台灣西南海域海底水系分析

Abstract

本研究利用水系分析技術建立台灣西南海域海底峽谷流徑分布與峽谷系統特性，並探討海底峽谷系統特性與區域構造作用及沉積作用的關聯。研究範圍為東經119度到121度，北緯21度到23度50度的海域，研究區域中包含多條海底峽谷，較大的有澎湖海底峽谷、高屏海底峽谷、枋寮海底峽谷及福爾摩沙海底峽谷等。 本研究利用地理資訊系統的地形及水文分析方法，包括坡度、坡向、水系等級、水系空間、水系密度及水系計測分析等，以地形學的原理來描述區域海底峽谷水系的分佈型態及特徵，進而探討地形、地質構造與水系發育受的關聯性，所使用的資料為科技部海洋學門資料庫2010年彙編出的200公尺×200公尺網格水深資料。 研究結果顯示，研究區域內海底峽谷的水系可以分為五個：澎湖峽谷水系（A區）、高屏峽谷水系（B區）、福爾摩沙峽谷水系（C區）、澎湖峽谷東側水系（D區）和澎湖峽谷西側水系（E區）。其中B區、D區和A區西半邊一小部分位於活動大陸邊緣，水系型態主要以平行狀、格子狀及矩形狀為主。平行狀水系的發育主要跟狹小的陸棚和陸坡的地形有關，格子狀水系的發育可能受地表裂隙或節理作用的影響，矩形狀水系的發育則推測是因構造作用產生褶皺、斷層，在海床上產生海脊和海谷所致。C區、E區和東半邊一大部份A區位於被動大陸邊緣，水系型態主要以平行狀和樹枝狀為主。平行狀水系分布在陸坡區，推測是陡峭的地形造成崩移作用，使海底水道的走向均沿同一方向發育；而樹枝狀水系主要是分布在未受構造活動影響的陸棚區域。 水系密度分析結果顯示，A、C及E區平均水系密度為0.34 (公里/平方公里)，略高於B及D區之平均水系密度0.31 (公里/平方公里)。平均坡度分析結果，A、C及E區平均坡度約2.8度，比B及D區平均坡度約4.3度要緩。水系數目分析結果，A、C及E區平均水系數目為661條，多於B及D區的平均水數目為563條。坡向分析結果，不考慮陸棚區（陸棚的坡向較無特定方向），A、C及E區的坡向主要反應海底峽谷和沈積物波的走向，B及D區的坡向則跟區域性的構造走向較有關。 水文資料分析結果顯示，西南海域活動大陸邊緣因構造活動造成陡峭的地形和線形的走向，影響水系的型態、海底水道的數目和長度，以及地形的坡度和坡向。本研究結果顯示GIS地理資訊系統的水系分析技術，可用以分析海底峽谷的分布與特性，進而探討海底峽谷系統的發育與構造的關係。

This study utilize the geographic and hydrological method of the Geographic Information System (GIS) to analyze the distribution and characters of the submarine system offshore southwest Taiwan. The study area is from 119°E to 121°E and 21°N to 23°50’N, where many submarine canyons such as Penghu, Kaoping, Fangliao and Formosa canyon have developed calculate the slope, aspect, numerical analysis on drainage, drainage density and spatial analysis on drainage pattern of seafloor topography, geomorphology theory is then applied to describe the regional characteristics and the features of the drainage related to the submarine canyons, and to discuss the relationship among the topography, structure and river developments. The bathymetry data with gird size 200×200 meters produced by the Ocean Data Bank in 2010 is used to this study. The results show that the drainage pattern of submarine canyons in the study area could be divided into 5 systems: Penghu (A), Kaoping (B), Formosa (C), Penghu East (D) and Penghu West (E). The B, D and a small part of A systems located on active margin are dominated by the parallel, lattice and rectangular drainage pattern. It is inferred that the parallel drainage pattern occurred in restricted areas on continental slope, while the lattice drainage pattern is possibly developed with the fractures or joints on seafloor. As for the rectangular drainage pattern, it could be related to the enlargement of fold or fault structures. The C, E and most parts of A system located on passive margin are dominated by the parallel and tree-like drainage pattern. We explained that submarine channels along the slope is easily developed on the steeper terrain, such as the continental slope in this area while the tree-like system pattern is mainly distributed on the flat continental shelf. The hydrological analyses reveal that the drainage density is on the average of 0.34 km-1 in A,C and E river drainage is a little higher than that in B and D river drainage (0.3 km-1). The average slope 2.8 degree at A, C and E river drainage is smoother compared to that 4.3 degree at B and D river drainage. 661 river numbers on the average for A, C and E are greater than that 556 river numbers on the average for B and D. The major aspects of A, C and E reflect the direction of canyons and sediment waves while the aspects of B and D may relate to the regions structure trend. The results of hydrological analyses show that the significant lineation or features are controlled by the steep seafloor caused from tectonics in the active margin off SW Taiwan. The topography will affect deeply the drainage pattern, the number and length of submarine channel, and the slope and aspect of topography. This study demonstrated that GIS could be successfully applied to analyze the drainage data of submarine canyon system and then to discuss the relationship between the development of the submarine anyon systems and regional structures.