Evolution of post-rifting uplift and sag pattern and differences of source-sink system under the compression condition in the Xihu Sag of the East China Sea Basin

The East China Sea shelf basin has undergone many tectonic movements since the Cenozoic, which have led to dramatic changes in the structure framework pattern and source-sink system of the Xihu Sag. The study on the evolution of its uplift and depression pattern and the difference of source-sink system can promote hydrocarbon exploration in the medium and shallow strata. In the perspective of the tectonic change, we analyzed the sedimentary pattern and source-sink system during the depression period of the Xihu Sag by using fined seismic profiles, paleogeomorphological restoration, and compilation of regional sedimentary maps. Results show that the evolution of the post-rift period of the Xihu Sag could be divided into four depression episodes of Ⅰ (\rm T_3^0 -\rm T_2^4 ), Ⅱ (\rm T_2^4 -\rm T_2^3 ), Ⅲ (\rm T_2^3 -\rm T_2^0 ), and Ⅳ (\rm T_2^0 -\rm T_1^0 ). Sedimentation during episodes Ⅰ and Ⅱ reflects thermal subsidence in the early post-rift period, forming a single depression, while that during episode Ⅲ features separation by polymorphic sags formed under compressional background, and that during episode Ⅳ represents back-arc extension background. Two major changes occurred in the source-sink system during the depression period. The first one took place at the end of the Oligocene epoch, sudden regime shifted from the “lakes in the south and rivers in the south” pattern of the Huagang Formation, with multiple marine transgressions in the southern part of the lake system, to the “rivers in the south and lakes in the south” pattern of the Longjing Formation, with an occurrence of terrestrial lake system. The second one occurred at the end of the Miocene, transitioning from meandering river network system of the Liulang Formation to the shallow sea environment of the Santan Formation. The evolution in the uplift and depression pattern of the Xihu Sag not only reflected in the abrupt change of the depositional environment, but also led to the change in depositional center position from single center, to multicenter, and to residual centers, as shown in the thickness of sediment. The multiple compression was the main reason for the formation of the uplift-and-depression framework and the source-sink system. The results reveal the dynamic process of continental margin controlled by the joint compression of four major tectonic plates, during which the Pacific Plate subducted in the NWW direction, the Eurasian Plate creep slowed down, the Philippine Plate wedged in, and the Indian Plate subducted in the near N-S direction. These findings provided an important geological basis for the hydrocarbon exploration in the medium and shallow strata.