A 3D model of the crustal structure of the region including the Chukchi Borderland with the adjacent ocean and Chukchi‒Alaska Shelf has been compiled and calculated. A specific feature of the crustal structure consists of three-sided isolation of the Chukchi Borderland and a transitional zone from the south connecting the Chukchi Borderland with the Chukchi‒Alaska Shelf. The connection between the Chukchi Borderland and Wrangel Rise of the Chukchi Shelf is traced through the North Chukchi Rise located between the North Chukchi Trough and the Hanna Trough. Clockwise rotation of the Chukchi Borderland began in the Early Cretaceous, because basalts at the bottom of sedimentary strata sections along the eastern and southern boundaries of the Chukchi Basin have reversed magnetization: their outpouring occurred before the beginning of the Cretaceous superchron, earlier than 121 Ma. Near the upper boundary of the Neocomian, there was a large-scale shear displacement of crustal blocks along the eastern boundary of the Chukchi Rise, which had a thrust pattern before that. The 3D model demonstrates that the Chukchi Borderland and Chukchi Basin are natural components of the continental margin, since they have a strong geological connection with the continental masses of the Chukchi Shelf.

In 2011‒2020 a significant number of seismic lines were carried out in the Eurasian Basin of the Arctic Ocean, which made it possible to study the structure of the junction zones of the Gakkel Ridge with the Nansen and Amundsen basins on a number of profiles. During 2019‒2020 15 sections of the Gakkel Ridge and its rift valley were studied using a sub-bottom profiler and seismo-acoustic profiling. New data on the relief of the basement, as well as the use of databases of bathymetry, gravity, and magnetic anomalies updated at VNIIOkeangeologia, made it possible to calculate the magnetization of the rocks of the Gakkel Ridge along a number of profiles crossing the ridge and to perform model calculations of the structure of the Earth’s crust using a complex of geological and geophysical data in the area of the southeastern termination of the ridge. The Gakkel Ridge is a structure that was isolated in the Early Oligocene (34 Ma)–Early Miocene (23 Ma) in the process of radical restructuring of the spreading kinematics in the existing ocean basins in the regions of the North Atlantic and the Arctic. The values of the calculated magnetization of the magnetic layer of the Earth’s crust show that this layer is partly composed of oceanic basalts, but mainly of deep-originated rocks, gabbro, and peridotites that were brought to the surface during detachment accompanying spreading. The Laptev Sea continuation of the rift valley of the Gakkel Ridge to the south of the caldera passes above many kilometers of sediments, at the base of which sedimentary rocks of Cretaceous and Late Jurassic age occur.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23050057

We propose a geodynamic model capable of explaining the formation of structural features of the Earth’s crust in the Amerasian Basin. The model relates the evolution of the crust of the Alpha-Mendeleev Rise and the Podvodnikov Basin in the Cretaceous to the dynamics of an isometric convective cell in the upper mantle, which can be interpreted as an upper mantle plume. The presented results of numerical modeling confirm the applicability of the presented approach for explaining the geodynamic evolution of the continental crust of the Alpha-Mendeleev Rise and the surrounding basins of the Amerasian Basin in the Cretaceous.

A 3D model of the Earth’s crust for the continental margin of the Laptev Sea and the adjacent part of the Eurasian Basin was developed using the latest seismic and gravity data. The thickness of the consolidated part of the Earth’s crust in the study area is estimated at 7–11 km, which corresponds to a highly extended continental or oceanic crust. The formation of the basement and sedimentation in this area most likely began in the Late Jurassic. The southeastern part of the Eurasian Basin is separated from the rest of the basin by a dextral shear zone, the displacement along which during the Paleogene was more than 100 km.

— The Chukchi Borderland is a tectonic unit of the eastern part of the Arctic continental margin of Eurasia that is part of the complex of the Central Arctic rises, along with the Lomonosov Ridge, the Alpha‒Mendeleev Rise, the Podvodnikov, Chukchi and Mendeleev basins. The study provides data on the structure of the Chukchi Borderland and the surrounding geological structures, morphology and geology, uses bathymetric materials, seismic materials from CDP and deep seismic survey, sampling and drilling data. A review of materials on the study region was carried out. The latest results of geomorphological analysis of bathymetric data and 3D modeling of the Earth’s crust of the study region using geophysical data are presented. To explain the identified features of the morphology and deep structure of the Chukchi Borderland, a tectonic model is proposed that explains the deep mechanisms of its formation and adjacent structures, as well as a structural-tectonic scheme of the Arctic Alaska–Chukotka microplate, which presents the morphological and geological connection of the Chukchi Borderland with the continental shelf.

