Lithospheric and upper mantle structure of the northeastern Tibetan Plateau

We use receiver functions calculated for data collected by the INDEPTH-IV seismic array to image the three-dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70km in northern Tibet. In addition, we observe a 20km Moho offset beneath the northern margin of the Kunlun Mountains, a 10km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong-Nujiang Suture at 80km depth which dips ∼10 to the north, reaching ∼120km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20km depression of the 660-km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10km depression of the 410-km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small-scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.