南海重力异常的沉积层密度改正及其对区域构造特征分析的意义

南海位于太平洋板块、印澳板块和欧亚板块交汇处,自晚中生代以来历经张裂作用、海底扩张以及印藏碰撞、菲律宾海板块西向运动等构造事件的叠加改造,不仅形成了复杂多样的构造格局,而且堆积了厚薄不均的沉积层.为了考察沉积层密度改正对利用重力资料分析南海不同尺度构造特征的影响,本文利用南海各区域不同深度沉积层的地震波速度及钻孔密度等数据,建立了沉积层与沉积基底密度差随深度变化的二次函数关系式,并基于该关系式,计算了南海沉积层相对基底密度低而产生的重力异常值.结果显示,南海沉积层的重力异常值在海盆区介于-40~-60 mGal,而在堆积巨厚沉积物的莺歌海盆地可达到-135 mGal;相对于空间重力异常、布格重力异常,经沉积层重力异常改正后的地壳布格重力异常更能突出深部不同尺度的密度结构和莫霍面的起伏特征,其总水平导数模更突显了南海西北部红河断裂带的海上延伸;利用谱分析技术估算岩石圈强度时,经沉积层重力异常改正的地壳布格重力异常数据获得的岩石圈有效弹性厚度值更为符合地质实际,特别是在长条形的巨厚沉积区如莺歌海盆地和马来盆地.分析表明,重力异常的沉积层密度改正对揭示南海构造特征具有重要的意义.

Sediment density correction of gravity anomaly in the South China Sea and its significance to analyze regional tectonic characteristics

Located in the interactional area of Pacific plate, Indo-Australian plate and Eurasian plate, the South China Sea has experienced a series of tectonic events such as rifting, seafloor spreading, India-Tibet collision and westward movement of Philippine Sea plate since the late Mesozoic. As a result, the South China Sea has not only developed a complex and various tectonic framework, but also accumulated sediments with uneven-thickness. To examine the effect of sediment density correction on the different scale tectonic characteristics analysis based on gravity data, by analyzing seismic wave velocities and drilling data collected from different parts of the South China Sea, this paper presents a quadratic function relationship to predict the density difference between sedimentary and sedimentary basement at different depth, and the gravity anomaly of the South China Sea due to the gravity deficit of less dense sediment. Our results show that the gravity anomaly due to the less dense sediment is in the range of -40~-60 mGal in the oceanic basin, while it could be larger than -135 mGal in the Yinggehai basin which has accumulated very thick sediments. With respect to the free-air gravity anomalies and the Bouguer gravity anomalies, the crustal Bouguer gravity anomalies, which has been corrected for the less dense sediment, can shed more light on the deep density structure at different scales and the fluctuation characteristics of the Moho surface, and its modulus of total horizontal derivative could present more details of offshore the extension of the Red River fault zone in the northwestern South China Sea. When estimating the lithosphere strength, spectrum method could estimate more reliable effective elastic thickness of the lithosphere with the crustal gravity anomaly data, especially in those elongated area with thick sediment such as the Yinggehai Basin and Malay Basin. These analyses indicate that the sediment density correction of gravity anomaly is necessary to reveal the tectonic characteristics of the South China Sea.