North Chukchi–Podvodnikov and Zhokhov–Wrangel Composite Tectono-Sedimentary Elements, East Siberian Arctic

<p>The growing interest of geoscientists to the Eastern Arctic shelf is caused one of the most important problems of the present time – the creation of a tectonic model for assessing the hydrocarbon potential of the Eastern Arctic basins. In this time, over the past decade, the study of the East Siberian sea seismic lines have increased. Now, we operated a new seismic data, the interpretation of which gives the key to understanding the structure of the East Siberian continental margin.</p><p>This paper presents an analysis of the tectonic structure and geological history of the shelf of the East Siberian continental margin based on the interpretation of seismic lines in conjunction with geological information.</p><p>The modern ideas of the East Arctic rift tectonic evolution and formation of sedimentary basins over the entire East Siberian shelf resulted from the large-scale tectonic and magmatism events took place and the intense rifting or stretching phase widespread the entire shelf in the Albian-Aptian.</p><p>The East Siberian basin includes the main structural elements, formed in a postcollisional destructive stage of development – the New Siberian rift, the De Long uplift, the Zhokhov Foredeep basin, the Melville trough, the Baranov rise, the Pegtymel trough, the Shelagskoe rise.</p><p><strong>The New Siberian rift</strong> is located between the elevations of the New Siberian Islands and the archipelago De Long. Rift extends in a southeast direction from the East-Anisin Trough deflection to the Islands of Faddeev Island and New Siberia Islands. The New Siberian rift is a bright negative structural element and clearly stands out on the maps of the anomalous magnetic and gravitational fields, contrasting with the positive anomalies of surrounding rises and ridges.</p><p><strong>De Long Plateau</strong> is a large positive structure. The uplift boundaries and internal structure are clearly visible in the gravitational and magnetic fields. The magnetic anomaly expressed in the De long, it is a typical for the areas of development of volcanogenic formations and basalts trap magmatism.</p><p><strong>The East Siberian Rift System</strong> located from the northwestern part of the De long Plateau to the eastern part of the North Chukchi basin. System includes the <strong>Melville trough</strong> in the southern part of the East Siberian Sea. The reflector packages on seismic lines in the De Long Plateau and The East Siberian Rift System indicate that continental rifting occurred over the mantle plum.</p><p>The length of the Melville trough is a 350-370 km; with a width of 100-150 km. Trough is the symmetrical deflection consists of two narrow rifts separated by a rise.</p><p>The eastern branch of the rift system of the Melville trough joins the <strong>Baranov rise</strong>. The Baranov rise has a block structure with the geometry of which is similar to the block structure of the De-Long Plateau.</p><p><strong>The Dremkhed</strong> <strong>trough</strong> is a deep rift structure transitional between the East Siberian and North Chukchi basins, the thickness of the sedimentary cover in central part of section is 7000 ms.</p><p>The study was funded by RFBR project - 18-05-70011.</p>

