Through structural shelf-ocean zones in the Russian East Arctic

Geological-geophysical features of the lithosphere of the Arctic Region Proceedings of NIIGA-VNIIOkeanologiya, 2010, Issue 7, p. 32-47

THROUGH STRUCTURAL SHELF-OCEAN ZONES IN THE RUSSIAN EAST ARCTIC V.A. Vinogradov1, Yu.V. Goryachev1, E.A. Gusev1 (1) I.S.Gramberg’s All-Russia Geological Institute for Geology and Mineral Resources of World Ocean (VNIIOkeangeologia), St. Petersburg, Russia Geological mapping of Eastern Arctic Shelf scaled 1:1 000 000 has revealed a sublongitudinal zoning pattern in the structure of sedimentary cover. Structural zones identified on the shelf well correspond to the morphostructures in the ocean, thus showing a structural skeleton of the region going through its oceanic and shelf segments. This zoning also involves the adjacent land at the north-east of Russia. The age of these zones is estimated as Barremian-Albian. Fig 16, references - 18.

The geological mapping in the Russian East Arctic shelf have been carried out by VNIIOkeangeologia since the end of the 1990s and during the last decade resulted in obtaining of a large amount of information both on structure, age of sedimentary cover of the region and on history of its formation. Seismic profiles were used by the compilation of sheets of geological maps at a scale of 1:1 000 000 provided by the Murmansk Arctic Geological-exploration Expedition (MAGE), Laboratory of the regional geodynamics (LARGE), Trust of Dalmorneftegeofisika (DMG), and the Federal institute of geology of natural resources of Germany (BGR). Besides, seismic lines, geophysical data were accounted for in full measure evidence on potential fields and all the available information for islands and bottom sampling. The interpretation of seismic profiles on the Russian part of the Chukchi shelf was inferred from the American data for seismic profiling and especially data on deep-sea drilling on the Chukchi shelf of the USA [14, 15, 17, 18]. The geological results of mapping of the Russian East Arctic offshore have been repeatedly published [3, 4, 6]. However, there is an aspect which has not been properly covered, namely, the relations between shelf structural zones and morphostructures of the ocean. This paper is aimed at successive discussion of these relations both over the entire Russian East Arctic offshore and the Chukchi shelf off the USA. So, let as discuss the earlier published Map of the tectonic zonation of the Russian East Arctic offshore (Fig. l). It shows uplifts separated into four groups according to the sedimentary cover thickness, monocline and steps and also four groups of grabens and troughs differing in the sedimentary cover thickness. But this map clearly shows the division of structure direction into two classes, i.e. sublatitudinal and submeridional.

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1 – uplifts with the cover of less than 1-3 km thick and the folded basement projections; 2 – uplifts with the cover of less than 6-7 km; 3 – with the cover less than 8-9 km, 4 – with the cover less than 16-17 km, 5-6 – monoclines and structural steps: 5 – with the cover less than 1-3 km, 6 – with the cover less than 4-5 km; 7-10 – grabens and depressions: 7 - with the cover up to 3-5 km; 8 –with the cover up to 5-7 km; 9 – with the cover up to 9-12 km; 10 – with the cover up to 20 km; 11 – Ocean morphostructures; 12 – Continental structures, ; 13-15 – boundaries: 13 – of the main structures, 14 – of the large structures, 15 – of the grabens and horsts on the De-Long rise and the Zhokhovsky depression, 16 – folded basements outcropes. Names of structures: I – Laptev riftogeneous basin; highs and horsts: 1 – Sardakh, 2 – Trofimov, 3 – Tumat, 4 – Minin, 5 – outer-shelf highs, 6 – North-Laptev perioceanic basin. II – New Siberian System of grabens and horsts; 7 – New Siberian basin, 8 – Anisin basin. III – De-Longa uplift, IV – Chukchi – East Siberian basin: 9 – Zhohovsky depression, 9a – Zhanetta high, 10 – North Chukchi uplift, 11 – North Chukchi depression, 11a – Andrianov uplift, 12-16 – East Siberian riftogeneous depression (12 – South Denbarsky depression, 13 – Chukchi uplift, 14 – Melvil graben, 15 – East Chersky uplift, 16 – Ambarchiksky depression), 17 – Baranov uplift, 18 – Shelaga depressions, 19 – Wrangel-Gerald uplift, 19 – Wrangel-Gerald horst and grabens zone, 20 – South Chukchi basin, 21 – Coastal monocline, 22 – Chokhuro-Chekurdakhsky row of granitoid intrusions. V – Perioceanic structures: 23 – Nansen Basin, 24 – Gakkel Ridge, 25 – Amundsen Basin, 26 – Lomonosov Ridge, 27 – Podvodnikov Basin, 28 – Mendeleev Rise, 29 – Chukchi abyssal Plain, 30 – Chukchi Plateau.

