Characteristics of Seamounts in the South China Sea: Implications on Mid-Ocean Ridge Magmatism during Cessation of Seafloor Spreading

Abstract:

We have analyzed bathymetric, seismic, and geophysical data of the South China Sea (SCS) to investigate the characteristics of seamounts and their implications on the magmatic evolution of a mid-ocean ridge from active seafloor spreading to post-spreading volcanism. Among the two dozens of seamounts that can be identified with ease on multi-beam bathymetry data, about half of them are located along the fossil spreading ridge while the remaining located off axis. The distribution of off-axis seamounts also shows strong asymmetry about the fossil spreading ridge with a much greater population of seamounts concentrated in the northern basin. The shape of individual seamounts is approximated as elliptical cones to yield best-fitting models: the population of seamounts in the SCS has an average major-axis length of 21 km (standard deviation σ = 8.3 km) and an average minor-axis length of 14 km (σ = 5.3 km); The above-seafloor height and volume of the seamounts have average value of 2.9 km (σ = 1.1 km) and 1.21*10¹² m³ (σ = 0.99*10¹² m³), respectively. However, limited seismic reflection data show that when the parts of the seamounts buried in the sediment are considered, the resulting estimations of the seamount dimension and volume would increase significantly. Overall, seamounts located on the fossil ridge in the East Subbasin have larger dimension and volume than those in the Southwest Subbasin, except for a seamount neat the Zhongnan Fault between the two subbasins. Adjacent seamounts on the fossil ridge have an average separation distance of 53 km (σ = 9.4 km). We are currently investing lithospheric and mantle melting mechanisms that might control the observed characteristics of the spatial distribution and dimension of seamounts in the SCS.

Keywords: the South China Sea, fossil spreading ridge, seamounts