Seismic, magnetic and gravity data indicate that the Chukchi and Beaufort epicontinental seas off northern Alaska overlie three sedimentary basins, or provinces, separated by structural highs of regional extent. The basins trend west to northwest and their enclosed sediments become increasingly marine from south to north. The Chukchi-Beaufort continental margin is similar to those of Atlantic type. Hope Basin, in the southern Chukchi Sea, overlies strongly deformed Paleozoic to mid-Cretaceous rocks of the Brooks Range orogen. The basin is inferred to contain nonmarine and marine clastic sedimentary rocks in a one km-thick Upper Cretaceous (?), a 1.5 km-thick Paleogene (?), and a 1.5/0.75 km-thick Neogene(?) sequence. A large anticline and many faults and smaller folds disrupt mainly the older sequences. The Hope Basin sedimentary units onlap Herald Arch, which trends northwestward from Cape Lisburne in the central Chukchi Sea. At the Herald fault zone Brooks Range rocks in the arch are thrust eastward or northeastward over Mississippian to Jurassic shelf carbonate and clastic rocks of the Arctic Alaska (Ellesmerian) basin and overlying Cretaceous flysch and molasse of the Colville Geosyncline. These Mississippian to Cretaceous rocks underlie the northeast Chukchi Sea and reportedly are about 10 km thick near the Herald fault zone on the Lisburne Peninsula. The great Chukchi syntaxis in western Brooks Range rocks and structures is thought to result from intersection of the west-trending Brooks Range orogen with the northwest-trending Herald fault zone. The Mississippian to Jurassic shelf sequence thins northward, and onlaps the Barrow Arch, which trends northwest from Point Barrow to 161° W. Long., thence west-southwest to the Herald fault zone. The Colville Geosyncline sequence oversteps both the pre-Cretaceous rocks and the Barrow Arch to form the North Chukchi Basin west of 161° W. Long, and the progradational Beaufort continental terrace to the east. The North Chukchi Basin may contain about 6 km of probable Cretaceous and Tertiary section, which may be deltaic in origin. Diapirs (of Cretaceous shale?) pierce the gently northward-dipping strata of this basin, in places reaching the sea floor. Thick Tertiary marine and nonmarine clastic rocks of the Camden Basin overlie the Cretaceous rocks of the North Slope and inner Beaufort continental terrace east of the Colville River delta. These rocks dip gently seaward west of 146° W. Long, but are deformed into long, high-amplitude, east-northeast-striking folds to the east.

Within the limits of the Chukchi Sea and the Amerasian Basin of the Arctic Ocean, study have been carried out to calculate D function anomalies. The result was the discovery of elongated faults that cut, according to the positions of the upper and lower extents of the disturbing masses, both the upper crust and the upper mantle. It is shown that these faults are right-lateral strike-slips continuing the Late Cretaceous-Paleogene structures of the same type in the Bering Sea. This suggests that the en echelon strike-slip system of the Bering Sea, Chukchi Sea, and Amerasian Basin is a relic of the Late Cretaceous-Paleogene transform fault zone between the Eurasian and North American lithospheric plates.

In our study, we have developed a new tectonic scheme of the Arctic Ocean, which is based mainly on seismic profiles obtained in the Arctic-2011, Arctic-2012 and Arctic-2014 Projects implemented in Russia. Having interpreted many seismic profiles, we propose a new seismic stratigraphy of the Arctic Ocean. Our main conclusions are drawn from the interpretation of the seismic profiles and the analysis of the regional geological data. The results of our study show that rift systems within the Laptev, the East Siberian and the Chukchi Seas were formed not earlier than Aptian. The geological structure of the Eurasian, Podvodnikov, Toll and Makarov Basins is described in this paper. Having synthesized all the available data on the study area, we propose the following model of the geological history of the Arctic Ocean: 1. The Canada Basin formed till the Aptian (probably, during Hauterivian-Barremian time). 2. During the Aptian-Albian, large-scale tectonic and magmatic events took place, including plume magmatism in the area of the De Long Islands, Mendeleev Ridge and other regions. Continental rifting started after the completion of the Verkhoyansk-Chukotka orogenу, and rifting occurred on the shelf of the Laptev, East Siberian, North Chukchi and South Chukchi basins, and the Chukchi Plateau; simultaneously, continental rifting started in the Podvodnikov and Toll basins. 3. Perhaps the Late Cretaceous rifting continued in the Podvodnikov and Toll basins. 4. At the end of the Late Cretaceous and Paleocene, the Makarov basin was formed by rifting, although local spreading of oceanic crust during its formation cannot be excluded. 5. The Eurasian Basin started to open in the Early Eocene. We, of course, accept that our model of the geological history of the Arctic Ocean, being preliminary and debatable, may need further refining. In this paper, we have shown a link between the continental rift systems on the shelf and the formation history of the Arctic Ocean.

