New Paleomagnetic Results From The Volcanic Rocks Of The Tiaojishan Formation In The Yanshan Area Of The North China And Their Tectonic Implications

The Upper Jurassic Tiaojishan Fm volcanic rocks are widely exposed in the Yanshan belt?YSB?of the northern margin of the North China Block?NCB?.The strata were well-dated,and its fresh volcanic rocks are amenable to paleomagnetic study.The late Jurassic paleomagnetic results from the NCB and Inner Mongolia region are crucial to understand the tectonic evolution of the Mongol-Okhotsk suture,but the existed data are conflict each other,partly because most strata are not well dated.One pole was newly published for a section of¹55 Ma volcanic rocks of the Tiaojishan Formation in the Chengde basin?Pei et al.,2011?.It is a very good opportunity to compare this pole to other two coeval poles from the northern side of the Mongol-Okhotsk suture.However,it indicates a significantly lower paleolatitude of the NCB and it is rather different to those late Jurassic poles obtained from southern part of the NCB.Thus,further studies are needed to test whether this Tiaojishan pole has averaged out the secular variation of the paleogeomagnetic field.New paleomagnetic study on well dated?¹55 Ma?volcanic rocks of the Tiaojishan Formation?Fm?in the northern margin of the NCB have been carried out,during which 194samples were collected from 26 sites in the Luanping,Beipiao and Shouwangfen areas.All samples were subjected to stepwise thermal demagnetization.After removed a low temperature viscous component acquired in the recent geomagnetic field,a stable high temperature component was isolated.The new data are significantly steeper than those published paleomagnetic directions observed from the Tiaojishan Fm in the Chengde basin?Pei et al.,2011?.Our analyses demonstrate that the paleomagnetic directions obtained from each section were strongly biased by the paleosecular variation?PSV?,but the PSV of normal or reversal polarity group can be averaged out by combing all the virtual geomagnetic poles?VGPs?of five sections from the Tiaojishan Fm in the region.The mean pole that is at 69.6°N,203.0°E(A₉₅=5.6°)passes a reversal test and a regional tilting test at 95%confidence and is thus considered as primary paleomagnetic record.This new pole of the Tiaojishan Fm is consistent with that previously published Late Jurassic poles obtained from the red-beds in the southern part of the NCB.But they are still distinguishable from the coeval poles of Siberia and the reference pole of Eurasia,indicating that the convergence between the Siberia and the NCB had not been ended by¹55 Ma.By comparison the reconstructions,the convergence were likely finalized during the Early Cretaceous.There had been¹600 km crustal shortening occurred after¹55 Ma,but not³000 km based on the previous published Tiaojishan pole?Pei et al.,2011?.In addition,no significant vertical axis rotation was found either between our sampled areas or between the Yanshan belt and the major part of the NCB after¹55 Ma.