| 摘要: | 台灣南部外海增積岩體有活躍的斷層與褶皺作用、塊體坡移、沈積與侵蝕作用以及流體或甲烷氣滲漏現象。最近研究指出此區的脫序逆斷層、板塊間大逆斷層以及前段滑脫斷層皆是孕震斷層，這些孕震斷層可能產生規模大於8 的大地震、進而引發海嘯，造成鄰近人口密集海岸地區重大的災害威脅。本計畫擬研究台灣南部增積岩體區的上述各種地質作用及增積岩體的內部構造，研究結果希冀可釐清上述孕震斷層帶的結構，並指出與孕震斷層錯移相關的地質現象，進而得知孕震斷層與增積岩體的構造或沈積發育史。本計畫擬針對恆春外海、高屏峽谷以南之上部與下部增積岩體的過渡帶進行細部研究。選擇此研究區的原因如下：（1）此過渡帶下伏一系列的脫序斷層。斷層上盤（即為上部斜坡）可能為曾經過深埋後，再被斷層作用往上抬升的已固化岩石；斷層下盤為膠結較差的年輕地層(即為下部斜坡)。推測此區海床應有多個斷層露頭，使得地下流體能經由這些斷層帶移棲至海床。換句話說，此區應有為數不少與海床冒氣相關的地質現象；（2）此過渡帶是一個廣大的斜坡，高程差可達1000 公尺、坡度可達10 度、寬約10~20 公里、長約數百公里。較高角度的斜坡，顯示此區應有旺盛的塊體坡移運動，研究這些塊體坡移現象，或許可得知增積岩體孕震斷層的發育史；（3）此區有數個斜坡盆地發育，研究這些斜坡盆地內的地層幾何形狀與沈積物堆積，或許也可推測孕震斷層發育史。本計畫擬以3 年的時程（2009-2012 年），利用整合的地物與地質方法，來釐清研究區的各項地質作用。研究方法包括：（1）底拖式側掃聲納與多音束海床測繪；（2）底拖式變頻聲納海床底質測繪；（3）多頻道反射震測測勘；以及（4）海床沈積物採樣與分析。這些資料最後將綜合解釋，來探討下列問題：（1）過渡帶下方的地層與脫序斷層幾何樣貌；（2）增積岩體內逆斷層錯移、塊體坡移、海床冒氣、沈積與侵蝕作用的交互關係；（3）脫序逆斷層的發育史。本研究結果將可增進科學界對於增積岩體內孕震斷層所產生的地質災害與地質現象的了解，也可提供未來可能的深海鑽探的基本參考資訊。 The collisional wedge off southern Taiwan is characterized by various dynamic geological processes. These processes may include active folding and thrusting, mass wasting, sedimentation and erosion, and fluid/methane expulsions. A recent study suggests that the out-of-sequence thrust, plate-interface megathrust and frontal decollement in this accretionary wedge are seismogenic faults. Slips on these fault zones are capable of generating large and probably great earthquakes, which may cause tsunamis with devastating effects on the populated coastal regions nearby the epicenters. This project proposes to study the various dynamic geological processes mentioned above which are operating in the accretionary wedge offshore southern Taiwan. The results will yield insights on the nature of seismogenic faults and the structural and sedimentary development of the accretionary wedge. The target area for this study is chosen to be in the transition zone between the upper and lower slope domains of the orogenic wedge and south of the Kaoping Canyon offshore the Hengchun Peninsula. The choice of this target area is based on the following reasons: (1) the transition zone is interpreted to host a few out-of-sequence thrusts that bring once deeply buried sediments on the hangingwall (i.e. the upper slope) into contact with the younger sediments in the footwall (i.e., lower slope). This indicates that there are multiple fault outcrops on the seafloor. These fault zones may therefore serve major fluid conduits that tap deeper fluid/methane into the shallow subsurface or into the seawater. In other words, there are probably abundant features that are related to fluid/methane venting and fluid migration on the seafloor or in the shallow subsurface in the target area; (2) this zone is marked by a topographic break (up to 1000 m change in bathymetry) of 10~20 km in width and a few hundred km in length, and it features a steep slope gradient (up to 10°). The steep slope gradient suggests that mass wasting processes, induced perhaps by earthquake loading, for example, are prone to occur in the transition zone; and (3) there are a few slope basins developed both in the hangingwall and footwall of the out-of-sequence thrusts. The history of fault movement for the out-of-sequence thrusts as well as mass wasting processes may be deciphered from analyzing the basin sediments. We propose, within three years (2009-2012), to employ various geophysical and geological approaches to decipher the geological processes in the target area. These approaches include (1) seafloor mapping by using deep-tow sidescan sonar and swath multibeam imageries; (2) shallow sub-bottom imaging by using deep-tow chirp sonar profiling; (3) seismic reflection profiling; and (4) sediment coring. The geophysical and geological data will be integrated to address, specifically, the following issues: (1) the structural and stratal geometry beneath the transition zone that is underlain by a major out-of-sequence thrust system; (2) the interplay between processes of thrusting, mass wasting, fluid venting and sedimentation in the target area; (3) the evolutionary development of the out-of-sequence thrusts. The research results will have implications for potential geological hazards such as earthquakes and perhaps tsunamis originated from the out-of-sequence thrusts, megathrust, and frontal decollement. The results will also provide important information for future planning on drilling the megasplay faults if it is to be proposed. 研究期間：9908 ~ 10007 |
