南海北部地殼構造與深海沈積物波之研究

Study of crustal structures and deep-sea sediment waves in the northern South China Sea

蔡慶輝(Ching-Hui Tsai)

指導教授 ： 許樹坤 ； 李昭興

國立中央大學/地球科學學院/地球物理研究所/博士(2007年)

摘要

本論文主要有兩個主題，其一是使用反射震測資料及重力模擬的方式來探討南海北部大陸邊緣的地殼構造變化；其二是透過高解析震測與水深資料來研究福爾摩沙峽谷下游南北兩側深海沈積物波的分佈和特性。另外，本文也展示南海最東北部新的多頻道水深資料的處理流程與整編結果，提供較詳細的海底地形資料。
南海北部的大陸邊緣地殼構造主要呈現出東北-西南走向，根據地殼厚度與莫荷面坡度的變化，可以將區域內主要的地殼構造區分為三部分：輕度變形的大陸地殼、地殼減薄帶與海洋地殼。地殼減薄帶的莫荷不連續面有較陡的坡度，深度變化從24公里深到14公里深之間，地殼厚度變化則從20公里厚到10公里厚之間。此減薄帶主要分佈在自由空間重力異常相對較低的區間。根據震測剖面的解釋與地殼厚度的比較，及配合磁力異常曲線的振幅變化，海洋地殼存在的範圍應延伸至南海最北端，而大陸地殼與海洋地殼的交界主要是沿著大陸斜坡的坡腳分佈。由於南海北部海底擴張之後有火成活動，使得東沙島附近及其東南區域因火成岩體的出露與侵入，以及下部地殼的板底作用（underplating），在海床上有很多海底火山的出現，其地殼厚度則較一般海洋地殼約6-8公里為厚，大約介於10-12公里之間。
南海北部深海沈積物波的發育主要位於兩個區域，其一為福爾摩沙峽谷下游與澎湖峽谷之間；其二為福爾摩沙峽谷下游與LRTPB（Luzon-Ryukyu Transform Plate Boundary）之間，深度分佈大都位於3000公尺等深線以下的區域，波前走勢主要為平行等深線的方向。其波形主要呈現不對稱上坡遷移的形貌，波高大都小於85公尺，波長大都小於6公里，坡度則小於0.8度。根據濁流模式，濁流主要沿著福爾摩沙峽谷與澎湖峽谷移動，並在峽谷間留下清楚的侵蝕凹槽痕跡。在兩峽谷轉彎處，濁流可能開始發生普遍的溢流現象，因而造成沈積物波的發育，順著重力效應，沈積物隨著濁流往下坡處沈積，使得其波前走勢約略與等深線垂直。由於LRPTB構造所造成的海床地形落差，使得濁流無法跨過LRTPB的南方而形成溢流，因此形成研究區域內沈積物波發育的西南邊界。

關鍵字

南海北部 ； 深海沈積物波 ； 多頻道水深 ； 被動大陸邊緣 ； 地殼構造

並列摘要

There are two main topics presented in this thesis. First is to use reflection seismic data and gravity modeling to study the crustal structures in the northern continental margin of the South China Sea (SCS). Another topic is to use high resolution seismic and chirp sonar data to study deep-sea sedimemt waves in the north and south sides of downstream of the Formosa Canyon. Additionally, we present the foundamatal procedures of data collecting and processing of the multi-beam bathymetry data and show a preliminary result of new bathymetry map off southwest Taiwan.
We can distinguish the crustal strutures in the northern margin of the South China Sea into three partions: the continental crust (CC), the thinned continental crust (TCC) and oceanic crust (OC). The TCC zone displays a steeper slope and Moho depth becomes shallow oceanward from about 24 km to about 14 km deep. The corresponding crustal thickness is from 20 km thick to about 10 km thick and is located in a relatively low free-air gravity zone. According to the seicmic data, crustal modeling results and magnetic anomaly curves, the OC could be existed further northward and the continent-ocean boundary is along the base of continental slope. Because the volcanism happened after sea floor spreading, there are many intrusive or extrusive igneous bodies around the Dongsha Island. Thus, we can find a possible underplating in this area. Volcanism in this area also causes the abnormal thick oceanic crust in the southeast of the Dongsha Island.
In the northern SCS, the deep-sea sediment wave fields are located in two zones, one is between downstream of the Formosa Canyon and Penghu Canyon, the other is between downstream of the Formosa Canon and LRTPB (Luzon-Ryukyu Transform Plate Boundary). The distribution of these deep-sea sediment waves are found below 3000 m deep and the wave front is rather parallel to the bathymetric contour lines. Base on high resolution sesmic data, we suggest that these sediment waves are caused by turbidity currents. The sediment waves have shown asymmetrical internal structures which may induced by high gravity flow activity. Most of these sediment waves are less than 85 m high, and most of wavelengthes are less than 6 km long. Most of slopes are less 0.8 degree. We suggest that the Formosa Canyon and Penghu Canyon are two major sediment transportation channels. The turbidity flows downward and may overflow directly from the turning points of the two canyons. The sediment deposits in the stoss side and erodes in the lee side. It proceedes downslope continuously because of gravity effect. Due to the bathymetric offset of the LRTPB, the turbidity current can not overflow southwards from this tectonic structure, which makes LRTPB a natural sediment wave dam.

並列關鍵字

multi-beam bathymetry ； crustal structure ； passive continental margin ； northern South China Sea ； deep-sea sediment wave

參考文獻

李昶緯: 南海北部深水區非線性內波之傳播特性. 國立台灣大學海洋研究所碩士論文, 台灣台北, 2007.

馮力中: 台灣南部海域增積岩體的構造特徵. 國立台灣大學海洋研究所碩士論文, 台灣台北, 2006.

Allen, J. R. L.: The Bouma division A and the possible duration of turbidity currents, J. Sedim. Petrol., 61, 291-295, 1991.

Asquith, J. M.: Nature and origin of the lower continental rise hills, off the east coast of the United States, Marine Geology, 32, 165-190, 1979.

Bagnold, R. A.: Auto-suspension of transported sediment; turbidity currents, Proc. R. Soc. Lond., A, 265, 315-319, 1962.