Distribution and Development of Mud Diapirs Offshore Southwestern Taiwan

The area offshore southwestern Taiwan is an incipient collision zone where the submarine Taiwan accretionary wedge encroaches 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. Then in late Pliocene, fold structures of the mudstone formation were formed by tectonic forces of the Penglai orogeny, and thick channel deposits began to fill the synclinal area in Pleistocene time. Unbalanced loading over synclines and anticlines of the thick underlain Pliocene mudstone formation then triggered mud diapirism in this region. We used over 100 seismic reflection profiles to map the distribution of the mud diapirs in the area offshore southwestern Taiwan. Our results reveal that 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. To understand the relationship between fold-and-thrust structures and mud diapir development, we examine the shape of the diapirs on seismic profiles. 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. Transitional mud diapirs in the upper slope area west of the Goaping Canyon. These 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. Initial stage is described by low dip angle of the strata around diapirs, accompanied by thin withdrawal basin and thick sediments above the crest. Mature diapir is described by high dip angle of the adjacent strata, thick withdrawal basin surrounding the dome, and thin or no sediment cover above the crest.