well as automation of model recalculation. The structural framework serves as the basis for mapping physical properties and parameters, assessment of the region resource potential, and study of other aspects. The proposed model being not conclusive, it offers rich opportunities for revision and refinement as to the area size (geological model domain), level of detail, database enlargement, thereby determining the directions of further development of the digital structural framework.

The magnetotelluric sounding (MTS) method implemented on drifting ice floes in the Arctic is suitable for detection of 3D inhomogeneities in crustal conductivity while recording the transverse magnetic (TM) mode potential of the electromagnetic field. High-conductivity layers of seawater and sediments shield the underlying 3D inhomogeneity. Their presence virtually does not affect changes in the standard responses of the medium used in MTS but is quite noticeable in the characteristics of the TM mode. To register them, one can use a circular electric dipole (CED) located at the surface of an ice floe. During the drift, the electric field can be measured on the ice floe using electrodes in seawater. We propose to lower the magnetic sensors beneath the ice, in seawater, because ice deformations interfere with the magnetic-field component measurements. The coordinates of the observation station during MT soundings on the ice floe in the Arctic (similar to earlier observations at North Pole stations) can change significantly. In order to take into account the effect of horizontal movements of the drifting station, we propose to complement all the recorded time series with the coordinates of measurement points. We developed a technique for processing such data to take into account nonplane-wave effects, which can occur in the Arctic because of the proximity of ionospheric current jets. We carry out the synchronization of all observations in the investigated area, using a model of spatial and temporal field variations and data accumulation. To test our approach, we use the synthetic experimental data for the model that considers the existence of seawater, sediment, resistive crust, crustal object, and the underlying mantle. We determine the crustal 3D object parameters with account of the TM-mode potential distributions at the seawater surface restored from the synthetic experimental data obtained at the drifting station during the drift. We use the Nelder-Mead method for optimization of the object characteristics. The parameters of the object become highly similar to their test values if the trajectory of the drifting station passes through an object, covering it most fully.