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

Data Used

[7]  The research of the deep structure of active continental margins of the Far East is carried out on the basis of combined interpretation of geological and geophysical data. To construct geodynamic models of sedimentary basins we use the results of geological, seismic, petrological, geothermal, magnetic, electromagnetic and gravimetric research. Geophysical fields, the geological structure of sedimentary cover, the structure of the crust and the upper mantle, location of deep faults, volcanoes and their magmatic centers, distribution of earthquake foci, depths of occurrence of the asthenosphere and diapirs, paleo and modern subduction zones, distribution of deep temperatures are shown in geodynamic models. To construct geological sections a great body of geological data were used including drilling and r/v voyages data summarized in papers [Biebow et al., 2000; Bogdanov and Khain, 2000b; Filatova and Rodnikov, 2006b; Obzhirov et al., 1999; Rodnikov et al., 1996, 2005; Sergeyev, 2006]. Consolidated crust was constructed on the basis of seismic profiles made in different years. They were interpreted with the use of new methods, which gave additional information on the structure of the crust [Piip and Rodnikov, 2004]. Seismic waves velocities abruptly change in the Okhotsk Sea region from 7.8 km s-1 beneath island arcs and sedimentary basins to 8.2 km s-1 beneath stable areas of the crust. The asthenosphere in the upper mantle is separated mostly from geothermal data [Rodnikov et al., 1996]. The asthenosphere is a layer in the mantle where the matter is under temperature close to melting temperature and in this case the viscosity in it decreases, and under certain conditions, magmatic centers appear. The upper surface of the asthenosphere is assumed to be 1000-1200o C that is the temperature of upper mantle rocks partial melting with consideration for deep fluids influence [Rodnikov et al., 1996; Smirnov and Sugrobov, 1980]. The separation of the asthenosphere is corroborated by tomography research data [Bijwaard et al., 1998] and seismic observations [Kosminskaya et al., 1987; Rodnikov et al., 1996]; electromagnetic research in the upper mantle separated high-conductivity layers, which suggests the existence of partially melting zone beneath sedimentary basins [Kaplun, 2002; Lyapishev et al., 1987; Nikiforova et al., 1980].

Tectonic position.

2007ES000224-fig02
Figure 2
[8]  The Okhotsk Sea region forms a lithosphere plate located between North American, Eurasian and Pacific plates (Figure 2).

[9]  The Okhotsk Sea plate is bounded by deep faults, for the major part strike-slip faults, and in the southeast it is bounded by a recent subduction zone that is Benioff zone. Its basement is heterogeneous from crystalline the Paleozoic-Mesozoic revealed in the continent, in Sakhalin and Kamchatka to the Mesozoic-Cenozoic in water area of the Sea of Okhotsk [Rodnikov et al., 1996]. The plate was finally formed in the Late Cretaceous and it was overlapped with a cover of sedimentary and volcanogenic sedimentary rocks in the Cenozoic. The thickness of the crust is approximately 28-32 km decreasing to 24 km in the Deryugin Basin and to 15 km beneath the Kuril Basin.

[10]  The Okhotsk Sea plate belongs to intensely deformed structures from geological and geophysical data and the results of analysis of the recent crustal movements. The origin of the plate structure is determined by general geodynamic settings which were formed by the end of the Paleocene [Filatova and Rodnikov, 2006b], and in this case according to [Gatinskiy and Rundkvist, 2004; Gatinskiy et al., 2005], the directions of horizontal displacement vectors in general agrees with the assumption that the range of velocities measured experimentally is related to the Pacific plate effects. But it is likely that to some extent, the deformation of the Okhotsk Sea plate structures was also caused by Eurasian continent moving eastwards with Baikal rift development [Tamaki and Honza, 1985]. Structural dislocations inside the plate may be related to extensions in the Kuril Basin, Tatar Strait and Deryugin Basin caused by upwelling of asthenosphere diapirs, which reached their peaks in the Miocene.

[11]  Sedimentary basins were formed in the conditions of the Okhotsk Sea crust destruction caused by rifting in the Cenozoic. It is assumed that extensional conditions started in the Paleocene but they were manifested most intensely in the Late Oligocene - Middle Miocene, which resulted in the formation of grabens, semi-grabens and pull-apart deep basins with oceanic and thin sub-continental crust [Rodnikov et al., 1996]. In the Late Miocene and Pliocene, compression conditions were activated, which resulted in the formation of reversed faults, strike-slip faults and thrusts [Biebow et al., 2000].

Heat flow.

2007ES000224-fig03
Figure 3
[12]  The distribution of heat flow values along the geotraverse is taken from the global heat flow catalog [Pollack et al, 1991] and heat flow maps [Smirnov, 1986; Smirnov and Sugrobov, 1980; Tuezov, 1988]. It is high in the deep basins and troughs of marginal seas and relatively low in the continental structures of the Far East and Kuril area of the Pacific (Figure 3).

