南海新生代构造演化及岩石圈三维结构特征

Cenozoic tectonic evolution and the 3D structure of the lithosphere of the South China Sea.

地震层析资料表明,南海地区,自红河口向南经南海、苏禄海到苏拉威西海,岩石圈速度低,底部横波速度仅4.4km/s,岩石圈厚度在60～80km之间,为薄岩石圈地区。软流层的速度也较低,在4.2～4.4km/s之间,但厚度较大,大于200km。从红河-莺歌海断裂带经南海到苏禄海,存在一条北西向宽约200km的上地幔北西向低速带,面波速度在4.05～4.25km/s之间。由上述资料可见,东亚大陆边缘及边缘海的上地幔存在一巨型低速带,在南海地区低速带的走向为北西向,在东海地区为北北西向。这种走向与地表的区域构造走向基本一致,反映这里新生代构造活动可能与地幔低速带分布有关,即上地幔低速带反映了岩石圈的区域流动。这类岩石圈区域流动引起岩石圈表层的张性构造,形成裂谷及稍后的海底扩张,在亚洲东部边缘形成一系列边缘海盆。

Seismic tomographic data indicate that the lithosphere velocity is low in the South China Sea region from the mouth of the Red River (Hong Ha River) through the South China Sea and Sulu Sea to Sulawesi Sea. The velocity of the transverse wave at the base of the lithosphere is only 4.4 km/s and the thickness of the lithosphere ranges from 60 to 80 km; so the region is the one with a thin lithosphere. The velocity of the asthenosphere is also relatively low, ranging from 4.2 to 4.4 km/s, but its thickness is great, >200 km. From the Red River-Yinggehai fault belt through the South China Sea to the Sulu Sea there exists a NW-trending, ～200 km wide, upper mantle low-velocity zone with a transverse wave velocity ranging from 4.05 to 4.25 km/s. The above-mentioned data suggest that there exists a gigantic low-velocity zone from the margin of the East Asian continent to marginal sea. It strikes NW in the South China Sea region and NNW in the East China Sea region. The approximate consistency of these strikes with those of the lineaments on the surface reflects that the Cenozoic tectonic activity there might be related to the distribution of the mantle low-velocity zone, i.e. the upper mantle low-velocity zone reflects the regional flow of the lithosphere, which gave rise to tensional tectonics in the superficial layer of the lithosphere and formed rifts and slightly later sea-floor spreading and a number of marginal sea basins on the eastern margin of Asia.