RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4002, doi:10.2205/2007ES000224, 2008
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Figure 11 |
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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.
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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.
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Figure 14 |
[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].
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Figure 15 |
[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].
[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].
[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].
[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].
[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].
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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.
Citation: 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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