東沙島以南大陸邊緣火成岩體時空分布之探討

Abstract

本研究使用OR1－840和OR1－883兩個航次所收集來的多頻道反射震測資料，以及2007年MD－162航次的水深與磁力資料，和日本地調所1996年整編的東亞磁力異常圖資料，來探討南海北部大陸邊緣在東沙島附近海域中的火成岩體分布，再利用ODP 184航次的鑽井定年資料，來推估研究區域內岩漿活動的大致時間。 研究區域中一共辨識出60個火成岩體，7個分布在大陸斜坡上部，28個分布在大陸斜坡下部，25個分布在深海海盆區。另外利用ODP 184航次在研究區域範圍中鑽井所獲得地層年代與本研究所用之震測剖面比對，認為研究區域內岩漿活動可分為四期：第一期發生在早漸新世，南海剛開始擴張的時候；第二期發生在早中新世南海正在擴張的時候；第三期發生在晚中新世南海停止擴張的時候；第四期的岩漿活動則發生在第四紀。 本研究認為南海北部大陸邊緣是屬於火山型大陸邊緣，下地殼高速層是南海擴張時期底侵作用所形成的薄層下地殼高速層，分布在整個南海北部大陸斜坡底下，但是到擴張結束後，地函岩漿的一個湧升事件，在研究區域內再次形成了底侵作用，而讓研究區域有比西側的大陸邊緣更厚的下地殼高速層，此次底侵作用（岩漿湧升）的位置主要位於為東沙島南邊的大陸邊緣東半部。

This research used multi-channel seismic reflection data（collected from OR1 840 and OR1 883）and magnetic anomaly data（from cruise MD162 and the gridded Magnetic Anomaly Map of East Asia compiled by the Geological Survey of Japan in 1996）to probe the distribution of igneous bodies in the continental margin and deep sea basin of the northern South China sea south of the Dongsha Island. We also utilized borehole data from ODP Leg 184 to provide age constraints of the sedimentary layers and propose the ages of various igneous activities based on the relationships of magma intrusion and sediment deposition. This study has identified up to 60 igneous bodies from the seismic reflection profiles used. Among them, 7 igneous bodies are distributed in the upper continental slope, 28 igneous bodies are distributed in the lower continental slope, and 25 igneous bodies are distributed in the deep sea basin area. Magmatic activities can be grouped into four stages. The first stage occurred in the early Oligocene when the South China Sea began to rift. The second stage occurred in early Miocene when the South China Sea was spreading. The third stage occurred in late Miocene when the South China Sea just ceased opening. The magmatic activity of the fourth stage took place in Quaternary Period. We thus suggest that the continental margin of the north South China Sea is regarded as a “volcanic rifted margin”, the high velocity materials in the lower crust reported from seismic refraction studies were formed during the South China Sea expansion period through underplating processes, and distributed beneath the continent slope along the whole northern South China Sea continental margin. After the seafloor spreading ceased, another magmatic event occurred in the eastern portion of the continental margin that thickened high velocity material of the lower crust south of the Dongsha Island.