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RHUM-RUM: Imaging a mantle plume under the hotspot of La Réunion

Final Report Summary - RHUM-RUM (RHUM-RUM: Imaging a mantle plume under the hotspot of La Réunion)

RHUM-RUM is a seismological experiment designed to image an oceanic mantle plume – or lack of plume – from crust to core beneath La Réunion Island, and to understand these results in terms of material and heat flows. The Réunion hotspot is one of the most active volcanoes in the world, and its hotspot track leads to the Deccan Traps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago. The genesis and the origin at depth of this mantle upwelling are controversial, and La Réunion stands exemplarily for the entire class of hotspot-type volcanism that occurs mostly in the world's oceans, far removed from plate boundaries. It remains unclear whether oceanic hotspots are fed by deep mantle plumes, and whether plumes are significant contributors to the solid earth's heat budget. From 2011 to 2014, we deployed 57 broadband seismometers on the ocean floor, complemented by 37 seismic stations on the surrounding land masses of Madagascar, Mauritius, the Seychelles, and the Iles Eparses. Data analysis has been underway since 2014, co-funded by this CIG grant.

RHUM-RUM has become the largest effort worldwide to image a plume under an oceanic hotspot. It started as a French-German cooperative effort that pools the marine resources, funding, scientific expertise, and local infrastructure needed to realize this large experiment. I initiated the project during my time as an Assistant Professor at LMU Munich, together with my French co-Principal Investigator Guilhem Barruol from the Université de la Réunion/CNRS. My move in 2013 to the University of Oxford as an Associate Professor meant that some of the German funding had to be left behind, which this Marie Curie Career Integration Grant has been used to mitigate, so that RHUM-RUM could continue uninterrupted. The part of the project carried out in my group at Oxford uses the most advanced methods of waveform tomography to turn conventional and unconventional body wave phases into 3-D structural models of the entire mantle column under La Réunion.

We have achieved:

1) a hemispheric, whole-mantle P/Pdiff wave tomography model for the Indian Ocean hemisphere, centred on La Réunion (Tsekmistrenko et al. 2019a, 2019b, in prep.).
This effort led by PhD student Maria Tsekhmistrenko yielded spectacular images of the whole-mantle upwelling(s) beneath La Réunion. It incorporates huge amounts of seismic observations from the RHUM-RUM deployment and stations around the world, and images down to the lowermost mantle adopting the technological developments (Pdiff tomography) of item 2). The findings leads us to propose a new model for how deep-mantle upwellings in the Indian Ocean (and elsewhere) act in concert over timescales of tens to hundreds of millions of years to split up continents and open new ocean basins.

2) a global P-wave tomography model that integrates Pdiff waves, a major technical milestone in mantle tomography (Hosseini & Sigloch 2015; Hosseini et al. 2019, GJI, in revision). An almost decade-long push to make core-diffracted body wave accessible to tomography has been capped by the achievement of the first such tomography model. During the Reporting Period, this effort was closely intertwined with item 1).

3) a high-resolution Rayleigh-wave tomography of the Indian Ocean, of isotropic and azimuthally anisotropic shear velocities (Mazzullo et al. 2017)
PhD student Alessandro Mazzullo, advised by my primary RHUM-RUM colleagues at IPG Paris, produced the first high-resolution tomography of the upper 300 km in this previously almost uninstrumented ocean basin.

4) Shear-wave splitting measurements for the area of the RHUM-RUM deployment, one of two primary proxies for mantle flow in the asthenosphere (Scholz et al. 2016, 2018). PhD student advised by my French co-PI Guilhem Barruol and co-advised by myself.

5) Observations of both predicted and unexpected, large-scale flow patterns of the asthenosphere under the Indian Ocean, arguably driven by deep mantle upwellings, with major implications for the (large) contribution of plumes to the solid earth's heat budget (Barruol, Sigloch et al., 2019 submitted to Nature Geoscience). Drawing together all structural imaging results from the RHUM-RUM exeriment for the upper 300 km.

6) A regional-scale surface-wave tomography of the oceanic crust and lithosphere in the Southwestern Indian Ocean, derived from ambient noise correlations of the RHUM-RUM data set (Hable et al. 2018, Hable et al. 2019 in revision). A demonstration of the surprisingly long-range correlations of ambient seismic noise across an ocean basin and it good recording on the ocean bottom, harnessed to the first large-scale tomography of its kind.

7) the first 3-D tomography of the island of La Réunion, from ambient-noise correlations (Hable et al., in prep).

8) The first observations of Earth's free oscillations on ocean-bottom seismometers (Deen et al. 2017). A heroic signal denoising effort my IPGP PhD student Martha Deen led to the first observation of this faint signal on an instrument type that has been generally deemed too noisy to show normal modes.

9) The demonstration that tropical storms over the oceans can be tracked and quantified from ocean-bottom seismometer networks (Davy et al. 2014)

10) The demonstration that seismic stations on ocean islands can monitor and quantify destructive swells on islands in the (Southern) ocean, acting as proxies for (non-existent) tide gauges (Barruol et al. 2016)

11) Several significant method improvements in processing and denoising waveforms from ocean-bottom seismometers, including:
* Horizontal orientation of ocean-bottom seismometers from a method making combined use of Rayleigh waves and the polarization of P-waves (Scholz et al. 2016).
* High-accuracy estimation of clock errors in OBS and land stations (Hable et al. 2018).
* Denoising of persistent frequency artifacts on some OBS recorders (Deen et al. 2017).

M.Sc. and B.Sc. theses have focused on the characterization and tracking of ships passing above the ocean-bottom seismometers; investigation of sound propagation in the deep sea below the SOFAR channel; the use of OBS as proxies for ocean-bottom current meters; and the analysis of whale song.

Public website of RHUM-RUM project: