Crustal radial anisotropy across Eastern Tibet and the Western Yangtze Craton

Abstract

[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 and Vsv in the crust and uppermost mantle from which 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 (À1.0% ± 1.4%) negative radial anisotropy (Vsh < Vsv) in the shallow crust mostly in the Songpan-Ganzi terrane, (3) negative midcrustal anisotropy (À2.8% ± 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 ± 0.050 km/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.

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

“…Like Agius & Lebedev (2014), we find the strongest anisotropy in west Tibet, although the anisotropy is found to be stronger in our study, and we observe relatively strong anisotropy in northeast Tibet, while this is where anisotropy is low or absent in their study. In eastern Tibet, where our study area coincides with that of Xie et al (2013), we observe a broadly similar pattern in the depth and lateral extent of the anisotropy, in particular the decrease in anisotropy at the northern edge of the Tibetan Plateau. The magnitude of anisotropy we observe is larger than that calculated by Xie et al (2013).…”

“…In eastern Tibet, where our study area coincides with that of Xie et al (2013), we observe a broadly similar pattern in the depth and lateral extent of the anisotropy, in particular the decrease in anisotropy at the northern edge of the Tibetan Plateau. The magnitude of anisotropy we observe is larger than that calculated by Xie et al (2013). Forward modelling suggests that our method may overestimate the magnitude of ξ , but the presence and depth of anisotropy are robust.…”

Crustal structure of the eastern Qinling orogenic belt and implication for reactivation since the Cretaceous

“…6c), which is roughly bounded by the BSF to the south and LLWF to the north, respectively. The mid-to lower crustal low velocity zone has long been suggested to be associated with the Phanerozoic orogenic activities (Xie et al, 2013;Yang et al, 2012;Ozacar and Zandt, 2004). For instance, crustal low velocity zone is observed broadly distributed beneath the interior of Tibetan Plateau (Xie et al, 2013;Yang et al, 2012).…”

Inverting Rayleigh surface wave velocities for eastern Tibet and western Yangtze craton crustal thickness based on deep learning neural networks

“…The data sets consist of phase velocities of Rayleigh waves from Xie(2013) at discrete frequency of 10.0,12.5,15.0,17.5,20.0,22.5,25.0,27.5,30.0,32.5,35.0 mHz 40 and derived group velocities of Rayleigh waves at discrete frequency of 10.0, 12.5, 15.0, 17.5, 20.0, 22.5, 25.0, 27.5, 30.0 mHz. We conclude that:…”

“…Tibet and western Yangtze craton from newest and high-resolution phase and group velocity maps (Xie et al,2013). As seismology points out that there are many factors affect phase and group velocity, and inverting them for discontinuities within the earth forms a non-linear inverse problem (Ueli Meier et al,2007).…”