Crustal structure of the Xisha block and implications for along-strike segmentation in the northern South China Sea continental margin

Abstract

Continental blocks complicate the pre-rift lithospheric inheritance of passive continental margins, hence exert unique influences on rifting process. Along-strike rifting contrasts of the continental margins may reflect the pre-rift crustal heterogeneity and segmented rifting processes. A wide-angle seismic profile acquired on the Xisha Block, the northwestern margin of the South China Sea (SCS), provides the first constrains on the along-strike crustal structures of the continental block. Seismic imaging of the profile reveals a continental crust with relatively uniform thickness of about 22 km and the thickness ratio of the upper crust to lower crust at 0.8. Crustal stretching factor is approximately 1.5, and high-velocity anomalies are absent in the lower crust. Crustal velocities of the upper and lower crust are approximately 5.4–6.5 km/s and 6.6–6.9 km/s, respectively. The results indicate that the Xisha Block underwent slight and uniform extension during rifting, and the crust has not suffered from strong magmatic activities that formed extensive magmatic underplating. As a rigid continental block, the Xisha Block has retarded the rifting onset and shortened the process from lithospheric rifting to breakup in the northwestern SCS margin. Hypothetic models for the rheological heterogeneity of the inherited crust are proposed tentatively to interpret the contrasting distribution of the high velocities in the lower crust along the northern SCS margin.