SKS splitting in the Western Indian Ocean from land and seafloor seismometers: Plume, plate and ridge signatures

We present SKS splitting measurements in the Western Indian Ocean, recorded on 20 land and 57 seafloor seismometers deployed by the RHUM-RUM experiment (Reunion Hotspot and Upper Mantle - Reunions Unterer Mantel). We discuss our splitting observations within their geodynamic settings and compare them to SKS splitting parameters predicted from an azimuthally anisotropic Rayleigh wave tomography model that includes the RHUM-RUM data. We find that anisotropic directions poorly correlate with the present-day motion of the Somali plate, which at <2.6 cm/yr may be too slow to cause strongly sheared fabric in the asthenosphere. Fast split directions (Phi) between La Reunion and the Central Indian Ridge (CIR) trend E-W and provide strong, first seismological evidence for near-horizontal flow in the asthenosphere that connects the Reunion mantle upwelling with the CIR, supporting a long-standing hypothesis on plume-ridge interaction. In the vicinity of the Reunion hotspot, we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the Reunion mantle upwelling and its consecutive asthenospheric spreading. We furthermore observe ridge-normal Phi along the CIR and ridge-parallel Phi along the Southwest Indian Ridge (SWIR), both mainly attributed to asthenospheric mantle flows. In the Mozambique Channel between East-Africa and Madagascar, we attribute E-W trending Phi to frozen lithospheric structures, recording the paleo-orientation of the spreading ridges that enabled Madagascar's separation away from Africa. Based on the synopsis of this and previous SKS splitting studies at mid-ocean ridges, we propose that ridge-normal Phi may develop at fast and intermediate spreading ridges (e.g., CIR and East Pacific Rise) and ridge-parallel Phi could be characteristic to slow spreading ridges (e.g., SWIR, Mid-Atlantic Ridge and the paleo-ridges in the Mozambique Channel). (C) 2018 Elsevier B.V. All rights reserved.