Characteristics and evolutionary model of Paleogene structures in southwestern Huizhou Depression, Pearl River Mouth Basin

The study area is located in the southwest of the Huizhou Depression of the Pearl River Mouth Basin next to the Xijiang Depression at the transitional part of two tectonic systems trending in NEE and NE respectively. Three sets of fault systems, trending in EW and NW as well as NE directions, have been identified within the Depression. The faults may be classified into flat-type and shovel-type according to their profile patterns. And the three sets of faults jointly control the structural features of the Paleogene fault depression in different stages. The activity of N-dipping faults is stronger than the S-dipping faults in the east of the study area whereas the S-dipping faults are more active in the west of the study area during the stage when the WenChang Formation was deposited. With a high settlement rate, the activity of S-dipping faults, i.e., the sea-faced-dipping faults, were obviously intensified during the Enping stage because of the spreading of the South China Sea. The settlement and sedimentation rate increased significantly in the Zhuhai stage, although the activities of all faults were weakened. The sediment infilling is obviously controlled by the tectonic framework and its evolution. For rapid growing faults, due to the large difference between the hanging wall and footwall, thick deltaic depositional system will be developed on the subsiding plate. However, for the faults which grow slowly, thin braided river and its deltaic depositional system will develop on the declining plate along the fault transfer zone.The structural pattern of the study area in the Paleogene is not a simple copy of the former, but a significant structural conversion taking T80 as an important tectonic interface, as a long distance response to the South China Sea spreading