Seismic Anisotropy and Upper-Mantle Velocity Structures Beneath the Qinling-Dabie Orogenic Beltand Surrounding Regions

The Qinling-Dabie orogenic belt was formed as the suture zone by the collision of the North China Block and the South China Block in the Late Triassic and it has been reactivated by the Indo-Asian collision since 60 million years ago. From July 2012 to May 2014 Peking University and Chinese Academy of Geological Science deployed a seismic array of 147 portable broadband seismic stations around the Qinling-Dabie orogenic belt. Here shear-wave splitting analysis of SKS data and finite-frequency tomography were applied to these data to imagine the seismic anisotropy and velocity structures in the upper mantle beneath the region. Large delay times were observed at the southwest corner of the Ordos Block and the northeast of Tibetan Plateau. Where the angles between the fast polarization directions and the GPS velocity vectors are relatively large but those in the Weihe rift and the North China plain are nearly parallel. Delay times are small, as expected, in the southeast of the Ordos Block and the north of Sichuan Basin with inconsistent fast polarization directions. Finite-frequency tomography clearly images a fast velocity anomaly under the Ordos block that extends down to 400 km depth and a slow velocity anomaly under the east of Weihe rift that can be traced down to 300 km depth. More importantly we observe a slow velocity anomaly in the upper mantle extending down to 200 km depth at the southwestern corner of the Ordos Block and the northeast of Tibetan Plateau, and the location corresponds well with the results of shear wave splitting showing large angles between the fast polarization directions and the GPS velocity vectors. These results suggest that it is possible that the mantle lithosphere of the southwestern corner of the Ordos Block could be thermally weakened by the eastward Tibetan mantle flow between the Ordos Block and Sichuan Basin.