| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 104 === In this study, we analyzed multichannel reflection seismic profile, gravity and magnetic data along a 217 km long line, to discuss the crustal characters in the northern offshore area of the Chenho Reefs in southern South China Sea. The roughly SW-NE trending seismic profile lies in the Chenho-Daoming trough north of the Chenho Reefs. We divide the seismic profile into three sections from southwest to northeast: seamount area, northern offshore of the Taiping Island and igneous intrusion area. All the seamounts and igneous intrusions disturbed the shallow sediments beneath the seafloor. The seismic reflection images in the northern offshore of the Taiping Island are characterized by several sets of strong reflections with amplitudes dropping off westward. These reflections show reciprocal stacking patterns which are typical seismic expressions at the edge of carbonate platforms. The Moho depth is about 13-19 km by inversion of satellite gravity data, suggesting a thinned continental crust. Gravity modeling reveals that a high density layer（2.97 g/cm3）lies in the lower crust. The magnetic anomalies in the seamount area and igneous intrusions area show high values about 80 to 160 nT, whereas -35 to -180 nT in the northern offshore of the Taiping Island. Magnetic modeling also reveals that the magnetic susceptibilities of the seamount area and igneous intrusion area are higher than that of the northern offshore area of the Taiping Island.
In this study, a high density layer which may be caused by underplating is simulated in the lower crust. Referring to ODP Site 1143, we suggest that the volcanoes and the igneous intrusions were not formed during the rifting but were developed after the rifting stage. In this study, we propose that an intermediate mode of rifting to fit characteristics of the Chenho Reefs, which is different from either volcanic margin or magma-poor margin.
The structural trend of the Chenho Reefs axis is perpendicular to the rifting axis of the Southwest Subbasin in the South China Sea. Moreover, the magnetic susceptibility of the crustal material in the northern offshore area of the Taiping Islands suggest a granite basement. The seismic features of seamounts and igneous intrusions do not appear in the northern offshore area of the Chenho Reefs. We propose that the basement beneath the Chenho Reefs should be a continental fault block caused by the rifting of the South China Sea, rather than volcanic seamounts.
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