Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

Abstract

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.