RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES5002, doi:10.2205/2005ES000190, 2005
[104] As follows from the reconstructions of the oceanic plates [Engebretson et al., 1985; Maruyama et al., 1997], the Farallon-Izanagi spreading range moved along the East Asian margin during the time of 220-180 Ma. Since that time to almost the middle of the Late Cretaceous (85 Ma), the Izanagi plate interacted with the continental margin. During the Triassic and Jurassic a large volcanic oceanic plateau was formed on the Izanagi plate in association with the Central-Pacific plume activity, known as the Sorachi Plateau [Kimura et al., 1994; Maruyama et al., 1997] and as the Okhotsk Plateau [Bogdanov and Dobretsov, 2002], and believed to have been associated with the central Pacific plume activity.
[105] The Izanagi plate moved at that time at the highest velocity (30.0 cm year-1 ) in the northwestern direction (see Figure 10), which caused its rapid orthogonal subduction under the island arc. Its subduction was stopped by the underplating of the large blocks of the above-mentioned oceanic plateau at the end of the Neocomian (possibly during the Hauterivian-Barremian). The high velocity of the Izanagi plate movement (20.7 cm year-1 ) caused the detachment of the arc rocks from their basement, as well as of the accretion prism with the fragments of the accreted plateau. All of these rock components continued their passive movement along the continental margin as the constituents of the oceanic plate. This was facilitated by a change in the direction of the Izanagi plate motion from the northwestern to the northern at the beginning of the Aptian without any changes in the velocity of their movement.
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Figure 13 |
[107] The global left-lateral movements resulted in the
formation of an extensive suture which had inherited the Early
Cretaceous convergent boundary and could be traced by the
fragments of the accretion-type Northern and Southern Chichibu
and Kurosegawa terrains and of the subduction-type Mikabu and
Sambagawa terrains of Southwest Japan. The granitoid
magmatism of the Rioke Belt developed with some delay in the
rear of this structural feature
[Isozaki, 1996;
Miyashiro, 1975].
Earlier, it was interpreted as a high-temperature and low-pressure
metamorphic belt, forming a pair with the Sambagawa high-pressure
metamorphic belt. According to this view, the
Sambagawa Terrain and the metamorphic accretion-type terrains,
conjugated with it, were "pressed" in the zone of transregional
strike-slip faults, whereas the Rioke granite belt was formed as a
syncollision tectono-magmatic structural feature. This is proved
by the intensive transformation of the granitoids to gneiss and by
their tectonic position. As follows from
[Maeda and Kagami, 1996;
Maruyana et al., 1997],
the formation of the syntectonic granitoids and the low-pressure
metamorphism might have been associated also with the development
of the Izanagi-Pacific oceanic ridge along the continental margin.
As follows from the age of the igneous rocks (100
5 million
years), the peak of this tectonic activity can be placed at the
end of the Early Cretaceous.
[108] The left-lateral strike-slip movements involved the whole of the continental margin. Its Jurassic-Early Cretaceous accretion structure experienced repeated lateral doubling with the formation of large saw-shaped protrusions in the NE direction [Golozubov, 2004]. One of these protrusions was formed by the Abakuma, North and South Kitakami, and Oshima terrains of Southeast Japan. It can be supposed that the formation of this doubling zone might have been accompanied by the Aptian-Albian alkaline volcanic activity of the Kem Terrain along its continental side. At the same time the Rebun-Kobato, Moneron, and Sorachi terrains were displaced along the oceanic side of the protrusion and accreted at the end of the Early to the beginning of the Late Cretaceous, having occupied the position close to the modern one, adding the abundant volcaniclastic material to the synchronous turbidite sediments of the southern segment of the Iezo-West Sakhalin Trough. The latter originated as a circum-shear turbidite depression which grew larger in the northern direction and overlapped the Sorachi Plateau accreted segments. The eastern segments of this plateau continued to move to the north and were subducted in the oblique subduction zone (Figure 13b).
[109] The process of the oblique subduction was accompanied by the left-lateral movements of the Early Cretaceous prism with the subduction and high-pressure metamorphism of the lower structural levels (West Susunai Subterrain). The upper structural levels experienced crowding and were thrust over the fore-arc turbidite and trench sediments. To sum up, the active continental margin of the Andean type existed in the east of Asia during the early half of the Late Cretaceous. Its accretion-type structural features were formed in the environment of the oblique subduction of the Pacific Plate.
[110] The Early Campanian time witnessed the reorganization of the Asia-Pacific ocean convergent boundary. A system of intraoceanic island arcs was formed in the northwestern part of the Meso-Ppacific Ocean at a latitudinal distance of up to 2000-3000 km from the continental margin [Bazhenov et al., 2001, 2002; Kovalenko and Chernov, 2003]. The compilation of the paleomagnetic data available for the Campanian-Early Paleogene island-arc rocks of Hokkaido, South Sakhalin, Kamchatka, and Koryakia shows that the island-arc systems had a NE strike, generally orthogonal to the movement of the Pacific Plate. The southwestern flank of the island arc garland, closest to the East Asian continental margin, was occupied by the Tokoro Arc. This arc developed at the leading edge of the Pacific plate which moved, at the end of the Cretaceous, in the NW direction at a rate of 13.1 cm year-1 [Engebretson et al., 1985]. The subduction zone was directed to the ocean, and the arc moved closer to the continental margin, together with the overhanging oceanic plate. Northeast of the Tokoro Arc, supposedly following the same kinematic model, the East Sakhalin island arc [after Zonenshain et al., 1990], or the Patience island arc [after Khanchuk, 1993; Parfenov et al., 1999], was formed and included in the early Paleogene into the accretion-collision structure of the East Sakhalin composite terrain. As to the further reconstruction and substantiation of the early collision of the Patience Arc with the continental margin, we have to admit that this margin had been advanced northwest relative to the Tokoro Arc and joined it along a transform fault (Figure 13b).
