Multilayered Crustal Anisotropy in Eastern Tibet Revealed by Receiver Function Data

It is widely accepted that the ongoing India-Asia collision since approximately 50 Ma ago has resulted in the uplift and eastward expansion of the Tibetan Plateau. Yet the interpretations of its dynamic process and deformation mechanism still remain controversial. Distinct models that emphasize particular aspects of the tectonic features have been proposed, including fault-controlled rigid blocks, continuous deformation of lithosphere and lower crust flow.

One possible way to reconcile these models is to investigate crustal deformation at multiple depths simultaneously, as well as crust-mantle interaction. Seismic anisotropy is considered as an effective tool to study the geometry and distribution of subsurface deformation, due to its direct connection to the stress state and strain history of anisotropic structures and fabrics. In the eastern margin of Tibetan plateau, previous studies of seismic anisotropy have already provided useful insights into the bulk anisotropic properties of the entire crust or upper mantle, based on shear wave splitting analyses of Moho Ps and XKS phases.

In this study, we went further to extract anisotropic parameters of multiple crustal layers by waveform inversion of teleseismic receiver function (RF) data from the western-Sichuan temporal seismic array using particle swarm optimization. Instead of directly fitting the backazimuthal stacking of RFs from each station, we translated the RF data into backazimuthal harmonic coefficients using harmonic decomposition technique, which separates the signals (of planar isotropic structure and anisotropy) from the scattering noise generated by non-planar lateral heterogeneity. The constant (k=0) and k=1, 2 terms of backazimuthal harmonic coefficients were used in our inversion. We also fixed the anisotropic model to slow-axis symmetry to avoid ambiguous interpretations.

Our results show that:

(1) Anisotropy with a titled anisotropy axis of symmetry is more commonly observed than pure azimuthal anisotropy in our data, which has been also reported by other RF studies across the surrounding areas of Tibetan plateau.

(2) The trends of slow symmetry axis vary from the upper to lower part of the crust in both Chuandian and Songpan units, indicating the deformation of the upper crust is decoupled from that of the lower crust in these two regions, while the trends are more consistent throughout the crust in the Sichuan basin.

(3) In the upper crust, the trends show a degree of tendency to lie parallel to the major geological features such as the Xianshuihe and Longmenshan faults, exhibiting a fault-controlled deformation or movement. In the middle and lower crust, the trends are NS or NW-SE in Chuandian unit and NE-SW in Songpan unit, which are coincident with the apparent extension directions of the ductile crustal flow.