[13]  Heat flow variations in Sikhote Alin only make 39-56 mW m-2. In Kuril area of the Pacific, the heat flow mean values are 52 mW m-2. The lowest values, which make 22 mW m-2, are noted in deep Kuril-Kamchatka trench. The heat flow mean values for the Kuril Island Arc are 118 mW m-2 and the highest values reaching 790 mW m-2 are noted in the western part of the island arc. Heat flow mean values in Sakhalin amount to 76 mW m-2. High heat flow is noted in Tatar Strait (123-132 mW m-2 ) and in the Deryugin Basin where it reaches 200 mW m-2. Besides, high heat flow values are established in the Kuril Basin of the Sea of Okhotsk, where it reaches 346-354 mW m-2 [Rodnikov et al., 1996].

Magnetic field

2007ES000224-fig04
Figure 4
[14]  (Figure 4). Anomalous magnetic field of the Okhotsk Sea region is characterized by various orientation of anomalies extension and different configurations and values of the anomalies [Rodnikov et al., 1996].

[15]  Magnetic field anomalies for the major part show linear northwestward and northeastward extension. Magnetic field of Sikhote Alin is characterized by positive anomalies extended along deep faults and reaching values from 300 to 600 nT, which are related to magmatic body masses. In Tatar Strait, a chain of individual maximums is distinguished, which approximately coincide with the axis of greatest depths of the strait. Generally, in Sakhalin, negative magnetic field is noted. Individual positive linear anomalies are related to intrusive and effusive basic and ultra-basic bodies. Along eastern Sakhalin in the Sea of Okhotsk, the East Sakhalin positive magnetic anomaly extends, which reaches values of 1200-1400 nT. This anomaly locates in the East Sakhalin ophiolite belt where ultrabasic and basic rocks are exposed in Schmidt Peninsular and in East Sakhalin mountains. This belt separates North Sakhalin from the Deryugin Basin. The Deryugin Basin and Kuril Basin are characterized by weak negative anomalies with the amplitude of -200 nT, which are related to nonmagnetic sedimentary rocks composing the basins. The anomalies are isometric. Nearer to the islands, they assume linear character.

[16]  Going toward Kuril Islands the magnetic field becomes differentiated and varies from -300 nT to +400 nT. Volcanic arc corresponds to a narrow zone of disturbed magnetic field with local positive and negative anomalies of individual volcanic constructions superimposing the general negative horizontal background [Gainanov et al., 1968]. Local positive anomalies and negative anomalies commonly associated with them are confined to submarine volcanoes. The span of the anomalies is often more than 1000 nT [Pushcharovskii, 1992a]. Northwest extension of the magnetic field anomalies is associated with deep faults breaking the Kuril Island Arc into separate blocks [Rodnikov et al., 1996].

[17]  In the Northwest Pacific Basin abutting the Kuril Island Arc, the systems of linear magnetic anomalies were revealed of the age ranging from 108 million years to 160 million years [Hilde et al., 1977]. The anomalies of the continental slope of the deep-sea trench have the general northeastern extension, which is disturbed by transverse anomalous zones. In the southern area of the trench slope, the linear northeastern anomalies parallel to the trench axis look as if they continue band anomalies of the Pacific plate but are less distinct [Sergeyev and Krasnyi, 1987].

Electromagnetic research.
[18]  The results of magnetic telluric sounding in Sikhote Alin [Kaplun, 2002; Nikiforova et al., 1980] showed that the lithosphere thickness beneath Sikhote Alin structures makes 100-120 km. In the west, it increases up to 220 km. Beneath Sakhalin, electric conduction layer in the upper mantle is located at depths of 80-90 km subsiding down to 120 km toward the west shore of the island [Vanyan and Shilovskiy, 1983]. Under the western area of Tatar Strait immediately beneath the volcanogenic belt, Sikhote Alin continental structures are likely to be in contact with transition zone structures from magnetotelluric observation data. In the crust of Sakhalin, a conduction layer is established as well, which is at a depth of 15 km and has total longitudinal conductivity of approximately 40 S [Vanyan and Shilovskiy, 1983].

[19]  In the Kuril Basin of the Sea of Okhotsk along the geotraverse, electromagnetic research was conducted with the use of gradient magnetic variation sounding [Lyapishev et al., 1987]. According to the geoelectrical model selected, a layer of specific conductivity of 0.3-0.5 S m -1 and integral conductivity of approximately 15,000 S was separated in the depth range from 30 to 65 km in the upper mantle. The nature of the layer is associated with partial melting and its occurrence is confined to the basin. At a depth of more than 100 km, one more conduction layer may be distinguished [Lyapishev et al., 1987]. The obtained results are in agreement with deep temperatures in the upper mantle, seismic and other geophysical data.

