Mascarene Plateau Research Articles

Transport and transformation of surface water masses across the Mascarene Plateau during the Northeast Monsoon season

The Mascarene Plateau lies in the south-west Indian Ocean between the islands of Mauritius and the Seychelles Bank, and is characterised by a series of shallow banks separated by deep (>1 000 m), narrow channels. The plateau acts as an obstruction to the general ocean circulation in this region, separating the westward-flowing South Equatorial Current (SEC) into two branches downstream of the plateau. In this article, we present the results of a survey conducted along the entire Mascarene Plateau during the Northeast Monsoon, in October–November 2008. In addition, data from Argo floats were used to determine the origin of water masses entering this region. The plateau contains three gaps through which branches of the SEC are channelled. The northern, central and southern gaps receive 14.93 Sv, 14.41 Sv and 6.19 Sv, respectively. Although there are differences in water-mass properties to the west and east of the Mascarene Plateau due to mixing, the SEC acts as a sharp boundary between water masses of southern and northern Indian Ocean origin. Mixing occurs in the central gap between intermediate water masses (Red Sea Water [RSW] and Antarctic Intermediate Water [AAIW]) as well as in the upper waters (Subtropical Surface Water [STSW] and Indonesian Throughflow Water [ITW]). Through the northern gap, mixing occurs between Arabian Sea High-Salinity Water (ASHSW), ITW and Tropical Surface Water (TSW), while through the southern gap, mixing occurs between STSW and ITW. North Indian Deep Water (NIDW) is present in the region but the plateau appears to have no effect on it.

Anisotropic Tomography Around La Réunion Island From Rayleigh Waves

AbstractIn 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.

How plume‐ridge interaction shapes the crustal thickness pattern of the Réunion hotspot track

AbstractThe Réunion mantle plume has shaped a large area of the Earth's surface over the past 65 million years: from the Deccan Traps in India along the hotspot track comprising the island chains of the Laccadives, Maldives, and Chagos Bank on the Indian plate and the Mascarene Plateau on the African plate up to the currently active volcanism at La Réunion Island. This study addresses the question how the Réunion plume, especially in interaction with the Central Indian Ridge, created the complex crustal thickness pattern of the hotspot track. For this purpose, the mantle convection code ASPECT was used to design three‐dimensional numerical models, which consider the specific location of the plume underneath moving plates and surrounded by large‐scale mantle flow. The results show the crustal thickness pattern produced by the plume, which altogether agrees well with topographic maps. Especially two features are consistently reproduced by the models: the distinctive gap in the hotspot track between the Maldives and Chagos is created by the combination of the ridge geometry and plume‐ridge interaction; and the Rodrigues Ridge, a narrow crustal structure which connects the hotspot track and the Central Indian Ridge, appears as the surface expression of a long‐distance sublithospheric flow channel. This study therefore provides further insight how small‐scale surface features are generated by the complex interplay between mantle and lithospheric processes.

Continuous movement behavior of humpback whales during the breeding season in the southwest Indian Ocean: on the road again!

BackgroundHumpback whales are known to undertake long-distance migration between feeding and breeding sites, but their movement behavior within their breeding range is still poorly known. Satellite telemetry was used to investigate movement of humpback whales during the breeding season and provide further understanding of the breeding ecology and sub-population connectivity within the southwest Indian Ocean (SWIO). Implantable Argos satellite tags were deployed on 15 whales (7 males and 6 females) during the peak of the breeding season in Reunion Island. A switching-state-space model was applied to the telemetry data, in order to discriminate between “transiting” and “localized” movements, the latter of which relates to meandering behavior within putative breeding habitats, and a kernel density analysis was used to assess the spatial scale of the main putative breeding sites.ResultsWhales were tracked for up to 71 days from 31/07/2013 to 16/10/2013. The mean transmission duration was 25.7 days and the mean distance travelled was 2125.8 km. The tracks showed consistent movement of whales from Reunion to Madagascar, demonstrating a high level of connectivity between the two sub-regions, and the use of yet unknown breeding sites such as underwater seamounts (La Perouse) and banks (Mascarene Plateau). A localized movement pattern occurred in distinct bouts along the tracks, suggesting that whales were involved in breeding activity for 4.3 consecutive days on average, after which they resume transiting for an average of 6.6 days. Males visited several breeding sites within the SWIO, suggesting for the first time a movement strategy at a basin scale to maximize mating. Unexpectedly, females with calf also showed extensive transiting movement, while they engaged in localized behavior mainly off Reunion and Sainte-Marie (East Madagascar).ConclusionsThe results indicated that whales from Reunion do not represent a discrete population. Discrete breeding sites were identified, thereby highlighting priority areas for conservation. The study is a first attempt to quantify movement of humpback whales within the southwestern Indian Ocean breeding range. We demonstrate a wandering behavior with stopovers at areas that likely represent key breeding habitat, a strategy which may enhance likelihood of individual reproductive success.

