- 學位論文
台灣西南海域泥貫入體之活動與演化
Development of Mud Diapirs Offshore Southwestern Taiwan
摘要
台灣西南海域在大地構造上屬於初期弧陸碰撞的階段，此處隱沒作用形成的增積岩體開始接觸到非活動的南海大陸邊緣。在更新世晚期，造山帶侵蝕下來大量的沈積物覆蓋在較古老的深海泥質沈積物上，因沈積荷重不平均而引發泥貫入體的活動。此區泥貫入體的分佈大多為北北東-南南西走向，與陸上的褶皺構造平行，部分的泥貫入體可由高屏陸坡向北延伸到台南高雄一帶陸地的背斜構造。為了理解泥貫入體的形貌與大地構造的關係，本研究利用反射震測資料來探討泥貫入體的發育。結果顯示，台灣西南海域泥貫入體的形貌與鹽貫入體有所差異，造成此差異的原因可能來自於源岩黏滯度與上覆沈積層比值的不同，或是受到貫入體發展初期環境擾動的影響。而泥貫入體在分佈上，因受大地應力影響，會垂直於大地應力方向，此與非活動大陸邊緣之貫入體多平行於海岸線的分佈不同。泥貫入體的形貌受構造與沈積環境控制，本研究顯示，在下部斜坡的泥貫入體發展過程受斷層控制，而造成不對稱的現象;上部斜坡則呈現對稱的泥貫入體。在高屏陸坡區高屏峽谷以西處，下方泥層逐漸變薄，不足以發展成泥貫入體，但可在下方形成滑脫面，使上方地層的褶皺逆衝構造傾向不一致。另外本研究觀察到台灣泥貫入體有不同階段的活動，進而建立泥貫入體之演化過程，台灣的造山活動在三百萬年前的活躍碰撞帶來大量的沉積物，覆蓋於深海平原的泥質沉積物上，形成泥貫入體，此為第一階段的活動。泥貫入體生長過程同時受沈積物覆蓋而逐漸壓密，最後停止活動。由於壓密後的泥岩層持續受到擠壓，泥貫入體受到側向應力使其抬升，開始第二階段的活動。下部陸坡持續受到活動斷層影響，因斷層抬升的作用，此區的泥貫入體可以看到第三時期的活動，並且持續進行中，而位於上部陸坡的泥貫入體則已停止抬升。
並列摘要
The area offshore southwestern Taiwan is an incipient collision zone where the submarine Taiwan accretionary wedge encroaches on the passive China continental margin. Thick sequences of deep marine sediment filled a rapidly subsided foredeep basin during the Pliocene time. The thick muddy sediments deposited provide source material of mud diapirs. In late Pliocene, unbalanced loading over synclines and anticlines of the thick underlain Pliocene mudstone formation triggered mud diapirism in this region. Most of the mud diapirs extend in a NNE-SSW direction, similar to the trend of the folds onshore, and some diapiric mud walls even extend to on land SW Taiwan. In order to understand the relationship between fold-and-thrust structures and mud diapir development, we examine the shape of the diapirs on seismic profiles. The study revealed that the different shapes between mud diapirs in Taiwan and salt diapirs in Gulf of Mexico, were probably by the different viscosities of the diapiric material and the overburden sediments, it could also cause by enviromental perturbations during initial diapir development. Different mud diapir patterns are observed in different areas of the Goping shelf and slope, we classify them into four patterns: 1. Symmetrical diapirs buried by sediments in the shelf area east of the Goaping Canyon. 2. Symmetrical diapirs extruded to seafloor in the upper slope area east of the Goaping Canyon. 3. Asymmetric and irregular shaped diapirs in the shelf area west of the Goaping Canyon. 4. In the upper slope area west of the Goaping Canyon, thin mud layer may have act as a detachment that caused those folds and thrust faults developed above the mud by mud layer without preferred vergence. We also recognized that different diapir patterns may represent different stages of diapir formation. By examining the dip angles of the strata adjacent to the mud diapirs, we are able to recognize different mud diapir development stages. In the first stage, muddy sediments were covered with thick oregenic sediments in Pliocene, and mud diapirs developed due to unbalanced sedimentary loading. Continuous sedimentation compressed the underlain muddy layer and made it rigid, and mud diapirs ceased their growth owing to reducing the density difference between overburden and mobile layer. In the second stage, continuous collision and lateral compression caused mud diapirs to be uplifted again. In the third stage, out-of-sequence-thrust seperated the accretionary wedge offshore southern Taiwan into upper slope domain and lower slope domain, and those active faults in the lower slope domain casused mud diapirs to be uplifted again and formed asymmetric shape.