Thin lithosphere–asthenosphere boundary beneath eastern Indian craton

Abstract

The lithosphere–asthenosphere boundary (LAB) separates the hard and rigid outer layer of the earth (lithosphere) and the weaker, hotter, and deeper part of the upper mantle (asthenosphere) and plays a pivotal role in plate tectonics. However, its definitive detection, especially beneath the cratons, is proving elusive. One of the geophysical tools used to map the LAB beneath the cratons is through magnetotelluric (MT) observations. The resistivity at boundary falls in the range of 5–25 Ω-m and can be explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Here, we report thickness of the LAB in one of the oldest dated ancient cratons of India—Eastern Indian Craton (EIC) of ~ 3.3 Gyr, from MT studies. The two prominent phase-sensitive strike directions, one each for crust and mantle, and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath EIC. Our results show that the LAB beneath the EIC is at 95 km. The region is interesting as the electrical properties of the crust and mantle and the Moho depth are similar to those of the Slave Craton, Canada (~ 4.0 Gyr) but the depth of the LAB beneath the EIC is half that of the Slave craton. As cratonic signatures, depicted by ultrapotassic rocks from Gondwana coal fields close to EIC, are preserved at least till early Cretaceous (117 Ma) it is likely that Himalayan orogeny could have played a major role in delamination of the lithospheric roots of the EIC in addition to attendant seismicity.