During the summer of 2006 TGS-NOPEC conjointly with “Geophysical Solutions Integrator” acquired new seismic data in the Russian part of the Chukchi Sea. The area of the Chukchi Sea studied includes (from S to N): South Chukchi sedimentary basin (1), Wrangel Late Kimmerian Arch (2), North Chukchi sedimentary basin (3). Due to the absence of offshore wells in the Russian sector of Chukchi Sea, the interpretation of acquired seismic data and definition of probable hydrocarbon potential must be based on the comparison with the US sector of the Chukchi Sea and the Alaska North Slope, as well as on the geology of Northern Chukotka and Wrangel Island. In the northern part of the Wrangel Arch most of the thrust faults are N-vergent, but double-vergent transpressional structures also occur. To the North of the Wrangel Arch, a clearly recognizable angular unconformity in the upper parts of the North Chukchi basin may correspond to the Lower/Upper Brookian (~Cretaceous/Tertiary) boundary, although it may be as old as Early Cretaceous (pre-Aptian) in age. The maximum Pz-Mz-Cz sediment thickness of the North Chukchi basin exceeds 16 km. In the South Chukchi basin the thickness of sediments (Late Cretaceous?-Cenozoic) mostly doesn’t exceed 3-4 km, but in some places reaches 5-6 km. The geometry of the faults indicates an extensional/transtensional setting of the South Chukchi rift basin development. The changes in phase or polarity in upper parts of the sedimentary cover, listric fault planes in the pre-rift sequences, associated with areas of reduced reflectivity in the upper sediments may point to a gas presence. The syn-orogenic (pre-rift) Upper Jurassic-Lower Cretaceous organic-rich terrigenous sequence (containing visible plant remnants), which is exposed onshore in Northern Central Chukotka and is probably present in the Chukchi Sea, may represent regional gas source rocks.

A reconnaissance grid of 24-channel seismic-reflection data indicates that most of the United States Chukchi shelf north of Point Hope, Alaska, is prospective for petroleum. The prospective rocks, which consist of four stratigraphic sequences, rest on the Arctic platform, a regional erosional surface cut across mildly metamorphosed lower Paleozoic rocks in Late Devonian time. The Eo-Ellesmerian sequence, interpreted to contain mainly Mississippian nonmarine deposits, is 5+ km (16,500 ft) thick and fills local sags and faulted depressions in the Arctic platform. Mississippian to Neocomian stable shelf clastic and carbonate beds of the Ellesmerian sequence, 0 to 7.7+ km (25,000 ft) thick, underlie most of the shelf but are absent from Barrow arch and the outer shelf of the ortheastern Chukchi Sea. Albian and Upper Cretaceous intradelta and prodelta deposits of the lower Brookian sequence, which thicken from 250 m (800 ft) on Barrow arch to 7.5+ km (24,500 ft) to the southwest, northwest, and north, underlie most of the shelf. The upper Brookian sequence, inferred to consist of marine and nonmarine clastic deposits of mainly or entirely Tertiary age, is 0 to 5.6+ km (18,500 ft) thick. It occurs only in Nuwuk and North Chukchi basins and locally as canyon fill beneath the central Chuckchi shelf. The northern Chukchi shelf contains seven provinces of contrasting tectonic origin and structural style. Nuwuk basin, a progradational clastic prism containing 12+ km (39,500 ft) of lower and upper Brookian strata and numerous growth faults, overlies a rifted margin of Neocomian age beneath the outer shelf and slope of the northeastern Chukchi Sea. North Chukchi basin, which underlies the outer shelf west of Nuwuk basin, contains Ellesmerian beds and 12+ km (39,500 ft) of lower and upper Brookian strata. It may also overlie a Neocomian rifted margin, but was deepened by Laramide extensional rifting. South of these basins, shelf structure is controlled by the geometry of the Arctic platform, which slopes gently southwest from a depth of 0.25 km (800 ft) on Barrow arch to about 13 km (4 ,650 ft) off Point Lay. In the central part of the shelf, the platform is somewhat faulted and folded and descends to a depth of 10+ km (33,000 ft) to form the north-trending Hanna trough. West of the trough the platform rises to within 1 km (3,300 ft) of the seabed and is broken by numerous normal faults. The southern part of the platform contains a thick lower Brookian section with numerous northwest-striking, northeast-verging detachment folds. The fold province is bounded on the southwest, off Cape Lisburne, by the northwest-striking Herald arch overthrust belt at which one or more southwestward-dipping thrusts brought Ellesmerian and older strata to the seabed. The seismic and extrapolated onshore data suggest that Nuwuk and North Chukchi basins, Hanna trough, and the Arctic platform east and west of the trough could contain significant deposits of oil or gas. The potential of the fold belt, however, is modest, and of Herald arch slight. Small areas on Barrow arch and the Arctic platform west of Hanna trough lack potential because they are underlain by less than 1 km (3,300 ft) of prospective section. End_of_Article - Last_Page 474------------

