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SCIENCE IN CHINA (Series D)
Vol. 46 No. 5
May 2003
Depth distribution of Moho and tectonic framework in
eastern Asian continent and its adjacent ocean areas
TENG Jiwen ()1, ZENG Rongsheng ()2, YAN Yafen ()1
& ZHANG Hui (
)1
1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100101, China;
2. Institute of Geophysics, Chinese Seismological Bureau, Beijing 100081, China
Correspondence should be addressed to Teng Jiwen (email: [email protected])
Received December 12, 2001; revised October 27, 2002
Abstract With the results of interpretation of seismic sounding profiles acquired in the past 30
years in the continent of China and its adjacent countries and ocean regions, such as Russia, Kazakhstan, Japan, India, Pakistan, Philippine ocean basin, Pacific and Indian Ocean, we compiled a
2D Moho distribution map for the continent and its adjacent areas of eastern Asia. From the features of depth distribution and undulation of Moho, it is suggested that the eastern Asian region
can be divided into 18 gradient belts with different sizes, 18 crustal blocks, 20 sediment basins and
depression zones. The depth of Moho varies smoothly in each block, while the boundary (separating different blocks) delineates the abrupt variation of Moho depth. Then, some subjectssuch as
oregen and sediment basin, fault system and rift, plate boundary, ocean-continent coupling and
tectonic framework, are discussed based on the distribution gradient belts and block partition features of Moho depth in the eastern Asia and its adjacent regions.
Keywords: Moho, block partition, sediment basin, boundary field effect, eastern Asian continent.
It is the essence of the study on the dynamics of eastern Asian continent to investigate the effects of geophysical field and the process of the deep block boundaries, and furthermore, to establish a geodynamic physical-mathematical model based on the depth distribution of Moho. So, special attention has been paid to lithosphere in a series of international projects (incorporating information from geophysics, geological tectonics and geochemistry). In particular, many hot topics
in geosciences have been involved in the researches on lithosphere with remarkable progress in
the International Upper Mantle Project, Geodynamics Project and Lithosphere Project. Numerous
important results and new recognitions about crustal structure and geodynamics have been obtained substantially from the Moho depth images of northern America, eastern and western parts
of Europe[1
4]
. Especially, the study on crustal thickness distribution of Indian subcontinent using
the gravity and some seismic data is useful for understanding the distribution of crustal thickness
of the Qinghai-Xizang Plateau and eastern Asian region[5].
The continent of China is located in the eastern Asia. The studied region (5N55N,
60E150E) in this work included most part of the eastern Asian continent, continental margin
and its southeast ocean, it is a key region for studying the global tectonics and continental dynam-
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ics. Therefore, many authors have compiled the maps of distribution of crustal thickness[6
10]
.
However, these maps are mainly compiled based on the results of the inversion from Buger gravity anomalies. These data of gravity observation suffer from the strong interference of the topography undulation, the difference between different periods results in different precisions of the
corresponding observations. In the other respect, the results of inversion are based on the simplified model of infinite homogenous parallel slab. In recent years, the regional distribution characteristics of crustal thickness in China have been obtained from deep seismic sounding (DSS),
earthquake data and gravity measurements in part (in the regions lacking for seismic data)[11
13]
.
Based on the data of surface wave of earthquakes, a 3-dimensional velocity structure in China and
surrounding areas was obtained. Recently, another map of Moho depth distribution drawn based
on the data of gravity survey has been published. The compilation of this map is on the basis of
the existing gravitational maps of China and the former Soviet Union, according to the data of
Geoscience Transect in China, as well as the gravity and geological tectonic data[14]. In this map, a
series of blocks are drawn, and the relationship between the distribution of Moho depth and the
volcano, and earthquake, as well as tectonics, is discussed. According to the outline map of distribution of Moho discontinuity from the data of seismic sounding profiles carried out before
1990[14], China continent is divided into 8 blocks[11,14]. The above-mentioned Moho depth maps
compiled based on various geophysical data are of importance for the study of the tectonic pattern
and continental dynamics features. At the same time, it must be admitted that, it is limited in respects of the amount and dependability of available information in the compilatory process of
these maps. In recent years (up to 1999), the reliable depth of Moho was obtained, with the precision of 1 km, from seismic phases Pm and Pn by using deep seismic sounding profiles with
high quality in the area of eastern Asia. With such a prerequisite, there is a great possibility to obtain the reliable fine structure and distribution features, which will play a significant role in the
study on deep structure, tectonics, deep process and continental dynamics in eastern Asia.
Since the 1970s of last century, seismic sounding profiles with the total length of 50000 km
(including seismic reflection profile over 2000 km long) have been carried out by the Chinese
Academy of Sciences, Chinese Seismological Bureau and the Ministry of the Land and Resources
29]
of China[6
. These data (up to the year of 2000) provide an important base for overall recogni-
tion of Moho undulation and new discoveries in this study. The location and distribution of the
profiles are certainly uneven (see fig. 1). In some study areas, there are too many overlapping profiles, so we must select the data by comprehensively analyzing them.
