Evolution of Oceanic Crust of the Marginal Seas in the South China Sea

西北太平洋地区有许多边缘盆地，其中对日本和南海的研究远远超过该地区的其他边缘盆地。人们经常提到，日本海和南海不仅在一般地形形状上，而且在地震和磁资料上都有很好的相似之处。两个边缘盆地都有洋壳和陆壳，并有一些清晰的磁力线。两海磁向趋势的不稳定变化的相似特征推断出在厚沉积物覆盖下隐藏的断层数量，仅通过测深观测很难发现。这些边缘海被认为是沿着大洋板块和大陆板块之间的边界处于张力引起的裂谷阶段。1993年初夏，“石研二号”遥感船在南海北缘进行了一系列海洋地球物理观测。此次科学考察的主要目的是收集资料，研究边缘海（即日本和南海）在大陆与深海盆地过渡边界的地震结构转变和地磁特征方面的比较地球科学。数据包括单通道地震反射、气枪- OBS折射剖面、全分量和三分量磁资料以及重力资料。由微机沿400公里长的轨道线每隔2秒采集一次质子、进动和磁通门磁强计的数据，利用水平陀螺仪和垂直陀螺仪测量装置的倾斜度。为了消除船舶磁化对现场数据的影响，在巡航过程中对船舶航向进行了完整的圆周旋转。然后可以参考IGRF90计算三分量异常。本文获得的一组新的三矢量地磁异常和迄今为止编制的总磁剖面表明，不对称和不连续的海底地理和声学基底的粗糙度反映了空间和暂时传播速率和方向的变异性。少

There are number of marginal basins in the Northwestern Pacific region of which the Japan and South Chine Seas are studied far better than other marginal basins of the region. It is frequently referred that Japan Sea and South China Sea have good similarities from the viewpoint of not only general topographic shape but also seismic and magnetic data. Both marginal basins have oceanic and continental crust with some clearly definable magnetic lineations. The similar characteristics of the erratic change in the trend of the magnetic trend in both seas infers number of faults hidden under the thick sediment cover hardly noticeable only by bathymetric observations. These marginal seas are thought to be in the tension-induced rifting stage along the boundary between oceanic and continental plates.A series of marine geophysical observations by the R/V Shi Yan II in the northern margin of the South China Sea were carried out in early summer of 1993. The objective of this scientific cruise is … More to collect data to study comparative geosciences on the marginal seas, i.e., Japan and South China Seas, in terms of the transition in the seismic structure and magnetic characteristics in the transient boundary between continental and abyssal basin. The data include single channel seismic reflection, airgun - OBS refraction profiles, total and three component magnetic data, and gravity data.The proton, precession and fluxgate magnetometer data, tilt of the measuring devices by use of horizontal and vertical gyroscope are collected by a microcomputer and recorded on a MO disk every two seconds along 400 km-long track lines. Number of complete circular turns of the vessels heading were run during the cruise in order to eliminate the effect of the ship's magnetization from the field data. The three component anomalies can then be calculated by referring to the IGRF90.The set of new three vector geomagnetic anomalies obtained by this study, and the total magnetic profiles compiled so far show that asymmetric and discontinuous seafloor geography and roughness of the acoustic basement reflect the variability of the spatial and temporary spreading rate and direction. Less