Tibet uplift and erosion

Abstract

The 5-km-high Tibetan plateau is an outstanding topographic feature on the Earth today. Its horziontal extent, elevation, and location cause significant effects on modern atmospheric circulation and climate, so the history of uplift of the surface of Tibet is linked to Cenozoic climate changes, at local, regional, and perhaps global scales. Geological and geophysical studies of the plateau are contributing data on the present and past deformation of the Tibetan lithosphere that has formed the plateau, primarily during the Cenozoic. The principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history. Different parts of Tibet probably had different uplift histories, but presently available data are not sufficient for distinguishing many contrasts. In one scenario, Cretaceous and Early Cenozoic north-south distributed shortening of Tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early Miocene. Thinning of the high-density mantle portion of the lithosphere during the Miocene may then have allowed the thick Tibetan crust to rise close to its present elevation (perhaps higher) before ∼8 Ma. Since then, slow east-west extension of Tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late Cenozoic.Erosion of Tibet, unlike narrow mountain belts, has been unable to match the uplift of a broad plateau. Orographic precipitation and efficient river networks concentrate erosion on the edges, while the interior is protected from significant erosion despite its lofty elevation. The southern edge of the plateau, the Himalaya, has suffered a minimum of 25 km of denudation since the Miocene, while central Tibet shows little or no sign of major erosion since that time. The Gangdese arc in southern Tibet was rapidly eroded during the mid-Miocene when >4 km of rock apparently were removed from the surface, as shown by mineral cooling ages. This pulse of erosion was probably caused by a combination of local thrust-system movement and changes in base level and precipitation due to relative elevation changes between the Gangdese and the Himalaya to the south. The modern long-wavelength flatness of Tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface.