南海北部陆坡高速堆积体的构造成因

南海北部陆坡是南海海域沉积活动最为活跃的地区之一,发育着迄今为止所发现的南海最高沉积速率堆积体。构成该堆积体的沉积物究竟来自何方,仍是南海沉积学研究中未解决的问题之一。通过对南海北部多道地震剖面的解释和海底底流观测资料的分析,指出南海北部中新统披覆层是南海北部断陷阶段结束后开始沉积的一套地层,直到上新世开始前,该地层在很大一个区域内是保持水平的;上新世后,由于构造抬升,中新统披覆层随之隆起,并在东沙隆起的部位遭受很大程度的剥蚀,其剥蚀量和南海北部珠江、韩江以及台湾西南部高屏溪和曾文溪向南海的输沙量相当,为南海北部一个非常重要的沉积物来源。研究分析指出,如果珠江和韩江所携带的沉积物全部沉积到南海北部陆坡区,则可获得的沉积速率为12 cm/ka,这一数值远低于该高速堆积体上的沉积速率值。从南海北部现今的沉积动力条件和地形上看,来自珠江和韩江的沉积物几乎不可能经过平坦的陆架区,再绕过东沙岛,优先沉积到该高速堆积体上。本研究中的高速堆积体的沉积物也不可能主要来自台湾西南的高屏溪和曾文溪,因为台西南河流所携带的沉积物被特殊的洋流体系圈闭于台湾周边一个较小的范围内沉积下来。观测数据表明,南海北部东沙隆起区有足够强的水动力环境能够剥蚀海底隆起的沉积地层,并将剥蚀下来的沉积物向南经陆架输运到陆坡区;该高速堆积体紧邻东沙隆起剥蚀区,其沉积物来源应该主要来自东沙隆起剥蚀区。

Tectonic Control on the Formation of High-deposition-rate Sediment Drift in the Northern Slope of the South China Sea

The northern slope of the South China Sea is one of the most active areas in the world oceans with modern sedimentary process.A high-deposition-rate sediment drift was found on the down slope of Dongsha islands.The material sources of this sediment drift were examined and discussed by several other papers,but not yet solved.Based on the multi-channel seismic data and the long term bottom current observation data,this paper finds a new source for this sediment drift.The seismic profiles show that a Miocene drape formation which was started from the end of Oligocene and ended to the Early Pliocene was very well and originally developed as a horizontal layer in the study area.Since about 3 Ma,the Miocene drape formation was uplifted and eroded along Dongsha area.The amount of the eroded materials from the Miocene drape formation could be as much as that from Pearl River and Hanjiang River to the northern South China Sea,and that from the rivers of southwest Taiwan to north South China Sea.The calculated result shows that a sediment rate of only 12 cm/ka could be reached if all the materials from Pearl River and Hanjiang River deposited on the northern slope of the South China Sea.So,it seems impossible that the sediments on the high-deposition-rate sediment drift are mainly from Pearl River and Hanjiang River.Considering the special oceanic current regime of the northern South China Sea and its adjacent areas,most part of the sediments from rivers of southwestern Taiwan was trapped in the area around Taiwan,and only small part could move far to and deposit on the high-deposition-rate sediment drift.Bottom current observation data show that the hydrodynamic environment of the northern South China Sea Shelf is powerful enough to erode the uplifted Miocene drape formation at the sea floor and transfers the eroded materials along the northern shelf of the South China Sea southward to the slope area,and these materials become the main source of the high-deposition-rate sediment drift.