Whole-mantle body-wave tomography under the hotspot of La Réunion
Final Report Abstract
Mantle plumes were conceived as thin, vertical conduits in which buoyant, hot rock from the lowermost mantle rises to the earth's surface, manifesting as hotspot-type volcanism far from plate boundaries. Spatially correlated with hotspots are two vast provinces of slow seismic wave propagation in the lowermost mantle, likely representing the heat reservoirs that feed plumes. Imaging plume conduits has proven difficult because most are located beneath the non-instrumented oceans, and they may be thin. We combined novel seismological data sets to resolve mantle upwelling across all depths and length scales, centred on Africa and the Indian and Southern oceans. Using seismic waves from one of the largest seismic ocean-bottom deployments to date, plus a novel type of data from waves that sample the deepest mantle extensively, we show that mantle upwellings are arranged in a tree-like structure. From a central, compact trunk below ~1500 km depth, three branches tilt outward and up toward the various volcanic hotspots of the Indian and Southern Oceans. Each branch is vertically underlain by a corridor-like root in the lowermost mantle. The ocean-bottom (RHUM-RUM) data also permit to resolve a thin, vertical plume conduit in the upper mantle beneath the Réunion hotspot, anchored in the termination of a mid-mantle branch. Our principal new observation is the three-fold repetition of the structural element tilting branch above narrow corridor in the lowermost mantle. We conclude that this element seems to reflect the general principle of how narrow, scattered mantle plumes are generated from compact, large LLVPs. Since these newly observed geometries are not accounted for by existing models for this process, we propose a new geodynamic model. Each tilting branch represents an alignment of vertically rising blobs or proto-plumes, which detached in a linear staggered sequence from their underlying low-velocity corridor at the core-mantle boundary. Once a blob reaches the viscosity discontinuity between lower and upper mantle, it spawns a "classical" plume head/plume tail sequence. We show that the spatiotemporal sequence of observed plume head eruptions in the region supports this explanatory model. A follow-up by quantitative geodynamic modelling should serve to constrain the admissible range of rheology parameters in the mantle that can explain these observations. A feature-length documentary film about the RHUM-RUM project has been produced by the Université de la Réunion in French language. It presents the RHUM-RUM project in general, and this sub-project in particular (featuring interviews with Ph.D. student and the two PIs). It also includes footage from the research cruises, including interviews (in German) with the German participants. The film can be viewed here and/or on YouTube: La Réunion, de la croûte au noyau terrestre https://numerique.univ-reunion.fr/nos-realisations/les-films-documentaires-pedagogiques-institutionnels/la-reunion-de-la-croute-au-noyau-terrestre; https://youtu.be/135O6SeWdNw EOS, the member magazine of the American Geophysical Union, recently produced a feature article titled 'The Unsolved Mystery of the Earth Blobs', which features the present project prominently, with 3 of its 6 figures were produced by us.'The Unsolved Mystery of the Earth Blobs' (EOS, 27/02/2019) https://eos.org/features/the-unsolved-mystery-of-the-earth-blobs
Publications
- 2016. Performance report of the RHUM-RUM ocean bottom seismometer network around La Réunion, western Indian Ocean. Advances in Geosciences, 41
Stahler, S.C., Sigloch, K., Hosseini, K., Crawford, W.C., Barruol, G., Schmidt-Aursch, M.C., Tsekhmistrenko, M., Scholz, J.R., Mazzullo, A. and Deen, M.
(See online at https://doi.org/10.5194/adgeo-41-43-2016) - (2018). SubMachine: Web-based tools for exploring seismic tomography and other models of Earth's deep interior. Geochemistry, Geophysics, Geosystems, 19(5), 1464-1483
Hosseini, K., Matthews, K. J., Sigloch, K., Shephard, G. E., Domeier, M., & Tsekhmistrenko, M.
(See online at https://doi.org/10.1029/2018GC007431) - (2019) Seismic tomography of the La Réunion mantle plume. Doctoral dissertation, University of Oxford
Tsekhmistrenko, M.
- (2020). Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves. Geophysical Journal International, 220(1), 96-141
Hosseini, K., Sigloch, K., Tsekhmistrenko, M., Zaheri, A., Nissen-Meyer, T., & Igel, H.
(See online at https://doi.org/10.1093/gji/ggz394)