1
Principle and purpose for compiling Moho depth map of eastern Asia
The understanding of the structure of the earth’s interiors, tectonics, plate movements and its
driving mechanism has been greatly transformed with the presentation of plate tectonics theory in
the 1960s of last century. The theory of plate tectonics renewed the traditional ideas of mankind
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Fig. 1. Distribution of seismic sounding profiles in eastern Asiadeep seismic sounding profile.
about the earth, and furthermore revealed the fact that, on the global scale, the movement and deformation of the lithosphere plates (outer circle of the earth) are dominated by the pattern of horizontal movement. In recent years, the discovery of the upwelling of effusive basalt under continent and ocean and mantle plumes compelled us to take the contribution of vertical force into account. So, unifying the radial and horizontal movements is very important for deepening our understanding about the earth itself, the tectonic pattern of intraplate, the resource, hazard, environment variation and global change, since the key of the study on geodynamics is to study the exchange of material and energy in the earth interior.
1.1 Purposes of mapping the crustal thickness (Moho depth)
(i) The map should provide important information of the deep intraplate for the study on
block zoning, tectonic framework, resource distribution, nature hazard and continental geodynamics in eastern Asia.
(ii) To compile the first detailed map of crustal thickness for eastern Asia using the updated
and reliable data.
(iii) By widespread application of the map to discover and propose new problems from block
zoning, analysis and interpretation of the coupling between the shallow and the deep structures,
the block boundaries effect and continental geodynamics in eastern Asia.
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1.2
Principles and rules for compiling the map of crustal thickness
(i) Based on the theory of plate tectonics, the compilation of the map of crustal thickness distribution in eastern Asia and its adjacent areas should embody the update results of the study on
lithosphere in China and surrounding regions, and emphasize the tectonic features of intraplate.
(ii) The data obtained from deep seismic sounding are used as the criterion to compile the
map.
(iii) The surface tectonic pattern is not taken into account so as to keep the regularity and
deep process revealed by geophysical data.
(iv) The weighted interpolation controlled by sufficient precise points was used in the regions
sparsely covered by observations data to ensure the collected data to be more reasonable and reliable. Additionally, other geophysical data were used as the reference, but not the standard.
(v) For the regions with discrepant data, we selected the data by analyzing the overlapping
data in small regions and other geophysical observations and taking into consideration the rationality of the data in surrounding areas.
(vi) On the southern margin of eastern Asia, especially the oceanic regions where seismic
sounding data are lack, data of the results by comprehensively analyzing the natural earthquakes,
potential field and large-scale seismic surface wave tomography are supplemented.
(vii) The permitted error range of contour is 2 km.
1.3
Basis and steps of compiling crustal thickness map
(i) The crustal thickness maps of various editions were collected for getting key points. This
can lead to the principles for compiling the map.
(ii) Collecting the data of seismic sounding profiles and Moho depth in the areas concerned
both at home and abroad to depict them on the sketch map showing the distribution of seismic
sounding profiles.
(iii) Considering the various resolutions of the blocks caused by the unevenness of data distribution and different resolutions of blocks to nomalize them on a background map with scale of
125000000.
(iv) After data treatment by the least square method, the contour map of the crustal thickness
is drawn.
(v) Analyzing, studying and checking the singular points (zones) on the map to compile a
formal map of crustal thickness.
1.4
Collection of data of crustal thickness
(i) According to the spatial distribution of seismic sounding profiles in the continent of China,
the unequal spacing interval method was used to collect crustal thickness in the areas with abrupt
variation of Moho depth and the zones with knee points. Totally, 3000 data were collected with
point intervals of 2050 km.
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(ii) In the oceanic areas and the regions surrounding China continent, the depth data of Moho
discontinuity were mainly collected from the previously published papers. Totally, 1000 data were
collected with point intervals of 50100 km.
1.5
Data source of the study of Moho undulation
Eastern Asia and its adjacent sea areas cover a very large region. A lot of work and investigations have been done on this region and also many publications were reported on the Moho discontinuity. The length limitation of the paper can only allow us to list some of them.
2 Basic pattern of the crustal thickness of eastern Asian continent
The overall tendency of the crustal thickness distribution in eastern Asian continent can be
briefed as follows: the crust is thin on the margin of the continent (just 10 km in its adjacent oceanic regions), thickens gradually toward the inland, and then becomes the thickest (about 74 km)
in the central part (the Tibetan Plateau). From fig. 2, we could see several belts with dense contour
Fig. 2. Thickness map of the crust under eastern Asia and its adjacent ocean areas. The unit of contour map: km.
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lines and also some regions with smooth and slight variation of crustal thickness in eastern Asia.
