Structural Comparison And Stretching Model Of The Southwest Sub-basin Conjugate Margins,South China Sea

The continental breakup is an important part of Wilson cycle,and its rifted structure and the control factors are the leading edge of international study.The stretch patterns are divided into two end models,pure shear and simple shear,and the developments of sequence faults are also applied to interpret the mechanism of asymmetry passive margin formation.The margins of South China Sea(SCS)were formed by Cenozoic extension of the South China continental margin,and it is small,young and well-preserved.The southwest sub-basin(SWB)with the smallest oceanic basin,the widest margins,the latest breakup and the best preserved margins is the best area to study the rifted structure and control factors of continental breakup.This study finishes kinematics analysis about structure,tectonic subsidence,stretch and thinning based on a high resolution seismic profile,which runs across the SWB conjugate margins,OBS data,heat flow data and gravity data.This thesis also discusses the dynamics of the margin formation using the dynamically numerical modelling.The conclusions are listed as follows:(1)Based on the interpretation of seismic profile,this study finds the greater sediment thickness in the northern continental margin than the southern margin,greater sediment thickness in the synrift period than the gross in spreading and post-spreading stages.The faults of similar scales nearly well distribute along the margins,with no obvious detachment fault,and the main faulting period is before 18.5 Ma.Many horst and graben structures develop in the northern continental margin,while half-grabens faulted in the south and overlapped in the north dominate in the southern continental margin.(2)From flexural-cantilever model,which considers the regional isostatic effect,the stretch factors in northern margin is larger than that in southern margin.The statistics of faults,which are from kinematic simulation,show that faults in northern margin have smaller dips and larger offsets than these in southern margin.Therefore,the stretch degrees of conjugate margins are different during rift stage.The OBS profiles show the decoupling of the upper and lower crust during the extensional thinning process.(3)The tectonic subsidence on conjugate margins are divided into three stages:slow subsidence during synrift stage;extremely slow subsidence/uplift during spreading stage;accelerated subsidence during post-spreading stage,which is different with two-stage tectonic subsidence in classical rifted basins.The subsidence center shifts from the proximal margin to distal margin with the tectonic evolution.The lower crust of the SWB margins was relatively weak,which would flow to the center of faulting and compensated for the upper crustal thinning caused by brittle faulting during the synrift period and thus reduced the tectonic subsidence rate.During the spreading stage,faulting attenuated rapidly and small-scale secondary mantle convection occurred at the necking zone,which raised the continental margin isotherms and increased the buoyancy.Simultaneously,secondary mantle convection lifted the overriding crust,so the overall subsidence rate decreased sharply or even reversed to uplift.After seafloor spreading,the effect of mantle convection vanished and vast sediment loading drove the lower crust to flow landward.Thermal relaxation,lower crust flow and the vanish of secondary mantle convection together lead to a rapid subsidence during this stage.(4)From the results of numerical simulation about dynamics,this study finds that if the lower crust is weaker,the time needed for rifting before breakup is longer and the rifted margin is therefore wider.Based on previous studies of other scientists,this study proposes that the lower crust in SWB margins is weaker than that in east sub-basin margins,and the lower crustal strength in SWB margins has a change from east to west,which may be the main reason for diachroneity of breakup in SWB margins.The stretch pattern of SWB margins is more like the wide rift mode.