[J08-P] Poster

[J08-P-17] Mantle heterogeneity in the oceanic lithosphere of the southwest sub-basin, South China Sea, from the wide-angle seismic and the gravimetric model

^○Chuanchuan Lu¹, Tianyao Hao¹, Jian Lin² (1.Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 2.Woods Hole Oceanographic Institution, Massachusetts, U.S.A)

The southwest sub-basin (SWSB) of the South China Sea (SCS), an oceanic basin, locates in the southwestern part of the SCS, separated by several rigid blocks, influenced by a direction changing spreading center and ridge propagation until the asthenosphere stopped upwelling around 15Ma. The processes of seafloor spreading through a series of mantle lithospheric dynamics in the oceanic lithosphere is generally considered to be the consequence of melt extracting and crustal accretion in the upper mantle structure. Lateral crustal heterogeneity only plays a minor role in geophysical attributes, since the crustal structure can be revealed by the modeling of wide-angle seismic traveltime and amplitude. After the calculation of the residual mantle gravity anomaly (RMA), we can do inversion about the upper mantle density variations across the axis in 3-D. Taking account of the crustal structure, cold thermal structure with a strong lithosphere means anisotropic magma distribution along the slow-spreading ridge, which leads a heterogeneity mantle's density structure across the ridge. Therefore, the upper mantle structure, inferred from wide-angle seismic and gravity can thus provide us with knowledge of the processes that have formed oceanic lithosphere at different times and under a variety of conditions which is a straightforward response of the deep mantle structure of the lithosphere. By inverting the RMA of the SWSB in the SCS, its distinct low anomaly over the fossil spreading centre, little variation along the direction of the paleo-spreading axis direction, and relatively wider area of high anomaly on the southeast side, the density variation indicates the changing of the direction may leave space for melt accretion in the northern flank of the spreading ridge, and squeeze the limited off-axis space in the southern side, which may be the reason that more melt was retained on southern flank of fossil ridge.