竹坑北坑河川襲奪及其在造構地形上的隱義

The Tectonic Geomorphic Implication of the River Capture on Jhukengbei Ditch

黃麗津(Li-Chin Huang) ； 楊貴三(Guey-San Yang) ； 蔡衡(Heng Tsai)

《地理學報》 70期 (2013/09) Pp. 23-46

https://doi.org/10.6161/jgs.2013.70.02

摘要

竹坑北坑位於台灣中部大肚臺地西南邊緣，彰化活斷層帶上，是侵蝕作用與斷層活動共同產生的地形。本研究透過航照判釋、野外實察、地形計測、造構地形指標等方法，揭露竹坑北坑河川襲奪的機制、過程，以及它在造構地形上的意義。研究結果顯示，大肚山背斜與彰化斷層、龍井線形活造構的發生有時間與空間上的差異。大肚山背斜先抬升而後彰化斷層錯動出露地表，龍井線形為彰化斷層的背衝斷層。彰化斷層與龍井線形出露地表後，竹坑北坑順向河呈現平行狀水系發育，竹坑北坑下游遇龍井線形往北偏移，穿過龍井線形與彰化斷層成為先行河。因為活造構持續向北擴展，支流高位河向北偏移發育，加上主流低位河側蝕，最後切割主支流間分水嶺產生河川襲奪。風口階地崖高約3m，是大肚臺地西翼最年輕的河川襲奪事件。由竹坑北坑河川襲奪的地形演育，可知大肚臺地西翼南段近期造構活動持續向北移動。

關鍵字

河川襲奪 ； 造構地形指標 ； 活造構 ； 地形演育

並列摘要

Jhukengbei Ditch is located on the west slope of the Tadu Tableland which is uplifted as an anticline by the western frontal thrust of the Changhua active fault. A river capture event of the Jhukengbei Ditch was discovered based on aerial photographs interpretation and field studies. Moreover, the fluvial evolution responded to the active tectonics and its geomorphic implications was revealed with further help from the application of morphometry including tectonic geomorphology indices. The result showed the TaduShan anticline and Changhua fault, Longjing lineament were developed in sequence with temporal and spacious variation. TaduShan anticline was uplifted prior to the outcropping of the Changhua fault to the surface Longjing lineament formed as the backthrust of the Changhua fault. The surface deformation changed a consequent stream of the Jhukengbei Ditch into a parallel river system in pattern. The lower reaches of the main channel turned northward by the barrier of Longjing lineament, behaving as an antecedent stream flowing through the Changhua fault and Longjing lineament. The continuing northward extension of the regional tectonics leds to the northward migration the tributary streams at higher altitude and the lateral erosion of the mainstreams at lower altitude, thus resulting in river capture. The wind gap of the beheaded river left a scarp about three meters height, leaving the trace for the youngest river capture occurred on the west limb of the Tadu Tableland. The example of the river capture and the drainage geomorphic evolution of Jhukengbei Ditch highlight the continuing lateral propagation of the western Tadu Tableland to the north.

並列關鍵字

river capture ； tectonic geomorphology indices ； active tectonics ； geomorphic evolution

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