Location of the Okhotsk Sea Geotraverse

Geophysical Center of the Russian Academy of Sciences, Moscow, Russia
World Data Center for Solid Earth Physics
The Geotraverse Project

The Okhotsk Sea Geotraverse


Explanatory Note

Introduction   |   Geology   |   The Crust   |   Geophysical Investigations
The Upper Mantle   |   Geodynamic Investigations   |   Conclusions   |   Bibliography



Introduction

The deep section profile crosses the Mesozoic structures of Sikhote Alin, the rift structures of the Tatar Strait, the Cenozoic formations of Sakhalin, the Kuril Basin in the Sea of Okhotsk, the volcanic structures of the Kuril Island Arc, the Kuril Trench, and the Mesozoic plate of the North - Western Basin of the Pacific Ocean (   Rodnikov et al.,   1994  ;   Rodnikov and Stroev,   1994   ).
Location of the Okhotsk Sea Geotraverse
Location of the Okhotsk Sea Geotraverse
The length of the profile is 2 000 km. The data obtained within the band 200 km wide are projected onto the section.

Geology

Tatar Strait   |   Sakhalin   |   Sea of Okhotsk
Kuril Island Arc   |   North West Pacific Basin

Tatar Strait

The Tatar Strait is a large trough - graben composed of a thick layer (up to 8 - 10 km) of Mesozoic - Cenozoic sedimentary formations (   Tronov et al.,   1987  ;   Khvedchuk,   1992  ;   Varnavsky,   1994   ). It is situated between the Mesozoic structures of Sikhote Alin and the Western Sakhalin Mountains, from which it is divided by deep faults. According to the available geological – geophysical data, the sediments filling the trough form four structural complexes divided by regional stratigraphic un conformities and differing by the structural and physical characteristics; i. c. Upper Cretaceous, Paleogene, Oligocene - Lower Miocene and middle Miocene – Quaternary

Geological - Geophysical Section of the Tatar Strait Trough (   Structure ...,   1996   )
Geological - geophysical section of the Tatar Strait Trough
The basement of the trough consists of Triassic - Lower Cretaceous volcanogenic - siliceous sediments with seismic velocities reaching 5.8 – 6.2 km/s (   Gnibidenko et al.,   1992   ).
The Upper Cretaceous structural complex forms the lower part of the sedimentary layer of the trough and is composed of dense sedimentary and volcanogenic - sedimentary rocks 2 – 4 km thick. The complex was studied by combined interpretation of data obtained by the seismic soundings and using the materials of MTS and gravimetry. The composition of the rocks was determined by studying outcrops of Upper Cretaceous deposits in the Western Sakhalin Mts that form the eastern edge of the Tatar Strait trough (   Tronov et al.,   1987   ).

The Paleogene structural complex is composed of mainly continental sedimentary and volcanogenic - sedimentary formations up to 1.5 km thick. There sediments usually fill the narrow (up to 5 - 10 km) grabens stretching along the eastern and western margins of the trough. There are indications that Paleogene complex contains hydrocarbon accumulations on the areas covered by the seal of the Oligocene - Lower Miocene clayey deposits (   Tronov et al.,   1987   ).
The Oligocene - Lower Miocene complex is formed by a thick (0.5 – 4.0 km) sedimentary layer dissected by faults with displacements reaching 100 - 800 m over the series of horsts and grabens. The lower part of the layer is composed of deepwater mostly clayey and cherty deposits 0.1 - 2.0 km thick and fills individual isolated grabens. The upper is composed of sandy and silt - sandy subcontinental and coastal - marine deposits 0.1 - 1.5 km thick and completely covers the formations of the lower part of the layer and the Upper Cretaceous and Paleogene formations.
The Middle Miocene - Quaternary complex is composed of folded sedimentary rocks almost horizontal in their upper part. The thickness of the complex varies from 0.1 to 5.1 km. The lower part contains mainly clayey and cherty - clayey deposits 0.2 – 2 km thick thinning out to the west. The middle part of the complex has sandy - clayey and clayey - sandy rocks that also thin out to the west from 1.7 to 0.1 – 0.3 km. The upper part of the complex is composed of mainly sandy and clayey - sandy sediments 0.5 – 1.9 km thick. This complex and the Oligocene - Lower Miocene complex are the main regional oil - gas regions of the Tatar Strait. In 1986, a gas field was discovered in the Upper Miocene rocks of the upper complex (   Tronov et al.,   1987   ).
By DSS data Tatar Strait Trough is a rift 50 km wide and 4 km deep (   Structure ...,   1996  ;   Piip,   1997   ). Moho discontinuity is located at a depth of approximately 30 km. Rift formation is related to the upwelling of the asthenosphere (   Rodnikov,   1997   ). The rift is located on the northern continuation of spreading center revealed in the deep - sea basin of the Japan Sea (   Isezaki et al.,   1976   ).

Sakhalin

The polygon of the Sakhalin geotraverse starts in the area of the Isthmus of Vetrovoi and in the north crosses the East Sakhalin Mts and in the south the Susunai Ridge. The structure of the island includes metamorphic, magmatic and sedimentary rocks ranging from the Paleozoic to Recent. The oldest rocks were observed in the East Sakhalin Mts. and on the Susunai Ridge. They are composed of serpentinized ultrabasites and metamorphic shales with jasper, quartzite and marble interbeds and change over to quartzites, green shales, metaspilites and metadiabases in the upper part of the section (   Rihter,   1986  ).

The formations are characterized by intensive plication dislocations of layers and an extensive development of dislocations with a break in continuity. The exact age of metamorphic formations is not fixed and is considered by many authors as middle - upper Paleozoic. Above them lies the Jurassic - lower Cretaceous complex of siltstones, claystones, cherty shales, greywackes, sandstones, jaspers, and tuffs of mainly basic composition. Along the faults, the complex contacts with more ancient deposits. The granitoid intrusions appeared much later, in the Paleogene, as stocks and dykes of biotite granites, plagiogranites, granodiorites, granite - porphyries, and granodiorite - porphyries. The K/Ar age determination reaches 58 – 66 Ma (   "Geologiya SSSR",   1970   ). The ultrabasic rocks of the Susunai Ridge compose dykes, layer bodies and small stocks of serpentinites pyroxenites, and hornblendites (   Bikenina et al.,   1987   ).

