南海转换断层成因机制及对南海扩张的启示

ORIGIN OF TRANSFORM FAULTS IN SOUTH CHINA SEA AND ITS IMPLICATIONS FOR SOUTH CHINA SEA SPREADING

转换断层是揭示边缘海盆地演化的一把钥匙,但其成因机制一直都是地质科学研究的难点和热点。基于高分辨率海底构造地貌、盆地发育几何学结构、重磁异常、磁条带分布样式、莫霍面埋深、海底热流和南海北部地震剖面等资料对比,并结合区域地质背景、板块重建、动力学来源,重新厘定了南海海盆转换断层和邻区走滑断层的分布,尤其是其走向,统一归结为NNE向。另外,从转换断层走向的角度入手,提出了边缘海盆地扩张的新模式,并修正了前人对新生代南海的板块重建方案。南海海盆真正的转换断层方向应该是NNE向,并不是传统上认为的NW向。NNE向转换断层及南海的扩张,可能为继承邻区裂解陆缘走滑断层方位的模式。NNE向转换断层实际上就是在南海陆架中广泛分布的NNE向大型右行走滑断层,也是华南地块上一系列NNE向右行走滑断裂在海上的自然延伸,进而提出南海转换断层实际上是继承了邻区陆地上大型走滑断层的走向。这种模式并不是类似于正常洋中脊那样的正向扩张,而是一种斜向扩张,转换断层不垂直于扩张中心。

Transform faults are the keys to reveal the evolution of marginal seas. However, the mechanism of their formation remains a puzzle and hotspot in geosciences. Based on high-resolution tectonomorpholo- gy, basin geometry, gravity and magnetic anomalies, magnetic lineation pattern, Moho contours, submarine heat flow, and seismic profiles in the Northern South China Sea, together with studies of regional geo- logical settings, plate reconstruction and geodynamic analysis, this paper tries to redefine the spatial distri- bution pattern of transform faults in the SCS and strike-slip faults in adjacent areas. Our data suggests that transform faults and strike-slip faults are in general in NNE-trending in the SCS. Therefore, a new model of marginal sea spreading is proposed against the previous Cenozoic plate movement models concerning the East Asian Continental Margin and SCS, which believed that the strike of transform faults was NW. The NNE-trending transform faults in the SCS may he the ones inherited from the strike-slip faults at the rif- ting continental margin. They are actually part of the large-scale NNE-trending dextral strike-slip faults distributed in the continental shelf of the SCS. Therefore, they could be treated as the natural extension of the NNE-trending strike-slip faults in the South China Block. They are not analogous to the positive expansion of the normal mid-ocean ridge, but a kind of oblique spreading. They are not perpendicular to the spreading axis.