Similarities And Differences Of Neotectonics Of The Northern South China Sea Slope And Its Effection Of The Distribution Of BSR

The South China Sea is one of the largest marginal basins in the West Pacific, inwhich numbers of Cenozoic basins developed. In this paper, seismic profiles and focalmechanism are used to carry through a systematic and comprehensive study about theneotectonics of the northern South China Sea. It is of great importance to understandthe dynamics mechanism of the neotectonics, migration and accumulation ofgashydrate and the prevention of geological disasters. This thesis got five collusionsas follow.(1) The igneous rocks can be divided into three segements, including theQiongdongnan-Xisha Region, the Shenhu Region and the Dongsha Region. Theoccurrences in different regions have distinct differences. The igneous rocks of theQiongdongnan-Xisha Region have an obvious rule of distribution controlled by thedextral Right-step pull-apart basin. The igneous rocks of the Shenhu Region aremainly located in the intersection of the NE and NW faults. There is a high correlationbetween the tensional stress field caused by the Dongsha Movement and the igneousrocks in the Dongsha Region.(2) Based on the distribution of earthquakes and focal machanism, the northernSouth China Sea seismic zone is divided into three regions. The strong NW-directedcompression driven from the Philippine Sea Plate along the Luzon Arc leads to thestrongest earthquake activity of the eastern part. Affected by the Red River Fault，thewestern part develops stronger earthquake activity. In the Pearl River Mouth region,which lies in the central between the transform plate boundary and collision plateboundary, earthquake activity is very weak. Both macro-earthquake andmicro-earthquake are not developed. The study of the focal mechanism solutionindicates that the controlling fault of the northern South China Sea is also different. Controlling faults in the eastern part are the NE-trend dextral slip, central seismicactivity is controlled by normal faults, controlling faults in the western are NW-trenddextral strike-slip faults.(3) Using the Coulomb software to simulate stress field of earthquake history inthe eastern, middle and western sections of northern South China Sea. The1936Lingshan earthquake in the Guangxi Province triggered the1988Beibuwanearthquake. The1605Qiongshan earthquakes’ aftershocks occurred in area where thestress contour developed relatively wide. Two earthquakes of magnitude6happenedin1994and1995, the reduction of stress field is given priority to, and the aftershocksdevelop in the main controlling faults. The aftershocks of the1962Heyuanearthquake developed mainly at the NW end, the Yangjiang earthquake in1969focused on on the north side of the fracture, which is in the middle of the northernSouth China Sea. In addition, the aftershocks of the two earthquakes locate in asmaller stress increased area and the boundary of increasing stress and reducing stress.There are less aftershocks in maximum stress increased area. In the east of the SouthChina Sea, the1878earthquake prodeces the NE-strike stress increased area whichcovers the1906earthquake. From the point of time, it is more likely to trigger the1906earthquake. In addition, the earthquake in1878produces a SW-striking archedstress increasement, which covers epicenter of a magnitude7.5earthquake in1918. Itis extremely likely to trigger an earthquake in1918. The1992earthquake in theDongsha Islands produces a NE-striking stress increasing zone, and a NW-strikingreducing area. The aftershocks developed on the west side of the controlling fault.Increasing stress zone produced by the Dongsha Islands earthquake covers the1994earthquake in shoal area of Taiwan, and the former may trigger the latter.(4) The eastern part of the northern passive margin of the South China Sea isinfluenced by the Dongsha Movement which high pressure released. So diapirs arenot well-developed, which make the transportation system of gas in the combinationamong faults. However, the western part is less affected by the Dongsha Movementand some diapirs are well developed, making the transportation system of gas in thecombination of faults and diapirs. (5)Neotectonics evolution of the northern South China Sea is restricted by thePacific Tectonic Domain and the Tethys Tectonic Domain. Characteristics of faultsdevelopment in the northern South China Sea are different due to its distribution. Themain fault activity in the eastern part is inherited. However, lots of newborn faults andreversed structures are developed in the western part. On time scale, the coverage ofthe two tectonic domains on the northern South China Sea is different. From lateCretaceous to early Oligocene (32Ma), the whole northern South China Sea isinfluenced by the Pacific Tectonic Domain, almost not affected by the TethysTectonic Domain. The research of the fault activity indicates that in Miocene the PearlRiver Mouth basin is influenced by the Tethys Tectonic Domain. As the DongshaMovement occurred at the end of the late Miocene, the eastern Pearl River MouthBasin began to be affected by the Pacific Tectonic Domain. After the increase of theconvergence rate of the Pacific plate, the influenced scope of the Pearl River MouthBasin gradually expands westward. The boundary of the two tectonic domains islocated at about112°E in neoid period.