Home – Home – Jornals – Russian Geology and Geophysics 2025 number 4

A long history of diverse magmatism of the Omolon Cratonal Terrane (OM) is traced from the Paleoproterozoic to the Late Miocene. The new isotope-geochronological (U-Pb, ⁴⁰Ar/³⁹Ar) dating and geochemical data allowed us to reliably identify nine main episodes and geodynamic settings of magmatism, as well as to discuss in detail potential sources of detrital zircon populations in sedimentary rocks of northeastern Russia. The earliest manifestations of fleck gneisses of the anorogenic Upper Оmolon complex with an age of about 1.9 Ga reflect the rifting and the beginning of destruction of the Precambrian basement which has the age of about 3.2 Ga. The ongoing rifting of the OM is traced in the Riphean, when swarms of dikes and small bodies of the gabbro-diabase Strelkin complex were intruded. In the Silurian (433-425 Ma), in oblique shears and local extension zones of the OM, plutonic syenite magmas of the Anmandykan and granitoids of the Abkit complexes ( E _Nd from -23 to -10) are intruded. The largest manifestations of plutonic and volcanic magmatism were recorded in the OM in the Late Devonian (from 375 ± 3 to 356 ± 4 Ma), when calc-alkaline suprasubduction volcano-plutonic complexes (Bulun hypabyssal and Kedon volcanic) were formed. The Nd isotopic composition of Devonian magmas ( E _Nd from -20 to -6), with an extremely low content of radiogenic lead isotopes (²⁰⁶Pb/²⁰⁴Pb = 17.2-15.7) indicates their melting from a mature crustal protolith in the OM basement. Early and Middle Jurassic rifting locally manifested on the OM, which are reflected by the Tummin trachybasalt and Omolon essexite-teschenite complexes. The Early Cretaceous (144-133 Ma, E _Nd from +7 to 0) Namyndykan and Egdegkych granodiorite-monzonite plutonic complexes form suprasubduction zones of an island-arc nature on the northeastern margin of the OM. In the Santonian-Campanian time (~ 85-77 Ma), the Early Cretaceous extension zone on the Omolon massif was healed by the Kongin and Viktorin calc-alkaline complexes of the Okhotsk-Chukotka volcanic-plutonic belt. The final stage of magmatism in the OM is represented by intraplate alkali-basaltic volcanism in the interval from 9 to 7 ± 1 Ma. Synchronization in time of the manifestation of the most voluminous Late Devonian suprasubduction magmatism of the OM and basaltic LIP-magmatism of the Vilyui rift in Siberian craton was established. Based on this and comparison with detrital zircon populations from Paleozoic sandstones of northern Siberia, we reconstruct that the OM broke off from the northeastern part of the Siberian craton during the Silurian to Devonian and migrated southeast (in modern coordinates). A vast area of crustal extension and thinning in the Alazeya zone during the Jurassic-Cretaceous time has been reconstructed.

We report new paleomagnetic data for the carbonate-terrigenous section of the Vorogovka Group at its type locality along the Vorogovka River in northwestern Yenisei Ridge. The data is synthesized with existing information on the age of the rocks, including our new determinations using detrital zircons from sandstones and Sr chemostratigraphy for carbonate rocks from all three stratigraphic units of the group: Severnaya Rechka, Mutnina, and Sukhaya Rechka formations. These findings definitively indicate a younger age for the strata, in the range of 580-535 Ma, with a relatively rapid rate of sedimentation, contrary to previous estimates. The anomalous paleomagnetic record typical for the Vendian (Ediacaran)-early Cambrian period and the proximity of the paleopoles established for Siberia both suggest that the rocks formed over an even shorter interval, between 580 Ma and 560 Ma. The data shows that the paleomagnetic record of the Vorogovka Group contains, in addition to a viscous component, a metachronous magnetization, presumably of Cambrian age, and two coeval components of primary origin. The coexistence of these components in a single section does not correspond to actualistic ideas about the prevailing geocentric axial dipole geometry of Earth’s magnetic field and is the subject of a long-standing debate. Our analysis of the new data and its comparison with existing paleomagnetic records for Siberia provided new insights into the interpretation of the Vendian geomagnetic phenomenon within an original hypothesis. According to this model, the second magnetization component in the Vendian-lower Cambrian section is explained by a sharp decline in the main dipole component of the geomagnetic field to values comparable to the intensity of the global magnetic anomalies. This phenomenon distorted the conventional paleomagnetic record to varying degrees, and during periods of ultra-low dipole strength, it sometimes entirely replaced the usual record. Accordingly, these paleomagnetic vectors no longer oriented toward the magnetic pole but instead aligned with the nearest magnetic anomaly. This hypothesis is used to interpret not only the paleomagnetic vectors observed in the Vorogovka Group but also the entire set of paleomagnetic data for the Vendian-early Cambrian formations of Siberia.

