台南近岸外海淺層地質構造

Abstract

台灣位於歐亞板塊和菲律賓海板塊交界，菲律賓海板塊以每年7.2公分往西北方向碰撞。板塊碰撞的結果造成地殼不斷上升，形成造山運動，造山運動形成褶皺逆衝帶，因此台灣西部平原以及麓山帶地區形成了數條南北走向的逆斷層，這些斷層越往西生成年紀越輕，傾角也越小。這些構造線都會往南向高屏地區延伸，地質學者認為最年輕的彰化斷層往南延伸至台南地區時，其斷層的延伸最零碎，因此無法在陸地上明確的判斷其走向;所以根據台南高雄外海陸坡地形與震測資料，將馬尼拉海溝位置線往北延伸至彰化斷層，組成海陸相連的變形前緣線，定義這條變形前緣線是台灣造山運動中最年輕的褶皺逆衝帶。我們於是在台南陸地上出現零碎逆斷層的近岸海域，利用多音束測深機收集大量的水深資料，描繪出水下地形，圖上可看到明顯垂直岸往陸上斷層方向延伸的地形線，以及一個拉張盆地。在這些區域另外使用3.5kHz地層剖面儀，又看到在淺層沉積層中出現因構造活動所造成的錯動現象。本文認為這些地形線以及淺層構造為陸上新化斷層和後甲里斷層向海域延伸，延伸至研究區域轉換成一個受右移運動影響的淺層正斷層，這些正斷層為匙狀斷層。它們又以馬尾狀排列，斷層尾端拉張出一個拉張盆地。

Taiwan is at the boundary between the Eurasian Plate and the Philippine Sea Plate, and the Philippine Sea Plate is moving northwest toward the Eurasia Plate at a rate of ~72 mm/yr. The collision between them resulting in Taiwan orogeny, and forming a fold-and-thrust belt aglined in nouth-south direction. Therefore, the western plains and foothills of Taiwan have formed by the action of the north-south strike reverse faults in the belt. More to the west, the age of fault is younger , and the dip is smaller. These structure lines will extend south to Kaoping area. Most geologists believe that the youngest Changhua fault extends southly to Tainan area, but becomes fragmented further to south; therefore, fault line is hard to located especially in Tainan area. According to the reflection seismic profiles, in the continental slope offshore the Tainan and Kaohsiung areas, some researchers believe the paleo-trace of the Manila Trench can be extending northwards and connected to the Changhua fault, marking a deformation front going from land to ocean floor. However, their conclusions lack nearshore topographical and geophysical data such that the doublt has been prevailed to support the Manila Trench was previously located in Taiwan Orogon. Therefore, nearshore geophysical survey was conducted off Tainan coast, firstly, the R2 Multi-beam Sounder was used and the significant topographical lineations related with those geological structures shown onland Tainan are found. Besides, by using 3.5 kHz bottom profiler, the dislocation in shallow sediments is also dominant. This study indicates there is less evidence to suggest the deformation front formed along Taiwan Isand can extends offshore into the continental slope and shall connect with the Manila Trench. We suggest that these topography and shallow structure are the extension of Hsinhua fault and Houchiali Fault. When they extend to the study area , they convert into a right lateral fault which affect by several extensional normal faults, these normal faults are called listric fault. Listric fault often arranged in house-tail. The end of the fault torn into a extensional basin.