Crustal structure and isostatic compensation beneath the South China Sea using satellite gravity data and its implications for the rifting and magmatic activities

Abstract

Opening of the South China Sea (SCS) was triggered by the breakup of south-eastern Eurasia and the southward drifting of the Palawan-Reed Bank microcontinent, as various prior studies have suggested. The young, moderately magmatic, rifted northern margin of the SCS is vital for investigating the relationship among magmatism, rheology, and structural evolution. This paper integrates satellite gravity anomaly, elevation/bathymetry, geoid, and seismic velocities to investigate continental breakup, magmatism, and rifting beneath the South China Sea. Source depths of 146, 31.5, and 8 km derived from the spectrum of Bouguer gravity anomalies suggest the average depths of the lithospheric base, continental, and oceanic crustal bases, respectively. Correspondingly, gravity Moho ranges from 8 km beneath the rifting center of SCS to 42 km in the Indo-China block. It is worth mentioning that the isostatic Moho from the Airy and flexural models were highly correlated with gravity Moho with correlation coefficients of ∼1. From the ratio between geoid and topography and our estimate of the vertical tectonic stress, the seamounts and reefs (Shuangfeng Basin: SB, Reed Bank: RB, Macclesfield Bank: MB) have a deep compensation depth. In contrast, the other parts (Manila Trench: MT, Phu Khan Basin: PK, East Sub Basin: ESB) have a smaller depth of compensation. 2D gravity modelling suggests the crustal thinning of the oceanic basin, and the thickening of the continental boundary implies the opening of the SCS, which is connected and happened at the same time as the northern subduction of the proto-SCS. The Gravity modelling also suggests a large rifting event within the lithosphere that favors mantle upwelling.