南海北部陆坡海底峡谷形成机制探讨

Forming mechanism of the submarine canyon on the north slope of the South China Sea

通过对南海北部地震剖面的解释,并结合地貌以及区域地质特征等,对发育于南海北部陆坡区的珠江口外、台湾浅滩南以及澎湖海底峡谷的地貌和构造特征进行分析和对比,并对其形成机制进行探讨。研究结果显示,各海底峡谷具有相似的走向,并均具有转向的特征,但是其形成机制却各不相同,由此形成的地貌特征也各不同:珠江口外海底峡谷的形成与珠江带来的大量陆上沉积物的搬运相关,形成了喇叭型的水道;台湾浅滩南海底峡谷的形成受到NW向断裂构造的控制,这些断裂构造形成了薄弱带,经过沉积流的侵蚀而形成狭长的水道,当进入下陆坡后由于海山的阻隔作用而转为近EW向;澎湖海底峡谷带的上段主要是由陆坡沉积流的下向侵蚀、崩塌和滑移形成的,而其下段则主要具有沿马尼拉海沟北向延伸段发育的特征。

Submarine canyons are common features on continental margins wordwide, either active or passive. Most of them are located entirely on the upper slope, while there are also some large canyons with head that deeply indent the shelfbreak. They are generally in ＂V＂ shape （erosional situation） or ＂U＂ shape （depositional situation）. What makes investigation of a submarine canyon interesting is that submarine canyons are prominent topographic features on the sea floors of continental margins. They not only shape the present morphology of continental margins, but also acted as the main conduits for transferring terrestrial sediments toward the deep sea and fed the fan system. Apart from constituting an important archive for documenting the sedimentation process history of an area, submarine canyons are also studied as modern analogues for deepwater hydrocarbon reservoirs because of their association with sand-rich turbidites.The geological and morphological characteristics of the submarine canyons on the north slope of the South China Sea were studied and the forming mechanism were discussed based on the interpretation of the multi-channel seismic profiles obtained during a ＂973＂ survey and other seismic data, including the off Zhujiang River Mouth submarine canyon, the south Taiwan Bank submarine canyon and the Penghu submarine canyon. It shows that these submarine canyons have the similar orientation, the character of direction-turning, but with different forming mechanism： The off Zhujiang River Mouth submarine canyon is related with the terrestrial downslope sediment flows carried by the paleo-Zhujiang River and has the horn shape. The south Taiwan Bank submarine canyon is controlled by the faults in NW direction. These faults weaken the surface strata, and the sediments flows will develop along them. The uplift of the sea mountain in the downslope blocks the flows＇ way and makes them turn into EW direction. The downward erosion, excavation, and sliding by down slope sediment flows are the major forming processes in the upper part of the Penghu submarine canyon, while the lower part mainly develop along the extending of the Manila Trench where the South China Sea is subducting under the Philippine Plate.