台灣造山帶之河流地形動態演育

Abstract

河流形貌演育深受構造與氣候等環境變化控制，而地形與地質學家能根據河流地形特徵重建過去環境演化歷史。分水嶺為流域的邊界，過去多數河流相關研究不討論分水嶺位置隨時間的移動，常假設流域範圍於研究時間尺度中不變，但如果分水嶺兩側的流域抬升與侵蝕作用不同，將移動分水嶺位置，從而改變兩側水系的流域面積、河流能量與下游沉積盆地紀錄。若能成功建立各流域範圍隨時間的變化，將有助於提升後續地形與地質解釋的完整性。 本研究建立一套討論流域範圍演育的流程，並進一步找出演育機制與時間尺度。利用近期新提出的河流地形參數χ值，搭配衛星影像判釋、地形測量、河流陡峭度指標（ksn）與野外調查等方式，能有效指出台灣地區各流域的穩定狀態以及其不穩定時的演育趨勢。此流程具有普遍性與發展潛力，能應用於不同構造環境。 於台灣的測試結果顯示，構造相對穩定的雪山山脈北段，河流地形展現不穩定的主分水嶺位置，分水嶺正持續往西北側移動調整。然而搭配地形測量與大地構造解釋，其實分水嶺位置已經與大地構造達到動態平衡，此區域的地形演育特徵可能反映北台灣垮山、沖繩海槽擴張作用以及伴隨的正斷層構造活動；構造活躍的中央山脈西翼，建立此區河流流向與流域面積隨時間的演育模式，河流由台灣南端發育後逐漸往北部河流狀態演育，主流流向與流域面積將會藉由分水嶺移動與河流襲奪事件大規模改變，並影響西部河流之能量與沉積盆地紀錄。此演育過程可能與台灣大地構造以及西部麓山帶的構造發育相關。 本研究建立台灣不同位置河流的演育模式與時間尺度，發現大部份河流展現活躍的流域範圍動態變化。建議未來河流地形或其沉積盆地相關研究時，都應該檢驗研究時間段的區域河流演育過程，才能使解釋更加精準與完備。

Landscape evolution is fundamentally coupled with processes within river channel networks, thus understanding the changes in river systems would provide important constraints on geomorphic and tectonic developments. Most previous studies assumed that river networks and watershed boundaries are stable through time. However, if rivers on both sides of the drainage divide have different erosion rates, the divide may move due to the erosion rate difference. In this study, we used a new method (the fluvial geomorphic index χ) to analyze the river networks of the Taiwan orogen. By comparing the χ values of river tributaries on both sides of the drainage divide, we can obtain the information of the divide stability. Based on the results of χ values, satellite image analysis, river steepness index, and field investigations, the major river basins of the Taiwan orogen appear to be adjusting their sizes and shape. The main drainage divide of the Hsuehshan Range may move from southeast to northwest. This movement is likely produced by the general tectonic setting of the area and the activity of the Northern Ilan structure. The results from the Zengwun River basin show that some parts of the drainage area had been captured by another river, thus the rivers change the flow direction from southward to westward probably due to river captures. From south to north in western Taiwan, the χ values of river basins gradually reach an equilibrium. This suggests changing the flow direction would make the channels more stable. Moreover, this development is likely produced by the general tectonic evolution processes of the fold and thrust belt in western Taiwan. The identification of past and ongoing changes in river networks creates a new opportunity to explore connections between geological and geomorphic systems. We hope this study would provide more information for reconstructing river basins in the past and understanding their developments in the future.