Figure 11 Schematic illustration of the possible material flow beneath the Sichuan-Yunnan region inferred from our tomography model. Blue arrows depict different directions of the lower crustal flow, whereas the red arrows indicate possible upwelling of warm material from the upper mantle beneath the eastern margin of the Songpan-Ganzi Block. The solid and dotted black lines depict the traces of the same major fault zones (as defined in Figure 1) on the surface and Moho, respectively.
In central Sichuan-Yunnan Rhombic Block, there is a low-velocity zone with some extremely slow velocity anomalies (below 6.0 km/s) surrounding the Lijiang-Xiaojinhe Fault Zone. The rather low-velocity materials may also be indicative of crustal flow in this region. As the upper crust in this region is mainly controlled by brittle deformation (e.g., Wu et al., 2006; Zhang, 2013), the ductile flow of the lower crust may be able to drag the upper crust to move. As a coherent tectonic unit, the Sichuan-Yunnan Rhombic Block is considered to not only have the south-southeast motion, but also a clockwise rotation (e.g., King et al., 1997; Shen et al., 2005). Despite the stress coming from the collision between the Indian and Eurasian plates, the ductile materials in the lower crust also help to drive the whole block to move and rotate. Figure 11 presents an illustrative sketch of the possible lower crustal flow based on the velocity patterns in our 3-D tomography model for the lower crust and uppermost mantle. The ductile flow is obstructed by locally strong crust and redirected to south-southeast, with a wide lateral distribution in the lower crust. The surface features and movements of the Sichuan-Yunnan region is generally associated with the distribution of the lower crustal flow of a large quantity of weak and ductile materials. Therefore, we suggest a coupling between the surface deformation and the material flow in the lower crust.
As we can see in Figures 9 and 10, the P-wave velocity at the shallower part of the uppermost mantle is quite different from that of the deeper part. At the depth of 45 km, the velocities are so slow that over half of the region is dominated by velocities of ~7.2 km/s, which is close to those along the southeastern margin of the model in the lower crust. Therefore, the region immediately below the Moho interface may be strongly influenced by the lower crustal flow, as illustrated by the yellow arrows in Figure 11.
The low-velocity materials in the southwestern corner of our model as shown in Figure 9 seem to be blocked by the surrounding high-velocity zone. The extremely high velocity (above 8.3 km/s) in the Sichuan-Yunnan Rhombic Block may indicate that this region becomes relatively cold and stable in the uppermost mantle below 60-km depth, which suggests that the lower crustal flow may not have obvious influence on the deeper part of the uppermost mantle in this region, except at depths above 60 km near the Moho interface.