The study area, located in 20°N to 41°N and 115°E to 130°E, includes the Bohai Sea, Yellow Sea, East China Sea, and parts of the South China Sea, Sea of Japan, the Philippine Sea and their adjacent regions. It involves the Eurasian plate, Philippine Sea plate and converging plate boundary consisting of trenches, isl and arcs and back arc basins. The continent shelves of the Bohai Sea, Yellow Sea and East China Sea are also the extensions of Eastern China mainl and toward into the seas. In the tectonic framework of China mainl and (Liu, 2007), this study area is located east of the gradient zone along the Da Hingan Ling-Taihang Shan- Wuling Shan mountains. Before the pattern-changing motion(Zhu, 1987), the collisions and convergence of North China, Yangtze, South China blocks and the South China Sea left signs in Tethys tectonic system. The structural features generated from these merging plates were well preserved there. During the period of new global tectonics, the study area was situated in the Circum Pacific tectonic domain, and exhibited the characteristic of the Jurassic. The Pacific plate sustainably grew among the four transform faults with a NS trend. During the Eocene, these faults trended in NW-SE, and the plate of Philippine Sea formed in the system of the Mariana trench, isl and arc and back arc basin. Lithosphere between Eurasian and Philippine Sea plates was thinned due to the convergence of plate boundary and extension within plates. It became a part of the Circum Pacific tectonic domain, and a series of fault basins were formed in China mainl and (Zhang et al., 2009). The imbrications of the Tethys and Circum Pacific tectonic domains in various geological periods developed as a result of strong crustal activity and complicated geological and geophysical structures in the seas east to China mainl and (Liu, 1992b).

Geophysical research is one of the most important means to delineate the regional tectonics in sea areas. The 3D structure model of China and adjacent areas using 3D surface and shear wave velocity inversion indicates that the lithosphere and asthenosphere of eastern China(105°E-110°E)exhibits the characteristics of mosaic structure(Zhu, 2002). The eastern part with thin lithosphere and thick asthenosphere is distinct from the western part with obvious layered structure with thick lithosphere and thin asthenosphere. According to the macro gravity and magnetic features, the East China mainl and and adjacent areas can be divided into two areas(pericontinental area and marginal sea transitional region), three strip zones and five blocks(Hao, 1997). The blocks with different geological structures demonstrate distinct differences in geophysical features, such as gravity and magnetic fields, velocity structure and electrical properties of lithosphere. During the compilation of map series of geology-geophysics and geochemistry of China seas and adjacent areas(Zhang, 2010; Wen, 2011), new gravity and magnetic data covering the Korea Peninsula, Taiwan Isl and , Japanese sea and Philippine sea have been added into the maps. Based on these data, we analyzed the regional gravity and magnetic anomalies, and calculated the Moho and Curie interface by inversion of gravity and magnetic data. Then, the distribution of thermal lithosphere base was computed. Furthermore, some processing for highlighting the faults was implemented to these gravity and magnetic data, which permits to recognize the trends of these faults, distinguish the blocks and suture zones. Based on these underst and ing, we explored the characteristic of lithosphere in the study area.

The free-air gravity anomalies with upward continuation 10 km highlight the regional field or the gravity field related to deep structure(Fig. 1). The free-air gravity field exhibits a general tendency of increasing from west to east or from the continent to sea. Another feature is that study area can be divided into south and north parts by a NEE line, which is coincident with the Jiang-Shao fault zone in the l and , while a boundary between the anomaly lines with NE and NEE trends in the sea areas. To east, this NEE separates the anomaly zone(IV)of the trench-arc-basin system of Japan-Ryukyu-Taiwan from the anomaly zone(VII)of the Japan sea. South of this NEE line, anomalies can be divided into six areas, including the South China area(IIIa)with strong positive local anomalies within a negative background, wide and gentle positive anomaly area(IIIb)of the continental shelf, anomaly zone(IV)of trench-arc-basin, wide and gentle positive anomaly zone(V)of the western Philippine sea(the western boundary of anomaly IV is coincident with the western fault zone of the Diaoyu Isl and fold), anomaly zone(IVa)close to the central line of the Ryukyu Isl and with local positive anomalies in a higher background, and the negative anomaly zone(IVb)between the eastern central line and trench, which indicates mass defect caused by huge water depth in the trench.

