4. Discussion
As can be expected from the many
major active zones criss-crossing the region and forming the boundaries
between the tectonic blocks, our P-wave velocity model exhibits a highly
heterogeneous pattern in the lower crust throughout the Sichuan-Yunnan
region. For instance, strikingly different velocities can be seen in the
regions located on opposite sides of the Longmenshan Fault Zone and the
Anninghe-Zemuhe Fault Zone (ANH-ZMHFZ), with the velocity changing from
6.3 km/s on the western side to above 6.6 km/s on the eastern side over
a short distance. Similarly, strong lateral heterogeneities are also
present in the uppermost mantle.
Apart from the high velocities in the area around the Sichuan Basin and
the Yangtze Block, the lower crust of the Sichuan-Yunnan region is
generally characterized by a low-velocity anomaly with a mean value of
6.7 km/s, which is rather low as compared with many other regions in the
world. For example, Deng et al. (2014) reported the P-wave speed value
of above 7.0 km/s in the lower crust of southeast China, Chulick et al.
(2013) yielded a P-wave speed of 6.9-7.3 km/s in the lower crust of
South America, and Tesauro et al. (2014) found that the P-wave speed in
the lower crust of the North America Continent is generally above 6.8
km/s. As can be seen in Figures 8-10, the P-wave velocity in the top of
the uppermost mantle is also much lower than the global average value of
8.04 km/s in model AK135 (Kennett et al., 1993). In the deeper part, the
whole region is largely featured by velocities of ~8.1
km/s. This is comparable to many other places, such as the values of
8.15 km/s in northern Eurasia (Schueller et al., 1997) and 8.2 km/s in
southeast China (Sun and Kennett, 2016) and South America (Chulick et
al., 2013).
As shown in Figure 9, our P-wave tomography model shows a high level of
lateral variation in the uppermost mantle (45–50 km) in the southwest
region where the velocity jumps from below 7.5 km/s to above 8.2 km/s
over a short distance. In contrast, the Sichuan Basin and the Yangtze
Block have weak lateral heterogeneity and are dominated by velocities
around 8.1 km/s. The variations of P-wave speed in the uppermost mantle
appears to have no apparent relationship with the surface feature, such
as the Sichuan Basin. Meanwhile, we cannot see any distinct features in
the velocity model marking the major fault zones. This means that those
giant faults may not have extended all the way to the Moho. We can also
see this in the high velocity band along the Longmenshan Fault Zone
which slowly vanishes with increasing depth.
In contrast to the upper mantle, the variation of the complex 3-D P-wave
velocity structures in the lower crust is closely related to the
regional tectonics. As shown in Figure 8, the old stable Sichuan Basin
located to the east of the Longmenshan Fault Zone is characterized by
relatively high P-wave velocity at lower crustal depths as compared to
other places in the Sichuan-Yunnan region. The Sichuan Basin was an
intra-cratonic basin from the late Proterozoic to the middle Late
Triassic times (Jia et al., 2006). It has undergone many different
phases of changes during its long evolution history, but has always
maintained as a single stable block with few large active faults and
relatively low seismic activity. In Figure 8, we can see the high-speed
feature associated with the stable basin extending from the depth of 25
km to almost 45 km. The stable Sichuan Basin has clearly resisted the
eastward movement of the Songpan-Ganzi Block at the Longmenshan Fault
Zone. The push between the Tibetan Plateau in the west and the Sichuan
Basin in the east results in the drastic relief across the Longmenshan
Fault Zone.
Similar to the Sichuan Basin, the Yangtze Block further south is also
characterized by higher P-wave speeds in the lower crust. The P-wave
velocity in this stable region varies smoothly at different depths
(Figure 10f). Because of the resistance of the Sichuan Basin and Yangtze
Block, the eastward movement of the Tibetan Plateau is redirected to
southward and drives the Sichuan-Yunnan Rhombic Block to rotate
clockwise, which can be seen in the velocity field of the crustal
deformation observed by GPS in the Sichuan-Yunnan region (Figure 1). The
Yangtze Block is not as stable as the older Sichuan Basin, and the
resistance may not be as strong as in the Longmenshan Fault Zone,
resulting in a much less sharp contrast in topography relief.
To the west of the Longmenshan Fault Zone, the Songpan-Ganzi Block
covered by a thick sequence of deep marine deposits is considered to be
part of the old Yangtze Platform and underlain by continental crust
(e.g., Burchfiel et a1., 1995; Zhang, 2001; Liu et al., 2006). The
average P-wave velocity in the eastern margin of this block at lower
crustal depths is about 6.2 km/s, with some extremely low-velocity
anomalies below 6.0 km/s. These low-velocity features are consistent
with the possible existence of lower crustal flow in this region. The
low-velocity materials are distributed around the higher-velocity region
near the Longmenshan Fault Zone, which may indicate that the flow is
blocked by the stable Sichuan Basin and bifercated to both north and
south along the fault zone. Yang et al. (2020) also found the existence
of a significantly low-velocity region and suggested that the thickening
and erosion of the crust-mantle transition zone may be due to the mixing
and metasomatism caused by the partial melt materials in the mid-lower
crust. We can also see a region of rather low velocity in the eastern
margin of the Songpan-Ganzi Block between the depths of 50 km and 70 km
in Figure 10(f). There is a large block of rather high velocity with
values above 8.2 km/s extending from 50-km to 70-km depth south of the
low-velocity channel beneath the Sichuan Basin. The rather stable basin
obviously plays an important role in preventing the materials from
flowing further east. Previous studies also found the low-velocity zone
in the uppermost mantle and suggested that it is the result of warm
material upwelling from the deep mantle based on tomography and
anisotropy studies using local and teleseismic data (e.g., Wang et al.,
2003; Wang et al., 2010; Lei et al., 2019). Based on our P-wave
tomography model, we suggest that the low-velocity anomaly in the lower
crust beneath the Songpan-Ganzi Block is resulted from the lower crustal
flow. The low-velocity materials in the uppermost mantle of this block
may come from the upwelling of warm materials from the deep mantle
(Figure 11).