Earth and Climate Science

Dr. Padma Rao Bommoju
National Centre for Earth Science Studies

The Indian Ocean Geoid Low (IOGL) that spans a vast areal extent south of the Indian subcontinentis a most prominent geoid anomaly (−106 m) on the globe, whose origin remains elusive. In this study, weinvestigate the shear velocity structure of the lower mantle beneath this geoid low and the adjoining geoidhigh region utilizing the differential travel times, amplitude residuals of high-quality S and ScS phases.Results reveal large variations in ScS-S differential travel time residuals indicating that the lower mantlebeneath the IOGL is heterogeneous. The obtained ScS-S differential travel times are slower than thosepredicted by IASP91 model, primarily due to velocity increase in lower mantle beneath the IOGL and higherthan IASP91 beneath the geoid high region, due to velocity decrease in the lower mantle. Modelling ofdifferential travel time residuals reveal the existence of high shear wave velocities in a 1000 km thick layerabove the Core Mantle Boundary (CMB). Also, the ScS/S amplitude residuals beneath the IOGL are positive,implying high impedance contrast at the CMB, owing to the presence of high velocity material.For a better understanding of the seismic structure of the D″ layer, we modelled the compressionaland shear wave velocities especially within the assumed D″ layer utilizing the PcP-P and ScS-S differentialtravel time residuals. The obtained ScS-S and PcP-P differential travel time residuals are corrected for thevelocity structure above the D″ layer with the help of global tomographic models. The corrected differentialtravel time residuals truly reflect the anomalies within the D″ layer (220 thick layer above the CMB).Modelling results of corrected differential travel time residuals indicate the variable shear and compressionalwave velocity perturbations within the D″ layer. Interestingly, the high compressional and shear wavevelocity perturbations sample the IOGL region while the negative velocity perturbations sample theadjoining geoid high region. The modelling results clearly suggest the existence of high velocity materialsituated above the CMB beneath the IOGL. This high velocity material may be attributed to the dehydratedTethyan subducted slabs at the CMB. This hypothesis also gains support from the study of Aitchison et al.(2007), which shows that the present day location of IOGL corresponds to the reconstructed position of theTethyan subduction.Further, we investigate the lowermost mantle anisotropy by analyzing high-quality ScS phasescorrected for source and receiver side upper mantle anisotropy to decipher the nature of D″ layer. Thesplitting results reveal significant anisotropy (~1.01%) in D″ layer. The observed fast axis polarizationazimuths in ray-coordinate system indicate a TTI style of anisotropy. Lattice Preferred Orientationdeformation of palaeo-subducted slabs experiencing high shear strain is a plausible explanation for observedD″ layer anisotropy beneath the IOGL.The observed results clearly indicate the existence of high velocity material at the CMB and it isanisotropic in nature, which we attributed to the dehydrated Tethyan subducted slabs. Since only 20% of thegeoid anomaly in the Indian Ocean is related to the shallow mantle structure (<1000 km) (Spasojevic et al.,2010), the candidate mechanisms to explain the remaining 80% anomaly should reside in the lower mantle.The sources are the high density materials (Tethyan slab graveyards) as witnessed by the high velocities atopthe CMB. The logical explanation for the deep source for the IOGL could be (i) due to the recirculation of the Tethyan slab graveyards through mantle upwellings that get influenced by the buoyant hydrated mantle environment lowering the velocities/densities above slab graveyards, (ii) evidence from mineral physics studies that there exist two dehydration sites in the lower mantle (Hirschmann, 2006) – one at the top of the lower mantle and the other at 1200–1500 km depth. Hence, we conclude that the lowering of density andshear velocity at these depths due to the dehydration of slabs could be one of the causative factors for theIOGL. However, it is essential to decipher the mid-to-upper mantle structure for a better understanding ofcauses responsible for the IOGL. Thus, we investigate the mantle transition zone (MTZ) structure beneaththe region using P receiver functions (PRFs), to examine its role in the genesis of this feature. Results from3‐D time to depth migration of PRFs reveal a thin MTZ primarily due to an elevation of the 660 kmdiscontinuity. This is suggestive of anomalously hot temperatures in the mid mantle beneath the IOGLregion, possibly sourced from the African Large Low Shear Velocity Province (LLSVP). The combinedeffect of the hot (low‐density) material in the MTZ, in the mid mantle and the (high‐density) cold slab gravesatop the core‐mantle boundary can possibly explain this geoid low. 

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