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由于缺少有效钻孔资料,对于南海扩张的时间一直存在较大的疑问.在南海三大海盆中,西北次海盆面积最小、磁条带特征不明显,因此对其扩张年代的争议最大.最新采集的高密度(小于10km测线间距)船测地磁资料清晰地显示了西北次海盆磁条带的存在.在OBS和多道地震资料的约束下,利用船测地磁资料,本文对西北次海盆的地壳年龄进行了重追踪.根据定量的比较,西北次海盆的主体扩张始于35.8Ma(C16n,2n),在34.7Ma(C15)时其西南部开始扩张,扩张最终同时终止于33.2Ma(C13n),整体的全扩张速率在40～50mm/a之间.这表明南海的扩张可能首先起源于西北次海盆,在其结束扩张后,东部次海盆才开始打开(约30Ma).得益于数据精度和密度的提高,利用化极后的磁力异常以及反演的磁化强度可以对西北次海盆进行二级中脊段的划分.我们共划分出六个中脊段和一个明确的转换断层.中脊的分段性与OBS反演的地壳厚度的变化相一致.转换断层东侧,中脊主体分为四个中脊段,每个中脊段长度均在30km左右.转换断层西侧,存在一个长约50km的中脊段和一个不确切的中脊段.中脊段上磁化强度的变化幅值和中脊段长度在整体上成正比.每个中脊段中央的磁化强度弱于中脊段两端的磁化强度,这与扩张速率相近的大西洋中脊的磁化强度特征一致.
Due to the lack of effective drilling data, there is a big doubt about the expansion time of the South China Sea.In the three major basins in the South China Sea, the area of the northwestern sub-basin is the smallest and the characteristics of the magnetic stripe are not obvious, so it is the most controversial point in its expansion. The high-density (less than 10 km line spacing) ship-based geomagnetic data clearly shows the presence of the northwest sub-sea basin. Under the constraints of OBS and multi-channel seismic data, According to the quantitative comparison, the main expansion of the Northwest sub-sea basin began at 35.8Ma (C16n, 2n), and began to expand at the southwest of 34.7Ma (C15). The expansion eventually ended at 33.2Ma (C13n ), And the overall rate of expansion is between 40 and 50 mm / a, indicating that the expansion of the South China Sea may first originate in the northwestern sub-sea basin and only open in the eastern sub-sea basin (about 30 Ma) after its expansion. We can divide the secondary ridges of the northwest subbasin by using the magnetic anomalies after the magnetization and the magnetization of the inversion.We have divided six ridges and a definite transitional fault Ridge points In the eastern side of the transitional fault, the main body of the mid-ridge is divided into four mid-ridge sections, each of which has a length of about 30km. On the west side of the transitional fault, there is an area of about 50 km Of the midrib and an inaccurate midrib.The amplitude of the magnetization in the midrib is proportional to the total length of the midrib.The magnetization at the center of each midrib is weaker than that of the midrib Magnetization, which is consistent with the magnetization characteristics of mid-Atlantic ridges with similar expansion rates.