Surface wave phase velocity variations underneath the Indian Ocean geoid low

The Indian plate has undergone a long tectonic journey since its separation from the Gondwanaland. Its protracted rift-drift motion has rendered several enigmatic features in the Indian Ocean. One such feature is the large geoid deficit (−106 m), observed in the south of Sri Lanka that is known as the Indian Ocean geoid low (IOGL). A broad consensus about plausible explanations behind the IOGL anomaly still remains elusive. For the first time, 17 Ocean Bottom Seismometers (OBSs) were deployed in the IOGL region to characterize the subsurface structural variations. Here, we examine the inter-station Rayleigh wave phase velocity variations within the period range of 15–197 s. We construct 1-D velocity-depth sections down to c.a.380 km or less along with N-S and E-W profiles, using ≥5.2 magnitudes (Mw) earthquake data that occurred at a depth ≤ 100 km at regional/teleseismic distances. Our result demarcates the regional Moho depths varying from c.a.11.6 ± 0.8 km to c.a.19.8 ± 1.2 km along the OBS array from south to north respectively. A thickened oceanic crust (c.a.9–10 km) is attributed to magmatic underplating in the lower crust near the aseismic Comorin ridge. The lithospheric thickness ranges between c.a.47.6 ± 2.9 km and c.a. 75.1 ± 3.6 km along a south-north profile. We observe a c.a. 20–23 km thick lithosphere-asthenosphere boundary (LAB). The depth interval between c.a.87 km and c.a. 280 km is characterized by a distinct low-velocity zone (LVZ). A thick LVZ appears to be associated with a thin lithosphere, which implies a hot thermal regime in the upper mantle. We envisage a possible connection of this hot thermal regime with the African LLSVP (large low shear velocity province) and/or a vertically deep mantle upwelling directly beneath the IOGL region. Our findings have significant implications for deciphering the possible causes behind the source of mantle upwelling, the large geoid anomaly and extensive lithospheric deformation across this region.