Mode of Extension During the Xisha Trough Rift in the South China Sea

Abstract

A seismic transect across the Xisha Trough failed rift provides information on processes active during the opening of the South China Sea (SCS). The rift basement gradually thins at the conjugate flanks from ∼25 to 15 km toward the central sector, where it abruptly thins to ∼10–6 km in thickness. The seismic velocity model supports that 6.5 km/s velocity separates an upper from a lower crust layer, where Vp reaches 7.1 km/s above Moho. The upper crust extends across the entire rift, but the lower crust layer does not occur in the thin crust of the central sector, which is underlain by mantle with anomalously low Vp, indicating serpentinization. Changes in tectonic structure mimic the Vp distribution. The rift flanks have comparatively small faults associated to gradual thinning, whereas the central sector has larger faults that possibly reach the mantle. The faulting in the central sector thus indicates whole-crust embrittlement, which possibly led to synrift mantle serpentinization. Despite the weaker rheology, serpentinized mantle did not result in the development of low-angle faulting or continental break-up. Furthermore, the rift overall thinning and faulting structure is asymmetric with respect to the rift center, supporting an initial widespread moderate extension creating the flanks. When extension thinned the crust to <15 km, the whole crust was brittle and deformation first focused and subsequently laterally migrated to create the central sector. The continental Xisha Trough rift evolution is similar to classical magma-poor margins, but synrift magmatism described in adjacent regions indicate abrupt segmentation of the rift system.