Radial Anisotropy in the Lithosphere and Asthenosphere Beneath the Northeastern Tibetan Plateau from Surface Wave Tomography

Abstract:

We analyzed Rayleigh and Love wave data from distant earthquakes recorded by 36 broadband stations that were operated in two years in the NE Tibetan Plateau. Phase velocity variations were determined in a period range of 20 to 91 s for Love wave and 20 to 100 s for Rayleigh wave using a two-plane-wave method, which can accounts for non-planar energy in the incoming wave field caused by structures outside the study area. 3-D shear wave velocity and radial anisotropy models were constructed from the obtained Rayleigh and Love wave dispersions. The mid-lower crust is characterized by strong positive anisotropy (V_SH>V_SV) beneath the Qilian-Qinling Orogen and relative weak radial anisotropy beneath the Kunlun Mountains. The mantle above 90 km is largely isotropic while positive anisotropy increases gradually with depth below 90 km, which probably marks the lithosphere-asthenosphere boundary (LAB). In contrast, shear wave velocity does not show an obvious reduction at 90 km and fails to detect the LAB. A low shear wave velocity anomaly and relatively negative radial anisotropy are imaged in the entire lithosphere under the Kunlun restraining bend, indicating a weak lithosphere under compressional deformation. The asthenosphere here is characterized by significantly lower velocity and stronger positive anisotropy than the surrounding region, suggesting a warmer and weaker asthenosphere block. Our observations are consistent with a geologic model in which the lithosphere under the restraining bend was thickened and subsequently delaminated, and the warm asthenosphere then filled up the space left by the delamination.