As the tectonic boundary belt

the south fault terrace

which consists of Wubao fault terrace and Zhenwu fault terrace

has complicated tectonic history and style.Based on the geometrical characteristics of the south fault terrace

guided by the stress-strain theory and extensional rate from the balanced section

we figured out the dominate extensional direction in different periods and analyzed the amount of extension and strike-slip of the south fault terrace

and their control on structural style.The evolution of the dominate extension direction of the Gaoyou sag can be divided into three stages since the late Cretaceous

including NW(K

2

t

2

-E

2

d)

NS(E

2

s)

and NW(Ny-Qd).Detailed understanding of the direction of stress field in the Gaoyou sag was obtained since the late Cretaceous in a quantitative way for the first time.Because the dominate extensional direction is not always parallel with the axis of fault terrace

Wubao fault terrace experie-nced 3 periods of tectonism

extension

right-lateral slip-extension

and extension.This resulted in the simple faults style with skew traces in plane view and horse tail-like in profiles.Similarly

Zhenwu fault terrace also experienced 3 periods of tectonism

left-lateral slip-extension

right-lateral slip-extension and left-lateral slip-extension

which resulted in the complicated faults style of grid fault trace in plane view and duplex horse tail-like in profiles.The study on the history of extension rate and strike-slip shows that E

1

f

E

2

d and E

2

s were the main periods of extension and strike-slip

which played a key role in the formation of structural style.