This implies the strong variation of the crustal thickness. For the convenience of discussion, we
divide the eastern Asia into four regions. They are the northern continent region of eastern Asia
(45N55N, 60E140E), the central continent region of eastern Asia(20N45N,70E
120E), southeast part of eastern Asia and marine region (10N22N, 75E130E), the
easern part of eastern Asia and marine region (10N55N, 125E140E). Each region can be
further divided into several blocks which have their own specific features.
2.1
Northern continental region of eastern Asia (bounded by 45N55N, 60E140E, and
its adjacent area)
From west to east, the main part of this region includes the southern margin of the west Siberia plain, the Kazakhstan hill and its southern neighboring regions (such as Balkhash Lake, Isaac
Lake and Baykal Lake), the Junggar Basin in China, the northern area of the Mongolia Plateau,
and the belt of Altay-Mongolia Plateau-Da Hingan Ling Mountains-Northeast Plain. The crustal
thickness of western part is the largest (48 km) of this region. Then crust reduces its thickness in
north, east and south directions. The crust thickness of Junggar Basin is 42 km. Baykal Lake has a
crustal thickness of 35 km. However, the values of the regions along this lake increase to 4044
km. Da Hinggan Ling is a gradient belt of crustal thickness. Its thickness ranges from 34 to 36 km.
The Northeast Plain is 3234 km in crustal thickness. There is an uplift zone of upper mantle at
the Daqing Oilfield. The crustal thickness there is 32 km.
2.2 Central continent region of eastern Asia (20N45N, 70E120E)
The main body of this region is the inland continent of China, which includes Pamir and the
Tibetan Plateau (with the thickest crustal thickness of 74 km in the central area) in the western
part. The region is separated from the Indian subcontinent by a gradient belt with a great variation
of crustal thickness. The plateau is separated from the Tarim Basin by a highly gradient belt with a
great variation of crustal thickness. The central part of the region is bounded by a nearly NS
striking gradient belt with a great variation of crustal thickness on the eastern margin of the
Tibetan Plateau. To the east, it neighbors with the Ordos block, the Sichuan Basin, Yangtze and the
South China blocks. The crustal thickness decreases abruptly from 44 km under the NS striking
velocity gradient belt to 2830 km on the eastern margin of the continent[15].
2.3 Southeast part of eastern Asia and marine region (10N22N, 75E130E)
This area mainly covers the Indian subcontinent, the Bay of Bengal, countries in the Indochina Peninsula, including Pakistan, Burma, Thailand, Laos, Cambodia, and Viet Nam, the southern margin of the Yunnan-Guizhou Plateau, the Hainan Island and the Beibu Gulf of China. The
crustal thicknesses are 3644 km in the Indian subcontinent, 4044 km in the central India with
smooth variation, 3632 km in Pakistan and 1830 km in the Bay of Bengal, which looks like a
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wedge whose tip is towards north. The area is separated from the Alagan Mountain to the east by a
nearly NNW striking gradient belt of crustal thickness. Its crustal thickness varies from 36 km to
38 km. The range of crustal thickness is 3438 km in the Indochina Peninsula. As for the south
part of the peninsula, the range of crustal thickness is 3236 km and the extension of contour
lines strikes NNW and nearly NS. To the north, the area covering Burma and Laos is a depression
block with crustal thickness of 38 km. To the east, the southern margin of the Yunnan-Guizhou
Plateau with a crustal thickness range of 3644 km strikes in nearly WE direction. There is a
transition belt with crustal thickness jump of 10 km located between southern margin of the YunnanGuizhou Plateau and the southern margin of South China block.
2.4
Eastern part of eastern Asia and marine region (10N55N, 125E140E)
From north to south, this region includes the Sakhalin Island, the Okhotsk Sea, the Kurilskiye
Islands, the Japan Sea, the Japan Islands, the Korea Peninsula, the Yellow Sea, the East China Sea,
Ryukyu-gunto Islands, the Taiwan Island, the Philippine Sea Basin, the South China Sea and the
Philippine Islands. The range of crustal thickness is 2834 km at the Sakhalin Island and the
Okhotsk Sea, the crustal thickness is 12 km at the central part for the Japan Sea. The thickness of
Japan is 20 km in north part. It increases southward and reaches 36 km, 3428 km at the north
part of the Korea Peninsula (D. P. R. Korea), 2830 km at the south part of the Korea Peninsula
(R. O. Korea). The crustal thickness is 1824 km at the Yellow Sea and East China Sea. The
crustal thickness is 24 km at the Taiwan Strait and 28 km at the Taiwan Island. The Philippine Sea
Basin and the region east to the Ryukyu-gunt belong to oceanic crust. Its range of crustal thickness
is 1018 km. To the south, the Philippine Islands show a broom-shaped pattern with an NS striking gradient belt of crustal thickness (with thickness range of 1224 km). The densely distributed
contour lines in the north of the area seem to be the stem of the broom. The contour lines disperse
southwards at the position of 15N and form a fan-like pattern of the broom composed by Sulu
Sea, Laut Sulaweisi, Laut Maluku, Laut Jawa and Laut Banda. The tectonic framework shows the
positive correlation between the principal compressive stress axis and the distribution of the active
and inactive micro-plates’ subduction. The broom-shaped and densely contour belt of crustal
thickness separates the South China Sea from the Philippine Sea. The crustal thickness varies
within the range of 2024 km in the South China Sea, 2024 km in the northern margin (near
20N), 10 km or so in the Zhongsha Islands and 1824 km in the Nansha Islands.
