红河断裂带海域延伸位置的地球物理证据及其与南海扩张的关系

Geophysical evidence of the Red River Fault extending position in the South China Sea and the relationship with seafloor spreading

为了解红河断裂带入海后的延伸位置及其与南海扩张的关系,基于南海地区地震、纵波速度、热流、重磁以及莫霍面深度特征,对红河断裂带入海后的延伸位置进行了综合地质与地球物理研究,认为在南海西南次海盆扩张前,红河断裂向东南方向与中建南断裂相接,并延伸至南沙中部地区。从红河流域陆区入海穿过莺歌海盆地往东南延伸至南沙地区表现为一条明显的北西向的纵波低速带和低热流带,在莫霍面、重磁异常特征上也有明显的反映,它们均与文章所提出的红河断裂延伸位置有很好的吻合度。结合前人对红河断裂走滑时代和位移、南海北部中生代俯冲带以及南海扩张时代的研究,提出了它们三者的演化关系模式:晚侏罗—早白垩世末期,南沙地块向华南大陆漂移、碰撞,形成了南海北部中生代俯冲带,南海的破裂从此处开始。在南海西南次海盆扩张前,红河断裂大规模左行走滑运动将南海北部中生代俯冲带左旋切割,当扩张到达西南次海盆时,红河断裂被分为南北两部分。考虑红河断裂大规模走滑运动和南海渐进式扩张的时间以及方向,认为红河断裂大规模走滑运动不能导致南海扩张,推测可能对扩张方向具有一定的制约,导致了南海渐进式扩张的中央海盆近S-N向扩张逐渐转变为西南次海盆的SE-NW向扩张。

In order to locate the Red River Fault zone (RRFZ) and study its relationship with the evolution of the South China Sea, we identified the RRFZ location in the South China Sea by comprehensive geological and geophysical studies. These studies were based on the characteristics of gravity and magnetic, heat flow, seismic P-wave velocity of different depths, and depth of Moho in the South China Sea. The RRFZ is connected with the southeast Zhongjiannan Fault, and extends to the Nansha area before the expansion of the southwestern sub-basin of the South China Sea. There is an apparent geophysical anomaly zone, with low velocity and low density from the Red River Valley land area to the southeast across the Yinggehai Basin and extending to the Xisha and Nansha areas. The two sides of the fracture have significantly different gravity and magnetic characteristics. Combined with previous research on the Red River strike-slip displacement and times, the Mesozoic subduction zone in northern South China Sea and the South China Sea expansion era, we propose an evolution model: in Late Jurassic-Early Cretaceous, the Nansha block drifted to the north, and collided with the South China continent. Before the expansion of the southwestern sub-basin of the South China Sea, the sinistral strike-slip events of the RRFZ cut the Mesozoic subduction zone in the northern South China Sea. When the Southwest sub-basin began to expand, the RRFZ was divided into two parts by the ocean crust. Taking consideration of the large-scale strike-slip movement of the RRFZ and the direction and time of the propagation expansion of the South China Sea, we do not think that the large-scale strike-slip movement of the RRFZ could lead to the expansion of the South China Sea, and speculate the movement may alter the expansion direction of the South China Sea. And the strike-slip movement of the RRFZ led to the S-N trending expansion of the Central sub-basin gradually turning to the SE-NW trending expansion of the Southwestern sub-basin.