Analysis On The Tectonic Pattern Of Wulungu Depression In Northeastern Junggar Basin

The Wulungu depression locates in the northeastern Junggar basin.Lacking of the common sedimentary formation of Permian in Junggar basin,suggests that the tectonic pattern of Wulungu depression has some particularity.Basing on the interpretation of strata and fault structure of 4500 km seismic profile about 45 NE inline and 38 NW cross line and the different types of the unconformity relationship among formations,this paper divides the tectonic stages of the Wulungu depression.According to the growth index of synsedimentary reverse faults in different period and the structure map of different reflection layers and the basic theory of thrust fault,We analysis the activity intensity and tectonic migration of main faults in different structural stages of Wulungu depression.On the base of the above studies,combining with regional geological background and the thickness map of different development periods,this paper gives a determination of the tectonic pattern and the tectonic control on sedimentary strata of Wulungu depression.Then come to the main understanding as follows:(1)The sedimentary formation in Wulungu depression can be divided into the sedimentary basement of Carboniferous and Carboniferous before and the sedimentary cover since Late Triassic.According to the unconformity relationship of the cover formation,six tectonic stages can be divided,the stage of the Early Permian to the Early Middle Triassic,the Late Triassic stage,the stage of early Jurassic to early Middle Jurassic,the stage of late Middle Jurassic to Late Jurassic,the Cretaceous stage and the Paleogene stage.(2)By analyzing of the growth index of synsedimentary reverse faults,it is revealed that the activity of Wudong fault,Hongpen fault and Kalasayi fault have the characteristic of piggyback propagation from northeast to southwest at the depositional stage of Baijiantan formation(T3b)and Badaowan formation to Xishanyao formation(J1b to J2x),which indicates the tectonic dynamic derived from the northeast to the Wulungu depression.In the direction of strike,the change rule of the growth index is not only related to fault attribution of different fault systems,but also related to the distance between them.The growth index of Kalasayi fault displays even big difference in the direction of strike,which owes to the combination of other faults.In period of the late Middle Jurassic to late Jurassic,the faults became active again.Since Cretaceous the South Eastern of Wulungu depression uplift and fault activities gradually weaken.(3)The Lun2 fault system is an arcuate thrust structure.The fault activities have the characteristics of piggyback propagation,migration vertical to the top of arc,with strike slip structures on both sides of the fracture zone.The arcuate thrust structure has important influence on the distribution of the sedimentary center.(4)In pre-Cretaceous,the local tectonic dynamics Wulungu depression is the NE compression with dextral slip,forming the tectonic pattern of Wudong fault and thrust fault in western Wulungu depression as the main fracture,Tusituoyila fault as the “P” fracture,and Kalasayi fracture as the “S” fracture.The triangle zone composed of Wudong fault and Kalasayi fault and the Lun2 south arcuate thrust structure have the important influence on this tectonic pattern.Since Cretaceous,the tectonic pattern of Wulungu depression is mainly under the control of the fault of South margin of Junggar basin,but the local tectonic pattern of Wulungu depression still play a role.(5)The sediments of Wulungu depression form the growth strata related to the fold caused by fault.According to the characteristics of sedimentary formation of Wulungu depression,lacking of large scale detachment structures.We determine that the sedimentary strata are mainly in the control of the basement involved triangular shear fold of growth strata.During Cretaceous,with the development of interlayer slide,the type of fault related folds may be triple basement involved fold.