南海西北次海盆的磁条带重追踪及洋中脊分段性

摘要

Since there is no valid deep sea drilling core, there are some debates about the opening age of the South China Sea (Nanhai). Owning to the small area, few magnetic polarity reversals and low magnetic anomalies amplitude of the northwestern sub-basin, the controversies about its opening age are very intense. The latest marine magnetic profiles which are closely spaced less than 10 km reveal the obvious magnetic lineation in the northwestern sub-basin. Constrained by newly measured ocean bottom seismic and multi-channel seismic data, we present a new identification to the latest marine magnetic profiles. According to the results of quantitative comparison, we prefer the northwestern sub-basin initially opened at 35.8 Ma (C16n, 2n), propagated to southwest at about 34. 7 Ma (C15) and all relict ridges ceased at the same time (33. 2 Ma, C13n). The full spreading rate is about 40~50 mm/a. The result implies the South China Sea was initially opened in the northwestern sub-basin and the eastern basin began to spread (about 30 Ma) after the cease of the northwestern sub-basin. The accurate and dense data give the great opportunity to study the segmentations with magnetic anomalies reducted to the pole and the inversed magnetization. Six segments and one obvious transform fault are revealed here and the segmentation is consistent with the crustal thickness variation based on OBS data. To the east of the transform fault, the main part of relict ridge includes four segments which are connected by non-transform offsets and each segment is about 30 km. To the west of the transform fault, one segment is about 50 km and another one is uncertain here. The variation of magnetization amplitude along each segment is correlated with corresponding segment length. Magnetization in the center of segments is lower than that of segments ends and its magnetic structure along spreading axis is similar to the slow-spreading mid-Atlantic ridge which also has similar full spreading rate.%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.