Analysis Of Morphotectonics At The North Continental Margin Of South China Sea And Its Tectonic Implication

The northern margin of South China Sea, which is one part of the West Pacific trench-arc-basin system, at the joint part of Indian-Australian plate, Eurasian plate and Pacific plate, is one of the four important tectonic boundaries in the South China Sea. The northern margin of South China Sea belongs to discrete type continental margin in Cenozoic Era. It records plenty of important geological information about lithosphere rifting and sea-floor spreading, and is the best natural laboratory for marginal sea tectonic evolution study.Strong tectonic movements generated complex geology structure and abundant geomorphologic landscape at the northern margin of South China Sea, such as structural platform, faulted basin, cliffs, submarine canyon, trough, sea basin, Seamounts and seamount chain. The often used term tectonic movement is a grab-bag expression that encompasses all type of deformation, including the motion of tectonic plates, slip on individual faults, ductile deformation, volcanism, and tectonic subsidence. Tectonic landform is a dynamic morphological type, which is directly controlled by tectonic movement. The study to tectonic landform is an analysis using integrated methods of geomorphology, structural geology and geo-dynamics. Tectonic landform analysis with its contrasting of seismic data, has becomes one of important tools in the researches for tectonic evolution of young marginal sea.The new complete coverage multi-beam bathymetric data collected mainly in the last two decades, in the northern margin of South China Sea allowed the identification and mapping of a wide range of morphologic seafloor features. Based on the joint inspection of swath bathymetry and multi-channel seismic profiles a detailed characterization of the seafloor shaping processes was carried out. Total propagated error computation algorithm was established to improve the efficiency of Multi-Beam Echo-Sounder data processing and to avoid losing of terrain detail in trend surface model and median filter model. According to the IHO standard, TPE algorithm eliminated noises automatically based on the total propagated error computation. TPE established Digital Terrain Model based on the combined estimation of the depth and related uncertainty of the MBES data. The data processing speed of TPE algorithm is 5 times of the man-machine editor. The TPE terrain model has much more detail than the man-machine model. Huidong-Beiweitan fault, Shantou-Taixinan fault, South-North Weitan islands, Tuandao island, Yitong Ansha rise, Jiulong submarine canyon, Jiulong seamount and Heli seamount were distinguished from the seafloor, and named according to the new international seabed features naming rules.On the basis of establishment of Digital Terrain Model and geomorphic unit and type division, we carried out the multi-scale analysis of tectonic landform of the northern margin of South China Sea, using the method of wavelet decomposition. The tectonic landform was divided into three scales, as follows big scale, middle scale and small scale. In the big scale, the continental shelf line and slope toe line show the direction of tectonic structure. It shows that three cycle tectonic stress field leads to three time different direction crustal movement in the northern margin of South China Sea in Cenozoic Era. The first phase tectonic stress field is in South-North direction, resulting in the S-N sea-floor spreading. The second phase tectonic stress field is oriented NW to SE, resulting in the NW to SE sea-floor spreading. The third phase tectonic stress field is in NW direction, causing the tectonic compression, NW active faults and volcanism in the late Cenozoic Era at the northern margin of South China Sea.Tectonic movements had great intensity and numerous types in the northern margin of South China Sea, and such primary internal force controlled form and evolution of seabed tectonic landform. In the middle scale tectonic landform, we analyze the types of tectonic movement such as tectonic subsidence, sea-floor spreading, fault and volcanic activity. NW faults such as Huidong-Beiweitan fault and Shantou-Taixinan fault formed fault cliff, fault terrace, seabed fault valley block mountain, graben basin and structural platform etc. Volcanic activity formed seamounts and seamount chain, such as Jiulong seamount, Heli Seamount. Tectonic subsidence formed large tectonic mesa, deep water terrace and continental slope basin etc.In the small tectonic landform, we distinguished protuberance, shallow fault, shallow gassy belt which is composed of partial enhanced reflection, acoustic blank patches and acoustic blank zone, in the sub-bottom profile PO in Baiyun slope. Contrasting the sub-bottom profile with the data of Chinese first gas hydrate expedition, we deem that the gas in the shallow gassy belt came from the decompounding of the gas hydrate in the deep sediment. We identified pockmark, seepage, fold and BSR in the single-channel seismic profile P2. The BSR under the Chinese first gas hydrate samples may be the bottom of the gas hydrate stability zone. We plotted out Oligocene, early Miocene, middle Miocene and Pliocene in the seismic profile, according to the sedimentary thickness, sedimentary rate, and age data of ODP site 1148. BSR, fold and first gas hydrate samples locate in the stratums of late Miocene and Pliocene in the single-channel profile P2, 12-2.6 Ma. The existence of pockmark, seepage and protuberance illuminates the gas escaping in the study area. Long BSR, fold and shallow gassy belt show the enormous resource of gas hydrate in the sediment in Baiyun slope at the northern margin of South China Sea.