LINEAMENT STRUCTURE OF THE MANTLE UNDER SOUTHEAST ASIA AND SOUTH CHINA

LARYSA ZAIETS

Institute of Geophysics, National Academy of Sciences of Ukraine

Abstract: The method of Taylor's approximation for solution of seismic tomography problem used by us to construct 3-D P-velocity mantle structure of the Southeast Asia and South China up to the depth 2600 km. Mantle velocity situation supports the thesis of connection of processes occurring in the crust structures with the mantle. This connection follows from location of fault zones systems in the crust having extension in the form of velocity boundaries in the region mantle. It is shown dependence of the mantle structure on fault tectonics is predominant in the velocity structure of the mantle under the studied region. Thus, the following lineaments have found reflection in the mantle structures: lineament 100-103°E - one of the largest ones in Asia, through lineament 109-110°E., lineament at 116°E., lineament stretching in the zone between meridians 120-122°E., as well as a number of lineaments of latitudinal extension.

I. INTRODUCTION

With the help of satellite image interpretation and geology and geophysics evidence analysis, a number of major disturbances crossing boundaries of large megablocks have been identified in the territory of Southeast Asia and its surroundings. Transstructural (through) zones play a significant role in the general tectonic situation of the region. The largest of them have become a kind of frame, within which intense tectonical events leading to the current appearance of the region [7] took place. Lineaments determine contours of continents, fold systems and disturbance zones and serve as boundary structure between separate elements of the Earth's crust, connection of mantle and deep parts of the crust with the surface is carried out through them. As far back as 1948 N.S. Shatskyi describing Transcaucasian lineament noted: "Availability of immense tectonic structures and movements covering various in structure parts of the Earth’s crust raises a question of commonness of movements and, probably, of commonness of matter alternation processes in the deep earth mantles under various surface structures”. Applying this approach to lineaments one should certify that existence of structures, in relation to which not only boundaries of continental and ocean earth’s crust, but also boundaries of the largest lithosphere plates are surface structures, allow to make assumption about under-lithosphere intramantle location of lineaments [1].

Certainly, the form of occurrence of lineaments in the mantle has little in common with usual notion of deep-seated fault resulting from active intramantle processes. In this work, we will use the term “velocity boundary” in the mantle, which refers to the zones bounding velocity mantle blocks, which characterize mantle divisibility into large velocity structures having particular behaviour of velocity characteristics.

This work is concerned with distinguishing and analysis of mantle velocity boundaries under the structures of the studied region and their correlation with the surface structures within the developed 3-D P-velocity model of the mantle under SE Asia and S China platform. 3-D P-velocity model has been obtained as a result of application of the method of Taylor approximation of solution of seismic tomography of P-waves arrival time introduced by V.S. Geyko [3-5]. The solution is represented in a form of vertical sections (latitudinal and longitudinal) up to 2500 km in depth with 1° spacing in residual in relation to one-dimensional referential model obtained as a result of seismic tomography analysis for Eurasia. The tectonic map of SE Asia is presented by Prof. Cao Dinh Trieu (Institute of Geophysics, VAST, Cau Giay, Ha Noi, Viet Nam).

II. THE MANTLE VELOCITY BOUNDARY

We distinguish the following mantle velocity boundaries under the region in question:

Figure 1. Pattern of spreading of high-velocity seismic lithosphere under SE Asia

Boundary between the areas with high-velocity and low-velocity upper mantle. Careful study of velocity structures within SE Asia and S China platform allows to distinguish large region with thin high-velocity lithosphere (Fig.1). Main velocity characteristics of the mantle of the studied region are low-velocity upper mantle and high-velocity transition zone of upper mantle, which is typical of Phanerozoic structures. At these areas are distinguished having two-layer structure of upper mantle – a thin (50-100 km) high-velocity layer of seismic lithosphere and a heavier low-velocity part (100-400 km). Such structure is observed under the central and eastern part of S China platform, East Sea, eastern and central part of Indochina.

