旗山斷層於海外延伸：轉換拉張機制之探討

Abstract

走向滑移斷層為在世界各地常見的斷層構造，許多地震皆是由於此類斷層的錯動所產生，所以了解走向滑移斷層的構造和其力學行為是非常重要的。過去研究中有關走向滑移斷層構造的探討多為陸地上斷層，而陸地上的野外露頭可能會因風化或人為作用造成破壞而使其資料受到影響。旗山斷層為一位於台灣西南部高雄地區的活動走向滑移兼逆衝斷層，此左移斷層延伸至高雄壽山外海的陸棚邊界然後停止，並於其斷層尾端拉張出一轉換拉張盆地。資料中顯示此轉換拉張盆地已停止向海外區域延伸並向陸地持續拉張的趨勢，由此我們可得知在高雄左營軍港外陸棚上正發育著一年輕的張裂盆地，而此張裂作用和旗山斷層的活動有關。在本研究中，我們想要以旗山斷層末端轉換拉張盆地的地質構造來探討走向滑移斷層的尾端所受到的力學行為，藉由高解析度的多音束測深系統我們可初步得知此區域的地質構造分佈，接著，我們利用3.5-KHz震測剖面資料來了解研究區域上分佈的正斷層、弧形匙狀斷層和匙狀斷層之間隆起區域的地下構造。利用這些資料我們推測出在此沉陷區域受到旗山斷層剪切應力所造成的持續應變和運動模式關係圖，研究結果中顯示此三角沉陷區域為旗山斷層末端應力所拉張出的轉換拉張盆地，我們利用其地形狀況來推測此區域構造形成的順序，而亦可發現此拉張盆地上所形成的構造為走向滑移斷層末端應力所形成，其與一般走向滑移斷層形成的地形特徵上有些構造上之不同。

Transtensional fault is a common geological structure around the world with many earthquakes occurs upon, so it is important to understand the structure and kinematics of transtensional faults. Tectonic of transtensional basin deformed in strike-slip settings overland have been described by many authors, but the field outcrop could be destroyed by Weathering and made the tectonic features incomplete. Chishan fault is an active strike-slip fault that located at Kaohsiung in the Southwestern Taiwan. This left-lateral strike-slip fault extends to shelf break and stop, with a transtensional basin at the termination. The transtensional basin has stopped extending to open sea, whereas it is spreading toward the inshore area. Therefore, we can know that a young extensional activity is developing at the offshore seabed of Tso-Ying Naval Port and the activity is relative to the transtension of left-lateral fault. In this study, we want to figure out the tectonic of transtensional basin at Chi-Shan fault and further discuss the kinematics about termination of strike-slip faults. By using the multibeam bathymetry data, we could initially understand the terrain features. Then, we use the 3.5-kHz sub-bottom profiler data to understand the subsurface structure of the normal faults, arc listric normal faults and the curved area between the listric faults. Finally, we illustrate a diagram for the incremental strain associated with simple-shear deformation and conjecture the mode of motion in the research area. We can figure out the evolution of the tectonic features in research area by 3D topography. The way that transtensional basin developed at continental slope of southwestern Taiwan is a structure of strike-slip fault termination and it is different from general left-lateral faults.