Tectonostratigraphic structure and crustal extension of the Qiongdongnan basin,northern South China Sea

Rifting of the Qiongdongnan basin initiated in the Cenozoic above the Pre-Cenozoic basement which was overprinted by rifting basin tectonics soon after becoming part of a rifted passive continental margin of the South China Sea. Using integrated data of sedimentary horizons,wells,seismic reflection,and the observed gravity field,we constructed a crust-scale structural model of the Qiongdongnan basin.Many characteristics of this model reflect the tectonostratigraphic history of the basin.The structural and isopach maps of the basin allow us to retrace the well-established history of the basin comprising a dominant structural trend ofgradually shifting depocentre from the Paleogene sequence(45Ma~23.3Ma)to the Neogene to Quaternary sequence(23.3 Ma~present)towards the west or southwest.The present-day configuration of the basin implies that the basin fill of Cenozoic sediments becomes thinner towards the east.By integrating several reflection seismic profiles,interval velocity and performing gravity modeling, we have modeled the sub-sedimentary basement of the Qiongdongnan basin.The results show some high-velocity bodies in the lower part of crust(vP7.0 km ·s-1),and most probably is igneous magma underplated to the lower stretched continental crust during the final rifting and early spreading phase.The crystalline continental crust spans from the weakly stretched domains(as thick as about 25km)near the continental shelf to the extremely thinned domains(of less than 2.8km thickness)in the central depression,representing the continental margin rifting process in the Qiongdongnan basin.The estimated distribution of the crustal extension factor(β)of the basin shows that the factor in central depression is maximum value(6.0),while that in northern and southern side wings is minimum value(2.0).This model can shed light on future numerical simulation researches,including the reconstruction of the evolutionary processes from the rifted basin to the passive margin one and the evolution of the thermal field of the basin.