RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4002, doi:10.2205/2007ES000224, 2008

Deep Structure

2007ES000224-fig11
Figure 11
[33]  The research in the deep structure was conducted along geotraverse constructed on the basis of combined interpretation of geological and geophysical data (Figure 11). The thickness of the crust in the Sea of Okhotsk varies from 35-40 km beneath Sakhalin and the Kuril Islands to 10 km beneath the Kuril Basin. The crust is divided into the basement and sedimentary cover. The basement rocks are exposed in the framing of the Sea of Okhotsk in Sakhalin, Kamchatka, Shantarskie Islands and the Kuril Island Arc and were brought up from underwater elevations with dredge. The basement age ranges from the Paleozoic to the Mesozoic. The sedimentary basin comprises individual abysses where its thickness reaches 12 km. For the major part it is composed of sedimentary rocks and partially of volcanogenic sedimentary rocks of the Late Cretaceous-Cenozoic. In the Late Cretaceous, sedimentation went on in rifting-causing conditions and was accompanied by considerable volcanic activity. Deep-sea basins were formed that were composed of volcanogenic-siliceous sediments gradually replaced with more shallow-water rocks up the section. In the Cenozoic, most of the sedimentary basins were formed. The deposits of those times as an unbroken cover overlapping the underlying formations contain almost all oil and gas complexes of the Sea of Okhotsk.

Sikhote Alin Region

2007ES000224-fig12
Figure 12
[34]  being western continental framing of the Okhotsk Sea plate encompasses Middle Cretaceous orogenic belt of Sikhote Alin [Bogdanov and Khain, 2000a], which in the west abuts the ancient continental margin of Asia represented there by Bureinskiy and Khankaiskiy massifs (Figure 12).

[35]  The orogenic belt of Sikhote Alin comprises rocks of different age (from the Paleozoic-Mesozoic to Neocomian) and of different origin (with predominant oceanic, marginal seas and island-arc complexes). All the formations were accreted to the eastern margin of Asian continent as a result of their dislocations on the shifts that accompanied transform boundary, which formed in the Middle Cretaceous between the Asian continent and Kula plate [Khanchuk, 2000; Natal'in et al., 1994; and others]. Middle Cretaceous orogeny, which was widely manifested in the periphery of the Pacific [Filatova, 1998], in Sikhote Alin manifested itself in the formation of complex scaly-thrust structures, shows of metamorphism and granite formation and in the appearance of strike-slip basins and magmatism. Middle Cretaceous accretion processes considerably enlarged the continent margin and increased its thickness almost up to 40 km.

2007ES000224-fig13
Figure 13
[36]  All those structures are overlaped by volcanic-plutonic associations of East Sikhote Alin sub-subduction belt, which marked the Asian continent edge in the Senomanian-Paleocene. Fragments of the oceanic plate of this belt that underwent subduction under the continent are now registered by seismic tomograph data at mantle depths as high-velocity objects [Bijwaard et al., 1998]. The Younger Cenozoic extension structures (including Tatar strait rift) upset the structures of continent margin and abutting perioceanic area formed before [Filatova, 2004] and are commonly accompanied by intense magmatic manifestations. Judging by heterogeneous composition of magmatic rocks, the latter are related to several levels of depth (lithospheric mantle, asthenosphere and lower mantle). In the East Sikhote Alin belt, magmatic activity went on from the Cretaceous until the Early Quaternary [Filatova, 1998]. The Paleogene-Quaternary basalt flows are the products of fissure eruptions; the thickness of basalt plateau sections reaches 800-1000 m. This effusive comprises tholeiites, sub-alkaline basalts and rocks of olivine-basalt series. Tholeiites are close to basalts of MORB type and apparently are associated with asthenospheric magmatic sources (Figure 13).

[37]  The crust thickness varies from 30 km beneath volcanogenic belt to 38 km beneath Sikhote Alin [Rodnikov et al., 1996; Zverev and Tulina, 1971]. The results of magnetotelluric sounding in Sikhote Alin [Kaplun, 2002; Nikiforova et al., 1980] showed that electric conductivity layer considered as the asthenosphere is located in the upper mantle at a depth of approximately 100-120 km.

