Crustal density structure of the northern South China Sea from constrained 3-D gravity inversion

The imaging of crustal structure is the basis of studying the extension mechanism of the continental margin. Although a large number of crustal-scale 2-D seismic surveys have been carried out on the northern South China Sea, there are still many areas lacking seismic coverage, and the results obtained from these surveys cannot provide a regional view of crustal structure. To resolve the crustal structure across the northern South China Sea, a 3-D regional inversion of the free air gravity data constrained by bathymetric and sediment thickness information was undertaken, and the results from seismic studies were used to assess the quality of the inverted results. Density anomaly slices along the seismic profiles show an excellent agreement between the inverted results and the seismic Moho. We select two density anomaly isosurfaces that match well with seismic constraints to represent the depth of Moho and Conrad and to provide Moho depth and the upper and lower crustal thickness. Then, the crustal stretching factor as a whole and upper and lower crust are computed by assuming the corresponding initial thickness. Comparing the Moho depth with the published Curie depth, the results show that the Curie depths are larger than Moho depths in most of the northern South China Sea. Among them, the Xisha Trough with crustal stretching factor greater than 3. 5 and starving post-rift magmatism is a potential area for serpentinization of the uppermost mantle. The crustal stretching factors (beta(w)) in the northern margin of the South China Sea show that it has experienced two groups of rifting with extension directions of 128 degrees and 160 degrees, corresponding to Shenhu event and the first and second episode of Zhugiong event, respectively. The stress field rotates clockwise during rifting. In addition, the stretching factors of the upper and lower crust indicate that the positive and inverse extension discrepancy are widespread in the northern continental margin. The shelf regions exhibit inverse extension discrepancy, and the oceanic-continental transition zones display positive extension discrepancy. It is conjectured that the positive and inverse extension discrepancy may be caused by the lower crustal flow, which is driven by a combination of the lateral gradient press arising from the crustal thickness variations, the asthenospheric buoyancy and the sediment loading.