Characterization of material changes with depth (profiles) in many landfill sites can be problematic for some conventional geophysical methods. Localized anomalies within the landfill can complicate mapping of underlying layers, and layered-model techniques are inappropriate for imaging laterally discontinuous landfills. Recently-developed geophysical hardware and software tools provide the opportunity to image the vertical structure of a landfill and its geologic setting. In May, 2000 a sequence of geophysical data sets were acquired at a landfill site at Camp Roberts, CA to test the benefits of new hardware and software for characterizing the three-dimensional boundaries of the landfill and the geologic setting. Conventional magnetic and electromagnetic measurements provided a backdrop for these new methods. A Geometrics G-858 magnetic gradiometer equipped with a real-time GPS positioning system was used to map the areal extent of the landfill. Resistivity, seismic refraction, and electromagnetic data were acquired along profile lines to characterize the vertical extent of the landfill and geology. Seismic refraction data were processed with conventional time-delay methods, and with newer tomographic methods. The multielectrode resistivity data were compared with data acquired with the capacitively-coupled OhmMapper system The landfill boundaries that are defined in map view by the magnetic data are supported in profile by the seismic refraction data and multielectrode resistivity data. The seismic data are most effective in identifying trench locations when a tomographic inversion is used, instead of a conventional delay-time approach to interpretation. This shows a localized high-velocity zone that coincides with the trench boundaries that are defined by the magnetic data. The multielectrode resistivity data show a disruption of layering where trenching has occurred. Both the seismic data and the multielectrode resistivity data provide evidence that the shallow geology is laterally discontinuous and heterogeneous. The high electrical conductivity of the near surface imposed limitations on the penetration depth of both the OhmMapper and multielectrode resistivity systems. The multielectrode system was better suited for penetrating this zone than was the OhmMapper.

The paper summarizes the results of geological and geophysical studies of the Siberian Arctic Shelf (Laptev, East Siberian and Chukchi seas), which is one of the largest continental shelves on Earth. This region consists of as many as 22 significant sedimentary basins of variable age and genesis which are expected to bear significant undiscovered volumes of hydrocarbons. Two major groups of the basins are identified based on the age of the underlying crustal basement: (1) post-Hauterivian/Barremian basins resting on the Late Mesozoic folded basement; and (2) older (Late Palaeozoic to Early Mesozoic?) basins preserved outboard of the Late Mesozoic deformational front in the northern part of the East Siberian and Chukchi seas. At least two significant tectonic events caused the overall tectonic pattern of the shelf as well as formation and structural styles of its individual crustal domains and the sedimentary basins: (1) Late Mesozoic convergence and subsequent collision of the Arctic Alaska–Chukchi Microplate with the Verkhoyansk–Kolyma/Omolon margin of the North Asia Continent around 130–125 Ma (the Verkhoyansk–Brookian compressional event); and (2) a series of Cretaceous and Cenozoic extensional events related to the origin of the Arctic oceanic basins. Based on 2D regional multichannel seismic reflection data constrained by onshore geology, plate tectonic models and inter-regional correlations, as well as on gravity and magnetic grids, the structural styles, lithostratigraphy and possible hydrocarbon systems of the offshore sedimentary basins are considered.