Depth to Curie temperature or bottom of the magnetic sources in the volcanic zone of la Réunion hot spot

We present an innovative study to generalize Curie Point Depth (CPD) determinations at the scale of oceanic volcanic islands, an approach which has previously focused largely on continental areas. In order to determine the validity of this technique in oceanic environments, we first tested the approach on sets of sea-floor-spreading anomalies. Assuming that magnetic anomalies are concentrated within the oceanic crust and uppermost mantle, the Curie depth should deepen as oceanic lithosphere increases in age and thickness away from spreading centers. The calculated depths to the magnetic bottom are in agreement with this general pattern. On the basis of this test, we then applied the method to La Réunion Island and surrounding oceanic lithosphere. The calculated extent of magnetic sources lies at depths between 10 and 30km and exhibits a complex topography, presumably caused by a combination of various magmatic and tectonic lithospheric structures. These calculations indicate that magnetic sources extend well below the crust-mantle interface at this location. To the first order, the bottom of the magnetic surface shallows beneath Réunion and Mauritius Islands due to the thermal effect of the hot spot, and deepens away from La Réunion edifice. On the scale of the Mascarene Basin, several discontinuities in the CPD correlate well with major fracture zones.

Volcanism of hot spots on the Réunion as the example

Reunion (French: La Reunion, formerly: Īle Bourbon) is beside the Mauritius the largest and the youngest island on the Mascarene Plateau in the Indian Ocean. The island is of volcanic origin, formed through volcanic activity initiated by deeply settled “hot spot” inside the Earth’s mantle. It was formed within the last 2 million years from lava effusions whose volume is up to now nearly 75,000 km3. Only 3% of its volume is emergent and the total height from the sea floor is 6,500 m. The island is about 40 km in width with about 800,000 inhabitants. The capital city is St Denis. The nature of the island is impressive: active volcano, mountains above 3,000 m in height and deep canyon, frequently exceeding 2,000 m in depth. Since 2010, about 40% of its area is announced a National park and a UNESCO World Heritage site.

Redescriptions of Polysteganus coeruleopunctatus (Klunzinger 1870) and P. lineopunctatus (Boulenger 1903), with two new species from Western Indian Ocean.

Five valid species in the genus Polysteganus (Pisces; Sparidae) from the Western Indian Ocean are currently known: P. baissaci Smith 1978, P. coeruleopunctatus (Klunzinger 1870), P. mascarenensis Iwatsuki & Heemstra, 2011, P. praeorbitalis (Günther 1859), and P. undulosus (Regan 1908). Although P. lineopunctatus (Boulenger 1903) has long been synonymized under P. coeruleopunctatus, both species are redescribed as valid. Two new species of Polysteganus were discovered in the course of this review. Polysteganus flavodorsalis n. sp. is described on the basis of six type specimens (143-265 mm SL) from Nazareth Bank, Mascarene Plateau, Indian Ocean, and P. cerasinus n. sp. is described based on the holotype (134 mm SL) from Saya de Malha Bank. Provisional distribution patterns of the eight valid species of Polysteganus in the Western Indian Ocean are discussed. A key to the species of Polysteganus is provided.