(Fig.1)

Fig.1 Free-air gravity anomalies with upward continuation 10km in sea areas east to China mainland (Unit: mGal)

In the north of the NEE line, anomalies can be divided into four areas, which are the North China area(Ia)with positive local anomalies within a negative background located west of the Tanlu fault zone, wide and plain anomaly area(II)of the lower Yangtze within a negative background, high-anomaly area(Ib)of the Korea Peninsula, wide and plain anomaly area(VII)within the low background of the Japan Sea.

Figure 2 shows the △T anomalies with upward continuation 10 km in the sea areas east China. A series of positive anomaly zones with NW trend, usually regarded as the product of seafloor spreading in NE-SW during sea basin development, are present in the western Philippine Sea and end in the trench. In the trencharc- basin system of Japan-Ryukyu-Taiwan, NE trending positive anomalies can be found in the west sides both of the basin and isl and . Compared the anomalies west of the isl and , larger anomalies are present west of the basin. The magnetic anomalies in the East China Sea shelf tend to be lower in the north and higher in the south, positive in the south and negative in the north. Metamorphic rock older than 16.8 hundreds million years was encountered in the Lingfeng No.1 well, indicative of the existence of the continental core. The lower anomalies in the north might be related to the deep magnetic basement. The North China area located in the west of the Tanlu fault with positive and negative local anomalies is distinct from the lower Yangtze area and southern portion of Liaodong Peninsula. In addition, positive anomalies were also found along the Qingdao- Wulian fault zone, and the higher positive and negative local anomalies were seen in the circum southern of Korea Peninsula. These anomalies are usually regarded as the products of igneous rock zones extending from the Zhejiang-Fujian-Shanghai area to the Korea Peninsula. In the lower Yangtze area, a distinct negative anomaly zone with EW trend gradually extends to the Yellow Sea. South of this anomaly zone, there are two positive anomaly zones with NE trend and larger amplitude, presumably attributed to the igneous rock zone along the Yangtze River and coast igneous rock, respectively. On the contrary, in the lower Yangtze area north of this anomaly zone, an obvious positive anomaly zone with NW trend might be the eastern boundary of the lower Yangtze area. The features mentioned above are more evident in magnetic anomalies reduction to the pole and upward continuation 10 km, especially for the positive anomaly zone regarded as the boundary of the lower Yangtze area(Fig. 3). The higher positive and negative local anomaly zones found in the southern part of the circum Korea Peninsula can be divided into the western local anomaly developed area generated by the extension of the Zhejiang-Fujian-Shanghai coast igneous rock, and eastern local anomaly developed area with NNE trend and the similar feature with the fold zone of the Diaoyu Isl and . The latter was used to locate the boundary between anomalies of Japan Sea and Sino-Korea. Additionally, the positive anomaly trending NE, located west of the isl and in the system of trench-arc-basin of Ryukyu-Taiwan, is much enhanced after magnetic anomaly reduction to the pole. The similar change can be also seen in south of the East China Sea shelf. The higher positive magnetic anomalies with NEE trend northeast of the South China Sea extend to Taiwan Isl and .

