利用反射震測探討南海東北部被動大陸邊緣地形與構造特徵

Abstract

海底地形的成因與地殼性質的劃分一直是南海東北部大陸邊緣一個讓人困擾的問題。典型的被動大陸邊緣地形上包含了陸地延伸的大陸棚、地形陡降的大陸斜坡、斜坡下方沉積物堆積的大陸隆堆與深海盆地，主要是依據地形的變化與坡度來做區別，其代表了大陸邊緣受到拉張應力的環境。南海東北部被動大陸邊緣地形與典型被動大陸邊緣相異，通常陸棚與陸坡的劇烈坡度變化會出現在水深約兩百公尺處，而在南海東北部陸棚與陸坡坡折處發生在水深較深的地區。大陸斜坡坡度也較附近地區的斜坡坡度為緩，範圍也較寬。大陸隆堆與深海海盆水深海底地形較高，且海盆中許多高低起伏，因此南海東北部是被動大陸邊緣在地形上是一個特殊的區域。 本研究利用新蒐集到11條，總共2853公里長的的反射震測資料調查東沙島東部的構造特徵，從構造特徵與分布解釋東沙地區地形是受到構造所控制，下部陸棚邊緣基盤抬升與陸地方向的基盤陷落形成一個的盆地，沉積物在此堆積形成下部陸棚。大陸斜坡下張裂盆地的基盤持續陷落造成斜坡坡度平緩。而東沙島南端斜坡上有一期範圍較大的火山活動，由推測的定年認為是發生在1.23個百萬年。深海盆地中基盤高度高於其他區域深海盆地，認為海盆的地形高區是受到火成活動與基盤的斷塊活動所導致。 在地殼型態方面，南海東北部被認為是有大陸地殼-海洋地殼過渡帶，南海的地殼過渡帶在被動大陸邊緣特徵呈現為變薄的大陸地殼上有火成活動發生且下部地殼有一高速帶的區域。但是在不同的地球物理調查中，過渡帶的位置卻也不盡相同。本研究認為過渡帶在基盤開始快速下陷的區域至海洋地殼之間。在研究區域內，由深海盆地的基盤斷塊運動方式，認為是受到拉張的大陸地殼，而非海洋地殼。因此本研究認為過渡帶範圍在深海盆地較類似於Wang et al. (2006)所提出來的版本。

Morphology and crustal type are annoying problems in continental margin off northeastern South China Sea . Typical passive continental margin includes land extended continental shelf, steep continental slope, continental rise and deep sea basins, they divide by morphology and the slope gradient variation Morphology in Passive margin northeastern South China Sea is different with typical passive continental margin, shelf break is usually occurs at 200 meters depth, at the continental margin northeastern South China Sea presents at deeper part. In continental slope, the gradient is slow than surrounding areas and the range is wider. Continental rise and deep sea basin is higher than other place, and it appear many ups and downs, so the passive continental margin northeastern South China Sea is a special area. In this study, a new data set have 11 lines, 2,853 km reflection seismic data to survey structural characteristics and distribution east of Dongsha region, to explain the morphology controlling by structure. The basement uplifted at the edge of lower shelf and landward basin created by basement rifting. Sediment accumulated in the basin and become a flat lower shelf. Under the continental slope, rifted basin induced by basement subsiding and it made the slope gradient becoming slowly. There were a regional volcanic activity at the continental slope south of Dongsha region, and the activity could be dating at 1.23 millions ago. Oceanic basin in study area, the basement depth is higher than other region's basins, we suppose the high morphology in the basin was caused by igneous activity and titled blocks activity on the basement. Northeastern South China Sea was supposed that have continental-oceanic transition zone crust(COT), the structure characteristics in the transition zone have some igneous activities in the thinned continental crust, and it also have a high velocity zone at the lower crust. But according to different geophysical surveys, the crustal type distributions are not the same. This study suggests that the transition zone in the northern South China Sea contains the area between the basement where subsides rapidly to the typical oceanic crust. In our study area, we suppose that the crustal type is thinned continental crust according to titled blocks movement pattern at the basement of deep ocean basin. Therefore, this study suggests that the continental-oceanic transition zone distribution is similar to the version publish by Wang et al. (2006).