5. Conclusions
In this study, we have used 43,540 P-wave traveltimes to build a 3-D velocity model for the lower crust and uppermost mantle beneath the Sichuan-Yunnan region. The traveltime dataset involves Pg and Pn onset times picked automatically by the deep machine learning model PickNet and verified by visual inspection. The two-stage iterative tomography inversion with event relocation greatly reduces the travel-time residuals. Statistical resolution matrix analysis shows that our dataset offers overall resolution lengths of 0.4° in the lower crust and 0.2° in the uppermost mantle.
Our 3-D P-wave tomography model exhibits strong lateral variations in both the lower crust and uppermost mantle. Widespread low-velocity features are seen in the lower crust which may indicate the existence of the lower crustal flow under the eastern margin of the Tibetan Plateau. The stable Sichuan Basin and the Yangtze Block are underlain by high-velocity structures and may act as a dam blocking the crustal flow in the west from moving further east. Patterns of lateral variation in P-wave velocity in the lower crust are closely associated to the distribution of the tectonic blocks on the surface, whereas the velocities in the uppermost mantle seem to have no relationship with the surface features. The thickening and erosion of crust-mantle transition zone beneath the eastern margin of the Songpan-Ganzi Block may be influenced by both the weak materials in the lower crustal flow and the upwelling of warm materials from the deeper mantle. The low-velocity zone extending from the lower crust to the uppermost mantle east and west of the Sichuan-Yunnan Rhombic Block suggests the materials immediately below the Moho interface may be strongly influenced by the lower crustal flow.