台灣西南麓山帶可發現同沉積正斷層構造曾經發育之跡象；觀察台灣西南外海震測剖面，能找到疑似正斷層構造，暗示此構造相當年輕。可是根據大地構造觀點，台灣西南麓山帶一系列緊密褶皺及逆衝斷層，反應此區曾經受過強烈應力擠壓，表示此區構造歷史與盆地演化過程必定相當複雜。因此，台灣西南部是了解盆地演育、斷層活動、大地伸張及擠壓應力相互影響關係良好的地區。 本文嘗試利用地層資料來繪製三維地層等厚圖，配合古應力分析，嘗試了解本區大地應力演變過程，藉此推估本區大地應力演變過程與前陸盆地演化之歷史，期望得到前陸盆地演化過程與斷層活動關係之啟示。 由上新世早期至更新世之地層等厚圖，發現中崙層與六重溪層於北港高區缺失，但於北港高區南界的A斷層以南則迅速增厚，暗示A斷層於上述兩層堆積時，重新再活動。六重溪層堆積時期之沉積速率，顯示本區進入造山前緣，基盤快速沉降。本區古應力大致上可分為3時期：第1期為 N 98 °E方向之伸張應力，第 2a 期為 N 08 °E之伸張應力，第 2b 期為橫移斷層系統之應力組，最大應力方向為 N 88 °E，最小應力方向為N 164 °E，第3期為N 106 °E方向之壓縮應力。 結合以上特徵，本文推論A斷層於前陸盆地發育時，其活動時期為6.5 ~ 5.3 Ma 與 3.5 ~ 1.2 Ma，並提出台灣西南部盆地演化可分成四階段。第一階段，約10~6.5 Ma，台灣地區仍處於被動大陸邊緣，呂宋島弧仍未靠近，盆地緩慢沉降。第二階段，約 6.5 ~ 3.5 Ma，本區因板塊聚合，受到輕微壓應力影響，產生南北伸張，觸發原來存在的正斷層再次活動，加深此時期的盆地。第三階段，約 3.5 ~ 0.5 Ma，本區快速沉降，形成深海區或是亞深海區。因為沉積物加厚、板塊撓曲，加劇正斷層活動。同時，靠近造山帶的前陸盆地也產生一系列橫移斷層，截切原來的正斷層系統。第四階段，約0.5Ma~至今，靠近造山帶之前陸盆地沉積物受壓應力影響形成褶皺逆衝斷層帶。 The fold-and-thrust belt of southwest Taiwan has incorporated strata that were previously deposited in the foreland basin. Present-day foreland basin strata lying to the west of the fold-and-thrust belt of SW Taiwan strata show the prevalent development of normal faulting. This suggests that normal faulting may develop prior to thrusting in a later stage that forms the fold-and-thrust belt. I integrated stratigraphic data and collected paleo-stress measurements on fault surfaces in the study area in a view to understand the sequential developments of normal and reverse faulting as well as their associated changes of paleo-stress directions in each stage. A series of isopach maps of foreland deposits and seismic data show that the major normal fault, A-Fault, lying in the foreland and striking in an east to west direction, reactivated during two periods of 6.5-5.3 Ma and 3.5-1.2 Ma, respectively. Paleo-stress analyses reveal three successive tectonic events. From older to younger events they are: (1) extensional event of E-W direction (meaning that normal faults strike in the N-S direction); (2) extensional event of N-S direction in the distal foreland accompanied by strike-slip movements with faults striking in an E-W direction in the proximal foreland; and (3) compressional event of NW-SE direction, which is consistent with present-day direction of plate convergence. Getectonic evolution of this area from the Eurasian passive margin to the foreland basin was beginning in 6.5 Ma. From 10 to 6.5 Ma, this area is subsidence slowly. From 6.5 to 3.5 Ma, the normal faults in the foreland basin were active again by the weak pressure of the arc-continent collision. From 3.5 to 0.5 Ma, this basin was subsidence rapidly, which led to continental rupture and the normal faulting. From 0.5 Ma to present, the sediment of this foreland basin was compressed and uplifted in this area.