The next structural complex is formed by the Upper Cretaceous deposits of siltstones and mudstones with sandstone and conglomerate interbeds, jaspers, radiolarites, lavas, and tuffs of various composition. They have the pre - Cretaceous hiatus in sedimentation, a washout, and the unconformities with underlying rocks. The upper structural complex is Cenozoic deposits; its lower boundary in the western regions of the island is fixed between the Paleogene and Cretaceous rocks, but it is not clearly traced everywhere. In the eastern part of the island, the Paleogene deposits are the most numerous, and the Neogene formations contain an unconformity located on the Paleozoic in the area of the Susunai Ridge and the Poronai Depression. The complex is composed of marine shallow - water deposits crumpled into gently sloping meridional folds. Of particular importance in the structure of the island and the adjoining equators are the fault systems of various deposition that caused the fold - block structure of the island.
In the Cenozoic, the magmatic activity caused the formation of two rock complexes associated with deep faults that appeared, obviously, in as early as the Mesozoic on the function of tectonic zones of various types. The Miocene complex has dykes, sills, extrusive cupolas of andesites, andesite - basalts, basalts, dolerites, less often andesite - dacites. They are located both in the West Sakhalin Mts and in the eastern part of the island. The early Pliocene essexite - monzonite complex is represented by platy bodies, loccoliths, dykes and veins of dolerites, monzonites, alcaline syenites, seldom by teschenites and crinanites. They are distributed along the coast of the Tatar Strait and confined to the West Sakhalin Deep Fault.
Seismogeological Model of the Eastern Zone
of the Central and Southern Sakhalin (   Bikenina et al.,   1987   )
Seismogeological model
This model demonstrates the generalized stratigraphic scheme of Sakhalin (a) and the seismic characteristic of rocks (b).
The systems of deep faults formed the fold - block structure of the island. The West Sakhalin, Central Sakhalin, East Poronaiskii and East Sakhalin Deep Faults are the most important of there systems (   Tyutrin et al.,   1986   ).

The West Sakhalin Fault stretches along the western coast of Sakhalin as a zone of normal faults and strike - slip faults with eruption of the Miocene plateau - basalts, dolerites, and dacites, the Pliocene intrusions of basic and alcalic composition. The Fault morphologically separates the West Sakhalin Mts uplift from the Tatar Strait. The formation of the Fault can be fined at Late Cretaceous and its activity in the Miocene and Pliocene can be traced by magmatic events and the distribution of Cenozoic sediments.

The Central Sakhalin fault runs along the eastern slopes of the West Sakhalin Mts and is marked by a gravity gradient. The fault also has complicated plication dislocations of the normal and strike - slip type, and magmatic and mud - volcanic sites. The plane of the fault is dipped to the west at 60 - 80 o and the amplitude of vertical displacement is 1.5 – 2 km. The fault is considered as a large structural suture dividing the structural - facial zones with the Upper Cretaceous and Cenozoic sediments of various thickness’ (   Tyutrin et al.,   1986   ). The Late Cretaceous is the earliest time of formation of the structural suture. The studies of the modern movements on the fault were conducted with the application of geodesy techniques in 1975 - 1983 (   Vasilenko and Bogdanova,   1986   ). The research demonstrated that in 1975 - 1978 on the fault a dextral strike - slip fault was noted that was replaced with a sharp sublatitudinal compression of the fault zone in 1979. In the following time period of 1979 - 1980 an extension was observed in the fault zone. From 1980 to 1983 deformations attenuation was noted; their intensity dramatically decreased as compared to 1977 - 1980 (   Vasilenko and Bogdanova,   1986   ).

The East Poronaiskii fault lies along the western slopes of the East Sakhalin Mts. The Neogene effusives situated along the Fault. Further south, it is traced across Terpeniya Bay to Sakhalin Bay, and with the Central Sakhalin fault composes a system of meridional grabens filled with sediments.
The larger part of the East Sakhalin Fault is located on the shallows of the Sea of Sakhalin and stretches further south to the Kuril Basin of the Sea of Okhotsk. The location of the fault is established by seismic data and linear intensive magnetic anomalies.

The Sea of Okhotsk

Terpeniya Bay   |   Kuril Basin

In the Sea of Okhotsk, the geotraverse crosses Terpeniya Bay and the Kuril Basin.

Terpeniya Bay .   The bottom of Terpeniya Bay is a flat shelf plain slightly dipping to northeast with a submeridional gently sloping uplift 25 – 30 m high in its central part. By seismic data the sedimentary layer consist of two strata (   Soloviev et al.,   1967   ).
The upper one has seismic velocities ranging from 1.8 to 3.0 km/s and is composed of loose Miocene - Quaternary deposits. The lower stratum with velocity 3.5 – 4.3 km/s is composed the Upper Cretaceous sedimentary rocks (   Bikenina et al.,   1987   ).
In the central part of Terpeniya Bay, the seismic was revealed a large buried Poronaiskoe uplift stretching north - northwest. In the north of the Terpeniya Bay is located Poronaiskaya lowland, which is covered by Quaternary deposits. The Poronaiskoe uplift divides the bay into two troughs: the western Makarovskii trough 2500 – 3000 m deep and the eastern Vladimirskii trough with a basement 2000 m deep. A Poronaiskoe uplift is traced from Terpeniya Bay to the Kuril Basin, where was dredged at the depth 550 – 850 m altered effusives similar to the Mesozoic effusives of Sakhalin (   Vasiliev,   1988   ). Basalts, andesites, their tuffs, tuffolavas, and lava breccias were recovered. By chemical composition the rocks correspond to highly aluminiferous basalts. The Cretaceous age of the basalts was determined by K/Ar method ( 108 Ma ). Among acid rocks are quartz porphyries, dacites, tuffolavas; their radiometric age is 136 Ma.

The Kuril Basin .   The Kuril Basin is a backarc depression. It is contoured by the isobath 3000 m; the average depths in the basin are 3200 m. More than 4000 m thick sediments overlie the acoustics basement ( by seismic data ) probably of volcanogenic - sedimentary rocks, below which lies the third layer of the oceanic crust with Vp = 6.4 – 6.8 km/s and 5 km thickness.
Seismic Profile Across the Kuril Basin of the Sea of Okhotsk
(   Structure...,   1996   )
Seismic profile across the Kuril Basin

