Paleomagnetism And Kinematics Study On The Cenozoic Qaidam Basin

The Qaidam Basin, locates among the Altyn Tagh, the Qilian and the Kunlun Mountain, is a big mountainous basin of Mesozoic and Cenozoic in the western China, with a special basin-mountain structure and a stereosphere geodynamics background, the structure pattern varied with the time, and has a polycyclic evolution history, its process is staged, it has varied plate tectonics borders and basin prototypes in different geohistory stage, the paleogeographic pattern has complicated superposition process. This study is based on the Qaidam Basin, the systematic paleomagnetic study was carried on, and the Cenozoic structure evolution within this basin was discussed.35 Cenozoic paleomagnetic sites have been collected from the Qaidam Basin in Qinghai province, North China Block (NCB).During stepwise thermal demagnetizations, the direction of magnetization in few samples (10%) which from NRM to 680 ℃ kept invariant basically, represented single magnetic component, that showed the typically characteristic of high hematite content. Tow magnetic components can be separated from most samples. The low temperature components can be isolated at 300℃ or 400 ℃, which showed the low temperature components are carried by goethite and/or siderite. The low temperature components lie close to the present geocentric axial dipole field direction in the present geographic coordinate system, which are probably viscid residual magnetism of the present magnetic field direction. The high temperature components can be separated from the temperature 300℃-680℃ in most samples, the high temperature components are carried by hematite, with normal and reversed polarity, those are symmetrical distributed in the stratigraphic coordinate system, and pass the fold test and reverse test. It suggested that the high temperature components were probably the primary magnetization, with the direction of residual magnetism (Dg=27.8~0,Ig=61.8~0,Kg=5.6,a95=4.1~0, Ds=4.7~0,Is=46.0~0,ks=7.1,a95=3.6~0), the corresponding paleomagnetic pole is at λ=80.3~0N,(?)=255.5~0E (dp=2.9~0, dm=4.6~0), which distributed in the primary APWP, thepaleolatitude is 27.4°N, it suggested that the Qaidam Block distributed in the northern hemisphere in the late Devonian.It can be concluded that the Qaidam Block has moved to north and the block itself has undergone a left and right lateral alternate process, rotated about 40° since the Cenozoic from the APWP and the variable curve. The latitude has changed 8.9°, which suggested the Qaidam Block has slipped to north about 890km from the Cenozoic to the present. The probably reasons are extrusion of the East Kunlun and Qilian Mountain and the settlement of the middle basin, which lead to the crust shortened from north to south, the himalaya block was pushed on to north and extrusion was carried on the Qaidam Block at the direction of NEE, and the collision of India and Euro-Asia Block resulted in the Altyn Tagh fault system slipped left on large scale.