1. The South China Sea is one of the largest marginal seas between Continental Asia and the Pacific Ocean, with NE to SW axis extended 1,500 km long, and 800km wide of east to west axis. It forms as a rhombus oceanic central basin due to deep tension fault with basalt of upper mental eruption accumulated as seamounts (fig.1). Water depth of the central basin is 3,400 m in the northern part, it reaches to 4,200 m in the southern part, and a few deepest parts of the basin are more than 4,400 m of water depth. The central basin surface is quite flat with gentle slope in the order of 0.3‰, - 0.4‰, but with deep valleys developed by fault tension on the NE and SW end (Fen, 1982). There are about 27 seamounts are 1,000 m high above basin surface, several sea mounts reaches to 3,400 - 3,900 m high above sea bottom, and more than 20 sites of sea knoll are with height ranged from 400 - 1,000 m (Zeng et al, 1986). Sediments and coral reefs have accumulated on the top of most volcanic sea mounts (fig.2).

　

(1) On the both side of central basin, there are block stepping shaped continental slopes formed by parallel faults subsidence.

It occupied about 49% of total area of the South China Sea. Water depth is 1,500 - 3,500 m on the North and Northeast slope, but it reaches to 3,800 - 4,000 m deep on the other side of continental slope (fig.3). The stepping slopes originated from a break off continental shelf by parallel - fault processes (Wang et al, 1996). Thus, the bedrock of the slpoe are granite -gneiss as same as the one of adjacent Asia continent.

The block - stepping shaped continental slope can be seen as submarine plateau several thousand metres height standing up from sea bottom.

(2) Xisha (Paracel Islands) - Zhongsha continental slope is located on the northwest side of central basin (fig.4). It is a block pattern subsidence follows a NE trending fault zone from Dongsha Islands (Pratas Island) to Taiwan bank and to PongHu Islands. Water depth is 900 m to 1,100 m on the slope steps, there are atolls (water depth less than 200 m) reefs and volcanic islands developed above the step surface. There is a 1,100 m deep channel between Xisha Islands and Zhongsha Islands, and a Sea trough, 1,500 m to 3,400 m deep, 8 - 10 km wide, 420 km long, and with steeper cliffs surrounded the trough, is located between the Xisha Islands and mainland of Asia in the direction from east to west.

(3) Along the South and Southeast side of central basin, there is Nansha Islands ( Spratly Islands), continental slope with a relative height about 2,400 m above sea bottom. There are NE and NW faults zone along both edge of it. Thus, the Nansha Islands block is also in a form of submarine plateau as same as Xisha Islands but separated by the central deep basin.

Water depth is about 1,800 m above the Nansha Plateau, and there are valleys, sea mounts, sea hills and about 200 coral islands, shoals, submerged reefs arranged in the NE trend developed on the top. The bedrock basement of the Nansha plateau is metamorphic granite - gneiss, which is a subsidence blocked continental shelf as same as the Xisha plateau, were breaked off from Asia continent, i,e, the Guangdong coastal area during late Tertiary period subsidence.

(4) North side of central basin is also a steped continental slope, with Dongsha Islands on the top (fig.5). Upper water depth is 700 m with slope in the range of 3.4‰, deeper parts the slope is in the order of 27.7‰.

Dongsha Islands stand up on the continental slope in a plateau from with area of 12,000 km², and water depth is between 300 - 400 m.

(5) On the East side of the central basin, there is narrow steeper slope off the Philippine Islands. The width of the slope is less than 38 nautical mile with slopes reach to 170.8‰ to 119.3‰ (Luzon Trough. Manila Trough).

The gentle slope is 17.7‰ located off Palawan Islands. These Islands margin slopes are cut by submarine channels, and with deep sea fan developed along foot parts of the slopes.

　

Continental shelf of South China Sea is mainly distributed (developed) on the North, west and south side, which is extended lower down from the Asia continent.

(1) East to the Pearl River Estuary, most parts of continental shelf is in the water depth of 200 m, but reach to 379 m on the west side of the Pearl River (fig.6). The width of continental shelfs is 150 nautical mile along mainland to China, but is narrow in surrounded islands area, such as 7.5 nautical mile along East side of the Taiwan Island, 50 nautical mile off south coast of the Hainan Island.

(2) Continental shelf on the south side of the South China Sea, ranged from offshore to 150 m water depth, 300 km wide. There are coral reefs: Hayes reef, South Luconia Shoals, Lydis Shoal, Parsons Shoal, ets. Located on the north part of the continental shelf: water depth of reef area is 8 - 9 m, or 20 - 30 m.

(3) East side is island marginal shelf, distributed along the margin of Philippine Islands (Luzon Islands, Mindoro, and Palawan), in the direction either north to south or NE to SW, outer parts of the shelf is in the water depth of 150 - 300 m, with channel cut down to the Palawan trough.

To understand the submarine geomorphology is meaningful for territorial and environmental protection. Intermittent line indicates the oceanic boundary of China, according to "The Location Map of South China Sea Islands" published in 1949 by Chinese government based on historical tradition of administration of the sea, and recognized by international government agreement "The Announcement of Potsdam" 1945 after the Second World War.