hidden plate tectonics

--> Fun geology term for the day - hidden plate tectonics

The Tibetan plateau is the largest topographic feature on Earth and is related to the ongoing collision between India and Eurasia that began some 55 million years ago. Construction of the plateau began earlier with the accretion of ocean island crustal blocks to the Eurasian continent in the Jurassic, although the extreme topography likely wasn’t created until the Cenozoic following the collision with India (see my earlier post about the debate over the timing of uplift). How and when the plateau crust became thickened and elevated is a major debate in geology. Tapponnier et al. (2001) suggest that the collision with India resulted in intracontinental subduction (“hidden plate tectonics”) of mantle lithosphere of the previously accreted terranes.

Image from figure 3 from Tapponier et al. (2001). The authors caution that the proposed continental subduction has not yet been imaged geophysically. 

Tapponier et al. (2001) suggest that the terranes act as coherent blocks, contradicting other ideas of plateau growth that suggest the Tibetan mantle lithosphere behaves more fluidly. They challenge an earlier hypothesis put forth by mainly by Philip England and Gregory Houseman of a “soft Tibet” that involves: a) the entire lithosphere thickening as a viscous sheet, b) eventual removal and sinking of this dense lithosphere into the asthenosphere, c) subsequent buoyant rise, and extension of the  plateau.

Image from figure 9 of Molnar et al. (1993) showing convective removal of the Tibetan lithosphere.

Another simple model of lithospheric removal beneath Tibet. From Harrison et al., 1992

The debate often centers on determining of the timing of plateau thickening vs. uplift and in the interpretation of the Late Miocene to recent faulting that appears to be forcing the plateau crust eastward. Tapponier et al. (2001) suggest that the observed extension by normal faulting due to gravitational collapse is negligible, and that the lateral extension of the plateau is largely occurring along strike slip faults, which are more in line with the continental subduction hypothesis. How the plateau is deforming has strong implications on the causes for the volumetrically minor, but widespread potassic volcanism on the plateau. Tapponnier et al. further argue that the post-collisional volcanism is localized in three different belts that line up well with the continental subduction model, and not so much with wholesale removal of the lithospheric mantle. How continental subduction promotes melting is not addressed in this paper, but later researchers do and will be discussed here shortly.    

Citations: 

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T. M. Harrison, P. Copeland, W. S. F. Kidd, A. Yin, Science 255, 1663 (1992).

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.

Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Yang, J.S., 2001, Oblique stepwise rise and growth of the Tibet plateau: Science, v. 294, p. 1671–1677.