Home – Home – Jornals – Russian Geology and Geophysics 2026 number 10

The Mid-Atlantic Ridge within Iceland differs significantly from others by its structures. It consists of a few active rift zones with different kinematics and internal structure. At the same time, there are also several inactive rift zones, separated by block uplifts. This structural diversity is caused by the thermal influence of the Icelandic plume, which manifests itself in conditions of asymmetric spreading. Physical modeling was used to identify the conditions for the development of Icelandic rift zones and the features of their structure in relation to the cyclicity of plume activity. The resulting model reflects the structure and development of Icelandic rift zones over the last 21.5 Myr. Results show that the kinematics and internal structure of rift zones are a consequence of the development of multi-scale overlaps of spreading centers. Between them, block uplifts arise, which in the present-day topography are expressed as elevated peninsulas, mainly in the northern part of the island. The sizes of block uplifts depend on the distance between overlapping spreading axes. As the distance decreases, large uplifts are replaced by a series of small en-echelon blocks. The formation of this structural ensemble is the result of periodic increase in plume activity and its eastward displacement relative to the boundary of the lithospheric plates, which is caused by spreading asymmetry. There are two cycles of plume activity with different durations. The period of 7-8 Myr reflects the complete cycle of formation and development of overlappings, and the period of 2-3 Myr determines the evolution of the rift zone structure within the entire structural ensemble.

This paper presents a synthesis of published and new geological, geochronological, biogeographic, paleomagnetic, and petrological-geochemical data. The goal is to substantiate the tectonic zoning of the Central Asian Fold Belt and its Vendian-Paleozoic geodynamic evolution as a complex folded structure composed of deformed fragments of collisional and accretionary orogens. It is shown that the belt formed at the junction of the collisional and accretionary margins of the Siberian Craton. This setting is analogous to the modern tectonic interactions between the Indo-Atlantic and Pacific global tectonic segments of the Earth and their respective interactions with Eurasia. During the Vendian and Early Ordovician, two active margins existed along the periphery of the Siberian Craton: the collisional Tuva-Mongolian margin and the accretionary Kuznetsk-Altai margin. These margins separate the craton from the tectonic plates of the Paleo-Asian Ocean (Indo-Atlantic segment) and the Paleo-Pacific Ocean (Pacific segment), respectively. A distinctive feature of the Paleo-Asian Ocean plate is its complex structure, which includes fragments of oceanic crust and Precambrian Gondwanan continental blocks that contain fragments of the Grenvillian orogeny. Grenvillian detrital zircons (1.3-0.9 Ga) are widely found in Vendian-Paleozoic sedimentary deposits from various geodynamic settings of the collisional orogen. In contrast, a distinctive feature of the accretionary orogen is the widespread presence of oceanic crust fragments and the absence of Precambrian Gondwanan blocks. Consequently, there are no Grenvillian detrital zircons in the Vendian-Paleozoic sedimentary rocks of various geodynamic settings that represent the accretionary orogen. The early to middle Paleozoic Charysh-Terekta-Ulagan-Sayan-Olkhon suture-strike-slip zone separates the collisional orogen from the shelf of a back-arc basin. This basin is located in the northern part of the Altai-Sayan Folded Area, which frames the Siberian Craton. The suture-strike-slip zone consists of fragments of Vendian-early Paleozoic oceanic crust from a back-arc basin, Ordovician blueschist, and Ordovician-Silurian turbidite, as well as late Silurian-Devonian collisional metamorphic rock and granite. Based on the structural position of the suture-strike-slip zone, petrology, and geochronology, this zone is interpreted as a subduction-collision complex. It formed in the back-arc basin of the Vendian-Cambrian Tuva-Mongolian island arc. Late Paleozoic strike-slip faults and thrusts manifested in the Central Asian Fold Belt during the Late Devonian and early Carboniferous. These structures resulted from the collision of the Kazakhstan-Baikal orogen (a composite continent) with the Siberian Craton, which led to the formation of the North Asian continent. Subsequently, during the late Carboniferous and Triassic, the collision of the East European and North Asian continents led to the formation of Eurasia. As a result of these collisional events, the originally linear structures of the collisional and accretionary orogens were deformed into large oroclinal folds. These structures were further fragmented by strike-slip faults and thrusts into numerous blocks. Within these blocks, the Vendian-Paleozoic structures are complicated by thrust-fold deformations.

