八卦山背斜集水區地形計測指標於活動構造意義之探討

Abstract

研究嘗試以集水區之地形計測指標探討八卦山背斜集水區地形特徵上之分區及該分區於活動構造上之意義，所採用之指標有面積高度積分、集水區不對稱性、一級河 之河道坡度、及水系密度、河流頻率（之後轉換為河流頻率密度比）、細長比、崎嶇數等。 研究結果顯示，七項地形計測指標於因素分析後，以一級河之河道坡度、水系密度、河流頻率密度比、崎嶇數等四項最具代表性，其後再以此四項指標利用集群分析 將八卦山背斜之125 個集水區分為I、II、III 三區，並推測背斜北段之區域I 及區域II 其分區主為反應集水區底岩岩性之差別，而西翼南段區域 III 與區域 I 之分區則主為反應地表抬升速率之差異。此外西翼中段有一計測值相對較小區，推測應為「八卦轉換斷層帶」通過八卦山背斜所致。

This study attempts to identify topographic subareas of Pakuashan Anticline by calculating geomorphic indices of watersheds to interpret their tectonic meanings. Seven indices, including hypsometric integral, drainage asymmetry factor, first-order channel gradient, drainage density, stream frequency (were transformed to frequency-density ratio), elongation ratio, and ruggedness number, have been used in this study. First-order channel gradient, drainage density, stream frequency-density ratio (F/D1.8129), and ruggedness number are selected based on factor analysis. According to this result, cluster analysis is applied to identify 125 watersheds on Pakuashan Anticline and dissects this area into three subareas. Subarea I and II mainly highlight the differences in lithology of bedrock; while subarea III and I mainly stress the differences in uplift rate of surface. There is an area with lower geomorphic indices than those of surrounding subareas in central part of western Pakuashan Anticline. It is inferred that this might be due to that Pakua Transfer Fault Zone passes through Pakuashan Anticline.