Paleomagnetic Data From The Middle-Late Jurassic Sedimentary Rocks Of The North Qiangtang Block,Tibet Plateau,China

The formation and evolution of the Tibet Plateau have undergone a series of complex geological events,and involved a variety of tectonic activities,including assembly expansion of blocks and evolution of the Tethyan oceans.Especially the time and method of collision between the Qiangtang block and the southern margin of Eurasia,and the paleogeographic location of the Qiangtang block is of great significance to understand the structural evolution of the Tibet Plateau since the Mesozoic.We sampled 371 sedimentary rock samples from the Yanshiping area and the Shuangquanhe area of the north Qiangtang block of the Tibet Plateau,and we also conducted on the detailed rock magnetic and paleomagnetism studies to these samples in this paper.The results of rock magnetism show that the magnetic minerals of the most of samples are dominatede by magnetite in the limestone samples of the Buqu Formation and the Xueshan Formation.However,the magnetic minerals of the most of samples are dominatede by hematite in the sandstone samples of the Quemco Formation and the Xiali Formation,and magnetite is also existed at this time.Through systemic thermal demagnetization experiment and alternating demagnetization experiment,we obtain stable characteristic remanent component from these samples.In addition,we also carry out the identification of the paleomagnetic inclination shallowing of sedimentary rock samples which were sampled from he Middle-upper Jurassic in the Yanshiping area.The result show that the shallow bias was existed in the Quemco Formation and the Xiali Formation possiblely,and the problem may be caused by the compaction of sedimentary rock,but the data obtained from the Buqu Formation and Xueshan Formation sandstone samples haven't this problem.We use the E/I method and the AIRM method to correct the data of existing the shallow bias,and it is found that there existed error of about 10°.The tilt corrected paleomagnetic results are as follow: the paleopole at ?=72.8°N,?=230.3°E,A95=10.4 for Quemco Formation:at ?=77.7°N,?=292.8°E,A95=7.9 for Buqu Formation;at ?=76.3°N,?=291.8°E,A95=10.9 for Xiali Formation;at =79.3°N,?=255.8°E,A95=12.0 for Xueshan Formation.In addition,this paper also obtain a paleoploe from the limestone samples of the Buqu Formation in the Shuangquanhe area.In addition,this paper also obtained an ancient geomagnetic pole in the limestone sample of the Buqu Formation in,it is basically consistent with the result from the Yanshiping area.The north Qiangtang block of the Tibet Plateau was located between ~21-23°N during the Middle-Late Jurassic,and hadn't occurred the large-scale latitude movement.Compared with the average expected ancient latitude in the same period of the Tarim,there is a latitude difference of 5.3° in the measured paleolatitudes during the Late Jurassic,indicating that have existed northward drift or structure shortens about 580 km between the north Qiangtang block and the Tarim block since the late Jurassic.Inaddition to,compared with the Tarim block,the north Qiangtang block mainly rotated counterclockwise from the Late Triassic to the middle Jurassic.And the north Qiangtang block mainly rotated clockwise during the Middle Jurassic.It turned counterclockwise during the Late Jurassic to the Cretaceous.