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.