南海東北部大陸邊緣的構造型態

Abstract

南海在地理上是位於歐亞大陸東南緣的一個邊緣海，可分成三大構造區塊：北部的大陸邊緣、中央的洋底盆地及南邊的大陸邊緣。其中，南海東北部緊連著大陸東南邊緣和臺灣褶皺逆衝帶。根據新的海洋磁力異常條帶顯示，南海東北角的擴張時間約在37個百萬年前左右，相對應的磁力異常條帶為C15到C17，並推論整個南海最老的海洋地殼應落在南海東北部。 本研究是利用美國藍賽斯號研究船2009年TAIGER計畫中所收集到的長支距多頻道反射震測剖面來探討南海東北部的火成岩體分佈及整個區域性的地體構造。從幾條垂直大陸邊緣的剖面中都可以清楚的描繪出正斷層、火成岩體及斜坡盆地。如果以福爾摩沙海底峽谷（東經118度左右）為界線，以西的大陸斜坡坡度緩且有較寬廣的斜坡範圍，區域內的火成岩體在東沙島以南的區域分佈最多，岩漿活動共分成四期；以東緊鄰台灣的部分，大陸斜坡坡度陡且漸趨狹窄，區域內的火成岩體多半是侵入岩的形式，大約是早漸新世活動。 多次的岩漿活動導致了此地區基盤分佈的差異性，基盤最高的區域為東沙島以南的大陸斜坡上。在斜坡盆地之下，則有異常厚的地殼分佈，推斷是南海擴張結束後再一次的岩漿入侵所造成的結果。整個南海最北部地殼最厚的地區落在東沙島以南附近，海洋地殼厚度約10到20公里左右，為火成活動影響的結果。甚至在MGL0905-10線的剖面中描繪出部分莫荷面的分佈位置，在莫荷面之上則觀察到一些疑似下地殼高速帶的強反射訊號，以上均能顯示火成活動的確在南海東北部構造演化上扮演著重要的角色。

The South China Sea (SCS) is a marginal sea offshore Southeast Asia which can be divided into three portions, the northern continental margin, the central oceanic basin and the southern continental margin. The northernmost SCS abuts the Southeastern Asia continental margin and is contiguous to the Taiwan accretionary wedge in the northeast. Based on latest marine magnetic studies, seafloor spreading of the northeastern SCS took place about 37 Ma, associated with magnetic anomalies C15 to C17 and suggesting that northernmost SCS is the oldest oceanic crust of the SCS. By analysis new deep seismic reflection profiles collected by the R/V Marcus G. Langseth during the 2009 TAIGER survey and together with previously published seismic sections, we have mapped many structures in the northeastern South China Sea continental margin where normal faults, igneous bodies, and slope basins are prevailing. Taking Formosa Ridge (118°E or so) for a boundary, morphology of the northeastern SCS continental margin shows distinctive variation. From wide and gentle dipping continental slope to the west of 118°E because of the most igneous activities in the region around south of Dongsha Island, to steeply dipping and narrow continental slope near Taiwan. Different basement patterns are observed in different areas of the northeastern SCS due to the multiple phases of igneous activities. We can also approximately define some deep strong Moho deflections on MGL0905-10. Beneath the continental slope, there lies unusually thickened crust which may be formed during the SCS spreading period through underplating process and later magmatic events. A regional maximum of crustal thickness is located around Dongsha Island area. There are at least four events of igneous activities in the northernmost SCS continental margin, and our study indeed shows that multiple volcanic activities have a great influence on the Cenozoic evolution of this region.