Abstract:

Assuming that the GPS velocity field is greatly affected by the interseismic elastic deformation of the upper crust, under the constraints of the existing GPS data, and referring to the elevation data and the NUVEL1A model of global plate movement, we have constructed the viscoelastic mechanical models to analyze the rationality of the explanation to the Tibetan presentday crustal movement by two different deformation mechanisms in the deep continental lithosphere, including the viscous flow in the lower crust and the ductile strain localization in the deep fault zones. Numerical experiments show that the presentday crustal movement in different regions of the Tibet may be attributed to different geodynamic mechanisms. In the Tibetan southeast area, although the lower crustal ductile strain localization could not be excluded, the viscosity of the lower crust should be much lower in order to reduce the model’s prediction error. In the middlenorth and northeast areas of the Tibet, the GPS velocity field is interpreted much better by the ductile strain localization in the deep fault zones, implying that the actual active faults may incise down deeply in these areas. Ignoring the lateral change of the viscosity in the lower crust, the model whose prediction of the movement has a firstorder similarity with the GPS data yields an estimate of (15)×1022 Pa·s for the average viscosity of the Tibetan lower crust, and about 1021 Pa·s for that of the deep fault zones. Due to the high elevation and the lower viscosity of the lower crust, the gravity plays an important role in the presentday movement of the Tibetan plateau.

Key words: Tibetan plateau, GPS data, viscous flow in the lower crust, ductile strain localization in deep fault zones, numerical models