The Types And Formation Mechanisms Of The Cenozoic Strike-Slip Structures In Jiyang Depression

The Jiyang Depression is located in the eastern part of the North China Craton and is the most important sedimentary center in the southeastern Bohai Bay Basin.It was generally believed that the Cenozoic Jiyang Depression was a rift basin.However,the dextral strike-slip deformation due to the subduction of the Pacific Plate and the collision of the Indian-Eurasian Plate cannot be ignored either.With the development of research,Cenozoic strike-slip structures in the Jiyang Depression have been proposed and widely accepted by scholars.Although many strike-slip structures and related oil and gas reservoirs have been reported in recent years,there is still a lack of systematic study on the distribution of strike-slip structures,the structure characteristics of strike-slip fault zones and the formation mechanisms of strike-slip structures.At the same time,the faults developed in Jiyang Depression have both extensional and strike-slip structural characteristics,and there is also a lack of quantitative identification indicators to determine whether the fault is mainly strike-slip motion or extension motion.Based on seismic data,this paper identifies the strike-slip fault zones,systematically analyzes the structural types and structural characteristics of the strike-slip fault zone,calculates the strike-slip amount of different types of strike-slip fault zone,and establishes a quantitative identification method of the strike-slip fault zone.Based on the expansion indices and fault displacement analysis,the strike-slip activity time was determined.According to the scale of strike-slip fault zone,associated structures and the relationship with the extensional structure,three types of strike-slip deformation mechanism are divided.Combined with the plate interactions around the Pacific-Eurasian-Indian plates,the dynamic mechanisms of the different strike-slip activity periods are analyzed.The main findings of this study are as follows:（1）Based on the results of seismic data interpretation,coherence slice images,and Ant-tracking images,the 29 strike-slip zones were identified,and the geometric features of strike-slip fault zone in the Jiyang Depression were systematically analyzed.According to the faults scale,fault linkages and motion of strike-slip faults,four-level strike-slip faults are divided.The 29 strike-slip fault zones are classified into 6 types of strike-slip structural types,namely T-type strike-slip fault zones,R-type strike-slip fault zones,C-type fault zones,P-type strike-slip fault zones,and N-Y combination type strike-slip fault zones and Y-Y type strike-slip fault zones,respectively.These strike-slip fault zones are formed in a unified dextral strike-slip system.Furthermore,this paper summarizes the structural and distribution characteristics of 6 types of strike-slip structures in detail.（2）Combined with the characteristics of strike-slip structures in the Jiyang Depression,this paper puts forward the calculation method of strike slip fault zones,which are echelon fault model,inclined stratum model and restricted fault model.In addition,the study improved the calculation method of the micro-strike-slip pull-apart basin.A quantitative identification system for strike-slip structures is established based on pitch angle and strike-slip factor.（3）Combined with the structural characteristics,fault-level division and results of the strike-slip structure quantitative identification,the Jiyang Depression is divided into three major structural systems,namely,the Tan-Lu strike-slip structural system,the superimposed strike-slip structural system,and the Yan-Lan（Yanshan-Lanliao）strike-slip structural system,respectively.（4）According to the cutting relationship between faults and strata,fault drop statistics and expansion indices calculation results of the normal fault related to strike-slip activity,three intense strike-slip motions in the Jiyang Depression are determined（E₂s₃^L-E₂s₃^U,E₃s₁-E₃d,and Qp-N₁m^L,respectively）.The far-field effect of the hard collision of the Indian Plate and the change in the direction and the rate of subduction of the Pacific Plate jointly regulated the dextral slip and shearing of the Jiyang Depression at ca.42 Ma.In addition,the second strike-slip activity of Jiyang Depression is closely related to the rate of subduction of the Pacific Plate increased from 55 mm/yr to 77 mm/yr and the SWW-compression force generated by the opening of the Japan Sea during the E₃s₁-E₃d period.During the deposition of the Qp-N₁m^L,the dextral strike-slip activity of Jiyang Depression was affected by the continuous activity of the Pacific plate and the increase of the collision rate of the Indian plate.（5）According to the scale and structure of the strike-slip fault zone,the relative development density of strike-slip structure and extensional structure,the strike-slip structures in Jiyang depression are divided into shear deformation mechanism,differential extensional-strike-slip deformation mechanism,and transitional deformation mechanism.The shear deformation mechanism includes strike-slip fault zones formed by the simple shearing and pure shearing,which are dominated by strike-slip structures and extensional structures and other secondary strike-slip structures are derived.The structures formed by this deformation mechanism include T-type,R-type,Y-Y-type and P-type strike-slip fault zones.The differential extensional-strike-slip deformation mechanisms is a small-scale strike-slip structure mainly formed by extensional activities.These strike-slip structures either connect two non-collinear normal faults or adjust the displacement difference of different segments of the hanging wall of the normal faults.The structures formed by this deformation mechanism includes R-type and N-Y combination type strike-slip fault zones.Transitional deformation mechanism related to the local reactivity of preexisting fault in extensional or strike-slip activity.The structures formed by this deformation mechanism include Gaoqing and Lijin strike-slip fault zones,and C-type fault zones.