南海岩石层及边界构造的地球物理特征

南海经历了中生代主动大陆边缘到新生代被动大陆边缘的转换,其岩石层地球物理场具有明显的块、带特征.本文通过综合分析南海地区深地震探测、面波层析成像、重磁异常以及地热与岩石层流变学等各种地质地球物理资料,对南海地壳及岩石层的综合地球物理特征进行了深入总结,发现深地震探测剖面所确定的洋、陆壳转换位置与空间重力异常梯级带位置较为一致,据此拟定了南海洋、陆壳的转换边界;依据多条地壳结构剖面中拉张减薄的程度确定了正常减薄陆壳、洋陆壳过渡带及洋壳等属性特征,并初步圈定了南海下地壳高速层的分布范围.对比分析了南、北陆缘地壳结构及其拉张减薄的变化特征,从综合地球物理特征的相似性上推测了北部陆缘的中西沙陆块与南部陆缘的南沙礼乐滩陆块具有共轭对称性.依据S波速度梯度变化确定了南海岩石层厚度分布情况,揭示出南海北部陆缘存在一条岩石层厚度的减薄带,且该减薄带与高热流带具有较好的一致性.在综合分析的基础上,以深地震探测剖面与重、磁异常变化的对应性为基础,划定了南海边界构造的位置.

Geophysical features of lithosphere and tectonic boundaries in the South China Sea

The South China Sea (SCS) has experienced a transition from an active continental margin in Mesozoic to a passive continental margin in Cenozoic, and its geophysical field of lithosphere has obvious features of zonation and blocking. In this paper, we make a comprehensive analysis with various geological and geophysical data of deep seismic sounding, surface wave tomography, gravity and magnetic anomalies, geotherm and lithospheric rheology in the SCS, and thoroughly summarize the integrated geophysical characteristics of crust and mantle lithosphere in this region. The results show that the position of the continent-ocean transition zone (COT) determined by deep seismic profiles is consistent with a free-air gravity anomaly gradient zone, and the transform boundary of continental-oceanic crust is proposed accordingly. The attribute features of normal thinned continental crust, the COT and oceanic crust are determined by the degree of thinning in crustal structure profiles. The variation characteristics of crustal structure and its thinning in the southern and northern continental margin are compared and analyzed. It is inferred that the Zhongsha and Xisha blocks in the northern margin is conjugate with the Nansha block in the southern continental margin from the similarities of their comprehensive geophysical features. The distribution of lithospheric thickness in the SCS is determined on the basis of velocity gradients of shear waves, and a lithospheric thinning belt is reveled in the northern margin of the SCS, which has a good consistence with the location of the high heat flow zone. Finally, we demarcate the boundary tectonic position of the SCS according to the correspondence between deep seismic profiles and gravity-magnetic anomalies on the basis of comprehensive analysis.