SKS splitting in the Western Indian Ocean from land and seafloor seismometers: Plume, plate and ridge signatures
Introduction
The Réunion hotspot in the Western Indian Ocean feeds the Piton de la Fournaise, one of the most active volcanoes in the world. Its age-progressive hotspot track is formed by La Réunion Island, Mauritius Island and the Mascarene Plateau on the Somali plate, and the Chagos, Maldive and Laccadive alignment on the Indian plate (Duncan, 1990, Duncan et al., 1990). The track leads to the Deccan Traps of India, one of the largest flood basalt provinces on Earth that erupted 65 Ma ago (Courtillot et al., 1986) and is likely linked to the Cretaceous–Paleogene extinction event (Richards et al., 2015).
The Western Indian Ocean presents an unusual variety of upper mantle phenomena to investigate. 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. The Southwest Indian Ridge (SWIR) is the other nearby spreading center. Despite its ultra-slow spreading rate and magma-starved dynamics, it also could be influenced by adjacent hotspots/plumes (La Réunion, Marion and/or Crozet; Sauter et al., 2009) and/or the South-African Superswell. Finally, in the regional context of the East African Rift System (EARS), the location of the diffuse plate boundary that connects the southern EARS to the SWIR remains subject to discussion (e.g., Kusky et al., 2010, Stamps et al., 2015), together with the synchronous volcanism occurring from the EARS to the Mascarene Basin at 10–20 Ma ago (Michon, 2016) that could suggest episodic, large-scale events of mantle upwelling.
To address these questions of upper mantle structures and dynamics, we analyzed seismic anisotropy via the splitting of the teleseismic, core-refracted shear waves such as SKS, SKKS, and pSKS phases (hereafter called XKS). Seismic anisotropy is accepted to result mostly from lattice preferred orientation (LPO) of rock-forming minerals in response to tectonic strain. In the upper mantle, olivine is the dominating phase. It is intrinsically anisotropic to P and S-waves (e.g., Mainprice et al., 2000) and controls large-scale patterns of seismic anisotropy (Nicolas and Christensen, 1987). In the lithosphere, LPO may record past tectonic episodes that produced deformation such as faults and shear zones, tectono-thermal interactions with the asthenosphere such as plume head arrivals, and/or plate accretion at mid-ocean ridges (e.g., Wolfe and Silver, 1998). In the latter scenario, rock fabrics acquired through ridge-parallel or ridge-normal mantle flow (i.e., ridge-parallel or ridge-normal LPO) could become “frozen-in” by lithospheric cooling and preserved during the seafloor's entire lifetime. In the asthenosphere, LPO may reflect present-day mantle flow, the subducting of mantle slabs, the shearing caused by motion of the overlying plate, and/or the flow induced by rising plumes spreading horizontally beneath the lithosphere (Morgan et al., 1995). In addition to LPO (or “intrinsic” anisotropy), shape preferred orientation (SPO, or “extrinsic” anisotropy) can contribute to observed shear wave splitting patterns. SPO can be generated by (liquid filled) cracks, oriented melt pockets, dipping discontinuities, and/or fine layering (e.g., Wang et al., 2013).
Seismic anisotropy may be also present within the layer in the lowermost mantle (e.g., Kendall and Silver, 1996), and this region is also sampled by XKS waves. There are, however, several seismological arguments why observed XKS splitting is dominantly caused by upper mantle anisotropy: i) XKS splitting parameters often display short-scale variations indicative of rather superficial causes of anisotropy (e.g., Alsina and Snieder, 1994); and ii) anisotropy measurements from XKS and (local) S-phases yield similar splitting parameters, putting an upper bound of s on the splitting contribution from the lower mantle (e.g., Vinnik et al., 1995, Savage, 1999, Long, 2009). XKS splitting is hence a suitable tool to investigate seismic anisotropy in the upper mantle.
Section snippets
Data set
Seismic data analyzed in this study were recorded during the RHUM-RUM experiment (Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel; Barruol and Sigloch, 2013). This French–German experiment in the Western Indian Ocean (Fig. 1) deployed 20 broadband, three-component land seismometers between 2011 and 2016, and 57 broad- and wideband, three-component ocean-bottom seismometers (OBSs) between October 2012 and December 2013. Detailed station information is provided in the on-line
Methodology
We refer to our measurements as XKS splittings, meaning we recorded splitting mostly on SKS phases but occasionally on SKKS and pSKS phases, too.
