The magnetic lineation identifications and segmentation of the northwestern sub-basin in the South China Sea

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 similar to 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.