Probing the Central Indian Ocean Basin: Subsurface Anomalies through Surface Wave Group Velocity

Summary

Surface waves are sensitive to the shear wave velocity and low-velocity zone (LVZ). Here, we analyze the subsurface anomalies in the upper mantle beneath the Central Indian Ocean Basin (CIOB) utilising the ocean bottom seismometer (OBS) data. The Rayleigh wave dispersion curve analysis between earthquake clusters and OBS stations shows a period range between 12 and 300 s for the fundamental modes. A significant decrease in group velocity is observed at an intermediate period (60-180 s). The estimated depth of the lithospheric base is ∼81 km, ∼68 km, ∼67 km, and ∼82 km for P1, P2, P3, and P4 profiles respectively. A significant reduction in Vsv velocity is observed beneath the lithospheric base (i.e. ∼22-24 km thick Lithosphere-Asthenosphere boundary). Our results show an anomalous LVZ between ∼80 km and ∼170 km depth interval beneath the CIOB. A ∼18–20 per cent reduction in Vsv velocity within the LVZ suggests the presence of ∼1.9–2.0 per cent melt fraction in the shallow asthenosphere along P2 (∼3.7 km/s) and P3 (∼3.73 km/s) profiles. An excess temperature of ∼230°C is inferred across the P1-P4 profiles in the vicinity of LVZ beneath the CIOB. Henceforth, we propose a mechanism in which the presence of an unextracted melt fraction, in conjunction with the northward movement of the Indian plate and supplemented by plume-lithosphere interaction, can account for the formation and persistence of anomalous LVZ in the upper mantle. An additional ∼1 per cent melt within the LVZ along the P2 and P3 profiles, relative to the P1 and P4 profiles, favours the possibility of the west-to-east channelized asthenospheric flow beneath the CIOB region.