Fig. 1. Main structures of the Russian East-Arctic Shelf

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North Chukchi Basin and Central graben of East-Siberian basin, 3 – Schmidt depocenters of South-Chukchi Basin, 4 – structural highs with reduced sedimentary cover, 5 – horst-graben structure in uplifted zones, 6 – De-Long massif, 7 – folded basement outcrops, 8 – acoustic basement outcrops in a shelf area, 9 – main structures boundaries, 10 – positive and negative structures boundaries on the shelf, 11 – normal faults and strike-slip faults, 12 – Cenozoic and Recent volcanoes, 13 – hydrothermal sources. Through Structural zones: I – Eurasian – Laptev, II – Lomonosov – New Siberian, III – Podvodnikov – East Siberian, IV – Mendeleev – Wrangel, V – West-Chukchi, VI – Central Chukchi, VII – West Nordwind, VIII – East Nordwind.

Fig. 2. Through Structural Shelf-Ocean zones in the Russian east Arctic. 1 – riftogenous structures of Laptev basin and east-Siberian Basin, 2 – Central depocenter of

Fig. 3. Fragment of seismic profile (87722, MAGE) in Laptev basin.

The sublatitudinal structures were reported from the northern part of the shelf. Those are the Chuckchi ana Zhokhov troughs emplaced on the Caledonian folded basement. The Caledonian age of the folded basement was recorded in the northern Alaska [15] and in the East Siberian sea on Henrietta island, where an absolute age of diorite porphyrites is estimated at 440-450 Ma (40Ar/39Ar and Sm/Nd) [9]. Sedimentary filling of these troughs are represented by the Elsemerian (D3 - K1) and Brookian (K1 br - KZ) seismic units identified by American researches on the northern coast of Alaska and Chukchi shelf [15, 17]. The American study shows that the formation of the Elsmerian seismic unit was caused by washout of sediments from the north, i.e. from the side of the present ocean while the Brookian seismic unit was accumulated due to the washout of sediments from the south. Hence, the emplacement of the North Chukchi and Zhokhov troughs took do place well before the opening of the Arctic ocean. The second class of submeridional structures rests mainly on the Late Mesozoic folded basement or is superimposed on the earlier sublatitudinal structures. These submeridian structures are directly related to similar striking oceanic morphostructures, together they form through structural zones. There are eight such zones stretching from west to east (Fig. 2). The first, the Eurasian-Laptev zone is well known and unlikely can be debated. The structural pattern of the Laptev Basin is well depicted on MAGE 87722 seismic profile (Fig. 5). The distinct deformation of all seismic sequences of the lower unit is its peculiar feature. The lower unit is unconformably overlain by the upper seismic unit represented by structureless sequence up to 1 km thick. The second Lomonosov - New Siberian zone extends along the shelf between the Laptev basin and East Siberian riftogenic depression. Its structural pattern is caused by the combination of horsts, grabens, semigrabens and structural terraces against a background of common reduction and discontinuity of the sedimentary cover. Grabens mainly not deep with a thickness of sediments of 1-

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3 km (Fig. 4), however, a thickness of the, for example in the New Siberian graben reaches 10 km (Fig. 5). There are multiple highs of the acoustic basement on the shelf, while the Late Mesozoic exposures of the folded basement occur on islands, such island-horsts as Kotelny, Belkovsky, Stolbovoy rise for the first hundreds of meters above the sea-level. All protrusions of the folded basement are grouped in a submeridional band of a sort of axial part of this zone.

Fig. 4. Fragment of seismic profile (008, LARGE) in New Siberian horst and graben system.

Fig. 5. Fragment of seismic profile (90800, MAGE) in New Siberian basin.

