Multi-Mode Waveform Tomography of the Indian Ocean Upper and Mid-Mantle Around the Réunion Hotspot

Réunion Island in the western Indian Ocean is well known as one of the most active volcanic hotspots on Earth. Its birth, ~65Ma ago, created the Deccan volcanic traps in India (almost 2 million km2), associated with the Cretaceous-Tertiary boundary and with the extinction of about 90% of life on the Earth, including dinosaurs. However, the deep structure of the underlying mantle, the potential presence of a rising plume and its exact geometry in the lower and in the upper mantle are still subjects of debates. The use of seismic data acquired by the French-German RHUM-RUM experiment in the Indian Ocean around the Réunion volcanic hotspot (2012–2013) and the collection of broadband seismic data from temporary experiments and from the FDSN (Federation of Digital Seismograph Networks) data center make it possible to investigate the deep structure of the Réunion mantle plume along its complete track, from its birth to its present stage, with a lateral resolution of ~600 km. So far, global seismic tomography models cannot provide such high resolution images of the transition zone or lower mantle
in this region. In this study, we used the spectral element method (SEM) to perform waveform forward modeling for several thousand paths beneath the Indian Ocean, and normal mode perturbation theory to compute the gradient and the Hessian for the inverse part of the tomography. Using this hybrid method, we derived a regional tomographic model (including teleseismic and regional events) beneath the Indian Ocean, down to ~1200 km depth, from simultaneous inversion of fundamental and higher mode three components waveforms down to 40 s period. Our model retrieves a low-velocity channel extending from West to East in the western side of the Central Indian Ridge, in the depth range of 150–250 km. It also reveals a plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle at 1,200 km depth or deeper. The connection between the Réunion hotspot and the South-Africa Large Low-Shear Velocity Province (LLSVP) is also brought to light. Our findings suggest a long-lived Réunion hotspot, since the lower part of the conduit appears to be anchored in the lower mantle, likely fed by the African LLSVP. Our results will guide further geochemical and geodynamic studies on the interaction between the lower transition zone (660–1,000 km) and the deep lower mantle beneath the Réunion hotspot.