Boundary region between the South Chinese continental suburb an

I.GEOLOGO-GEOPHYSICAL PERFORMANCE OF A BOUNDARY REGION BETWEEN SOUTH CHINA PLATFORM AND PHILIPPINES OCEANIC PLATE

The Luzon-Taiwan-Rjukju system, as boundary part between southeast continental borderland of Asia and northwest borderland of the oceanic plate of Philippines Sea estimed in the limits of 15-35^oN and 117-127^oE. This territory contains the northern end of the Luzon arc, the Taiwan Island and the southwest end of Ryukuy island-arc system. The Philippines Sea is presented by the West Philippines basin, a continental part - Southeast China and wide shelf of South China and East China Seas.

Heterogeneous structure of the continental borderland of SE Asia is result of an intensive extension it, which is connected with the formation of the South China and East China seas and others oceanic basins in Cenozoic. They were closed and dipped into the east zones of subductions in late Cenozoic. The west of northern borderland of Luzon island-arc is surrounding by deep-water basin of the South China Sea. This basin rests on oceanic lithosphere, which dipped in east direction along the Manila trough under the plate of Philippines Sea. Volcanic activity of the Luzon arc is related to this dipping. The oceanic plate of Philippines Sea dipped under the continental shelf of the East China Sea along Ryukuy trough. Okinawa back-arc basin locates to the north from the Luzon arc, which one the extension and modern volcanic activity is linked. The Taiwan Island has arisen on continental borderland because of a collision of a northwest angle of an oceanic plate with the continental lithosphere.

The oceanic crust of the West Philippines basin is of Eocene age. It is supposed, that the Luzon arc which is setting from the west by the Manila trough, existed since early Miocene. It is most likely that activity has stopped in the Quaternary period. The most northern peak of modern volcanic activity of the arc is Batan Islands (nearby 21.2°N).

The South China Sea was opened in Oligocene-Miocene time (32-15 Ma). The subduction of lithosphere of the South China Sea was since early Miocene along the Manila trough under the Philippines Sea plate. To the north, with approach to Taiwan Island, there is a consecutive transferring from subduction of oceanic crust of the sea to a collision continent-ocean and to underthrust of southeast continental borderland of China under island.

Active area of Taiwan Island generated because of dynamic interaction of a northwest angle of an oceanic plate of Philippines Sea and borderland of the Asian continent, to the southeast from elongated in a north-northeast direction structures of China Continent.

The Ryukuy trough, fore-arc ridge Jaema, intra-arc basin Nanao and Ryukyu island-arc extended to the east from northern part of Taiwan Island. They are structural elements characterized by a zone of subduction of a northwest part of the Philippines plate under Asian, along the Ryukyu trough. The interaction area between Ryukyu system subduction and the Taiwan collision zone stretches from the Okinawa trough in the north, to the southern edge of Gagua ridge in the south, and in the east is confined about 124.5° E. Boundary between the zone of active deformation referred to a collision, and the West Philippines basin is Gagua ridge. It is confirmed by smaller seismic activity to the east of the ridge. Structural members of the Ryukyu arc zone subduction deformed and partially disappear to the west of 123°E in the area of interaction of tectonic processes subduction and collisions. Ryukyu trough morphology is well expressed in the northeast part, at approach to Taiwan it becomes essentially wider, but not deeper, and to the west of the meridian 123°E its clear bathymetry outlines practically disappear.

The East China Sea washes the inside of Ryukyu arc introduced by vast continental sand bank on the southeast borderland, where lays the trough Okinawa. The trough by morphology is expressed by the depression extending along Ryukyu island-arc. Its bottom is enough flat, in the middle part is well traced a graben, along its axis a young volcanic construction in the form of a small basalt height extends. The most expressed is the southwest part of the trough. Near to Taiwan Island, its max depth is noted as 2719 m. A thick sedimentary layer is evaluated as 2 km and more. Depth and cross-sectional dimensions a trough to the northeast on a direction of strata is systematically relieving to a full pinching out. It is considered that trough forming has originated because of two phases of rifting the South Japanese volcanic arc and now it is at a stage rifting a continental crust of the East China Sea.

It is supposed, that ridge Jajame is a subduction product of the Philippines plate under an island-arc extending along Ryukyu trough and represents an accretionary prism.