Biodiversity and distribution of leptocephali west of the Mascarene Plateau in the southwestern Indian Ocean

Marine eels that live on the continental shelf, slope and deep-sea and in the pelagic realms of the ocean are distributed worldwide at tropical and subtropical latitudes, but little information is available about their biodiversity and ecology in most areas. Collections of their leptocephalus larvae that all live together in the upper few hundred meters of the ocean are one way to learn about the distributions, reproductive ecology, and life histories of these difficult to observe fishes. This study evaluated the biodiversity and spatial distributions of leptocephali collected during two surveys conducted to the west of the shallow banks of the Mascarene Plateau in the southwestern Indian Ocean. 1140 leptocephali of at least 94 species of ⩾14 families of anguilliform and notacanthid fishes were collected in Nov.–Dec. 2006 and Jan.–Feb. 2010 in overlapping areas west of the plateau. In 2006, 565 leptocephali of at least 71 species were collected at 42 stations in two transects from 8 to 18°S, with larvae of shallow-water eel species of the Congridae, Muraenidae, Ophichthidae and Chlopsidae being most abundant at the northern stations in warmer water to the west of the Mascarene Plateau. Some small leptocephali of those eel families appeared to be transported offshore by currents from areas including the Seychelles, but the distribution of the leptocephali of mesopelagic eels of the Nemichthyidae and Serrivomeridae were more widespread. In 2010, 575 leptocephali of at least 53 species were collected in a dense grid of stations with the smallest larvae of the Muraenidae and Ophichthidae being near the banks, and spawning occurring at the Cargados Carajos Bank. Small larvae of Conger collected in the southwestern edge of the grid indicated offshore spawning, which is similar to Conger species in the Atlantic and Pacific. Seasonal and geographic differences in the characteristics or compositions of leptocephali were apparent, with higher proportions of small larvae in 2010 than in 2006, but cluster analysis of assemblage structure found similar geographic structure between the two surveys. Thalassenchelys larvae were only collected in the same southern area in both surveys and nettastomatid and notacanthid larvae were only collected in the north in 2006. The two surveys suggested that there is a relatively high biodiversity of eels in the Mascarene Plateau area compared to some regions and that colder water temperatures may inhibit spawning of eels on the southern banks. The two types of sampling strategies (transects or grid survey) each had different advantages, which can be considered for use in future oceanographic surveys for leptocephali in other regions of the world.

Stable isotopic composition of anguilliform leptocephali and other food web components from west of the Mascarene Plateau

Leptocephali are the poorly-understood transparent larvae of elopomorph fishes that live in the ocean surface layer throughout the world’s tropical and subtropical oceans. Their feeding ecology has been difficult to understand because they appear to primarily feed on particulate organic material (POM), which contains few identifiable objects, and there have been few studies on their diets or trophic positions. This study presents the first comparative results on the stable isotope ratios of 7 families of leptocephali in relation to the ratios of 30 taxa of other marine animals and POM samples. The carbon and nitrogen stable isotope ratios were analyzed using 50 specimens of leptocephali, 354 specimens of mesozooplankton, cephalopods, fishes, and POM collected west of the Mascarene Plateau in the western Indian Ocean. Nitrogen and carbon isotopic ratio analyses indicated that the 12 taxa of DNA barcoded leptocephali (⩾15 species) could be separated into 2 groups of species with either higher (Group 1: 9 taxa of 7 families, 25–91mm) or lower (Group 2: 3 taxa of 2 families, 43–275mm) δ15N ranges. Group 2 exclusively included species that reach much larger sizes of >150–200mm (Nemichthys and Avocettina, 3 species of Ariosoma-type), whereas Group 1 included Anguilla bicolor bicolor, Serrivomeridae, Muraenidae, Congridae (3 species), Chlopsidae, Ophichthidae (2 species), and Thalassenchelys. Differences in feeding depths, the types of POM ingested by preference or because of different jaw morphology, or the transport of larvae from other regions with different isotopic signatures are possible reasons for the differences between the two groups. The isotopic signatures of 14 taxa of copepods had higher but slightly overlapping δ15N and δ13C signatures compared to leptocephali and most crustaceans and other mezozooplankton, cephalopods and mesopelagic fish taxa had even higher values. The δ15N and δ13C signatures and composition of POM were variable spatially and with depth and may have been influenced by particulates originating from the shallow banks of the Mascarene Plateau. The two apparent isotopic groups of leptocephali should be examined in relation to their consumption of POM, which can include various proportions of prokaryotes, phytoplankton, protozoans, discarded appendicularian houses and other materials, by conducting further studies in different regions and using a variety of techniques.