[38]  The major part of Primor'e belongs to seismic zone of 5-6 intensity [Ulomov and Shumilina, 1999]. Several strong earthquakes with M =6.0 (1914), M =5.6 (1924), and M =5.0 (1968) were registered there that were associated with deep faults, along which terrains of different types forming Primor'e moved. Deep-focus earthquakes noted at depths in the range from 300 to 600 km are lower margins of two seismic focal zones the Kuril zone and Indzu Boninskaya zone subsiding under the continent.

Tatar Strait

2007ES000224-fig14
Figure 14
[39]  (Figure 14) is a large rift structure, which is approximately 50-km wide and 4-km deep [Piip and Rodnikov, 2004]. It is composed of a thick bed up to 8-10 km of the Mesozoic-Cenozoic sedimentary formations [Khvedchuk, 1992; Tronov et al., 1987; Varnavskiy, 1994]. The rift is located between the Mesozoic structures of Sikhote Alin and West Sakhalin Mountains and is separated from them by deep faults. Sediments composing the trough from available geological and geophysical data are divided into four structural complexes separated from each other by regional stratigraphic disconformities and having different structural- compositional and physical characteristics: Upper Cretaceous, Paleogene, Oligocene-Lower Miocene and Middle Miocene-Quaternary. The trough basement is granite-metamorphic layer with seismic boundary velocities in the range of 5.8-6.2 km s-1 [Gnibidenko et al., 1995].

[40]  The earth's crust is broken by faults. Recent tectonic activity is emphasized by high heat flow, magmatic activity and seismic manifestations. In this context, the thickness of the crust is lowered as compared to the bordering areas and decreases to 25 km, and velocities in M -boundary make 7.4-7.6 km s-1. Deep faults revealed with the use of deep seismic sounding are corroborated by geological data. Thus in the area of West Sakhalin fault bordering Tatar Strait in the east, the Cenozoic sediments steeply tilt westwards (up to 50-80o) as compared to the rest of the trough and are intensely dislocated by faults and reversed faults. Displacements on faults vary within tens and hundreds of meters, reaching 4-5 km. Volcanic centers of the Lower and Upper Miocene and Pliocene are confined to the fault zone. Increased seismic activity and permeability (fluid conductivity) are characteristic of the faults [Rodnikov et al., 1996]. Calculations of deep temperatures showed that the sedimentary trough is associated with the rise of hot asthenospheric diapir causing the splitting of the earth's crust, rift structures formation in the trough basement, magmatic activity manifestations and sedimentary bed heating. The asthenospheric diapir may have been an additional source of hydrocarbons and fluidal flows providing intense hydrothermal activity and contributing to oil and gas deposits formation [Rodnikov et al., 2001]. The formation of Tatar Strait rift structure is associated with the upwelling of the asthenosphere to the earth's crust [Rodnikov, 1997]. The rift is the northern continuation of the spreading center located in the abyssal basin of the Sea of Japan, which was revealed from the studies of magnetic field anomalies profiles of the Sea of Japan [Isezaki et al., 1976]. It is believed that spreading processes went on there 15-25 million years ago and were accompanied by basalt lavas eruptions [Maruyama et al., 1997; Jolivet et al., 1995]. In the middle of the Oligocene the processes of the earth's crust extension started and in the Miocene they finished in Tatar Strait as a result of rift formation accompanied by area basalt volcanism manifested in Moneronskiy rise located in the central part of Tatar Strait. The chemical composition of effusive shows that the effusive belongs to tholeiitic and alkali olivine-basalt type [Piskunov, 1977]. Moneronskoe earthquake that occurred in Tatar Strait in 1971 is characterized by upthrust motions [Arefiev, 2003]. Earthquake hypocenters depth was on the average in the range from 5 to 20 km [Zlobin, 2005].

2007ES000224-fig15
Figure 15
[41]  In Figure 15 the deep structure of the lithosphere is shown beneath the sedimentary trough of Tatar Strait where Izylmetievskoe gas field was discovered. Increased heat flow, magmatic activity and sedimentary bed heating caused by asthenospheric upwelling became an additional source of hydrocarbons and fluidal flows fostering hydrocarbon deposits formation in the sedimentary beds of Tatar Rift.

