東沙島以南大陸邊緣構造及沉積物分布之探討

Structure and Sediment Distribution in the Continental Margin South of the Dongsha Island, South China Sea

陳政儀(Cheng-Yi Chen)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.10640

摘要

本研究處理、分析南海北部東沙環礁以南大陸邊緣帶所收集來之多頻道反射震測資料，以觀察該地區淺層地殼構造與沉積物分布的情況，繪製研究區域沉積物等厚度圖，並藉由沉積物厚度分布與周圍環境地質構造間的相關性，探討火成活動對南海北部大陸邊緣形貌與構造之影響。
研究區域位在南海北部東沙島附近之大陸邊緣帶，包含大陸棚、大陸坡至南海海盆，在大地構造上為典型之非活動型大陸邊緣。從震測剖面可以觀察到研究區域內有許多正斷層構造及火成岩體的分布。正斷層分布範圍從大陸斜坡到深海海盆靠近馬尼拉海溝處都有，其形成原因與南海擴張時地殼張裂有關；火成岩體大致分布於大陸斜坡坡腳，成因可能為南海擴張後期及擴張停止後，上部地函岩漿沿著地殼縫隙及破碎處流動，向上湧升而成。
在整編沉積物分布等厚圖方面，本研究首先在震測剖面上辨識基盤之位置，計算出沿測線沉積物之厚度變化，在由各測線沉積物厚度分布繪製出研究區域內之沉積物等厚度圖。本研究再利用海底地震儀在研究區域內所獲觀測資料模擬出之地殼速度構造資訊，將研究區域分成大陸斜坡及深海平原兩個部分進行討論，推求震波在沉積物間的速度函數，以將原來以來回走時(秒)為單位之沉積物厚度值轉換為以深度(公尺)為單位之厚度值，並為日後探討南海北部鄰近地區沉積物提供速度參考資訊。
沉積物分布等厚圖顯示研究區域內沉積物厚度大致呈現向南海海盆增厚的趨勢，其分布明顯受到正斷層及火成岩等構造活動的影響。研究區域內沉積物厚度變化約250~3400公尺，有三處沉積物較厚的地方：一處在東沙島南方大陸斜坡上，其成因可能是受到大陸棚邊緣及大陸斜坡坡腳火成岩，阻擋住部份由大陸棚向深海海盆傳輸之沉積物，而在大陸斜坡上形成盆地；另一處在東沙島西側大陸斜坡上部，此區域為珠江口盆地的一部分；第三處在南海北部深海平原，此區沉積物有越靠近馬尼拉海溝越厚之現象，可能因南海海洋地殼在海溝處漸深而造成沉積物厚度越厚。

關鍵字

沉積物等厚圖 ； 反射震測 ； 大陸邊緣 ； 東沙島

並列摘要

In this study, we use multi-channel seismic reflection data to investigate shallow geological structures and sediment distribution in the continental margin and northern South China Sea Basin south of the Dongsha Island and discuss the relationships between the sediment distribution and geologic structures observed.
From the seismic profiles, we have identified many normal faults, volcanic bodies and igneous basement in the study area. Most of the normal faults are distributed in the continental slope and in the abyssal plain; they were formed in the syn-rift crust when seafloor spreading of the South China Sea was active. The volcanic rocks are distributed near the foot of the continental slope. They were formed by magma upwelling from the mantle after the seafloor spreading activities of the South China Sea had ceased.
The sediment thickness were determined by identifying the basement depth along each of the seismic profiles, then an isopach map was generated. Velocity information derived from ocean-bottom seismometer data were used to estimate the P-wave velocity profile of the study region. We use this function to convert the sediment isopach map from two-way travel time (second) to depth (meter).
From the isopach map compiled in this study, the sediment thicknesses vary between 250~3400 meters, and the sediment distribution was largely affected by the development of small basins formed due to normal faulting and volcanic activities. The sediment isopach map shows three areas in the study region where thick sediments are present. One of them is the South China Sea abyssal plain where influence of the Manila Trench subduction is apparent. Another one located to the west of the Dongsha Island is the Pearl River Mouth Basin. The third area is located in the continental slope south of the Dongsha Island, where small basins formed by the normal faults in the landward side and uplifted volcanic rocks near the bottom of the continental slope.

並列關鍵字

sediment isopach map ； reflection seismic ； continental margin ； Dongsha Island

參考文獻

Briais, A., Patriat, P., Tapponnier, P., 1993, Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: Implications for the tertiary tectonics of Southeast Asia. J. Geophys. Res., 98, 6299–6328.

Hao, H., Zhang, X., You, H., Wang, R., 2009, Characteristic and hydrocarbon potential of Mesozoic strata in Eastern Pearl River Mouth Basin, Northern South China Sea. Journal of Earth Science, 20, 117-123

Hayes, D.E., Nissen, S.S., 2005, The South China sea margins: Implications for rifting contrasts. Earth Planet Sci. Lett., 237, 601–616.

Hsu, S.-K., Yeh, Y.-C., Doo, W.-B., Tsai, C.-H., 2004, New bathymetry and magnetic lineations identifications in the northernmost South China Sea and their tectonic implication. Marine Geophys. Res., 25, 29-44.

Hamilton, E.L., 1980, Geoacoustic modeling of the sea floor. Naval Ocean Systems Center, San Diego, California 92152.