LAUSR

The plastic flow in the Sichuan–Yunnan fragment (SYF) is the consequence of the eastern expansion of the Tibetan Plateau. The SYF area can be divided into three sections by the Xiaojin River fault zone and Red River fault zone, which both contain the normal fault component, consistent with GPS observations. The average vertical rates of the three sections exhibit a step-like distribution, and the southern subsidence rate is about 1–2 mm/year with respect to the northern area. To explain the vertical movement in the SYF, we subtract the vertical rates caused by the horizontal movement from the observed vertical rates and then use a two-layer viscous model to constrain the viscosities of the crust and upper mantle in the SYF. The fitting results between the vertical observed and model data show that the viscosity of the upper mantle is about 2.8 × 1022 Pa s and the viscosity of the crust is less than 1.0 × 1025 Pa s. Furthermore, we find that the vertical tectonic stresses dominate the driving mechanism for the vertical movement in the SYF.