TOPOGRAPHIC AND MORPHOLOGIC CHARACTERISTICS AND GENESIS ANALYSIS OF XISHA TROUGH SEA AREA IN THE SOUTH CHINA SEA

Detailed topography and geomorphology of Xisha Trough sea area were mapped with multibeam sounding survey data that were acquired by Guangzhou Marine Geological Survey since 1997. The study area can be divided into three geomorphic units. The northern unit is a slope of the continental slope, mainly formd by accumulation and extending to NE, and its topography is simple, the slope surface is smooth and its geomorphic type is single. The middle unit is Xisha Trough and extending to NNE, whose landform change is great. Xisha Trough consists of trough bottom plain and trough slopes that have some sub-order geomorphic units such as scarp, eroded canyon and terrace etc. The southern unit is Xisha plateau and extending near EW, whose landform changes greatly and geomorphic type is complex, with a lot of seamounts, seaknolls, mesa and depressions that are arranged alternatively. The two seafloor spreadings of the South China Sea controlled the origin and evolvement of geomorphic types of the study area. During the first seafloor spreading, Xisha Trough probably rifted. During the second seafloor spreading, Xisha Trough had three stages of rift-fault, and the intensity differed greatly between east and west parts, and consequently, the Xisha Trough has had the present geomorphology, deeper, narrower, steeper in the east part than in the west which reprensents the basic geomorphology of the study area. And collective effects of endogenetic and exogenic forces such as geologic structure, volcanic activity and sea level change controlled the origin and evolvement of the sub-order geomorphic types.