DEEP PENETRATION SEISMIC-SOUNDINGS ACROSS THE NORTHERN MARGIN OF THE SOUTH-CHINA-SEA

Twenty reversed, two-ship expanding spread profiles (ESPs) with maximum source-receiver offsets of similar to 100 km were collected in three transects across the rifted northern margin of the South China Sea, Source-receiver offset versus two-way travel time (X-T) data were mapped into the intercept time versus lay parameter (tau-p) domain, and velocity-depth solutions were obtained by a combination of tau-sum inversion in the tau-p domain and ray tracing in both the tau-p and X-T domains. Arrivals from the Moho were detected on 17 of the ESPs, The depths to Moho determined for individual ESP interpretations have reproducibilities of +/-0.1 km to +/-3 lan; in most cases the Moho depth has been determined to within +/-1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data, Variations in present-day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin, The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent-ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (V-p<6.4 km/s) and lower crust (V-p>6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures, Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure, Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.