Sakhalin Island
[42]  is a fragment of Asian continental margin separated from it by the Cenozoic rift structure of Tatar Strait. In this context, the Paleozoic and Mesozoic - Early Paleogene structures can be followed in the island, which are abundant in Sikhote Alin, though they are considerably dislocated there by a system of faults drawn together and having north-south extension. The western Sakhalin is occupied by thick (up to 10 km) the Cretaceous-Paleogene turbidites of the fore-arc trough of East Sikhote Alin magmatic belt and under it basement rocks like intensely dislocated the Jurassic-Neocomian and Paleozoic oceanic formations are buried. To the east those rocks of ancient oceanic plates, which underwent intense greenschist, glaucophanitic and locally eclogitic metamorphism, form the sublongitudinal zone bounded by faults of East Sakhalin mountains from where it can be followed to the south of Sakhalin Island to Susunaiskaya zone and farther to Kamuikotan zone of Hokkaido Island. The extreme east of Sakhalin Island is occupied by fragments of the Campanian-Paleocene island arc and they together with fragments of the Cretaceous oceanic plate are thrust on Saklhalin structures from the Okhotsk Sea plate. Basites-ultrabasites participating in the scaly-thrust structures and separated from the Cretaceous oceanic crust (earlier had belonged to the Okhotsk Sea plate), apparently create the linear magnetic anomaly along the eastern shoreline of Sakhalin Island. These ultrabasic rocks are apparently remnants of the Late Cretaceous-Early Paleogene zones of subduction. In deep seismic sounding profiles, listric faults sloping approximately at an angle of 15o are prominent that penetrate into the upper mantle from the sedimentary cover; discontinuities are noted in M -boundary and thickness variation of the upper layer of the crust and blocks of crust with high values of seismic velocities are noted [Piip and Rodnikov, 2004].

[43]  The thickness of the earth's crust makes up 30-35 km. Velocities on M -boundary vary from 7.8 to 8.3 km s-1. In Sakhalin, earthquakes are confined to the earth's crust and are related to deep faults extending along the whole island and serving boundaries of lithospheric plates. Geodetic observations of 1975-1983 showed regularities of horizontal movements in the zone of Central Sakhalin deep fault [Vasilenko and Bogdanov, 1986]. It was established that in 1975-1978 a right-lateral shift had been observed on the fault that had been replaced by compression by 1979. Subsequently in 1979-1980 in the fault zone expansion was observed that was replaced by attenuation in 1980-1983 [Vasilenko and Bogdanov, 1986]. Generally in Sakhalin, migration is noted of strong earthquakes sources from east to west and directed upwards from great depths to the surface [Zlobin, 2005]. Mud volcanoes eruptions are confined to deep faults as well [Melnikov et al., 2005].

Kuril Basin
[44]  of the Sea of Okhotsk belongs to back-arc depressions. In plan, it has a form of wedge narrowing northwards. It is outlined by isobath of 3000 m, the average depths in the test site under investigation are 3200 m. Thick (more than 4000 m) sedimentary beds overlie "acoustic basement'', which evidently is a volcanogenic-sedimentary layer, and beneath it, the third layer of oceanic crust is observed with seismic velocities 6.4-6.8 km s -1 and thickness of 5 km in the middle of the basin. High heat flow is characteristic of the basin [Smirnov and Sugrobov, 1980]. "The acoustic basement'' is intensely rugged; scarps associated with faults are abundant on the slopes. From data obtained with the use of reflected wave method [Snegovskoi, 1974] the sedimentary cover is subdivided into two sedimentary complexes. The upper one, which is likely of Pliocene-Quaternary age, with thickness in the range from 800 m to 1000 m is characterized by thin layering. Sediments of the lower complex in the central part of the basin have thickness more than 3000 m and make transparent acoustic layer.

[45]  Subsequent seismic research showed that seismic velocities range from 1.7 to 4.3 km s -1 for sedimentary bed of thickness of approximately 5 km. It is underlain by a layer of thickness of 2.0-2.8 km with velocities of 4.8-5.2 km s -1, which is apparently volcanogenic-sedimentary section of the oceanic crust [Baranov et al., 2002]. A layer with velocities 6.4-7.2 km s -1 and thickness 4-5 km is located below; some researchers correlate it with oceanic layer 3 [Baranov et al., 2002]. M -boundary is noted at the depth of 11-13 km [Galperin and Kosminskaya, 1964]. Data on the basement composition and age are lacking. Insufficient data are only available on the basement structure in the basin edges. Samples dredged up from Akademii Nauk Elevation showed that the northern slope of the Kuril Basin is composed of magmatic rocks of calc-alkali series. K-Ar method testifies to their Cretaceous age [Gnibidenko et al., 1995]. According to [Baranov et al., 1999], isotopic analysis Sr-Nd-Pb of volcanic rocks testifies to the effect that the basement may be a thinned continental crust. Mean velocity of subsidence in the Pliocene-Quaternary apparently associated with back-arc basin extension ranges from 0.5 to 2.0 mm year -1 [Baranov et al., 2002].

