左鎮斷層位於新化丘陵ㄧ帶，斷層呈西北-東南走向，由西向東經過埔羌坑、左鎮村、三角潭至心仔寮一帶，斷層長度約10公里，曾文溪與其支流菜寮溪為該區內之主要河川，其區域內有發育顯著的曲流地形。前人利用地表地形與衛星影像觀察，發現地形上有落差，線性邊界明顯，但是從地表地質調查，發現斷層兩側岩性並無明顯差異，也無法了解其活動年代，故將其歸類於存疑性活動斷層。因野外地質調查未能提供完整證據，因此仔細調查左鎮斷層與其鄰近區域之地下構造，將有助於釐清左鎮斷層所扮演之角色。 本研究採用反射震測法進行地下構造測繪，收集左鎮斷層附近23條淺層反射剖面，配合地表地形及地質，並參考中油重力資料與地調所兩條深部平衡剖面，探討全區的地下構造形貌。結果發現在崙後斷層西側，曾文溪流域的區域(即大內鄉一帶)，其地下構造南北側向變化不大，東西方向地層向下有明顯變陡同一地層厚度有些許不同的形貌，反映出是個沉積物斜向沉積且構造均勻抬升的區域，本文稱之為「大內均勻抬升區」，該區地層無突然的轉折變化，代表斷層並沒有穿越此區，因此，此區可視為為一個構造分界，分隔北邊的六甲斷層與南邊的左鎮斷層。而左鎮斷層兩側地下構造型態有明顯不同，北側為受到擠壓彎曲變形所形成反覆向斜背斜構成之褶皺串列；南側則為向西傾斜的單斜構造，地層傾角變化不大(約15∘左右)，應力有明顯的釋放的情形。左鎮斷層西北端可能被崙後斷層截切，東南端則被龍船斷層截切。 综合上述研究可得知，左鎮斷層無明顯的錯動，活動性並不強烈，但是其地質上的意義卻十分重要，斷層區隔南北兩側不同構造模型，為台灣西南部麓山帶主要構造界線之一。 The Tsochen fault lies in the Shinhua hill area of the southwestern Taiwan, about 10 kilometers long. Due to lacking of surface geologic evidences, the fault is difficult to identify. The fault is ranked as a ‘suspect’ fault in the Central Geologic Survey’s (CGS) active fault list. This research uses the seismic reflection method to study the Tsochen fault including its location and underground configuration. The possible mechanism of the fault’s evolution is also under investigation. The results show that the fault can be divided into the northern and southern parts. At the northern part, the fault structure is controlled by the Peikang High which interferes the fault in a ‘blocking’ pattern. At the southern part, the structural style displays as a ‘triangular’ structural pattern. The Tsochen fault doesn’t cross the Chukou fault. To the northwest of the Tsochen fault, the variation of the structural strata is relatively gentle, forming the “Da-nei evenly risen area”. The strata is only crooked and out of shape slightly in this area. Both sides of the Tsochen fault have obvious difference. There is very complicated structure in the north side of the fault , where exists a series of folds. In the southern side of the fault, the structure is relatively flat. Integrating above-mentioned results, the Tsochen fault seems not being in an immediately active stage, but its importance should not be underestimated in the geology.