(Fig.2)

Fig.2 Magnetic anomalies upward continuation 10 km in sea areas of the East China (Unit: nT)

(Fig.3)

Fig.3 Magnetic anomalies with reduction to the pole and upward continuation 10 km in sea areas east of China mainland (Unit: nT)

With reference to the Moho depth from the shear wave velocity structure inverted by earthquake data and the results of global geo-science transect research(Zhu et al., 2002; Xi et al., 2006), the Moho depth in eastern China seas was inverted in a series of sub regions and blocks in terms of the fourth-order approximation of wavelet decomposition of the Bouguer gravity anomalies(Fig. 4). The result shows that the Moho depth in most of the study area is less than 30 km except for the northwest, southwest corners and northern areas of Korea Peninsula(more than 30 km). The Moho depth ranges from 30 km to 10 km or less than 10 km from east to west, which indicates that the transition from continental crust to oceanic crust. In addition, the Moho depth ranges from 29~26 km in the Yellow Sea, Bohai Sea and west of the East China Sea, and from 22 to 16 km between the Okinawa trough and Ryukyu arc. The Ryukyu trench is a gradient zone of Moho, east of which the Moho depth is less than 10 km in the Philippine Sea, which is regarded as typical oceanic crust. In the northeast of Japan Sea, the Moho depth ranges from 16 to 24 km and can be regarded as the transitional crust.

(Fig.4)

Fig.4 Moho depth in sea areas east of China mainland (Unit: km)

The Curie surface is the temperature interface where ferrimagnetic rocks become paramagnetic, and they lose their ability to generate detectable magnetic anomalies, which result in strong magnetic fluctuations at the Curie surface. Thus, the deepest level within the crust at which materials create discernible signatures on a magnetic anomaly map is generally interpreted as the depth of the Curie point isotherm. Therefore, the inversion of the Curie surface using the △T magnetic anomalies can be regarded as a method for calculating the lowest layer of magnetism. Here, the depth of the Curie surface was inverted from the third-order approximation of wavelet decomposition of magnetic anomalies using the spectral moment method(Fig. 5). The general trend of the Curie surface is mainly in NE. Along the boundary of the tectonic plates(such as deep and large faults), the Curie isothermal surface can be defined as a series of gradient zones and uplifted zones, such as the Sulu orogen zone, Jiangshao fault zone, Tanlu fault zone, the fault zone along the eastern margin of the Yellow Sea, the West Lake Keelung fault zone and the Ryukyu arc belt. An uplifted region of the Curie isothermal surface with a NE trend extends between the South Yellow Sea and East China Sea. This uplift is consistent with the Jiangshao fault zone which extends into the sea. The deep Curie isothermal surface can be found in the Bohai Sea, Westlake depression, Subei basin(part of the lower Yangtze region) and South Yellow basin. The shallowest Curie isothermal surface lies in the ultra-high-pressure metamorphic belt in the Jiangsu-Sh and ong(Sulu)area(the northern part of the lower Yangtze), Korea Peninsula(the east part of lower Yangtze) and Japan sea. In particular, the very shallow Curie isotherm with a NW trend can be found in the western and middle parts of the Cheju Isl and . In addition, the depth of the Curie isotherm is only 20 km and becomes shal-lower in the middle region of the research area. The uplifted region of the Curie isothermal surface is also present in the southern region of the East China Sea shelf basin; however, it is significantly different from that in the northern part of the shelf basin of the East China Sea, and the Yushan-Kume fault zone constitutes the boundary between the southern and northern regions. The Curie surface in the Ryukyu trench contains a wide depression, which may be caused by an increase in thickness and a decrease in heat flow within the ocean lithosphere at the trench subduction zone.

(Fig.5)

Fig.5 Curie depth in sea areas of east China (Unit: km)