Analysis of the hypsometric positions of lava flows of different ages (~8 and ~4 Ma) in the Iya-Uda volcanic field indicates that the main phase of intense relief dissection and activation of block movements along the Main Sayan Fault within the Biryusa Block occurred from the Late Miocene to the Pliocene. The geochemical characteristics of the lavas, including elevated FCKANTMS values (0.46-0.77) [Yang et al., 2019] and the positions of data points on CaO-MgO and TiO₂/Al₂O₃-SiO₂ diagrams, point to melting of a garnet-pyroxenite mantle source rather than a typical peridotite mantle. The trace-element composition of the rocks is consistent with OIB-type intraplate basalts. Variations in Th/Nb and TiO₂/Yb ratios suggest that the role of garnet in the melting zone increases for the 4 Ma lavas, whereas the contribution of the lithospheric mantle is more significant for the 8 Ma magmas. The origin of the enriched pyroxenite component in the lithosphere of the Biryusa Block is most likely linked to Cenozoic tectonic convergence and rifting. These processes led to foundering dipping of lower crustal material into the mantle of the Siberian Craton. Volcanism in the (dripping) Iya-Uda interfluve is thus the result of melting of a heterogeneous and enriched lithospheric mantle.

Experimental studies modeling the interaction of garnets from eclogitic and lherzolitic parageneses with reduced and oxidized sulfur-rich fluids were conducted using a split sphere multianvil high-pressure apparatus (BARS). The experiments were performed in garnet-sulfur-carbon and garnet-anhydrite-carbon systems at a pressure of 6.3 GPa and temperatures of 1150, 1300, and 1450 °C, using graphite capsules as well as platinum capsules with a buffering hematite container. We demonstrated that interaction of both reduced and oxidized sulfur-rich fluids with garnets of mantle paragenesis leads to their partial dissolution and recrystallization, resulting in the formation of zoned crystals with reduced iron concentrations in the rims. In addition, this metasomatic interaction results in (1) garnet carbonation, accompanied by the formation of a magnesite ± dolomite + kyanite + coesite assemblage or a carbonate-silicate melt, and (2) garnet sulfidation, with the formation of pyrite (1150-1300 °C) or a sulfide melt (1450 °C) as inclusions within garnet and in the interstitial space. The experiments yielded indicator characteristics for mantle-paragenesis garnets that have been affected by sulfur-bearing fluids. These characteristics include decreased FeO contents in the peripheral zones of the crystals relative to their cores, as well as the formation of sulfide inclusions that trace the boundaries between zones. Here we present the first study of diamond dissolution features under lithospheric mantle P-T conditions in a silicate-sulfur-carbon system that models mantle metasomatic alteration of garnet-bearing rocks by sulfur-rich fluids. We conclude that, under these conditions, diamond crystals undergo partial dissolution, producing characteristic etch pits in the form of negative trigons on {111} and rectangles on {100}.

We studied chemical and isotope (C, O, Sr) compositions of carbonate sediments in three sections of the lower Baratal Group in the southeast of Gorny Altai, which are considered to be one of the oldest known fragments of the carbonate cover of oceanic uplifts preserved in the structure of the Central Asian Orogenic Belt. The contents of Fe, Mn, Sr and their ratios to each other, and to isotope parameters indicate postsedimentary alterations varying within and between studied sections. For the least altered Akkaya section, the δ¹³C values are -0.4…+0.7‰, and the ⁸⁷Sr/⁸⁶Sr ratios vary from 0.70818 to 0.70833; this limits the age of sedimentation to the range of 550-520 Ma. In the Kurai and Chagan-Uzun sections, along with similar values, anomalously low values of ⁸⁷Sr/⁸⁶Sr down to 0.70662-0.70701 are documented, which are often accompanied by a decrease in δ¹⁸O values and an increase in Mn content. These unusual isotope and geochemical characteristics of carbonates are supposed to be the result of the influence of metasomatic fluids due to underlying basalt and spatially related to the Kurai regional fault zone. In the Kurai section, skeletal small shelly fossils Cambrotubulus were found, which are not known to be older than 550 Ma; this confirms the Precambrian-early Phanerozoic age of the basal sequences of the Baratal Group.