[46]  Geophysical research of the basement shows that it is complicated with a number of dislocations with a break in continuity revealed in near-border areas and individual ledges of the basement that are commonly isometric in plan and conic in vertical section. Sediments overlap them and evidently they are buried volcanic constructions [Tuezov, 1975]. It is corroborated by characteristics of the magnetic and gravitational field anomalies [Krasnyi, 1990]. Rocks of "acoustic basement'' are apparently composed of basic volcanics (basalts and their tuffs) alternating with volcanogenic-sedimentary and siliceous formations whose fragments were dredged up from the basin's slopes. T. A. Emelianova [Emelianova et al., 2003] studied the material composition of volcanogenic rocks obtained by dredging in the expeditions of research vessels "Pegas'', "Pervenets'', "Akademik Lavrentiev'' [Biebow et al., 2000; Krasnyi et al., 1981; Tararin et al., 2000]. In the Kuril Basin they compose numerous volcanoes of the Pliocene-Pleistocene, which for the major part are located in the framing of the basin. They are in the southern slope of Akademii Nauk Elevation [Emelianova et al., 2003], in the rear zone of the Kuril Basin [Avdeiko et al., 1992] and Geophysicist seamount volcano located in the northeastern Kuril Basin at a depth of approximately 3200 m [Baranov et al., 2002; Tararin et al., 2000]. In the southern slope of Akademii Nauk Elevation, Pliocene volcanics are represented by andesite-basalt, andesite and locally basalts and andesite-dacite [Emelianova et al., 2003]. Similar rocks compose volcanic formations of the rear zone of the Kuril Basin and Geophysicist seamount volcano. K/Ar dating of whole rock samples shows the range from 0.9 to 1.6 million years. Volcanoes are located at intersections of transverse and longitudinal faults. The chemical composition of volcanic rocks composing volcanoes is more alkali as compared to effusive of calc-alkaline series of the Kuril Island Arc, which gives evidence in favour of their relation to the formation of Kuril back-arc basin [Emelianova et al., 2003].

[47]  Palynological studies of rocks along seismic profiles allowed dating the sedimentary bed of the Kuril Basin [Bezverkhniy et al., 2003]. It was established that the basin formation started in the Late Paleogene - the Early Oligocene. Coastal and marine sediments with volcanogenic bands were deposited at that time.

[48]  From seismic data, a rift or spreading structure is distinguished in the central part of the Kuril Basin [Piip and Rodnikov, 2004]. This structure is pronounced in the upper sedimentary layers. Faults forming it penetrate into the upper mantle where zones of anomalous low velocity (7.0-7.5 km s -1 ) are likely to be the asthenospheric diapir containing magma-formation sources. Electromagnetic research testifies to a partial melting area in the upper mantle beneath the Kuril Basin [Lyapishev et al., 1987].

[49]  High heat flow is characteristic of the basin. The highest temperatures reaching 1200o C in the mantle are noted beneath the Kuril Basin at a depth of approximately 25 km, forming an area of partial melting [Smirnov and Sugrobov, 1980]. On the floor surface of the Kuril Basin, the rise of hot anomalous mantle corresponds to rift structures and basic magmatism. The Kuril Basin is a back-arc basin where we assume suboceanic crust associated with arc-rear spreading [Khain, 2001]. If this model is correct, we may expect layers of depleted tholeiite of composition close to MORB in combination with hyaloclastic rocks. The studies on the deep structure of the Kuril Basin show that the thickness of the crust reaches approximately 10 km. The asthenosphere forms diapir ledges immediately approaching the earth's crust. Rifts that are spreading centers are located in the basin's basement. The rise of asthenospheric diapirs to the crust caused high heat flow.

[50]  Seismic activity in the Kuril Basin is considerably lower than in Kuril subduction zone. Shallow-focus earthquakes are only located there. Most of the earthquakes are located in the northwestern slope of the Kuril Island Arc and form a narrow belt running parallel to the arc. Seven focal mechanisms are known in the Kuril Basin. The focal mechanisms are of two types: reversed faults and strike-slip faults. The epicenters of the former group are located near the continental rise of the Kuril Island Arc and in the northern slope of the Kuril Basin. Epicenters with strike-slip movements are located in the central part of the basin and near the continental rise of the arc [Baranov et al., 2002].

