Differential activity characteristics and genetic evolution of faults across stages and segments in Pingbei area of Xihu Sag, East China Sea Basin

The Pingbei area is a significant oil and gas enrichment zone in the Xihu Sag of the East China Sea Basin. Influenced by pre-existing structures and regional transitional stress fields, its fault structures and evolutionary processes are highly complex and distinct. Currently, there is a lack of in-depth and systematic understanding of the development characteristics of NW-trending concealed strike-slip faults, the mechanisms of differential fault activity across segments, fault evolution processes, and their dynamic background in this area, which restricts the effective summarization of oil and gas accumulation patterns and accurate evaluation of exploration targets in this region. Through detailed structural interpretation of high-resolution 3D seismic data and quantitative analysis of fault activity, combined with regional dynamic background and basin evolution processes, this study systematically investigated the structure types, spatial distribution characteristics, mechanisms of differential activity across stages and segments, and genetic evolution patterns in the Pingbei area. The research results showed that: (1) Faults in the Pingbei area could be classified into three major types based on their dynamic properties: extensional, transtensional, and strike-slip faults. Based on hierarchy and scale, they could be categorized as secondary, tertiary, and quaternary faults. Based on activity periods, they could be divided into faults of the rift stage, faults of the fault-depression transition stage, faults of the depression stage, and faults of the regional subsidence and reformation stage. (2) The spatial distribution of faults exhibited distinct zoning, banding, and segmentation differences. The northern fault slope zone was dominated by co-directional normal faults; the southern fault slope zone primarily developed co-directional and anti-directional transtensional faults; and the southern fault uplift zone was mainly characterized by co-directional extensional and transtensional normal faults. (3) The activity characteristics of faults differed significantly across stages, and could be divided into three stages comprising six phases, overall showing strong early activity and weak late activity with a composite strike-slip-extensional pattern. (4) The differential activity of faults across segments was directly controlled by pre-existing fault-block structures and NW-trending concealed strike-slip faults, and its evolution process was closely related to the interactions among the Pacific, Eurasian, and Philippine plates, as well as the transformation of their stress fields.