Discussion
Seismic characteristics of the crust of four high-grade granulite terranes of the south Indian shield: the Eastern Ghat, the Nilgiri-Madras, the Madurai, and the Kerala Khondalite belts (KKB) have been determined by jointly inverting the receiver functions and surface-wave dispersion data and compared with the structure of the Eastern and Western Dharwars. Converting the P-wave receiver functions in the H-σ domain show that the crusts beneath the Eastern Ghat, the western Nilgiri-Madras, and the Madurai belts are generally thicker by ~8 km compared with that of the Archaean crust in the eastern Dharwar Craton, even though the shield has gentle topography with a maximum relief of ~850 meters. Variations in the crustal thickness along traverse AB (Fig. 1) plotted with respect to elevation (Fig. 7b), show that the lower crust beneath the western Dharwar Craton, the western NMGB, and the EGGB, are not topographically compensated and must therefore, be denser and more mafic. This is further substantiated by the presence of higher shear wave velocity (3.85-4.15 km/s) beneath lower crust of western Dharwar and granulite terranes in contrast with the ~3.7 km/s shear wave speed beneath the eastern Dharwar. The crust beneath two of the coastal sites, TRV in the KKB and MDR the eastern NMGB, are thinner (36.5 and 38 km) compared with the crust beneath the coastal site VIZ (51 km) which lies north of MDR in the EGGB. Our analysis shows that the crust of the granulite terranes is not as seismically transparent as those of the eastern Dharwar Craton, as determined in earlier studies (Rai et al. 2003). In particular, the inverted models at these sites, require the incorporation of a shear wave velocity boundary at a depth of 20±2km, with a low shear wave velocity layer in the middle and upper crusts for of KOD, and VIZ. These low-velocity features at upper to mid-crustal depths could have been resulted from the influx of CO2 rich incipient fluids trapped at these depths, as noted in various studies in granulite terranes (Touret, 1995, Bolder-Schrijver et al. 2000; Santosh & Tsunogne, 2003, Friend and Nutman, 1992, Mohan et al. 1996). Retrograde metamorphism of granulites to amphibolite and greenschist facies, due to deep seated thrusting and lateral shearing during a transpressive regime (Tenczer et al. 2005), can also reduce seismic velocities at upper crustal depths. This indicates possibly, that an extensive crustal reworking has taken place during the upliftment and exhumation of the lower crust in these areas of the "stable" south Indian shield.
Comparison of the seismic characteristics of the south Indian granulite crust with those of the Archaean Dharwar crust, shows that while the granulites share significant similarities with the western Dharwars, notably their larger thickness that are distinctly different from the ~35 km-thick, seismically-transparent Archaean crust of the eastern Dharwar. Furthermore, both the granulite and western Dharwar crusts possess significant intra-crustal features and have higher shear velocities (3.85-4.15 km/s) in their lower crust, representing a more mafic average crustal composition. While these characteristics are understandable for the granulite crust, they are rather surprising for the western Dharwar, believed to have been stabilized by the end of the Archaean. Apparently, this stabilization was predated by significant magmatic invasion notwithstanding the inferred refractory nature of the base of Archaean crusts.
These observations when viewed in the geological perspective of the area, particularly the Pan-African affected transitional region from PCH to as far north as MTP which lies on the southern fringe of the Nilgiri massif apparently uplifted against a north dipping fault at ~650 Ma, can be reconciled by visualizing a soft continental welding of two distinctly different geological realms, brought about by the counter under-thrusting of the intervening Nilgiri block by both the Dharwars and the Madurai-Kerala block, which in the process sheared and uplifted the sandwiched terrane on their leading edges. This process, if it did take place, must have been more intense in the west than in the east along the supposed northern boundary of the Pan-African imprinted Madurai block, to explain the apparently less affected Madras crust.