Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography

Abstract

[1] Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T = 10-40 s) and teleseismic surface waves (T = 20-150 s) for 3-D heterogeneity and azimuthal anisotropy in the lithosphere to ∼150 km depth. Our surface wave array tomography reveals (1) deep crustal zones of anomalously low shear wave speed and (2) substantial variations with depth of the pattern of azimuthal anisotropy. Upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern Himalayan syntaxis, with fast directions generally parallel to the main strike slip faults. The mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south. The tomographically inferred 3-D variation in azimuthal anisotropy helps constrain the source region of shear wave splitting. South of ∼26°N (off the high plateau) most of the observed splitting can be accounted for by upper mantle anisotropy, but for stations on the plateau proper (with thick crust) crustal anisotropy cannot be ignored. On long wavelengths, the pattern of azimuthal anisotropy in the crust differs from that in the mantle. This is easiest explained if deformation varies with depth. The deep crustal zones of low shear wave speed (and, presumably, mechanical strength) may represent loci of ductile deformation. But their lateral variation suggests that in SE Tibet (localized) crustal channel flow and motion along the major strike slip faults are both important.Citation: Yao, H., R. D. van der Hilst, and J.-P. Montagner (2010), Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography,

Complex, multilayered azimuthal anisotropy beneath Tibet: evidence for co-existing channel flow and pure-shear crustal thickening

“…The presence of substantial azimuthal anisotropy beneath Tibet is well established in studies using different techniques and data types, including surface-wave imaging (e.g., Griot et al 1998;Huang et al 2004;Su et al 2008;Yao et al 2010;Yi et al 2010;Yang et al 2010b;Ceylan et al 2012;Legendre et al 2015;Pandey et al 2015;Schaeffer et al 2016;Xie et al 2016;Chen et al 2016), shear-wave splitting analysis (e.g., McNamara et al 1994;Hirn et al 1995;Sandvol et al 1997;Sol et al 2007;Zhao et al 2010;Leon Soto et al 2012;Eken et al 2013;Chang et al 2015;Wu et al 2015a;Chen et al 2015;Liu et al 2016;Singh et al 2016;Ye et al 2016), receiver functions (e.g., Vergne et al 2003;Levin et al 2008;Shen et al 2015;Liu et al 2015;Kong et al 2016), attenuation studies (Bao et al 2012) and P-wave arrival times (e.g., Wei et al 2013;Huang et al 2014;Zhang et al 2016b;Wei et al 2016). Radial anisotropy (the difference between the vertically and horizontally polarized waves: V SV and V SH , respectively, in the case of S waves) is also well documented (e.g., Shapiro et al 2004;Huang et al 2010;Duret et al 2010;Guo et al 2012;Xie et al 2013;Li et al 2016).…”

Analytical and numerical simulations of uplift processes at the Tibet-Sichuan boundary

“…Furthermore, small elastic thickness and high Q value of Tibetan plateau both support a weak lower crust for the Tibetan plateau (Jordan and Watts 2005;Wang et al 2007;Bao et al 2011;Zhao et al 2013). Third, the lower crust of the plateau has strong seismic anisotropy, where the velocity of SH wave is much higher than SV wave (Shapiro et al 2004;Huang et al 2010;Yao et al 2010). This radial anisotropy may be explained by the horizontal arrangement of anisotropic minerals such as mica (Shapiro et al 2004;Huang et al 2010;Shen et al 2015).…”

A method for inversion of layered shear wavespeed azimuthal anisotropy from Rayleigh wave dispersion using the Neighborhood Algorithm

“…V PH (or A) and q are linked to V SV (or L) using some empirical relationships in the crust (Brocher 2005) and upper mantle (Masters et al 2000). We refer to Yao et al (2008Yao et al ( , 2010 for the details of this step. (2) Step 2: Perform the NA to estimate to G c and B c (or G s and B s ) from the azimuthally anisotropic part of Rayleigh wave dispersion data, a c (x, M) (or a c (x, M)), using the perturbation Eq.…”

“…Montagner and Nataf (1986) proposed a linearized inversion method for inverting azimuthal anisotropy of surface wave dispersion for shear wavespeeds with azimuthal and radial anisotropy at depths. This method has been used to obtain upper mantle azimuthal anisotropy regionally (e.g., Montagner and Jobert 1988;Silveira and Stutzmann 2002;Yao et al 2010) and globally (e.g., Montagner and Tanimoto 1990). Another approach to obtain the depth-dependent V SV and azimuthal anisotropy is following a two-step procedure based on waveform inversions: (1) nonlinear surface waveform inversion to obtain 1-D path-averaged V SV models and (2) tomographic inversion to invert all 1-D path-averaged V SV models for 3-D V SV structures and azimuthal anisotropy (e.g., Simons et al 2002;Debayle et al 2005).…”

“…In regions with good azimuthal path coverage, perioddependent 2-D phase/group velocity maps with azimuthal anisotropy can be obtained either from direct inversion of phase/group velocity dispersion measurements (e.g., Montagner 1986;Su et al 2008;Fry et al 2010;Yao et al 2010;Yi et al 2010;Endrun et al 2011;Lu et al 2014) or from localized estimation of phase velocity and its azimuthal anisotropy by solving the Eikonal equation (Lin et al 2009) or the Helmholtz equation (Lin and Ritzwoller 2011). Montagner and Nataf (1986) proposed a linearized inversion method for inverting azimuthal anisotropy of surface wave dispersion for shear wavespeeds with azimuthal and radial anisotropy at depths.…”