X-Discontinuity Beneath the Indian Shield—Evidence for Remnant Tethyan Oceanic Lithosphere in the Mantle

Beneath the continents and island arcs, a seismic discontinuity is often detected around

300 km depth, referred to as the X-discontinuity. In order to investigate this discontinuity

beneath India, we analyzed 10,216 P wave receiver functions at seismological stations deployed on the Indian shield and the Himalayas. We detect the X-discontinuity as a sporadic and thin feature, in the depth range of 246–335 km, with a sharp shear velocity jump of 2.5%–3.6%. It neither bears a clear tectonic affinity nor has any correlation with the transition zone discontinuities. Interpreting its origin due to a single mineralogical change warrants a large spatial variation in the mantle temperature. Therefore, we suggest that the observed widespread X-discontinuity beneath the Indian shield owes its origin to lower pressures (∼2 GPa) due to the presence of water (0.13 wt% H₂O) in MgSiO₃ and coesite-stishovite transition that is, Orthoenstatite to high pressure Clinoenstatite transformation transition occurring at 8–11 GPa due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during lower Eocene. The identification of such a discontinuity could allow tracking of subducted material within the upper mantle providing a measure of mantle geochemical heterogeneity.

Schematic diagram showing the origin and disposition of X-discontinuity beneath the Indian shield from north to south

Citation: Srinu, U., Kumar, P., Haldar, C., Kumar, M. R., Srinagesh, D., & Illa, B. (2021). X-discontinuity beneath Indian Shield—Evidence for remnant Tethyan oceanic lithosphere in the mantle. Journal of Geophysical Research: Solid Earth, 126, e2021JB021890. https://doi. org/10.1029/2021JB021890