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The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn- tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low- Pn -velocity zones (<8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S -velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High- Pn -velocity zones (>8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region. [ABSTRACT FROM AUTHOR]

In the western Indian Ocean, the Réunion hot spot is one of the most active volcanoes on Earth. Temporal interactions between ridges and plumes have shaped the structure of the zone. This study investigates the mantle structure using data from the Réunion Hotspot and Upper Mantle-Réunions Unterer Mantel (RHUM-RUM) project, which significantly increased the seismic coverage of the western part of the Indian Ocean. For more than 1 year, 57 ocean bottom seismometer stations and 23 temporary land stations were deployed over this area. For each earthquake station path, the Rayleigh wave fundamental mode phase velocities were measured for the periods from 30 s to 300 s and group velocities for the period from 16 s to 250 s. A three-dimensional model of the shear velocity in the upper mantle was built in two steps. First, the path mean phase and group velocities were inverted, to obtain regionalized velocity maps for each separate period. Then, all of the phase and group velocity maps were combined and inverted at each grid point, to obtain the local S wave velocity as a function of depth, using a transdimensional inversion scheme. The three-dimensional anisotropic S wave velocity model has resolution down to 300 km in depth. The tomographic model and surface tectonics are correlated down to ∼100 km in depth. Starting at 50 km in depth, a slow velocity anomaly beneath Rodrigues Ridge and the east-west orientation of the azimuthal anisotropy show a connection between Réunion upwelling and the Central Indian Ridge. The slow velocity signature beneath La Réunion is connected at greater depths (150-300 km) with a large slow velocity zone beneath the entire Mascarene Basin. This develops along a northeast direction, following the general motion direction of the African Plate. These observations indicate nonisotropic spreading of hot plume material and dominant horizontal flow in the upper mantle beneath this area. [ABSTRACT FROM AUTHOR]

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations. [ABSTRACT FROM AUTHOR]

The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until <1 Ma, but the sources of which remains uncertain. Combined analysis of three complementary surface wave methods (ambient noise, Rayleigh wave cross-correlations, and two-plane-wave) illuminate the upper mantle down to depths of 150 km. The phase-velocity measurements from the three methods for periods of 8–182 s are combined at each node and interpolated to generate the first 3-D shear-velocity model for sub-Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50–150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism. [ABSTRACT FROM AUTHOR]

Abstract: Azimuthal anisotropy is the dependence of local seismic properties on the azimuth of propagation. We present the azimuthally anisotropic component of a 3D SV velocity model for the Pacific Ocean, derived from the waveform modeling of over 56,000 multi-mode Rayleigh waves followed by a simultaneous inversion for isotropic and azimuthally anisotropic v sv structure. The isotropic v sv model is discussed in a previous paper (A. Maggi, E. Debayle, K. Priestley, G. Barruol, Multi-mode surface waveform tomography of the Pacific Ocean: a close look at the lithospheric cooling signature, Geophys. J. Int. 166 (3) (2006). doi:10.1111/j.1365-246x.2006.03037.x). The azimuthal anisotropy we find is consistent with the lattice preferred orientation model (LPO): the hypothesis of anisotropy generation in the Earth''s mantle by preferential alignment of anisotropic crystals in response to the shear strains induced by mantle flow. At lithospheric depths we find good agreement between fast azimuthal anisotropy orientations and ridge spreading directions recorded by sea-floor magnetic anomalies. At asthenospheric depths we find a strong correlation between fast azimuthal anisotropy orientations and the directions of current plate motions. We observe perturbations in the pattern of seismic anisotropy close to Pacific hot-spots that are consistent with the predictions of numerical models of LPO generation in plume-disturbed plate motion-driven mantle flow. These observations suggest that perturbations in the patterns of azimuthal anisotropy may provide indirect evidence for plume-like upwelling in the mantle. [Copyright &y& Elsevier]

The mantle plumes, their origin, their surface signature (as hotspots) and their role inglobal geodynamics are considered as one of the &#x0022;last frontier&#x0022; challenges in Earthsciences. To first order, seimsic and volcanic activities are well understood by platetectonics. But mantle plumes cannot be explained by plate tectonics. Their birth,life and death play a fundamental role on the evolution of biological life on Earth,on plate tectonics reorganization. In that respect, La Réunion island is one of thelargest hotspot on Earth. Its birth, 65Ma ago, creating the Deccan volcanic traps inIndia (almost 1million km2) are associated with the Cretaceous-Tertiary boundaryand might have provoked the disappearance of more than 90% of life on Earth(including dinosaurs). However, the deep structure of this geological object and theexact geometry of volcanic plumes are still the subject of controversies. The useof seismic data acquired by the French-German RHUM-RUM experiment in theIndian ocean around La Reunion volcanic hostpot (2012-2016) and the collection ofbroadband seismic data by Incorporated Research Institution for Seismology (IRIS)enable us to investigate the deep structure of La Réunion Plume along its completetrack from its birth to its present stage in the Indian Ocean around La Reunion.The recent progress in inversion techniques enables a better resolution of thesestructures, by taking into account the full information of seismograms. We presentpreliminary results with an enhanced resolution in the transition zone (410-660 km). [ABSTRACT FROM AUTHOR]

We present two high-resolution body-wave tomography models of the whole mantle column, beneath the western Indian Ocean, centered on the volcanic hotspot of La Reunion, the presumed location of a deep mantle plume. The P-wave model shows a source in the lowermost mantle for the upwelling beneath La Réunion, rooted in the African LLSVP. The model is consistent with previous studies observing 'fat' plumes in the lower mantle but reveals more detail and considerable complexity in the transition zone and upper mantle beneath the Réunion hotspot. Rather than being near-vertical, the upwelling shows a tilt in the lower mantle (2000-1000 km depth) and splits into branches closer to the surface (upper 600 km).The new S-model consists of three datasets: teleseismic multifrequency waveform measurements from the ocean-bottom and island-based RHUM-RUM experiment around La Réunion (2011-2015, Barruol & Sigloch, 2013); cross-correlation measurements of teleseismic S-waves from S40RTS (Ritsema et al., 2011); and picked S-phases from the ISC/ISC-EHB catalog (Weston et al., 2018). Like the P-model, the S-model reveals a whole-mantle, low-velocity upwelling rooted in the African LLVP. The two models are highly correlated throughout the mantle but additional low-velocity features are imaged by the S-model in the upper mantle. [ABSTRACT FROM AUTHOR]