Generally, the crustal structure in eastern Asia and adjacent area can be divided into continental (crustal thickness is greater than 30 km), subcontinental (crustal thickness is greater than 15
km and less than 30 km) and suboceanic (crustal thickness is greater than 10 km and less than 15
km) types. The oceanic crust (thinner than 10 km) mainly distributes in the Pacific.
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3 Block division of crustal thickness for eastern Asian continent
The distribution of crustal thickness at the continental and oceanic areas of eastern Asia depicts that this area can be divided into 18 blocks. The Moho depth varies slightly within each
block, but their crustal thickness changes abruptly at their boundaries.
3.1
Distribution features of steep gradient belts of crustal thickness contours
The crustal thickness of east Asia varies strongly. The contours strike in many directions. The
gradient variations of the belts of densely distributed contours are quite heterogeneous. They have
different intensities, directions and shapes. They are the boundaries of different blocks. 18 steep
gradient belts of crustal thickness were recognized for eastern Asia (fig. 3).
Fig. 3. Steep gradient variation belts of crustal thickness in eastern Asia and its adjacent oceanic areas. Number X and so
on represent the steep gradient variation belts of crustal thickness. V, X, etc. are serial numbers of gradient belts.
3.1.1 Kazakhstan Hill-Altay Mountains steep gradient belt (belt I in fig. 3). The gradient variation of crustal thickness is smooth in this belt, and its crustal thickness varies within the range of
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3848 km. The gradient belt strikes in nearly east-west direction, and extends eastward after intersecting orthogonally with gradient belt IX with striking in nearly NS direction.
3.1.2 Tianshan Mountains steep gradient belt (belt II in fig. 3).
The gradient variation of belt
II with striking in nearly EW direction is stronger than that of belt I. Its crustal thickness ranges
between 48 km and 53 km. It is located between the Junggar Basin to the north and the Tarim Basin to the south. It is a long and narrow belt where the crust is about 10 km thicker than that of the
basin area.
3.1.3 Altun Mountains steep gradient belt (belt III in fig. 3).
The gradient variation of the
belt is very strong and strikes in NNE direction. Its crustal thickness ranges within 64 km and 48
km. It separates Junggar and Tarim basins and forms the typical pattern: the uplifting Moho beneath the sediment basin while depressing under orogenic belt.
3.1.4 Himalaya steep gradient belt (belt IV in fig. 3).
The gradient variation of this belt is
very strong and wide, and strikes in nearly EW direction. The crustal thickness ranges between 64
km and 48 km. It is a transition belt of collision and compression between two plates (the plates of
northern margin of the Ganges River and the Tibetan Plateau)[16], and also the strongest variation
belt of modern tectonic movement. The exchanges of energy and material are strongest.
3.1.5 Steep gradient belt on the eastern margin of Indian Peninsula (belt V in fig. 3).
The
gradient variation of the belt is relatively strong, and strikes in NE direction. The crustal thickness
ranges between 32 km and 40 km. The belt is the boundary between the Indian subcontinent and
the ocean. At the north of the belt is the Himalaya area with abrupt thickening of crustal thickness.
3.1.6
Alagan steep gradient belt (belt VI in fig. 3).
The gradient variation of the belt is smooth
and small in scale, and strikes in NNW direction. The crustal thickness ranges within 3640 km.
The Bay of Bengal is located between the two gradient belts of V and VI.
3.1.7 Steep gradient belt of eastern Indochina Peninsula (belt VII in fig. 3).
The gradient
variation of the belt is relatively smooth, and strikes in nearly NS direction. The crustal thickness
ranges between 28 km and 32 km. The belt combining with the Alagan belt (belt VI) forms the
continental crust area of the southern part of eastern Asia.
3.1.8 Philippine Sea steep gradient belt (belt VIII in fig. 3).
The gradient variation of crustal
thickness contour of the belt is relatively abrupt, and strikes in NS direction. The crustal thickness
ranges between 12 km and 24 km. Northward it connects with the Taiwan Island. The broomshaped gradient belt of contour in this area separates the South China Sea and the Philippine Sea.
3.1.9 Baykal-Seleng River steep gradient belt (BS) (belt IX in fig. 3).
The gradient variation
of crustal thickness of the belt is relatively strong, and the belt strikes in nearly SN direction. The
crustal thickness thickens from the north to the south (4048 km), and is 38 km at the Baykal
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Lake. It extends southward to connect with the south-north tectonic belt, then further southward
with gradient belt VII to form a huge tectonic belt (IX+XI+VII) striking in NS direction and crossing the eastern Asia.
