Injection of Indian crust into Tibetan lower crust: A two‐dimensional finite element model study

Abstract

We propose a model in which the stronger Indian crust injects into the weaker lower crust of Tibet. If the viscosity of the Tibetan lower crust is below a critical value, the continuous advancing of the Indian crust beneath Tibet would produce negligible variation in stresses on the bottom of the stronger brittle Tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed Tibetan Plateau surface. This process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber. We find that if the viscosity of the Tibetan lower crust is assumed to be constant throughout, the computed critical viscosity is 6 × 10 18 Pa s for the present‐day lower crustal thickness of 58 km. In addition to uniform viscosity, two hypothetical depth‐dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust. Our model also interprets the 4‐km elevation contrast between the Tibetan Plateau and the Tarim block. A 1∶160 viscosity contrast between the Tibetan and Tarim lower crust is required in order to maintain the current thickening rate of the Tibetan crust (1.2 mm/yr) while keeping the thickness of the Tarim crust unchanged.