Structures of Indo-Australian plate surrounding SE Asia in the south and west are characterized by high-velocity upper mantle. Velocity boundary dividing high-velocity Indo-Australian plate and low-velocity structures of SE Asia is distinguished along latitudinal sections in the upper mantle. Beginning under the eastern part of Sibolga block (95°E) and south-western part of Sumatra block, it is distinguished in the upper mantle (to 450 km) by the change of cover thickness of roof upper mantle transitional high-velocity layer (450 km under Sibolga block to 150-300 km under Sumatra block), in the transition zone of upper mantle - by change of behaviour of velocity layer down to the depth of 1650 km. Further, under the blocks of Andamans and Shan-Thai, the velocity boundary is observed down to the depth of lower mantle (2500 km), with 21-23°N - to the 1000 km depth; the boundary is further shifted under the western boundary of Sino-Burma block (95°E) and is observed down to 2000 km (Fig. 2).

Figure 2. Pattern of spreading of velocity boundaries
in the mantle under SE Asia and S China

The velocity boundary in the mantle between Indo-Australian plate and Andaman block is observed at 94°E. In the high-velocity layer of the upper mantle transition zone it is distinguished by the abrupt leap of depths of velocity isolines spreading. Thus, the roof of high-velocity layer of upper mantle transition zone under Nicobar block (Indo-Australian plate) located at the depth of 450 km raises under Andaman to 250-300 km in depth. To the north, the velocity boundary between Indo-Australian plate and structures of SE Asia shifts to the west. Low-velocity layer dipping to the side of Indo-Australian plate structures with residual -0, 1 km/sec accompanied by earthquakes is distinguished in the upper mantle from the side of Andaman Sea. Starting from 14° N velocity boundary in the upper mantle transition zone shifts by 1° to the west as compared to the velocity boundary appearing at the depth of 50-100 km in the upper mantle. No earthquakes are observed in this region. Under Shan-Thai block no dipping layers in the upper mantle are distinguished, but considerable rising of high-velocity layer of mantle transition zone up to the depth of 250 km takes place.

Let us analyze the behaviour of velocity boundaries under separate structures of SE Asia and S China platform.

Indochina, as mentioned before, is characterized by inhomogeneous structure horizontally: two-layer structure of upper mantle - thin high-velocity layer corresponding to seismic lithosphere and thick low-velocity layer - characterizes the most part of Indochina - western and central parts. Central part of the block is described by interchange of consistent in thickness high and low-velocity layers. In the west and east this part of Indochina block is separated by the velocity boundaries: at 104-105°E (low-velocity layer of the upper mantle with residual -0,05-0,075 km/sec located at 100-400 km depth under Indochina, raised under Khorat block to the depth of 50-350 km) and at 109-110°E (the same raising of low-velocity layer under the block of Central East Sea is observed). It is worth to mention that boundaries 105-106° and 109-110° have extension in the northern and southern directions.

The boundary at 105-106°E (being inhomogeneous boundary, aggregate of separate boundaries joined by their features) in the upper mantle runs along the western boundary of low-velocity disturbance bound by isolines with -0,075-0,125 km/sec residual at the 150-275 km depth spread under the blocks Natuna (depth 100-150), North Kalimantan and Indochina (depth 150-275). Velocity boundary starts under western part of the blocks Natuna and North Kalimantan, and from 9°N to 19°N it bounds Indochina block in the west. Under Indochina one more velocity boundary is distinguished in the high-velocity layer of the upper mantle transition zone; it is identified by abrupt dipping of velocity isolines with 0,025 km/sec from 900 km down to 1200 km depth (in places to 1400 km). Beginning from 20°N the boundary in the mantle shifts eastwards to 106°E - it is distinguished in low-velocity part of the upper mantle and transition zone of the upper mantle. In high-velocity seismic lithosphere no mantle velocity boundary is identified. Further, mantle velocity boundary goes under northwestern part of Indochina and western part of S China platform bounding area of impact of Alpine-Himalayan mobile belt structures in the west.