It was found there exists a negative correlation between the depth of the Curie point and the heat flow. The lesser the depth of the Curie point, the greater the heat flow. This trend is consistent with the prediction of the theoretical heat conduction model. Obviously, ground surface heat flow is influenced by the depth of the isothermal surface. According to one-dimensional steady state heat conduction equation and the given thermal conductivity and heat productions of lower crust and mantle, the geothermal field dominated by heat conduction and the thickness of the regional thermal lithosphere have been calculated(Fig. 6). It shows that the depth of the base of the thermal lithosphere ranges from 62 to 112 km, and the general trend of the depth contours is in NE. Generally, the depth of the thermal lithosphere base decreases along a NE trend, and the pattern alternates between deep and shallow. From west to east, uplifts with a NNE trend along the Tanlu fault zone, a NE trend along the Korean Peninsula and a NEE trend along Ryukyu Isl and s Arc can be found. The thicknesses of the thermal lithosphere in these three regions are 85~90 km, 80~85 km and 75~80 km, respectively. On the contrary, the deeper thermal lithosphere base can be found in the Bohai Bay west of the Tanlu fault zone, the Subei basin in the lower Yangtze region and the South Yellow Sea basin, the shelf basin of the East China Sea, the Ryukyu Trench and adjacent sea basin. The thicknesses in these four regions are 95~100 km, 90~95 km, 85~90 km and 95~100 km, respectively. A depression zone of the thermal lithosphere base with a substantial width and depth of 90~100 km provides evidence for the subduction zone in the southeast, which might be related to the increasing thickness of the oceanic lithosphere and a decrease in heat flow at the trench. The thermal lithosphere becomes shallower(only 75 km)towards the eastern region of the ocean.

(Fig.6)

Fig.6 Calculated depth contours of the thermal lithospheric base in sea areas of east China (Unit: km)

The boundaries between blocks with different basement structures can generate gravity and magnetic anomalies due to the different density and magnetic susceptibility, which makes it possible to recognize faults and locate boundaries of blocks(Hao et al., 1996). Considering the influence of Mesozoic and Cenozoic sedimentary basins, some information extraction and enhancement methods have been applied to highlight the characteristics of the faults and igneous rock in raw observation data, and the corresponding boundary faults have been located using feature extracting.

The subduction zones of the Philippine Sea and the eastern-edge of the Taiwan are the components of trench-arc-basin system of the West Pacific, which are all plate boundary faults and the important boundaries between continental and oceanic crust. Here, obvious geophysical features can be seen, including narrow and long steep gravity gradient zones and magnetic anomaly trends meeting together.

Figures 7 to 9 demonstrate 13 boundary faults or subduction zones related to tectonic unit division, including:(1)Tanlu fault zone, (2)Qingdao-Wulian fault zone, (3)Xiangshui-Chuhe fault zone, (4)east-edge fault zone of the Yellow Sea, (5)fault south of Cheju Isl and , (6)Jiangshao fault zone, (7)Changle-Nan0ao fault zone, (8)West Lake-Keelung fault zone, (9)East uplift fault zone of Diaoyu Isl and , (10)subduction belt of east margin in Taiwan Isl and , (11)subduction belt of the Philippine Sea, (12)west-edge fault in the Sea of Japan, and (13)south-edge fault in the Sea of Japan.

(Fig.7)

Fig.7 Free-air gravity anomalies with the processing of marginal enhancement and distribution of major faults in sea areas of east China

(Fig.8)

Fig.8 Magnetic anomalies with processing of marginal enhancement and distribution of major faults in sea areas of east China

(Fig.9)

Fig.9 Magnetic anomalies reduction to the pole with the processing of marginal enhancement and distribution of major faults

The fault zone of Zhongjieshan-Jiushan archipelago and Zhejiang-Fujian coast fault zone developed along the west edge of the East China sea. The fault zone of Zhongjieshan archipelago is located at the Zhejiang coast, parallel to Zhoushan archipelago and coastline. Also the fault zone formed in the Mesozoic, and exhibits a trend of NE 20°~25° and extends along the bathymetric contour of 20 m. It is the west boundary of higher and positive coast magnetic anomalies with a NE trend. In this area, the seafloor terrain is dominated by the fault zone and exhibits plain and stepped characteristics. The other fault zone, Zhejiang-Fujian coast fault zone is also called the fault zone of the East China Sea shelf base. It is located on the east side of the Zhongjieshan fault zone and extends along the depth contour of 40 m. Similarly, it is formed in the Mesozoic, and exhibits a trend of NE 20°-25°. The fault zone is regarded as the eastern boundary of higher and positive coast magnetic anomaly with a NE trend. It dominates the seafloor terrain and demonstrates plain and stepped characteristics. Both these fault zones extend to the southeast coast of Fujian, and coincide with the fault zone of Changle- Nan'ao.