3.1.10 Da Hinggan Ling Mountains steep gradient belt (belt X in fig. 3).
The gradient variation of crustal thickness contour lines is relatively strong and the belt strikes in the NNE direction.
The crustal thickness varies between 34 km and 42 km. At the surface, it is the Da Hinggan Ling
orogenic belt. At the place of about 55N, its north part connects with the Outer Hinggan Ling
Mountains tectonic belt striking in nearly EW direction.
3.1.11 South-north steep gradient belt (belt XI in fig. 3).
The gradient variation of crustal
thickness contour lines is extremely strong and strikes in the NNE direction. The crustal thickness
varies between 44 km and 60 km. The belt is the boundary between the eastern and the western
parts of China continent. Further study is needed for checking the possibility whether it extends
northward to the Baykal Lake, and southward to connect with the steep gradient belt on the eastern margin of the Indochina Peninsula and then to the South China Sea.
3.1.12 Taihangshan-Xuefengshan-Yunkaidashan steep gradient belt (belt XII in fig. 3).
The
gradient variation of crustal thickness contour lines is relatively strong and strikes in the NNE direction. The crustal thickness varies between 36 km and 44 km. The belt may connect with Da
Hinggan Ling Mountains steep-radient-belt (X) northwards to form a huge gradient belt striking in
NNE direction and crossing the eastern part of eastern Asia continent.
3.1.13 Sikhote gradient belt (belt XIII in fig. 3).
The contour of crustal thickness parallels
with the Yilan-Ytong steep gradient belt and forms a narrow gradient belt with strikes of NE and
the gradient variation of crustal thickness contour is relatively smooth. The crustal thickness varies between 30 km and 34 km. The former belt is a continent-ocean transition belt of eastern Asia,
which turns over in the South Korea area, the latter extends from north to south to cross the Pohai
Bay and South China and finally to reach the Hainan Island block.
3.1.14 Yanshan steep gradient belt (belt XIV in fig. 3).
The gradient variation of crustal
thickness contour striking in NWW-EW direction is relatively strong. The crustal thickness varies
between 28 km and 34 km. Cut by a nearly NS striking tectonic belt, the belt is not continuous. It
is shown in fig. 3 that the belt may extend westwards. The relationship between the belt and the
Ordos tectonic belt is still being debated.
3.1.15 Steep gradient belt of southeastern continental margin (belt XV in fig. 3).
The gradient variation of crustal thickness contour striking in NE direction with eastward arc-shaped
prominence is smooth. The crustal thickness varies between 28 km and 32 km. The belt is a transition belt of the dynamical boundary of the southeastern continental margin of eastern Asia. The
magma activity is strong, and may be the cause for few earthquakes in South China.
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3.1.16 East China Sea steep gradient belt (belt XVI in fig. 3).
The gradient variation of
crustal thickness contour striking in NEE-EW direction is smooth. The crustal thickness varies
between 16 km and 24 km. In nearby EW direction the belt is the tectonic boundary between the
East China Sea and the Yellow Sea, the Japan Islands and the Ryukyu-gunto Islands.
3.1.17 Bashi Channel steep gradient belt (belt XVII in fig. 3).
The gradient variation of
crustal thickness contour lines striking in NW direction is smooth. The scale of the belt is small.
The crustal thickness varies between 12 km and 14 km. It is a boundary between the Philippine
Sea Basin and the Ryukyu Trench to the north.
3.1.18 Steep gradient belt of Japan and neighboring islands (belt XVIII in fig. 3).
The belt
strikes in NNE-NE direction. The crustal thickness varies between 20 km and 36 km. The belt is
the transition belt of the Japan Islands and Pacific marine areas.
3.2
Variation of crustal thickness, block partition and its feature of eastern Asia
According to the distribution of steep gradient belts, we divided eastern Asia into 18 blocks
(fig. 4).
3.2.1 Block A.
It is mainly composed of the Balkhash Lake and the Junggar Basin with the
strike of nearly EW. Its range of crustal thickness is 4244 km, which is 46 km thinner than
that of the surrounding orogenic belt. The block is located in the uplift zone of the upper mantle
and belongs to the Kazakhstan plate (A) and borders upon the Mongolia block (H) to the east.
3.2.2
Block B.
It is composed of the Tarim Basin with the strike of nearly EW. The crustal
thickness of the main body of Block B is 44 km, which is 810 km thinner than that of the surrounding orogenic belt. The block (old continental nuclear) is a separation from south to north for
the material movement of crustal and mantle and it is also the dynamic boundary of northern part
acting on the Tibetan Plateau block and compressing southwards.
