Anisotropy in the lowermost mantle beneath the Indian Ocean Geoid Low from ScS splitting measurements

Abstract

The Indian Ocean Geoid Low (IOGL) to the south of Indian subcontinent is the world's largest geoid anomaly. In this study, we investigate the seismic anisotropy of the lowermost mantle beneath the IOGL by analyzing splitting of high‐quality ScS phases corrected for source and receiver side upper mantle anisotropy. Results reveal significant anisotropy ( ∼1.01%) in the D′′ layer. The observed fast axis polarization azimuths in the ray coordinate system indicate a TTI (transverse isotropy with a tilted axis of symmetry) style of anisotropy. Lattice Preferred Orientation (LPO) deformation of the palaeo‐subducted slabs experiencing high shear strain is a plausible explanation for the observed anisotropy beneath the IOGL.

Seismic Evidence for a Hot Mantle Transition Zone Beneath the Indian Ocean Geoid Low

“…Our findings affirm the existence of hot material at the depths of MTZ. These results and the observations from previous studies beneath the study region prompt us to interpret that 10.1029/2020GC009079 the world's largest geoid anomaly could be explained by the combined effect of (i) high-velocity/density anomalies in the lowermost mantle associated with slab graveyards (Rao & Kumar, 2014;Rao et al, 2017) and (ii) the low-velocity/density material in the mid mantle (present study) sourced from the African LLSVP. The interpretation supports the outcome of geodynamical modeling (Spasojevic et al, 2010), which advocates that the slab graveyards at the CMB alone cannot explain the long-wavelength geoid anomaly, necessitating the existence of a low-velocity/density anomaly at mid-to-upper mantle depths.…”

“…The present study vindicates the presence of hot anomalies at the mid mantle depths beneath this region. Therefore, the origin of the world's largest geoid anomaly, that is, IOGL, could be due to a combined effect of (i) high-velocity anomalies in the lowermost mantle due to slab graveyards (Rao & Kumar, 2014;Rao et al, 2017) and (ii) lowvelocities/densities in the mid mantle due to hot materials sourced from the African Large Low Shear Velocity Province (LLSVP) (Grand, 2002;Masters et al, 2000;Montelli et al, 2006;Ritsema et al, 2004Ritsema et al, , 2011Simmons et al, 2007Simmons et al, , 2010Su & Dziewonski, 1997), as also evidenced by the results from the present study.…”

“…The exercise suggests that the high-velocity slab graveyards atop the CMB alone cannot explain the IOGL anomaly, necessitating a low-velocity/density anomaly above 1,000 km, which may contribute to 20% of the anomaly (Spasojevic et al, 2010). The shear wave velocities in the lowermost mantle and the D′′ layer anisotropy (Rao & Kumar, 2014;Rao et al, 2017) indicate the presence of anisotropic high-velocity anomalies in the lowermost mantle, related to slab graveyards at the CMB beneath the IOGL region. The present study vindicates the presence of hot anomalies at the mid mantle depths beneath this region.…”

“…The IOGL, located to the south of the Indian continent, is the most prominent geoid anomaly (−106 m) on Earth (Mishra, 2014;Mishra & Kumar, 2012). Various hypotheses, like depression in the core-mantle boundary (CMB) (Negi et al, 1987), low-density subducted materials of the Tethyan lithosphere within the mid-toupper mantle (Mishra, 2014;Mishra & Kumar, 2012), uncompensated depression in the upper mantle (Ihnen & Whitcomb, 1983), dehydration of slabs at mid-to-upper mantle depths-with these slabs now ponded at the CMB (Rao & Kumar, 2014;Rao et al, 2017), and a large palaeo-back-arc basin in the central Neo-Tethys ocean inferred from geodynamic modeling (Nerlich et al, 2016), have been proposed to explain this feature. Further, recent analysis of SS and PP precursors (Reiss et al, 2017) revealed the existence of a hot anomaly in the mantle transition zone (MTZ) beneath the IOGL.…”

The Evolution of Mantle Plumes in East Africa

“…The Tethyan slab has been subducting to the lowermost mantle since the late Jurassic (~150 Ma; Van der Voo et al, 1999), and its seismic signature has been reported by several studies. For example, Rao et al (2017) found substantial anisotropy in the lowermost mantle beneath the Indian Ocean from ScS splitting measurements, which they attributed to lattice preferred orientation deformation of the Tethyan subducted slab dipping southwestwardly. This geometry is consistent with the migration of the subducted Tethyan slab toward the African LLSVP with time, which in turn may have pushed the LLSVP westward.…”