Kuril Island Arc
[51]  comprises the Greater island arc and the Lesser island arc separated by an inter-arc trough. The islands of the Greater island arc are composed of the Cenozoic volcanogenic and volcanogenic sedimentary rocks that, judging by xenoliths, overlie the basement composed of metamorphic rocks, crystalline schist, hornfels, gabbroid rocks, diorite and plagioclase granite. The crust thickness reaches 30-35 km. The thickness of the crust beneath the inter-arc trough decreases to 15 km. Submarine volcanoes confined to faults complicate the Okhotsk Sea slope of the island arc. They are composed of the Quaternary basalt, andesite-basalt and andesite lavas with layers of loose sediments. In dredged up fragments and blocks of effusive, impregnation of sulfide minerals was established: pyrite, marcasite, pyrrhotite, chalkopyrite, digenite and covellite [Kononov, 1989]. The Lesser island arc for the major part is composed of the Upper Cretaceous formations. Basement rocks are composed of banded gabbro, gabbro-norite, and serpentinous peridotite. They form allochthon plates, whose top part contains complexes of parallel dykes [Pushcharovskii and Melankholina, 1992b]. The inter-arc trough is situated between the outer and inner island arcs and their contact is noted by the fault system. The trough width is 45-60 km. It is composed of the Neogene and Quaternary tufogenic sedimentary structures. The thickness of sediments in the axial zone is more than 3 km but seismic research in the sedimentary layer base has not been conducted. Evidently seismic research did not reveal the total section of sedimentary volcanogenic structures. On the basis of the calculation of the upper edges of anomaly-forming bodies the occurrence of geological basement is likely in the most subsided parts of the trough at depths of 5-6 km. Abundance of volcanogenic rocks in the trough sediments is related to rifting with structures at present overlapped by thick loose sediments, intermediate effusive as well as tuff, tuff breccia and tuff sandstone.

[52]  Andesite-basalt and andesite composing the volcanoes of the Kuril Island Arc belong to moderate potassic calc-alkaline series and basalt have chemical parameters close to rocks of tholeiitic series [Avdeiko, 1994]. In the Kuril Island Arc rear zone, depleted varieties disappear and calc-alkali volcanics have enriched composition at the expense of increased content of various incoherent and rare elements. In the same direction from front to rear zone, value 87Sr/86Sr increases in lavas and value 143Nd/144Nd decreases. The process of the Pacific plate subduction genetically determines volcanic rocks of the Kuril Island Arc. Their magmatic sources are located in the wedge above subduction in the upper mantle and partially they may be located in the asthenosphere as well [Martynov et al., 2005], which in the form of a mantle diapir immediately approaches the earth's crust of the inter-arc trough of the Kuril Island Arc [Rodnikov et al., 2005].

[53]  In the Kuril Island Arc earthquake sources form a distinct focal zone dipping at an angle of 40o from the Kuril Trench toward the continent to a depth of 700 km. The most of earthquake sources form a wedge narrowing at a depth of approximately 200 km [Yunga and Rogozhin, 2000]. The studies of earthquake focal mechanisms showed that generally in Kuril-Kamchatka island arc subhorizontal compression is noted oriented across the strike of the arc [Balakina et al., 1996]. According to data on Kronotskoe earthquake focal mechanism in Kamchatka the major axis of compression gently subsides under the trench and is oriented from southeast to northwest. Extension axis steeply slopes to northeast. Repeated shocks encompassed the upper part of the lithosphere to a depth of 40 km. Earthquakes caused strike-slip motions accompanied by upthrust processes [Yunga and Rogozhin, 2000]. In the southern Kuril Islands, where oblique subduction is noted, Shikotan earthquake of 1994 caused upthrust motions [Arefiev, 2003]. The earthquakes were accompanied by vertical and horizontal movements that resulted in Shikotan subsidence for 0.5-0.6 m [Yunga and Rogozhin, 2000].