To the west from its central part, zone structure is more contrasting, than that to the east. Most horsts and grabens are grouped there. The transverse profile of the zone on the shelf is in fact similar to that of the Lomonosov Ridge but the latter has steeper and contrasting western flank. The New Siberian system of horsts and grabens on the shelf and the blocky structure of the Lomonosov Ridge in the ocean in our opinion form a single structural zone. Noteworthy to pay attention to a continental projection of this structural zone as well known from publications the meridional Chokhchur-Chekurdakh series of granitoid intrusions on the Yana-lndigirka lowland [13]. This series is expressed (reflected) in the topography by a number of uplifts gradually decreasing from north ( Cape Svyatoy Nos) to south where they are terminated within the lowland. The Podvodnikov - East Siberian structural zone is identified farther east. On the shelf, it is represented by the East Siberian riftogenic depression similar in its inner structure to the Laptev riftogenic basin. The depression structure is shown by two geological sections based on seismic lines. One of them is the cross section of the depression and another is a diagonal section (Fig. 6).

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Fig. 6. Geological sections crossed East-Siberian Basin.

On the northern end of the diagonal section where the East Siberian basin onlaps the sub-latitudinal Zhokhov trough, the Ellesmirian seismic unit is depicted under the Brookian seismic unit. The Ellesmirian unit is shown on the LARGE seismic profile, it was indirectly confirmed by fossiliferous fragments of the Middle - Upper Carboniferous limestones and Cenozoic volcanites from Zhokhov island [11]. The latter is located 40 km north of the western flank of the Zhokhov trough. The Podvodnikov - East Siberian zone intersects the De-Longa massif underlain by the Caledonian basement and even older cores. Debris of metashales, microcline granites and gneisses were reported from greenstone-sandstone sequence of Henrietta island [5]. The disintegration and probably expansion of the crust on the De-Longa massif is reflected in a series of grabens and by the manifestation of the Cenozoic up to the recent alkali-basalt volcanicity in the north-western part of the massif. Those are the Aptian-Albian basalts of Bennetta island, Late Miocene - Early Pleistocene basalts of Zhokhov island containing abundant spinel peridotite nodules up to 10 cm in size, and the youngest Late - Middle Pleistocene basalts of Vilkittsky island. A bubble-train over Bennetta island fixed by the American satellite in the 1980s should be noted as well. Most likely it was a water vapour cloud caused by issue of basalt melt at the contact with the ice cap of the island. Recent volcanoes issuing alkali basalts were reported from the continental projection of the zone discussed [8].

Fig. 7. Geological section crossed North Chukchi Basin.

The large Baranov uplift limited on the east by the Shelaga zone of depressions is identified on the shelf east of the East Siberian basin. The relation between these shelf structures with morphostructures of the ocean has not been established as yet. In some publications [10] the Shelaga zone of depressions with the cover thickness up to 8-10 km is referred to as the Dremhed rift. This statement has resulted from misinterpretation of single DMNG profile ESS-91-01 crossing this structure in sublatitudinal direction, a signature in the form of pseudo reflections having shape deep and steep curves which could be taken for real reflections is discernible on the profile below the clearly identified acoustic basement. If one considers a real acoustic basement than the cover thickness is about 3 km there and according to gravity data it might increase up to 4 km in the northern Shelaga zone. Farther east the authors have established the Mendeleev - Wrangell zone of uplifts. Within the shelf, Wrangell Island rises to heights of 1000 m above the sea-level and the acoustic basement comes to the surface over an extensive area around the island, Wrangell and Gerald islands as manifestation of ascending tectonic movements occupy an extensive sublatitudinal Wrangell-Gerald arc. Unfortunately, there is no latitudinal seismic profile crossing this zone in the vicinity of the

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continental slope, but the authors suggest the existence of a large uplift there, that is similar to the Andianov uplift on the Chukchi shelf.

Fig. 8. Fragment of seismic profile (SC-90-12, DMNG) along southern slope of North Chukchi Basin.

