Numerical simulation of mid-lower crustal flow model in Sichuan-Yunnan constrained by GNSS observations

The Sichuan-Yunnan region, located at the southeastern margin of the Qinghai-Xizang Plateau, serves as a key channel for the southeastward extrusion of plateau material. The characteristics of crustal deformation and the mechanisms of deep material flow have been central topics of interest in geoscience research. In this work, a three-dimensional viscoelastic-plastic finite element model including the upper and mid-lower crust was established, constrained by GNSS horizontal crustal velocity observations and incorporating major active faults and geophysical survey data to explore the contribution of mid-lower crustal flow to surface deformation and its coupling with faults. Comparison of modeling experiments shows that relying solely on boundary loading or uniform layering assumptions fails to reproduce the GNSS observed velocities. We introduce a mid-lower crustal low-velocity weak zone, derived from the latest seismic velocity structure models. The new model improves the fit to GNSS observations. Tests of different viscosity coefficients in the low-velocity zone indicate an optimal viscosity range of 7.5 × 10¹⁹–1 × 10²⁰ Pa·s. Vertical profiles reveal that mid-lower crustal material motion is mainly concentrated at depths of 20–40 km, forming localized channelized flow in low-velocity zone with a typical Poiseuille velocity profile which indicates a ductile, fluid-like behavior with the low-velocity zone serving as primary pathways for deep material transport. The results further show that under the geometric constraints of upper-crustal faults, the mid-lower crustal flow contributes approximately 1–3 mm/a to surface deformation, primarily concentrated along major faults. This indicates that faults play a key role in constraining and modulating the transmission of deep-seated dynamics to shallow surface deformation. However, the contribution of mid-lower crustal flow is also significant; neglecting its influence on surface deformation would lead to an incomplete understanding of the deformation pattern and bias the interpretation of block boundaries and crustal kinematic segmentation.