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.