Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

International audienceThe relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment ''Horn of Africa'' and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar ''plume'' originates from deeper than 400 km and is characterized by enrichment in primordial 3 He and 3 He/ 4 He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3 He/ 4 He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere

Anisotropic structure of the African upper mantle from Rayleigh and Love wave tomography

International audienceThe geodynamics of the mantle below Africa is not well understood and anisotropy tomography can provide new insight into the coupling between the African plate and the underlying mantle convection. In order to study the anisotropic structure of the upper mantle beneath Africa, we have measured phase velocities of 2900 Rayleigh and 1050 Love waves using the roller-coaster algorithm [Beucler, E., Stutzmann, E., Montagner, J.-P., 2003. Surface-wave higher mode phase velocity measurments, using a roller-coaster type algorithm. Geophys. J. Int. 155 (1), 289–307]. These phase velocities have been inverted to obtain a new tomographic model that gives access to isotropic SV-wave velocity perturbations, azimuthal and radial anisotropies. Isotropic SV-wave velocity maps have a lateral resolution of 500 km. Anisotropy parameters have a lateral resolution of 1000 km which is uniform over Africa for azimuthal anisotropy but decreases at the West and South of Africa for radial anisotropy. At shallow depth, azimuthal anisotropy varies over horizontal distances much smaller than the continent scale. At 280 km depth, azimuthal anisotropy is roughly N-S, except in the Afar area, which might indicate differential motion between the African plate and the underlying mantle. The three cratons of West Africa, Congo and Kalahari are associated with fast velocities and transverse anisotropy that decrease very gradually down to 300 km depth. On the other hand, we observe a significant change in the direction and amplitude of azimuthal anisotropy at about 180 km depth, which could be the signature of the root of these cratons. The Tanzania craton is a shallower structure than the other African cratons and the slow velocities (−2%) observed on the maps at 180 and 280 km depth could be the signature of hot material such as a plume head below the craton. This slow velocity anomaly extends toward the Afar and azimuthal anisotropy fast directions are N-S at 180 km depth, indicating a possible interaction between the Tanzania small plume and the Afar. The Afar plume is associated with a very slow velocity anomaly (−6%) which extens below the Red sea, the Gulf of Aden and the Ethiopian rift at 80 km depth. The Afar plume can be observed down to our deepest depth (300 km) and is associated with radial anisotropy smaller than elsewhere in Africa, suggesting active upwelling. Azimuthal anisotropy directions change with increasing depth, being N-S below the Red sea and Gulf of Aden at 80 km depth and E–W to NE–SW at 180 km depth. The Afar plume is not connected with the smaller hotspots of Central Africa, which are associated either with shallow slow velocities for Mt Cameroon or with no particular velocity anomaly and N-S azimuthal anisotropy for the hotspots of Tibesti, Darfur and Hoggar. A shallow origin for these hotspots is in agreement with their normal 3He/4He ratio and with their location in a region that had been weakened by the rifting of West and Central Africa

Features caused by ground ice growth and decay in Late Pleistocene fluvial deposits

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The SISFRANCE database of historical seismicity. State of the art and perspectives

International audienceThe SISFRANCE database aims to collect and interpret archives relating the historical seismicity in metropolitan France. The database presently contains some 12,500 documents that allow to define 108,832 macroseismic data points, for a total of 6,427 events, of which 5,743 can be qualified as real earthquakes. Although half of these events are and will probably remain poorly known, completing our knowledge of historical earthquakes remains a fundamental element for the definition of seismic hazard and the understanding of seismicity in France and low to moderate seismicity regions in general. The first aim of this paper is to explain the state of the art of the database and remind end-users how the limits introduced by the necessary interpretation of archival data are translated into quality factors that should be carefully considered when using such data for seismotectonic and seismic hazard purposes. Originally built as part of the necessary datasets to define the seismic hazard for nuclear facilities, the SISFRANCE database is the result of 40 years of work within a consortium bringing together the French civil nuclear operator (EDF), the French geological survey (BRGM) and the French institute of nuclear safety (IRSN). SISFRANCE is also the heir to archival research carried out since the second half of the 19th century, thus constituting the richest collection of data related to historical seismicity in France. The second aim of this paper is to explain to end-users the current state of archival research within the SISFRANCE consortium, present avenues for future research strategies and list potential improvements of the database structure. Archival research within the SISFRANCE consortium, no longer carried out by a dedicated historian since 2018, is presently being pursued by engaging individual researchers or academics, focusing research on specific archives and regions, as well as by developing data-mining techniques to exploit the increasing wealth of accessible numerical archives (i.e. Gallica, etc.). Given the new impetus of French governmental research agencies that encourage transdisciplinarity research, SISFRANCE is also pursuing the idea of further engaging the academic community of historians and seismologists, presently greatly underrepresented in the field of historical seismicity in spite of the importance of this data, fundamendal for any seismotectonic and seismic hazard study in France