Crust-mantle coupling mechanism beneath the Qinling Orogen Belt revealed by SKS-wave splitting

The Qinling Orogenic Belt (QOB) is an east-west extended Proterozoic mountain range located between the Ordos block in the north and the Yangtze block in the south. It is featured by complicated crustal deformation structures due to its long history of tectonic evolution. Measuring the subsurface deformation field beneath the QOB thus becomes essential for deciphering major tectonic events that shaped the belt. Seismic anisotropy is generally believed to be caused by fabric structures that are close related to the subsurface strain field. Seismic anisotropy is often measured with two parameters, the fast polarization direction φ and delay time δt between the fast and slow directions, from splitting or birefringence of shear waves. In this study, we measured (φ, δt) from SKS recorded by 41 broadband stations that cover the QOB and it surrounding areas. We employed a 2-D grid searching method which searches for an optimum (φ, δt) that minimizes the transverse energy or the second eigenvalue of the covariance matrix. Stacking of SKS data from multiple earthquakes provides robust measurements of the two splitting parameters. The measured seismic anisotropy shows an overall E-W fast polarization direction across the QOB with slight variations at its western and eastern ends. The fast polarization direction is consistent with a eastward mantle flow that escapes from the NE margin of the Tibetan plateau and flows between the Ordos and Yangtze blocks. We also compared SKS splitting with GPS measurements and crustal anisotropy, and found that the NE marging of the Tibetan plateau and western part of QOB exhibit a vertically coherent deformation (VCD) across the entire lithosphere. In the eastern part of the QOB, there is a significant difference (>15°) in the fast polarization direction measured from SKS and Pms, a P-to-S converted wave at the Moho, indicating some decoupling of deformation between the crust and upper mantle. © 2021, Science Press. All right reserved.