For those fault zones mainly located in the continental area, including Tanlu fault zone, Qingdao-Wulian fault zone, Xiangshui-Chuhe fault zone, West Lake-Keelung fault zone and the east uplift fault zone of Diaoyu Isl and , our underst and ing is consistent with results given by previous studies. However, we emphasized that the extension of the Jiang-Shao fault zone toward the sea area and the east-edge fault zone of the Yellow Sea with a NNW-NW trend are the important boundary between the Yangtze plate and Sino-Korea plate.

As the boundary between Yangtze and the South China blocks, the Jiangshao fault zone with a NE 40°-60° trend extends along the Zhejiang-Jiangxi railway line. Crossing the Jinghua-Chuzhou basin and submerging in Quaternary along the south edge of Jinhua, the fault zone becomes the south boundary of the Changhe-Wang-panyang depression and approaches Hangzhou Bay. Then, it continues to extend toward NE till meeting the fault zone of the East China Sea shelf base. After crossing the fault zone of the East China Sea shelf base, it extends further and might be regarded as the fault south of Cheju Isl and . The Jiangshao fault zone in the sea area is probably the boundary between southern and northern gravity and magnetic fields due to its obvious gravity-magnetic anomalies. The southern regional gravity field is dominated by negative anomalies and exhibits lower gravity. On the contrary, the northern regional gravity field is dominated by higher positive anomalies. The fault zone is located at the gradient belt between higher and lower gravity anomalies. In the southern area of the fault zone, the magnetic anomaly zone is dominated by lower values with local higher values. Compared with the northern area of the fault zone, the regional background field in southern area is higher and overlapped by high and positive magnetic anomalies. After crossing the fault zone of the East China Sea shelf base, the similar features are seen along the south fault zone of Cheju Isl and , which can be regarded as the extension of the Jiangshao fault zone. Previous studies show that an obvious low-velocity zone was found at the depth of 77km from Yangtze estuary to Cheju Isl and , extending to the neighboring Kyushu of Japan(Xi et al., 2006). If this zone is considered as a deep tectonic boundary, it seems reasonable that the Jiangshao fault zone extends to Kyushu of Japan through the south-edge fault zone of Cheju Isl and .

The Dabie-Sulu orogen zone is the boundary between the Yangtze and Sino-Korea blocks. Some scholars believed that the Sulu orogen zone should be related to the tectonic zone of Linjinjiang located at the middle of Korea Peninsula(Yang 1989; Liu, 1992a; Cai, 2002). The northern area of Linjinjiang fault belongs to Sino- Korea block, and the southern area belongs to Yangtze and the South China blocks. Other scholars considered that the Jingji block located south of the Linjinjiang fault has typical crystalline basement of the Sino-Korea plate, sedimentary rock series of lower Paleozoic, and pseudo-stratigraphic contact relationship between Middle Ordovician and Carboniferous(Wan 2001). The Sulu orogen zone should be related to the Wochuan tectonic zone of Korea Peninsula, or the southern main body of Korea Peninsula still belongs to the Sino-Korea block(Zhai et al., 1997; Hao et al., 2001; Wu et al., 2002); or the Dabie-Sulu orengon zone did not extend into Korea Peninsula due to the shear movement of the large-scale dextral strike-slip zone with a NNE trend in the Yellow Sea(Lee, 2003; Ishiwatari and Tsujimori, 2003). According to the gravity anomaies, depth distribution of Moho and results of tomography, a fault zone with a NS trend can be recognized at the west edge of Korea Peninsula. This fault zone, and the Wulian-Qingdao fault zone and fault zone of south of Cheju Isl and jointly define the boundary between the Sino-Korea and Yangtze blocks in Yellow Sea area(Hao et al., 2004). Our study illustrates that the east-edge fault zone of the Yellow Sea is also located at the west edge of Korea Peninsula, which consists of the northern part with a NNW trend and southern part with a NE trend, and separated by the northern South Yellow Sea fault zone at the junction of these two parts. The east-edge fault zone of Yellow Sea can be easily seen in the free-air gravity anomalies upward continuation 10 km(Fig. 1), and exhibits a gradient zone with a trend of higher in east and low in west. Similarly, this fault zone can be also seen in the magnetic anomalies and its reduction to the pole upward continuation 10 km(Fig. 2 and Fig. 3), and exhibits distinct difference on both sides. The higher magnetic anomalies with a NEE trend suddenly disappear. Higher regional magnetic anomalies appear along the fault zone. On the other h and , the fault can be also recognized in Moho depth distribution(Fig. 4), the Curie surface depth distribution(Fig. 5) and the depth contours of the thermal lithosphere base(Fig. 6). There are obvious differences of depth and trends of these interfaces on both sides. A narrow low-velocity zone with a NS trend north of the west-edge of Korea Peninsula is easily recognized at the velocity anomaly figure at 77 km depth. However, it becomes a high-velocity zone with a NNE trend in the velocity anomaly figure at 120 km depth. Both in these velocity anomaly figures, the low-velocity zone with a NW trend is consistent with the east-edge fault zone of the Yellow Sea. Therefore we deduced that the east-edge fault zone of the Yellow Sea is also a deep tectonic boundary, which separates Sino-Korea and Yangtze blocks.