The basin has a high heat flow (   Smirnov and Sugrobov,   1982   ). The acoustic basement is dissected by faults. According to seismic soundings (   Livshits et al.,   1972  ;   Snegovskoi,   1974   ) the sedimentary layer has two complexes. The upper complex, probably of the Pliocene - Quaternary age, is up to 800 – 1000 m thick with fine stratification. The layers of the lower complex in the central part of the basin are more than 3000 m there and are acoustically transparent. Rock dredged from the bottom of the basin produced data on its geologic structure (   Zhuravlev,   1984  ;   Vasiliev,   1988  ;   Savostin et al.,   1978  ;   Krasny,   1990   ).
Along the western slope of the basin were dredged Neogene rocks with Middle Miocene sandstones and claystones and the Upper Miocene - lower Pliocene siltstones, diatomites, tuffs, taffograverites overlain by the Quaternary sands and silts with gravels and pebbles. In the central part of the basin, the lower sedimentary beds are probably composed of conglomerates of a submarine uplift 110 km northeast of Iturup Island (   Savostin et al.,   1978   ). The conglomerates contain pebbles of eftusives, green schists, granitoids, and other rocks not discovered on the Kuril islands, which probably build up the volcanogenic - sedimentary layer of the basin. As a result of dredged, lumps and fragments of granodiorites of 94 - 219 Ma were recovered from the southeast slope of the basin (   Krasny,   1990   ).
At the base of the western slope of the basin, fragments of gray sandstones typical of the Sakhalin Cretaceous rocks were dredged (   Structure ...,   1981   ). The sedimentary layer is composed of Miocene clays with interbeds of volcanic ashes. Other areas of the western edge of the basin have conglomerates, tuffodiatomites, sandstones, and siltstones.
  I.I. Khvedchuk,   (1992) divides the sediments of the basin into two layers. The upper layer (Miocene - Quaternary) is composed of turbidities and volcanogenic sediments (ashes). The lower layer is composed of pelagic clays and claystones with interbeds of volcanic material. The age of the rocks is Cretaceous - Paleogene.
The acoustic basement are, apparently, composed of basic volcanic rocks (basalts or their tuffs) alternating with volcanic - sedimentary and cherty formations, the fragments of which were dredged from the sides of the basin. The submarine volcanic structures identified by   I.K.Tuezov,   (1975) and the high heat flow indicate active volcanic processes that took place on the sea bottom.
It seems very likely, that the formation of the Kuril Basin is associated, as of all back - arc basins, with the formation of rifts usually reflected on seismic profiles (   Piip,   1997   ) and with the presence of basalts at the base of the Kuril Basin. The linear magnetic anomalies and high heat flow, confined to the axial zone of the Kuril Basin. were used to substantiate the presence of the axial spreading zone in the central part of the basin (   Smirnov and Sugrobov,   1982  ;   Bogdanov,   1988  ;   Rodnikov,   1997  ;   Baranov et al.,   1999   ).

The Kuril Island Arc

Great Kuril Island Arc   |   Inter-Arc Trough   |   Lesser Island Arc   |   Kuril-Kamchatka Trench

The geotraverse runs within the Great Island Arc in the region of the Iturup Island and crosses the Lesser Island Arc and the Kuril Trench. The description of the geological structure is represented according to the following publications (   Rodnikov,   1968, 1973, 1979  ;   Belyaevsky and Rodnikov,   1972  ;   Sergeev,   1976  ;   Geology - Geophysical ...,   1987  ;   Vasiliev,   1988  ;   Zlobin,   1989  ;   Piskunov,   1987  ;   Frolova et al.,   1989  ;   Krasny,   1990  ;   Pushcharovsky and Melankholina,   1992  ;   Avdeiko et al.,   1992   ). Within the Kuril Island Arc there are the Great Island Arc, the Inter - Arc Trough, the Lesser Island Arc and the Kuril Trench. The geological structure of the enumerated structures is shown below.

Correlation Scheme of Sections of the Major Tectonic Structures of the Kuril Island Arc
(   Vasiliev,   1988   )
Correlation scheme of sections of the major tectonic structures of the Kuril Island Arc

The Great Kuril Island Arc .   The islands of the arc are composed of Cenozoic volcanogenic and volcanogenic - sedimentary formations pierced by small intrusive bodies of gabbroids, diorites, and granodiorites (   Vasiliev,   1988   ). The evidence of xenoliths in lavas testify that the basement of the arc is built up by metamorphic rocks: crystalline shales, quartzites, hornblendites, gabbroids, diorites, and plagiogranites of an older age (   Fedorchenko and Rodionova,   1975   ). The Cenozoic formations are subdivided into four lithological - stratigraphic formation. The lower formation of "green tuffs" is represented by propilitised lavas and tuffs of basalt and andesite composition, liparites, dacites, acid tuffs, tuff - sandstones, tuff - aleurolites, agglomerate breccias, argillites.Their age is determined by analogy with the "green tuffs" formation of Japan dated the Lower Miocene.
The next formation is volcanogenic - flyschoid complex and is deposited with angular unconformity on the underlying rocks. It is composed of volcanogenic breccias and conglomerates with sandstone, tuff and aleurolite interbeds associated with diatomites and ash tuffs. These rocks are covered by volcanic breccias, tuff - sandstones and tuffs of the basic composition and by horizons of basalt and andesite - basalt lavas with diatomite and aleurolite interbeds. The age of these rocks is Lower - Middle Miocene.
The next is the volcanogenic - sedimentary formation of the Upper Miocene - Lower Pliocene age; it is composed of tuffs of andesites, dacites and rhyolites and of volcanic sandstones, gravelites and small - detritus breccias, diatomites, and aleurolites.

The last in the Neogene deposits is the volcanogenic complex composed by tuff and lava breccias of basalts, andesite basalts with the lava covers of the same composition, among which upsection appear the interbeds and lenses of tuffogenic gravelites, sandstones, aleurolites, and basic tuffs.
The Quaternary formations are represented by thick volcanogenic and volcanogenic - sedimentary beds of mainly mean composition: breccias, tuffs, tuffogenic gravelites and sandstones, andesites. Apart from the volcanogenic formations there are marine and alluvial, mainly sand - gravel deposits that compose terraces.
On the Great Kuril islands, there are 105 land volcanoes, 42 of which are active, and 81 submarine volcanoes (   Avdeiko,   1994   ). The volcanoes stretch in chains, which are usually oriented towards the strike of the arc and which reflect, apparently, the ruptures in the Earth's crust. Two zones of volcanism are distinguished: the frontal and the rear. The land volcanoes dominate in the frontal zone, whereas the submarine ones in the rear zone. Between them lies the zone of weaker volcanism. Three series of volcanic rocks are distinguished ranging from the lime - alkaline (dominant) to the island - arc tholeiitic and island - arc alkaline.

The Sea of Okhotsk slope of the Great Kuril Arc has submarine volcanoes, apparently confined to faults. They are built up by the Quaternary basalt, andesite - basalt and andesite lavas, interlayered with sedimentary deposits. The fragments and blocks of andesites and andesite - basalts, recovered by dredged, show impregnation by sulfide minerals: pyrite, marcasite, pyrrotine, less frequently by chalcopyrite, which associates with bornite, digenite and covellite (   Kononov,   1989   ).

The Inter - Arc Trough .   The Inter - Arc Trough is located between the outer and the inner island arcs. The subsidence is 45 - 60 km wide. It is built up, apparently, by the Neogene and Quaternary tuffogenic - sedimentary formations. The thickness of the sediments in the axial zone exceeds 3 km, but the base of the sediments could not be traced by seismic studies (   Snegovskoi,   1974   ). According to the seismic sounding two beds are distinguished in the section of the sedimentary cover. The upper one is from 70 to 80 m thick. The reflecting boundaries in it are practically horizontal. The lower bed is composed of layered deposits. The structure of the Inter - Arc Trough is shown below.
Seismic Section Across the Minor Kuril Arc (   Snegovskoi,   1974   )
Seismic section across the Minor Kuril Arc

The upper part of the lower bed in the Trough is composed of tuffogenic aleurolites, tuffs, tuffites and tuff - diatomites. Basalts locally occur among the tuffogenic - sedimentary rocks (   Vasiliev,   1988   ). The upper layer is built up of gray and greenish - gray aleurites with Quaternary pebbles and gravel (   Krasny,   1990   ).

