Comprehensive analyses of unconformities

faulting systems and basin tectonic framework were carried out to characterize tectonic transformation mechanisms during the periods of Early-Middle Eocene and Late Eocene-Early Oligocene in the Zhu 1 Depression of Pearl River Mouth Basin.The results show that the Indochina block's rotation and extrusion and southward subduction of the proto-South China Sea resulted in a regional stress field clockwise change from NW-into SN-extension

and a shift in fault-controlled basin from NE- and NEE-trending to near EW and NWW-trending

with numerous new faults of EW-trending developed.The lithosphere extensional deformation transformed from homogeneous wide rift mode to centralized narrow rift mode

and migrated to north margin of the South China Sea.As a result

the Basin expanded as the isolated half-grabens or narrow grabens tended to get connected.Under a SN-trending stretching

the stretch boundary at the north of the depression was defined by basin-controlling faults.As a pre-existing weak zone with concentrated stress

the northern margin experienced strong and northward faulting and significantly spread northwards

causing the sedimentary center migrating northward as well.The tectonic changes during the faulting activities of the Basin may explain the characteristic differences of the two source rocks:the Wenchang and the Enping Formations

as well as the disparity of oil and gas distribution in the area.