Epilogue
The Indian continental lithosphere is an extraordinary feature of the globe. Its stolid crust continues to cleave through Tibet at 2 metres a century. Over the past 50 Ma of this longstanding process, south Asia has been thickened and raised into the largest high altitude plateau on the globe apparently resting on a large expanse of cold mantle below 100km, which would most likely survive as a cratonic core insulating the overlying remnants of the eroded plateau from the convecting mantle underneath. The continental Indian plate exposing 5 cratonic blocks separated by mobile belts, basins and a vast cover of Deccan basalts, in seismic images, is also supported at depth, by an extensive more or less continuous cratonic core stretching from the Himalayan arc southward to the Dharwar craton and further south, notwithstanding theirapparent absence in the east, most likely a result of sparser data coverage. In a future time these two extensive cratonic cores of India and Tibet would most likely fuse to once again create a large composite lithospheric unit, a testimony to their extraordinary encounter in the Cenozoic. Meanwhile, Tibet would continue to spread arc-normally over India as it does today, sliding over the Himalayan decoulement, and more haltingly in a stick-slip manner, on encountering the weight of the Great Himalaya, creating every few hundred years a great earthquake along some segment of the arc where accumulating strains have reached the failure limit.
The crustal structures of the various cratonic crusts, both exposed and buried, as gleaned from joint inversions of receiver function and surface wave dispersion data, allow one to create a perspective for insightful rooting of potentially more consequential new questions. The eastern and the western Dharwar cratons, both containing vestiges of early Archaean but stabilized by the late archaean, and separated by a ~50km wide transition zone between the narrow 400km long Closepet granite and the Chitradurg schists, represent the two extreme crustal types of the continent. The eastern Dharwar craton is a seismically transparent, classical Archaean crust with a thickness of ~32-38km and average shear velocity of 3.8km/s. The western, in contrast, except for sites near the coast, is thicker (48-52km), less felsic with a higher average shear wave speed of ~3.96km/s and has a significant intra-crustal layer at 10-20km depth below which the shear wave speeds are >4km/s, suggesting the operation of plate tectonic processes as early as the 3.3 Ga age of the overlying greenstones (Charlotte et al., 2009). Other cratons of the continent including the undercarriage of the Deccan basalts, have thicknesses and average shear velocities within and near the ranges characterizing the eastern Dharwar craton, except that those north of the CITZ are more felsic requiring mafic cleansing, most likely by forging the development of eclogites in their deeper roots and their subsequent loss by gravitational foundering. These results are broadly similar to those obtained for other cratonic and Proterozoic crusts in the world (Christensen and Mooney, 1995), supporting the basic hypotheses to explain the processes of melt depletion that created a buoyant crust resting over a depleted and therefore refractory protective upper mantle. However, the evolutionary history of individual cratons must vary in detail as to the extent of their material processing depending on the prevailing thermodynamic conditions at the time of their formation.
The foregoing account of the Indian continental lithosphere is not intended to be exhaustive and necessarily exposes a limited perspective that highlights the aspects considered significant by the author, particularly in the context of enquiry driven experiment design and investigations. Their results too have been summarily stated and further details of the studies would be found in the corresponding publications referred to in the text. Because of the limitations on space, the text has been largely devoted to what is considered germane to the larger picture rooted primarily in Archaean complexes, supported by more definitive results, albeit their severely limited representation and yet to be improved resolutions, at the expense of many other contributions that appeared tentative or merely confirmatory.
Pointed investigations to determine the deep structure of the Indian lithosphere have been woefully inadequate, partly because of the disproportionately small volume of earth science activity in the country compared for example to biological sciences. Hypothesis driven studies have occupied an even lesser space in preference to diagnostical which has of course greatly expanded in recent years with better availability of analytical systems. However, even these endeavors have shied away from the potentially more revealing earth archives of the Indian Archaean, notably the bastar and Singhbhum cratons Even factual data about their structural and metamorphic structures are too sparse for serious hypothesis formulations. Meanwhile, the proportion of data generation activity focused on ad hoc problems ending in superficial interpretation, to incisive analysis, remains overwhelmingly large.
Critical research issues, even their articulation, therefore remain in being without which one of the potentially most evocative lithospheres of the globe is likely to remain the least well understood. To take just one example, there is very little systematic work on mantle zenoliths to determine the geotherm within the Indian cratonic lithospheres, an insightful knowledge to spur hypotheses about their stabilization history.