The Lesser Island Arc .   The Geotraverse runs within the Lesser Island Arc the submarine continuation of the Shikotan Island, Zeleny Island and other small islands composed mainly by the Upper Cretaceous formations. The rocks of the basement are studied in xeholith brought up by basalts. They are mainly amphibolites, to a lesser amount hornblende gabbro and serpentinites. On the oceanic side of the Shikotan I., the rocks of the basement complex, according to (   Pushcharovsky and Melankholina,   1992   ), are represented by striated gabbro and gabbro - norites with small amounts of serpentinised peridotites, whereas in the upper part of the complex there are also gabbroids. They form allochthone plates, the upper parts of which contain complexes of parallel dykes of several generations. It is presumed that in the Maastrichtian these tectonic covers moved to the north and dipped to south - east (   Pushcharovsky and Melankholina,   1992   ).
The geological section of Shikotan I. represented by Campamian alternating basalt pillow - lavas and coarse pyroclasts, which change upsection to tuffs and lava - breccias. Further lie the mainly sandy - aleurolite bed with the olistoliths of gabbroids, and then clastic volcanites of the Maastrichtian - Paleocene. The total thickness of this section is about 2 000 m. The rocks form a monocline dipping towards the through.
As a result of seismic research of recent years, the considerable thickness of the crust of the Kuril Island arc was established at 25 - 40 km (   Zlobin and Zlobina,   1991   ).

The Kuril - Kamchatka Trench .   The Kuril - Kamchatka Trench in the region of the geotraverse is contoured by the 7 000 m isobath and has a characteristic asymmetric transversal profile. The island slope is steeper ( 7° - 10°, locally up to 15° ) than the oceanic ( 5° - 7°, in the upper part 3° - 5° ) (   Vasiliev,   1988   ). From the oceanic side of the trench, along the very edge of the Pacific Ocean, stretches a slightly inclined uplift, the Zenkevich Rise, which rises 200 - 400 m above the ocean bed and is up to 300 km wide. A large area of the trough is covered with the sedimentary layer of the first oceanic bed, which is often covered by thin (tens of m) turbidite formations (   Tectonics...,   1983   ).
The oceanic slope is broken up by numerous faults , most of which cut only the second layer, but some of them also cut the first layer and are expressed in the relief as scarps 50 - 200 m high. The planes of the faults are usually inclined to the axis of the trench under the angle of 30° - 60°. The distance between the faults is 1 - 5 km (   Vasiliev,   1988   ). The axial part of the trench is characterized by low heat flow values. Most of the seismic profiles (   Vasiliev,   1988  ;   Structure of the floor...,   1984  ;   Tectonics...,   1983   ) in the axial part of the trough indicate the sinking of the top of the second seismic layer of the oceanic slope under the island slope to the traced distance up to 12 km with the angle of inclination 5 - 7 degrees.

The North West Pacific Basin

The geotraverse runs within Zenkevich Rise and a vast plain stretching eastward to Shatsky Rise with average depths of 5000 - 5500 m. Zenkevitch Rise is a marginal oceanic uplift separated from an isobath of 5500 m which is 300 - 350 m wide and is 200 - 400 m above the ocean floor (   Structure of the floor...,   1984   ). The thickness of sediments is insignificant (300 - 350 m) and the oceanic crust second layer rocks composed of tholeiite basalts (   Tectonics...,   1983   ). When dredged, besides basalts fragments of tuffs, aleurolite, graywacke, argillite, siliceous rocks, metaschists, andesites, diabases, porphyry, granodiorite, granites, and aplites were lifted (   Vasiliev,   1988   ). The age of the dredged basalts from the outcrops of the acoustic basement is in the range of 80.1 - 32.6 million years ( from the Late Cretaceous to the Oligocene inclusive ), and the age of granodiorites is 103 million years (The Early Cretaceous).

The North West Pacific Basin having the oldest crust from geological and geophysical data (approximately 150 million years) is covered with a continuous sedimentary layer of an average thickness of 300 - 400 m. It is composed from boreholes 303 and 580 data (   Larson et al.,   1975   ) of diatomite and radiolarite ooze and laminated clays enriched with Late Miocene - Quaternary ashes overlying zeolite clays, clay muds and siliceous rocks. At a depth of 211 m these sediments are underlaid with Late Cretaceous pelagic zeolite clays having in the lower section interlayers of siliceous slates and plankton limestones. At a depth of 284.75 m sediments are overlaid by pillow lavas of basalts. Several layers of pillow lavas are separated in the borehole, composed of fine - grain aphyric basalts with enclosure and veins of calcite, celadonite, montmorillonite and chalcedony.

The structure of basalts varies from intersertal to doleritic. Plagioclase is mostly presented by labradorite. The basalts by their mineralogical composition and analysis data are defined as tholeiites. The borehole is made in the area where M-4 magnetic anomaly is observed. The age of the basalts is unknown, but since they underlie sedimentary rocks the lower section of which is dated Early Cretaceous it is assumed that the basement age is pre-Cretaceous (   Tectonics...,   1983   ).
The basement is intensely broken by faults which are traced in the cover. The horst - graben structure of the basement is locally caused by the fault tectonics (   Tectonics...,   1983   ) .
The relation between the seismic section data and the data of borehole 303 is shown below.
Correlation of Seismic Reflection Profiles with Drilling Results at Site 303
(   Larson et al.,   1975  )
Correlation of seismic reflection profiles with drilling results at Site 303
The upper stratified layer corresponds to unconsolidated sediments of the Late Miocene - Quaternary. The acoustic basement correlates with the opaque (nontransparent) layer composed of Cretaceous pelagic clays with a considerable amount of siliceous interlayers, whereas the top of the basalt basement is difficult to distinguish (   Structure of the floor...,   1984   ).

The earth crust of the transition zone is noted for its very different thickness varying from 10 to 40 km, its complicated relief of M boundary with seismic velocities ranging from 7.8 to 8.1 km/s (   Deep ...,   1987   ). Large structures of the M boundary relief mainly coincide in plan with the major geostructures. The uplifts correspond to large troughs in M relief. Negative structures are correlated with the uplifts of M boundary (   Semenova et al.,   1986   ).

