Constructing the Eastern Margin of the Tibetan Plateau During the Late Triassic

The steep eastern margin of the Tibetan Plateau is thought to have developed in the Cenozoic by brittle crustal thickening or lower crustal flow related to India-Asia collision. However, our data indicate that Late Triassic crustal shortening and voluminous magmatism contributed to crustal thickening and topography of the region. Sr/Y ratios of intermediate to silicic magmatic rocks reflect mineral assemblages in the magma source and can be used to constrain crustal thickness. The Sr/Y ratios from the Songpan-Ganzi Fold Belt in the eastern Tibetan Plateau indicate that the eastern margin of the plateau achieved a crustal thickness ranging from 446 to 679km during ca. 220 to 190Ma. An average elevation of 2,600300m at ca. 211Ma is deduced based on average crustal thickness of 552km assuming Airy isostatic compensation. The thickened crust and high topography of the eastern margin of the Tibetan Plateau during the Late Triassic is supported by intense shortening and voluminous magmatism, contemporaneous Barrovian metamorphism, and the development of a foreland basin. Our data indicate that the eastern margin of the Tibetan Plateau was initially built during the Late Triassic, and this area has a long history of crustal thickening and mountain building.