Subduction of the Indian slab into the mantle transition zone revealed by receiver functions

A large volume of continental lithosphere should have been emplaced beneath the Tibetan Plateau during the convergence between the Indian and Asian plates. However, geophysical studies have imaged an obviously thickened crust and a normal to slightly thickened mantle lithosphere beneath the plateau. Most of the additional mantle lithosphere is proposed to have detached and descended into the deep mantle to the mantle transition zone (MTZ). Recent studies suggest that the Indian lithospheric slab has torn into sections with different subduction angles. However, the locations of these slab sections, especially the high-angle one, require further structural constraints. Here, we used newly collected data from the broadband seismic network SANDWICH in central Tibet to study the thickness of the MTZ using the receiver function method. Our results show that there are two MTZ regions that are approximately 15 km thicker than the average one. One region is beneath the Lhasa terrane centered at 90 degrees E, 31 degrees N, and the other one is beneath the Qjangtang terrane centered at 881,33 degrees N. Because the upper and lower boundaries of the MTZ are both rock phase transition discontinuities that are sensitive to temperature. We interpret the thickened MTZ beneath the plateau as evidence of the presence of cold Indian slab and mantle lithosphere. The high-angle Indian lithospheric slab between the Yadong-Gulu Rift and the Pumqu-Xianza Rift is subducting deep into the MTZ, producing a decrease in temperature and thickening the MTZ beneath Lhasa terrane. In contrast, the Qiangtang anomaly corresponds to a piece of delaminated thickened Tibetan lithosphere. Based on the extensive distributions of Eocene to Oligocene potassic rocks and Mid-Miocene to Quaternary potassic rocks, we suggest that the thickened mantle lithosphere beneath the northern Qjangtang terrane and Songpan-Ganzi terrane detached in a piecewise manner, resulting in subsequent plateau uplift and recent potassic volcanism. (C) 2017 Elsevier B.V. All rights reserved.