Viscous Lower Crust and Lower Crustal Shear Strain Localization in the Tibetan Plateau: From the Constraint of the GPS Velocity Field

On the assumption that the GPS（Global PositioningSystem）velocity field is affected by the interseismic elasticdeformation of the upper crust to a great degree, under theconstraint of the existing GPS data, by the numerical simulationmethod we construct the viscoelastic mechanical models toanalyse the rationality of the explanation to the Tibetanpresent-day crustal movement by the two different deformationmechanisms in the deep continental lithosphere, such as theviscous flow in the lower crust and the shear strain localizationin the deep fault zones, in this paper. Numerical experimentsshow that the present-day crustal movement in the differentregions of the Tibetan plateau may be attributed to the differentgeodynamic mechanisms. In the Tibetan southeastern area,although the shear strain localization in the deep fault zonescould not be rejected, the viscosity of the lower crust should bemuch lower in order to diminish the model’s prediction error. Inthe central-northern and northeastern areas of the Tibetanplateau, the GPS velocity field is interpreted much better by theshear strain localization in the deep fault zones, implying thatthe actual active faults may incise down deeply in these areas.Ignoring the lateral change of the viscosity in the lower crust,the preferred model by which the GPS velocity field could beaccounted for with a first-order similarity yields an estimate of1022—5×1022Pa·s for the mean viscosity of the Tibetan lowercrust, and an estimate of about 1021Pa·s for that of the deep faultzones. Due to the high elevation and the lower viscosity of thelower crust, the gravity plays an important role on thepresent-day movement of the Tibetan plateau