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The west-east contrast of magmatism in northern Tibet suggests that the lithospheric root has been removed in the west, following continental collision that led to lithospheric thickening and removal, but not in the east where paradoxically larger convergence occurred. Here we show a full-wave P-n tomography model for the eastern Tibetan Plateau, which reveals a high-velocity layer beneath the Moho extending to 150-km depth. The anomalously high velocities and its northward dipping top surface suggest a very depleted and cold mantle consistent with an underthrusted Precambrian Lhasa lithosphere. A high-velocity column connects this layer to another high-velocity layer below 190-km depth, representing early-stage removal of the Tibetan mantle lithosphere and its interaction with the underthrusted Indian lithosphere. The west-east contrast is thus attributed to different stages of lithospheric removal, which may be controlled by varying angles of Indian subduction from the west to the east.

Plain Language Summary The northern Tibetan Plateau can be divided into two parts: the western part with abundant young volcanic rock (<17 million years) and the eastern part where young volcanic rock is absent. Using compressional seismic waves that mainly travel in the upper mantle, we model the three-dimensional wave speed structure of the upper mantle of the eastern Tibetan Plateau. Our model reveals that a very old and cold Tibetan mantle layer above 150-km depth is experiencing foundering under eastern Tibet, and those foundered materials have downwelled onto the underlying Indian Plate below 190-km depth. The mantle structure beneath the plateau east of similar to 93 degrees E is different from that in the west where most of the Tibetan mantle lithosphere may have been removed already. This difference is consistent with the lateral contrast in recent volcanic activities and is presumably due to different styles of subduction of the Indian Plate between the west and the east.