Cenozoic Deformation Characteristics Of The Yingxiong Range Anticlinal Belt In The Qaidam Basin And Its Significance

The Yingxiong Range is adjacent to Altyn Tagh Fault in northwest and the Qiman Tagh System in south side.Its critical tectonic position is ideal for understanding the structural deformation of Qaidam Basin bounded by compression and strike-slip tectonic condition.It is also the highest topography in the whole Qaidam.The average elevation above the basin is nearly 800 meters,and its northeastern part has been subjected to strong erosion of wind.Not only has unique features in topography,but also important to recognize the internal deformation and transfer process in the Qaidam Basin and understand the relationship among Qiman Tagh System,Altyn Tagh Fault,Qaidam Basin.To study structure and deformation of Yingxiong Rang is great significance for understanding the deformation characteristics and basin-mountain coupling in the northern Tibet Plateau.In this paper,we present a detailed structural analysis,shortening and detachment depth calculation based on two-dimensional seismic data,three-dimensional seismic data,well log and field investigation.We discuss the relationships between the Yingxiong Range Anticlinal Belt and the Qiman Tagh System and Altyn Tagh Fault.Finally,the conclusions are summarized as follows:(1)The deformation of Yingxiong Range Anticlinal Belt is mainly controlled by the two main basement-involved thrusts that are north-directed thrusts including XI Thrust and Yinbei Thrust.Its deformation are also affected by the distribution of gypsum layer developed in the Upper Xiaganchaigou Formation.In west part,Shizigou Anticline is mainly controlled by slipping of the Shizigou Fault along the gypsum layer in shallow but little effect by the XI Fault and its branch faults in deep.The Ganchaigou Anticline and Xianshuiquan Anticline are mainly controlled by the basement-involved Yinbei thrust and its reverse-directed fault.In the east part,Youshashan Anticline is controlled by tectonic wedge which consist of ?? Thrust and Youshashan fault.Ganchaigou Anticline tends to be gentle and gradually disappears to the east;the Youquanzi Anticline is a fault propagation fold in the hanging wall of Yinbei Thrust.The shortening deformation of the Yingxiong Anticlinal Belt is the largest in the west part,which can reach 5.5 km(13%of the strain)and decreases gradually to 1.5 km(the strain is reduced by 3.9%).(2)Combining the development time of the unconformity and growth strata revealed by seismic profiles and field geological surveys,it is believed that the XI fault deep in the Yingxiong Anticlinal Belt began to be weakly active during the deposition of Upper Xiaganchaigou Formation and was accelerated with Yinbei Fault during the deposition of the Shizigou Formation(<8.1Ma).Subsequently,strong fold and uplift occurred,forming the current Yingxiong Range.(3)There are five secondary anticlines in the Xianshuiquan Anticline at the northernmost tip of the Yingxiong Range and a right-step en echelon distribution.It is show that the main fault which controls the fold has dextral strike-slip component.Four groups of thrust are developed,the first group which has north dip and NEE-trending has a left-lateral component and is apart of Altyn Tagh Fault system.The second group which has west dip and NNW-trending is located in eastern Xianshuiquan Anticline.The structural configuration of Xianshuiquan is controlled by the second group which is the part of the Yinbei Fault and has a certain right-lateral strike-slip component.The third and fourth groups are a series of thrust which have NE-dip or NNE-dip and arranged in an en echelon.This two groups faults are the reverse fault of the second set of faults.We divide the Xianshuiquan Anticline into the north and the south parts.The south part is little influenced by the Altyn Tagh Fault system,and characterized by the development of NNW-trending secondary anticlines and NW-trending back-thrusts with small horizontal shortening.The north part is greatly affected by the Altyn Tagh Fault system,and characterized by NWW-trending sub-anticlines and back-thrusts with much larger amount of shortening.Our results support that the Qaidam basin is a compression-dominated basin,and the influence of the sinistral strike-slip Altyn Tagh Fault system on its deformation is only confined within a limited area in the Cenozoic.(4)Through the structural analysis of Yingxiong Range,detachment calculation,and combine with previous research,we build,two large cross-sections across the Qiman Tagh,southwestern Qaidam and Yingxiong Range.They constitute a large intracontinental north-direct thrust system above the brittle-ductile transition zone of 14-18km.Combining previous studies on the activity time of this area,it was found that the initial activity time of the thrust system also has a significant transfer process from south to north.In the Late Cretaceous-Early Cenozoic era,the Qiman Tagh began to rise rapidly.The southwestern Qaidam began to active during the deposition of the Lulehe Formation and gradually transfer to north and east.The activities in the Yingxiong Range were stable until 8.1 Ma.Tectonic Activity in the southwestern Qaidam began to weaken after 8.1Ma.These observations indicate that the Yingxiong Range Anticlinal Belt is front end of the Qiman Tagh thrust belt,and the shortening of the Yingxiong Range Anticlinal Belt is mainly caused by the thrust system.Shortening of Yingxiong Range Anticlinal Belt in the Cenozoic era shows a significant decrease from west to east.It also shows that the Altyn Tagh Fault system also has some influence on its formation.