Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain,India, from Isostatic Considerations |
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| Authors: | Niraj Kumar A P Singh B Singh |
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| Institution: | (1) National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad, 500007, India; |
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| Abstract: | The Southern Granulite Terrain of India, formed through an ancient continental collision and uplift of the earth’s surface,
was accompanied by thickening of the crust. Once the active tectonism ceased, the buoyancy of these deep crustal roots must
have supported the Nilgiri and Palani-Cardamom hills. Here, the gravity field has been utilized to provide new constraints
on how the force of buoyancy maintains the state of isostasy in the Southern Granulite Terrain. Isostatic calculations show
that the seismically derived crustal thickness of 43–44 km in the Southern Granulite Terrain is on average 7–8 km more than
that required to isostatically balance the present-day topography. This difference cannot be solely explained applying a constant
shift in the mean sea level crustal thickness of 32 km. The isostatic analysis thus indicates that the current topography
of the Southern Granulite Terrain is overcompensated, and about 1.0 km of the topographic load must have been eroded from
this region without any isostatic readjustment. The observed gravity anomaly, an order of magnitude lower than that expected
(−125 mGal), however, shows that there is no such overcompensation. Thermal perturbations up to Pan-African, present-day high
mantle heat flow and low Te together negate the possible resistance of the lithosphere to rebound in response to erosional
unloading. To isostatically compensate the crustal root, compatible to seismic Moho, a band of high density (2,930 kg m−3) in the lower crust and low density (3,210 kg m−3) in the lithospheric mantle below the Southern Granulite Terrain is needed. A relatively denser crust due to two distinct
episodes of metamorphic phase transitions at 2.5 Ga and 550 Ma and highly mobilized upper mantle during Pan-African thermal
perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography. |
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