Seismic anisotropy and discontinuities of the lithosphere-asthenosphere system beneath La Réunion and the southern Mascarene Plateau
Final Report Abstract
This project is based on seismological data obtained from the deployment of temporary seismic networks on Mauritius, Rodrigues and Seychelles as part of the Reunion Hotspot and Upper Mantle– Reunions Unterer Mantel (RHUM–RUM) project. Additional data from the permanent stations were also used in the studies. The investigations and results are presented under three themes, namely: (1) crustal structure beneath Mauritius, (2) upper mantle anisotropy below Mauritius, Rodrigues and Seychelles and (3) intraplate seismicity in the Rodrigues–CIR region. Previously, information on upper mantle anisotropy in south-west Indian Ocean region was limited, especially for the islands of Mauritius and Rodrigues. From a joint-splitting analysis of the new data, we obtained a fast-polarisation direction (φ) dominant in N80°E and a delay time (δt) of ≈ 0.85 s for Mauritius and φ trending East–West in Rodrigues with δt ≈ 1.1 s. A parabolic asthenospheric flow model explains the orientation of the fast-polarisation direction beneath Mauritius, whereas deep mantle circulation patterns can explain the horizontal alignment of the fast-polarisation direction in Rodrigues slightly better than a model based on plume-ridge interaction. From Seychelles data, the results show φ trending NE and δt ≈ 0.74 s, even for the islands close to the Amirante Ridge, suggesting an asthenospheric deformation induced by relative motion between the plate and the deeper mantle. It has recently been suggested that the volcanic island of Mauritius may be underlain by a remnant of continental origin termed “Mauritia.” To constrain the crustal thickness beneath Mauritius, we derived 382 P-receiver functions from data recorded by the stations on the island. We applied the H–κ stacking technique and obtained the crustal thickness values varying between 10 and 15 km. The observed considerable variation in the VP/VS ratio (κ) is attributed to the lack of clearly identifiable multiples in the receiver functions. Using forward modelling of receiver functions, we show that the lack of clear multiples can be explained by a transitional Moho with a gradual increase of velocity. The modelling further indicates that the thickness of this gradient zone is approximately 10 km. The findings suggest oceanic crust thickened by crustal underplating due to the mantle plume currently located beneath La Reunion rather than a remnant of crustal origin. Seismicity around Rodrigues Island has been generally associated with events near the Central Indian Ridge as recorded by the global networks. Using seismological array techniques on data collected from a temporary seismic array on Rodrigues Island 62 new events were detected and located, not reported by any global network. Determinations of backazimuth and apparent velocity were performed by applying a time-domain array analysis. Event distances were calculated using a 1-D velocity model and the measured travel-time differences between S- and P-wave arrivals. Local magnitudes of the events were obtained by removing the velocity response from the seismographs and then convolving with the Wood–Anderson transfer function to obtain ground displacements. Most of the newly-detected events are located off the ridge axis and can be classified as intraplate events. Three different seismic clusters were observed around the island. The majority of the events were localised to the north-east of Rodrigues at a distance of approximately 138 km from the reference station. A distinguishable swarm of earthquakes was observed on the west of the spreading segment from March to April 2015. The local magnitudes (ML) of the events varied between 1.6 and 3.7. Degassing of CO2 form the mantle is suggested as a possible mechanism for the earthquake swarms.
Publications
- (2020) Seismic gaps and intraplate seismicity around Rodrigues Ridge (Indian Ocean) from time domain array analysis. Solid Earth 11 (6) 2557–2568
Singh, Manvendra; Rümpker, Georg
(See online at https://doi.org/10.5194/se-11-2557-2020) - (2015), Crustal and uppermost mantle structure variation beneath La Réunion hotspot track, Geophys. J. Int., 203, 107-126
Fontaine, F. R., Barruol, G., Tkalcic, H., Wölbern, I., Rümpker, G., Bodin, T. and Haugmardet M.
(See online at https://doi.org/10.1093/gji/ggv279) - (2015), Generalization of H-κ stacking method to anisotropic media, J. Geophys. Res., 120 (7), 5135-5153
Kaviani, A. and Rümpker, G.
(See online at https://dx.doi.org/10.1002/2014JB011858) - (2016), Limitations of H-κ stacking: ambiguous results caused by crustal layering, Journal of Seismology 21 (1), 221-235
Wölbern, I. and Rümpker, G.
(See online at https://doi.org/10.1007/s10950-016-9599-z) - (2016), The crustal structure beneath Mauritius from teleseismic P-receiver functions - oceanic or continental?, Geophys. Res. Lett., 43, 9636–9643
Singh, M., A. Kaviani, and Rümpker, G.
(See online at https://doi.org/10.1002/2016GL070529) - (2017), Seismic Anisotropy, Crustal Thickness Variations and Intraplate Seismicity in the South-West Indian Ocean, Dissertation, Goethe-University Frankfurt
Manvendra Singh
- (2017), SplitRacer: MATLAB code and GUI for semiautomated analysis and interpretation of teleseismic shear-wave splitting, Seismological Research Letters, 88
Reiss, M. and Rümpker, G.
(See online at https://doi.org/10.1785/0220160191)