The boundary at 109°-110°E disturbs boundary alternation of high-velocity and low-velocity layers under Indochina; low-velocity layers of the upper mantle raise from 250-300 to 200 km depth, and high-velocity layers of the upper mantle transition zone dip down to 900 km. It is observed down to 2500 km, and is distinguished by Y.G. Gatinsky et al. in the crust structures as a submeridional structure - fault Hainan-Natuna, and is identified by A.G Rodnikov et al. as submeridional Indosinian (Boundary) fault. Submeridional extension faults, with which Van Eza line coincide in space (it is confined to the part of submeridional lineament at 109°-110°E), were also distinguished by Vietnamese and Chinese scientists.

On longitudinal section velocity boundary is observed under the southern part of Indochina at 8-10°N; it is distinguished by change of thickness of low-velocity layer of the upper mantle from 50-250 km (under North Kalimantan) to 50-400 km (under Indochina) with closing error -0,025 km/sec. It is distinguished in the transition zone by dogleg of velocity isolines. The velocity boundary of sublatitudinal extension continues under Pattani block at 10°N in the west and under deepwater basin of East Sea in the east ending within 120°E. In the upper mantle transition zone the boundary shifts to 1-2° northwards. At 10°N boundary dividing Pattani block into northern and southern parts is distinguished, as well as a number of velocity boundaries passing through the centre of Khorat block (at 14-15°, 16°, 19°N). At the surface these boundaries correlate with latitudinal faults zone of Indosinian massif. It is worth mentioning that mantle velocity boundary at 9-10°N beginning under Pattani block (98°E) and ending under Truong Sa archipelago (119°E) is observed down to the depth of lower mantle. Velocity boundary is especially clearly distinguished in the upper mantle transition zone by velocity isolines dogleg.

Central part of Indochina block constituting interchange of consistent in thickness high and low-velocity layers is detached by velocity boundaries at 10°N and 20°N (105°-111°E). At that, high-velocity layer of upper mantle transition zone is lowered to the 1000 km depth. Here velocity boundaries do not coincide with the borders of Indochina block in crust.

S China platform as a young structure is characterized by inhomogeneity both vertically and horizontally. According to Hsu Kenneth J. et al. [6], S China platform consist of 3 blocks: folded belt of Yangtze, deformation zone of Hainan, Dongnanga block, which are separated by sutural zones (Xianggangzhe (Banxi), Gunanhai). The platform mantle may also be divided into separate blocks according to behaviour of velocity characteristics. Here the central block belonging to Precambrian structure and activated margin are distinguished. Activation in the west apparently takes place under the influence of pressure of Indian indentor [2]. Western part of S China platform (up to 109°-110°E) has low-velocity upper mantle, which, due to high-velocity anomaly (residual 0,025-0,05 km/sec) at the depth of 100-250 km, is divided into two arms, lower part of which reaches 400 km depths losing in some places its continuity. Velocity boundary between the platform and Sino-Burma block in the upper mantle is not distinguished clearly (to 104°E).

Under the central part of the platform the upper mantle has two layers - thin high-velocity layer of seismic lithosphere down to 50-75 km and thick low-velocity layer. In the low-velocity layer of the upper mantle low-velocity anomaly with residual 0,025 km/sec is distinguished. It lowered to the depth of 400-600 km, which led to lowering of high-velocity layer of the upper mantle transition zone down to 700-1300 km. Both high-velocity and low-velocity layers of the upper mantle under the central part of the platform have subhorizontal extension with consistent thickness. Identifying the area of sharp change of depth of velocity layers occurrence with the velocity boundaries in the mantle one may observe borders of central block of S China platform to the depth of separation zone -2.

The block in the south of the platform is identified separately. Here the upper mantle is a two-layer one with a thin (to 50 km) high-velocity seismic lithosphere layer and thick low-velocity layer. At 19°-22°N in the low-velocity layer of the upper mantle and high-velocity layer of transition zone of the upper mantle interlaid low-velocity and high-velocity longitudinal anomalies with residual -0,1 km/sec - -0,075 km/sec are distinguished at the depth of 100-250 km and 0,05 km/sec at the depth of 300-650 km, which detach the block corresponding at the surface to Dongnanga block of S China platform [6]. Southern end of these anomalies under margin of S China platform is distinguished in the upper mantle as a velocity boundary, which is identified along high-velocity layer and is traced down to the depth of the lower mantle. Only partial correlation with southern margin of S China platform at the surface is observed. Here up to 120°E platform margin is correlated with the velocity boundary distinguished at 23°N.

