Reactivity Features and Patterns of Basement Faults in the Qingdong Sag,Jiyang Depression

The Qingdong sag of the Jiyang depression is a Cenozoic faulted basin in the southern Bohai Bay basin.The sag contains nearly EW,NW and NNE striking fault systems.The NW and NNE striking faults belong to basement faults respectively resulting from foreland deformation in the Indosinian period and the Tancheng—Lujiang (Tan-Lu) sinistral faulting in Late Jurassic.The sag was under NS extension and undergone intense rifting during the Paleogene,leading to formation of the EW faults.The two sets of basement faults reactivated by a way of oblique extension under the NS extension.The NW and NNE striking basement faults have sinistral and dextral movement respectively.Reactivity of the NW striking basement faults predominate in the Qingdong sag during the Paleogene,which had more intensive activities and controlled the NW to SE sedimentary framework during the early rifting stage,then replaced by large numbers of EW faults and resultant EW sedimentary framework in the later rifting stage.However,the larger WNW striking faults with lower strength and in more favorable extended direction have the sustained and intensive activities.It is for the low strength that the large-scale Qingdong 4 fault zone and Tan-Lu fault zone also showed strong and sustained activities.Moreover,they present left or right en echelon arrangement on horizontal and flower liking structure on profile as being with large proportion of strike-slip movement,displaying the large-scale basement reactivated features.After more detail studies,reactivity patterns of the basement faults can be classed into three types,such as the direct activity,formation of new en echelon faults and direct activity plus propagating into EW faults.It can be seen that basement faults have important effect in the evolution and distribution of fault systems and sedimentary fdling characteristic in a basin.