1. Gravitational field

In Fayer reduction over water areas of the East China Sea and the West Philippines basin is quiet. In the first case, it positive also is inflected from 0 to 40 mGl, and in the second - basically, negative (from -10 to -15 mGl). The Okinawa trough and system of island-arc - Ryukyu trough, in the gravitational field dedicated with adjoining strip anomaly zones of opposite signs on different intensity. To a trough valley, there corresponds depressing of a positive going field to zero values, and island-arc positive anomalies to 40-50 mGl. The Nanao Interarc basin and Ryukyu trough are expressed by the intensive adjoining minima, partitioned by maximum over ridge Jaejea (Fig. 2). The amplitude of Nanao anomaly can reaches the value of -180 mGl (at the Taiwan island - to -235 mGl), and Ryukyu - more than -100 mGl. This entire anomaly zone prevented at east coast of the Taiwan island where they are closed with the positive and negative extending local anomalies answering to sub-latitude structures of the crust orogenic belt of Taiwan Island [2, 3].

2. Heat flow

The heat flow is changeable in the West Philippines basin. It is probably caused by adjective carrying of heat to sediments circulating waters. The Okinawa trough as a whole is characterized by the raised heat flow, which one magnitude exceeds observed in middle-oceanic ridges and others young rift structures and testifies the intensive modern hydrothermal activity linked with young connected by lying down sources of magma island-arc type. A weak heat flow is marked in the southwest part of the trough, at Taiwan Island.

3. Seismicity

The seismicity of Philippines Sea is located within bordering its island-arc systems and has different character and depth. Fore-arc Ryukyu island system has low seismicity. It is explained by the oceanic platform stability. The high seismicity is marked in the Taiwan collision zone. Earthquakes with contraction focus-mechanism are observed in the western part of Ryukyu arc and in the Okinawa trough - with focus-mechanism expansion. High density of such earthquakes in the trough is limited by the longitude 123^o,5. It is proposed, that it is caused by the release pressure and faulting and temperature reduction. It illustrates the weak heat flow.

II. 3D P-VELOCITY MODEL OF THE BOUNDARY ZONE BETWEEN SOUTH CHINA CONTINENTAL REGION AND PHILIPPINES OCEAN PLATE

The 3D P-velocity model of the mantle under Taiwan region and its surrounding is received in context 3D P-velocity model of the mantle under Eurasia by Taylor¢s approximation method for the seismic tomography problem of the arrival times, which was developed by V.S. Geyko [1]. The used method has a number of advantages before a method of classic linearization of the decision of the similar problems, one of which is independence of a choice 1D reference velocity model. Our model is performed by horizontal sections through 25 km at the depth of 50-2500 km and vertical sections (longitudinal and latitude) through 1^o in the same depths. Horizontal sections introduced by true velocity with separation on high and low, concerning 1D reference model, which received for our 3D mantle P-velocity model under Eurasia as average velocities, vertical sections - in residuals, concerning the same reference model.

Velocity layers characterize the mantle under Taiwan region as a whole: 1) Low-velocity upper mantle (depths of 50 - 400 km); 2) High-velocity transition zone of upper mantle (400 - 600 - 650 km); 3) Low-velocity middle mantle (600 - 1700 km); 4) High-velocity zone of deviation-II (1700-2200 km); 5) And low-velocity low mantle.

1. Horizontal sections

The boundary position of island-arc systems is fixed by morphological features of top of low-velocity upper mantle at the depth of 50 km a chain of velocity anomalies union by an isoline of 7.9 km/s (Fig. 1).

These features permit to estimate the morphology and depth of the seismic lithosphere bottom. It increases to 70-80 km under oceanic and continental regions and decreases more than 50 km under island-arc system. The bottom of underlying layer has conformed turn and thickness of the depth about 200 km. In a mean part of this layer anomaly region has longitude direction as continuity Luzon-Taiwan part of boundary and flow together with low-velocity mantle layer East-China Sea. Its observed velocity inversion in the part of the Ryukyu arc (horizontal sections of 100-150 km (Fig. 1) is detected.

The boundary of velocity anomaly layers, which include island-arc system, restores to the depth of 175-225 km (Fig. 1) at the same time velocity inversion to the relatively high.

Below it is a spreading of high-velocity region under Luzon Island and East China Sea. Between East China Sea and Luzon island one can see the region, which separates the continental upper mantle from oceanic one. This boundary structure passes to the north-west under south-east China margin. North-east its direction changes to longitude to the south. Transitional zone of upper mantle observed at the depth of 250-450 km and characterizes by high velocity.