Superimposed Structure Of Multiphase Extension From Different Orientations:Insights From Numerical Modelling

Superimposed structure of multiphase extension from different orientations is a complex and valuable structural pattern in the evolution of rift basins,controll ing the migration and accumulation of hydrocarbon in extensional zones.Based on discrete element method,a series of theoretical models are established to investigate the effect of the extensional orientation and the presence of a ductile layer on the deformation of the superimposed structure.A rolling resistance model is used to simulate the behavior of shear faulting in extensional process.The results of benchmark models show that adding a rolling resistance coefficient into the linear model will signi ficantly increase the angle of repose,which equals to the internal friction angle in loose granular materials.The direct and final effect of the rolling resistance coefficient is to produce a shear zone under the Mohr-Coulomb failure criterion.The results of multiphase extension models indicate that the middle ductile layer divides the extensional fault systems between the upper and lower brittle layers.Ductile deformation of the middle layer makes itself thin and pulls upper brittle layer from two sides to the central depression extending the upper fault zone,while has no extra influence on the lower brittle layer,and hence producing the vertical superimposed fault systems in two sets of brittle layers.One stage of orthogonal extension following by a nother oblique extension produces two sets of superimposed faults with different fault strikes.Changing the extensional orientation not only produces new oblique faults but also enlarges fault zones with the increase of fault displacement.Thinning the ductile layers leads to continuous development of large-scale faults without division.A markedly growth of elasticity modulus produces more tiny faults instead of large-scale ones.Declining the extension velocity decreases the difference of development of fault systems in both sides,but does not affect superimposed extensional deformation.Based on stress field of the Nanpu Sag in Bohai Bay Basin by finite element method,comparison of various discrete element modelling results with the structural and sedimentary characters of the Nanpu Sag in Bohai Bay Basin indicates that the layers with different mechanical properties through multi-stage extensions from different directions is responsible for the formation mechanism of the superimposed fault system.