Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain, India, from Isostatic Considerations

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.

The South India Precambrian crust and shallow lithospheric mantle: Initial results from the India Deep Earth Imaging Experiment (INDEX)

“…Receiver function modelling from two stations in the SGT shows crustal thickness and Vp/Vs ratio variation of 43.5 ± 0.7 km and 1.753 ± 0.015 for station KOD and 36.5 ± 0.9 km and 1.722 ± 0.017 for station TRV that correspond to felsic to intermediate composition respectively (Rai et al 2008). Modelling of gravity field study also shows Moho depth variation of 33-48 km in SGT with the maximum of 53 km beneath the Nilgiri hills (Kumar et al 2011).…”

Crustal density structure across the Central Indian Shear Zone from gravity data

“…It would now be imperative to search for imprints of major tectonic event that made the upper mantle comparatively weak. Recently, a similar upper mantle density beneath the Southern Granulite Terrain is hypothesized as a highly remobilised lithosphere due to crustal delamination (Singh et al, 2006, in press;Niraj Kumar et al, 2009a,b, 2010. Chemical stratification of the Indian lithospheric mantle with the upper lithosphere (up to 80 km) comprising of the spinel peridotite (of lower density) and the lower lithosphere with that of garnet peridotite composition (of higher density) may be the other distinct possibility (Jagadeesh and Rai, 2008).…”

Three‐dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

“…To the south below MB and ACSZ, the moho was taken from velocity model by Behera [2011]. Bouguer gravity anomaly [Kumar et al, 2010] are plotted along a profile from north to south. The conductors C1-C4 and resistors R1-R4 are explained in the text.…”