Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

Flow of weak lower crust has been invoked to reconcile observed topographic gradients, uniform elevations, slow seismic velocity, and high conductivity measured in the Tibetan Plateau, with viscosity estimates of 10(16)-10(21) Pa.s. Here we investigate the dynamic response resulting from a range of lower crust viscosities in a 3-D lithospheric-scale geodynamic simulation of the India-Eurasia collision zone to determine bounds of physically viable lower crustal strengths. We show that thickening of the plateau is accommodated through viscous buckling of the upper crust in response to lower crustal flow for a lower crustal viscosity on the order of 10(20) Pa.s. This generates two east-west trending bands of surface subsidence and dilatation consistent with observed normal faulting and estimates of vertical velocity. These results suggest viscous buckling of the upper crust, induced by lower crustal flow from gravitational pressure gradients due to high topography, is responsible for the observed extension in Tibet.