Results and interpretation
20 land seismometers and 40 usable ocean-bottom seismometers (OBSs) yielded 74 non-null and 205 null XKS splitting measurements from 101 earthquakes (Fig. 1). The signal-to-noise ratio for all these measurements averages 9.9, the dominant frequency 0.1 Hz. The smallest earthquake magnitudes yielding splitting were on land station MAYO, and on OBS RR29 (Fig. 2). Individual measurements can be found in the supporting material and in our on-line XKS splitting data-base (Wüstefeld et
Discussion
Generally, for the Western Indian Ocean our XKS splitting observations largely coincide with the azimuthal anisotropies determined by global waveform studies (e.g., Debayle et al., 2016, Schaeffer et al., 2016) and mantle flow computations (e.g., Becker et al., 2008, Conrad and Behn, 2010 – the latter with constraints on the Mozambique Channel for example).
In Table 2, we classified our interpretations of our XKS splitting measurements performed on active or fossil mid-ocean ridges in the
Conclusions
As part of the RHUM-RUM project that investigates whole-mantle structure beneath the Réunion hotspot in the Western Indian Ocean, we presented XKS splitting measurements for 20 terrestrial and 40 ocean-bottom seismometers (Fig. 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 (
Acknowledgments
The presented XKS splitting measurements can be found in the on-line supplements and in our splitting data-base (http://splitting.gm.univ-montp2.fr/DB/public/searchdatabase.html), which is also mirrored at IRIS (Incorporated Research Institutions for Seismology, https://ds.iris.edu/spud/swsmeasurement). The RHUM-RUM project (http://www.rhum-rum.net) was funded by ANR (Agence Nationale de la Recherche) in France (project ANR-11-BS56-0013), and by DFG (Deutsche Forschungsgemeinschaft) in Germany
References (86)
- et al.
Bathymay: la structure sous-marine de Mayotte révélée par l'imagerie multifaisceaux
C. R. Géosci.
(2006) - et al.
Mantle flow beneath La Réunion hotspot track from SKS splitting
Earth Planet. Sci. Lett.
(2013) - et al.
Radial seismic anisotropy as a constraint for upper mantle rheology
Earth Planet. Sci. Lett.
(2008) - et al.
Deccan flood basalts at the Cretaceous/Tertiary boundary?
Earth Planet. Sci. Lett.
(1986) - et al.
Three distinct types of hotspots in the Earth's mantle
Earth Planet. Sci. Lett.
(2003) - et al.
Joint seismic-geodynamic-mineral physical modelling of African geodynamics: a reconciliation of deep-mantle convection with surface geophysical constraints
Earth Planet. Sci. Lett.
(2010) - et al.
Some comments on the effects of lower-mantle anisotropy on SKS and SKKS phases
Phys. Earth Planet. Inter.
(2004) - et al.
Active tectonics of the Alaotra–Ankay Graben System, Madagascar: possible extension of Somalian–African diffusive plate boundary?
Gondwana Res.
(2010) - et al.
Azimuthal anisotropy and phase velocity beneath Iceland: implication for plume–ridge interaction
Earth Planet. Sci. Lett.
(2003) Complex anisotropy in D″ beneath the eastern Pacific from SKS–SKKS splitting discrepancies
Earth Planet. Sci. Lett.
(2009)
Strain pattern and late Precambrian deformation history in southern Madagascar
Precambrian Res.
Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography
Earth Planet. Sci. Lett.
Earth structure and instrumental seismicity of Madagascar: implications on the seismotectonics
Tectonophysics
Forward modeling of the development of seismic anisotropy in the upper mantle
Earth Planet. Sci. Lett.
Identifying global seismic anisotropy patterns by correlating shear-wave splitting and surface–wave data
Phys. Earth Planet. Inter.
SplitLab: a shear-wave splitting environment in Matlab
Comput. Geosci.
Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations
Geophys. J. Int.
NOAA Technical Memorandum NESDIS NGDC-24
Geologically current motion of 56 plates relative to the no-net-rotation reference frame
Geochem. Geophys. Geosyst.
Investigating La Réunion hot spot from crust to core
Eos
Mapping upper mantle flow beneath French Polynesia from broadband ocean bottom seismic observations
Geophys. Res. Lett.
Toward a generalized plate motion reference frame
Geophys. Res. Lett.
ObsPy: a Python toolbox for seismology
Seismol. Res. Lett.
An updated digital model of plate boundaries
Geochem. Geophys. Geosyst.
Structure, Age and Evolution of the Mascarene Basin, Western Indian Ocean
Shear wave splitting across the Iceland hot spot: results from the ICEMELT experiment
J. Geophys. Res.
Seismic anisotropy beneath the Juan de Fuca plate system: evidence for heterogeneous mantle flow
Geology
First results from the UnderVolc High Resolution Seismic and GPS network deployed on Piton de la Fournaise volcano
Seismol. Res. Lett.
Oceanic lithosphere–asthenosphere boundary from surface wave dispersion data
J. Geophys. Res.
Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E)
J. Geophys. Res.
Shear wave splitting at the Hawaiian hot spot from the PLUME land and ocean bottom seismometer deployments
Geochem. Geophys. Geosyst.
