The island of La Réunion has been created by one of the most active volcanoes in the world. Its well-dated hotspot track leads unambiguously to the Deccan trapps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago and seems to have had a massive impact on global climate and life at the time. But the genesis and the origin at depth of the mantle upwelling and of the hotspot are still very controversial.
We wish to establish the origin of the heat source that has been fueling this powerful hotspot: Is there a direct, isolated conduit into the deepest mantle, which sources its heat and material from near the core-mantle boundary? An isolated plume conduit connecting to the African superswell at mid-mantle depths? Or might the volcanism reflect merely an upper mantle feature? We also want to study the hotspot’s interaction with the neighboring ridges of the Indian Ocean. There is a long-standing hypothesis, not yet examined seismically, that channelized plume flow feeds the Central Indian Ridge at 1500 km distance. The aseismic Rodrigues Ridge is presumed to be the surface manifestation of this asthenospheric flow channel (though others conted that it is just a leaky transform fault). The goal of the RHUM-RUM experiment is to image a mantle plume – or lack of plume – from the surface down to the core-mantle boundary, and to understand these results in terms of material and heat flow. This comprises the vertically ascending flow in the plume conduit, as well as any lateral flow spreading into the asthenosphere of the western Indian Ocean. For this, we propose to use seismology and gravity methods.
This project is particularly ambitious through the scientific objectives we plan to tackle, through the number of broadband instrument we propose to deploy and through the size of the recording system. RHUM-RUM is the largest experiment so far aimed at imaging an oceanic mantle plume. It is fundamentally a German-French collaboration, allowing to tap into complementary scientific expertise, technical capabilities (land and ocean seismometers), and logistics (oceanic vessels). So, we can attain a critical mass which should enable to fulfill our scienitifc objectives of image the Reunion plume with the required resolution.
We propose to deploy 57 French and German ocean-bottom seismometers (OBS) over an area of 2000 km x 2000 km2 centered on La Réunion Island. The OBS deployment will be augmented by 15 land seismographs in Madagascar, La Réunion and the Iles Eparses, and by 12 stations in the southern Seychelles proposed by our German partners. A data sharing agreement with U.S. collaborators will give us access to 33 additional stations in Madagascar, the Comores and Mozambique. A significant number of OBS placed along the Central and South West Indian Ridges are aimed at lower mantle imaging beneath the hotspot, but also provide independent opportunity for the study of these ridges and plume-ridge interactions.