南海东北部陆缘浊流活动的地貌记录及其形成机制分析

Geomorphological records of turbidity current activity in the northeastern margin of the South China Sea and analysis of triggering mechanism

超临界浊流影响下的周期阶坎广泛分布在南海东北部台西南盆地的西澎湖峡谷中。由于频繁的构造活动,常年的台风影响,以及来自中国台湾河流的大量沉积物供给,导致这个区域的浊流活动经常发生。本文利用高分辨率的地貌资料,对西澎湖峡谷发育的23个净侵蚀周期阶坎和10个净沉积周期阶坎进行了形态的定量分析,并且统计了流经这些周期阶坎的浊流流速。结果显示,浊流在流经净侵蚀周期阶坎过程中的速度有明显突变,而在净侵蚀到净沉积周期阶坎的过程中速度也发生了明显的降低,前者由峡谷中的坡折带导致,后者则是由限制性环境到非限制性环境的转变造成的。此外,净沉积周期阶坎主要分布于靠近峡谷口外的西南侧,这是由于科氏力对浊流影响的结果。研究此区域周期阶坎的形态特征和形成机制能够帮助我们更好地理解海底地形地貌与浊流的相互关系,对于理解峡谷中的地貌演化具有重要意义。

Cyclic steps caused by supercritical turbidity currents are distributed widely along the West Penghu Canyon in the Taixinan Basin of the northeastern South China Sea. Turbidity currents occur frequently in this area due to high tectonic activities, marine factors such as typhoon and delivery of large sediments from rivers. Using high-resolution bathymetric data, we conduct quantitative analysis on the morphology of 23 net-erosional cyclic steps and 10 net-depositional cyclic steps along the West Penghu Canyon, and compute flow velocity of turbidity currents flowing through these cyclic steps. We find that the flow velocity of turbidity currents has an abrupt change in the transition of net-erosional cyclic steps, while the velocity decreases significantly in the transition from net-erosional to net-depositional cyclic steps. The former is mainly caused by the slope break in the canyon, while the latter is caused by the change from confined to unconfined environment. In addition, the net-depositional cyclic steps are located closer to the southwestern flank of the West Penghu Canyon;and we propose that this phenomenon should be mainly caused by turbidity currents affected by the Coriolis force. Investigating the evolution and controlling factors of cyclic steps in this area can help us better understand the interaction of submarine bedforms and turbidity currents, which plays a significant role in the geomorphological evolution along submarine canyons.