Abstract: This research focuses on investigating the genetic mechanisms and evolutionary processes of the superimposed multi-stage fault systems in Block 3 of Nanpu Sag,Bohai Bay Basin.The workflow herein adopted a variety of modern techniques such as 3-D seismic interpretation,physical simulation of structure and finite element numerical simulation,and took the theories of structural geology and geomechanics as guidance.We recognized the geometry and combination modes of faults,restored the paleostress state of different tectonic periods,analyzed the development and evolution patterns of faults under variable stress field,discussed the control of pre-existing structures on later fault formation and evolution,and established geodynamic models of the superimposed multi-stage fault systems in the study area.The results showed that,bounded by unconformity on the top of Shahejie Formation,the X-pattern normal faults could be divided into two vertically superposed fault systems with short-term inversion characteristics.The upper tone is a SN-trending composite extensional and sinistral strike-slip fault system with pure shear deformation being predominant,while the lower tone is a NW-SE-trending extensional fault system with single shear deformation being predominant.Distribution of geomechanical layers,later tectonic stress intensity,and angle between pre-existing fault strike and later principle stress direction were regarded as the three main factors controlling the development of superposed fault systems.The pre-existing faults could influence the redistribution of local stress field and the development of later faults,which either continued to expand along early faults,or created new faults.As a mechanical weak surface,the unconformity was easy to generate uncoordinated deformation or slipping,resulting in significant vertical discontinuous development of faults.Regionally,the formation and evolution of the superimposed fault system in Block 3 of Nanpu Sag was directly controlled by transition of tectonic stress field by Tan-Lu fault since the Paleogene.

Key words: superimposed faults, genetic mechanism, physical simulation, numerical simulation, Nanpu Sag, Bohai Bay Basin