[54]  Seismic focal zone in the Kuril-Kamchatka Island Arc is located in the area of increased values of seismic velocities [Tarakanov, 2005]. Separated areas of significant velocity gradients are characterized by strong earthquakes manifestations [Gontovaya et al., 2004] and the island arc areas characterized by considerable attenuation and low velocity of waves are located beneath the volcanic zone above subsiding slab [Fedotov and Chernyshev, 2002]. In the subsiding slab, a compression area is noted in the top part, which is replaced by expansion after the seismic focal zone bent at a depth of approximately 200 km [Balakina, 1981; Zlobin, 1987, 2002].

Kuril-Kamchatka Trench
[55]  in the geotraverse test site is outlined with isobath of 7000 m and has an asymmetric profile. Near-island slope is steeper (7-10o, and locally up to 15o) than the ocean side (5-7o, in the top part 3-5o) [Vasiliev et al., 1979]. From the ocean side of the trench along the edge of the Pacific a gently sloping uplift of Zenkevich swell extends with elevation of 200-400 m above the ocean floor and width of 300 km. The major part of the trench is covered by the sedimentary bed of the first oceanic layer commonly overlapped by thin (tens of meters) turbidite structures or a landslide lens [Belousov and Udintsev, 1981; Rodnikov et al., 1996]. The oceanic slope is broken by numerous faults of strike-slip type; the majority of them only cut the second layer but some of them cut the first layer as well and manifest themselves in the relief as ledges 50-200 m high. Fault planes are commonly tilted with respect to the trench axis at an angle of 30-60o. The distance between faults ranges from 1 to 5 km [Vasiliev et al., 1979]. Low values of the heat flow are typical of the axial part of the trench. In the most of the seismic profiles [Belousov and Udintsev, 1981] in the axial part of the trench, the subsidence is observed of the roof of the second seismic layer of oceanic slope under the island slope for a distance traced for 12 km with inclination angle of 5-7o.

Northwest Pacific Basin.
[56]  The geotraverse runs within Zenkevich swell and a vast plain extending eastwards to Shatskii elevation with mean depths of 5000-5500 m. Zenkevich swell is a marginal oceanic elevation separated by isobath 5500 m of width of 300-350 km with the elevation above the ocean floor in the range from 200 to 400 m [Belousov and Udintsev, 1981]. The thickness of sediments is insignificant (300-350 m), and rocks of the second layer of the oceanic crust composed of basalts belonging to tholeiite are commonly exposed on the floor surface near the trench axial zone. Those rocks are commonly associated with horst ledges [Rodnikov, 1983]. Besides basalts, fragments of tuff, aleurolite, greywacke, argillite, siliceous rocks, hornfells, metaschist, andesite, diabase, felsitic porphyry, granodiorite, granite and aplite [Vasiliev et al., 1979] were dredged up. K-Ar dating of basalts dredged up from acoustic basement outcrops of the boundary swell shows the age range from 80.1 to 32.6 million years (from the Late Cretaceous to Oligocene) and granodiorite dating is 103 million years (Early Cretaceous). Specialists, who studied the boundary swell, believe [Rodnikov, 1983] that such spread of dating bears evidence on duration of the final stage of magmatic activity in the swell. In the southeast, the swell smoothly gives way to the ocean floor forming a plain complicated by smaller and larger hills.

[57]  Having the most ancient crust from geological and geophysical data (about 150 million years), the whole area of northwestern basin is covered through by sedimentary cover of thickness 300-400 m. Judging by boreholes DSDP 303 and 580 [Larson et al., 1975], the cover is composed of diatom and radiolarian ooze and laminated clays enriched with Late Miocene - Quaternary ash overlying zeolitic pelagic clays, clayey nanosilt and siliceous rocks. At a depth of 211 m, those sediments are underlain by the Lower Cretaceous pelagic zeolitic clays and in the bottom of the section with interlayers of flinty slate and nanoplankton limestone. At a depth of 284.75 m, sediments are underlain by pillow lavas of the Jurassic and Cretaceous basalts of MORB type accumulated with the activity of spreading axes of different orientation [Khain, 2001].

The upper mantle
[58]  beneath the Sea of Okhotsk is characterized by both horizontal and vertical heterogeneities. It is somewhat less packed as compared to the Pacific [Boldyrev et al., 1993].