The deep model of the Earth’s crust and upper mantle of the Arctic basin is represented by a series of velocity sections along the DSS profiles and a set of maps showing the thickness of the sedimentary cover, the thickness of the Earth’s crust as a whole and the distribution of the continental and oceanic types of the Earth’s crust in the Circumpolar Arctic. Crustal Thickness Map is based on results of deep seismic studies and gravity field anomalies in the Circumpolar Arctic. Over 300 profiles of total length of about 140,000 km and equations of correlation, which link the depth of the Moho discontinuity occurrence with Bouguer anomalies and the topography, were used for the map compilation. Correlation sketch map of crustal types, which differ in velocity and density parameters, structure, and total crust thickness, has been compiled based on the data of deep seismic studies on continents and in oceans. The sketch map of crustal types distribution, which was compiled based on seismic profiles in the Arctic, demonstrates the position of the oceanic and continental crust in the structures of the Circumpolar Arctic. Summary geotransect is composed of DSS seismic line fragments and supplemented with density modelling. The geotransect demonstrates structure of the Earth’s crust and upper mantle along the line 7600 km long, which crosses the continental crust of the East European Platform, Barents-Kara shelf seas, Eurasian Basin oceanic crust, reduced crust of the Central Arctic Submarine Elevations, shelf seas of Eurasia passive margin, and crust of the Chukotka-Kolyma folded area.

Over a period of the past 15-20 years, the Russian Government implemented the Arctic Mega Project for geological and comprehensive study of the Arctic Ocean. In this paper we discuss the methods that were used in the implementation of this project. In the course of several expeditions, multiple types of data were acquired, which included: (1) seismic data of different types, (2) subbottom profiler data, (3) geological sampling on slopes of the Mendeleev Rise with the use of special equipment, (4) borehole drilling, (5) gravity and magnetic anomalies, (6) offshore geodetic data, (7) multi-beam bathymetry surveys, and (8) field surveys on multiple Arctic islands. Several nuclear icebreakers and a scientific research submarine were deployed in these operations. Specifically, more than 23,000 km of 2D multi-channel seismic lines and more than 4000 km of wide-angle refraction/reflection seismic lines were acquired, in addition to subbottom profiles for the Eurasia Basin and new bathymetric data of the Arctic Ocean. The new database is intended to facilitate the development of new insights into Arctic geology and geodynamics and contribute to a better understanding of the structure and tectonic evolution of the Arctic Ocean as a whole.

New seismic, magnetic and gravity data of the continental margin of the Laptev Sea shelf indicate: (1) Absence of the Lomonosov-Khatanga transform fault between the Eurasia Basin and Laptev Sea shelf. On a number of new seismic lines we do not observe evidence for transtension or transpressional deformation along this lineament whereas some typical deformation for the continental slopes is recognized. Recent seisimicity is absent along the lineament. (2) The pull-apart Laptev-Gakkel continental basin along the Laptev Sea continental slope is in an orthogonal position to the Gakkel Ridge axial rift. This pull-apart basin was tectonically active during Eocene-Oligocene times. (3) Evidence exists for number possible intrusions just below the rift/postrift (break-up) unconformity (56 Ma) on some seismic lines in the area between the Taimyr Shelf and the continental slope of the Eurasia Basin. Evidence is also found for the existence of possible volcanics just below the break-up unconformity in this area. (4) Intrusions might also be present just below the 56 Ma break-up unconformity recognized on some seismic lines in the area between the Lomonosov Ridge and the continental slope of the Eurasia Basin. Buried volcanoes are likely present as well. These two magmatic provinces are symmetric to each other on both sides of the Eurasia Basin and well expressed on the new magnetic anomaly map.(5) The Eurasia Basin has a conical shape in its Southern near-Laptev domain. Opening of the basin appears to be controlled by propagation of oceanic crust spreading to the south. (6) We assume that the continental margin between the Laptev Sea Shelf and the Eurasian Basin could be a passive volcanic margin. This margin is characterized by a structure that is very similar to the North Atlantic margin of almost the same age. This study was supported by RFBR grant (18-05-70011).

The seismic structure of the complex underlying the stratified sedimentary cover in the Amundsen Basin in Siberia indicates that the formation of a large-scale rift-related basin lying on a hyper-extended continental crust preceded the spreading process in the Eurasian Basin.

The purpose of this work was to study the seismic attributes of the Podvodnikov Basin basement. It was found that these attributes, first of all, P-wave velocities of 5.9–6.2 km/s, the ratio VP/VS = 1.71, and the diffraction nature of the reflections from the basement indicate that the basin has a continental origin and that its tectonic development began at the pre-oceanic stage of evolution of the Arctic.

Main positive morphostructures of the Amerasia Basin, the Lomonosov Ridge, Alpha Ridge, Mendeleev Rise, Chukchi Plateau and Northwind Ridge, have been considered from geomorphological, geological and geophysical aspects. Time and Depth seismic sections have been provided up to the Moho discontinuity from the Lomonosov Ridge and its junction with the Greenland and East-Siberian shelves. Time and Depth seismic sections of the Mendeleev-Alpha rises and Chukchi Plateau are presented too. The sections were set up based on seismic surveys: deep seismic sounding and multichannel seismic reflection. Some similarities have been reflected for the foregoing land structure depth sections and typical sections of the continental crust. Brief geological and geophysical data have been presented for the positive morphostructures of the Atlantic Ocean such as the Rockall and Vring plateaus, the continental nature of which is established beyond all doubt. Genesis of positive morphostructures in the northern Atlantic Ocean and the Arctic Ocean has been connected with processes of continental rifting and concomitant intraplate magmatism.

