Paleomagnetic Results And Their Tectonic Significance From Miocene-Pliocene Lacustrine Strata And Basalt Of Baoshan Terrane And Tengchong Block,southeastern Edge Of The Tibetan Plateau

For further restriction on characteristics of rotation of Baoshan Terrane and Tengchong Block since the Miocene,we performed tectonomagnetism studies on Pliocenen lacustrine strata,Pliocene Basalt in Baoshan terrane and Miocene lacustrine strata in Tengchong Block.The paleomagnetic research was carried out on Pliocene lacustrine strata of Yangyi Group in the Baoshan Terrane.Results of Anistropy of Magnetic Susceptibility show the original sedimentary magnetic fabric to the initial deformation magnetic fabric.The direction of K1 axis of the magnetic susceptibility ellipsoid stretch in the same direction as the basin in the NWWSEE direction,which reflects the nearly NS-trending tectonic compression and the east-west extension.Direction of average remanent magnetization of ChRMs of YYN after tilt correction is:Ds/Is=20.21°/37.1°,Ks=59.7,?95=4.8°,N=16;The ChRMs of YYN section are passed the Fold Test and Reversal Test,indicating that it represents the original remanent magnetization in the formation of sedimentary rocks.The result reveals that Baoshan Terrane have a clockwise rotation about 19.2°±6° relative to the paleomagnetic reference pole of east Asia tectonic region since Pliocene.By comparison with the results of the magnetic strata of science drill(Report of Chinese geological survey project 12120114002301)in the east margin of Baoshan basin,we propose that the stratigraphic age of YYN section was 6±0.2Ma,and average rotating velocity is about 3.2°±1.0°/Ma on average.This result is in accord with paleomagnetic study from Baoshan Terrane and Tengchong Block,which revealed large angle of clockwise deformation since Miocene.In the eastern part of the Baoshan Terrane,Pliocene Yingpan basalt was collected for paleomagnetic research.The result of Ar-Ar chronology study is 4.22±0.01 Ma.Thermal demagnetization results yield a single reversed polarity,and it can be predicated that the basalt was errupted in late Gilbert reversed polarity epoch.The data of basalt samples failed to average out geomagnetic secular variation.Site-mean direction of ChRMs of YP before tilt correction is:Dg/Ig=166.8°/19.9°,N=27,Kg=18.1,?95=6.7°;Site-mean direction after tilt correction is:Ds/Is=167.8°/21.7°,N=27,Ks=41.9,?95=4.1°.The data initially reflects that the area has underwent counterclockwise rotation since 4Ma,which may be affected by the left-strike slip of the Kejie fault and wanding fault in northeast direction.The paleomagnetic study of LHN section of Miocene Nanlin Formation in Lianghe county get a site-mean direction of Dg/Ig=339.1°/31.9°(N=3,Kg=941,?95=4.0)before tilt correction,and Ds/Is=317.8°/3 1.5°(N=3,Kg=1299,?95=3.4°)after tilt correction.The site-mean direction of high temperature component is Dg/Ig=158.2°/29.1°(N=7,Kg=75.2,?95=7.0)before tilt correction,and Ds/Is=139.1°/29.1°(N=7,Ks=78.3,?95=6.9°)after tilt correction.The midtemperature and hightemperature component can pass through C-class inversion test under 95%confidence,indicating that high teperature components and midtemperature components were obtained in a close time,or between the time they were recorded this area we study did'nt undergo a obvious rotation.Lianghe area have underwent counterclockwise rotation about 43.6 °±3.3° relative to the paleomagnetic reference pole of east Asia tectonic region since Miocene,and this maybe is due to left-strike slip of the large yingjiang fault.