Seismic evidence for the crustal deformation and kinematic evolution of the Nansha Block, South China Sea

Abstract

To understand the crustal deformation of the Nansha Block (NB), this study presents an interpretation of a recently acquired multichannel seismic profile that traverses the Southwest Subbasin (SWSB), the NB, and the Nansha Trough (NT). A total of six Cenozoic unconformities and three stratigraphic sequences are identified. On the basis of tectonic, structural and stratigraphic features, the NB is divided from northwest to southeast into two structural domains, namely, the Hyper-stretched and Forebulge Domains. Based on the interpretation of the seismic profile, the fault-derived and whole crustal extension factors (βf and βc, respectively) for the NB are calculated. The calculation results show that βc is greater than βf in a limited space of appr. 200 km, where the crust underwent depth-dependent extension. We suggest that the crustal extensional pattern of the NB may be a combination of distal limited depth-dependent extension and proximal uniform extension. The interpretation of the unconformities is also used to calculated the fault-derived crustal extension at various stages. The two main extension stages, the syn-rift and syn-drift, account for appr. 70% and 25% of the total extension, respectively. A comparison with the seismic profiles of the eastern NB shows that the crustal extension along the continental margin of the NB is heterogeneous, with more significant crustal deformation on the west side of the NB. The heterogeneity of the crustal extension along the depth and margin may be related to the thermal condition and rheological structure of the crust or lithosphere.