New Electromagnetic Results From The Sedimentary Rocks Of The Tuchengzi Formation In The Yanshan Area Of The North China And Their Tectonic Implication

The Tuchengzi Formation(Fm)sedimentary rocks are widely exposed in the Yanshan belt(YSB)of the northern margin of the North China Block(NCB).The recent geochronologic data have been reported with ages ranging from ~153-137 Ma.The strata were well-dated,and its remains fresh without metamorphism and penetrative deformed fabrics,and thus these conditions are good for our paleomagnetism research.The paleomagnetic investigations have been carried out in the upper,middle and lower layer of Tuchengzi Fm sandstone in Beipiao basin,Chengde basin and Yanqing basin.In these three areas,we collected 294 samples from 30 sites.All the samples were subjected to stepwise thermal demagnetization.After removal of a recent geomagnetic field viscous component,a stable high temperature component was isolated.The results of Beipiao basin passes a reversal test(level C)and a fold test at95% and 99% confidence.The characteristic remanent directions of Chengde basin pass a fold test at 95% confidence.And the high temperature component of Yanqing area is almost the same with the other two basins.That means the high temperature component of the three areas could be the primary paleomagnetic record.The mean pole of NCB(~145 Ma)is at 83.3°N,233.2°E(A95=4.4°)by averaging all the site-level VGPs.The pole of Tuchengzi Fm(~145 Ma)revised the Apparent Polar Wander Paths(APWP)of the Mesozoic for the NCB.We got a ~145Ma fitting pole of the Siberia by using running mean method for the APWP of the Siberia.After compared our new~145 Ma pole and the coeval pole of the Siberia,we can calculate that there had been~1200 km latitudinal plate movement and crustal shortening between NCB and Siberia after ~145 Ma.