基于南海北部大陆边缘珠江口-琼东南盆地深水区实施的14条近垂直深反射地震探测叠加速度谱,利用Dix公式将叠加速度剖面转换为地壳层速度剖面,并利用时深转换方法构建了深度域地壳层速度模型,综合各地壳速度剖面分析了南海北部大陆边缘珠江口与琼东南盆地不同深度层次的P波速度变化趋势以及地壳几何分层特征.结果表明,琼东南盆地区可分为4～8 km沉积层(V_P为1.7～4.7 km/s)、4～10 km厚的上地壳层(V_P为5.2～6.3 km/s)、5 km左右的下地壳层(V_P为6.4～7.0 km/s)以及2～6 km厚的高速下地壳底层(V_P＞7.0 km/s).V_P＞7.0 km/s下地壳高速层的存在被认为是岩石圈伸展、下地壳底部底辟构造或者是残存的原始华夏下地壳基性层的地震学指示;综合研究区地球物理探测成果构建了跨越华南大陆与南海北部陆坡区剖面莫霍和岩石圈底界图像,揭示出岩石圈上地幔在华南大陆与南海北部大陆边缘的减薄特征.%Base on stack velocity spectrum dataset of 14 near-vertical deep reflection profiles in the Zhujiang River estuary-Qiongdongnan basin in the continental margin of northern South China Sea, we use Dix formula to transform the corresponding stack velocities into the crustal-layer-velocities (in TWT domain) and convert the layer velocities into the depth domain by time-depth transformation scheme. Integrating all the crustal velocity models, we analyze the spatial variation of P wave velocity in different depths and the stratified geometry of crust in the Yinggehai and Qiongdongnan basin area in the continent margin of northern South China Sea. The results demonstrate that the kinematic structure of Qiongdongnan basin can be divided into 4 ~8km thick sediment layer (V_p is 1. 7~4. 7 km/s), 4~10 km thick upper crust (V_p is 5. 2~6. 3 km/s), about 5 km thick normal lower crust (V_p is 6. 4~7. 0 km/s) and about 2~6 km thick anomalous lower crust with high velocity (>7. 0 km/s). The existence of anomalous lower crust layer with high velocities >7. 0 km/s can be considered as seismic signature of the underplating at the lower crust bottom induced by lithosphere extension in South China and its continental margin, or the residual mafic layer of the extended Cathaysia lower crust. Integrating the related geophysical sounding results, we construct the depth images of Moho and the lithosphere bottom across the continent of South China and the continental slope of South China Sea, which reveals the lithosphere thinning across continental South China and the continental margin of the South China sea.
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