orogeny and volcanism in Tibet

Fun geologic relationship of the day: orogenic plateaus and volcanic triggers

The processes occurring beneath the world’s great plateaus such as Tibet and the central Andes are difficult to decipher. How the deep crust and mantle lithosphere are behaving, and how they interact with the underlying lithosphere has major implications for how the plateaus grow and why they contain widespread volcanism. In one of the first major studies of volcanism in Tibetan, Turner et al. (1996) interpreted highly potassic lavas to indicate melting of biotite-rich, garnet bearing mantle lithosphere that had been metasomatized (enriched in volatiles and hydrous fluids) during earlier (pre-collisional) periods of subduction. Evidence includes negative Nb-Ta and Ti anomalies, fractionated LREE/HREE, high K contents, and isotopic considerations. Turner et al. (1996) suggest that the hydrated mantle lithosphere most likely melted due to temperature increases related to convective removal of its lower regions, as proposed by England and Houseman (1988, 1989). In this model, the collision between India and Eurasia (beginning ~52 million years ago) thickened the cold, relatively dense lower mantle lithosphere until it detached and was replaced by warm, buoyant asthenosphere. This influx of asthenosphere then raised the plateau, and led to gravitational collapse of its edges. Turner et al. proposed that the newly exposed (undetached) mantle lithosphere heated up and melted, resulting in the plateau volcanism. They further suggest that the age of the lavas (< 13 Ma) puts a time stamp on the removal process, as shown in this figure. 

 Figure 13 of Turner et al. (1996).

The timing of plateau uplift has major implications concerning the evolution of our atmosphere and the life it harbors, as a rising plateau may lead to increased global cooling. However, it is very difficult to pin down the dates of plateau uplift (it does not necessarily correlate with crustal thickening alone) and is the focus of numerous studies.  

An alternatives to the convecting thinning / mantle lithosphere melting model discussed in this post was addressed on this blog earlier, and el volcán tranquilo will, no doubt, return to this controversy repeatedly.  

References

Turner, S., Arnaud, N., Liu, J., Rogers, N., Hawkesworth, C.,Harris, N., Kelley, S., Van Calsteren, P. & Deng, W. (1996). Post-collision, shoshonitic volcanism on the Tibetan plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts. Journal of Petrology 27(1), 45--71.

Molnar, P., P. England, and J. Martinod, Mantle dynamics, the uplift of the Tibetan Plateau, and the Indian monsoon, Reviews of Geophysics, 31, 357-396, 1993.

England, P. C. & Houseman, G. A.; 1988. The mechanics of the Tibetan plateau. Philosophical Transactions of the Royal Society of London A326, 301-319.

England, P. C. & Houseman, G. A., 1989. Extension during continental convergence, with application to the Tibetan Plateau. Journal of Geophysical Research 94, 17561-17579.