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).