The Crust

Institute of Earth Physics and Sakhalin Integrated Institute carried out the seismic works along some profiles at the region of the geotraverse in 1963 - 1964 years. The seismic measurements along four profiles were made in the period of International Geophysical Year in 1958 - 1959 by Institute of Physics of Earth of Academy of Sciences of USSR. The seismic works were made at most high level for that time. Over 20 profiles were investigated in that period.
Map of Location of Deep Seismic Sounding Profiles at the Geotraverse Region
Map of location of Deep Seismic Sounding profiles at the geotraverse region

Interpretation of the data fulfilled in that time had served by basis for studying of geological structure of the region (   Structure ...,   1964   ). Traveltime curves of first arrivals along the profiles were published (   Structure ...,   1964  ;   Deep ...   1971   ).
Later structure of the crust at the region of the geotraverse was received by V. B. Piip (the detailed description of interpretation method and results are here as the supplement) from reinterpretation of data of 11 Deep Seismic Sounding (DSS) profiles that are disposed at the area of geotraverse and neighbouring areas. Previous published traveltime curves now have been digitised and reinterpreted using new 2-D computer technique. Detailed cross-sections have been obtained down to 30 - 60 km depth. The new seismic cross-sections correspond to old ones in depth of major boundaries and velocity values in average only. Blocks of subducted oceanic plate and relic subduction zone are distinguished in the seismic cross-sections and in the velocity maps of the region.
Seismic Cross-section of the Crust along the Geotraverse (   Piip V.B.   )
Seismic cross-section of the Crust along the geotraverse

From the East to the West we can trace following structure in the geotraverse. In the region of Kuril Trench thick accretion prism is present. Its sizes reach 200 x 20 km2. Subducting third layer of oceanic plate is broken by numerous faults and its blocks relaxed from 15 km to 30 km. Moho is disposed from 20 to 30 km here. Hang wing of subduction zone in its frontal part forms a several normal faults and in back part of it there are a series of reverse faults. Depth of Moho is 25 km here. At the 149 o of longitude a sub - vertical fault exists that divided the hang wing of subduction zone from the oceanic crust of Kuril Basin. Moho is founded at the depth of 15 - 20 km here and the crust is divided into three layers. In east part of Kuril Basin a mantle plume is present. Anomalous low velocity mantle (v = 7 - 7.5 km/s) reaches 300 km here. Possibly there are partially melting rocks here that served by source of lava for volcanoes of Kuril Island Arc. Above this anomalous mantle in the crust a rift zone exists. Faults foming of it penetrate from sedimentary layers to upper mantle. The rift zone is disposed in centre of large lifted block of crust that is founded above mantle plum. At west edge of Kuril basin the zone subduction is present in that oceanic crust of Kuril Basin was embedded under continental crust of Sakhalin Island. In Tatar Strait the lower crust forms steep steps relaxing toward Sikhote Alin shores. Possibly a subduction zone existed here in past geological time. Basement is disposed in the region of the geotraverse at the depth about 10 km in average.

Geophysical Investigations

Geothermal studies of deep structure   |   Gravity Field
Magnetic Field   |   Electromagnetic studies of the deep structure

Geothermal studies of deep structure

Heat Flow   |   Deep Temperatures

Heat flow .   The distribution of heat flow values along the geotraverse is presented by the data selected from the world heat flow data catalogue (   Pollack et al.,   1991   ) and the heat flow maps (   Smirnov,   1986  ;   Tuezov,   1975   ). The heat flow density greatly varies across the strike of the main structures of the geotraverse. It is high within the deep basins and troughs of the marginal seas and rather low in the continental structures of the Far East and the Kuril part of the Pacific Ocean
Distribution of Heat Flow Values (in mW/m 2 ) along the Geotraverse
Distribution of heat flow values along the geotraverse

The heat flow variations within the Sikhote Alin reach only 39 - 56 mW/m 2. Near the Kuril part of the Pacific Ocean the average heat flow values are 52 mW/m 2.

The lowest values of 22 mW/m 2 are recorded in the Kuril - Kamchatka Trench. The average heat flow value for the Kuril island arc is 118 mW/m 2, and the highest values were recorded in the western part of the island arc reaching up to 790 mW/m 2. The mean heat flow values within the Sakhalin area are 76 mW/m 2. A high heat flow was recorded in the Tatar Strait, where it reached within the geotraverse 123 - 132 mW/m 2. A high heat flow was also observed in the Kuril Basin of the Okhotsk Sea, where it reached 346 - 354 mW/m 2 ( Structure ...,   1996 ).

Deep Temperatures .  The calculations of deep temperatures along the geotraverse are shown in the publications Structure ...,   1996;   and by   Smirnov and Sugrobov,   1982 .

Deep temperatures along the geotraverse



Deep Temperatures along the Geotraverse
( Structure ...,   1996; )


The deep temperatures on the Moho boundary vary from 100o C in the Pacific Ocean to 1000 o C below the Tatar Strait Trough. Under the Kuril basin with a thin crust they reach 800 o C. The depth to the roof of the partial melting area, identified as the asthenospheric layer of the upper mantle (isotherm 1100 o C, according to Ya.B. Smirnov, and 1200 o C, according to I.K. Tuezov and V.D. Epaneshnikov ( Structure ...,   1996 )) along the geotraverse varies from 25 km to 100 km. The area of partial melting forms two asthenospheric diapirs, i. e., under the Tatar Strait and the Kuril Basin, thus causing an active tectonic regime manifested in the volcanic and seismic activity. Moreover, above the asthenospheric diapir in the Tatar Strait the hydrocarbon deposits were recorded in the sedimentary layer, and in the Kuril basin a sulfide mineralization was established on the tops of the submarine volcanoes ( Kononov,   1989 ). The volcanoes are composed of the Pliocene - Quaternary basalts, andesites, and dacites. Sulfide mineralization coincides with the summits of submarine volcanoes. The prevailing mineral is pyrite. Also marcasite, pyrhotite, and chalcopyrite are encountered. Thus, the maximal temperatures and the minimal thickness of the lithosphere are typical of deep water basins of the Sea of Okhotsk. In the axial parts of these structures the asthenospheric layer rises up to 25 km and probably even to 10 km (under the interarc trough); on the flanks it sinks to the depths of 40 - 50 km, whereas under the Pacific Ocean it reaches the depth of 100 km.

Scheme of Depths of Deposition of the Asthenosphere under the Sea of Okhotsk
( Structure...,   1996   )
Scheme of depths of deposition of the asthenosphere under the Sea of Okhotsk

Gravity Field

The main features of the deep structure of the Okhotsk Geotraverse area can be seen also from the field of the geoid heights. To make the regional features (smaller spatial size) of the deep density structure of the region more easy visible we construct the field of the gradients of the geoid heights. Lower degrees up to 15 power spherical harmonics are removed to make the regional structure more easy visible.

