1. Introduction
The Sichuan-Yunnan region located in the southwestern margin of the Tibetan Plateau has been under continuing crustal deformation since the collision between the Indian and Eurasian plates ~65 Ma ago. As an extremely complex transition zone between the uplifted Tibetan Plateau in the west and the relatively stable Yangtze craton in the east, a series of large-scale active fault zones have formed in the region (Figure 1), including the Longmenshan Fault Zone (LMSFZ), Xiaojiang Fault Zone (XJFZ), Lancangjiang Fault Zone (LCJFZ), Luzhijiang Fault Zone (LZJFZ), Lijiang-Xiaojinhe Fault Zone (LJ-XJHFZ), Jinshajiang Fault Zone (JSJFZ), and Red River Fault Zone (RRFZ). Most of the large earthquakes in the Sichuan-Yunnan region occur in and around these fault zones. Furthermore, these fault zones act as boundaries between the main tectonic blocks of this region. As a result, the region as a whole is under large-scale shear deformation, resulting in complex patterns of block motions involving translation, rotation and extrusion. For example, the Sichuan-Yunnan Rhombic Block (Figure 1) is moving in south-southeast direction with clockwise rotation and extrusion (Xu et al., 2003). The tectonic environment in this region is thus complex and requires continuous efforts to gather information from various disciplines, including and in particular seismology, to resolve the three-dimensional (3-D) structures at depth in order to understand the evolution and interaction of different tectonic blocks.
Previous studies have shown that the crust and upper mantle in the Sichuan-Yunnan region are generally characterized by specific influences that can be attributed to the Indian and Eurasian plate collision, such as the wide-spread low-velocity (e.g., Kan et al., 1986; Wang et al., 2003; Wang et al., 2016) and high-attenuation zones (e.g., Wei and Zhao, 2019; Zhao et al., 2013), large variation of crustal thickness (Wang et al., 2010), and the existence of a layer of high heat flow and high conductivity (Bai et al., 2010). Large-scale surface deformation with an obvious clockwise rotation can be seen in GPS observations in this region (e.g., Wang et al., 2001; Zhang et al., 2004; Wang and Sheng, 2020; Wang et al., 2021). However, various aspects of the movement of the deep crust and uppermost mantle, including the depth extent of the surface deformation, the existence and pattern of the lower crustal flow, and the nature of the interaction between the lower crust and uppermost mantle, still remain to be resolved.
With the increased deployment of digital broadband seismometers at the turn of the century, the Sichuan-Yunnan region has been the focus of a number of seismic studies (e.g., Zhang and Wang, 2009; Yao et al., 2010; Lu et al., 2013; Zhao et al., 2013; Chen et al., 2014; Liu et al., 2014; Bao et al., 2015; Legendre et al., 2015; Wei and Zhao, 2019). Based on the lithospheric P-wave velocity structure obtained from wide-angle seismic profiles, Zhang and Wang (2009) suggested that the surface deformation in Yunnan is decoupled from the lower crust flow, which is consistent with Royden et al. (1997) which, via geodynamic simulation, indicated that the lower crust in this region is weak enough to allow the upper crustal deformation to be decoupled from the materials below. Lu et al. (2013) found distinct velocity differences between the southern and northern parts of the uppermost mantle in the Sichuan-Yunnan region from 2.5-dimensional inversions of the P- and S-wave velocity structures. Chen et al. (2016) reported a weak lower crust which allows for material flow there and revealed a complex crust-mantle coupling mechanism in the Yunnan region based on a 3-D azimuthally anisotropic S-wave velocity model constructed from teleseismic surface waves. Using ambient noise as well as teleseismic body and surface waves, Yang et al. (2020) obtained a crustal S-wave velocity model with a best horizontal resolution of 0.5°. They found widespread low-velocity zones in the mid-lower crust in the Sichuan-Yunnan region, and that the boundaries of the low-velocity zones correlate well with major fault systems. The seismic velocity models of the crust and uppermost mantle obtained in different studies are generally in good agreement in large-scale features, such as the high-velocity anomaly beneath the stable Sichuan Basin. However, models with higher resolution are necessary to confirm the existence of material flow in the lower crust, and to understand the interactions between the lower crust and uppermost mantle, which is still not well resolved by the existing seismic experiments in the region.