Seismic anisotropy in the Tibetan lithosphere inferred from mantle xenoliths

Shear wave splitting measurements in the Qiangtang terrane of central Tibet revealed extraordinarily large delay times of >2.0 s for SKS splitting and an abrupt change of the fast polarization direction from a NNE-SSW direction into an E-W direction in an about 100 km wide zone south of the Banggong-Nujiang Suture. Peridotite xenoliths brought to the surface from a depth of 55 km to 70 km by Cenozoic potassic volcanism in Sailipu in the north-western Lhasa terrane show lattice preferred orientation of their olivine with a concentration of [010] directions and a large circle girdle of the [100] and [001] directions defining an important fabric plane with a polarization anisotropy of 4.6% on average in directions parallel to the plane. In contrast, the polarization anisotropy is very weak in the direction perpendicular to this fabric plane. According to the seismic anisotropy of the peridotite xenoliths, to match the delay times ranging from 1.5 s to more than 2.0 s in the Lhasa and Qiangtang terranes would require an anisotropic layer at least 150 km to 200 km thick even if the fabric plane with maximum seismic anisotropy were oriented vertically and the effect of the intrinsic elastic anisotropy of the peridotites on SKS splitting were maximized. This is unrealistic, and we propose to attribute the large delay times to an extraordinarily strong seismic anisotropy of the lithospheric mantle underneath central Tibet. We propose that this anisotropy is due to the presence of partially molten domains with high aspect ratio that are aligned sub-vertically enhancing the effect of the intrinsic elastic anisotropy of the peridotites on SKS splitting. The abrupt change of the fast polarization direction south of the Banggong-Nujiang Suture may reflect the change of the fabric of olivine which is fundamentally different in the northward subducting Indian lithospheric mantle and in the lithospheric mantle beneath northern Tibet. (C) 2019 Elsevier B.V. All rights reserved.