Дана геоморфологическая и геолого-геофизическая характеристика основных положительных морфоструктур Амеразийского бассейна — хребта Ломоносова, хребта Альфа, поднятия Менделеева, Чукотского плато и хребта Нортвинд. Приведены временные и глубинные сейсмические разрезы до поверхности Мохо хребта Ломоносова и его зон сочленения с Пригренландским и Восточно-Сибирским шельфами, а также временные и глубинные разрезы хребта Альфа, поднятия Менделеева и Чукотского плато. Разрезы построены на основании профильных сейсмических исследований ГСЗ и МОВ-ОГТ. Показано сходство глубинных разрезов вышеперечисленных морфоструктур с типичными разрезами континентальной коры. Приведены краткие геолого-геофизические данные о строении положительных морфоструктур Атлантического океана — плато Роколл и Воринг, континентальная природа которых не вызывает дискуссий. Отмечена связь образования положительных морфоструктур северной части Атлантического океана и Северного Ледовитого океана с процессами континентального рифтогенеза и сопутствующего ему внутриплитного магматизма.

—The available body of geological and geophysical data indicates that the morphologic structures of the Central Arctic submarine elevations complex (CAE) form a single complex block of continental crust that broke away from the Barents–Kara continental margin in the late Paleocene. Seismostratigraphic interpretation of the multichannel seismic reflection data acquired within the CAE, based on seismostratigraphic benchmarks confirmed by drilling and continuous tracing of pre-Cenozoic unconformities from the offshore North Chukchi Trough to its deep-water extension (Vilkitsky Trough), makes it possible to draw the following conclusions: The sedimentary-basin depocenters of the Vilkitsky Trough and Chukchi basin include pre-Upper Jurassic sediments in addition to Cretaceous complexes. However, the former are not common in the rest area of the CAE. Synrift extension of the continental crust is the key factor that affected the tectonic evolution of morphologic structures of the Central Arctic basin. Multichannel seismic reflection data show the clearest signs of the synrift extension in the Lomonosov Ridge, Mendeleev Rise, Chukchi plateau, and their flanks sloping to the sedimentary basins of the Vilkitsky Trough and Chukchi basin. At the same time, the depocenters of these sedimentary basins formed by pre-Upper Jurassic deposits are characterized by an almost undisturbed bedding of all sedimentary complexes. Pre-Upper Jurassic deposits might be interpreted as a relic of the Ellesmerian structural stage preserved in the deep-water extension of the North Chukchi Trough since the preoceanic evolution stage. Pre-Upper Jurassic complexes seem to be affected by deep rift activity only within the elevations of the Central Arctic area and near-flank zones of the depressions separating them. Pre-Upper Jurassic deposits in the sedimentary basin depocenters of the Vilkitsky Trough and Chukchi basin structurally linked to the shallow-water shelf were barely affected by the rifting processes. The tectonic evolution of the depocenters and their submergence relative to the flank zones might have been affected not only by crustal extension processes but also by compensation mechanisms.

The junction zone between Lomonosov submarine ridge and the shelf of the East Siberian Sea was studied. The aim was to prove the absence of a strike-slip fault in the junction zone between Lomonosov Ridge and the East Siberian shelf. The problem of the existence of a fault zone in the region where the Lomonosov Ridge joins the East Siberian shelf is still currently debateable. The geological and geophysical data used to solve it are insufficient. To remove this ambiguity, seismological data obtained in neighboring areas were used. An analysis of the map of earthquake epicenters in the region showed that, if the fault zone mentioned existed, present-day intraplate seismic activity would certainly be observed within its limits. The absence of seismicity in the junction zone between Lomonosov Ridge and the adjacent shelf clearly indicates the genetic unity of these structures.

Объект исследований - зона сочленения подводного хребта Ломоносова и восточно-сибирского шельфа. Цель работы - доказательство отсутствия сдвигового разлома в зоне сочленения подводного хребта Ломоносова и восточно-сибирского шельфа. Проблема существования разломной зоны в области сочленения хребта Ломоносова с шельфом Восточно-Сибирского моря до настоящего времени является дискуссионной. Использовавшиеся для её решения геолого-геофизические данные оказались недостаточными. Для снятия неоднозначности были привлечены сейсмологические данные в соседствующих районах. Анализ карты эпицентров землетрясений региона показал, что в случае существования указанной разломной зоны в её пределах непременно отмечалось бы современная внутриплитная сейсмическая активность. Асейсмичность зоны сочленения хребта Ломоносова с прилегающим шельфом однозначно свидетельствует о генетическом единстве указанных структур.

Summary Map of thickness of the sedimentary cover of the Arctic Ocean is based on the results of analysis and reprocessing of multi-channel seismic reflection and wide-angle refraction/reflection data collected by both Russian and foreign researchers. It can serve as a factual basis for further tectonic constructions and predictive hydrocarbon estimations. This digital map of the sedimentary cover thickness differs from the previous maps of this region by significantly better detail - for making it a much larger amount of new seismic data has been used. This map is a new stage in the process of compiling and analyzing seismic data obtained in the Arctic Ocean. No doubt it will be refined and improved as new geological and geophysical data appears.