Gravity anomalies, flexure of the Indian Plate, and the structure, support and evolution of the Himalaya and Ganga Basin

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Abstract
Bouguer gravity anomalies along four profiles across the Western Himalaya and Ganga Basin show large deviations from local isostatic equilibrium. A deficit of mass characterizes the Ganga Basin, and an excess underlies the Lesser Himalaya. Both can be understood if the Indian plate is flexed down by the distributed load of part of the mountains. The cross sectional shape of the Ganga Basin seems to be controlled by the deflection of the Indian plate, which we compute assuming the Indian plate to overlie an inviscid fluid. From the shapes of both Bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate. If the Ganga Basin is a steady state feature, then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin. If the rate of convergence of India and the Himalaya were constant, that rate should equal the distance divided by the corresponding age. We find a rate of 10 to 15 mm/a for the last 15 to 20 Ma, which is consistent with a large part of the 50 mm/a rate of convergence between India and Eurasia being absorbed by the eastward extrusion of parts of Tibet. Profiles of Bouguer gravity anomalies show only a small peak or plateau over the southern edge of the Lesser Himalaya, implying that the boundary between the light sediments of the Ganga Basin and the heavier crustal rocks of the Lesser Himalaya is not sharp and that there exists some light material beneath the range. We infer that some sediment deposited in the Ganga Basin has been underthrust beneath the Lesser Himalaya, but the quantity is small; most of this sediment probably is scraped off the Indian plate to make the foothills of the range. We find that the load of the High Himalaya is too large to be supported solely by elastic stress in the Indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate. The observed gradient in Bouguer gravity anomalies increases from an average of about 1 mGal/km over the Ganga Basin and Lesser Himalaya to about 2 mGal/km over the High Himalaya. This increase in the gravity gradient implies that the Moho dips more steeply (10°–15°) beneath the High Himalaya than beneath the Lesser Himalaya (2°–3°). We interpret this steepening of the Moho to be due to a weakening of the plate, which allows it to bend more sharply beneath the High Himalaya than farther south. With the inclusion of a weak segment of Indian plate beneath the High Himalaya, calculated anomalies show a somewhat increased gradient beneath the High Himalaya, but when the weight of the entire Himalaya is used as a load on the plate, calculated anomalies are more negative than observed. Therefore an external force system is needed to support much of the weight of the High Himalaya, as well as to bend the plate sufficiently beneath the High Himalaya. The magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of India's mantle lithosphere, stripped of its crust and underthrust beneath southern Tibet.