The Magnetostratigraphy And Paleomagnetic Study Of Cretaceous Chela Basin In The West Qinling Orogen,China

When the Qinling orogen accomplished its main convergence in the Triassic,there have formed abundant intercontinental basins in the West Qinling orgen.Especially the sediments accumulated in the Late Mesozoic may hold important information of the lithosphere behaviors related to the Late Triassic collision between the North and South China blocks and the following intra-continental evolution information.More and more researchers are going to focus on the nature of these basins,and its tectonic evolution,and the sedimentary histories.In this study,the high-resolution magnetostratigraphy,rock magnetism,and paleomagnetic study of the tectonic evolution had been carried out on the profile of Chela-Sima(N:34°5.5",E:104°28.3"),which is located to the north of Chela basin with a thickness of 1450 m.The rock-magnetism experiment of representive samples indicated the hematite to be the main magnetic mineral of the Chela-Sima section,including the saturation isothermal remanent magnetization,and temperature dependent magnetic susceptibility.According the step-wised thermal demagnetization of 669 oriented samples,a total of 633 samples yielded primary magnetizations.A few samples could be separated two components,the low temperature and the high temperature components.Especially,the low temperature ones have a similar direction with the geomagnetic dipole field directionin the direction of geographical coordinates,which may be resulted from the viscous remanent magnetization of the earth's magnetic field.A large number of oriented samples have a single component,and the analyzed magnetic field direction is closed to the high temperature component of samplesisolated two components.Through the magnetostratigraphy and analyzed deposition rate,the geological time of Chela-Sima section could be traced to 145 Ma relative to the reported fossils from previous studies.A total of 182 oriented palaeomagnetic samples from 19 sites indicated a clockwise rotation of?18°,relative to the reference pole for coeval Eurasia plate,which is remarkably consistent with previous studies.The Lower Cretaceous sites recorded a lower rotation rate of 0.69°/Ma according to the 0.89°/Ma of the Upper Cretaceous ones.These rotation patterns of Chela basin were interpreted to result from either the activities of eastward rotational extrusion along curved strike-slip faults,or the response to the regional strike-slip in northwest of China during the Cretaceous thrusting and colliding of Qiangtang-Changdu block and Lhasa block.