3.2.3 Block C.
The main body of the block is the Tibetan Plateau. The block is subdivided
into the Qiangtang, Tsoqing and Tarim basins. From north to south, the crustal thickness is 52 km
in the north part, 68 km in the north-central part, 74 km (south-central part) and 56 km in the south
part, respectively. The concave downward at the centre of Moho with the strike of EW forms a
quasi-symmetrical pattern with a depression in the central part. The block is named the Tibetan
Plateau block.
3.2.4 Block D.
It is located in the south of the Tibetan Plateau, namely the Ganges Plain and
the Indian subcontinent. The crustal thickness is 44 km in the central part and thins toward the
eastern, the western and the southern margins of the block. It is also named the Indian subcontinent.
3.2.5 Block E.
It is the Bay of Bengal between the eastern margin of the Indian plate and the
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Fig. 4. Block partitions in eastern Asia and adjacent areas. The letters in capital on crustal thickness contour lines
represent the partitions according to the crustal thickness, namely the blocks.
western margin of the Indochina Peninsula. Crustal thickness of the continental and oceanic areas
thins from north to south (36 km18 km), forming a wedge-shaped pattern. Crustal thickness of
the Pakistan zone in the north to the top of wedge ranges between 32 km and 36 km. The block is
surrounded by three gradient belts: striking NNE gradient belt (western margin), striking NNW
gradient belt (eastern margin) and striking EW gradient belt (northern margin), and could be
named the Bay of Bengal as well.
3.2.6
Block F.
The main body of the block is the Indochina Peninsula. It covers many coun-
tries in northern part of the peninsula. The crustal thickness is from 32 km to 36 km. At central
part of the peninsula it reaches 38 km. The southern part is a narrow plateau belt with relative uplifting.
3.2.7 Block G.
The crustal thickness ranges from 10 km to 16 km in the central part of the
South China Sea block and thickens abruptly northward to the Hainan Island and southward to the
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Nansha Islands. The submarine risen belt is formed in the central uplifting zone. Its ridge runs in
the NNE direction. The block is named the South China Sea block.
3.2.8 Block H.
It is the Mongolia Plateau block. The variation of crustal thickness is smooth.
The crustal thickness of almost the whole plateau ranges between 40 km and 48 km. Located between the Baykal Lake to the north and the Da Hinggan Ling, it is an uplift belt as a whole. The
western part of the block between Altay Mountains and Ulan Bator is an uplifting zone of upper
mantle with thick sedimentation.
3.2.9
Block I.
With the strike of nearly EW at the central part, this block is mainly com-
posed of the Ordos and Fenwei rift system. The crustal thickness is about 4244 km. The block is
a wide and smooth variation zone between eastern and western China steep gradient belts of
crustal thickness in China, and also the intersection zone of two deep gradient belts with strikes of
nearly NS and EW, respectively. The strikes of deep tectonic belts in both the northern and the
southern parts of the block are perpendicular to the block. Although the block belongs to the
China-Korea block, its crustal thickness is thicker than that of North China.
3.2.10 Block J.
Equivalent to the Yangtze block, it basically consists of the Sichuan Basin,
the Chuan-Dian tectonic belt and the western margin of the South China block. The crustal thickness is within the range of 3652 km, it is 4042 km in the central part. It is a belt with strong
variation of crustal thickness in the step form of undulation, and the deep and shallow tectonic
frameworks of the block are very complicate. It connects with the Daba Mountains and the
Qinling Mountains to the north and the Indochina Peninsula and South China Sea block to the
south; borders upon western border of the South China block to the east and the Aba-Maerkang
region to the west with a boundary of the Xiaojiang fault. The crustal thickness increases steeply
westward.
3.2.11
Block K.
It is the northeastern block of China with the range of crustal thickness of
3234 km, a broad area with smooth Moho variation. The crustal thickness at Daqing and Fuyu
oilfields is 24 km thinner than that of the surrounding areas, presenting as an uplifting zone.
3.2.12 Block L.
It is the Japan Sea block. The crustal thickness is 12 km in the central oceanic area, and reaches 38 km as the thickest part in the northern part of the Japan territory, 36 km
in the southern part, and 20 km in the surrounding areas. So, the crustal thickness varies relatively
greatly. The block is located between continental and sub-continental crusts.
3.2.13
Block M.
It is the Pohai Bay block. As a particular block, the crustal thickness is 28
km in the central part and 3234 km in the surrounding areas with a central uplifting.
3.2.14
Block N.
It is the southeastern block of China with crustal thickness between 25 km
and 34 km, and strikes in NNE direction. The Moho discontinuity uplifts in the central part of the
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block. The difference of the crustal thickness between Block N and the surrounding areas is about
24 km. This block also includes the Jianghan and Hefei basins. After crossing the Yangtze River,
the Tan-Lu deep fault with crustal thickness of 3234 km extends southwards along the strike of
nearly NS to the South China Sea at the Hainan Island finally.
3.2.15
Block O.
