臺灣-呂宋島聚合帶沈積物散佈系統及其從源到匯之意義

Abstract

本論文利用臺灣-呂宋島間深海區域的水深與震測剖面資料，首先辨識出沿著臺灣-呂宋島聚合帶發育的海底沈積形貌特徵，包括南海棚坡 (被動大陸邊緣) 與高屏棚坡 (活動大陸邊緣)，兩陸坡之間的澎湖海底峽谷、澎湖深海水道、及北馬尼拉海溝。此三段南北走向的槽狀線性溝渠，分佈在鄰近臺灣-呂宋島西側南海海盆東緣最深處，可以相互連接排列，形成一條直接的沈積物輸送管道。從鄰近臺灣造山帶及中國大陸邊緣侵蝕風化而來的沈積物，會經由此沈積物散佈路徑，最終傳輸到馬尼拉海溝堆積。澎湖深海水道位於樞鈕位置，北邊為臺灣弧陸碰撞帶，南邊為馬尼拉海溝隱沒帶。南海陸坡上的福爾摩沙峽谷主要承接來自中國大陸的陸源物質，高屏棚坡上的高屏峽谷所傳輸的陸源物質主要來自臺灣，而位於軸部的澎湖峽谷可接受來自兩側陸坡的淺海沈積物，此三條峽谷皆可成為主要沈積物散佈管道，主要的造山帶沈積物透過軸部峽谷之縱向傳輸，並可經由兩側峽谷及小型溝渠側向補充沈積物，匯集到南海海盆。此沈積物散佈管道在由北到南反應由碰撞到隱沒的區域大地構造背景，同時受到構造與沈積作用的相互影響下，其沈積形貌特徵逐漸改變：水深由淺到深，槽狀溝渠橫剖面由窄到寬，縱剖面坡度由陡到緩，沈積作用由侵蝕到以堆積為主。本文比較南海最北部與新幾內亞-所羅門海板塊聚合帶胡安灣的沈積物散佈系統兩者之沈積物傳輸路徑，凸顯沈積物散佈系統的架構。兩者有相似的兩側陸坡聚合海床形貌，呈現一個三角形且開口向海溝處傾斜的聚合型海盆，軸部峽谷-水道沿著聚合帶發育，陸坡上發育的海底峽谷及溝渠水道朝軸部峽谷匯流，形成區域性且完整的樹枝狀沈積物散佈系統。此系統特徵為軸部縱向傳輸及沈積物側向補充，為活動大陸邊緣從源到匯研究提供一理想之類比範例，從造山帶源頭到海溝匯集處距離僅不到三百公里即有完整系統，可類比至其他地區聚合帶研究從源到匯的相關議題。澎湖峽谷、澎湖深海水道和北馬尼拉海溝為散佈系統主軸，其輸送管道位置及走向主要受到大地構造的影響，而系統形貌受控於構造及沈積作用。沈積物散佈系統的發育，始於形成數條南北走向位於臺灣造山帶西南側深水區的海底峽谷，距今約1.6Ma;澎湖深海水道自東側地勢抬升擠壓後形成，年代大約少於1Ma；距今約0.4Ma古澎湖峽谷約形成在今日澎湖峽谷東北方，位置及走向確立後，整體沈積物散佈系統大致成形。馬尼拉海溝約在15Ma開始形成並持續隱沒作用，並逐漸向西移動到臺灣造山帶南邊。至今澎湖海底峽谷和澎湖深海水道相接，且往南和馬尼拉海溝北端相接，形成完整之沈積物散佈系統。

Examining bathymetric and seismic reflection data from deep-sea region between Taiwan and Luzon, this study identified prominent morpho-sedimentary features that mainly include the South China Sea Shelf/Slope (passive margin) juxtaposed by the Kaoping Shelf/Slope (active margin) intersected by Penghu Canyon and the deep-sea Penghu Channel and northern Manila Trench farther down-slope. These three negative relief troughs of canyon, channel and trench can be interconnected and aligned in a nearly N-S direction, forming a sediment pathway along the western Taiwan-Luzon margin in the deepest east rim of the South China Sea basin. Sediments derived from adjacent Taiwan orogen and Chinese continental margin are longitudinally transported via this sediment dispersal route and are finally deposited in the northern Manila Trench. The deep-sea Penghu Channel is pivotal of the sediment dispersal system because it links the Penghu Canyon in Taiwan arc-continental collision zone to the Manila Trench in the subduction zone west of Luzon, forming a continuous regional sediment dispersal system. The Formosa Canyon on the South China Sea Slope serves mainly as a conduit for sediments derived from mainland China, feeding sediments into the Penghu Canyon, terrestrial sediments derived from southern Taiwan are mainly transported by Kaoping Canyon to the deep-sea Penghu Channel and the axial Penghu Canyon can receive shallow marine sediments from both flanked slopes, serving as a marine transport route. Most orogenic sediments derived from Taiwan are transported longitudinally by axial canyon and down-slope deep-sea channel, and other sediments are supplied laterally by canyons and gullies from flanking slopes, joining into the axial marine transport route and finally debouching to the South China Sea Basin. The morpho-sedimentary features change their morphology and sedimentary processes progressively from collision in the north to the subduction in the south due to effects of tectonics and sedimentation. Shallow canyon evolves to deep oceanic trench following increase in water depth. Narrow V-shaped canyon changes to wide asymmetrical cross-sectional morphology of the trench. The downslope sediment transport system changes from dominant erosion in the canyon to deposition in the deep-water channel and trench. This study emphasizes the significance of the sediment transport routes of the collision zone in the northernmost South China Sea by comparing that of Huon Gulf in Papua New Guinea-Solomon Sea collision zone. Both sediment dispersal systems reveal remarkable similarities in morphology of the collisional and juxtaposed slopes, forming a triangle and tilted trench-ward collisional marine basin. This study combines the axial canyon-channel developing along the convergent slopes, and canyons and gullies on the slopes merging laterally into the axial canyon-channel to form a regional sediment dispersal system with a dendritic pattern. This system is characterized by axial transport and lateral supply of sediments. This study demonstrates a sediment dispersal system in a short distance less than 300 km from the Taiwan orogen to the northern Manila Trench and provides an ideal and analog example for studying sediment dispersal system in the perspective of source-to-sink studies of other active continental margin. The Penghu Canyon, deep-sea Penghu Channel and northern Manila Trench form the axis of sediment dispersal system. Overall, tectonics controls the orientation and location of the sediment dispersal system and structure and sedimentary processes control the morphology and route of this system. About 1.6 Ma several N-S oriented submarine canyons developed in the offshore deep-water region southwest of Taiwan and initiated the early phase of development of the sediment dispersal system. Farther south, the deep-sea Penghu Channel has developed west of the uplifted bathymetric ridge produced by westward compression along the convergent zone around 1 Ma. Approximately 0.4 Ma ago, the paleo-Penghu Canyon developed with a N-S course northeast of the present-day Penghu Canyon, forming the proto-system of sediment dispersal route. The Manila Trench began to form about 15 Ma and progressively migrated westward to the present position south of the Taiwan orogen. At present, the modern Penghu Canyon links to the deep-sea Penghu Channel southward and debouches to the northern Manila Trench, resulting in a complete sediment dispersal system along the Taiwan-Luzon convergent margin.