| Xihu sag is located in the eastern depression of the East China Sea shelf basin.Since the Oligocene,the Philippine plate subducted to the East China Sea with NW direction,replacing the subduction of the Pacific plate,resulting in a significant tectonic transition in the Cenozoic.At present,the study on the structure and genetic mechanism of the Xihu sag before and after the tectonic transition is indistinguishable.Therefore,the Cenozoic structure and tectonic transition process of the Xihu sag was analyzed by using seismic,drilling,logging,gravity and magnetic data.Fatherly,the basin tectonic transition mechanism was analyzed by combined analysis of regional geological setting and structural physics simulation experiments.The structure feature of the Xihu sag was revealed by the seismic and drilling data.Three groups of faults developed in the Xihu sag,including NNE-,NE-and NW-trending.According to the characteristics of faults,NNE-trending faults are the main basin controlling faults of Xihu sag,including strike-slip,extension-slip,strike-slip extensional and extensional fault.The NE-striking faults are generally secondary extensional faults.According to the characteristics of faults combination pattern,there are strike-slip structural style,strike-slip extension structural style and extending structural style in the study area.The strike-slip structural style and strike-slip extension structural style mainly developed in the eastern part of the sag,whereas the extension structural style mainly develops in the west and east of the sag.Due to the influence of the faults in different formations,the depressions show a double-layered structure as a whole,the lower part shows a graben rift structure,and the upper part shows an ‘east-faulting,west-overlapping' single-faulted half-graben rift structure.Based on the analysis of fault activity rate and balance section,the transition process of Xihu sag was clarified.The tectonic evolution of the Xihu sag can be divided into the extensional faulted depression in Eocene(graben rift style),slip-compressive depression in Oligocene-Middle Miocene(single-faulted half-graben rift style)and stable subsidence in Pleistocene.During the Eocene,the east and west boundary faults of the Xihu sag began to develop under the extension stress,which indued the formation of graben,and mainly developing extension structural style.During the Oligocene-Middle Miocene,under the influence of the right-lateral strike-slip extrusion caused by the Philippine plate,the fault activity in the basin is greatly reduced,the eastern boundary fault continue active,the western boundary fault inactivate,and the sag structure changes from the graben to half-graben,and then a large number of strike-slip structural style have developed in the eastern part of the depression.During the Middle Miocene-Pleistocene,the sag was in a stable sedimentary stage and the second transition of the depression occurred.The basin tectonic transition mechanism was analyzed by combined analysis of regional geological setting and structural physics simulation experiments.The tectonic transition in different evolution stages of the Xihu sag was controlled by the large regional dynamic settings of the East China Sea Basin.During the Eocene,the Pacific plate subducted to the Eurasia and India plate collision with the Eurasia plate,thus,the deep mantle material of the east China continue to move eastward which resulted in a strong extension effect.All of these induced the migration of the East China Sea Basin from the west to the east,and graben formed.During the Oligocene-Middle Miocene,the subduction of Philippine plate caused the East China Sea under right-lateral strike-slip shearing,which induced the formation of the strike-slip structure,and the basin entered the slip-compressive depression stage.During the Miocene-Pleistocene,the subduction rate of the Philippine plate decreased,the deep mantle material continued to move eastward,and the Xihu sag entered into the stable sedimentary stage. |
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