This block consists of the Taiwan Island, the East China Sea, and the Yel-
low Sea, and is a transition zone from continental to suboceanic crusts. The crustal thickness
ranges from 18 km to 28 km. Gradient belt XVI separates Block O from the block of suboceanic
crust.
3.2.16
Block P.
It is the east block of the Ryukyu Trench. Its crustal thickness is within the
range of 1014 km. It belongs to oceanic and suboceanic crusts. With the southern boundary of
Bashi Channel, it is separated from the Philippine Sea Basin (Block Q).
3.2.17
Block Q.
It is the block of the Philippine Sea Basin. The crustal thickness of this
rhombus sea basin is about 1016 km. With the Philippine Islands as boundary, it is separated
from the South China Sea block (Block G). This is a transition zone from suboceanic and oceanic
crust to oceanic crust, and shows more typical features of oceanic plate.
3.2.18 Block R.
It is composed of the Sulu Sea-Selat Sulaweisi Island, Laut Jawa and Laut
Banda and is a triangle zone bounded by the NW striking Maluku Islands in the east, the Nansha
Islands and the NE striking Zengmu Shoal in the west, Laut Jawa, Kep Nusa Tenggara islands in
the south. The crustal thickness is 20 km in the northern part of the block and 16 km in the Sulu
Sea, and then thins gradually southward. It is a special block between the Philippine Sea and Laut
Jawa, Laut Banda, and the separation ridge between the South China Sea and the Philippine Sea,
belongs to the suboceanic type crust.
Blocks S1 and S2 belong to the Western Siberia Plain and Central Siberia-Sayan Ridge Plateau blocks, respectively. The crustal thickness is within the range of 4044 km. As they are located on the northern margin of the studied area, we do not discuss it in detail in this paper.
It should be mentioned that blocks P and Q belong to the basin area of the Philippine Sea as a
whole, a rhombus block (the Philippine Sea Plate) composed of the Ryukyu-gunto Islands, the
Philippine Islands, the Japan Trench-Mariana Trench, and Yap Island-Maluku Islands.
The analyses of Moho depth distribution, its shapes and strikes of steep gradient belts, the
block partition and the boundary field effects demonstrate that all these blocks can be bounded by
the steep gradient belts. Within some blocks, the crustal thickness varies strongly. This is caused
by differentiation, deep material readjustment and inhomogeneous exchange of material and energy from the strong collision and compression in Paleozoic. The opening-closing of ancient
Tethys and the aggregation and subduction of Pacific in Cenozoic is the main reason[16
18]
.
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3.3 Distribution features of sediment basins and Moho depth of eastern Asia
In eastern Asia, crustal thickness (Moho depth) of orogenic belts is quite different from that
of sediment basins. Crust is thicker at orogenic belts, while thinner at sediment basins. The tectonic pattern of Moho uplift exists widespread at the sediment basins[19
22]
. From fig. 5, we can
see that there are 20 zones where the crustal thicknesses are relatively thinned (Moho uplift
zones).
Fig. 5. Distribution of the main sediment basins in eastern Asia and its adjacent oceanic areas. B4, B17 et al. are
the serial numbers of sediment basins.
1) Bs1 is the Tyumen Oilfield and its adjacent areas in Russia. The crustal thickness is 3640
km. The Moho uplifts 48 km.
2) Bs2 is the Baykal rift belt. Its range of crustal thickness is 3638 km. The Moho uplifts
48 km with respect to its surrounding areas.
3) Bs3 is the Balkhash Lake. Its crustal thickness is within the range of 4244 km and Moho
uplifts 46 km with respect to the surrounding areas.
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4) Bs4 is the Hyargas Nuut-Dzavhan River-Tayshir areas in the western part of Mongolia. It is
a slight depression zone at the top of upper mantle with smooth variation between the Altay
Mountains and Ulan Bator. Showing as an eastward-rise monocline tectonic belt, its crust thins
eastwards.
5) Bs5 is the Junggar Basin. Its crustal thickness is 42 km. Moho uplifts 48 km with respect
to the surrounding areas.
6) Bs6 is the Loess Plateau zone. Its crustal thickness is about 4042 km. Moho uplifts 24
km with respect to the surrounding areas.
7) Bs7 is the Northeast Plain with the Daqing and Fuyu basins as the main blocks inside. The
range of crustal thickness is 3032 km. Moho uplifts 46 km with respect to the surrounding
areas.
8) Bs8 is the Japan Sea. Its crustal thickness is 12 km and Moho uplifts at least 10 km with
respect to the surrounding areas.
9) Bs9 is the Pohai Bay Basin. Its crustal thickness varies within the range of 2832 km and
Moho uplifts 46 km with respect to the surrounding regions.
10) Bs10 is the Tarim Basin. Its range of crustal thickness is 4244 km and Moho uplifts 5
10 km with respect to the surrounding areas.
11) Bs11 is the Qaidam Basin. Its crustal thickness is 52 km and Moho uplifts 48 km with
respect to the surrounding areas.
