Lithospheric structures beneath the Tibetan Plateau constrained by Sn propagation efficiency

The uppermost mantle is a crucial region linking the lower crust and lithospheric mantle and carries important dynamic and structural information (e.g., upwelling and delamination) on interactions between the crust and mantle. An intriguing observation of an inefficient Sn wave zone beneath the Tibetan Plateau was reported but had unclear spatial distributions. By analyzing the Sn wave propagation efficiency from 34,917 Sn wave arrivals, we provide strong constraints on the lateral and depth variations in the inefficient Sn wave zone in the Tibetan Plateau. Our results show that with increasing frequencies, the inefficient Sn wave zone becomes larger, expanding to the eastern Songpan-Ganze terrane and then to the Sanjiang area and the southern margin of the Qilian orogenic belt. The northern and eastern boundaries of the inefficient Sn zone, particularly a small inefficient zone in the Qilian orogenic belt (37°N, 102°E), are clearly depicted, indicating the widespread distribution of the seismic attenuation structure or partial melt in the uppermost mantle. Moreover, we provide the depth variations in the inefficient Sn wave zone by simulating 31,538 3D raypaths with the fast marching method. We therefore constrain the depth range of the partial melt region in the uppermost mantle beneath the Tibetan Plateau. The seismic attenuation structure in the northern-central Qiangtang terrane and western Songpan-Ganze terrane extends to 130 km from the bottom of the crust, whereas that in the eastern Songpan-Ganze terrane and Sanjiang area is thinner and mainly concentrated from 90 to 130 km. Combining this information with the temporal and spatial variations in potassic-ultrapotassic magmatic rocks, it is suggested that asthenospheric materials might upwell and flow laterally beneath the Tibetan Plateau. This behavior provides a thermal driving force for the uplift of the plateau and the deformation of the crust-mantle structure.