Lithospheric Deformation of Continental China from Rayleigh Wave Azimuthal Anisotropy

Abstract

AbstractThe tectonic history of continental China is complicated and the associated lithospheric deformation is large; the use of the anisotropic propagation of surface waves is an important way to investigate the history of lithospheric deformation pattern. We measured inter‐station Rayleigh‐wave phase velocity dispersion from cross‐correlation of narrow band‐pass filtered surface wave records and an image analysis technique. The dispersion data were then used to invert for the distribution of azimuthal anisotropy at periods 20~120 s of continental China and adjacent regions. Checkerboard tests show that the lateral resolution of phase velocity azimuthal anisotropy is about 5° (~500 km) in most area of continental China. Our results show that the pattern of azimuthal anisotropy in the study area displays clear spatial variation. The tectonic deformation of the crust and upper‐mantle east of 104°E is generally weaker than that to the west as inferred from the relative magnitude of azimuthal anisotropy, and strong tectonic deformation had occurred in the crust and upper‐mantle beneath Qinghai‐Tibet Plateau and its eastern margin. In eastern China relatively large amplitude of azimuthal anisotropy appears in South China block and Pearl River Estuary region, southwestern margin of Ordos basin and Qinling‐Dabie orogenic belt, indicating strong tectonic deformation occurred there in the geological history. At short and intermediate periods the fast propagation direction shows a rotational pattern from the west to the east within the Qinghai‐Tibet block consistent with the clockwise extrusion (or escape) of the crustal material in the block after the Eurasian and Indian collision. The deformation pattern in the mid‐lower crust of western Qinghai‐Tibet block is similar to that of the upper mantle, inferring possible vertically coherent deformation across the Moho interface. However, the central and eastern Qinghai‐Tibet block and its eastern margin show different deformation patterns in the crust and upper mantle, which suggests that deformation is not vertically coherent in the crust and upper mantle in eastern plateau region. Strong tectonic deformation had occurred within Tarim and Qaidam basins as well as Qilian fold belt north of Qinghai‐Tibet block. The fast propagation direction at the middle crustal depth is clearly different from those both in the lower crust and upper mantle underlying the upper Yangtze block including Sichuan basin. This suggests different tectonic deformation processes between the middle crust and lower crust beneath this block, whereas the lower crust and upper mantle appears to be coupled as inferred from the similar fast propagation direction at periods above 30 s. The Longmen Shan tectonic zone can be divided into southwestern and northeastern segments at about 103°E. The southwestern segment has relatively lower phase velocity while the northeastern area has higher phase velocity and stronger anisotropy. This suggests that the NE striking unilateral rupture propagation of the WenchuanMs8.0 earthquake on May 12, 2008, may be related not only to the cumulated high stress of the northeastern section but also to the underlying medium property along the segment with high phase velocity suitable for strain energy accumulation and concentrated release.