New high resolution multi-beam bathymetry data and previous research data are used to study the distribution characteristics of pockmarks and mud volcanoes, such as location, size and morphology, in the northern continental margin consisting of Southwest Taiwan Basin and to the west and the western continental margin consisting of Zhongjiannan Basin and to the north. Combined with the tectonic evolution and topography of the northern and western margin of the South China Sea (SCS), this work made statistics, and classification to these characters, and linked them with the geological evolution of the South China Sea and the distribution of oil and gas resources. The SCS is surrounded by different types of continental margins. To the north is the extensional continental margin which consists of Southwest Taiwan Basin (I), Pearl River Mouth Basin(II) and Qiongdongnan Basin(ID; to the west is the strike-slip continental margin which consists of Beibuwan Basin(IV), Yinggehai Basin(V) and Zhongjiannan Basin(VI); to the east is the subducted continental margin and to the south is the margin of continental collision. All the basins above have experienced almost the same stages during evolution of the SCS, which include Paleocene to mid-Miocene rifting followed by post-rift thermal subsidence and the neotectonic movement in recent similar to 5 Ma. In the study area, 16 distribution areas (a-p) of pockmarks and mud volcanoes are recognized. The data of areas a, b, c, d, and h were collected from previous researches and the other data were collected by the Guangzhou Marine Geological Survey in 1998-1999. The acquisition system SeaBeam2112 was used to acquire data and the processing sequence includes; navigation filter, parameter calibration, correction for transducer draft, correction of sound velocity, data filtering and so on. The cell size of raster grids is ca. 100 m and the vertical resolution is ca. 3 parts per thousand of the water depth. The multi-beam bathymetry is utilized to study the morphology, size and shape of the pockmarks and mud volcanoes. Statistical and fitting methods are used to study their distribution features and regional differences according to the pockmarks' width and depth and the mud volcanoes' diameter and height. The characteristics of the pockmarks and mud volcanoes in different areas vary from area to area apparently, which are shown respectively in the result sections. Compared with the western margin, pockmarks in the northern margin are fewer and smaller with width of no more than 100 m. A few giant pockmarks with widths Of 400 similar to 1200 m are found in Qiongdongnan Basin. All of them tend to develop on the continental slopes or near the continental shelf break. Pockmarks in the western margin show various shapes in planar view, like circular, elliptical, crescent, elongated and irregular, and have larger size, which usually exceed 1000 m in width and 100 m in depth. Some of them are composited, arranged in a chain or gathered in a group. Some of them grow near canyons or channels, in the soft sediment deformation areas or beside the mud volcanoes. Mud volcanoes with widths more than 100 m widely spread singly or in groups in Southwest Taiwan Basin, Yinggehai Basin and Zhongjiannan Basin. Fitting results reveal that the slopes and coefficients of determination (R-2) of pockmarks' fitting curves are different in each area, showing relatively bad linear relationship, however, the pockmarks of all the study areas show a good linear relationship. On the contrary, mud volcanoes in individual areas show good linear relationships, but all of them show a relatively bad linear relationship. Besides, according to the statistics result, a comprehensive pockmark classification scheme is proposed based on (a) their shape in planar view, which includes circular, elliptical, crescent, cornet-shape, elongated and irregular pockmarks; (b) their magnitude, which includes small, conventional, giant and mega pockmarks; and (c) their composite pattern, which includes composite pockmarks, pockmark chains and pockmark groups. Different fitting characteristics of pockmarks in different areas may reveal various seafloor sediments, shallow structural activities, continent slope dip and bottom current and so on. A good linear relationship of mud volcanoes in the same area may indicate that mud volcanoes suffer from minor environmental impact and have a strong coherence of morphologies, fluids components and fluids origin, which are probably different in other regions. Results indicate that. the appearance of pockmarks and mud volcanoes are strongly coherent with the active activities of strike-slip faults in southern Taiwan and the western margin in recent ca. 5 Ma. They are widespread with a relatively larger scale in strike-slip faults zones, like Southwest Taiwan Basin, Yinggehai Basin and Zhongjiannan Basin, but fewer and smaller in other extensional basins. Their distribution characteristics which may become important signs of hydrocarbon resources exploration, on one hand may indicate large quantities of fluid escape and on the other hand would indicate that continuous regional tectonic activities are in favor of shallow fluids accumulation and forming reservoirs resulted from deeply buried fluids escaping.
