Tectonic Numerical Modeling And Genetic Mechanism For The Continent-ocean Transition Zone Of The Passive Continental Margin: A Case Study Of The Kaiping Sag

The Kaiping Sag is located in the western Zhu-? depression in the Pearl River Mouth Basin and is situated in the continental-ocean transition zone of the northern margin of the South China Sea.The structural features are extremely complex and the research degree is low.Based on the latest drilling and seismic data,combined with previous research results,this paper analyses the geological structure and tectonic evolution of the Kaiping Sag and its adjacent area,the tectonic subsidence of different tectonic units has been calculated by means of basin modeling,and on this basis,calculating the crust and lithospheric mantle stretching factor respectively,in order to discuss the extensional pattern and extensional mechanism of the Kaiping Sag and its adjacent area.With the study,we get to the following results:1.Based on the high resolution 3D seismic profile and drilling data,the analysis of unconformity shows that the Kaiping Sag developed several unconformities.There are six major unconformities in the main sub-sag and northern sub-sag,while there are five major unconformities in the western sub-sag and southwestern sub-sag.Based on the major unconformities,the Kaiping Sag are divided into seven(or six)structural layers: the basement structural layer,structural layer at stage I of rifting,structural layer at stage II of rifting,structural layer at stage III of rifting,structural layer at the transitive stage,structural layer at the stage of depression,and structural layer at the accelerated subsidence stage.The characteristics of unconformities are different among the tectonic units,and the characteristics of tectono-stratigraphic sequence are also different.2.On the basis of the theory of fault-related folds,combined with the interpretation of faults and ?T0 map,it is considered that the Kaiping Sag is controlled by a large extensional detachment fault and is a typical detachment basin.Based on the geometry and kinematics characteristics of the detachment fault,the detachment fault can be divided into three segments: southwest,middle and northeast segments.The southwest segment is mainly listric,ramp-flat and spoon-like,the middle segment is mainly ramp-flat and spoon-like,the northeast segment is mainly listric.The different segments began acting during the deposition of the middle member of the lower Wenchang Formation,and almost stopped acting at the end of the deposition of the Enping Formation,and had a hard connection at this time.The western and southwestern sub-sags are typical half grabens with southward-dipping northern border faults.By contrast,the main and northern sub-sags are grabens that are controlled by the faults in the southern and northern,characterized by two-way rotation.3.The subsidence history of the basin has been restored based on artificial wells by means of basin modeling.It shows that tectonic subsidence rate reached maximum in the 49-39 Ma,and decreased significantly at 39-30 Ma,and increased slightly in the 30-17.5Ma.The tectonic subsidence rate increased at 17.5Ma,which may be related to the cessation of sea floor spreading combined with regional geological background.The breakup unconformity that formed by the seafloor spreading of SCS at 30 Ma separated the syn-rift sediments and post-rift sediments.The Kaiping Sag and its adjacent area entered a regional thermal subsidence stage after 30 Ma.4.Based on the history of tectonic subsidence,calculating the crust and lithospheric mantle stretching factor respectively by using uniform stretching model,and drawing the distribution curve of the stretching factor.It shows that stretching factor of the lithospheric mantle is significantly larger than that of the crustal stretching factor.The stretching factor of the lithospheric mantle varies between 1.28-1.45,while the stretching factor of crust varies 1.08-1.3.The distribution curve of the crustal stretching factor is roughly mirrored with the Cenozoic sedimentary basement.Integrated the previous research and the results of this study shows that the upper crust of the Kaiping Sag and its adjacent area was a simple shear stretching model with asymmetric structure.The lower crust and lithospheric mantle were pure shear stretching model.The lithosphere as a whole was characterized by non-uniform and different extensional deformation.