In the north velocity boundary is clearly distinguished in the mantle under the central block of S China platform at 30°N on the section 110-115°E by the change of thickness of low-velocity layer (residual -0,025 km/sec) from 250 to 400 km. Further to the east, at 30°N velocity boundary is distinguished along the southern margin of low-velocity anomaly with residual -0,075 km/sec. At the surface it partially coincides with the northern border of S China platform. Velocity boundary running along the western margin of mantle block under the Philippines plate, which is distinguished by the change of depth of occurrence of low-velocity layer from 100 to 200 km depth, corresponds to the eastern margin of S China platform. Boundary between S China platform and Indochina is distinguished in the low-velocity upper mantle and transition zone of the upper mantle at 19-23^oN by interlayering of concentric high-velocity and low-velocity anomalies with residual 0,75 km/sec at the 100-600 km depth (belt of anomalies is observed further westwards to 94°E). Velocity boundary corresponding at the surface to the boundary between Indochina and S China platform is not identified in the high-velocity seismic lithosphere. Intrusion of low-velocity anomaly with residual 0, 25 km/sec from the side of S China platform into high-velocity layer of transition zone under Indochina and fault of its consistency come under notice.

At 115-116°E belt of velocity heterogeneities is distinguished in the mantle under S China platform, it reaches deepwater basin, where it shifts westwards by 1° (at 114°E).

At 119°E (starting from 30°N) the velocity boundary is distinguished in the mantle along the eastern margin of S China platform, starting at 23°N to the south velocity discontinuity in the mantle shifts to 120-122°E.

There are no extended clearly distinguished at long distances lineaments of east-west strike in the region. However, the boundary is distinguished in the upper mantle at longitudinal sections. It starts at 23-24°N and goes though Indo-Burma block, Sino-Burma block (26-27°N) and further at 30° N passes northern part of S China platform. Through velocity boundary at 30°N (indentified by bends of velocity isolines) is observed down to the 2500 km in depth.

Limited by the above mentioned mantle velocity boundaries 9-10°N and 19-20°N the mantle of deep-water basin of East Sea is characterized by some peculiar features: in the upper mantle and transition zone of upper mantle the situation of mutual intrusion of high-velocity layer of upper mantle transition zone from the south and low-velocity layer of the upper mantle from the north (S China platform) is observed. Starting at 112°E to 118°E within the range of 10-20°N to the 750 km depth interlaid high-velocity and low-velocity layers - the result of interaction of velocity structures from the south and the north - are distinguished.

III. CONCLUSIONS

In conclusion, we will note the following:

1. According to the seismic tomography model, the mantle of SE Asia is characterized by complicated structure both horizontally and vertically, which is characteristic of Phanerozoic structures. Central and eastern parts of S China platform, East Sea, eastern and central parts of Indochina are regions with two-layer mantle structure consisting of thin seismic lithosphere layer (50-100 km) and low-velocity layer. There are areas where high-velocity layer of seismic lithosphere is absent, and the upper mantle is represented by low velocities down to the 400 km depth. In the upper mantle velocity boundary dividing low-velocity structures of SE Asia and high-velocity structures corresponding to Indo-Australian plate is distinguished.

2. Two-layer structure of upper mantle - thin high-velocity layer corresponding to seismic lithosphere and thick low-velocity layer - characterizes the most part of Indochina - western and central parts. Central part of Indochina block is described by interchange of consistent in thickness high-velocity and low-velocity layers. In the west and east it is separated by the velocity boundaries: at 104-105°E (low-velocity layer of the upper mantle with residual -0,05 – 0,075 km/sec located at 100-400 km depth under Indochina, raised under Khorat block to the depth of 50-350 km) and at 109-110°E (the same raising of low-velocity layer under the block of central part of East Sea is observed (Fig. 2; Fig. 4, 108°E; Fig. 3, 16°N). At that, the velocity boundary 104-106°E has extension in the southern direction under the blocks Natuna, North Kalimantan, and in the north - under the western part of South China, and the one at 109-110°E is observed in the mantle from Kalimantan to S China platform. Under Indochina block southern and northern margins of the central part of the block are detached by the velocity boundaries at 10°N and 20°N (105°-111°E).

