Study of crustal structures and deep-sea sediment waves in the northern South China Sea

• Main Authors: Ching-Hui Tsai, 蔡慶輝
• Other Authors: 許樹坤, 李昭興
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/90322780924963176476

博士 === 國立中央大學 === 地球物理研究所 === 95 === There are two main topics presented in this thesis. First is to use reflection seismic data and gravity modeling to study the crustal structures in the northern continental margin of the South China Sea (SCS). Another topic is to use high resolution seismic and chirp sonar data to study deep-sea sedimemt waves in the north and south sides of downstream of the Formosa Canyon. Additionally, we present the foundamatal procedures of data collecting and processing of the multi-beam bathymetry data and show a preliminary result of new bathymetry map off southwest Taiwan. We can distinguish the crustal strutures in the northern margin of the South China Sea into three partions: the continental crust (CC), the thinned continental crust (TCC) and oceanic crust (OC). The TCC zone displays a steeper slope and Moho depth becomes shallow oceanward from about 24 km to about 14 km deep. The corresponding crustal thickness is from 20 km thick to about 10 km thick and is located in a relatively low free-air gravity zone. According to the seicmic data, crustal modeling results and magnetic anomaly curves, the OC could be existed further northward and the continent-ocean boundary is along the base of continental slope. Because the volcanism happened after sea floor spreading, there are many intrusive or extrusive igneous bodies around the Dongsha Island. Thus, we can find a possible underplating in this area. Volcanism in this area also causes the abnormal thick oceanic crust in the southeast of the Dongsha Island. In the northern SCS, the deep-sea sediment wave fields are located in two zones, one is between downstream of the Formosa Canyon and Penghu Canyon, the other is between downstream of the Formosa Canon and LRTPB (Luzon-Ryukyu Transform Plate Boundary). The distribution of these deep-sea sediment waves are found below 3000 m deep and the wave front is rather parallel to the bathymetric contour lines. Base on high resolution sesmic data, we suggest that these sediment waves are caused by turbidity currents. The sediment waves have shown asymmetrical internal structures which may induced by high gravity flow activity. Most of these sediment waves are less than 85 m high, and most of wavelengthes are less than 6 km long. Most of slopes are less 0.8 degree. We suggest that the Formosa Canyon and Penghu Canyon are two major sediment transportation channels. The turbidity flows downward and may overflow directly from the turning points of the two canyons. The sediment deposits in the stoss side and erodes in the lee side. It proceedes downslope continuously because of gravity effect. Due to the bathymetric offset of the LRTPB, the turbidity current can not overflow southwards from this tectonic structure, which makes LRTPB a natural sediment wave dam.