Farther east the west Chukchi structural zone seems to be most easily discernible. It begins at the foot of the continental slope of the Chukchi abyssal plain and in fact a tectonic depression between the Mendeleev Rise on the west and Chukchi Plateau on the east. The southern projection of the zone in the North Chukchi trough is occupied by the Central syncline of this trough. The syncline consists of a thick lens of the Lower Brookian seismic sub-unit of the Barremian-Albian age (Fig. 7). According to A. Grantz [15] the lens thickness is 10.5 km and the authors estimate it at 11.5 km [3]. The southern extension of the discussed zone is detected in the southern flank (limb) of the North Chukchi trough as a disintegration and subsidence zone recorded on CS-90-12 DMNG seismic profile (Fig. 8). It is well reflected there in bathymetry as sub-meridional ridges and valleys within a band 150-160 km wide (Fig. 9). Even farther south at the intersection of the sublatitudinal Wrangell - Gerald ridge there is the submeridional Wrangell-Gerald system of grabens and horsts 150-160 km wide distinct within the gravity field and depicted on the DMNG seismic profiles [7]. The deepest North- and South-Schmidt depressions are located on the farthest southern extension of the west Chukchi structural zone in the south Chukchi trough. And finally, the Kolyuchin Bay is located at the continent boundary within the discussed zone, while centers of the recent alkali-basalt volcanism and a large number of hydrothermal springs with a water temperature up to +97°С and a high content of the mantle helium were reported from the southern Chukchi peninsula [12]. The same zone of the recent tectonic-magmatic activization on the Chukchi shelf was recorded by geologists who studied the Chukchi peninsula and the Chukchi sea shelf [2]. Based on the recent shelf deposits, this zone is characterized by high content of gold, silver and platinum, one sample of Fe-Mn nodules from the Gerald trough (westernmost graben of the Wrangell-Gerald system of grabens and horsts) contained l,46 g/t of platinum [1]. 39

Fig. 9. Chukchi sea bathymetry.

Farther east the authors have recognized the Central-Chukchi structural zone which includes the Chukchi Plateau at the shelf-ocean boundary, Andrianov transverse uplift in the North Chukchi trough and a large uplift within the Wrangell-Gerald ridge, the American researches call it the Chukchi platform [17]. The Andrianov uplift was formed in Barremian-Albian time, it is during this time interval (Lower Brookian seismic subunit) the thickness of the above deposits drastically decreases (Fig. 7) up to the complete erosion in the northern part (Fig. 10). The Lower Brookian seismic subunit on the so called Chukchi platform is also strongly reduced up to the first hundreds meters, and highs of the acoustic basement were recorded from some parts of the surface of the sea floor [7].

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Fig. 10. Fragment of seismic profile (SC-90-21a, DMNG) crossed Andrianov high in North Chukchi Basin.

The West North Wind structural zone shows itself still farther east, on the American part of the Chukchi shelf, it encompasses abyssal plain of the North Wind avantshelf and the Hanna trough zone located farther south in terms of Torsten and Theiss [17]. The last on the east, the East North Wind structural zone includes the North Wind Ridge of avantshelf and the North Chukchi uplift on the shelf reported by the American geologists [17]. Hence the question about the time of the onset of shelf-ocean through structural zones in the Russian East Arctic. The answer should be found in reliable dating of seismic horizons and is being resolved by means of the American studies carried out on the Chukchi shelf, where seismic profiles are combined with log sections of deep boreholes. Let us discuss the subdivision of the Brookian seismic unit with the LCU key seismic unit (Lower Cretaceous unconformity) at the base. This horizon is related in log sections to the base of the Barremian (Lower Cretaceous) deposits [18]. The next MBU seismic horizon (Middle Brookian unconformity) forms a boundary between the Lower and Upper Brookian subunits. The Upper Cretaceous deposits are absent from most of the American Chukchi shelf, and the MBU horizon forms the Cenozoic base. However, the American researchers have recorded a distinct UBU marker horizon (Upper Brookian unconformity) at the transition into the North Chukchi trough above the MBU horizon (Fig. 11). Of interest is that clinoforms within the range of 1.3 s of two-way travel time are discernible in seismic signature between the MBU and UBU horizons. A seismic train of parallel reflectors is recorded above the UBU horizon (Fig. 12). The authors take this train for the Cenozoic seismic subunit, while the underlying seismic subunit containing clinoforms is the Upper Cretaceous in age. A. Grantz [14, 15] has assumed the presence of the Upper Cretaceous deposits in the North Chukchi trough. The Upper Cretaceous and Cenozoic seismic subunits are identified on the Andrianov uplift where clinoforms are present among the Upper Cretaceous deposits within the two-way travel time interval of 1.5 s (Fig. 10). The Cenozoic seismic subunit rests always unconformably on the underlying deposits discernible on seismic profiles of the Chukchi, East Siberian and Laptev shelves (Figs. 3, 13, 14). Its thickness is 1,0-1,5 km and reaches 3 km (10 th. feet) only in the North Chukchi trough as infirred both from the authors and American data for the UBU horizon. On some maps, the American scientists show the thickness of the Tertiary deposits in the North Chukchi trough equal to 20-30 th. feet [18] considering the MBU horizon as their base. In the authors viewpoint this is a total thickness of the Upper Cretaceous and Tertiary deposists as has been mentioned above.

