Seismic Structure of a Postspreading Seamount Emplaced on the Fossil Spreading Center in the Southwest Subbasin of the South China Sea

Postspreading seamounts are unique seamounts built by volcanism after seafloor spreading had ceased. Limited understanding of their formation processes is mainly due to the lack of investigation. Therefore, we conducted a three-dimensional wide-angle seismic experiment in the Southwest Subbasin of the South China Sea and four profiles were acquired within the inactive spreading center over a postspreading seamount. Based on ocean bottom seismometer data, we report two-dimensional velocity structures of the Longnan seamount. Our results show that a low P wave velocity body (2.9-5.0 km/s, 4 km thick) is observed within the seamount summit, suggesting that material has extensively erupted as volcaniclastic rocks and high-porosity basalts. The extrusive/intrusive boundary is depressed beneath the seamount, indicating the absence of an intrusive core. Our results further show that the ratio of extrusive to intrusive volume is as high as 3.0 and that the accretion to the oceanic crust occurs in layer 2. Combined with an analysis of velocity structures of seamounts worldwide including seamounts in the South China Sea, we propose that the extrusive/intrusive ratio and type of crustal thickening are related to the magma supply mechanisms, and intrusive cores are more likely to be associated with a hot spot. The above analysis also suggests that the Longnan Seamount is a nonplume origin seamount and structurally different from other seamounts in the South China Sea. Ridge jumps in the South China Sea increase the duration or amount of volcanism, which could explain seamount structural differences.