[59]  From seismic tomography data [Anderson and Dzevonskiy, 1984; Bijwaard et al., 1998], we note decreased values of seismic velocities in the upper mantle beneath the Sea of Okhotsk as well as beneath the Sea of Japan and the Philippine Sea; in the Kuril Basin on the basis of electromagnetic research in the upper mantle in the depth range of 30-65 km a layer is distinguished of specific conductivity 0.3-0.5 S m-1 and integral conductivity of approximately 15000 S [Lyapishev et al., 1987]. The nature of the layer is related to partial melting and it only occurs in the basin. At a depth of 100 km, the second conductivity layer may be separated. Obtained results are in agreement with deep temperatures in the upper mantle, seismic research and other geophysical data [Maruyama et al., 1997].

2007ES000224-fig16
Figure 16
[60]  The asthenosphere in the upper mantle is separated for the major part from geothermal data [Rodnikov et al., 1996]. The term asthenosphere is used in reference to the layer in the upper mantle where the matter is under temperature close to temperature of melting. Thus initial magmatic centers are located there, increased electric conductivity and drastic absorption of elastic waves is noted, decrease in their velocities and in rock density is observed. For the first time, B. Gutenberg substantiated the existence of the asthenosphere as a low-velocity layer on the basis of experimental data [Gutenberg, 1953]. The asthenosphere is separated with the use of various methods of research: seismic, geothermal, and electromagnetic. The notion of asthenosphere as the partial melting zone is very important for the studies of the geological evolution of sedimentary basins because besides heat the asthenosphere is the source of hydrocarbon fluids. The upper boundary of the asthenosphere is assumed to be isothermal line of 1000-1200o. Under such temperatures the upper mantle rocks partially melt with account for deep fluid influence [Rodnikov et al., 1996; Smirnov and Sugrobov, 1980]. From the calculations the asthenosphere is located in the upper mantle in the Sea of Okhotsk at a depth of 50-70 km and beneath Northwest Pacific Basin it is revealed at a depth of approximately 100 km. From the asthenosphere, diapirs of partially melted matter come off, which reach a depth of 20-30 km beneath the sedimentary trough of Tatar Strait, the Deryugin Basin and the Kuril Basin and cause active tectonic conditions manifested in volcanic, seismic and hydrothermal activity (Figure 16).

[61]  Beneath the North Sakhalin sedimentary basin containing almost all oil and gas fields of Sakhalin, the asthenosphere is located at a depth of approximately 70 km. Besides, hydrocarbon deposits were noted above asthenospheric diapirs in the sedimentary cover of Tatar Strait and the Deryugin Basin, and sulfide mineralization was revealed in the Kuril Basin in submarine volcano peaks. Mantle fluids of asthenospheric diapirs determine the geodynamic evolution of sedimentary basins and the formation of hydrocarbon deposits in them.

[62]  In Sakhalin, the asthenosphere conductivity layer occurs in the upper mantle beneath the whole island and Tatar Strait where it is noted at a depth of 80 km. Along the continent eastern margin in the upper mantle the junction is noted of the asthenosphere high-conductivity layers and the continent rigid high-resistance upper mantle. Besides, beneath Sakhalin in the depth range of 300-500 km anomalous high-resistance areas are noted, which may be related to the cold subsiding plate of subduction zone occurring there.

[63]  Beneath South Kuril Islands in the geotraverse area, the depth down to conductivity layer in the upper mantle is 60-80 km [Alperovich et al., 1978]. Geothermal observations corroborate the results of electromagnetic research. Highest temperatures are observed beneath the Kuril Basin where partial melting area is located at a depth of approximately 25 km. Lowest values are noted beneath the deep trench [Smirnov and Sugrobov, 1980]. On the sea-floor surface of the Kuril Basin anomalous mantle rise corresponds to rift structures and basic magmatism. Deep temperatures in Moho boundary vary from 100o C in the Pacific to 800o C beneath Tatar Strait and Kuril Basin.

[64]  Thus maximum temperatures and minimum thickness of the lithosphere are characteristic of deep basins of the Sea of Okhotsk. In axial areas of the structures the asthenospheric layer rises to 15 km; in the sides it subsides to depths of 40-50 km and beneath the Pacific it goes down to a depth of 100 km.


RJES

Citation: Rodnikov, A. G., N. A. Sergeyeva, L. P. Zabarinskaya, N. I. Filatova, V. B. Piip, and V. A.  Rashidov (2008), The deep structure of active continental margins of the Far East (Russia), Russ. J. Earth Sci., 10, ES4002, doi:10.2205/2007ES000224.

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