Low-lying Tibet before India-Asia collision

The Tibetan plateau lies between the Himalayan...
The semi-arid Tibetan Plateau from spaceImage via Wikipedia

The vast Tibetan Plateau at an average elevation of 4500 m is a major influence on the climate of Asia, being central to the annual monsoons, as well as one the world’s largest continental tectonic features. When it formed is crucial in palaeoclimatic modelling as well as to geomorphologists and structural geologists. Whether or not it was present before the Indian subcontinent collided with Asia at 50 Ma has been the subject of perennial debate; it could have formed during the more or less continual accretion of terranes to southern Eurasia since the Jurassic Period. A novel approach to timing uplift of Tibet is obviously needed to resolve the controversies, and that may have been achieved (Hetzel, R. et al. 2011. Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift.  Geology, v.39, p.983-986). North of Lhasa is an area of coincident small plateaus at around 5200-5400 m into which are cut valleys a few hundred metres. It has the hallmarks of a peneplain stripped to the base level of erosion, and developed on Cretaceous granites. The German-Chinese-South African team applied a range of geochronological techniques to date the emplacement of the granites and their cooling history. U/Pb dating shows the granites to have crystallised between 120 to 110 Ma; U-Th/He dating of zircons in them indicate their cooling from 180° to 60°C between 90 and 70 Ma; apatite  U-Th/He and fission-track dating show that the granites experienced surface temperatures by around 55 Ma during a period of erosion at a rate of 200-400 m Ma-1. The clear inference is that an area >10 000 km2 became a peneplain by the end of the Palaeocene, to be unconformably overlain by Eocene continental redbeds.

By the Eocene the northern Lhasa Block had become a low-elevation plain from which a vast amount of sediment had been removed to be deposited elsewhere – Palaeocene and Eocene sediments are not common throughout the whole Tibetan Plateau. This is strong evidence that uplift of the Plateau only began after the India-Asia collision during the Eocene. Despite that and the erosion that would have taken place, much of the peneplain remains; given resistant bedrock peneplains can be very long-lived.

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