Lower Crust High Velocity Layer's Relationship with Magmatic Activity of Nansha block，South China Sea

The South China Sea is a unique marginal sea not only due to its special location at the junction of Eurasian plate, Indo-Australian Plate and Pacific Plate, but also because of its much-debated origin. The southern part of the South China Sea drifted from South China during Cenozoic rifting and seafloor spreading, is a key region to understand the evolution of the South China Sea. The Nansha Block is surrounded by the East Sub-basin and Southwest Sub-basin in the north, and NW Borneo Trough in the south. As revealed by previous deepseismic studies, the continental margin in the northeastern SCS is interspersed with small quantities of volcanic bodies, including a high-velocity layer (HVL) in the lower crust.But the origin of the HVL is controversial,and the magmatic underplating, serpentinized mantle material and subduction of the paleo-Pacific plate are considered.

For the purpose of understanding the origin of high velocity layer in the lower crust of southwestern Nansha block, the joint inversion of gravity and seismic data is performed by using Oasis Montaj software. The seismic profile is across the southwestern Nansha Block. The Moho undulation, crustal thickness and the distribution of the high velocity layer are recorded. The regional extension factor and the temperature at the high layer depth are calculated, and the age of volcanic eruption is determined by seismic features. The whole crustal stretching factor of Nanweixi basin and Liyue basin area is 1.5 4 which is impossible to develop serpentine. The whole crustal stretching factor is up to 11.2 in the Nansha trough. The serpentine is formed by the reaction of peridotite and sea water which enters the mantle through the fractures. The temperature near the high velocity layer is lower than the stable temperature of the serpentine. The volcanic eruption was found in the same place in the seismic profile, and the volcano formed after the cessation of the spreading of the South China Sea. Research shows that the high velocity layer of the southwestern Nansha block can be divided into two types. The high velocity layer in Nanweixi and Liyue basin was formed by volcanic eruption after the cessation of the spreading of the South China Sea; and the other section in Nansha trough basin is a mixture of serpentinized peridotite and late magma.