Crustal radial anisotropy across Eastern Tibet and the Western

[1] Phase velocities across eastern Tibet and surrounding regions are mapped using Rayleigh (8–65 s) and Love (8–44 s) wave ambient noise tomography based on data from more than 400 Program for Array Seismic Studies of the Continental Lithosphere and Chinese Earthquake Array stations. A Bayesian Monte Carlo inversion method is applied to generate 3-D distributions of Vsh andVsv in the crust and uppermost mantle fromwhich radial anisotropy and isotropic Vs are estimated. Each distribution is summarized with a mean and standard deviation, but is also used to identify “highly probable ” structural attributes, which include (1) positive midcrustal radial anisotropy (Vsh>Vsv) across eastern Tibet (spatial average=4.8%±1.4%) that terminates abruptly near the border of the high plateau, (2) weaker (1.0%±1.4%) negative radial anisotropy (Vsh<Vsv) in the shallow crust mostly in the Songpan-Ganzi terrane, (3) negative midcrustal anisotropy (2.8%±0.9%) in the Longmenshan region, (4) positive midcrustal radial anisotropy (5.4%±1.4%) beneath the Sichuan Basin, and (5) low Vs in the middle crust (3.427±0.050km/s) of eastern Tibet.Midcrustal Vs< 3.4 km/s (perhaps consistent with partial melt) is highly probable only for three distinct regions: the northern Songpan-Ganzi, the northern Chuandian, and part of the Qiangtang terranes. Midcrustal anisotropy provides evidence for sheet silicates (micas) aligned by deformation with a shallowly dipping foliation plane beneath Tibet and the Sichuan Basin and a steeply dipping or subvertical foliation plane in the Longmenshan region. Near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying Tibet