The Qiongdongnan Basin(QDNB) is situated in the extensional zone at the vertex of the V-shaped northwest sub-basin, non-volcanic northern margin of the South China Sea(SCS). From north to south, the thickness of the continental lithosphere decreases from 22 km on the northern continental shelf to 17 km at the deepest area of the central depression. A sharp change on the crustal structure is of importance to hydrocarbon exploration yet the dynamic causes remain unknown. A comprehensive study including (1) interpretation of seismic profiles,(2) P-wave velocity data modeling, and (3) magnetic anomalies analysis reveals that there are some high-density intrusions along the lithospheric thinning belt. Chaotic reflections can be found in the southwest of the QDNB, with a low velocity(<3.4 km/s), while in the center and the east, the intensively deformed strata passing towards the diapir flanks and their high velocities(>6 km/s) suggest the existence of igneous diapirs. Diapirism differentiation are primarily achieved through analysis of the contact relationship and the thickness variations in the surrounding strata. The first phase of diapirism along the Songnan low uplift occurred in the Late Mesozoic, and the second phase of diapirism in a form of subsequent gas movement remained active until the Late Quaternary. The distribution and the evolution of the diapirs would have major implications for post-rift emplacement.
