Thickness change of the Tibetan lithosphere since Early Cretaceous as revealed by Ce/Yb and Sm/Yb ratios of volcanic rocks

Abstract

ABSTRACT The growth of lithospheric thickness beneath the Tibetan Plateau since Early Cretaceous is demonstrated by changes in partial melting depth of the mantle as reflected in the Ce/Yb and Sm/Yb ratios of volcanic rocks. The ratios indicate that the lithospheric thickness of the Tibetan blocks ranged between 50–90 km in most areas and between 100–120 km in several areas during the Cretaceous. It increased from 60–110 km in Eocene to 170–200 km in Early Miocene in the Lhasa block, from 110–120 km in Early Oligocene to 170–180 km in Early Miocene in the Qiangtang block, and from ~130 km in Late Palaeogene to 160–190 km in Early Miocene for the Hohxil and Kunlun blocks. The thickness of the Lhasa block thus increased ~100 km at an average rate of 3.0 mm/a during Early Cenozoic. The indicated increase in Early Miocene for the Lhasa, Qiangtang, and Hohxil and Kunlun blocks to 160–195 km nearly matches the present 160–200 km plateau lithospheric thickness as inferred from seismic tomography. It is reasonable to assume that the crust, as well as the lithospheric mantle for all blocks thickened during the Palaeogene and reached its present thickness in Early Miocene.