Palaomagnetism Of The Early Cretaceous Yixian Formation In Xiushui Basin,North China Block

The multi-plate or block aggregation in Tethys,Mongol-Okhotsk,and the Paleo-Asian Ocean controls the evolution of palaeogeography in Asian.However,time of Mongol-Okhotsk Ocean closure and the strike-slip movement pattern of the Tan-lu fault zone are controversial.In addition,since the Cretaceous,the inclination anomaly in Asia has brought difficulties to the reconstruction of paleogeography.In this paper,we carried out paleomagnetic study on Early Cretaceous Yixian Formation in the Songliao-Xiushui Basin,North China Block,aiming to reveal the primary remanence,and directly compare the inclination of sedimentary rocks and volcanic rocks.The"Potbellied"hysteresis loop and low coercivity mineral indicated by Isothermal-magnetization-acuisition curves infer that pseudo-single-domain magnetite is the main magnetic carrier in Yixian Formation;The anisotropy of magnetic susceptibility measurements identify primary sedimentary fabrics.Low temperature viscous remanence was isolated in 0°C-200°C:Dg/Ig=3.8°/59.0°,consistent with the present geomagnetic field(D=14.62°,I=64.73°).High temperature component retreats to the origin in 500°C-580°C:Dg/Ig=68.7°/59.7°(kg=59.2,?95g=4.5°);Ds/Is=7.3°/64.9°(ks=67.9,?95s=4.2°).Combining with the coeval results from basalt flow,high temperature components pass the statistical fold test at 99%confidence limit(ks/kg=6.73>F(2×(n2-1),2×(n1-1))=1.8012)and Mc Fadden's(1990)fold test(?99%=9.45,?1(IS/TC)=31.31/1.28,?2(IS/TC)=31.11/0.35),The Direction-Correction fold test(Enkin 2003)gives a positive feedback with DC slope at 96.15%±8.56%untilting,and the stepwise untilting model shows the optimal degree of unfolding at 96.3%±4.0%.Samples possessing dual-polarity can be observed in both sedimentary and igneous rock.The positive result of baked test reveals that existing of primary detrital remanence in sedimentary rocks,and thermal remanence in igneous rocks.Based on the paleomagnetic results of this paper and the Cretaceous paleomagnetic data from North China Block,Amuria Block,and Eastern Liaoning-Korean(ELK)Block,the following conclusions are obtained:(1)The paleolatitude of Songliao-Xiushui Basin in Early Cretaceous(c.a.125 Ma)is 44.7°±1.3°N,corresponding paleomagnetic pole is:?o=82.1°,?o=184.7°,A95=3.7°);(2)Mean inclination of the Cretaceous sedimentary rock(I=62.54°,n=14)is consistent with the igneous rock(I=63.03°,n=16)from North China Block,Amuria Block and ELK Block,indicating there is no inclination shallowing occurred in sedimentary rock due to compaction after their diagenesis;(3)Cretaceous paleomagnetic poles of North China Block,Amuria Block and ELK Block are:?ncb=82.7°,?ncb=195.9°(A₉₅=2.4°,n=24);?amu=81°,?amu=189.5°(A95=6.5°,n=7);?elk=63.7°,?elk=201.2°(A₉₅=3.4°,n=11),respectively.Overlapping of paleomagnetic poles from the North China Block and Amuria Block,and consistent with the reference pole derived by apparent polar wander path of Eurasia,indicating that the Mongol-Okhotsk ocean closed before the Early Cretaceous(c.a.125 Ma).(4)Since the Cretaceous,paleomagnetic pole from western side of Tan-Lu fault zone is:?_W=82.5°,?_W=201.5°(A₉₅=2.8°,n=16),and that of eastern side is?_E=84.2°,?_E=185.5°(A₉₅=6.3°,n=7).Highly consistent of two poles indicate that there is no latitudinal displacement along the Tanlu fault zone,only a relative clockwise rotation(?25°)can be observed in ELK.Strike-slip motion up to hundreds of kilometers along the Tan-Lu fault zone is not supported by our data.