Cenozoic Structural Deformation And Its Mechanism In Western Part Of North Qaidam Basin

The Northern Qaidam Basin,with well exposed Cenozoic strata,is one of the most complicated areas in structural deformation in the northern Tibetan Plateau,and is of great significance in understanding the Cenozoic deformation and expansion of the northern Tibetan Plateau.At present,there is still much controversy about the mechanism of the Cenozoic tectonic deformation in north Qaidam Basin.Based on field geological survey and seismic reflection profile interpretation,this paper constructed 14 geological sections passing through the western part of north Qaidam Basin.On this basis,the Cenozoic deformation characteristics were segmented studied in detail.The results show that the study area can be divided into Xiaosaishiteng Mountain segment(near EW trend),Lenghu-Jielusu segment(near EW trend),Jieliisu-Lulehe segment(NW trend)and Lulehe-Yuqia segment(EW-NWW trend)from the northwest to the southeast.Xiaosaishiteng Mountain segment,the westernmost part of the study arera,is close to the Altyn Tagh fault and develops an angular unconformity in the mid-Miocene and the tectonic deformation is weak after mid-Miocene,which is similar to the structural characteristics of the Altun slope area studied by the predecessors.Therefore,we consider that the Cenozoic deformation in Xiaosaishiteng Mountain segment should be mainly influenced by the activity of the Altyn Tagh fault.However,in Lenghu-Jielusu segment,Jieliisu-Lulehe segment and Lulehe-Yuqia segment,the Cenozoic strata are continuously deposited without obvious angular unconformity.And in Lenghu-Yuqia segment,there develops a series of high-angle basal reverse faults and related folds.Besides,all strata in the Cenozoic are involved in deformation,reflecting strong tectonic deformation in the late Cenozoic.The calculation results of the horizontal shortening strain and deformation width of the regional geological profiles indicate that the shortening strain and the deformation width are relatively large in Lenghu-Jielusu segment and Lulehe-Yuqia segment,and the surface shows strong fold thrust.The Cenozoic strata in Jielusu-Lulehe segment shows a simple monocline structure with a small deformation width and a relatively small shortening strain.The above analysis of geometry and deformation strength indicates that a typical anti-"S" type right-lateral oblique thrust system is formed in the Lenghu-Yuqia region in western part of north Qaidam Basin:The deformation at both ends,Lenghu-Jielusu segment and Luluhe-Yuqia segment(near the EW trend),is dominated by thrusting,supplemented by strike-slip,which is characterized by strong basal uplift and fold deformation;however,the middle part,Jieliisu-Lulehe segment(NW trend),is dominated by right-lateral strike-slip,supplemented by thrusting.The sedimentary contrast of the Cenozoic in the Yuqia region and the results of the Mesozoic clastic apatite fission track test show that the Cenozoic deformation began in the early Cenozoic in western part of north Qaidam Basin,and the overall deformation was weak.During this time,the area was characterized by the paleo-uplift structure.Its strong deformation occurred in the Miocene and later,forming a series of fold-tectonic belts in the western segment of north Qaidam Basin,such as the Yuka-Jiulongshan giant box-shaped anticline structure.In addition,this paper also constructed a regional geological section through the Suganhu depression,the Saishiteng Mountain and the North Qaidam anticline belt.Based on this,we analyzed the tectonic coupling relationship between the Saishiteng Mountain and the depressions of its two sides.The results show that the Suganhu depression and the North Qaidam anticline belt were connected in the early Cenozoic and both were weakly deformed.During the Miocene,the Saishiteng Mountain began to uplift and separated the Suganhu depression and North Qaidam anticline belt.The strain was mainly absorbed by the Saishiteng Mountain and the Danghe nanshan on both sides of the Suganhu depression,therefor the interior becomes a relatively stable sedimentary area with weak deformation.