The mechanism of the present-day crustal deformation in southeast Tibet: from numerical modelling and geodetic observations

SUMMARY The mechanism of present-day crustal deformation in southeast Tibet remains controversial. 3-D high-precision geodetic data can provide significant clues to analyse the key driving forces. Here, we conduct a series of 3-D finite-element modelling to investigate the influences of gravitational collapse, tectonic extrusion and mid-to-lower crustal flow on crustal deformation in southeast Tibet. The numerical results show that the gravitational collapse leads to predominant N-S extension and surface subsidence in the northern region, and predominant NW-SE compression and uplift in the southern region, which can explain the normal-faulting earthquakes in the interior. The gravity-driven horizontal velocity depends on the upper crustal viscosity, while the vertical velocity is determined by mid-to-lower crustal viscosity. The eastward tectonic extrusion causes slight southeastward rotation and predominant E-W compression in the northern region but has a little effect on the deformation in the southern region. By considering the joint effects of gravitational collapse and tectonic extrusion, we simulate the crustal deformation that reconciles with present-day geodetic observations. Both the two driving forces lead to positive shear strain rates along the major fault zones, with more contributions from the tectonic extrusion of the Tibetan Plateau. Constrained by the 3-D geodetic observations, the numerical results argue against the presence of massive fast mid-to-lower crustal flow from the Tibetan Plateau. Overall, the present-day crustal deformation in southeast Tibet is jointly driven by gravitational collapse and tectonic extrusion, which play distinct roles in shaping the faulting kinematics and regional strain partitioning.