Shortening budgets and the role of continental subduction during the India–Asia collision

The India–Asia collision provides a remarkable example of diffused deformation because the zone of compressive strain extends far into the Hinterland of the Himalaya. Molnar and Tapponnierʹs [Science 189 (1975) 419] concept of India as a rigid indenter responsible for the diffused deformation has been widely accepted, perhaps uncritically. This review gives a reappraisal of landmark papers by Dewey et al. [Philos. Trans. R. Soc. London, A 327 (1988) 379; Eclogae Geol. Helv. 82 (1989) 717] which discussed the shortening budget in SE Asia and the evidence for the amount and the rate of the convergence between India and Asia taking place after the initial collision of these continents at roughly 55–50 Ma. Evidence for the total convergence comes from magnetic anomaly, palaeomagnetic and volumetric balancing studies. The results are consistent in showing about 1800–2100 km convergence between India and Asia in the western sector, 2475 km in the central and 2750–2800 km in the eastern sector. The rate of convergence between India and Asia over the period since initial collision is roughly 5 cm/year. However, the possible long-term rate of convergence within the Himalaya is considerably less, i.e. 1.5–2.0 cm/year, a fraction of the total convergence. This shortfall can be made up by adding to the rate for the Himalaya alone the convergence rates in other parts of the diffused zone of compressive strain, i.e. Tibet, Tien Shan, Altai. This works well for the post-Oligocene but not for the earlier post-collisional history when the areal extent of thrust/fold tectonics was much more limited. Possibly, the apparent shortening deficit before the Miocene was made up by strike–slip faulting but with the notable exception of the Red River Fault evidence for widespread pre-Miocene strike–slip faults is not convincing. ion of the amount of shortening in thrust/fold regimes in the Himalaya and its Hinterland falls short of that required by the total convergence figures quoted above. The deficit arises firstly because of the likelihood that pre-collisional crustal shortening (of uncertain areal extent) in Tibet reduces Tibetʹs contribution (>40% according to Dewey et al. [Philos. Trans. R. Soc. London, A 327 (1988) 379; Eclogae Geol. Helv. 82 (1989) 717]) to the shortening budget for the Cenezoic. Revised shortening budgets are presented on the assumption that most of the Lhasa Block (Southern Tibet) was already thickened prior to collision: this thickening is difficult to quantify. Secondly, balanced cross-sections from various parts of the Himalaya give 60–70% shortening, that is only about 500 km, roughly half of the shortening proposed by Dewey et al. The conclusion of Le Pichon et al. [Tectonics 11 (1992) 1085] that only Indian mid/upper crust is involved in the Himalayan thrust sheets is supported but it needs to be reassessed in the light of the recognition of Eocene–Oligocene thickening in the Himalaya. This early crustal thickening antedates the sequence of thrusts which started with the Main Central thrust and therefore the shortening estimates based on balanced cross-sections probably underestimate the total shortening in the Himalaya. The prospect of substantial underthrusting of India beneath Tibet minimizes the amount of horizontal shortening of Tibetan lithosphere during the Cenozoic. The conclusion is that either the total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening mechanisms other than thrust/fold regimes, i.e. strike–slip faulting which serves to move rocks out of the way of India.