Heterogeneous lithosphere and the underlying mantle of the Indian subcontinent

Abstract

P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph stations of the India Meteorological Department (IMD) illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the Indian sub-continent. Our analysis demonstrates that the Archean and Early Proterozoic lithospheric keel of the Indian shield has been reworked by younger processes. We find very low S-wave velocities in the uppermost mantle (from 4.0 to 4.3km/s) to the north of the Deccan Volcanic Province (Kutch region and Aravalli Craton) (1), in the south (Southern Granulite Terrain and Sri Lanka) (2) and in the north-east (Gangetic Plane, Bengal Basin and Singhbuhm Craton) (3). The anomalies 1 and 2 may extend into the transition zone. Early arrivals of the S410p seismic phase are indicative of anomalously high Vp/Vs ratio (~1.9) in the upper mantle of the low-velocity regions, whereas late arrivals in the western Himalaya, Ladakh and western Tibet are consistent with the previously found indications of anomalously low Vp/Vs ratio. A transition from the high-S-velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere–asthenosphere boundary is problematic. A mafic S velocity in the upper crust and a pronounced low-S-velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the Deccan Traps. A separation in depths between the 410-km and 660-km discontinuities varies laterally in a range from 240 to 270km. The largest uplift of the 410-km discontinuity (up to 390km) is observed beneath the foothills of the Himalaya where it is caused by cooling of the transition zone by the ongoing continental collision.