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Fig. 11. Fragment of seismic profile 6 from [17], crossed American part of Chukchi shelf.

Fig. 12. Fragment of seismic profile 7 from [17], crossed American part of Chukchi shelf.

Fig. 13. Fragment of seismic profile (SC-90-11, DMNG) crossed southern part of North Chukchi Basin.

The above also shows that the Brookian seismic unit is composed mainly of the Cretaceous deposits whose formation began since the Barremian age. All submeridional shelf structures were emplaced during the second half of the Early Cretaceous epoch. Considering that they are directly mate with morphostructures of the ocean it would be logically to assume that the emplacement of a same structural framework both in the ocean and on the shelf has taken place since the BarremianAptian. It has been confirmed by a number of seismic and geological data. The formation of the Cretaceous sedimentary basins on the shelf occurred under conditions of high geodynamic activity as evidenced by their contrast degree, and in fact in the onset of tectonic relief. So, morphologically the western slope of the East Siberian riftogenic depression was similar to the recent continental slopes, as indicated by progradation layering of the Cretaceous deposits on the western side of the 42

depression (Fig. 15). The presence of thick landslides in a seismic sequence of above 1 km thick in the eastern slope of this depression also implies a high geodynamic activity(Fig. 16).

Fig. 14. Fragment of seismic profile (BGR-94-15) [16] crossed East Siberian Basin.

Fig. 15. Fragment of seismic profile (ESS-91-04, DMNG) crossed western slope of East-Siberian Basin.

Fig. 16. Fragment of seismic profile (ESS-91-05, DMNG) crossed eastern slope of East-Siberian Basin.

