A Paleomagnetic Study Of Early Cretaceous Strata From The Eastern Yanshan Fold-and-thrust Belt And Its Geological Implications

The Yanshan fold-and-thrust belt, Located at the northern margin of the North China block, has often been referred to as the cradle of Chinese geologists and becomes famous due to its strong intraplate deformation in the Mesozoic and Cenozoic. From the beginning of the last century, many researchers have been focused on the Mesozoic stratigraphy and tectonics in this area and much progress was done and achieved. However, the correlation of the Mesozoic strata and the tectonic progress are still controversial, and more researches are needed to clarify these problems.The study of paleomagnetism has been developed for the Early Cretaceous strata at Luanping basin, eastern Yanshan fold-and-thrust belt. On the base of the new zircon SHRIMP U-Pb dating of the Dadianzi formation in the Luanping basin, the top of the Dadianzi formation was constrained to 131±6Ma. The Sandaogou section with Dabeigou, Dadianzi and Xiguayuan Formations at the Luanping basin, northern Hebei province, has a well-exposed and complete sedimentary sequence with rich fossils. Based on the magnetostratigraphic study at this section, a magnetic polarity sequence of the Dabeigou-Dadianzi Formation has been established. The integration of biostratigraphy, lithostratigraphy, chronosratigraphy and sequence-stratigraphy with high resolution magnetostratigraphic results suggest new ages of Dabeigou, Dadianzi and Xiguayuan Formations. It provides basic data for the stratigraphic classification and correlation of related strata in the northern Hebei province as well as China. The correlation between the magnetostratigraphic sequence of Sandaogou section and the MPTS indicates that the age of Aptian/Barremian boundary is 120 Ma, Barremian/Hauterivian is 125.1 Ma, Hauterivian/Valanginian is 130.4 Ma. The Dabeigou formation corresponds to the early of Valanginian, and the Dadianzi formation corresponds to the middle-late of Valanginian, and the lower part of Xiguayuan formation correspond to the Valanginian/Hauterivian.A stable magnetic component was isolated by stepwise thermal demagnetization of 123 samples from the Dabeigou and Dadianzi formations, which is characterized by a positive reversal test. A tilt-corrected mean direction is Ds=5.3°, Is=50.2°, k =36.8, a₉₅=2.1, corresponding to a paleopole atλ_p=79.3°N,φ_p=272.1°E, dp=1.9°, dm=2.8°. Comparisons of this paleomagnetic poles from the Luanping basin with those from western Liaoning, the North China Block (NCB), the South China Block (SCB) and Eurasian plate imply that there were no relative latitudinal displacements and tectonic rotations between Luanping basin and western Liaoning, NCB, SCB, however, an anticlockwise rotations occurred between the Luanping basin, western Liaoning and NCB and SCB.The determination of paleocurrent directions is a robust approach to establish the sediments transport and of the deposition progress. A total of 123 samples from the Dabeigou and Dadianzi formations, the Sandaogou section at the Luanping basin have been measured, which provide Anisotropy of Magnetic Susceptibility data and paleocurrent directions information. The dramatic changes in paleocurrent directions between Dabeigou and Dadianzi formations imply that the slope direction change of Luanping basin resulted from the tectonic transfer of the Yanshan fold-and-thrust belt occurred at the early Cretaceous.In order to better understanding the block rotations between the Luanping basin and adjacent regions, we analyses mid-early Cretaceous, late-early Cretaceous and late Cretaceous paleomagnetic data respectively. The eastern Liaoning and Korean peninsula were rotated about 11°clockwise relative to the North and South China blocks between the mid-early Cretaceous and late-early Cretaceous. No significant rotations have been occurred since the late Cretaceous between Korean peninsula and the North and South China blocks. About 13°counterclockwise rotations of the northern Hebei-western Liaoning with respect to the North and South China blocks took place since the mid-early Cretaceous. These rotations are accommodated by the lithosphere thinning and extension beneath the Northeast China and adjacent regions, which resulted from the strong upwelling of asthenosphere caused by the subduction of the Pacific plate.