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.