Abstract:

Based on geological structural interpretation of 2D and 3D seismic profiles, activities of main faults in Qiongdongnan
basin (QDNB) have been studied by using throw rate plots and displacement-distance plots. The results show that fault activities
corresponded  to evolution  stages of  the basin, and  the  syn-rifting  stage of QDNB can be  further divided  into Eocene-Early
Oligocene rifting episode and Late Oligocene rift-sag  transition episode, which controlled  the structural pattern  in deep QDNB.
The center of single fault activity (such as No. 6 fault and No. 11 fault) migrated from east to west during Early Oligocene to Late
Oligocene, and  the center of  the whole  fault system  in QDNB also migrated  from east  to west during Late Oligocene  to Early
Miocene. The composite type faults, such as No. 2 fault, showed segmentation feature. The modes of fault development are of two
kinds: The non-interacting mode: the fault system grew as a single segment during its development and evolution, such as No. 6
fault and No.11  fault. In  this mode,  the  fault simply grew  from  the middle  to both sides and  it become  larger due  to systematic
increase  in both maximum displacement and  length, and  the displacement-distance plot of  the  fault  system  is  semi-elliptic
during the whole process; The interacting mode: the fault zone is composed of several segments, and its growth was mainly due to growing-connecting of  the adjacent segments. The development process of  the  fault system shows  that, along  the  fault surface  in
vertical direction, the maximum displacement migrated from each segment center to each transition position, which led to fracture
propagation of each fault segment and finally joined the separated fault segrnents together to form a fault zone.

Key words: Qiongdongnan basin, fault activity, quantitative calculation, fault growth