Maps of the geoid heights and its gradients

Map of the geoid heights for the area of the Okhotsk Sea Geotraverse was obtained from the EGM-96 International Geoid Model (that has up to 360 degree expansion in spherical harmonics). As it can be seen from the figure, the geoid heights (given in meters) change rather essentially throughout the region under study. The main regional feature which can be seen at the figure is the strong decrease in the geoid heights above the deep trench area. This decrease is more abrupt at the island arc side.
To make the regional (smaller spatial size) features of the deep density structure of the region under study more visible we construct the field of the gradients of the geoid heights. It is known that the geoid height depends not only from the mean density value at the point but also from the first moment of the density distribution with depth. It means that isostatically compensated blocks that have different density variations with depth also cause a difference in the geoid heights (   Turcotte and Schubert,   1982   ).
Thus the boundaries between ancient (and thus isostatically compensated) blocks of different inner structure can be shown at the map of gradients of the geoid heights. In the used procedure, first 15 long - waves harmonics were removed to enlarge the effect of detecting of smaller size structures. At the figure obtained this way a number of the regional structures can be seen clearly. The main features of the figure are the recent Japan subduction zone. But this zone have a complicated structure. It can be seen that the subduction zone consist of a number of fragments divided by areas where gradients of the field of the geoid heights are essentially higher.
Such structure of the subduction zone correlates with the distribution of the earthquakes foci. The areas of higher density of the earthquakes foci and the focal zones of the major earthquakes (see   Aver'yanova,   1975  ;   Fedotov et al.,   1980  , for example) have evident segmentation along the deep trench and correlate with segmentation of the same area by the local zones of higher gradients of the geoid heights.
Besides, a few weaker anomalies can be seen. There are few weak anomalies located along the Asian coast near Sikhote - Alin mountain belt. These weak anomalies can be connected with the suggested here an ancient subduction zone (   Romanovskii,   1989;   ).

The Magnetic Field

The anomalous magnetic field of the Okhotsk Sea region along the geotraverse is characterized by anomalies with differently oriented elongations of various configurations and values. ( Structure...,   1996 ). Most of the observed magnetic field anomalies are linearity elongated in the north - western and north - eastern directions.

The Sikhote - Alin' magnetic field is characterised by positive anomalies elongated along the deep faults and reaching the values from 300 to 600 nT, which are associated with magmatic bodies. Within the Tatar Strait, a chain of separate maximums is identified, which approximately coincides with the axis of the largest depths of the Strait.
Within the Sakhalin territory, a negative background of the magnetic field is recorded caused mainly by the "granite" layer well - developed there. Individual linear positive anomalies are associated with the distribution of the intrusive and effusive bodies of basic and ultrabasic composition. Along the eastern Sakhalin stretches the East - Sakhalin positive anomaly reaching 1200 - 1400 nt values. This anomaly coincides with the East - Sakhalin ophiolite (hyperbasite) massif the ultrabasic and basic rocks of which are denuded on the Schmidt Peninsula and in the Eastern Sakhalin Mountains.
Anomal Magnetic Field of the Okhotsk Sea Region
( Makarova and Shmiyarova,   1978  )
Anomal magnetic field of the Okhotsk Sea region

The Kuril basin is mainly characterised by low negative anomalies reaching up to -150 ...-200 nT and associated with the non - magnetic sedimentary rocks filling the depression. With the advancement towards the Kuril Islands, the magnetic field becomes differentiated and varies from -300 nT to +400 nT. In this locality the magnetic field anomalies are conditioned by volcanic structures and stretch along the arc. The magnetic field anomalies of north - western strike are associated with the deep faults that cut the Kuril island arc into separate blocks ( Structure...,   1996 ). In the Northwest Pacific Basin, which is adjacent to the Kuril island arc, the systems of linear magnetic anomalies are recorded, the age of which varies from 108 Ma to 160 Ma ( Hilde et al.,   1977 ).
In the area of the geotraverse, combined geological and geophysical research into submarine volcanic zones of the Kuril Island Arc was carried out in nine voyages of the research vessel "Vulcanolog" ( Avdeiko and Rashidov,   1992 ). One of the techniques applied in the research was hydromagnetic survey. The measurements of the total vector of geomagnetic field intensity were conducted with a quantum magnetometer KM-2 and magnetometer PMIV with a testing unit accuracy of 1 - 3 nT. Standard measuring technique was applied. Tacks were coupled with coastal reference points with the use of the satellite navigation system and vessel radars.

Magnetic field (DT) in the geotraverse zone of the Okhotsk Sea in the area of the Kuril Islands generally has a negative background complicated by the magnetic field effects of submarine magnetic constructions. Anomal magnetic field in the Area of the Iturup Island



Anomal Magnetic Field (DT)  in the Area of the Iturup Island

In the larger part of the water area adjoining Iturup I., a negative magnetic field with intensity of -100 nT - -200 nT is developed. A negative anomaly of intensity of -300 nT complicated by two extremum reaching -470 nT and -510 nT is noted to the northwest of Antipin Cape and Natalia Bay. The dimensions of the anomaly are 48 x 18 km2
In the Okhotsk Sea water area adjoining Urup I. four areas are noted in which magnetic field positive anomalies are developed.
An intense alternating magnetic field anomaly with an amplitude exceeding 800 nT is located at the northeastern edge of Iturup 8.3 km away from Tigrovyi Cape. This anomaly is located at an undersea mountain, which apparently is an undersea volcano.
Against the negative field background distinct local anomalies are noted which do not affect the general character of the field and are located at the undersea volcanic constructions. The dimensions of these anomalies are comparable to the dimensions of the undersea volcanoes. The amplitudes of anomalies recorded above the undersea volcanoes vary from 100 nT to 800 nT. Both positive and negative meanings of the magnetic field may correspond to the undersea volcano, and above its peak the positive anomaly is commonly noted.
The gulf of Lvinaya Past' is an undersea caldera formed in the Holocene ( Bondarenko,   1991 ). The anomalous magnetic field of the gulf is characterized by positive values in the north and by negative values in the south, which is characteristic of layers magnetized in the direction of modern magnetic field. Anomalous magnetic field of the Caldera Lvinaya Past'



Anomalous Magnetic Field of the Caldera "Lvinaya Past'" (Iturup Island)


The maximal value of the anomaly exceeds 700 nT. Minimal values are noted in the northwest and southwest of the caldera and reach -900 nT and -1500 nT respectively. An intense positive anomaly with northeastward extension which coincides in space with a fault revealed from seismic data is noted to the north of the gulf outside the caldera. The amplitude of the anomaly reaches 1500 nT.

Electromagnetic studies of the deep structure

The results of magnetotelluric sounding within the Sikhote - Alin' area ( Nikiforova et al.,   1980 ). have shown that the conducting layer, i.e. the asthenosphere, lies in the upper mantle at the depth of about 120 km below the eastern part of Sikhote - Alin', and its depth grows to the west and reaches 220 km.

The electroconducting layer in the upper mantle under Sakhalin is located at the depth of 80 - 90 km and dips towards the western coast of the island down to 120 km ( Van'yan and Shilovskii,   1983 ). Under the western part of the Tartar Strait, just below the volcanogenic belt, the magnetotelluric sounding data imply a contact of the Sikhote - Alin' continental structures with those of the transition zone.
Geoelectric Structure under the Transition Zone ( Van'yan and Shilovskii,   1983   )
Geoelectric structure under the transition zone
A conducting layer is also recorded in the Sakhalin crust at the depth of 15 km, and its total longitudinal conductivity is about 40 Sm ( Van'yan et al.,   1983 ).