The Bilibin massif is a Mesozoic intrusion located in the southeastern part of the Aldan high-potassium igneous province of the Aldan-Stanovoy Shield of the Siberian Platform. This massif consists of the alkaline-mafic-ultramafic and granite-syenite phases, which form a concentric structure with the rocks becoming more silica-rich from periphery to center. Earlier studies proposed that these phases formed either from different parental magmas or from a common lamproitic magma via magmatic differentiation. In this study, we examined a representative set of rock samples from the Bilibin massif: phlogopite clinopyroxenites, melashonkinites, shonkinites, alkali syenites, quartz syenites, and granites. Mineralogical-petrographic, geochemical, and isotope-geochemical data imply that the series of differentiation of lamproitic magma comprises rocks from clinopyroxenites to shonkinites and possibly alkali syenites, which form the first phase. The quartz syenites and granites, which form the second phase, belong to a separate magmatic series. According to geochemical data and the Nd, Sr, and O isotope composition of the quartz syenites and granites, the magmas which formed the syenites and granites were derived from the lower crust rather than from the lithospheric mantle and originated owing to either crustal melting during mantle-derived magmatism or collisional tectonics at the southern margin of the Siberian Platform in the Mesozoic Era. Rocks of the lamproite series allow us to consider it as an example of complete differentiation of a mantle lamproitic melt, with the following stages of cotectic crystallization: olivine + chromite, olivine + clinopyroxene + chromite, olivine + clinopyroxene + phlogopite, clinopyroxene + phlogopite + leucite, and clinopyroxene + phlogopite + K-feldspar. Spot analyses of trace elements in clinopyroxene, phlogopite, leucite, and apatite allow estimation of the melt-mineral partition coefficients in such a system.

The history of development of the Cis-Yenisei sedimentary sub-basin, the presence of several stratigraphic levels in the section of organic-rich Cambrian rocks, and high level of catagenetic transformations (i.e. high maturity level) of the organic matter suggest intensive hydrocarbon generation processes in these deposits in the geological past. The migration processes are shown to be responsible for high variability of bitumoids (chloroform-extractable part of organic matter), both in terms of their content and group/ hydrocarbon composition. This is evidenced by three variety of bitumoids (allochthonous, autochthonous, and sedentary autochthonous) defined according to the Uspenskii-Vassoevich regularity. In order to attain more precise differentiation between these bitumoids, we propose here their classification by group and hydrocarbon composition, into seven classes. Allochthonous bitumoids (class VII) are described as hydrocarbon-dominated and migration-prone accounts for about 30% of the total amount of bitumoids, another (mixed) group (6%) is made up by autochthonous-allochthones bitumoids (classes V, VI), while the last group includes autochthonous and sedentary autochthonous bitumoids, with their composition dominated by asphaltenes and resins. Results of the molecular analysis of the composition of bitumoids generally attest to the genetic unity of autochthonous and allochthonous bitumoids. For allochthonous bitumoids, however, there is a wider range of values for molecular indicators, which evidence the influence of migration processes on their composition.

This study presents a geological model of the North Kara sedimentary basin, which was developed based on the integrated interpretation of seismic and geological data on islands and Siberian continental margin, and provides seismostratigraphic and structural-tectonic characterization of the basin. The results of the study indicate that the sedimentary cover of much of the basin is composed of Paleozoic sediments of Cambrian to Permian age, reaching 13-14 km in thickness in depression zones. The thickness of Mesozoic sediments over much of the basin is < 1 km; in the northwest, where the North Kara basin borders the Barents Sea basin, the thickness of Mesozoic sediments increases to 5-6 km, and the sedimentary cover comprises Paleozoic, Triassic, Jurassic and Cretaceous geoseismic sequences. In terms of its hydrocarbon potential, the North Kara sedimentary basin is regarded as part of the potential oil and gas-bearing area of the same name, having a good hydrocarbon potential based on seismic stratigraphic and structural-tectonic considerations. A number of large depressions and uplifts recognized in the structural geometries of different stratigraphic levels can be associated with petroleum generation and accumulation zones; a number of anticlinal, structural-tectonic, structural-stratigraphic traps that are distinguished in different sedimentary complexes ranging in age from Cambrian to Jurassic can be considered as promising exploration targets. The proposed quantitative assessment shows that the most probable initial in-place hydrocarbon resources of the North Kara sedimentary basin are equal to about 10.7 billion tons of hydrocarbon equivalent.