Based on results mentioned above, the study area can be divided into portions of the Eurasia and pacific plates. Furthermore, it can be divided into 8 blocks and 4 plate conjuncture zones(Figure 10): Sino-Korea block(I), Yangtze block(II), South China block(III), Japan Sea block(V), Okinawa trough block(VI), Southwest Japan-Ryukyu-Bachongshan block(VII), west Philippine block(VIII) and the South China Sea block(IV); Sulu conjuncture zone(IX), Diaoyu Isl and conjuncture zone(X), Taiwan-Luzon conjuncture zone(XI) and Ryukyu trench conjuncture zone(XII).

(Fig.10)

Fig.10 Map showing tectonic unit division in sea areas of east China

It is necessary to develop integrated geophysical interpretation for studying the lithosphere and block structure of eastern China seas and adjacent areas, such as mapping gravity and magnetic anomalies in eastern sea areas, gravity and magnetic data inversion, depth distribution of Moho, Curies surface and the thermal lithosphere base inferred from gravity and magnetic anomaly inversion and integrated interpretation. Guided by principle of integrated geophysical interpretation, the basic idea for the research on geophysical features of lithosphere study and block structure in eastern China seas is the joint inversion using different geophysical data and integrated geological geophysical interpretation.

Our study results show that 13 important fault zones are distributed in the study area, which is divided into 8 blocks and 5 conjuncture zones by these fault zones. These blocks and conjuncture zones are the products in two periods, i.e. the old global tectonic period and new global tectonics period, and they are also the specific performance of frontal line effect of Tethys and Pacific tectonic domains.

The Jiangshao fault zone and the east-edge fault zone of the Yellow Sea exhibit distinct geophysical features in different resultant figures, including the new gravity-magnetic anomalies in eastern China seas, postprocessing results of gravity and magnetic data, Moho depth distribution figure, the depth distribution figure of the Curie surface and the depth figure of the thermal lithosphere base. These two fault zones are the most important boundaries which can be used to demarcate the Yangtze block and Sino-Korea block, the Yangtze block and South China block.

Authors are grateful to Liu Guangding of Chinese Academy of Science for sharing his academic idea of leading edge. We thank Zhong Huizhi and Gao Dezhang for their participation to the research. The authors also appreciate the valuable comments from anonymous referees, which improved the article. This work was supported by the National Science and Technology Major Project(2011ZX05035-003-006), Map Series of Geology-Geophysics of Chinese Seas and adjacent areas(GZH200900504) and National 863 project(2010AA09Z302).