南海海底地形與年齡關係之探討

Abstract

南中國海 (South China Sea)位於歐亞大陸的東南邊緣，為西太平洋最大邊緣海 (marginal sea)，環繞在南海的外圍西、北、南三面為歐亞板塊的大陸棚區，海洋地殼則是分佈在其平均水深最深的南中國海海盆 (South China Sea Basin)內，海盆的東面則為隱沒帶-馬尼拉海溝與菲律賓海板塊為界。南海整體發展歷史由原本的聚合到後來的張裂，張裂的歷史約從32百萬年前開始，25百萬年發生洋脊跳躍 (ridge jump)，並在約16百萬年前結束，擴張結束後沿著擴張中心發生一連串火山作用，其複雜的擴張歷史在南中國海東部產生了複雜的地體構造。 海盆擴張隨著遠離中洋脊 (mid ocean ridge)海洋地殼之熱構造因為海水的冷卻隨之不同，深度也不斷加深，而在其他大洋也有許多前人探討洋盆海洋地殼深度與年齡之間的關係，發現在海洋地殼深度與年齡的開根號呈線性關係，並隨著海洋地殼年齡增加而有些微的變化。本研究利用分布於南海的22條反射震測剖面，搭配前人研究的磁力異常條帶，並在處理資料過程中挑選合適之資料，經由各個資料點並對其上覆沉積物進行沉積物修正，使南海海洋地殼回到均衡狀態擴張下應有的深度，並對應到南海海盆海洋地殼磁料條帶上，來探討南海海洋地殼深度與年齡之間的關係，找出關係式並與其他大洋做比較。研究結果顯示南海海洋地殼深度 (d)與年齡 (t)開根號亦成比例，其深度與年齡之關係式為 d(t)=-3024-387√t，與其他大洋相比，整體沉降曲線較陡，而初始張裂洋脊中心的深度較深，約3024公尺深，比現在所觀察到南海殘餘擴張中心深度還深，為後擴張之火山作用所造成。利用南海海洋地殼深度與年齡關係式得知南海海洋地殼形成時之深度，發現南海洋脊下方地函岩石圈厚度比起其他大洋還薄約3公里，可能與南海形成時洋脊下方區域溫度較高有關。

South China Sea (SCS) is the largest marginal sea located to the southeast of the Eurasian continent. The west, north and south portions of the South China Sea belong to continental shelves, while the Manila subduction zone exists in the east of the South China. The onset of seafloor spreading in the South China Sea was suggested to be ∼32 Ma. After a ridge jump around 25 Ma, the Southwest sub-basin started to open. The spreading of the entire basin ended at ~16 Ma, then a phase of post-magmatic seamount formation has occurred. As shown in the magnetic stripes of the ocean floor, the age of the oceanic crust increases with the distance away from the mid-ocean ridges. The thermal structures of the oceanic curst away from mid ocean ridge are different because of the plate cooling. Previous studies suggested that the oceanic crust depth is proportional to the square root of the mean age. In this study, we use 22 reflection seismic profiles distributed in the South China Sea and the associated magnetic age. After sediment correction, we suggest that the oceanic crust depth in the South China Sea is also directly proportional to the square root of the mean age of the South China Sea. The subsidence rate in the South China Sea is slower and the initial depth of the ridge is deeper than other oceans. However, the mantle lithosphere thickness beneath ridge in South China Sea is thinner than other ocean basins. We suggest that it would be due to a hotter mantle beneath the South China Sea.