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.