台灣西南海域之地體構造由南向北為隱沒作用漸變至初期弧陸碰撞的階段，為南中國海北部大陸邊緣及呂宋火山島弧之間的增積岩體區域。增積岩體有顯著的褶皺與逆衝斷層構造，同時由台灣造山帶提供的大量沉積物堆積於增積岩體上，產生海底峽谷與水道及斜坡扇沉積系統。位於增積岩體下部並且靠近南中國海大陸邊緣的澎湖峽谷/斜坡扇系統，其於全球低海平面時，經由台灣南部以及中國東南部之陸上河流提供沉積物傳輸至深海環境之中，而在現今之高海平面時，河流供應源與峽谷分離。貫穿增積岩體之上部斜坡及下部斜坡之高屏峽谷/深海扇系統，無論海平面低或高，其峽谷源頭接直接連於高屏溪出海口。 本研究利用海研一號研究船於2012年4月至2013年8月於台灣西南外海蒐集之多批多頻道反射震測資料MCS994、MCS1000-6、MCS1014及MCS1046，研究區域中逆衝斷層/褶皺構造及水道深海扇系統，利用震測相分析，以及由南中國海北緣之大陸棚所獲得的鑽井資料對比上新世底部及更新世底部之關鍵地層面。 研究結果顯示，上部高屏峽谷位置受限於構造海脊的分布，而下部高屏峽谷斜坡扇系統於更新世早期有多期水道溢堤堆積，尤其在更新世晚期下部高屏峽谷水道沉積物側向加積非常明顯。位於下部增積岩體之澎湖峽谷斜坡扇系統，震測相判釋為水道切割與充填、不活躍水道堆積及水道溢堤系統。澎湖水道系統似乎受到地層受構造抬升而改道至更西側。現今澎湖水道系統及高屏水道系統各為獨立的系統，經由震測相分析解釋，兩系統於早期共同供應沉積物形成斜坡扇。早期至中期更新世於下部增積岩體形成的斜坡扇稱為古高屏斜坡扇扇，現今下部增積岩體地形起伏小的澎湖灣，即位於此高屏斜坡扇的位置。澎湖灣最早於更新世時受到局部之構造抬升，使得澎湖峽谷向西側移動至變形前緣位置。 ;The accretionary wedge off SW Taiwan is the result of incipient arc-continent collision between the Luzon volcanic arc and the northern rifted margin of the South China Sea (SCS). Dynamic interactions of thrusting, folding and a rigorous sediment supply from the Taiwan mountain belts have resulted in two arrays of canyons/channels and slope-fan systems in the accretionary wedge. The Penghu canyon/fan system lies in the lower wedge and near the northern rifted margin of the SCS. The Penghu canyon is a river-fed canyon and receives sediments from southern Taiwan and SE China during eustatic lowstands. It becomes detached from river inputs during eustatic highstands as it is in the present-day. The Gaoping canyon/fan system in the south traverses both the upper slope and lower slope domains of the accretionary wedge. This system is a river-fed system during a full eustatic cycle and it drains sediments from the onshore Gaoping River. We interpreted multiple grids of multichannel seismic reflection data of MCS994, MCS1000-6, MCS1014, MCS1046 collected onboard Ocean Research I during 2012 April to 2013 August to map out thrust/fold structures and channel/fan systems in the study area. Seismic facies analyses were performed on seismic sections and key stratal surfaces of the bases of Pliocene and Pleistocene, respectively, are correlated from boreholes drilled in the shelf of the northern SCS margin. Our results show that the upper Gaoping Canyon has been confined by structural ridges with limited switching of canyon courses, whereas the lower Gaoping canyon/fan system has been developed on lower slope with channel/levee deposition in multiple slope fans since early Pleistocene. Pleistocene lateral aggrading channel-and-levee systems are especially evident near the modern canyon course in the lower slope. The Penghu canyon/slope fan system in the lower accretionary wedge is also evident by seismic facies showing channel cuts-and-fills, channel abandonment and channel-and-levee systems. This system seems to migrate westwardly in response to in-sequence thrusting and westward migration of the thrust front. The Penghu and Gaoping systems are separated features in the present-day. Seismic analyses show that these two canyon/channel system jointly to feed a slope fan, termed paleo-Gaoping fan in the lower accretionary wedge, in early to middle Pleistocene. Major part of this slope fan lies in an area of low relief in the Penghu Embayment. This slope fan has been abandoned since Pleistocene because of tectonic uplift of the Penghu Embayment and switching of the Penghu canyon to the west and along the thrust front.
