Crustal stress field in Yunnan: implication for crust-mantle coupling

Abstract

We applied the gCAP algorithm to determine 239 focal mechanism solutions 3:0 M W 6:0 ð Þwith records of dense ChinArray stations deployed in Yunnan, and then inverted 686 focal mechanisms (including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum (r 1 ) and minimum (r 3 ) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations (i.e., maximum and minimum horizontal compressive stress axes: S H and S h , respectively) accordingly and found an abrupt change near *26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress (i.e., maximum strain axis) S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further to NE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.

Strain Accommodation in the Daliangshan Mountain Area, Southeastern Margin of the Tibetan Plateau

“…The combined solution uncertainties should be kept in mind because of framework alignment issues, which may cause data incompatibility and artificial distortion (deformation) within the block. We note that the pattern (e.g., the direction of compressional and extensional components) of estimated permanent strain rate in block CD (Figure ) is consistent with strain rate pattern estimated by interseismic GNSS velocity (e.g., Figure 7e of Kreemer et al, ) and strain pattern estimated by the focal mechanism of historical earthquakes (e.g., Figure 5b of Xu et al, ). Such consistency should exclude the framework alignment issue.…”

P Wave Anisotropic Tomography of the SE Tibetan Plateau: Evidence for the Crustal and Upper‐Mantle Deformations

“…However, the upper-crustal stress field in the SE Tibetan Plateau (Figure 16) inverted from focal mechanism solutions and global position system (GPS) observations shows a comparable pattern to the rotated FVDs in the deep crust and uppermost mantle (e.g., Xu et al, 2016;Zhao et al, 2013). The orientations of the maximal horizontal extension (Sh; Figure 16) change from roughly N-S near the Tibet-Sichuan boundary to NE-SW in the southwest Yangtze Craton, further to E-W in the northern Indochina Block, and finally to NW-SE in front of the eastern Himalayan syntax.…”

“…Thus, accompanying the thrusts in the upper crust, a significant portion of the shallow-crustal materials (or block) may sink into the deep crust due to the gravitational potential. This process also explains a locally extensional region in the high plateau to the northwest (Figure 17; Xu et al, 2016;Zhao et al, 2013). It could also produce notably negative radial anisotropy (V SH < V SV ) in the deep crust along the Tibet-Yangtze boundary (Figures 17 and 18; Xie et al, 2017).…”

“…Large earthquakes (M≥6.0) generally occurred to the northwest of a NE-SW Figure 17. The blue bars show the maximum horizontal stresses (SH) in the SE Tibetan Plateau (Xu et al, 2016). The yellow bars show the minimal horizontal stresses (Sh; i.e., maximum extension) in an extensional environment while the red bars show the Sh orientations in thrust and strike-slip environments.…”

Sharp Lateral Moho Variations Across the SE Tibetan Margin and Their Implications for Plateau Growth

“…Extensional basins developed widely in the Yangtze block of Yunnan Province (Y. Zhang & Li, 2016), suggesting a change in tectonic regime from earlier compression to later extension. This mechanism also explains the local extension in this region to the northwest (Figure 10) (Z. Huang et al, 2018; Z. Xu et al, 2016). We propose that the Moho gradient across the Tibetan margin was mostly established during the late Miocene extrusion‐related orogeny.…”