New Middle-Late Jurassic Paleomagnetic And Geochronologic Results From The Northern North China Block And Their Tectonic Implications

True polar wander（TPW）is a rotation of entire crust-mantle system respect to a virtual spin axis on the Earth.It raised from redistribution of the Earth’s mass either on the surface or within the body.Paleomagnetic evidence from both continental and oceanic crusts indicates that a TPW event occured in the Jurassic.In Jurassic,the convergence between the Eurasian and eastern Asia blocks existed.Therefore,Jurassic witnessed two synchronous events on the Earth,which were the TPW and closure of the Mongol-Okhotsk ocean（MOO）between Eurasia and eastern Asia blocks.The paleogeographic change of the north China block（NCB）should thus include both TPW and plate tectonic components in Jurassic.In this study,we performed a new combined paleomagnetic and U-Pb geochronological study for volcanic rocks of Lanqi Formation and Haifanggou Formation in Middle Jurassic from the northern NCB,obtained three new well-dated paleopoles:82.7°N,282.6°E,A₉₅=3.2°（～160 Ma）;84.1°N,228.9°E,A₉₅=6.6°（～165 Ma）;and 89.3°N,216.8°E,dp/dm=5.4°/7.1°（～170 Ma）.A positive untilting test indicates a pre-folding origin interpretation for the ca.160 Ma pole.Quality of these poles are enforced by three positive conglomerate tests,the presence of magnetic reversals and secular variation analyses.Considering of the consistence of the coeval poles from the Mesozoic sedimentary and volcanic rocks from NCB,we suggest that there was no significant inclination shallowing problems need to be discussed.Based on the new poles obtained in this study and published poles flitered by the revised seven-point criteria system（"R"index）,we refined a segment of apparent polar wander path of NCB in Late Triassic-Early Cretaceous.Comparison of the apparent polar wander paths of Eurasia and the NCB suggests that the NCB accreted to Eurasia around 140 Ma and the convergence distance was no less than 3800 km during the Jurassic.The NCB and its then connected blocks did not undergo any significant paleogeographic shift which was predicted by the Jurassic TPW hypotheses during Jurassic because of the counteract of tectonic motion and TPW.After the correction of TPW,we estimate a relatively constant net northward plate moving velocity at 0.50-0.75°/Ma for the NCB in Jurassic.The TPW corrected seismic tomography indicated the long-term accumulation of MOO subducted slabs at the high paleolatitudes and roughly orthogonal to the TPW axis may have critically contributed to,or perhaps led to,the Jurassic TPW.