12) Bs12 is the Qiangtang Basin. Its crustal thickness is 68 km and Moho uplifts 46 km
with respect to that in its southern areas as a northward uplift monocline tectonic pattern.
13) Bs13 is the Sichuan Basin. Its range of crustal thickness is 4042 km and Moho uplifts
24 km compared with the western regions.
14) Bs14 is located in South China with Hefei and Jianghan basins as the main blocks inside.
The crustal thickness is 32 km and Moho uplifts 24 km with respect to its surrounding areas.
15) Bs15 is the continental slope, offshore of the southeastern China and some parts of the
oceanic regions. From north to south, there are the Yellow Sea, the East China Sea, the Taiwan
Strait, the Hainan Island, the Ryukyu-gunt and its adjacent sea areas westwards with a series of
sedimentary basins in marginal sea. The crustal thickness is within the range of 1020 km. From
east to west, there are three uplift zones with crustal thicknesses of 10 km, 18 km and 20 km, respectively, and the Taiwan Strait uplifting belt. The Moho uplifts 46 km with respect to the surrounding areas.
16) Bs16 is the South China Sea Basin and the Beibu Gulf area. The crustal thickness is within
the range of 1016 km in the central area and Moho uplifts 48 km with respect to the surrounding areas.
17) Bs17 is the broad and flat Philippine Sea Basin. The crustal thickness is within the range
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of 1012 km and Moho uplifts 24 km with respect to the surrounding areas.
18) Bs18 is the Ganges Plain. The crustal thickness is about 36 km. It strikes in the EW direction. Moho uplifts 4 km with respect to the surrounding areas.
19) Bs19 is the Bay of Bengal. The crustal thickness is about 18 km in the central part. Moho
uplifts 68 km with respect to the surrounding areas.
20) Bs20 is the area of Sulu Sea and Selat Sulaweisi. The crustal thickness is about 12 km and
Moho uplifts 48 km with respect to the surrounding areas.
By analyzing the 20 sedimentary basins or depressions above (Moho uplift zones), it is seen
that many sedimentary basins are known oilfields explored. They are Bs1 (Tyumen oilfield of Russia), Bs7 (the Daqing and Fuyu oilfields), Bs5 (the Karamay oilfield), Bs10 (the Tarim oilfield), Bs11
(the Qaidam oilfield), Bs9 (the Pohai oilfield), Bs15 (the East China Sea and the Yellow Sea) and
Bs16 (the South China Sea) which are the petroleum fields in the adjacent sea of China. However,
in some other sedimentary basins and crustal uplift zones, no oilfield has been found yet. These
basins include Balkhash Lake (Bs3), Hyargas Nuut-Dzavhan River-Tayshir area in the western part
of Mongolia (Bs4), the Japan Sea (Bs8), the Qiangtang Basin (Bs12), the Taiwan Strait and two uplifts in its western sea area (Bs15), Ganges Plain (Bs18), the Philippine Sea Basin (Bs17), the Bay of
Bengal area (Bs19) and Sulu Sea and Selat Sulaweisi (Bs20). Therefore, these basins and Moho uplift zones may be the prospecting places for finding oidfields and should be paid special attention
to.
4 Conclusions
Based on the distribution of Moho depth, deep process and the boundary effects, we divided
the eastern Asia into 18 gradient belts and 18 blocks. By analyzing and studying the blocks and
gradient belts, it is suggested that there are 20 basins or Moho uplifting zones in the eastern Asia.
They belong to the regions of convergence, mixing and alternating of blocks with continental,
subcontinental, suboceanic and oceanic crusts. It is obvious that whether in the orogenic belts and
23]
basins, or in the Tibetan Plateau and on the southeastern margin of the continent[20
, the varia-
tion of the crust-mantle boundary (Moho discontinuity) is very obvious and strong. This implies
that the distribution of crustal thickness is inhomogeneous and anisotropic in both vertical and
horizontal directions[23
29]
.
This study used the deep seismic sounding data as the basis. The distribution of crustal
thickness in eastern Asia is obtained by inversion. It is noted that 3D distribution and coverage of
observational areas are heterogeneous. So, even though we compiled the first distribution map of
the crustal thickness in eastern Asia with resolution of 2 km based on high-resolution seismic
sounding data at principal parts, it is still a primary result and necessary to be improved by new
data.
The presentation of the distribution of crustal thickness in eastern Asia will play an important
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guiding role for the study on deep background of seismic activity, stress distribution, deep process
of tectonic movement, tectonosphere and continent-ocean coupling. This result will helpful to understanding tectonic patterns from shallow to deep and geodynamical system in eastern Asia. It is
profoundly significant for the investigation on the biotic province, tectonic framework, potential
prospect of resource and energy, earthquake hazard, atmospheric transition and global change.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
40074020) and the National Basic Science Research Project (Grant No. 95-S-05).
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