Réunion Hotspot Upper Mantle experiment Réunions Unterer Mantle – RHUM-RUM

Pour imager la structure du manteau sous La Réunion, nous allons déployer des stations sismologiques sur une superficie de 2000x2000 km2. A l'aide du navire océanographique Marion Dufresne, 57 stations de fond de mer seront déposées autour de La Réunion et sur les rides centrale et sud-ouest indienne, pour une durée d'une année. Les déploiements de stations en fond de mer seront complétés par 15 stations terrestres déployées sur La Réunion, Madagascar et les îles Éparses.

Le projet RHUM-RUM représente essentiellement de la recherche fondamentale axée sur une meilleure connaissance de la dynamique profonde de notre planète, et en particulier sur la structure et de l'origine des point chauds et des panaches mantelliques associés. L'expérience proposée autour de La Réunion est l'une des plus grandes jamais envisagée et se place dans une forte compétition internationale concernant l'imagerie des panaches mantelliques.

Bien qu'étant un projet de recherche fondamentale, RHUM-RUM aura un impact sur les technologies de sismologie de fond de mer et de sismologie utilisant les bouées dérivantes, deux techniques qui seront mises en œuvre. Il apportera évidemment des nouvelles idées et données sur l'origine de l'activité volcanique actuelle de ce point chaud actif de façon quasi continue depuis 65 Ma.

Les résultats obtenus seront présentés en congrès internationaux et publiés dans des revues scientifiques internationales.

Abstract

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.

Here we apply for funding for 9 OBS and 10 island stations, personnel costs, and for ship operating costs (one out of two required cruises). Our ship time proposal for the “Marion Dufresne”, submitted in September 2010, has been classified as priority #1 by the national committee. We have also secured independent funding and ship access for 5 more land stations on the Iles Eparses (from the INSU SYSTER program, the OSU-Réunion, and INEE-INSU "programme Iles Eparses"). Our German colleagues are submitting a funding request for 48 OBS stations to DFG in February 2011. They have already submitted a ship time proposal for the second required cruise (OBS recovery), which could be done by the German vessel “Meteor” or through ship time barter with the “Marion Dufresne”.