Several times in Earth Pages News the topic of how erosion contributes to uplift has cropped up. That is more than just the iceberg-like bobbing up of the crust as the load on the underlying asthenosphere is eased by surface rock removal. One oddity is that as large valleys are carved the ridges and peaks that they separate can rise higher than the original lands surface from which they developed (see Erosion and plate tectonics in May 2005 issue of EPN). Now it is becoming clear that sideways movement of the crust beneath mountain ranges can also be a response to erosion; thrusts and nappes can respond to erosion as well as to plate tectonic forces. The most likely place where this might be happening is in the Himalaya, which produce a huge contrast in climate and erosion rate between their southern and northern sides by creating the world’s largest rain shadow. The evidence for this possibility is nicely reviewed by Kip Hodges of Arizona Sate University (Hodges, K. 2006. Climate and the evolution of mountains. Scientific American, v. 295 (August 2006 issue), p. 54-61).

The highest erosion rates take place where rainfall during the Indian monsoon is greatest, on the SSW face of the Himalaya, especially in the foothills between about 1000 and 3500 m. The Tibetan Plateau lies in the rain shadow of the Himalaya, and erosion is far less intense. Yet the Tibetan plateau is buoyed up by crust that is double the normal thickness, to an average elevation of around 5 km. In a crude way Tibet can be regarded as having a pressure head ‘dammed’ to the north of the Himalaya. Intense erosion at the foot of the mountain ‘dam’ is likewise akin to one cause of landslides: erosion of the toe of a slope. The gravitational potential of Tibet, combined with continual undermining of the Himalayan front must create a lateral force. Where the crust is able to behave in a plastic fashion, i.e. at depth, and if there are surfaces on which movement is possible — the north-dipping frontal thrusts of the Himalaya — then deep crust should be extruded sideways. In fact there are faults systems just to the north of the Himalaya that have the same dip as the thrusts, but an opposite sense of movement, directed northwards to create extensional detachments. The crustal zone in-between is the most likely to undergo extrusion. GPS measurements there and cosmogenic dating of the surface reveal that indeed this zone is experiencing  anomalously high rates of uplift. It is producing extremely high gradients on both hillsides and valley floors.