Quantitative Sn-wave Attenuation Beneath the Tibetan Plateau and Lithospheric Rheology

Sn wave, a regional seismic phase, propagates horizontally in the uppermost mantle and is sensitive to lateral variations in mantle lid thickness, temperature, and melt. The physical properties of the lithosphere can be indicated by Sn propagation efficiency or attenuation. The inefficient Sn propagation has been typically used to describe the regions with high-temperature anomalies in the uppermost mantle and infer the subduction front of the Indian lithosphere in the north Tibetan plateau. Here we collect 122,481 tangential-component digital seismograms, isolate the geometric spreading and attenuation for SH-type Sn wave, and construct a broadband uppermost mantle shear wave attenuation model in the Tibetan region. Beneath the central and north parts of the Tibetan plateau the Sn waves are strongly attenuated, while relatively weaker attenuation can be observed in the perimeter of the plateau, i.e., the Himalaya mountains in the south, Tarim and Qaidam basins and Eastern Kunlunshan terrain in the north, and Sichuan basin in the east. These weak attenuation regions are likely where the old crustal fragments were deposited during the collision between the Indian and Asian plates. In contrast, strong Sn attenuation likely indicates local lithospheric delamination in central and eastern Tibet. Furthermore, the correlation between strong Sn and Lg attenuation zones reveals the potential mantle upwelling with deep heat sources invading the crust.