Along the geotraverse in the Kuril basin of the Sea of Okhotsk the electromagnetic research was carried out by applying the method of the gradient magnetovariation sounding ( Lyapishev et al.,   1987 ). According to the selected geoelectrical model, a layer with specific conductivity of 0.3 - 0.5 Sm/m and integral conductivity about 15000 Sm was located in the upper mantle within the 30 - 65 km depth interval.

Geoelectric model beneath the Kuril Basin



Geoelectric Model beneath the Kuril Basin
( Lyapishev et al.,   1987   )


The nature of the layer is associated with partial melting, and its distribution is confined to the limits of the basin. At the depth of more than 100 km, a second conductivity layer can be identified ( Lyapishev et al.,   1987 ) The obtained results correlate with deep temperatures in the upper mantle, with seismic research, and other geophysical data.

Below the Iturup I. of the Great Kuril Arc, the depth to the electroconducting layer in the upper mantle is 100 - 130 km, whereas under the Shikotan I. of the Lesser Kuril Arc it is 75 - 80 km ( Borets,   1972 ). After I.M. Al'perovich et al.,   (1978) the depth of the electroconducting layer under the Iturup I. is 60 - 80 km and, according to B.E. Mardel'fel'd,   (1977) it reaches 159 km under the Shikotan I.

The Upper Mantle

The Upper Mantle is characterized by both horizontal and considerable vertical heterogeneities. From data on deep - focus earthquakes, V.N. Burmin (   Burmin et al.,   1992 ) constructed the velocity section of the upper mantle.

Distribution of the P - wave velocity in the 0 - 700 km depth range



Distribution of the P - wave Velocity in the 0 - 700 km Depth Range
(   Burmin et al.,   1992   )


On the whole, the velocity curve of the Okhotsk Sea upper mantle approximates the velocity curve for the standard model of the Earth, but in the depth range of 220 km to 600 km the velocity of seismic waves propagation is lower than the average for the Earth. Wave guides are missing from the Okhotsk Sea upper mantle ( Burmin et al.,   1992 ) Below the recent volcanism zones of the Kuril Island Arc, low - velocities zone is revealed to the depths of 150 - 250 km ( Fedotov and Kuzin,   1963;   Boldyrev and Kats,   1982 ). In this area local magmatic centers appear that supply numerous volcanoes of the Kuril Island Arc. On the whole, the upper mantle below the Sea of Okhotsk is slightly less dense as compared with the Pacific ( Boldyrev et al.,   1993 )

Scheme of distribution of density anomalies in the upper mantle under the Okhotsk Sea region

Scheme of Distribution of Density Anomalies (in g/cm3) in the Upper Mantle under the Okhotsk Sea Region
(   Boldyrev et al.,   1993   )

From data of seismic tomography (   Anderson and Dzevonsky,   1984   ) the upper mantle below the Okhotsk Sea is characterized by higher velocity of seismic wave propagation than the mantle beneath the Sea of Japan and Philippine Sea. Electromagnetic and geothermal data indicate considerable heterogeneities of the upper mantle of the Okhotsk Sea region. Under the deep basins, the diapirs of heated anomalous mantle (asthenosphere) of high conductivity are encountered, causing the magmatic activity of the region. Thus, a conduction layer was revealed at a depth of 30 - 65 km by magnetovariation research in the Kuril Basin ( Lyapishev et al.,   1987 ). The resistance of the layer is 2 - 3 Ohm . m and its longitudinal conductance is 15000 S. Besides, one more conduction layer is supposed to be found in the mantle at a depth of 100 - 120 km with a resistance of 2 Ohm . m ( Lyapishev et al.,   1987 ).
In figure below we can observe an active subduction zone. "This is a cross-section through China and the seismically active Kuril. The seismicity exactly follows the subducting Pacific Ocean plate until about 500 km depth where the plate appears to lie flat. From the layered Earth structure it is known that a jump in seismic velocity occurs at 660 km depth. That jump is presumably caused by a phase transition at which upper mantle material transforms to a denser structure. It has long been uncertain what exactly happens to plate material at that depth: Is it possible for a plate to penetrate through the phase transition and sink into the lower mantle or will it get stuck above it? Although the cross-section becomes less accurate below 660 km depth, the plate material seems to have broken through the discontinuity and has been sinking to the core - mantle boundary (2800 km)" ( Bijwaard ).

Tomograph structure along Okhotsk Sea Geotraverse



Tomograph Structure along Okhotsk Sea Geotraverse
( Bijwaard )


In Sakhalin the asthenospheric conduction layer occurs in the upper mantle below the whole island and below the Tatar Strait where it is located at a depth of approximately 80 km. Along the eastern margin of the continent in the upper mantle a junction of high - conductivity layers of the asthenosphere with the rigid high - resistance upper mantle of the continent is encountered. In addition under Sakhalin in the depth range of 300 - 500 km anomalous high - resistance areas are revealed that may be associated with the cold subsiding plate of the subduction zone that passed there ( Rodkin and Semenov,   1995 ).
Below the South Kuril Islands in the geotraverse area the depth down to the conduction layer in the upper mantle is 60 - 80 km ( Al'perovich et al.,   1978 ). The data of electromagnetic research are confirmed by geothermal observations.
The distribution of deep temperatures along the geotraverse is shown in the figure below.
Deep Temperature Distributions along the Sea of Okhotsk (   Smirnov and Sugrobov, 1982   )
Deep temperature distributions along the Sea of Okhotsk
The highest temperatures are encountered beneath the Kuril Basin, where the area of partial melting is located at a depth of approximately 25 km. The lowest values are noted beneath the Kuril Trench (   Smirnov,   1986   ). On the Kuril Basin floor rift structures and base magmatism correspond to the anomalous mantle uplift.

The anomalous mantle in the upper mantle beneath the Kuril Basin is apparently a rising asthenospheric diapir. Its upper part forms a partially melting zone ( Tuezov,   1975 ) where primary magmatic reservoirs are formed. Thus maximal temperatures and minimal thicknesses of the lithosphere are characteristic of deep basins like the Kuril Basin and Tatar Strait Trough. The asthenosphere there rises up to 20 km. The lowest temperatures and the greatest thickness of the lithosphere are established within the northwestern area of the Pacific. Models of tectonosphere density constructed by different authors ( Anomalous...,   1974;   Sychev,   1979;   Krasny,   1990 ) also demonstrated that under the deep basins of the Okhotsk Sea the asthenosphere of increased thickness is found. Residual mantle anomalies obtained by excluding gravity affects of the crust from the observed values of gravity field are close to zero above the oceanic basins, are positive above Kuril Trench, and are negative above deep basins. High heat flow and mantle thinning may be associated with the rise of the lighter and hotter anomalous mantle, i.e. asthenosphere.

