The seismic survey and analysis of the Dongsha rise

Project Duration	2011-12-31 ~ 2012-12-30
Research Title	The seismic survey and analysis of the Dongsha rise
Contracted Organization	National Central University
Principal Investigator	Hsu Shu-Kun
Co-Principal Investigator	Yeh Yi-Ching
Project Participants	Liu Chia-Hsuan, Tu Wen-Pin, Chiu Hsieh-Tung, Tsai Ching-Hui
Keywords	Dongsha Atoll, northern South China Sea, Dongsha Uplift, Quaternary
Abstract	Dongsha Atoll is located on the continental shelf in the northern South China Sea, at water depths of less than 100 meters. It is composed of a ring-shaped coral reef structure. Due to its geographic position directly facing the Luzon Ridge, internal waves generated in the South China Sea basin can reach amplitudes of several tens of meters near Dongsha Atoll. These oceanographic conditions contribute to the atoll’s rich ecosystems and high biodiversity, making it one of Taiwan’s most important natural assets. Although extensive seismic and drilling data exist in the Pearl River Mouth Basin north of Dongsha Atoll—primarily collected for petroleum exploration—the origin of the Dongsha Uplift beneath the atoll has previously been inferred only from low-quality seismic profiles and distant borehole data. Earlier studies suggested that the Dongsha Uplift occurred approximately six million years ago; however, systematic research on the basement characteristics and evolutionary history of the uplift has been lacking. Commissioned by the Marine National Park Headquarters, this project collected six shallow seismic reflection profiles around Dongsha Atoll in 2012. After suppressing multiple reflections, these profiles provided clearer imaging of deeper strata at depths between 1,000 and 2,500 meters. Time-to-depth conversion was also conducted to facilitate comparison with surrounding borehole data. Correlation results indicate that the basement of Dongsha Atoll likely consists of Mesozoic strata. During the boundary between the Miocene and Pliocene epochs, an igneous intrusion event—identified as the Dongsha Uplift event—occurred. This event caused intense erosion of the overlying strata, followed by the deposition of a thin Quaternary sedimentary layer above the erosional surface. Numerous high-angle normal faults cutting through the seafloor suggest that Dongsha Atoll is currently situated in an extensional tectonic environment. Additionally, large submarine mud volcano outcrops were discovered for the first time on the southeastern side of Dongsha Atoll. This finding may be related to the presence of natural gas hydrates in the nearby Shenhu area; however, the exact relationship and geological significance require further investigation.