南海共轭被动大陆边缘洋陆转换带构造特征

Structures of Continent-Ocean Transition at the Conjugate Passive Margins of the South China Sea

南海被动大陆边缘洋陆转换带与典型洋陆转换带相比有其特殊性,南海共轭被动陆缘的构造运动、火山活动以及海底地形的差异表明南、北洋陆转换带具有不同的构造特征。本文利用多条多道地震反射剖面和重力异常数据,研究南海共轭被动陆缘洋陆转换带构造特征差异性,探讨南海被动陆缘的类型和伸展模式。南海北部陆缘洋陆转换带可以划分为两种类型:一类以前缘铲状断块为界与洋壳区分,发育有裂陷期断陷、火山带(埋藏海山带)和不十分发育的向海倾的掀斜断块带;另一类则以海山为界,发育裂陷期断陷、宽缓的低凸起和火山或海山。深反射地震特征显示出了下地壳高速层的存在。南部以裂陷期断陷和明显的向海倾的掀斜断块为特征,以前缘铲式断层为界与洋壳区分。自由空间重力异常由陆向海从高异常值过渡为低异常值再到高异常值,预示着地壳由陆壳→过渡壳→洋壳的变化。结果表明,南海共轭被动陆缘更可能属于非火山型被动大陆边缘,其洋陆转换带的构造差异主要受岩石圈差异伸展变形控制。

Compared with typical passive margins, the South China Sea （SCS） has its particular characteristics in the Continent-Ocean Transition zone （COT） at the conjugate margins. Several tectonic events and volcanic activities occurred in the SCS, and differentials of seafloor topographies show that COTs at the northern and southern margins have diverse structures. Based on several crustal-scale multi-channel seismic reflection profiles and gravity anomaly data, this paper analyses the structures of the COT at the conjugate passive margins of the SCS and discuss the pattern and extensional mechanism. The COT was bounded by shelf slope-break at the landward, whereas the COT at the northern margin can be divided into two types according to the seaward boundary. One was distinguished from oceanic crust with frontier listric normal fault, and composed of syn-rift depressions with gentle highs, volcanic zone （buried seamounts） and not well developed rotated fault blocks mainly dip seaward. The other type was bounded by seamount, and characterized as syn-rift depressions, broad and gentle high and volcano or seamount. Two sets of strong reflection events in the deep seismic reflection profile indicate the existing of High Velocity Layer. In contrast, the COT at the southern margin was bounded by frontier listric normal fault, and occurred as syn-rift depressions, while the obvious rotated fault blocks mainly dip seaward which possibly merged into a detachment fault in the deep crust. Free air gravity anomaly shows that the anomaly values varied from high in the typical continental crust to low in the transitional crust then to high again in the oceanic crust. Results show that the SCS conjugate passive margins highly close to the magma-poor passive margins. The lithospheric differential extensional deformation controlled the structures of COT at the conjugate margins of the SCS.