A tree of Indo-African mantle plumes imaged by seismic tomography

Mantle plumes are commonly envisioned as thin, buoyant conduits rising vertically from the core-mantle boundary (CMB) to the earth's surface, where they produce volcanic hot spots. Most hotspots are located in the sparsely instrumented oceans, creating poor prospects for the seismic resolution of thin conduits in the deep mantle. 

The RHUM-RUM experiment remedied this issue around the hotspot island of La Réunion by instrumenting 2000x2000 km2 of seafloor for 13 months with 57 broadband ocean-bottom seismometers (OBS). We present a 3-D P-wave tomography model computed from the RHUM-RUM waveform data, supplemented by a global data set of P-diffracted measurements and a selection of ISC picks. Multifrequency travel times were measured on the waveforms and inverted in a finite-frequency framework. We achieve high image resolution beneath the Indian Ocean hemisphere, and especially beneath La Réunion, from upper mantle to CMB.

We observe the Large Low-Velocity Province (LLVP) rising 800 km above the CMB, forming a cusp beneath South Africa. A low-velocity branch undulates obliquely from this cusp region towards the uppermost mantle beneath La Réunion. Hence La Réunion's connection to the lower mantle is more complex than previously envisioned, being neither a thin vertical conduit nor projecting down to an edge of the LLVP. The deep-mantle connections of the Afar and Kerguelen hotspots emerge from the same LLVP cusp beneath South Africa and extend towards the surface through tilted low-velocity branches. 

Our results provide the first high-resolution image of a western Indian Ocean plume cluster from the surface to the CMB. This represents a key advance for linking geophysical, geodynamic and geochemical observations.