In the Warming Arctic Seas

The Arctic Alaska region includes three composite tectono-sedimentary elements (CTSEs): (1) the Arctic Alaska Basin (AAB), (2) the Hanna Trough (HT) and (3) the Beaufortian Rifted Margin (BRM) CTSEs. These CTSEs comprise Mississippian–Lower Cretaceous (Neocomian) strata beneath much of the Alaska North Slope, the Chukchi Sea and westernmost North Slope, and the Beaufort Sea, respectively. These sedimentary successions rest on Devonian and older sedimentary and metasedimentary rocks, considered economic basement, and are overlain by Cretaceous–Cenozoic syn- and post-tectonic strata deposited in the foreland of the Chukotka and Brooks Range orogens and in the Amerasia Basin. (1) The Mississippian–Neocomian AAB CTSE includes two TSEs: (a) the Ellesmerian Platform TSE, comprising mainly shelf strata of Mississippian–Middle Jurassic age, and including a relatively undeformed domain in the north and a fold-and-thrust domain in the south; and (b) the Beaufortian Rift-Shoulder TSE, which includes Middle Jurassic–Neocomian deposits related to rift-shoulder uplift. (2) The HT CTSE includes four TSEs: (a) the Ellesmerian Synrift TSE, which comprises Late Devonian(?)–Middle Mississippian growth strata deposited in graben and half-graben during intracontinental rifting; (b) the Ellesmerian–Beaufortian Sag-Basin TSE, which comprises Middle Mississippian–Upper Triassic strata deposited in a sag basin following cessation of rifting; (c) the Beaufortian Synrift TSE, comprising Jurassic–Neocomian graben-fill deposits related to rifting in the Amerasia and North Chukchi basins; and (d) the Beaufortian Rift-Shoulder TSE, which comprises Jurassic–Neocomian strata related to rifting and deposited outside rift basins. (3) The BRM CTSE includes two TSEs: (a) the Beaufortian Synrift TSE, comprising Middle Jurassic–Neocomian synrift strata deposited on attenuated continental crust associated with opening of the Amerasia Basin; and (b) the Ellesmerian Platform TSE, comprising mainly shelf strata of Mississippian–Middle Jurassic age that lie beneath Beaufortian synrift strata. The AAB, HT and BRM CTSEs contain oil-prone source rocks in Triassic, Jurassic and Cretaceous strata, and proven reservoir rocks spanning Mississippian–Lower Cretaceous strata. A structurally high-standing area in the northern AAB CTSE, northern HT CTSE and southernmost BRM CTSE lies in the oil window, whereas all other areas lie in the gas window. Known hydrocarbon accumulations in the three CTSEs total more than 30 Bboe, and yet-to-find estimates suggest that a similar volume remains to be discovered.

In the Arctic, global warming is 2–3 times faster than over other regions of the globe. As a result, noticeable changes are already being recorded in all areas of the environment. However, there is very little data on such changes in the Russian Arctic. Therefore, to fill the gap in the data on the vertical distribution of the gas and aerosol composition of air in this region, an experiment was carried out on the Tu-134 Optik flying laboratory in September 2020 to sound the atmosphere and water surface over the water areas of all seas in the Russian Arctic. This paper analyzes the spatial distribution of methane. It is shown that during the experiment its concentration was the highest over the Kara Sea (2090 ppb) and the lowest over the Chukchi Sea (2005 ppb). The East Siberian and Bering Seas were slightly different from the Chukchi Sea in terms of the methane concentration. Average values of CH4 are characteristic of the Barents (2030 ppb) and the Laptev Seas (2040 ppb). The difference between the concentrations at an altitude of 200 meters and in the free troposphere attained 150 ppb over the Kara Sea, decreased to 91 and 94 ppb over the Barents and Laptev Seas, and further decreased over the East Siberian, Chukchi, and Bering Seas to 66, 63, and 74 ppb, respectively. Horizontal heterogeneity in the distribution of methane over the Arctic seas is the greatest over the Laptev Sea, where it attained 73 ppb. It is two times higher than over the Barents and Kara Seas, and 5–7 times higher than over the East Siberian and Bering Seas.

