Gravitational instability of thickened yet compositionally buoyant Tibetan mantle lithosphere

The Tibetan plateau, one of the major topographic features of the Earth, presents many unanswered questions regarding its evolution, present structure, and continuing geological activity. The extensive crustal thickening that is attributed to continental convergence between India and Eurasia sustained for the past 55 Myr, and continuing today, is underlain by an upper mantle in which relatively fast shear-wave speeds are measured to depths of 200 km and more. Although the high elevation of the plateau can be attributed mainly to the thickness of the crust, the observation that the plateau is now extending is evidence that the lithosphere has increased its gravitational potential energy since it was thickened. Extensive volcanism during the current continental collision additionally implies that the lithosphere and/or uppermost mantle of Tibet was re-heated. Replacement of mantle lithosphere by asthenosphere in a convective thinning process has been advanced as an explanation of that re-heating, but that process has been difficult to reconcile with the high shear-wave velocities measured in the upper mantle beneath the plateau. However, a model based on a restricted overturn of a depleted, metasomatised, mantle lithosphere in the upper 200 km of the mantle can reconcile these apparently contradictory observations. Such overturn can explain how the observed distribution of seismic velocities has been produced, how heat is injected into the lithosphere producing transient uplift, and how mantle-derived melts can rapidly reach the base of the crust in widely distributed locations across the plateau.