[111] The space between the ensimatic arcs and the continental margin was occupied by the microplates of the marginal seas, representing large Paleo-Meso-Pacific segments. The Velikan plate identified in this region (Figure 12b) experienced subduction in the Campanian-Maestrichtian both under the continental margin and under the Tokoro Arc, which contributed to the rapid convergence of these structural features.
[112] To sum up, at the end of the Late Cretaceous the environment of the active Andean-type continental margin coexisted with the environment where the continental-margin and intraoceanic subduction zones existed together with epioceanic marginal seas being located between them. The latter situation has no analogs, yet can be compared conventionally with the modern Polynesian or Philippine Sea situation [Bazhenov et al., 2001, 2002; Kovalenko and Chernov, 2003; Melankholina, 2000].
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Figure 14 |
[113] It should be noted that almost simultaneously, namely, from the end of the Paleocene to the beginning of the Eocene, the collision of the Achaivayam-Valaga ensimatic island arc with the continental margin rearranged the convergent margin with the formation of a new subduction zone dipping under the continent and extending in the direction of South Sakhalin [Konstantinovskaya, 1999, 2003].
[114] To sum up, the beginning of the Paleogene witnessed the extinction of the Cretaceous subduction zone, the migration of the convergence boundary to the south, and the formation of a new Paleogene continental margin, known as the Okhotsk one, at the latitude of the South Sakhalin plate.
[115] The subduction of the Velikan Plate under the Tokoro Arc continued up to the Early Eocene, as indicated by the magmatic activity of the suprasubduction structural features and by the synchronous emplacement of fore-arc tuffaceous turbidite in the Tokoro Terrain [Kiminami et al., 1992].
[116] In South Sakhalin the collision occurred from the end of the Middle Eocene to the Early Oligocene. Southward, in Hokkaido, the collision of the Tokoro and Hidaka terrains terminated only in the Oligocene, as follows from the ages of the syncollision intrusions [Maeda et al., 1990]. In the southern segments of the collision zone, the accretion-type rocks of the Tokoro and Hidaka terrains experienced progressive metamorphism with the formation of the Hidaka collision-type metamorphic terrain. The oldest synkinematic anatectic granites have been dated Early Eocene (56 Ma, Ar-Ar), [Komatsu et al., 1992; Osanai et al., 1994]. This suggests that the collision might have developed from the south to the north, concordantly with the convergence of the island-arc and continental margin structural features, yet, terminated more rapidly in the north.
[117] The Aniva composite terrain, produced by the amalgamation of the accretion structural features, moved in the northern direction along the Idonnappu and Merei suture zones, and was finally included into the continental margin at the Eocene-Oligocene boundary, or at the beginning of the Oligocene, being located in the junction zone between the Okhotsk and proto-Japanese continental margins. Later the accretion-type structural features of Southeast Sakhalin and East Hokkaido were overlapped by the Late Eocene-Early Miocene neoautochthon.
[118] The proto-Japan continental margin began to form synchronously with or slightly earlier than the Tokoro Arc and continental margin collision, that is, at the end of the Early-Middle Eocene. It was recorded in the slow inherited subsidence along the inverted Cretaceous basin and in the formation of the Middle-Late Eocene zone of calc-alkaline volcanism (proto-Japan volcanic arc), displaced eastward relative to the Cretaceous-Paleocene East Sikhote Alin Belt (Figure 14b). The specific geochemistry of the volcanic rocks suggests their suprasubduction origin and their association with some previously subducted Paleo-Mezopacific slab [Okamura et al., 1998; Yasnygina, 2004]. At the back of the volcanic zone, extension began in the later half of the Eocene or at the beginning of the Oligocene, and the proto-Tatar and Central Sea of Japan rift basins began to form, with plateau basalts residing at their continental shoulders. On the contrary, in front of the proto-Japan Arc, en-echelon upthrust-overthrust zones were formed, which marked the axes of the synsedimentation structural highs. The comparison of the specific structural and petrological features of the proto-Japan Arc and the adjacent structural features suggests their evolution as a result of the break or the abrupt curvature of the previously subducted slab of the oceanic crust and its subsequent "floating up" or rolling away toward the trench. These movements "shaped" the late metamorphic structural features in the Susunai Terrain and controlled the final exhumation of the metamorphic rocks.
[119] The Okhotsk arc-trench system was being formed at that time in the area of the Sea of Okhotsk continental margin. Its frontal elements are preserved in the East Susunai and Tunaicha subterrains. Its Eocene-Oligocene volcanic rocks (46-26 Ma), marking the zone of the suprasubduction igneous activity, have been found in the southern part of Central Sakhalin and in its sea-floor extension, recorded in the Academy of Science Rise [Emel'yanova, 2003].
Citation: 2005), South Sakhalin tectonics and geodynamics: A model for the Cretaceous-Paleogene accretion of the East Asian continental margin, Russ. J. Earth Sci., 7, ES5002, doi:10.2205/2005ES000190.
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