Cenozoic Tectonic Characteristics Of Shizigou Area, Western Qaidam Basin, Qinghai Province, NW China

Western Qaidam basin is bounded by Altyn shan to the north,and Kunlun shan to the south,its evolution has been affected by both Kunlun thrust fault and Altyn Tagh Fault(or ATF),therefore,study on deformation features,tectonic stress field as well as interpretations of deep structures of Western Qaidam will definitely play an important role in better understanding the tectonic activities of ATF,and uplifting histories of Tibetan plateau as well as oil and gas exploration in this area.On basis of field observations,in conjunction with kinematic and dynamic analyses of striations from Western Qaidam,analyses of magnetic fabrics,reinterpretation of seismic profiles,passive seismic tomography as well,following conclusions can be reached.Faults of Western Qaidam are featured with NW striking thrust faults.Faults strikes NW can be classified into two primary categories,first,such as Kunbei thrust,Alaer thrust,which belongs to syndepositional thrust faults which have been formed earlier;the typical example of second type is Qigequan thrust,which is less greater in size,and formed later.Shizigou anticlines consist of shallow and deep layers,shallow layer formed thinning thrust to the south of Yingxiongling,is a thinning thrust slab small in scale will join the detachment fault beneath the Paleogene unit.Nappe resulted from thrust slab constituted echelon folds, such as Quanyagou,Shizigou,Huatugou et al.The results of magnetic fabric analyses from Cenozoic sedimentary rocks in western Qaidam basin show that the magnetic foliation is well-developed and the P value symbolizing the degree of magnetic anisotropy is low,suggesting the ductile deformation is relatively weak.The maximum compressive stress is mainly flattening deformation across the NE-SW trending,consistent with the arrangement of the anticlines at study area.The magnetic fabrics reflect mainly sedimentary fabrics,indicating that the magnetic foliations are mostly the beddings.However,some of them are formed during folding and faulting.According to the analyses of the relationships between magnetic fabrics and strata beddings,the Shizigou anticline is suggested as a fault-propagation fold due to the SW thrusting of Huatugou Fault underneath.According to striations analyses of study area,three phases of deformation can be identified,these deformations happened before deposition of Shizigou Fm.(Ⅰ),after deposition of Shizigou Fm.(Ⅱ),after deposition of Qigequan Fm.(Ⅲ),respectively.PhaseⅠindicates NE-SW compressive stress,PhaseⅡdemonstrates NNE-SSW compressive stress,PhaseⅢindicates NWW-SEE compressive stress.Distribution patterns of faults can be located by tomography results by presenting enormous velocity gradients.A great deal of fault-related folds can be found in study area,folds usually formed on the top of two thrust faults with opposite trend,which can also be explained by conjugate kink-band model.At the depth of approximately 2 km,there is apparent detachment fault,which could accommodate the strain caused by thrust faults,and could provide unusually ideal place for enrichment of oil and gas.According to growth strata in seismic profiles and unconformity relationships in the field,two primary phases of activities can be summarized.PhaseⅠinitiated at the end of paleogene(30 Ma),and lasted until the end of Xiayoushashan Fm.(14.5 Ma);PhaseⅡwas indicated by intensive strike-slip faulting of ATF, namely left-lateral compressive deformation of EW striking basement faults transformed into extensive shortening deformation which strikes NE.Flower-like structures in the depth and detachment faults of Shizigou,as well as intensive activities of Shibei thrust andⅪthrust are representive of the specific structures resulting from the PhaseⅡ.Dating of loess samples from units bounding the unconformity offered an age interval between 1.534 Ma and 0.277 Ma,which could record the latest uplift of Tibetan plateau.Up-to-date OSL dating for outcrop of Huatugou thrust fault assigned age of 70.6±8.5 ka indicating the latest activity of this fault.