Paleomagnetic Research On The Upper Triassic Yingshuiquan And Kanglu Formation From Northern Qiangtang Block,Tibetan Plateau

The Qiangtang block,the largest block in the Tibetan Plateau,is a key area not only for studying the collision between the Qiangtang block with the Tarim Block,the Qamdo block and the Lhasa block,but also for clarifying the forming and evolution of Tethys.The Triassic is a critical period for the northern Qiangtang block rifting away and northward drifting from the northern margin of Gondwana.During this period,the block drifted from the southern hemisphere to the northern hemisphere.The Triassic is also a crucial period for the opening of the Meso-Tethys and the closing of the Paleo-Tethys.In addition,it's a key period for the collision between the Qamdo block and the northern Qiangtang block,and between the Qiangtang northern block and the Lhasa block.It is in this context that systematic paleomagnetic and rock magnetism study is carried out on the early Triassic Yingshuiquan formation and Kanglu formation in the northern Qiangtang block.A reliable paleomagnetic pole location and paleo-latitude value has been obtained for the northern Qiangtang block in early Triassic,which provide the basis for reconstructing the paleogeographic location of this key block in the early Triassic.A systematic study of paleomagnetism has been conducted on 6 sites(76 samples)of the early Triassic Yingshuiquan formation and 2 sites(14 samples)of the early Triassic Kanglu formation in the northern Qiangtang block.The results of rock magnetism show that the main remanence mineral is magnetite within both formations.Integrating paleomagnetic results of 8 sites(90 samples)in Yingshuiquan formation and Kanglu formation,the high-temperate component is Ds=225.8°,Is=28.1°,?95s=11.6°,Ks=23.9 after tilt-correction.And paleolatitude corresponding to this direction is 14.9°S.The paleomagnetic data pass the Watson and Enkin(1993)and Enkin(2003)fold tests.Therefore,it is considered that the high-temperate component is primary NRM.Paleomagnetic pole position corresponding to this direction is ?p=24.7°N,?p=217.0°E dp/dm=12.7/7.0,A95=9.4°.Calculation shows that paleolatitudes of the late Permian-early Triassic northern Qiangtang block and the late Permian Qamdo block are very close and Latitude difference is negligible within the confidence interval,indicating that the Qamdo block may had entered the land-land collision stage and had completed the accretion in late Permian-early Triassic.Analyzing the Triassic data of the Lhasa block and the northern Qiangtang block,it can be found that positions of paleomagnetic pole of the northern Qiangtang block and Lhasa block were very close and the relative latitude difference between the two blocks is small within the confidence interval in the early Triassic.This shows that the Lhasa block and the northern Qiangtang block might be adjacent to each other and were in the lower latitudes of the southern hemisphere in this period.In the late Triassic,the northern Qiangtang block rapidly northward drifted to the mid-low latitudes of the northern hemisphere,but the Lhasa block still wandered in the mid-low latitudes of the southern hemisphere.During the entire Triassic,the northern Qiangtang block significantly northward moved with respect to the Lhasa block and northward drifted 30° about 3300 km away from the Lhasa block.