<p>The Eastern Arctic is poor studied by offshore drilling. There are some wells drilled on the Alaska shelf, but Russian sedimentary basins are separated from Alaska basins by tectonic structures, therefore seismic complexes could not be traced confidently from Alaska to the North Chukchi Basin. Nevertheless, seismic lines in the Eastern Arctic acquired in last decade, samples from seafloor scarps on the Mendeleev Rise (Skolotnev et al., in preparation) and geologic data from adjacent onshore geology allows to assume the mechanisms and timing of the Eastern Arctic Basins forming. According to data from De-Longa Islands and from sampling on the scarps of the Mendeleev rise, the wide basalt volcanism was acting during ±125-100 Ma. The volcanism related to forming of rift basins all over the Eastern Arctic. On the seismic lines crossing the Mendeleev Rise some structures that could be interpreted as volcanos and Seaward Dipping Reflectors (SDR) are identified at the base of geological section. The top of these structures are traced on the seismic lines, and continue from the Mendeleev rise to the North Chukchi Basin where they are covered by clastic complexes that prograde from the territory of the Early Cretaceous Verkhoyansk-Chukotka Orogen. On this account the North Chukchi Basin started to form not earlier than in Barremian-Aptian. Continuation of Mendeleev Rise into the North Chukchi Basin is confirmed by the data of magnetic anomalies. To the south of the North Chukchi Basin on the Wrangel-Gerald High the volcanic build-ups and associated intrusions are interpreted. Presence of magmatic features in this area is confirmed on the magnetic anomaly map. The volcanic horizons lay below the sedimentary cover of the North Chukchi Basin. Our main conclusion is that Mendeleev Rise and North Chukchi Basin started to form nearly simultaneously during Aptian (Barremian) - Albian time and they compile connected geodynamic system.</p>

