由水系談蘭嶼的地形演育

The Evolution of the Lanyu Volcanic Island as Revealed by the Drainage Network Analysis

蕭謙麗(Chi'en-Li Hsiao) ； 林明聖(Ming-Sheng Lin) ； 楊榮堃(Rong-Kuan Yang) ； 德方丹(Benoit Deffontaines)

《東台灣研究》 3期 (1998/12) Pp. 31-49

https://doi.org/10.6275/JETS.3.31-49.1998

摘要

蘭嶼地區是個十分典型且相當年輕的火山島嶼。由於交通不便、缺乏露頭、且地層與岩性不易分辨，造成蘭嶼區域的地質調查工作相當困難。本研究採用水系形態的分類及河流發育等級兩種獨立的分析方法，並參考地形局部高區，進行綜合分析各放射中心的發育先後，繼而探討蘭嶼的地形發育史。
由水系分析結果所判釋的蘭嶼火山島的地形演育過程如下：最先露出水面的是青蛇山－山田山－紅頭山－線的山脊，這也是該火山島的初期水系發育中心。接著，東北方山嶺開始發育；稍晚，西北與東南方山嶺再相繼發育，形成今日的西北－東南向之地形主軸，緊接著，尖禿山和相愛山也隨即發育，構成本島另一個呈東北－西南向的主軸，此時期的蘭嶼島西北部與東南部尚未連接在一起，有如今日的蘭嶼本島與小蘭嶼的縮影。由於地殼運動十分活躍，奧本嶺與山田山逐漸銜接起來演育成今日的地貌。

關鍵字

蘭嶼 ； 水系形態 ； 河流等級 ； 火山島 ； 地形發育史

並列摘要

A survey has been done on the recent geology in the Lanyu Island, southeastern Taiwan. In addition to field observations, physiographic methods were applied and an analysis was made of the drainage pattern. In the morphometric analysis, this enables the recognition of neotectonic discontinuities and lithological contrasts. The analysis of drainage patterns that they include their perturbations and anomalies can also reveal the presence of features due to neotectonics, structural control or lithological variation.
The study is based upon 1:50,000 topographic maps. Throughout the drainage analysis, first we knew that a whole Lanyu volcanic island is a radial drainage pattern. It can be divide into many radial drainage centers, some dendritic patterns and several parallel ones by regional analysis. The radial centers are located on each volcanic dome and the parallel ones distributed around the island. On detailed drainage analysis, we picked out the anomalies of drainage network. It shows that the trend of anomalous lineations is near northwestern to southeastern of the whole island. We take together the trend of anomalous lineations and the distribution of radial centers. Then we can predict the Yeyu dextral strike-slip fault which is trending NEE to SWW.
According to Strahler's stream ordering, the highest-order stream in Lanyu is designated 5. The lowest order is 1. Most of the highest order streams are located in the middle part. The results of the radial drainage center study are shown on a map where the respective stream order is also indicated.
Concerning the study, we reached the conclusion that we can make sure the inferred the fault-line and activity by the analyses of the morphometric and the drainage pattern. Advanced, we can get more information about the evolution of topography.

並列關鍵字

Lanyu ； drainage network analysis ； evolution ； physiography