Paleomagnetic Results Of The Upper Cretaceous Jingzhushan Formation Red Beds From The Northern Margin Of The Lhasa Terrane

As the southernmost margin of the Asian continent during the Late Cretaceous,the paleogeographic reconstructions of the Lhasa terrane are of great significance for the investigation of the Indo-Asian continental collision process.In order to quantitatively describe the late Cretaceous paleolatitude of the Lhasa terrane,different scholars have conducted detailed paleomagnetic researches on the Upper Cretaceous strata of the Lhasa terrane,and the obtained paleolatitudes vary widely in the interval of 9.9°N-26.7°N.Considering the potential discrepancies in paleomagnetic dates,this paper presents a detailed paleomagnetic study of 415 oriented samples from two sections 35 sites of the Late Cretaceous Jingzhushan Formation red beds on the northern margin of the Lhasa terrane.The results of Amsotropy of Magnetic Susceptibility（AMS）show that the samples have primitive sedimentary composition.The petrographic results show that the magnetic minerals in the sample have not suffered strong post transformation.The results of rock magnetism show that the main magnetic-bearing mineral in the sample is hematite.A set of stable high-temperature component direction(D_g=336.2°,I_g=27.3°,k_g=7.8,α₉₅=11.8°)was obtained from 22 sites by stepwise thermal demagnetization,the average direction after tilt correction is D_s=334.7°,I_s=32.6°,k_s=35.2,α₉₅=5.3°.Isothermal remanence anisotropy analysis shows that there may be no magnetic inclination shallowing the red beds of the Jingzhushan Formation in the study area.After syntectonic sedimenatation corrected,the high-temperature component orientation(D_c=335.6°,I_c=31.2°,k_c=52.8,α₉₅=4.3°)was indicated by the Mc Elhinny and Mc Fadden fold test at both the 95%and 99%confidence intervals;Watson&Enkin and Enkin Direction-Correction fold test show that the best spreading of the formation is at about 100%.The remanent magnetic decay curve is consistent with the remanent magnetic decay characteristics of clastic hematite.The results indicate that the direction is the primary remanence direction,corresponding to the paleopole at 63.3°N,329.4°E,A₉₅=3.6°.Combined with previous studys,it was defined that the late Cretaceous paleo latitude of the Lassa terrane was 16.6±2.1°N（reference point 31.8°N,87.7°E）.Thus,combining the paleomagnetic dates and geological datas of the surrounding terranes,it is inferred that a north-south tectonic shortening of 970±400 km may have occurred north of the Lhasa terrane since the Late Cretaceous...