[20]  Beneath Iturup Island of the Greater Kuril Ridge the depth up to electric conduction layer amounts to 100-130 km and beneath Shikotan Island of the Lesser Kuril Ridge it is 75-80 km [Rodnikov et al., 1996]. From data of [Alperovich et al., 1978] the depth down to electric conduction layer beneath Iturup Island is 60-80 km.

Gravitational field

2007ES000224-fig05
Figure 5
[21]  (Figure 5). Gravitational field of the Sea of Okhotsk shows medium values of free-air anomalies. Gravitational field is characterized by dramatically contrasting structure in the area of modern subduction zone and relatively even changes in other areas. In the northern Okhotsk Sea positive gravitational anomalies prevail. The largest values of anomalies (up to +50 mGal) are associated with basic rocks exposed on the Akademii Nauk uprise. A narrow minimum of anomalies is located to the north of the elevation, separating it from the Instituta Okeanologii uprise. Weakly anomalous gravitational field characterizes the Kuril Basin, for the major part, but in the northeast a positive anomaly up to 25 mGal was discovered [Boldyrev et al., 1993; Rodnikov et al., 1996]. Positive values of anomalies (up to 20-30 mGal) characterize a narrow zone extending along East Sakhalin and showing ophiolite belt (Mesozoic subduction zone); in the Deryugin deep basin, negative values of anomalies are noted, which is typical of deep trenches. Seismic focal zone identified as subsiding oceanic plate has increased density. Decreased density is noted beneath the Kuril Island Arc. In geoid heights map, the transition zone region under investigation is located in the area of positive values of geoid heights reaching 20 m in the Sea of Okhotsk and bounded by negative values from continental and oceanic sides [Sandwell and Smith, 1997].

Seismicity.

2007ES000224-fig06
Figure 6
[22]  The location of the Okhotsk Sea plate in the contact zone of three lithospheric plates (Eurasian, North American and Pacific) caused high seismicity in its margins (Figure 6).

[23]  In the active continental margin of the Far East a large number of earthquakes commonly occur. It accounts for eighty percent of the total energy of earthquakes in North Eurasia [Yunga and Rogozhin, 2000]. The largest earthquakes that occurred in the last 10 years are Shikotan earthquake in southern Kurils of 1994 (magnitude M =8.4 and the source depth is approximately 65 km), Neftegorsk earthquake in Sakhalin of 1995 ( M =7) and Kronotskoe in eastern Kamchatka of 1997 ( M =7.9).

2007ES000224-fig07
Figure 7
[24]  The highest seismic activity is noted along the Kuril Island Arc. There the Pacific plate subducts under the continent, forming a seismofocal zone, which is traced to the depth of 700 km. In the west, the Okhotsk Sea plate is bounded by deep faults extending along Sakhalin. There the earthquakes for the major part are localized in the crust. In the Kuril Island Arc the majority of earthquakes are confined to depth up to 100-150 km and seismic activity maximum is at depths of approximately 30-40 km [Tarakanov, 1978]. At depths greater than 100-150 km seismic activity abruptly decreases, and a sharp fracture of focal surface is noted at depths of 200-300 km. Sakhalin seismicity is associated with sub-meridian deep faults [Rodnikov et al., 1996] bounding the Okhotsk Sea lithospheric plate and separating it from the Eurasian plate. Plate movements relative to each other as well as spreading processes going on in the rift structure of Tatar Strait cause active seismicity. Earthquakes distribution along the geotraverse is shown in Figure 7.

Volcanism.
[25]  The Kuril Island Arc is an area of intense recent volcanism manifestations. Different authors distinguish from 68 to 160 surface volcanoes [Aprodov, 1982; Fedorchenko et al., 1989; Gorshkov, 1967; Pushcharovskiy, 1992a]. Twenty-nine of them erupted in historic time and six are in solfatarie stage [Fedorchenko et al., 1989; Simkin and Siebert, 1994]. From different assessments, the number of submarine volcanoes varies from 96 to 104 [Bondarenko and Rashidov, 2004; Gorshkov, 1967; Pushcharovskiy, 1992a; Rashidov and Bondarenko, 2003; Zatonskii et al., 1961].

[26]  In the opinion of V. A. Rashidov [Rashidov and Bondarenko, 2004], reliable data on submarine volcanic activity manifestations in this region are lacking although data of various catalogs are available [Gushchenko, 1979; Simkin and Siebert, 1994; and others].

