Anisotropic Tomography Around La Réunion Island From Rayleigh Waves

Abstract

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.

Seismic Wave‐Based Constraints on Geodynamical Processes: An Application to Partial Melting Beneath the Réunion Island

“…A high‐temperature upper mantle below Réunion is in line with local seismic tomography studies, which confirm a low shear‐wave velocity zone (LVZ) below the oceanic crust (Fontaine et al, ; Mazzullo et al, ). The LVZ phenomenon is not unique to Réunion but has been observed globally below hotspots, rifts, and mid‐ocean ridges in areas such as Afar, the East Pacific Rise, Hawaii, the Galapagos, and Iceland.…”

SKS splitting in the Western Indian Ocean from land and seafloor seismometers: Plume, plate and ridge signatures

“…1; Table 1), installed temporarily between 2011-2016 (land stations) and 2012-2013 (seafloor stations). We compared measured XKS splitting parameters with predicted XKS splitting parameters computed from a regional, azimuthally anisotropic Rayleigh wave tomography (Mazzullo et al, 2017). Our first order conclusions are as follows:…”

“…The effective thicknesses of the respective asthenospheric flows may therefore range d REU-CIR =100-165 km and d RODRID = 110-180 km, assuming typical anisotropy of 3-5% (Mainprice et al, 2000;Tommasi, 1998) along the vertical direction sampled by XKS waves. Interestingly, the tomographic models of Mazzullo et al (2017) allows to predict only weak lithospheric anisotropy (δt pred_lithos ≤ 0.15 s) that could reflect frozen lithospheric structures as discussed, but stronger asthenospheric anisotropy (δt pred_asthenos ≥ 0.4 s) at depths between 50-200 km. That is also fully consistent with the hot asthenosphere seen by the Rayleigh wave model at these depths.…”

“…Our model does well at explaining the rotating Φ around La Réunion, but not the N-S trending Φ in the Mascarene Basin that may reflect fossil lithosphere structure. Such modeling has, however, strong limitations because the actual source of mantle upwelling is probably neither nearly point-like, as suggested by surface wave tomography (Mazzullo et al, 2017), nor vertical.…”

“…The Réunion volcanic hotspot has been proposed to be fed by a "primary" (Courtillot et al, 2003) mantle plume (Morgan, 1972) -a deep rooted upwelling of mantle material that may be connected to the South-African Superswell (Forte et al, 2010). A recent, regional Rayleigh wave tomography study indicates that the Réunion hotspot could also be an expression of mantle material rising from beneath the Mascarene Basin, where a broad low-shear wave velocity anomaly at asthenospheric depths is observed (Mazzullo et al, 2017). Morgan (1978) also hypothesized that some of the hot material rising beneath La Réunion may be feeding the nearest spreading ridge, the Central Indian Ridge (CIR) at 1000 km distance, through a sub-lithospheric, channeled mantle flow.…”