No abstract

[1] The S wave velocity structure of the crust and uppermost mantle of the Indian shield has been investigated by jointly inverting P wave receiver functions and Rayleigh wave group velocities at 38 broadband stations in the subcontinent. The Indian shield is an amalgamation of several terranes of Archean and Proterozoic age that were partly flooded by Deccan Trap volcanism during Cenozoic times and that make up a natural laboratory for assessing models of Precambrian crustal evolution. Our results reveal significant variations in crustal thickness and deep crustal velocities: 45-50 km thick under the Archean West Dharwar craton and Southern Granulite Terrane, with lower crustal velocities around 4.1 km/s; 32 -35 km thick beneath the Archean East Dharwar and Bundelkhand cratons, with lower crustal velocities around 3.8-3.9 km/s; 50-65 km thick under the Proterozoic Bhandara craton, with lower crustal velocities around 4.2-4.3 km/s; and $55 km thick under the Proterozoic Aravalli-Delhi belt, with lower crustal velocities around 4.2 km/s. S velocities in the 4.1-4.3 km/s range in the deep crust can be attributed to mafic lithologies, suggesting there has been no secular change in the Precambrian evolution of the south Indian shield. Moreover, pervasive mafic dike swarming throughout the Indian shield suggests that the layer of mafic cumulates is 2.5-1.6 Ga old and that it delaminated from some Archean terranes. Our interpretation is that mafic underplating of the terranes making up the Indian shield occurred in Proterozoic times and that a refractory root developed under the Archean terranes after the Proterozoic event.Citation: Julià, J., S. Jagadeesh, S. S. Rai, and T. J. Owens (2009), 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,

[1] We investigate crustal properties beneath the deep crustal ($35 km) Jabalpur earthquake of 21 May 1997, Narmada-Son Lineament (NSL), central India, in search of a possible cause of stress accumulation in the region. Teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests Moho downwarp to $52 km across the width of the lineament, in contrast with an average 40 km depth elsewhere. In addition, the crust beneath the NSL has higher V p /V s of 1.84 compared to $1.73 in the surrounding, suggestive of a high-density mafic mass at depth that compensates the crustal root, also supported by small topographic variation (200 m) across the lineament. Presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre-existing Narmada-Son fault leading to the earthquake in deep crust.

No abstract