This work is dedicated to the development of a numerical inversion methodology for electrical logging data in sub-vertical and slightly inclined oil and gas wells. It examines the specific features of joint inversion of galvanic and induction logs combined in various configurations, depending on the equipment used. The axisymmetric two-dimensional geoelectric model of medium consists of homogeneous blocks separated with horizontal and coaxial cylindrical boundaries. The blocks are characterized by horizontal and vertical electrical resistivity, as well as dielectric permittivity. Numerical inversion is carried out until a minimal discrepancy is achieved between the measured signals and those simulated within the interpretive formation model. Unlike the traditional layer-based approach, this inversion method involves not only adjusting the sounding curves but also accounting for signal variations along the borehole. In this case, the resulting model aligns as closely as possible with the actual measurements. Inversion based solely on galvanic measurements can potentially yield unstable results near boundaries with high resistivity contrasts, underestimating the resistivity of layers adjacent to highly resistive ones. The ambiguity can be reduced through joint inversion of galvanic and induction logs, although this often requires the base formation model to be more complex. In particular, during drilling with fresh clay-based mud in reservoirs with mixed oil and mineralized formation water saturation, a low resistivity annulus forms. This zone is more electrically conductive than both the invaded zone and virgin formation due to the higher content of saline formation water. It cannot be detected through galvanic well logs analysis, but its influence on induction logs is significant. Therefore, the presence of an annulus may be a crucial factor to consider when constructing a geoelectric model of the formation that aligns with resistivity logs obtained with different logging methods. These specifics are illustrated with the results of inversion of real data measured in vertical wells in the intervals of Lower Cretaceous and Jurassic deposits of the Shirotnoe Priobie oil fields.

Experimental studies were carried out in the permafrost zone using the method of reflected electromagnetic waves based on multi-offset data acquisition with common distance-point (REW CDP), referred to here for short as electromagnetic CDP (ECDP). Comparison of the results of processing several ECDP hodographs taken within the same site at a distance of up to 6 km, specifically, the vertical velocity distributions in the 0-10 μs time intercept reveals some similarities in the structure of permafrost and electrical properties of the underlying rocks, which agrees with the drilling data. The velocity analysis data served as the basis for the geophysical model of the CDP stacked section built as a 500 m deep “virtual borehole” with a step size of 50 ns (2-5 m), describing the change in the electrical resistivity parameter with depth.

The paper gives a brief description of a system of spatially arranged laser strainmeters, united by the standard time signal system into a single network, installed permanently in Schultz Cape in Primorsky Krai, in Svobodny Cape in Sakhalin Island, in an underground mine near Krasnokamensk, Transbaikalia. As the whole, this system represents the eastern part of the planetary laser-interferometric seismoacoustic observatory, which additionally includes laser strainmeters, located in the western part of Russia. The paper discusses a technique to locate the signal, recorded sequentially by these laser strainmeters. Using the results of detecting the location of the signal and the obtained data of a two-coordinate laser strainmeter allows us to restore not only the place of origin of the recorded signals, but also to determine their amplitudes at each measurement site, the magnitude of the signal at the place of its generation, aperture and attenuation over the propagation distances from the source to the receivers.

This paper presents the basic principles of recording processes occurring in the Earth’s crust during the occurrence and development of catastrophic phenomena using an earthquake as an example; these principles allow one to determine the coordinates of the source and epicenter. Basic approaches are revealed to creating a laser interferometric seismoacoustic observatory (LISO) designed to monitor continental-scale seismic events. This study also describes the instrumental equipment (laser and optical strainmeters, accelerometers, and hydrophones) developed and used at the Institute of Automation and Control Processes FEB RAS, the V.I. Il’ichev Pacific Oceanological Institute FEB RAS, and the O.Yu. Schmidt Institute of Physics of the Earth RAS; this equipment is intended for use in the LISO.