Submarine volcanoes in the Xisha region of the northwestern South China Sea: sourced from a mantle plume or partial melting?

Abstract:

Submarine volcanoes are the significant structures in the Earth, which could reflect the mantle magmatic processes. In this study, we used multi-beam bathymetric data of the northwestern South China Sea (SCS) to document the morphometry of submarine volcanoes. Most of the submarine volcanoes are circular and elliptical cones in plan-view, while some of them are elongated in shape and consist of many small volcanoes. They have average diameters from 0.625 to 36.14 km and heights from 0.04 to 1.73 km; and their volcanic volumes range from 0.005 to 236.56 km³. These volcanoes produce chaotic seismic reflections which are distinguished from layered sedimentary deposits. Because rarely scientific and petroleum drills sampled the volcanic rocks in the study area, we estimated the timing of volcanoes mainly using their contact relationships with the sediments and the overlying sedimentary thickness through the seismic profiles. We found that most of the volcanoes were formed after seafloor spreading cessation of the SCS (~15.5 Ma). Among them, the most extensive volcanism may occur during 10.5 – 5.5 Ma, probably corresponding to the Dongsha event when the northern SCS margin was suffered from intensely magmatic activities.

The E-W oriented cross section of full-waveform tomography FWEA18 (Tao et al., 2018) across the study area show that low-velocity anomalies are generally located beneath the Indochina Peninsula, East Vietnam margin and the SCS sea-basin within the upper mantle. Directly under the northwestern SCS, a N-S oriented cross section of tomography imaged several low-velocity anomalies (<-3% dVp; <-4% dVs) are distributed below the SCS margin and sea-basin within the upper mantle and transition zone. The tomographic results do not show an obvious plume-style tomographic body below the crust in/around the Xisha region. Instead, we proposed that in this region magmas were formed by partial melting caused by buoyancy-driven upwelling of hot lithospheric and asthenosphere mantle material after the cession of SCS spreading (Castillo et al., 2010; Tian et al., 2011; Gao et al., 2015). The magma migrated though weak zones of lithosphere such as pre-existing faults formed by rifting, extension or strike-slip motions, and resulted in intensely post-seafloor spreading volcanism in/around Xisha region.