Palaeomagnetic Result Of Middle Jurassic Red Bed In The Southeast Margin Of Qinghai-Tibet Plateau And Its Tectonic Significance

The tectonic evolution of the Tethyan Ocean has been controversial for decades.The southeastern margin of the Qinghai-Tibet Plateau is a southward extension of the Qinghai-Tibet Plateau(the northern of ibumasu/Indosinian Block).It is of great significance to determine the palaeogeographic location of the Middle Jurassic in the southeastern margin of the Qinghai-Tibet Plateau for the study of the evolution of the Tethys Ocean.The southeastern margin of the Qinghai-Tibet Plateau was affected by the Indo-European collision in the late stage,and complicated rotational deformation occurred in the interior.In order to limit the palaeogeographic location of Sibumasu/Indosinian Block in the Middle Tethyan Ocean and its intraplate deformation characteristics in the northern part of the plate.Palaeomagnetism of the Middle Jurassic red beds in the Ximeng(Baoshan terrane)and Yunxian(Simao terrane)region southwestern Yunnan Province has been studied.In this study,235 samples were collected from 23 sampling sites in the red beds of the Huakuizuo Formation of the Middle Jurassic.The results of triaxial thermal demagnetization,isothermal remanence and reverse field curves show that hematite is the main magnetizing mineral in the samples,and some samples contain a small amount of magnetite.Systematic thermal demagnetization was performed on all samples,and stable characteristic remanence components were obtained in both regions.The mean direction after tilt correction from Ximeng sections is Ds=24.7°,Is=31.5°,k=83.0,?95=5.3°,N=10.These directions passed the fold test of Mc Elhinny(1990)at 99% confidence level and the reversal test of Mc Fadden and Mc Elhinny(1990)at the 95% confidence level in classification C.The mean direction after tilt correction from Yunxian section is Ds=44.4°,Is=31.0°,k=72.6,?95=5.7°,N=10.These directions passed the reversal test of Mc Fadden and Mc Elhinny(1990)at the 95% confidence level in classification C.The reliability tests indicate the directions are primary magnetization.E/I test of high temperature component in Yunxian section shows that there is no obvious phenomenon of inclination-shallowing.Paleomagnetic pole correlated with the result of Ximeng sections: ?p = 69.0°N,?p = 199.5°E,A95=4.5,Paleomagnetic pole correlated with the result of Yunxian section: ?p =47.9°N,?p = 190.9°E,A95=4.8.The results show that the study area(reference point: 23 N,100.7 E)was in low latitude(about 17°N)during the Middle Jurassic.There was no obvious zonal movement between the Baoshan terrene and Simao terrene from the Late Triassic to the Middle Jurassic,but there was northward movement of about 10 degrees between the Middle Jurassic and the early Cretaceous.Since the Middle Jurassic,Yunxian and Ximeng regions have different rotation from their northern regions.