Paleomagnetism And Its Implication Of The Sedimentary Of Lalongla Formation In The Tethyan Himalaya

The formation and evolution of the Tibetan Plateau has always been the frontier and hot topic for geoscientists at home and abroad,and the evolutionary history of the Neo-Tethyan Ocean,which is represented by the Indus-Yarlung suture zone,is of great importance to the study of the tectonic evolution of the Tibetan Plateau.Noticeably,although a large number of geological and geophysical investigations have been carried out in the Lhasa terrane and the Tethyan Himalaya over the past four decades,the opening time of the Neo-Tethyan Ocean still remains debated because the movement process of these two terranes during the Mesozoic could not been constrained.To clarify the mentioned issue above,a combined paleomagnetic,rock magnetic and petrographic study has been performed on the Lalongla Formation(174.1?168.3Ma)sedimentary rocks in the Tethyan Himalaya.Based on our new results,combined with the published geological and geophysical data from the Tethyan Himalaya,Lhasa terrane and Australian plate,several achievements may be summarized as follows:(1)A reliable Middle Jurassic paleomagnetic pole constrained by positive fold tests has been obtained from the Lalongla Formation sedimentary rocks in the Nyalam area(28.6°N,86.1°E)of the Tethyan Himalayan.This paleomagnetic pole is located at 23.9°N,292.8°E with A₉₅=2.7°,showing that the Nyalam area was located at 31.5°±2.7°S at?174.1-168.3 Ma.(2)Comparing the Middle Jurassic(?174.1-168.3 Ma)paleolatitude observed from the Tethyan Himalaya with the coeval paleolatitude expected from the Indian apparent polar wander paths indicates a paleolatitude difference of 2.4°±5.7°,corresponding a north?south crustal shortening of 270±630 km.The crustal shortening deduced from the Middle Jurassic paleomagnetic data is basically consistent with one within the Himalayas estimated by the balanced cross?sectional analyses,supporting that no ocean basin existed between the Tethyan Himalaya and Indian craton during the Middle Jurassic and the Tethyan Himalaya should be the passive margin of the Indian craton.(3)Comparing the Middle Jurassic pole obtained from the Tethyan Himalaya with those yielded from the Lhasa terrane shows a paleolatitude difference of 37.5°±8.1°,implying that the Neo-Tethyan Ocean would open a north-south width of 4200±900 km at?171.2 Ma.Assuming that the Lhasa terrane after separating from the Tethyan Himalaya maintained a constant northward rate of?10.1 cm/yr,the initial opening of the Neo-Tethyan Ocean should occur at?200 Ma.