Unusual lithospheric structure and evolutionary pattern of the cratonic segments of the South Indian shield

Abstract

The southern Indian shield, characterised by several prominent geological and geophysical features, can be divided into three distinct tectonic segments: Western Dharwar craton (WDC), Eastern Dharwar craton (EDC) and Southern Granulite terrain (SGT). With the exception of WDC, the entire crust beneath EDC and SGT has been remobilized several times since their formation during the mid-to late Archeans (3.0-2.5 Ga). In order to understand the evolutionary history of these segments, a multiparametric geological and geophysical study has been made which indicates that the south Indian shield, characterized by a reduced heat flow of 23-38 mW/m 2 has a much thinner (88-163 km) lithosphere compared to ∼200-450 km found in other global shields. In the EDC-SGT terrain, high velocity upper crust is underlain by considerably low mantle velocity with a thick high conductive/low velocity zone sandwiched at mid crustal level. Our study reveals that the entire EDC region is underlain by granulite facies rocks with a density of about 2.85 to 3.16 g/cm 3 at a shallow depth of about 8 km in the southern part and at even shallower depth of about 1 to 2 km below the Hyderabad granitic region in the north. Cratonic mantle lithosphere beneath EDC may contain a highly conductive, anisotropic and hydrous metasomatic zone between the depth of 90 and 105 km where estimated temperatures are in the range of 850-975• C. It is likely that before the early Proterozoic, the entire south Indian shield was a coherent crustal block which subsequently got segmented due to persistent plume-induced episodic thermal reactivations during the last 2.7 Ga. These reactivations led to self destruction of cratonic roots giving rise to negative buoyancy at deeper levels which may have been responsible for crustal remobilisations, followed by regional uplifting and erosion of once substantially thick greenstone belts. Consequently, the crustal column beneath the EDC has become highly evolved and now corresponds closely to SGT at depth.

Deep crustal structure of the Indian shield from joint inversion of P wave receiver functions and Rayleigh wave group velocities: Implications for Precambrian crustal evolution

“…The inferred crustal thicknesses compare well with Moho depths between 40 km and 45 km obtained from deep seismic reflection and refraction profiling [ Reddy et al , 2003]. This active source profile, however, also reports a significant low‐velocity zone between 20 km and 30–35 km depth, interpreted as a granitic layer resulting from subduction of the WDC underneath the SGT during Archean times [ Agrawal and Pandey , 2004]. This low‐velocity layer is not observed in our velocity‐depth profiles.…”

“…The crustal evolution of the WDC, on the other hand, was primarily during mid‐Archean times (3.4–3.0 Ga) and lacks geological signatures of subsequent tectonic perturbations [ Gupta et al , 2003b], so it cannot be regarded as a Karelian‐type exception to Durrheim and Mooney's model of crustal evolution. The SGT, on the other hand, was affected by the Pan‐African Orogeny about 550 Ma [ Agrawal and Pandey , 2004] but, as it will be shown below, it cannot be regarded as a Karelian‐type exception either.…”

“…The craton is cut in the north‐south direction by the 2.6 Ga old Closepet Granites, a ∼350‐km‐long granite‐migmatite intrusive that formally separates the East Dharwar Craton (EDC) from the West Dharwar Craton (WDC). The metamorphic grade of Dharwar metasupracrustal rocks increases toward the south across the approximately east‐west trending gneiss‐granulite transition (GGT) zone into the Archean Southern Granulite Terrane (SGT), where preexisting 3.4 Ga old rock assemblages were metamorphosed into high‐grade facies also at 2.6 Ga and reworked during the Pan‐African orogeny about 550 Ma [ Agrawal and Pandey , 2004]. The EDC, WDC and SGT terranes have behaved as a consistent unit since 2.5 Ga and are collectively referred to as the south Indian shield [ Mahadevan , 1994].…”

Cenozoic epeirogeny of the Indian peninsula

“…Similar denudation histories are recovered across eastern India with a late Cenozoic peak observed in the T ‐ t (temperature‐time) relationships derived from many samples [ Sahu et al ., ]. The inferred low geothermal gradient for India (i.e., 15 °C km −1 ) suggests that, for a stable geothermal profile, exhumation of 4.7–6.7 km is required to cool samples now found at the surface through their closure temperatures (i.e., , depending on cooling rate) [ Dodson , ; Roy and Rao , ; Agrawal and Pandey , ; Lisker et al ., ]. The paucity of Cenozoic ages suggests that no more than 4.7 km of exhumation has occurred since 65 Ma.…”

“…(b) Calculated exhumation plotted as function of AFTA age. Solid line = mean exhumation since 50 Ma required to record 50 Ma AFTA age; gray band with dashed lines = envelope reflecting uncertainties in mean surface ( ) and closure temperatures ( ), assuming constant geothermal gradient of 15 °C km −1 [ Roy and Rao , ; Agrawal and Pandey , ]. Exhumation since 50 Ma calculated using v = m 0.26 Ma −1 and m = 0.37 for .…”