Episodic Lithospheric Deformation in Eastern Tibet Inferred From Seismic Anisotropy

Abstract

Mechanisms for uplift and deformation of the Tibetan Plateau remain vigorously debated; hypotheses include stepwise growth, distributed thickening, and crustal channel flow, each with a distinct anisotropic signature. We have developed a new azimuthally anisotropic shear velocity model for the lithosphere beneath eastern Tibet, based on ambient noise tomography from 643 seismic stations. In our model, the Tibetan upper crust is characterized by strong anisotropy with fast axes that correlate with surface geology and mantle anisotropy, suggesting the occurrence of coherent deformation. However, a much different picture emerges in the middle and lower crust, where anisotropy is disordered and weaker beneath the plateau than along its margins, inconsistent with the prediction of large‐scale eastward crust flow in eastern Tibet. Our observations are best explained by heterogeneous crustal thickening beneath the plateau with complex flow in the middle and lower crust, accompanied by asthenospheric upwelling along the southeastern plateau margin.

Weak Crust in Southeast Tibetan Plateau Revealed by Lg‐Wave Attenuation Tomography: Implications for Crustal Material Escape

“…These results suggest that low‐ Q Lg anomalies are connected in spite of strong inhomogeneity, probably indicating complicated connections for crustal material flow. The inhomogeneous crustal flow was also suggested by seismic anisotropy investigations (X. Bao et al., 2020; H. Huang et al., 2010, Huang, Wang, Xu, Ding et al., 2015; W. Wei et al., 2013). W. Wei et al.…”

“…These regions are characterized by intense crustal deformation, intricate fault systems, and rough topography, which may result in remarkable Lg‐wave scattering and contribute to the low‐ Q Lg anomalies observed near major faults, including the Xianshuihe, Xiaojiang, Lijiang, and Sagaing faults (Wu et al., 2007). However, many studies have reported anomalies of high Poisson’s ratio, low S ‐wave velocity, and high conductivity within the crust in these regions (D. H. Bai et al., 2010; X. Bao et al., 2020; X. W. Bao et al., 2015; H. P. Chen et al., 2016; M. Chen et al., 2014; Q. Y. Liu et al., 2014; Qiao et al., 2018; Sun et al., 2012, 2014; W. L. Wang et al., 2017). The strong Lg attenuation obtained in this study also coincides with these anomalies, which seems to indicate that the intrinsic attenuation associated with thermal status and partial melting is an important origin of the Lg‐wave dissipation, in addition to the scattering effects.…”

“…(2013) observed remarkable low velocity zones beneath Songpan‐Ganzi and Chuan‐Dian at depths of 20–40 km but a disordered anisotropy with inconsistent directions of fast velocity with low‐velocity zone trends (W. Wei et al., 2013). The disordered pattern of the fast velocity direction probably reflects the nature of the complicated connections within the corridor of lower crustal material flow because the occurrence of lower crustal flow would result in the strain‐induced lattice‐preferred orientation of minerals at these depths (X. Bao et al., 2020). By simulating shear‐wave splitting based on a 3‐D anisotropic S ‐wave velocity model from Rayleigh surface‐wave tomography, Yao et al.…”

“…The highly heterogeneous crust and material transfer in the southeastern margin of the Tibetan Plateau could be a consequence of both crustal flow and strain partitioning across faults (Q. Y. Liu et al., 2014; P. Z. Zhang, 2013). However, localized and isolated geophysical anomalies and disordered crustal anisotropy were observed, questioning the pervasive and uniform channel flow of the lower crust (X. Bao et al., 2020; M. Chen et al., 2014; Qiao et al., 2018). In contrast, the Indochina Peninsula was assumed to be relatively stable because of its low elevation and very small crustal deformation, as revealed by GPS observations (Iwakuni et al., 2004; Morley, 2002).…”

The Role of Pre‐existing Crustal Weaknesses in the Uplift of the Eastern Tibetan Plateau: 2D Thermo‐Mechanical Modeling

“…In the early phase of the India‐Asia continental collision, the pre‐existing CWZs were reactivated and developed a series of thrust faults and crustal folds in the eastern Tibetan Plateau (Burchfiel et al., 1995; Hubbard & Shaw, 2009; Jia et al., 2010). The crust and lithosphere mantle coherently deformed in the form of pure shear (Bao et al., 2020). By late Cenozoic, a thick partially molten mid‐lower crust layer with low viscosity and low density is generated in the Songpan‐Ganzi crust (Bai et al., 2010; Diao et al., 2018; Jia et al., 2014; Li et al., 2009; Liu et al., 2014; Wang et al., 2018; Yang et al., 2012; Zhang et al., 2014; Zhao et al, 2012, 2013).…”