Shear Attenuation of the lithosphere beneath the southeastern Tibetan Plateau

Abstract:

The rheology of the lithosphere is key to understand the deformation mechanisms and tectonic evolution of the Tibetan Plateau. Although different geodynamic models such as rigid block extrusion, continuous deformation, and mid-lower crustal flow have been proposed, deformation mechanisms of the southeastern Tibetan Plateau are still under debate. Compared with velocity and Vp/Vs ratio, seismic attenuation has a very different dependence on temperature, partial melting, and volatile content, which can provide new insights into the rheology of the lithosphere.

Based on the 17 s – 143 s Rayleigh wave data of 350 ChinArray temporary stations in the southeastern Tibetan Plateau, we employed the two-plane-wave method to invert the shear velocity model and shear wave attenuation of the crust and uppermost mantle. In order to compare the lithospheric structure of different blocks, we divide the research area (97˚E - 108˚E, 21˚N – 30˚N) into subregions. The results show that in the southeastern Tibetan Plateau, the Rayleigh-wave attenuation coefficient reaches the peak around 20 s, and gradually decreases to the minimum at 45 s, and then increases again at longer periods to about 143 s. This reflects the variation of seismic attenuation with depth from the middle crust to the upper mantle. Our preliminary attenuation results imply that the lithosphere in the southeastern Tibetan Plateau has a stratified rheological structure with a weak mid-lower crust, probably caused by partial melting, on top of a strong uppermost mantle.