We introduce a new method for constructing a structural model of an oil and gas field, focused on the consistent integration of well and seismic data represented in different domains-depth and time. To address the problem of reconciling heterogeneous data, we propose the application of 3D approximation methods such as 3D splines and collocated cokriging. These methods combine well and seismic data in a single variable space. This approach simplifies the workflow for structural model construction by eliminating the need for both explicit generation of average velocity maps and introducing adjustments to regression relationships. We have also compared the quality of structural models obtained by the proposed methods - 3D spline approximation and collocated cokriging with results produced by traditional approaches, including linear regression and methods based on average velocity maps generated in various ways. The comparison is based on error analysis using cross-validation. The 3D spline method, which has not previously been applied to structural mapping problems, provides the best results. The traditional approach based on linear regression with subsequent modification of the structural map using interpolation and adjustments demonstrates a comparable accuracy. We have obtained the results by structural modeling of the upper part of the Bazhenov Formation in the Kazan oil and gas condensate field. High well density allows us to assess the accuracy of different structural modeling methods according to varying levels of well data availability, typical for different stages of field development.

This paper presents the results of developing a pulse height analyzer for a downhole geophysical instrument intended for spectrometric gamma-ray logging. The module is implemented using a field-programmable gate array and enables real-time signal processing. An algorithm for converting biexponential responses of a scintillation detector into trapezoidal pulses is proposed. The efficiency of the implemented algorithm has been demonstrated in resolving pile-up pulses with a time delay of 300 ns. Using calibrated gamma-ray sources - Am²⁴¹, Eu¹⁵², Cs¹³⁷, Co⁶⁰, and Th²²⁸ - the system’s energy coverage in the range of 50 to 3000 keV is confirmed. The pulse height analyzer features universal architecture and is compatible with various types of gamma-ray detectors.

This study extends the classical skin-factor concept originally proposed by M.F. Hawkins. A fundamental limitation of the Hawkins model is identified: the permeability of the drilling-fluid filtrate invasion zone is assumed to remain constant ( k _skin = const). Based on an analysis of recent experimental evidence, an integrated skin-factor model is developed in which the permeability ( k ) of the near-wellbore region is represented as a function of both the radial distance ( r ) from the wellbore wall and pressure ( p ): k = k ( r , p ). The proposed model comprises three characteristic invasion zones, each governed by a distinct mechanism of permeability variation. Comparative simulations reveal substantial differences between the classical Hawkins model and the proposed three-zone model, k = k ( r ), as well as the integrated model, k = k ( r , p ). Assuming a linear radial variation in permeability, comparative relationships are obtained between the normalized production rate and the skin factor, as well as between the skin factor, the production rate, and pressure drawdown. The classical Hawkins model is shown to overestimate the production rate by an average of 15-25%, with deviations increasing to 40-50% in the zone of maximum invasion. The results are of fundamental importance for the theory of flow through porous media and have practical significance for oilfield development planning by providing a basis for selecting well stimulation methods according to the characteristics of individual invasion zones.

We present our first experimental work with the GABL-PM absolute ballistic laser gravimeter on a pier, in ports and on board of an ice-resistant platform (IRP) in the expedition North Pole-42 (NP-42). During the tests, we parametrically adjusted the gravimeter to adapt it for the operation on the IRP without using a gyrostabilized platform. The results of the GABL-PM measurements on the pier in St. Petersburg are shown; they are required as reference to test the gravimeter on the vessel, as well as to tie variations in gravity acceleration measured on board of the IRP using the Chekan-AM relative gravimeter to absolute values. The estimation of the root-mean-square (RMS) error of the GAB-PM measurement on board of the IRP at various pitching parameters in the ports of St. Petersburg and Murmansk, as well as in the early days of the start of the drift of the North Pole-42 expedition, is given. Based on the measurement results, we conclude that a gravimeter without a gyrostabilized platform with reduced accuracy values, on average up to 0.60 mGal, can be used for measurements after freezing the vessel in ice and when all the requirements of a specially developed manual are met.