固物研究生论坛 -Crustal shear velocity model in Southwest China
Southwest (SW) China is located in a transition site from the southeastern margin of the active Tibetan Plateau to the stable Yangtze Craton, which has complicated tectonic deformation and severe seismic hazards. For the first time, we combine the data from ambient noise, teleseismic body and surface waves, and industry well logging profiles to better constrain the crustal shear velocity structure in SW China. Using data predominantly from the China National Seismic Network, we jointly invert Rayleigh wave dispersion (5-40s period), Rayleigh wave ZH ratio (20-60s period), and receiver function with a stepwise-linearized inversion method. Our new model has revealed several distinct features. Compared to previous tomography results, we observe higher VS in the sedimentary basins beneath the Sichuan Basin, consistent with well log measurements. Our model reveals widespread low-velocity zones in the mid-lower crust with distinct spatial variations and some of their boundaries correlate well with major fault systems. Between two mid-crustal low-velocity channels, a prominent high-velocity region surrounded by earthquakes is observed in the inner zone of the Emeishan large igneous province (ELIP). We suggest this mechanically strong crust of ELIP is obstructing the crustal ductile flow in southeastern Tibet and leading to its bifurcation. Integrating all the results, we suggest mid-lower crustal ductile flow and upper crustal fault movement play equally important roles in controlling the regional deformation styles and earthquake distribution in SW China. Our results also resolve thick crust-mantle transition zones beneath the eastern Tibet Plateau and the inner zone of ELIP, resulting from magmatic inflation from mid-lower crust and underplating from upper mantle, respectively.