Structural architecture and Palaeogene tectonic evolution of the Shanan sag, Bohai Bay basin, eastern China

The Palaeogene tectonic sag evolution of the Bohai Bay Basin (BBB) is still debated and has been interpreted in terms of lithospheric extension, strike-slip pull-apart mechanisms or hybrid processes involving Pacific–Eurasian–Indian plate interactions. In the western BBB, the Shanan Sag is located at the intersection of the Tan–Lu Fault Zone (TLFZ), Zhang–Peng Fault Zone and Cangdong–Lanliao Fault Zone, providing an ideal natural laboratory for testing these models. Using integrated 3D seismic and well data, we characterise the fault geometry, activity and basin evolution. Our results document the absence of the Kongdian Formation (Ek) and the lowest Shahejie Formation (Es4), which is consistent with early Palaeogene uplift and erosion, although non-deposition cannot be excluded. The analysis also reveals a temporal migration of fault activity from NW-trending to nearly EW-, NE-, and NNE-trending systems, accompanied by systematic shifts in depocentres. Structural restoration reveals a four-stage evolution: (1) early transpressional uplift and erosion (Kongdian–Es4); (2) extension with early sinistral-influenced NW-trending faults, followed by dextral modification and activation of nearly EW- and NNE-trending faults (Es3–Es(1 + 2)); (3) structural reorganisation and southward depocentre migration (Dongying); and (4) regional thermal subsidence (Neogene–Quaternary). These observations preclude a simple extensional rift model. Instead, we propose that the Shanan Sag represents a fault-zone intersection sag , where the interaction of major strike-slip fault systems reorganises the regional stress field and significantly modifies rift-basin architecture. This model helps reconcile the conflicting interpretations of the western BBB and provides a tectonic template applicable to similar intracontinental rift systems worldwide.