Anisotropic Signature of Falling Indian Lithosphere Beneath Western Lhasa Block.

Continental collisions displace and deform large volumes of mantle rock, making it seismically anisotropic due to the systematic alignment of olivine crystals under strain. In the India-Asia collision region evidence for deformation-induced anisotropy is widely observed, e.g. in the form of splitting of core-refracted shear waves. Throughout Tibet fast shear wave polarizations (and by inference the direction of mantle deformation) are found to be oriented NE-SW or nearly E-W.

This pattern largely holds for the western-most part of the plateau, with one notable exception. Sites at ~82°E display nearly N-S fast directions. A new S wave speed model of the upper mantle beneath western Tibet contains a major fast velocity anomaly. Near-vertical, and columnar in shape, it is located beneath and to the south of the sites with unusual fast directions. Positioned roughly beneath the India-Asia plate boundary, the fast anomaly likely represents down-welling Indian lithosphere.

Observations of shear wave splitting reflect cumulative influence of anisotropic properties along the ray, and depend on the mutual orientation of the anisotropic symmetry axis and the ray path. In Tibet most clear core-refracted phases arrive from a range of backazimuths (95° – 115°), necessarily biasing the inferred orientation of reported seismic anisotropy axes in the mantle.

We investigate lateral and vertical variation of seismic anisotropy in the upper mantle of the western Lhasa Block by developing models of anisotropic parameters for ~30 sites of the western Tibet seismic network operated from 2007 to 2011. We assemble directionally representative suits of SKS, PKS and SKKS phases, and deliberately cull the data in backazimuth range 100°-110°. We use a synthetic seismogram matching technique to invert resulting data sets for anisotropic parameters (fast symmetry axis and strength) of models that contain one or two layers of anisotropy, and where symmetry axes can be inclined from vertical.

Our unbiased anisotropic structure solutions confirm that fast symmetry axes in most of the western Lhasa block are close to E-W, and nearly horizontal. We find relatively modest (2% over 100 km thickness) levels of anisotropy. We also confirm that the area above the fast shear wave anomaly in the mantle has a distinct anisotropic signature.