Crustal Compositional Variations From Continental to Oceanic Domain: A VP/VS Ratio Study Across the Zhongsha Block, South China Sea

The V-P/V-S ratio is an important property for understanding magmatic and tectonic processes at passive continental margins as it is an indicator of the crustal composition. To classify the dominant lithologies in the Zhongsha Block, South China Sea (SCS), we present a detailed V-P/V-S crustal model based on the independent tomographic inversion of P wave and S wave data. The average V-P/V-S in the crust of the Zhongsha Block is similar to 1.77, indicating an overall felsic to intermediate composition lacking remnant magmatic intrusive rocks. The V-P-density relationship from gravity modeling suggests that the lower crust of the extended continental domain contains more greenschist and hence may have experienced metamorphism resulting from an elevated geotherm in the Northwest Sub-basin either during the syn-spreading or postspreading stage. The variability of the V-P/V-S ratio in the continental block is larger than that in the oceanic basin, showing distinct crustal properties. Several low V-P/V-S ratio anomalies (V-P/V-S < 1.7) were found near tectonic boundaries and are interpreted to either result from felsic metamorphism during an interval of rifting, or during the migration of magma along faults and cracks in the postrift period. V-P/V-S ratios occurring in concert with high V-P anomalies in the continent-ocean transition zone support a mafic composition of metapelitic granulite, which was either formed by magmatic intrusions or contact with mantle melting that stem from the upwelling of the asthenospheric mantle during the initial break-up and onset of the seafloor spreading stage in the SCS.