High-resolution Vp/Vs Tomography of the Crust and Uppermost Mantle beneath South China

Compared to separate Vp and Vs models, Vp/Vs model is more sensitive to fuilds, melts and rock compositions. For south China, many studies have created Vp and Vs models of different resolutions, but no high-resolution Vp/Vs models are available. In this study, to better constrain tectonics and presence of fluids and melts in the south China lithosphere, especially the control of lithospheric structure on the formation of metal minerals, we adopt a modified double-difference seismic tomography method for directly inverting for Vp/Vs using body wave P and S arrival times.  We assembled earthquake arrival times recorded by the China Seismic Network (CSN) between 2008 and 2018. A total of 25,023 earthquakes were analyzed within the study region (107°–123°E, 18°–35°N), recorded by 1,998 seismic stations. After strict quality control and phase picking by the deep-learning based USTCPicker that is retrained from PhaseNet, 617,143 P-wave and 583,628 S-wave arrival times were obtained. In addition, more than 14 million P- and S-wave differential travel times were constructed from event pairs recorded at common stations, providing strong constraints for earthquake locations and velocity structures around the source region.

The inversion started from the USTClitho2.0 lithospheric model, and the inversion is parameterized on a three-dimensional grid extending from the surface to 180 km depth with grid intervals of 1° in latitude and longitude. After 14 iterations, the root-mean-square travel-time residual is reduced from 1.59 s to 0.17 s, indicating a substantial improvement in data fit. Checkerboard resolution tests demonstrate that the Vp/Vs structure is well resolved throughout most of the crust and the uppermost mantle down to ~80 km depth. At the depth of 20 km, wide-spread high Vp/Vs values are imaged in the Jiangnan orogen. Which is consistent with the reworked crust regime delineated by medium zircon Hf isotope values. At the depth of 40 km, high Vp/Vs values are mostly distributed in the southeastern Cathaysia block and along the southeast coast, corresponding to juvenile crustal domain with high zircon Hf isotope values. These correlations indicate that the reworked crust mainly occurs in the middle curst while juvenile crust happens in the lower crust. These different processes actually have some control on the formation of different metal ores. Overall, the resulting Vp/Vs model offers new insights into the distribution of fluids, lithological variations, and tectonic processes in south China.