Seismic Imaging of the Mantle Discontinuities Beneath India: From Archean Cratons to Himalayan Subduction Zone

Abstract

We image the mantle discontinuities in the depth from ∼200–800 km beneath India from its southernmost Archean and Proterozoic cratons, Proterozoic mobile belts to the Himalayan subduction zone using P to S converted phases in the 1957 teleseismic waveforms recorded over 54 broadband seismograph locations. These phases are generated from the velocity contrast at the layer boundary. Our results show presence of Lehmann discontinuity at a depth of 220–250 km beneath southern part of India. The 410 discontinuity is sharp and at its normal depth beneath Precambrian terrains and is elevated by ∼10–15 km in the Ganges basin and the Himalaya. This suggests progressive cooling or thickening of the Indian lithosphere towards its northern margin. We observe a complex 660 km discontinuity with a broad double peak beneath the Himalaya and southern India that may be due to presence of non-olivine component in the deep mantle. Apart from the above mentioned global discontinuities a velocity interface is mapped at 475 km depth beneath the Ladakh. The mantle transition zone show ∼10 km thickening beneath the Ganges basin, suggestive of the presence of cold material within. The elevated 410 discontinuity beneath the Ganges basin and the Himalaya is interpreted as the signature of north-east subducting Indian slab and perhaps part of the Tethyan oceanic lithosphere in front of it. The Tethyan subducted slab broke off from the overlying Indian slab and rolled back southward, crossed the mantle transition zone discontinuity and lies presently beneath the Ganges basin. This relict Tethyan slab is possibly responsible for the thickened transition zone beneath the Ganges basin and the complex 660 km discontinuity beneath the Himalaya.