| Summary: | 碩士 === 臺灣大學 === 海洋研究所 === 98 === 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.
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