Cenozoic Deformation Of The Northern Qaidam Basin And Its Spatial Variation

The middle segment of the Northern Qaidam Basin(NQB)is a complex deformation zone affected by the long range effect of the Cenozoic Indo-Asian collision,and is mainly composed of three sub-units with distinct characteristics:Mahai-Dahongou uplift,Dachaidan depression and Luliang shan.Based on high-resolution seismic reflection data,combined with field investigation,remote sensing interpretation and drilling data,we constructed in this dissertation nine geological sections crossing these three sub-units in the middle segment of the NQB,and accordingly analyzed the characteristics and mechanism of the Cenozoic deformation in that area.The Conclusions are summarized as follows.(1)The Mahai-Dahonggou uplift is located in the EW-striking Maxian fault,the NW-striking Lunan fault and Lingjian fault in a triangular area.It is controlled by the high-angle thrust fault system from the NE to the SW.There are many NW-striking folds and faults in the uplift,and a small number of EW-striking faults and folds developed near the Maxian fault;The Cenozoic strata distribution above the uplift change greatly,and the thickness of the core was relatively small.The thickness increased rapidly to more than 5000 meters to the center of the Qaidam Basin.Dachaidan depression is a closed basin confined by Luliang shan and Qaidam shan.The Cenozoic strata in Dachaidan depression is thin and the Cenozoic deformation is generally weak.The Luliang shan are controlled by steep thrust faults which thrust to Mahai-Dahonggou uplift and Dachaidan depression.(2)The Cenozoic deformation strength of Mahai-Dahonggou uplift is generally smaller in the western segment(EW-striking)which is greatly affected by the Maxian fault(shortening rate is only 6%).The shortening amount in the middle segment(NW-striking)increases rapidly to 12%,and gradually decreases to 5%eastward.It may be related to the fact that the Maxian fault in the western segment is an EW-striking sinistral strike-slip thrust fault,while the middle and eastern part is mainly controlled by the NW-striking basement thrust system.The Cenozoic deformation in Dachaidan depression is weak,and the shortening rate is generally less than 2%.(3)The main boundary faults of the Mahai-Dahonggou uplift are characterized by multi-phase activities.The Maxian fault in the northern boundary of the uplift occurred remarkable activity in the Late Cretaceous.The activity form the angle unconformity between Cenozoic and underlying strata and control the sedimentary pattern in the early Cenozoic.It was weak in the early Cenozoic.Strong activities occurred again in the Miocene and later,which made the Mahai-Dahonggou uplift strongly,forming a Cenozoic Saikun depression with a thickness of 10,000 meters.The Lunan fault also has the characteristics of Mesozoic and Late Cenozoic activities.Other major basement faults,such as Lingjian fault,Wudong fault and Xiannan fault,also occurred in the Miocene and later time.(4)Based on the analysis of the deep and shallow fault system of the Nanbaxian anticline at the western of the Mahai-Dahongou uplift,we found that two sets of shallow normal fault systems,NW-striking and nearly EW-striking,are well developed within the Cenozoic strata in the Nanbaxian area,with very small fault distances.The NW-striking faults are mainly echelon distributed on the southern side(hanging wall)of the Maxian fault.The other is EW-striking,and confined in the core of Nanbaxian anticline above the Xiannan fault.It cuts the NW-striking normal fault system,and consists of a series of parallel normal faults,forming a small graben structure in the Cenozoic strata above the basement-involved Xiannan fault.We interpret that the NW-striking normal faults are related to extension at the western tail of the Maxian fault caused by the sinistral strike slip,while the EW-striking normal faults result from local extension caused by thrust of the near high-angle Xiannan fault.Combined with previous research,we conclude that the widely distributed shallow fault systems are of various origination with a complicated coupling with the deep fault systems in the Qaidam Basin,and that the pre-existing faults played an important role in the Cenozoic deformation.(5)The Mahai-Dahongou uplift,Lvliang shan and Dachaidan depression may have been connected in the early Cenozoic,forming the northern slope of the Qaidam Basin and depositing the relatively thin Cenozoic strata in the Dachaidan depression.During the Miocene,due to the strong compression on the northern edge of the Tibetan plateau,the Luliang shan and Maxian faults,as pre-existing weak belts,were strongly active,while the internal deformation of the Dachaidan depression was weak.The Mahai-Dahonggou uplift was controlled by both EW-striking Maxian fault(sinistral strike-slip thrust fault,constituting the northern strike-slip boundary)and NW-striking Lunan fault(mainly thrust,constituting the northeastern thrust boundary),resulting in strong NW-striking folds and faults deformation,and the deformation intensity gradually weaken from west to east.