Upper Mantle Heterogeneity and Radial Anisotropy Beneath the Western Tibetan Plateau

Abstract

It is generally considered that the tectonic evolution of the Tibetan Plateau is influenced by continental collision (e.g., Yin & Harrison, 2000), but many fundamental issues are still not resolved. For example, what happens in the advancing Indian lithospheric mantle (ILM), and what are the responses of the Eurasian lithospheric mantle (ELM)? Zhou and Murphy (2005) suggested that the ILM has subducted beneath the whole Tibet, whereas Kind et al. (2002) detected the ELM in northern Tibet. In recent years, some studies have revealed significant east-west variations of the northward advancing ILM (e.g., J. T. Li & Song, 2018; X. F. Liang et al., 2016). A subvertical high-velocity zone has been revealed at depths of 100-400 km by seismic tomography (Tilmann et al., 2003), and it was interpreted as a downwelling lithosphere in eastern Abstract P-wave radial anisotropy (RAN) tomography of the upper mantle beneath the western Tibetan Plateau is determined using teleseismic arrival time data recorded at 71 temporal seismic stations of the ANTILOPE-I and Y2 arrays, which reveals a complex deformation pattern in the upper mantle. A high-velocity anomaly with prominent positive RAN is visible to the south of 31°N, which may reflect the underthrusting Indian lithospheric mantle (ILM). Further north, another high-velocity zone appears at depths >200 km, which is possibly related to the foundering ILM, or the northward advancing ILM affected by previously attached Tethyan oceanic slab. Considering the continuity of the high-velocity zones and similarity of the RAN anomalies, we suggest that there is no slab tearing or breakoff at present beneath western Tibet. To the north of 31°N, E-W trending variations of velocity and RAN at shallow depths (<150 km) suggest that small-scale convection occurs between 79°E and 81°E, whereas the residue of the Eurasian lithospheric mantle is located in the eastern side. We deem that the formation of the Ya-Re Rift and Puran Graben was caused by variations of the lithospheric thickness instead of slab tearing or asthenospheric upwelling, considering locations of the rift and the graben and our present results. Plain Language Summary Our study region, the Tibetan Plateau, is a hot and ideal region to study the mechanisms of the convergence between continental plates. It is generally considered that the Tibetan Plateau is a product of convergence between the Indian and Eurasian plates, but some fundamental issues are still not resolved. For example, what happens in the northward advancing Indian lithospheric mantle (ILM), and what are the responses of the Eurasian lithospheric mantle (ELM)? Because of the extremely hard environment in the western Tibetan Plateau, seismic tomography is an effective way to illuminate the underground structure by processing earthquake waveforms recorded at seismograph stations. Although the interpretations of velocity anomalies are consistent, the anisotropy features result in different conclusions. Our results reveal both horizontal and vertical variations in the ...

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