Deep Imaging of Extensional Structures in the SW South China Sea

We process a 230-km-long MCS section and a coincident OBS refraction profile acquired across the Spratly Islands south of the SW SCS oceanic basin. By joint interpretation between MCS profile and OBS derived P-wave velocity field, we identify pre-rift units, two stages of syn-rift and post-rift sediments in the basins. Normal faults tilting pre-rift units root in a shallow decollement south of the profile. This shallow decollement is directly imaged by a thick band of reflectivity and also identified by a velocity contrast in the refraction derived P-wave velocity model. Deeper down (~7s twt), significant reflectivity may be interpreted as the top of the lower crust with ~7 km/s velocities. Even deeper reflectors around 10-11s may be related to the Moho or mantle. The images show moderate finite extension in the pre-rift sedimentary infill, although large faults penetrate the basement. Syn-rift units show that the tertiary rifting of the SW SCS may be characterized by rapid and short-lived activity of the faults or more subaerial conditions. Deeper, we infer that multiple heterogeneities present in the continental crust are used as decollement levels at different depths down to the ductile lower crust.