[27]  Both surface and submarine volcanoes form volcanic chains oriented at different angles to the general strike of the Kuril Island Arc. Surface and submarine volcanoes of the Kuril Island Arc are composed of rocks from basalt to rhyolite composition. Normal and sub-alkali rocks are separated ranging from low-potassium to high-potassium series. Low titanium and magnesia contents and high aluminum hydrate content are characteristic of the Kuril Island Arc lavas [Fedorchenko et al., 1989; Pushcharovskiy, 1992a].

2007ES000224-fig08
Figure 8
[28]  Forty submarine volcanoes and mountains of different age are located immediately in the geotraverse area [Bondarenko and Rashidov, 2004; Pushcharovskiy, 1992a]; about 40 large surface volcanic constructions and more than 160 minor volcanic cones are situated in Iturup Island [Luchitskii, 1974], across which the deep section runs. Nineteen surface volcanoes are the Quaternary [Fedorchenko et al., 1989] and in nine of them historic eruptions were noted beginning since 1778 [Gushchenko, 1979; Simkin and Siebert, 1994]. Among submarine volcanoes, we find both table mounts and point mounts. The former seem to be encompassing Iturup Island and the latter are at a distance from the island. Depths above table mounts increase with distance from the island, which may testify to subsidence of the island arc slope on the Okhotsk Sea side towards Kuril deep basin and may be related to its formation [Pushcharovskiy, 1992a]. The structure of three volcanoes is discussed in the paper (Figure 8).

2007ES000224-fig09
Figure 9
[29]  Krylatka submarine volcano is located at 17 km to north-northwest of Przhevalskiy Cape of Iturup Island. It is noted that at the end of the last century this volcano was at the stage of gashydrothermal activity [Rashidov and Bondarenko, 2004]. It is a cone-shaped construction of sub-meridian extension with a flat top at a depth of 300 m. The dimensions of the top are 4.3 times 6.5 km and it is somewhat extended in sub-meridian direction (Figure 9). The volcanic structure is separated from Iturup Island with a neck of a depth of approximately 900 m. The steepness of slopes increases from 15o in the lower part to 20-25o in the upper part [Pushcharovskiy, 1992a; Rashidov and Bondarenko, 2004]. The basement of the structure in the north and west is covered with 700-meter thick sedimentary deposits of sub-horizontal bedding. In records of echo-sounder measurements in the center of Krylatka submarine volcano flat top, acoustic anomalies are noted in the water mass in the depth range of 210-250 m, which may be caused by gas-hydro-thermal activity. From dredging the volcano structure, basalts, andesite-basalt, andesite, dacite-andesite and rhyolite were revealed [Erokhov et al., 1975; Ostapenko et al., 1986; Pushcharovskiy, 1992a]. Krylatka submarine volcano is located in the area of negative anomaly magnetic field (DT)a.

[30]  The base of the volcano is outlined by negative isolines of the anomalous magnetic field of intensity 100-130 nT. To the west, southwest and south of the near-top part of the structure positive local anomaly is located, which amounts to 96 nT, and to the east of the near-top part of the structure, local positive anomalies of intensity of 230 nT and 150 nT are noted. Above the near-top part of the structure a change is observed from smooth to high-frequency alternating anomalous field. High-frequency anomalous magnetic field may be caused by young lava flows [Babayants et al., 2005; Rashidov and Bondarenko, 2004]. Rocks composing the submarine volcano structure are magnetized in the direction of modern magnetic field. Effective magnetization of rocks corresponds to rocks of andesite-basalt series.

2007ES000224-fig10
Figure 10
[31]  Atsonupuri volcano (Figure 10) is situated in the southern Iturup to which it is linked with an isthmus of height of 30 m [Gorshkov, 1967]. It is a stratovolcano with the central cone in caldera. The base diameter at sea-level is about 6 km. Absolute height is 1205 m. The crater dimensions are 400 times 600 m and the depth is approximately 150 m [Aprodov, 1982; Gorshkov, 1967; Gushchenko, 1979]. In the eastern part of the cone a small area of flat atrio and remnants of soma crest are saved. The size of soma is approximately 2 km. The height of soma crest is 900 m. The elevation of the central cone above the soma is approximately 300 m [Aprodov, 1982; Gorshkov, 1967; Gushchenko, 1979].

[32]  Berutarube volcano is located at the southern end of Iturup Island. It is a shield strato-volcano of a diameter of approximately 10-11 km superimposed on Neogene rocks. The absolute height of the volcano is 1222 m [Gorshkov, 1967]. On the top, a destroyed crater of a diameter of 2.5-3 km is located with truncated Holocene cone of the base diameter of about 1 km in it. Two lava flows are noted that erupted from the cone, and in its base in craters, solfataras are found [Aprodov, 1982; Gushchenko, 1979]. The volcano is composed of andesite-basalt [Fedorchenko et al., 1989].


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.

Copyright 2008 by the Russian Journal of Earth Sciences

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