The fine velocity structure of sediment-basement layer in the Three-Gorges Region of the Changjiang River (Yangtze River)

Abstract

In this paper, the study on the fine velocity structure of sedimental and basement layers along 4 deep seismic sounding profiles in the Three Gorges Region of the Changjiang River (Yangtze River) are presented.

The velocity of sedimental cover is larger in hills of western Hubei in the western profiles, the total thickness is about 0–0.3 km. However, it becomes thick in southern part of Zigui basin and Zushui river valley, about 5.0 km and 4.0 km thick respectively. The sedimental cover is very thick in Jianghan plains in the eastern profiles, about 5–8 km, and the velocity is lower. The velocity of basemental plane is greater than 6.0 km/s over the whole region. An interface can be divided within the sedimental layer, it is about 3–4 km deep in Jianghan plains, while it approximates to surface in other regions. The profiles are cut by faults in many positions. Where the faults pass, the velocity isopleth varies sharply, and the velocity is obviously low.

The basement layer is characterized by high velocity and low gradient, there exist 3 high-velocity anomalous zones within the layer, which are located at the west, south and east of Huangling Anticlinorium respectively. They are the upwelling materials of basalt magma with high velocity from deep crust. Perhaps, this process took place before formation of Huangling Anticlinorium. Its action produces the significant variation of basement plane depth and the correspondent development and action of faults.