3. Latitudinal velocity boundaries 9-10° N, 19-20° N separate southern and northern margins of deep-water basin of East Sea. Limited by the above mentioned velocity boundaries mantle of the deep-water basin of East Sea is characterized by some peculiar features: in the upper mantle and transition zone of upper mantle the situation of mutual intrusion of high-velocity layer of upper mantle transition zone from the south and low-velocity layer of the upper mantle from the north (S China platform) is observed. Starting at 112° to 118°E within the range of 10-20°N to the 750 km depth interlaid high-velocity and low-velocity layers – the result of interaction of velocity structures from the south and the north - are distinguished (Fig. 2; Fig. 4, 114°E).

4. The S China platform is not observed in the mantle as an integral structure, it is divided by the velocity boundaries into blocks. In the mantle under the platform Precambrian core (central block of S China platform - between 22-23°E and 29-30°E) limited by the velocity boundaries 111°E and 118°E and activated margins (Fig.4, 114°E) are distinguished. At 29-30°N the velocity boundary corresponding at the surface to the northern margin of the platform is identified.

5. At 119°E (starting at 30°N) the velocity boundary is distinguished in the mantle along the eastern margin of S China platform, starting at 23°N mantle velocity boundary shifts southwards (120-122°E) Mantle velocity boundaries 120-122° correspond at the surface to the zone of meridional lineament where Luzon-Taiwan chain of Kalimantan-Taiwan archipelago is located.

6. Mantle velocity situation supports the thesis of connection of processes occurring in the crust structures with the mantle. This connection follows from location of fault zones systems in the crust having extension in a form of velocity boundaries in the region’s mantle. Dependence of the mantle structure on fault tectonics is predominant in the velocity structure of the mantle under the studied region. Thus, the following lineaments have found reflection in the mantle structures: lineament 100-103°E - one of the largest ones in Asia, through lineament 109-110°E, lineament at 116°E, lineament stretching in the zone between meridians 120-122°E, as well as a number of lineaments of latitudinal extension.

7. It is worth to mention that distinguished velocity horizons in the mantle only partially correspond to the boundaries of the surface structure. In most cases mismatch of tectonic zonation on the crust and on the mantle structures is observed. For instance, northern and southern borders of Indochina block according to the crust data do not correspond to the mantle velocity structures, boundaries in the west and east correlate only partially.

REFERENCES

1. Bush V.A., 1983. Systems of transcontinental lineaments of Eurasia. Geotectonics, 3: 15-31.

2. Gatinsky J.G., Rundkvist D.V., 2004. Geodynamics of Eurasia: Tectonics of plates and tectonics of blocks. Geotectonics, 1: 3-20.

3. Geyko V.S., 1997. Taylor approach of the wave equation and the eikonal equation in inverse seismic problems. Geoph. Mag., 19/3: 48-68.

4. Geyko V.S., 2004. A general theory of the seismic travel-time tomography. Geoph. Mag., 26/1: 3-32.

5. Geyko V.S, Tsvetkova Т.А., 1989. About uniqueness of the decision of an one-dimensional inverse kinematic problem of seismicity. Geoph. Mag., 11/6: 61-67.

6. Hsu Kenneth J., Li Jiliang, Chen Haihong, Wang Qingchen, Sun Shu, Sengor A.M.C., 1990. Tectonics of South China: Key to understand W Pacific geology. Tectonophysics, 183: 9-39.

7. Kulinich R.G., Zabolotnikov A.A., Markov J.Z., Zhuravlevlev A.V., Zdorovenin V.V., Golovan A.A., Obzhirov A.I., Nikolaev N.A., 1989. Cenozoic evolution of earth crust and tectogenesis of SE Asia. Science, Moscow, 256 pp..