| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 104 === Southern Taiwan and its offshore area is situated in a convergent plate boundary and have been suggested to be an accretionary wedge, which is characterized by numerous folds and thrusts. The unique geological setting makes Hengchun Peninsula, which is the southernmost tip of Taiwan island, a great place to study the neotectonics of Taiwan. The eastern part of the Hengchun Peninsula is the southern continuation of the Central Range; the western part of the Hengchun Peninsula lies the West Hengchun Tableland that was formed by collision uplifting. It seems that the morphology of the western Hengchun Peninsula and its offshore area are related to a large anticline. A curved depression valley, called "Hongtsai Canyon", was found on the west side of the tableland. This study suggests that this arc-shaped small submarine channel could be associated with collapsing blocks of western part of West Hengchun tableland formed by a normal faults. In this study, we first reprocess more than 10 seismic reflection profile data for signal enhancement offshore Southern Taiwan. We then analyze the seismic data for understanding the structural styles of the West Hengchun tableland offshore area. In the northern Hongtsai Canyon, a series of normal faults that are parallel arc-shaped depression have been recognized, and the channel widens westward. In the central and southern Hongtsai Canyon, shallow water depth coupled with strong deformed strata and overlying coral reefs make it difficult to clarify how the the sedimentary and structural processes affect the development of the Hongtsai Canyon from seismic profiles. Nevertheless, seafloor features such as anticlines and eroded channels imply that this depressed channel is formed by block sliding.
By comparing the bathymetric and seismic data, we suggest that this arc-shaped depression is a block sliding along a normal fault with a strike-slip component, and then this arc-shaped depression has been eroded and deposited by turbidity currents. As southern Taiwan is situated in the active collision stage, extensional normal faults are not expected to appear in this area. This study suggests that the extensional strike-slip faults may related to anticline top extention and the tectonic escape in southwestern Taiwan, which can lead to the surface rifting and sliding structures by transtension.
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