埔里盆地的地形面、活動構造與地形演育

Abstract

埔里盆地位在台灣地理中心附近，屬於雪山山脈南段；大肚溪（烏溪）上游支流南港溪和眉溪流經其間。本研究利用航照判讀、野外實察、地形計測等方法，將埔里 盆地的地形面分為5 段，包括高位階地（LT）3 段，低位階地（FT）和沖積平原（FP）各1 段。由老到新依序分布如下︰LT1 分布在盆地西北部大坪頂一帶者，形成向斜與背斜構造，向東南及南傾動，分布在盆地西南部暨南大學一帶者向東及東南撓曲。LT2 分布在盆地中央略靠西北側水蛙窟一帶及盆地南側蜈蚣窟附近一小塊。LT3 則分布於烏牛欄及坪仔頂。FT 主要分布於盆地西口的眉溪南岸和南港溪兩岸。盆地底部大部分為FP 面，包括氾濫原、現生河床和沖積扇。在LT 面上變位的活動構造，包括背斜和線形各5 段，向斜2 段，多呈北北東走向。背斜及向斜使地形面彎曲，其中，赤崁頂背斜由台糖赤崁頂農場向南南西延伸經烏牛欄階地西部，轉西南至外大坪，長約6 公里。背斜軸部偏西，越老的地形面變位愈大。外大坪（LT1 ）東側撓曲崖高約100 公尺。本區之線形構造地形包括線形谷、線形崖、撓曲崖、地形面傾動、構造隆起、反斜崖等。由地形面與活動構造的相互關係，可推論埔里盆地自LT1 面形成以來的地形演育：（1）由埔里盆地北緣礫石沈積的覆瓦狀構造顯示LT1 形成時，北港溪南流入埔里盆地，南港溪及眉溪也注入盆地堆積，分別形成沖積扇，包括北側的大坪頂扇，南側的外大坪扇及眉溪扇。（2）嗣後，隨著LT1 面的隆起，北港溪轉向西流，遺留下盆地北邊的大坪頂殘面。南港溪及其支流眉溪在盆地內的持續下切，則依次形成LT2 、LT3 。由變位地形的證據可知，顯著的背斜、向斜構造及傾動運動，一直持續至LT3 面形成。FT 形成以來的地形面則未發現任何變形。（3）嗣後，各河扇階面受順向河切割，形成丘陵或殘面。目前盆底各溪盛行埋積，現生的眉溪沖積扇及南港溪沖積扇 （FP）廣大分布。

Based on aerial photograph interpretation, field investigation and morphometry, the Quaternary geomorphic surfaces of the Puli Basin have been recognized. Three steps of lateritic terrace (LT), one step of fluvial terrace (FT) and one step of fluvial plain (FP) are correlated mainly according to their relative height. Deformed features in the Puli Basin are characterized by the anti-synclinal forms and lineaments including linear valleys, linear scarps, tilting surfaces, tectonic bulges and range-facing scarplets, with a NNE orientation. They have been identified from LT1 to LT3 surfaces but not from the younger surfaces (FT and FP). Among them, surface Tapinding (LT1) in the northern basin tilts to S-SE and surface Waitaping (LT1) in the southern basin flexes to east. Based on the relationship between active structures and geomorphic surfaces, the evolution of the Puli Basin since late Pleistocene is interpreted as follows: (1) According to the imbrication of gravel layers, the Peikang River flowed southwardly into the Puli Basin while the Nankang River and the Mei River drained westerly into the basin, and the Tapinding, Waitapin and Meihsi alluvial fans were formed around basin accordingly. (2) Under the effect of uplifting, the Peikang River avulsed and flowed westward and surface LT1, LT2 and LT3 around Tapinding and Waitapin formed subsequently. They have also been continuously compressed and tilted by the stress mainly from southeast. (3) Deformed surfaces have been dissected and modified since then and the modern fans and flood plains dominate the basin bottom.