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

Abstract

On the assumption that the GPS（Global Positioning
System）velocity field is affected by the interseismic elastic
deformation of the upper crust to a great degree, under the
constraint of the existing GPS data, by the numerical simulation
method we construct the viscoelastic mechanical models to
analyse the rationality of the explanation to the Tibetan
present-day crustal movement by the two different deformation
mechanisms in the deep continental lithosphere, such as the
viscous flow in the lower crust and the shear strain localization
in the deep fault zones, in this paper. Numerical experiments
show that the present-day crustal movement in the different
regions of the Tibetan plateau may be attributed to the different
geodynamic mechanisms. In the Tibetan southeastern area,
although the shear strain localization in the deep fault zones
could not be rejected, the viscosity of the lower crust should be
much lower in order to diminish the model’s prediction error. In
the central-northern and northeastern areas of the Tibetan
plateau, the GPS velocity field is interpreted much better by the
shear 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 preferred model by which the GPS velocity field could be
accounted for with a first-order similarity yields an estimate of
1022—5×1022Pa·s for the mean viscosity of the Tibetan lower
crust, and an estimate of about 1021Pa·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 on the
present-day movement of the Tibetan plateau.