Two-Mode Growth of the Tibetan Plateau

Abstract:

Previous models of continental collision predict that Tibetan Plateau has grown progressively northward following the Indo-Asian collision since ~60-50 Ma. Increasing evidence of crustal shortening and uplift in northern Tibet soon after the collision, however, challenges these models and thus our current understanding of the fundamental mechanics of continental collision. We show that these discrepancies may be reconciled in a viscoelasto-plastic geodynamic model that incorporates the vertically variable rheology of the continental lithosphere. Our results show that, upon the Indo-Asian collision, stress transmits instantly across the brittle upper crust to cause localized shorting and uplift over preexisting fault zones, including those in northern Tibet. Broad plateau uplift, however, results from crustal thickening, which occurs mainly in the viscous lower crust and propagates gradually northward. Such two-mode deformation of the Tibetan lithosphere explains both early crustal shortening in northern Tibet and the progressive northward growth of the Tibetan Plateau; the results provides a new framework for understanding the broad impacts of the Indo-Asian collision.