Geometries And Kinematics Of The Middle Part Of Foothill Fault-Fold Structural Belt In The Southern Junggar Basin

The southern Junggar basin is located in the northern of Tian Shan,and the foothill fault-fold belt is its central structural belt.The region has complex piedmont structure and multi-row fold belts.In addition,it is rich in petroleum and gas resources and has frequent seismic activities.The study of structural characteristics is of great significance to the study of active tectonics,seismic disaster assessment,oil and gas exploration and the kinematic study of Tian Shan ranges.In this paper,through field geological reconnaissance,seismic interpretation and drilling data application,combined with structural geometric and kinematic analysis methods,we got the structural geometry characteristics,kinematic mechanism,deformation time and structural shortening in the middle part of the foothill fault-fold belt:(1)The structural pattern of the foothill fault-fold belt is mainly controlled by decollements within the Lower Jurassic Badaowan Formation and the Paleogene Anjihaihe Formation.Fault slip propagates forward along Badaowan Formation,forming the deep structure of second row anticlinal belt,the third row anticlinal belt,the concealed structure belt between the first row and the second row anticlinal belts.Fault slip propagates forward along Anjihaihe Formation and reform the second row anticlinal belt.In the deep structure of the first fold belt,there are superimposed structural wedges or fault-bend folds.The structural deformation time is late Mesozoic,paleogene and quaternary respectively,and the total slip on the fault ramp reachs to ?13km.At the same time,the multi-bend fault can form a semi-plunging anticline on the front flank of fold(Qingshuihe nose structure).(2)Dongwan anticlinal belt is a hidden structural belt between the first and the second row of structural belts.It is found that its deformation time is the Anjihaihe Formation in the Paleogene,through the analysis of growth strata,and the formation mechanism is the detachment fold of wing rotation.The results of Area-depth-strain analysis show that the detachment is located in the Lower Jurassic Badaowan Formation,about 9km depth.The fold is mainly controlled by the pure shear in parallel layers and formation bending.The structural slip reaches to 1239 m in the core of anticline belt.In addition,the slip distribution of the structural belt mainly rely on the growth of decollement folds and fold-accommodation faults.(3)The deep structure of the second anticlinal belt is formed in the process of slip transfer along Badaowan Formation.According to the analysis of area-depth method,the structural slip can reach to ?3km.After the formation of the second raw of detachment fold,the rest of slip will continue to transfer forward along the detachment and form the third raw of anticline.The growth strata developed on the top of the anticline reveals that it was formed since Pleistocene and Holocene.The development of the third structural belt depends on the slip of the piedmont structure and the transformation of the slip between the first structural belt and the second structural belt.According to the calculation,the total absorption slip of the buried Dongwan structural belt,the deep structure of the second anticlinal belt and the third anticlinal belt is about 5km,which is consistent with the slip from piedmont structure.(4)The kinematic mechanism of Huo-Ma-Tu thrust is fault-bend fold.The slip on the fault ramp is about 7?8.5km,and all the slip is transmitted forward along the detachment of the Anjihaihe Formation.According to the growth strata in Pleistocene and Holocene,the deformation time of the Huo-Ma-Tu thrust can be determined to be? 2.58 Ma.In addition,the growth strata can not only reveal the time and mechanism of structural activity,but also determine the displacement amount of the late structure.The deformation mechanism and time of the early structure can be determined by restoring its initial geometries and position if the growth strata is reconstructed by the later structure.(5)There are complicated fold-accommodation faults in the two flaps of Dongwan anticline,and the different positions of the structural belt are different stages of fold-accommodation faults.Fold-accommodation faults propagate with the growth of anticline and its propagation direction depends on the development and thickness of the weak layers.In addition,the fold-accommodation faults play an important role in sealing and preserving oil in structural quiet period,but also play a role in hydrocarbon migration or trap destruction in structural activity period,which may be an important reason for the failure of hydrocarbon prospecting of Dongwan1 well.