Paleomagnetic Results Of The Mesozoic-Cenozoic Strata In The Northeastern Tibetan Plateau And Their Tectonic Implications

The study of tectonic deformation in the northeastern Tibetan Plateau is an important way to understand the uplift process and mechanism of the plateau and the remote effect of India-Eurasia plate collision.The northeastern part of the plateau,which is more than 1500km away from the collision boundary of India-Eurasia Plate,is the frontal zone of the northeast extension of the the Tibetan Plateau,and a relatively young and active area of tectonic uplift,with a unique"basin-mountain"geomorphic pattern.The formation and evolution of the basin is closely related to the surrounding orogenic belt,and the extremely thick continental sediments in the basin which are mainly from the surrounding mountains completely record the information related to the tectonic uplift of the surrounding orogenic belt and the rotational motion of the block since the Mesozoic-Cenozoic Era.Therefore,this thesis chooses the Lulehe profile at the northern margin of Qaidam Basin,the Chetougou and Riyueshan profiles in Xining Basin in the northeast Tibetan Plateau as the research object.Based on the detailed magnetostratigraphic age constraints,we use the unique advantages of paleomagnetism in studying the rotation of blocks to study whether these profiles rotate,whether they rotate clockwise or counterclockwise,and how much they rotate,which provides basic data for understanding the tectonic evolution of the basin and its surrounding areas,the deformation and uplift of the northeasternTibetan Plateau,and the remote deformation effect caused by the collision and compression of the India-Eurasia Plate.In this study,a total of 165 sites were arranged and 1548 pieces of paleomagnetic samples were collected.Combining detailed rock magnetic analysis,magnetic fabric analysis demagnetization analysis and data reliability test of paleomagnetic samples,with other existing research results,the following understandings were obtained:1.Through the rock magnetic analysis,including the CLG analysis for IRM acquisition curve,inverse field demagnetization curve and isothermal remanent magnetism curve?6samples?,hysteresis loop?17 samples?,k-T curve?12 samples?and three-axis isothermal thermal demagnetization test?12 samples?.It is concluded that the dominance magnetic carriers of most samples are characterized by coexistence of magnetite and hematite;a small number of samples take mainly magnetite or hematite as the main magnetic-carrying mineral;some samples contain a small amount of goethite or hematite.2.Mesozoic-cenozoic strata distributed continuously in the Lulehe section of the Qaidam Basin retain the characteristics of initial sedimentary magnetic texture,and the maximum axial direction of the magnetization ellipsoid indicates the paleocurrent direction.The paleocurrent direction has changed 4 times during the Dameigou Formation?J₂d?of the Middle Jurassic to the Lower Youshashan Formation?N₂¹y?of the Early Miocene.Firstly,during the middle to late Jurassic?Dameigou formation?J₂d?to Hongshuigou formation?J₃h??,the paleocurcurrent rotated clockwise by nearly 22°.Then,up to the early Cretaceous?Quanyagou Formation?K₁q??stage,the paleocurcurrent rotated counterclockwise by nearly65°.And then the paleocurrent rotated clockwise about 63°at 31.76 Ma;at 23.02 Ma,the paleocurrent rotated counterclockwise about 56°;and its paleocurrent direction lasted to 20.40Ma.3.Through the characteristic remanence separation scheme designed according to the results of rock magnetism analysis,the low-temperature remanence components and stable high-temperature remanence components were separated by systematic thermal demagnetization of the samples.The low-temperature remanence components without stratigraphic correction were closed to the direction of the modern geomagnetic field in this area,which should be due to the viscous remanence superimposed by the modern geomagnetic field.The direction of high-temperature characteristic remanence,which can generally pass the reversal test,fold test or conglomerate test,indicated that it may represent the primary remanence direction.4.Paleomagnetic results of Lulehe profile in the northern Qaidam Basin showed that,From the early Cretaceous Dougou Formation to the Lulehe Formation,there was a significant counterclockwise rotation of 51.4°?-28.9°to 22.5°?.From Lulehe Formation to Xiaganchaigou Formation?44.41-31.76Ma?,obvious counterclockwise rotation continues,with the amplitude up to⁶6.1°?22.5°to 88.6°?.From the Xiaganchaigou Formation to the Shangganchaigou Formation?31.76 to 23.02Ma?,an obvious clockwise rotation occurred,with a amplitude up to⁹6.4°?88.6°to-7.8°?.From the Shangganchaigou Formation to the Xiayoushashan Formation?23.02-20.40Ma?,no significant or minor clockwise rotation?².5°?occurred.The rotation mode changed from counter-clockwise to clockwise at 31.76Ma,which may be due to the continuous compression of the Indian plate and the large-scale left-lateral strike-slip of the Altun fault in the Early Oligocene.The change of paleocurrent direction is caused by block rotation and surrounding tectonic uplift.5.The paleomagnetic results of the Chetougou section and the Riyueshan section of Xining Basin show that,from the Mahalagou Formation?36.4-34 Ma?to the Xiejia Formation?34.0-24.0Ma?,a relatively obvious counterclockwise rotation occurred in the Xining Basin,with a amplitude of¹4.5°?22.8°to 37.3°?.From the Xiejia Formation?34.0-24.0Ma?to the Chetougou Formation?24-21.70 Ma?,clockwise rotation occurred in Xining Basin,with a amplitude of¹0.2°?37.3°to 27.1°?.From the Chetougou Formation?24-21.70 Ma?to the Guider Group?^<21.70 Ma?,the clockwise rotation amplitude of Xining Basin was as high as 43.6°?27.1°to-16.8°?.The rotation mode changed from counterclockwise to clockwise during early to late Oligocene?³4-24 Ma?,which may be due to the Xining Basin is located in the east of northeast Tibetan Plateau,far from the Altun Tagh Fault Zone,so the effect caused by Altun Tagh Fault Zone didn't occur until²8 Ma.6.The paleomagnetic results indicate that the rotation of the blocks in the northe ast of the plateau may have complex tectonic background and further factors.The occ urrence of basin rotation,the change of rotation mode or amplitude,and the occurrenc e of thrust or strike-slip fault system activities around the study area may be correlate in time,which may be the comprehensive reflection of the remote effects caused by the periodic northward collision and compression of the southern Qiangtang,Lhasa an d India Plates.