The article presents the results obtained by field tectonophysical methods applied to study tectonic stresses of the Northern Eurasia regions, including young and ancient platforms (West European, Timan–Pechora, Turan, West Siberian, East European, and East Siberian) and orogenic frame structures (Caucasus, Northern Tien Shan, Mongolia-Okhotsk system of mesozoids, and Sakhalin Island). Tectonic stress reconstructions provided the basis for analysing the influence of spreading in the North Atlantics and the Arctic on the stress state of the platforms in Northern Europe. A spatial boundary of the influence goes approximately along the margins of the Fennoscandian shield and the Russian plate in the north. Further southwards, the boundary is submeridional and extends from the western wing of the Byelorussian anteclise almost to the Eastern Carpathians. The stress reconstructions for this boundary show the WNW and W-E-trending axes of compression. The boundary line does not coincide with the Teisser-Tornquist line that represents the boundary between the platforms with heterochronous basements. However, it correlates well with heat flow anomalies. The boundary area is confined to the Baltic coast [Sim, 2000. Along the boundary area, near the Baltic Sea, there is an area wherein faulting is mainly caused by extension [Sim, 2000. In this setting, helium permeability is the highest, as shown by the crust map of the European part of the USSR [Eremeev,1983. Extension in this area is probably related to formation of young grabens in the Baltic shield. Changes in the compression axis orientation may be due to the alternating activations of the grabens in the submeridionalBotnicGulf and the latitudinalGulf of Finland. Reconstructions for individual faults show contradictions in the directions of shear displacements: both right- and left-lateral displacements are possible on the same fault segments, and the axes of compression can have either latitudinal or meridional orientations. The focal mechanisms of the Osmussaar andKaliningrad earthquakes (meridional and latitudinal axes of compression, respectively) give evidence of specific current neotectonic stresses in this area. Another zone is distinguished at 52°N from the above-described area. It is mainly sublatitudinal and detected along the southern flank of the Byelorussian anteclise. Further to the east, its orientation changes to SSW, and it roughly follows the SW boundary of theVoronezh anteclise. Reconstructions for the Ukrainian Shield, located south of this zone, show mainly the unstable orientations of the axes of compression. For the platforms inNorthern Eurasia, the tectonophysical methods reconstructed neotectonic stresses in the structures formed under the influence of intraplatform tectonic stresses. These are the residual gravitational horizontal compression stresses released by long-term denudation and uplifting of the structures, including the Khibiny massif of the Baltic Shield, theOlenek and Munsky massifs of the East Siberian platform. These structures are composed of the ancient Archaean-Proterozoic rock complexes, which have been subjected to predominantly vertical displacements for a long time, from the Paleozoic to the modern stage. Special attention should be given to the tectonic stresses ofSakhalin located at the boundary between the Eurasian and North American lithospheric plates. At the edges of these two largest plates, there are the Amur and Okhotsk microplates separated by theCentral Sakhalin fault, as described in some publications. Neotectonic stress reconstructions forSakhalinIsland show sublatitudinal compression and submeridional extension in the common stress field of shearing. The tectonophysical studies show that the neotectonic stresses differ in large structures: horizontal compression and shearing are typical of the uplifts (Kola Peninsula, Tien Shan, Sakhalin), while horizontal extension and extension with shearing are characteristic of depressions (Kandalaksha graben, depressions of theTatarGulf and theSea ofOkhotsk). Our studies provide the data on spacious ‘white spots’ in the modern stress maps ofNorthern Eurasia. The stress reconstructions for practically all the studied structures show that shearing is the dominant geodynamic regime in the study region.

New estimates of weight-at-size, maturity-at-size, fecundity, and biomass of snow crab, Chionoecetes opilio, in the Arctic Ocean off Alaska

<p>On the seismic lines acquired in 2011-2020 for the North-Chukchi Sea and East Siberian Sea basins plenty of low-amplitude normal faults is identified. Maximal apparent throw of the faults is 100-200 ms, and occasionally reaches up to 300-400 ms. Dip angles of the faults are often directed towards each other, the resulting flower structure is related to strike-slip tension. For individual faults it is possible to ascertain strike azimuth – near 350° for the North Chukchi basin and near 340° in East Siberian basin. By the seismic data, the faults are distributed within an area of ~1.500 km long- and ~350 km wide.</p><p>According to interpretation, the faults activation occurred from 45 Ma to 34 Ma. This time corresponds to a regional tectonic rebuilding, that is observed across all the region. For example, a sharp slowdown of the Eurasian Basin spreading had place then. Formation of the North-Chukchi and East Siberian basins is related to Aptian-Albian (~125 Ma) rifting, that manifested itself on the De Long Islands and the Mendeleev Rise. Isometric form of the basins could indicate the conditions of pull-apart tension. Data of gravity and magnetic anomalies support this assumption – a long linear anomaly of ~285° strike is identified to the North of the Wrangel Island (in Chukchi, the last is called Umkilir – “White Bear Island”). The anomaly is interpreted as regional strike-slip that was formed ~125 Ma. The angle between the strike-sleep and the multiple low-amplitude Eocene faults is about 55-65°. It is possible to relate the low-amplitude faults to the reactivation of the great strike-slip.</p><p>This study was supported by the Russian Science Foundation (Grant 22-27-00160).</p>