Paleomagnetic Constrains On The Convergence And Collision Process Of The Qiangtang And Lhasa Blocks In The Tibeten Plateau

The Tibetan Plateau is comprised of several terranes and was formed by the collision-amalgamation of these terranes since the early Paleozoic.The collision process and subsequent impacts are the main issues in the studies of the formation and evolution of the Tibetan Plateau.The Qiangtang-Lhasa collision came together with the closure of the Bangong-Nujiang Ocean,and resulted in a large amount of crustal shortening which caused a significant portion of central Tibet was raised to a high elevation prior to the India-Asian collision,and formed the“Proto-Tibet”during the Cretaceous.Hence,the processes involved in the Qiangtang-Lhasa collision and the evolution of the Bangong-Nujiang Ocean are key factors to understanding the early evolution history and geodynamics of the central Tibetan Plateau.Knowledge of the Mesozoic drift history of the Qiangtang block is critical to understand these questions.In this paper,we present the Mesozoic paleomagnetic studies in the Qiangtang block.Based on the available paleomagnetic data for the Qiangtang and Lhasa blocks and geological evidence,several conclusions can be summaried as follows:?1?The reliably Middle Jurassic and Early Cretaceous paleomagnetic results from the Qiangtang block in this study are Ds=245.2°,Is=54.1°,ks=113.9,?₉₅=3.9°?Tilt-Corrected?and Ds=61.7°,Is=45.4°,?s=76.0,?₉₅=4.3°?Tilt-Corrected?,respectively.The results indicate that the Qiangtang block was located at 34.6±4.6°N?reference point 32.7°N/89.4°E?and 26.9±4.4°N?reference point 32.9°N/83.5°E?during the Middle Jurassic and the Early Cretaceous,respectively.?2?The Qiangtang block moved to north with mean south-north plate movement speed of⁷ cm/yr during the Early to Late Triassic.The Qiangtang block was located at a stable paleo-latitudinal position?³0°N?and did not undergo significant N-S movement between the Late Triassic and the Middle Jurassic.The Qiangtang block reached a paleolatitude of²5°N during the Early Cretaceous.?3?Combined with the Mesozoic drift history of the Qiangtang and Lhasa blocks and the geological evidence indicates the Qiangtang-Lhasa collision occurried in the Early Cretaceous.The crustal shortening may reach to⁵00 km and¹000 km between the western and the eastern part of Qiangtang block and the Lhasa block since its collision,respectively.The crustal shortening of 1020±280 km may have taken place between the present-day southern margin of the Qiangtang block and the stable Asian continent since the Early Cretaceous.The N-S crustal shortening between the Qiangtang and Lhasa blocks may have provided the material for the early growth and vertical movement of the Tibetan Plateau.?4?The Bangong-Nujiang Ocean have expanded during the Early Triassic to the Late Triassic and reached its maximum width in the Late Triassic.The Bangong-Nujiang Ocean was shrinking after the Late Triassic due to the Lhasa block breakup from Gondwana and moved to north.Up until the Early Cretaceous,the Lhasa block had collided with the Qiangtang block and came together with the closure of the Bangong-Nujiang Ocean.