Geodynamic Investigations

Seismicity  |  The Recent Volcanism

Seismicity

Distribution of Earthquake Epicenters in the Region of Geotraverse
Earthquake spatial and depth distribution in the region of the Geotraverse
Depth Distribution of 2756 Earthquake Hypocenters along the Okhotsk Sea Geotraverse in 2o Zone according to the World Earthquake Catalog PDE for the period 1904 - 1999
All earthquakes were grouped with respect to magnitude (Mb) as follows: Mb<4, 4<Mb<5, 5<Mb<6, Mb6. To make seismic conditions representation more complete and to avoid taking the whole space by the earthquake centers, the width of the profile was different for the latitudinal and sloping sections of the geotraverse and for different ranges of magnitudes Mb. The width of the latitudinal section of the geotraverse with weak seismicity was 2° on the whole magnitude range. Events with Mb 5 were selected for the whole range with the profile width of 2° as well. Events with magnitude 0<Mb<5 (approximately 1800 centers) were constructed by sample data of a more narrow profile of 1°.
Magnitude ranges for earthquakes presented in the profile in scale 1:1 and the seismicity diagram in the geotraverse area are shown in corresponding figures. Designations of the main profile are given in the legend. In the bulk of the presented materials only earthquake of large magnitudes are shown in order to avoid encumbering of figures. All the earthquakes are shown for the geotraverse latitudinal section of weak seismicity. When mapping earthquakes in depth the average sea depth was taken into account.

The location of the Okhotsk Sea Plate in the contact area of the three lithospheric plates: Eurasian, North American and Pacific caused high seismicity on its margins. The highest seismicity is noted along the Kuril island arc. The Pacific Plate there subsides under the continent, forming a seismofocal zone which is traced to the depth of 600 km. In the west the Okhotsk Sea Plate is bounded by deep faults extending along Sakhalin. Earthquakes there are mostly located in the crust.

In the Kuril island arc the bulk of the earthquakes are located at depth ranging from 100 to 150 km, having their peak seismic activity at depth ranging from 30 - 40km ( Tarakanov,   1972 ). At depth lower than 100 - 150 km seismic activity sharply decreases, and at depth of 200 - 300 km a break of focal surface is noted.

Let us consider some issues of geodynamic conditions of two disastrous earthquakes in the South Kuril Islands in 1994 and in Sakhalin in 1995.

The disastrous Shikotan earthquake of October 4, 1994, originated in the area located to southeast of Minor Kuril Island Arc on the northwestern border of the Kuril Trench. Its magnitude was 8.1 ( Starovoit et al.,   1994 ).
Detailed description of the earthquake was made by E.A. Rogozhin,   1996 . According to Rogozhin the field of the earthquake was an oval of 200*100 km extending along the trench. Depthward the hypocenters of earthquakes embrace the upper part of the lithosphere to 60 km.
Section of Tectonosphere across the Kuril Island Arc at the Area of Shikotan Earthquake
The section of tectonosphere across the Kuril Island Arc at the area of Shikotan earthquake

The center is oriented along the island arc. A displacement in it is a dextral strike - slip fault ( E.A. Rogozhin,   1996 ).
The seismicity of Sakhalin is related to deep faults ( Oscorbin,   1977 ). separating the Okhotsk Sea Plate from the Eurasian Plate. The movements relative to each other of these plates as well as spreading development in the rift structure of the Tatar Strait result in active seismicity. The intensity of modern contrasting movements is such that 5 - 10 considerable shocks are noted annually and a disastrous earthquake occurs every five years ( Soloviev et al.,   1967 ).
In North Sakhalin in the night from May 27 to May 28 a disastrous earthquake occurred that destroyed the city of Neftegorsk ( Semenov et al.,   1996 ;   Rogozhin,   1996 ). Its magnitude was 7.1; intensity in the epicenter was 9; on the surface a seismic rupture formed extending to the north - northeast with a total length of 35 km. The seismogenic displacement is a dextral strike - slip fault with the amplitude of the horizontal shift component of up to 8 m, and vertical fault component of up to 2 m ( Rogozhin,   1996 ). The rupture formed in the Verkhnepiltunskii fault area which is a segment of the meridional Sredinno - Sakhalinskaya suture zone. The Neftegorsk earthquake formation is supposed to be associated with Okhotsk Sea plate movement relative to Eurasian plate (or the Amur plate located between those plates) ( Semenov et al.,   1996 ;   ).

The Recent Volcanism

The characteristic feature of the Kuril island arc is the high tectono - magmatic activity manifested on the surface by recent volcanism and seismicity. There are 105 land volcanoes on its territory, 42 of which are active, the others are of Quaternary age, and 96 submarine volcanoes ( Avdeiko et al.,   1992 ). The part of the geotraverse, covering the three islands Kunashir, Iturup and Urup, has 34 land volcanoes (16 of which are active) and 23 submarine volcanoes.

Distribution of Kuril Islands Volcanoes (   Fedorchenko and Rodionova,   1987   )
Distribution of Kuril Islands volcanoes

In the course of this century more than 100 eruptions of different strength and types were recorded on the Kuril Is. (   Gorshkov,   1967   ). The products of eruptions form a number of calc - alkalic rocks ranging from basalts to dacites and rhyolites. The content of alkalic metals (mainly potassium) grows with the distance from the ocean thus reflecting the manifestation of the petrochemical zoning (   Fedorchenko and Rodionova,   1987   ). Model of magma formation



Model of Magma Formation
(   Avdeiko,   1994   )


According to the data presented by G.P. Avdeiko ( Avdeiko,   1994 ), the magma under the Kuril arc is generated in the mantle wedge over the subduction zone where the earthquake foci are concentrated, which under effect of the volatile substances are separated from the subducting plate of the Pacific Ocean. At the initial stage of the oceanic plate subduction, the water in the sediments and in the pores and fissures of magmatic rocks is separated. It emanates at temperatures more than 100o C and at the depths of about 30 - 40 km. This water is used for the formation of metamorphic rocks. At large depths, in the 200 - 700 km interval, the water containing minerals are dehydrated, and the fluid solution is formed thus lowering the temperature of rock melting.

It is assumed that the formation and accumulation of magma takes place at these depths.

Conclusions

The geotraverse of the Okhotsk Sea area demonstrates the tectonosphere structure to a depth of 100 km.

The Okhotsk Sea Geotraverse
The Okhotsk Sea Geotraverse

More detailed view of the geotraverse

The crustal thickness ranges from 35 - 40 km under Sakhalin and the Kuril Islands to 10 km under the Kuril Basin. The asthenosphere forms diapir under the Kuril Basin and Tatar Strait Trough. Rifts, spreading structures are located in the base of these structures. The initial stage of the Kuril Basin formation and Tatar Strait Though is associated with magma extension and intrusion. The asthenospheric diapirs rising toward the crust caused high heat flow. The formation of Tatar Strait Trough is related to the extending northward spreading center found in the deep basin of the Japan Sea. The formation of the Kuril Basin is also related to spreading processes taking place in the Late Cretaceous. The formation of the Interarc trough dividing the Kuril Islands arc into the Larger and the Smaller ones is also related to spreading.

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