Comment on “Restoration of Cenozoic deformation in Asia and the size of Greater India” by D. J. J. van Hinsbergen et al.

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[1] Despite decades of investigation and literally thousands of research papers, there remain strongly divergent opinions on a number of key features/phenomena associated with the India-Asia collision system and the Himalayan MountainTibetan Plateau orogen that resulted. One particularly contentious issue concerns the timing of contact between the two continents; although many favor an early Eocene/ early–middle Eocene age, recent proposals have ranged from 65 Ma [e.g., Cai et al., 2011] to 35 Ma [Aitchison et al., 2007]. [2] In order to deduce the collision timing, we argue that detailed understanding is required first of the elements associated with the suture zone, and second, the broader geodynamic configuration before and during the contact. Regarding the former, the following are critical: (i) age of the youngest marine sediments; (ii) age of the youngest subduction-generated calc-alkaline volcanic rocks; (iii) identification and full interpretation of any trapped terranes; (iv) age and clast-assemblage lithologies of any molasse formations; (v) ages of the youngest entrained blocks in any melange deposits plus that of the matrix. From a platemodeling perspective, we need to know (i) past position of stable Eurasia; (ii) extent of deformation (shortening, extrusion etc) Eurasia’s leading edge experienced after the contact; (iii) motion path of the Indian block; (iv) size of the Indian sub-continent prior to the collision, which is commonly referred to as “Greater India.” [3] Recently, Van Hinsbergen et al. [2011a, 2011b] have proposed a new collision scenario. In their model, contact is interpreted to have taken place at 50 Ma, and following a structural restoration of Asia’s southern margin, a subcontinent extension is proposed that bridges the gap between the Indian craton (to the south of the Himalayan frontal thrust) and the Lhasa Block; 1350 km in the west, 2600 km in the east. By corollary, a huge amount of lithosphere must therefore have been subducted/underthrust beneath the Asian margin in Tibet. The model has provoked our interest, not only because the hypothesized Greater India is the largest so far presented [see Ali and Aitchison, 2005], but we consider it to suffer from two “unintendedconsequence” flaws.