Constraints on lithosphere net rotation and asthenospheric viscosity from global mantle flow models and seismic anisotropy
Geochem. Geophys. Geosyst.
An automatically updated S-wave model of the upper mantle and the depth extent of azimuthal anisotropy
Geophys. Res. Lett.
40Ar/39Ar geochronology of basement rocks from the Mascarene Plateau, the Chagos Bank, and the Maldives Ridge
Proc. Ocean Drill. Program Sci. Results
The volcanic record of the Reunion Hotspot
Proc. Ocean Drill. Program Sci. Results
Crustal and uppermost mantle structure variation beneath La Réunion hotspot track
Geophys. J. Int.
Upper-mantle flow beneath French Polynesia from shear wave splitting
Geophys. J. Int.
Shear-wave splitting beneath the Galápagos Archipelago
Geophys. Res. Lett.
Young tracks of hotspots and current plate velocities
Geophys. J. Int.
Variations in shear-wave splitting in young Pacific seafloor
Geophys. Res. Lett.
Timescales for the evolution of seismic anisotropy in mantle flow
Geochem. Geophys. Geosyst.
Mantle upwellings, melt migration and the rifting of Africa: insights from seismic anisotropy
Geol. Soc. (Lond.) Spec. Publ.
Cited by (17)
An isotopically enriched mantle component in the source of Rodrigues, Réunion volcanic hotspot
2023, Geochimica et Cosmochimica ActaMantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves
2022, Physics of the Earth and Planetary InteriorsCitation Excerpt :Such a contribution has been recently revealed by Barruol et al. (2019) investigating a possible plume-ridge interaction, linked by a channel flow near Réunion, as proposed by Morgan (1978, 1971). Revealing a coincidence between SWS (based on Scholz et al., 2018 and surface-wave fast orientations (based on Mazzullo et al., 2017, this study has found a roughly 100-150 km thick asthenospheric flow narrowing eastwards toward the ridge. The related depth distribution of fast orientation showed a spindle-shaped behavior.
Nature of the crust beneath the islands of the Mozambique Channel: Constraints from receiver functions
2021, Journal of African Earth SciencesCitation Excerpt :From the Middle Jurassic to the Early Cretaceous, the Davie Ridge accommodated the opening of two basins (Coffin and Rabinowitz, 1987). In the north, the Somali Basin resulted from the southward drift of Madagascar leading to oceanic accretion between ∼150 and 120 Ma (Ségoufin and Patriat, 1981) with N–S crustal extension along E-W trending ridges (Davis et al., 2016) supported by magnetic anomalies (Phethean et al., 2016) and SKS splitting measurements (Scholz et al., 2018) compatible with ridge-parallel asthenospheric flow. In the south, the Mozambique Basin is due to the separation of Africa and Antarctica that started at the end of the Jurassic (König and Jokat, 2010; Leinweber et al., 2013) and finished during the Early Cretaceous (e.g. Rabinowitz et al., 1983).
Depth dependent azimuthal anisotropy in Madagascar island from ambient noise tomography
2020, TectonophysicsCitation Excerpt :Using Rayleigh and Love wave dispersion measurements extracted from seismic ambient noise correlations, they obtained radial anisotropy patterns that broadly reflect the different geodynamic histories of the Morondava basin and the Precambrian shield. Seismic anisotropy have also been investigated for some of Madagascar's neighbouring regions like Seychelles (e.g., Barruol and Ismail, 2001), La Reunion (e.g., Barruol and Hoffmann, 1999; Barruol and Fontaine, 2013; Mazzullo et al., 2017), western Indian ocean (e.g., Scholz et al., 2018), Ethiopia (e.g., Hammond et al., 2005), East Africa (e.g., Bagley and Nyblade, 2013), etc. In East Africa, a NE-SW aligned fast polarization direction consistent with mantle flow from the African superplume was observed.
A trace of recycled continental crust in the Réunion hotspot
2019, Chemical GeologyCitation Excerpt :Small variations in the historical volcanic activity of the Réunion hotspot have been interpreted as being due to interaction of Réunion magmas with the oceanic crust or the volcanic edifice (Pietruszka et al., 2009). In contrast with this apparently simple geochemical model, recent seismic tomography and SKS splitting results depict the Réunion hotspot as a complex low-velocity structure interacting with the Central Indian Ridge (Mazzullo et al., 2017; Scholz et al., 2018). In addition, the recent discovery of Archean zircons in Mauritius has raised the possibility that plume magmas may also be contaminated by fragments of continental crust left behind when India separated from Madagascar (Torsvik et al., 2013; Ashwal et al., 2017).
Heterogeneous seismic anisotropy beneath Madeira and Canary archipelagos revealed by local and teleseismic shear wave splitting
2023, Geophysical Journal International