New Late Jurassic To Early Cretaceous Paleomagnetic Results From The Mongolia And North China And Their Tectonic Implications

The evolution history of the Mongolia-Okhotsk suture zone(MOS)formed by the collision between the Mongolia-North China block and Siberia has received long-term and wide attention.To better constrain the tectonic evolution of the MOS,we carried out new paleomagnetic studies on ~155 Ma Sharilyn Formation(Fm)sandstones and ~130 Ma Tsagantsav Fm basaltic lavas in southern Mongolia block(MOB),and the ~140 Ma Tuchengzi Fm sandstones and the ~130 Ma Dadianzi/Yixian Fm sedimentary rocks and basaltic lavas in the Yanshan area of the North China block(NCB).All 719 samples(from 100 sites)were subjected to stepwise thermal demagnetization.After a low temperature component of viscous magnetic remanence was removed,a stable high temperature component(HTC)was isolated from most samples.Combining with the rock-magnetic results reveals that both magnetite and hematite are abundant in the sandstone specimens and the remanent magnetizations of the lava specimens are mainly carried by magnetite.The HTCs from each rock unit passed a fold test and a reversal test,indicating their primary origins.Our statistical analyses demonstrate that the sandstones from the Sharilyn Fm and Tuchengzi Fm are free of inclination shallowing influence and the VGPs calculated from the HTCs of the Tsagantsav Fm and Dadianzi/Yixian Fm lavas have well averaged out paleosecular variations.The new poles of the MOB are 74.7°N/232.5°E(A95=3.7°)for ~155 Ma and 74.6°N/194.7°E(A95=2.9°)for ~130 Ma,and those of the NCB are 82.7°N/208.6°E(A95=4.3°)for ~140 Ma and 80.5°N/197.4°E(A95=2.3°)for ~130 Ma.By combining these results with the previously published paleomagnetic data and using a critical data quality assessment and selection,we refined the 155-100 Ma segment of the apparent polar wander paths(APWPs)for the MOB and NCB,respectively,demonstrating that these two blocks have been tectonically coherent(MOB-NCB)during 155-100 Ma.The comparison of these APWPs also suggests that there were 14.3°±6.9°(ca.1587±766 km)latitudinal plate convergence and ca.19.0° relative tectonic rotation between Siberia and the MOB-NCB after ~155 Ma.Their large scale latitudinal convergence likely had ceased by ~130 Ma,while some relative rotation between them ceased till ~100 Ma.Combining with the geological evidences indicates that the evolution process was accompanied by a large-scale Mongol-Okhotsk sinistral strike-slip motion between Siberia and MOB-NCB.In this region,the compression setting had turned into the widely extension setting by ~130 Ma.The Jehol Biota,which originated from the Yanshan area,commenced the extensive radiation evolution across the MOS and reached the Transbaikalia area after ~130 Ma.