Sedimentary Filling Response to Detachment Structural Deformation in Shallow-Water Continental Shelf of Pearl River Mouth Basin: A Case Study of Enping Sag

The Pearl River Mouth Basin developed many types of Cenozoic detachment depressions from shallow-water area to deep-water area. Among them, the detachment depression in continental shelf shallow water area is an important window to explore the detachment structural deformation and its sedimentary filling response. Based on the detailed analysis of seismic and drilling data, the characteristics of low-angle detachment fault system in Enping Sag are studied, and the control factors, deformation process and sedimentary filling response mechanism of structural deformation in detachment fault depression are discussed. The research shows that the low-angle boundary normal fault in Enping Sag is an inter-crust detachment fault, with a length of about 50 km and an average dip angle of 17.5°, deepest reaches to the middle-lower crust. During the rifting period, the fault-dip transformed from medium-low angle to low angle. The detachment fault was formed on the basis of the pre-existing thrust fault in the Mesozoic era, associated with the ductile shear dome of the middle-lower crust and the boundary regulating strike slip fault, and jointly controlled the formation of three types of subbasin structures, such as balanced deep detachment, forward spreading wide detachment and migration compound detachment subbasin. It can be divided into three rifting stages with different sedimentary filling responses due to different structural deformation. (1) In the homogeneous rifting stage at Early Wenchang period, the extension stress was concentrated on the weak surface of the pre-existing thrust fault, and the rapid rift formed narrow and deep homogeneous half-grabens, which developed thick medium-deep lakes. (2) In the detachment extensional stage at late Wenchang period, the sag developed into a wide and shallow fault depression with differential sedimentation and filling in the east and west due to differential structural deformation. In the western sag, the weak uplift of the ductile shear dome and the strong strike slip of the boundary strike slip fault controlled self migrating medium-deep lacustrine source rocks and large-scale braided river delta turbidite sedimentary system developed in Enping 17 Subsag. In the western sag, the strong uplift of ductile shear dome controlled the jumping and allochthonous migration of sedimentary center from Enping 12 Subsag to Enping 18 Subsag. A large fan delta was developed in the transition trending slope. (3) In the detachment-depression transition stage at Enping period, the structure changed from simple shear to pure shear deformation, and the sedimentation changed from internally drainage to large shallow braided river delta lacustrine filling dominated by externally drained conditions. The low-angle detachment depression has different structural deformation and sedimentary filling from the high-angle brittle fault depression. The research results have significance guiding value for the prediction of source rocks and deep reservoirs in Enping Sag, and for the study of structural deformation and sedimentary filling of similar detachment fault depressions in the northern continental margin of the South China Sea.