Introduction
The Indian continental lithosphere is distinguished by spectacular boundaries: the crown of the majestic Himalaya on the north and two Passive continental margins that taper it southward into a peninsula. It is surfaced entirely by a shield, itself a mosaic of Archaean cratons and intervening mobile belts that are exposed over much of the continent except for tracts of sedimentary covers of which the most notable is the Himalayan foredeep. The shield that was mostly amalgamated by early Proterozoic, had apparently remained largely stable up till the Eocene continental collision, except for a few regional extensions resulting in the formation of basins and rifts: the Cuddappah in the eastern Dharwar craton and Bhima & Kaladagi in the western, in addition to the perhaps younger east coast inter-cratonic Godavari and Mahanadi rift basins. More recently, ~50 Ma ago, upon the arrival of its leading continental edge face to face with the Eurasian, the Indian plate, apparently more stolid, underthrust the latter, creating the world’s largest high altitude plateau and its southern Himalayan rim. The flexed shield on their southern front was, in time, filled with the material eroded from the rising Himalaya to become a foreland basin superficially intervening the shield and the mountains, except in the southeast where it opens out into the Bengal basin abutting the northern Bay of Bengal. Apart from its flexure, the Indian shield also remained largely undeformed south of the collision boundary, requiring the existence underneath of an ultra strong cratonic core. The shield’s outlying limb in the northeast, south of the Brhmaputra valley, was of course fractured along mantle reaching faults (Bilham et al., 2001 and Mitra et al. 2005) and up-thrust to become the uncompensated Shillong plateau. However, this process not enacted elsewhere along the arc, could have been the result of a much lesser width of the shield in this region between the collision front and the strong oceanic lithosphere of the Bay of Bengal.
An outstanding question being addressed by earth scientists today begs the understanding as to how continental lithospheres formed in the early earth and, in particular, how did they come to acquire their distinctively processed geochemical and rheological structure from those of the primitive earth, which would guarantee an extraordinary immunity to disruption despite a vigorously convecting mantle. For, a significant proportion of those created by ~3,000 Ma have managed to survive being dragged down the mantle, through the vicissitudes of the earth’s turbulent history, to constitute at least 14% of their total expanse (Rudnick, 1995) visible on earth today. An interesting speculation stimulated by the new knowledge of the deep structure of Tibet (Priestley et al., 2006), that it represents an early stage in the evolution of cratonic lithospheres, opens up many interesting possibilities of testing this hypothesis by interrogating potentially responsive tracts of continental lithosphere. The Indian continental lithosphere which is a collage of several cratons occupying over 10% of the global cratonic lithospheres, offers particularly instructive possibilities to explore this idea.