5. Discussion
The Rayleigh-wave group velocity (9-15s) (Fig. 6 a) and the S-wave velocity at a depth of 10 km show relatively low velocity in the Sichuan Basin (Fig. 7 a). The thickness of the whole sequence of sedimentary layers in Sichuan Basin is about 15 km (Fig. 8 a, b). This is in good agreement with the estimated thickness of continental strata in this basin (Liu et al., 2016). There is a large-scale low-velocity anomaly in the middle-lower crust beneath the Songpan-Ganzi Block and the CYRB, which is consistent with the velocity structure in the Tibet Plateau (Kind et al., 1996). The high-velocity anomaly persists in the central Sichuan Basin within a depth range of about 20-40 km, which may reflect the rigidity crust of this basin (Fig. 7, Fig. 8). Previous studies on surface wave results also indicated that there is a high velocity body at this depth (Yao et al., 2008; Lu et al., 2014). The crustal S-wave velocity structure below 20 km shows low-velocity structure in the western part of the study region (Fig. 8 a-f), whereas eastern part or beneath the Sichuan Basin is characterized by high-velocity structure (Fig. 8 a and b). Pn wave tomography also demonstrated that there is a low-velocity anomaly beneath the SYDB and the Songpan-Ganzi Block (Lei et al., 2014).
In Fig. 6 and Fig. 7, the low-velocity anomaly (LV2) in the Songpan-Ganzi Block extend to the Yangtze Block, which may imply the extrusion of the Tibet Plateau or the eastward flow in the middle and the lower crust. Wu et al. (1988) obtained 14 terrestrial heat flow data which show high heat flow values in western Yunnan and Panxi areas. Hu et al. (2000) further demonstrated this result based on a new compilation of heat flow data in mainland China. These results suggest melt or partial melt in the crust beneath the Songpan-Ganzi Block and Yangtze Block. Sun et al. (1989) proposed that there are high-conductivity layers in the lower crust and/or upper mantle in western Yunnan and western Sichuan, which are considered to be related to partially molten materials.
As shown in Fig. 8, the seismic activities are mainly distributed at the boundary between the high- and low-velocity structure and the nearby boundary region. This might suggest the ductile deformation of the Songpan-Ganzi Block induced by the eastward extrusion of the Tibet Plateau which is obstructed by the rigid crust of the Sichuan Basin, leading to the stress accumulation and release or the seismic activities.
Although a number of tomographic studies have been carried out in this area and adjacent regions (e.g., Shen et al., 2016; Xie et al., 2013; Chen et al., 2014), our study is the first to define the obvious low-velocity anomaly, which is connected with the channel flow in the crust of the Chuandian region.