Triassic magnetostratigraphy of the north slope of Mount Qomolangma, Tibet

Abstract：A total of 728 oriented paleomagnetic samples have been collected from Triassic sedimentary strata in the Tulong area on the north slope of Mount Qomolangma (Everest) in southern Tibet. All the sampling was carried out with a portable GDS-1 Type air-powered core drill after measuring the section. The cores were oriented with a magnetic compass and an inchnometer. Samples were commonly taken at 0.5～3 m intervals, and the intervals reached 0.1～0.2 m near some important geological boundaries. The cores taken were 24～25 mm in diameter and 30～60 mm in length. The results of analysis show that the natural remanent magnetization of the sedimentary rocks in the study area is medium, generally reaching 1.9×10-2～9.4×10-1 A/m and their characteristic remanent magnetization reaches 3.9×10-2～5.0×10-1 A/m. Thermal demagnetization study indicates that when samples are thermally demagnetized at 550～600℃ the remanent magnetization of most samples is reduced by 90～100%, which shows that the carrier of this magnet in the rock is mainly magnetite. Magnetostratigraphic study indicates that there are 12 normal polarity zones and 11 reversed polarity zones in the Tulong Group, which form an Early Mesozoic mixed normal and reversed magnetic polarity superzone with an intermediate frequency, called the Tulong mixed polarity superzone polarity superzone. It coincides with the Illavara mixed polarity superzone. Only two small reversed polarity zones were found in the Kyunglung Goinba and Derirong formations, while the other samples all show normal polarity zones. In this paper this stratigraphic unit is named the Tulong normal polarity superzone, which is equivalent to the GRAHAM normal polarity superzone. The upper part of the Derirong Formation is a reversed polarity zone. Based on these studies, the boundaries of the Lower, Middle and Upper Triassic are determined. The intensive study of the Triassic magnetostratigraphy on the north slope of the Mount Qomolangma area has provided a scientific basis for the multiple stratigraphic division and correlation of the Triassic and establishment of a Triassic paleomagnetic database and Triassic magnetostratigraphic polarity column in the Mount Qomolangma area, as well as the in-depth study of the evolution of the supercontinent Gondwana, Formation and disappearance of the Paleo-Tethys and uplift mechanism of the Qinghai-Tibet Plateau.