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Basalt issues covering an extensive area of the De-Longa massif and recorded on Bennetta island began in Aptian-Albian time. Fractures that caused the destruction of the earth's crust, have affected the mantle in the end of the Cenozoic. The mantle matter in the form of spinel peridotite fragments turned out in basalts of Zhokhov island. The shaping of the ocean in the Cenozoic as a deep-water basin bounded by the continental slope has strongly obscured the outlined unified structural zones. However, purposeful studies will make it possible to find new evidence of their existence. REFERENCES 1. Astakhov, A.S., Goryachev, N.A., Mikhalitsyna, T.I., 2010. Conditions of the formation of gold-enriched horizons of ore-hosting black shale sequences (illustrated by Permian and recent marine sediments of northeastern Asia) // Doklady Earth Sciences. Vol. 430. N. 1. P. 9-14. 2. Astakhov, A.S., Rujan, W., Aijou, G., Ivanov, M.V., 2008. Lithochemical evidence of recent geological activity in the Chukchi Sea // Doklady Earth Sciences. Vol. 423. N. 1. P. 12681272. 3. Vinogradov, V.A., Goryachev, Yu.V., Gusev, E.A., Suprunenko, O.I., 2008. Russian EastArctic shelf sedimentary cover and paleoenvironmental conditions in ocean-continent system // 60 years in Arctic, Antarctic and World Ocean (Ed. V.L. Ivanov). St. Petersburg. VNIIOkeangeologia. P. 63-76 (in Russian). 4. Vinogradov, V.A., Gusev, E.A., Lopatin, B.G., 2004. Structure of the East Russian Arctic shelf // Geological-geophysical features of the lithosphere of the Arctic Region. St. Petersburg, VNIIOkeangeologia, № 5, p. 202-212. 5. Vinogradov, V.A., Kameneva, G.I., Yavshits, G.P., 1975. About Hyperborean platform in light of new data from geological structure of Genrietta Island. // Tectonics of Arctic. L. NIIGA Publ. Vol. 1. P. 21-25. (in Russian). 6. Vinogradov, V.A., Lopatin, B.G., Bursky, A.Z., Gusev, E.A., Morozov, A.F., Shkarubo, S.I., 2005. Main summary of geological mapping of Russian Arctic shelf in 1 Mln. scale // Prospect and protection of mineral resources. N. 6. P. 38-43. (in Russian). 7. State geological map of Russian Federation. Scale 1:1000 000. Sheet S-1,2 - Chukchi Sea. Explanatory note. St. Petersburg. VSEGEI Publ. 2005. 60 p. 8. Evdokimov, A.N., Korago, E.A., 2002. Late Cenozoic volcanism of the Northern Eurasia and associated deep xenoliths // Russian Arctic. Geological history. Minerageny. Geoecology. St. Petersburg. VNIIOkeangeologia. P. 252-266. (in Russian). 9. Kaplan, A.A., Coppland, P., Bro, E.G., Korago, E.A., Proskurnin, V.F., Vinogradov, V.A., Vrolijk, P.J., Walker, J.D., 2001. New radoiometric ages of igneous and metamorphic rocks from the Russian Arctic, VNIGRI/AAPG regional international conference, July 15–18, SaintPetersburg, Russia, 06-2. 10. Kosko, M.K., Butsenko, V.V., Ivanov, V.L., Korago, E.A., Poselov, V.A., Suprunenko, O.I., 2008. Arctic ocean and its continental margins tectonics // 60 years in Arctic, Antarctic and World Ocean (Ed. V.L. Ivanov). St. Petersburg. VNIIOkeangeologia. P. 16-43. (in Russian). 11. Makeev, V.M., Davidov, V.I., Ustritsky, V.I., 1991. Middle-carboniferous rocks founding contains tropical fauna on the De-Long islands // Arctic Paleozoic stratigraphy and paleontology. L., VNIIOkeangeologia. P. 167-170. 12. Polyak, B.G., Lavrushin, V.Yu., Cheshko, A.L., 2009. Modern magmatism localization on the eastern Chukotka (isotopes He, Ar, С, N data from hydrothermal gases) // Geology of polar regions. Proceedings of XLII tectonic conference. Vol. 2. P. 125-129. (in Russian). 13. Prohorova, S.M., Ivanov, O.A., 1973. Tin-bearing granitoids from Yana-Indigirka lowland and associated deposits. L., Nedra Publ. 232 p. 14. Grantz, A., Eittreim, S., Whitneey, O.T., 1979. Geology and physiography of the continental margin North of Alaska and implications for the origin of the Canada Basin. // The Ocean Basins and Margins. Vol. 5. The Arctic Ocean. P. 439-492.

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15. Grantz, A., May, S.D., Hart, P.E., 1990. Geology of the Arctic Continental Margin of Alaska. - In: The Arctic Ocean Region. The Geology of North America. Volume L. P. 257-288. 16. Franke, D., Hinz, K., Oncken, O., 2001. The Laptev Sea Rift // Marine and Petroleum Geology. V. 18. P. 1083-1127. 17. Thurston, D.K., Theiss, L.A., 1987. Geologic report for the Chukchi Sea Planning Area, Alaska. United States Department of the Interior Minerals Management Service. Anchorage, Alaska. 18. Undiscovered Oil and Gas Resources, Alaska Federal Offshore (As of January 1995), Sherwood, K.W. (ed), U.S. Minerals Management Service, OCS Monograph MMS 98-0054, 531 p. Reference to this paper:

Vinogradov V.A., Goryachev Yu.V., Gusev E.A. Through structural shelf-ocean zones in the Russian East Arctic. Geological-geophysical features of the lithosphere of the Arctic Region (Proceedings of NIIGAVNIIOkeanologiya, Vol. 210), 2010, Issue 7, p. 32-47 (in Russian).

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Виноградов В.А., Горячев Ю.В., Гусев Е.А. Сквозные структурные зоны шельф-океан Восточной Арктики // Геолого-геофизические характеристики литосферы Арктического региона. Вып. 7. Тр. ВНИИОкеангеология. 2010. Том 210. С. 32-47. В результате геологического картографирования масштаба 1:1 000 000 ВосточноАрктического шельфа России в структуре осадочного чехла выявлена субмеридиональная зональность. Структурные зоны шельфа коррелируются с морфоструктурными зонами океана, обнаруживая сквозной структурный каркас. Установленная зональность затрагивает и прилегающую сушу северо-востока России. Время заложения этих структурных зон относится к баррему-альбу.

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