Towards 3‐D fabric in the continental lithosphere and asthenosphere: The Tien Shan

Abstract

[1] As shown by previous observations of shear wave splitting of the SKS phase, the fast direction of azimuthal anisotropy beneath the Tien Shan tends to be aligned with the strike of the belt. This direction implies that the anisotropy is related to the current orogenesis. Now we describe an attempt to constrain the distribution of the anisotropy with depth by combining observations of SKS and P receiver functions. The data are inverted for the parameters of anisotropy with the aid of a genetic algorithm. The results of the joint inversion suggest that the actual distribution of anisotropy with depth can be more complicated than usually thought. The deformed mantle layer can be a few hundred kilometres thick. In the most reliable models fast direction of anisotropy in the shallow mantle is significantly different from that inferred from the observations of SKS. At some stations the data require anisotropy in the crust.

Thermo-Mechanical Models for Coupled Lithosphere-Surface Processes: Applications to Continental Convergence and Mountain Building Processes

“…We therefore conclude that the localized growth of a range like the Tien Shan indeed could result from the coupling between surface processes and horizontal strains. We do not dispute the possibility for a complex mantle dynamics beneath the Tien Shan as has been inferred by various geophysical investigations (Vinnik and Saipbekova, 1984;Vinnik et al, 2006;Makeyeva et al, 1992;Roecker et al, 1993), but we contend that this mantle dynamics has not necessarily been the major driving mechanism of the Cenozoic Tien Shan orogeny.…”

“…2). This estimate is based on the assumption of ductile rheology of the lower crust, which is supported for this area by multiple data starting from seismic data (Vinnik and Saipbekova, 1984;Makeyeva, 1992;Roecker et al, 1993;Vinnik et al, 2006) and ending by gravity-flexural analysis (Burov et al, 1990(Burov et al, , 1993Avouac and Burov, 1996). Only short topographic wavelengths, typically less than a few tens of kilometres that can be supported by the strength of the upper crust, would be maintained over geological periods of time, yet provided that they are not removed by erosion (which is faster on short wavelength topography).…”

Stratification of anisotropy in the Pacific upper mantle

“…There are several reasons to be cautious with this postulated interpretation of seismic anisotropy as a proxy for mantle flow. First, evidence for a depth‐dependent distribution of anisotropy has been found both beneath continents [e.g., Savage and Silver , 1994; Levin et al , 1999; Vinnik et al , 2002] and oceans [e.g., Wolfe and Solomon , 1998]. In addition, in most areas the observed seismic anisotropy is more complicated than a system with a fast horizontal axis of symmetry and may require additional radial anisotropy which has a vertical slow axis of symmetry [e.g., Ekström and Dziewonski , 1998; Shapiro and Ritzwoller , 2002].…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…Another type of phase that holds promise for shear wave splitting practitioners is converted phases (e.g., Girardin and Farra 1998;Vinnik et al 2002), including those converted at seismic discontinuities such as the Moho, or the 410 and 660 km transition zone discontinuities. Converted phases are already used to detect sharp changes in anisotropic structure by looking for backazimuthal variations in transverse component receiver functions that are the manifestations of so-called P-to-SH conversions at shallow (crust or uppermost mantle) depths beneath a station (e.g., Levin and Park 1998;Park et al 2004).…”