Paleomagnetism of Triassic rocks in the western Lhasa terrane, Tibetan Plateau, and its tectonic implications

A paleomagnetic study of the Triassic marine sediments on the western margin of the Lhasa terrane was carried out in this paper. Six sites (67 samples) were investigated. The rock magnetic data reveal that most of the samples are dominated by pyrrhotite and some are dominated by magnetite. A recent remanent magnetization (component A) was identified at four sites (47 samples), and two main components were identified: one of them (component B), which was obtained at six sites (66 samples) and characterized by a negative fold test, was identified as a Cretaceous remagnetization. The second (component C) could only be identified in some magnetite-dominant samples in the high temperature range (525-585℃). The mean directions obtained at five sites (28 samples) by combining characteristic remanent magnetization directions (ChRM) and great circles yielded a positive fold test which constrains the magnetization acquisition time. The calculated paleomagnetic north pole for the Triassic of Lhasa terrane has the following data: φρ=17.4°N，λρ=205.9°E，（dp=6.7，dm=3.7）. These pole positions are different from the positions of other poles in the Lhasa terrane, suggesting that the Lhasa terrane was paleogeographically situated at low latitudes in the southern hemisphere in the Triassic. Based on a comparison with the previously published paleomagnetic results from the Triassic rocks in the northern Qiangtang region, the authors hold that the Paleo-Tethys separating the Lhasa and northern Qiangtang terrane was probably opened in the Early Triassic, and then expanded rapidly during the early Triassic to late Triassic period. Owing to the opening of the Bangong Co-Nnujiang River Ocean, the Qiangtang terrane drifted northward quickly, and merged into southern Eurasia plates in the Late Triassic.