A new species of Saurida (Pisces: Synodontidae) from the Mascarene Plateau, Western Indian Ocean.

A new species of lizardfish, Saurida tweddlei n.sp., from the Mascarene Plateau, Western Indian Ocean, is described and figured. The new species is characterised by the following combination of characters: dorsal fin with 12-13 rays; pectorals with 14-15 rays; lateral-line scales 53-55; transverse scale rows above lateral line 4½, below lateral line 5½; pectoral fins moderately long (extending to or just beyond a line from origin of pelvic fins to origin of dorsal fin); 2 rows of teeth on outer palatines; 0-3 teeth on vomer; tongue with about 4-5 rows of teeth posteriorly; caudal peduncle compressed (depth greater than width); stomach and intestine pale whitish. A key to the species of Saurida of the Western Indian Ocean is provided.

Re-examination of geophysical data off Northwest India: Implications to the Late Cretaceous plate tectonics between India and Africa

Abstract The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models.

Absolute plate motion of Africa around Hawaii-Emperor bend time

Numerous regional plate reorganizations and the coeval ages of the Hawaiian Emperor bend (HEB) and Louisville bend of 50–47 Ma have been interpreted as a possible global tectonic plate reorganization at ∼chron 21 (47.9 Ma). Yet for a truly global event we would expect a contemporaneous change in Africa absolute plate motion (APM) reflected by physical evidence distributed on the Africa Plate. This evidence has been postulated to take the form of the Réunion-Mascarene bend which exhibits many HEB-like features, such as a large angular change close to ∼chron 21. However, the Réunion hotspot trail has recently been interpreted as a sequence of continental fragments with incidental hotspot volcanism. Here we show that the alternative Réunion-Mascarene Plateau trail can also satisfy the age progressions and geometry of other hotspot trails on the Africa Plate. The implied motion, suggesting a pivoting of Africa from 67 to 50 Ma, could explain the apparent bifurcation of the Tristan hotspot chain, the age reversals seen along the Walvis Ridge, the sharp curve of the Canary trail, and the diffuse nature of the St. Helena chain. To test this hypothesis further we made a new Africa APM model that extends back to ∼80 Ma using a modified version of the Hybrid Polygonal Finite Rotation Method. This method uses seamount chains and their associated hotspots as geometric constraints for the model, and seamount age dates to determine APM through time. While this model successfully explains many of the volcanic features, it implies an unrealistically fast global lithospheric net rotation, as well as improbable APM trajectories for many other plates, including the Americas, Eurasia and Australia. We contrast this speculative model with a more conventional model in which the Mascarene Plateau is excluded in favour of the Chagos-Laccadive Ridge rotated into the Africa reference frame. This second model implies more realistic net lithospheric rotation and far-field APMs, but fails to explain key details of the Atlantic Ocean volcanic chains. Both models predict a Canary plume influence beneath the Madeiras. Neither model, when projected via the global plate circuit into the Pacific, predicts any significant change in plate motion around chron 21. Consequently, Africa APM models do not appear to provide independent support for a chron 21 global reorganization.

Long-term observations of the East Madagascar Undercurrent

An array of five moorings was deployed at 23°S off eastern Madagascar and maintained for about 2.5years as part of the “INdian-ATlantic EXchange in present and past climate” (INATEX) experiment. The observations reveal a recurrent equatorward undercurrent (during 692 of 888days), the East Madagascar Undercurrent (EMUC), flowing below the poleward surface East Madagascar Current (EMC). The average core of the undercurrent was found near the continental slope, at a depth of 1260m and at an approximate distance of 29km from the coast, with mean velocities of 6.4 (±4.8)cms−1. Maximum speeds reach 20cms−1. The mean equatorward volume transport is estimated to be 1.33 (±1.41)Sv with maxima up to 6Sv. The baroclinic/barotropic partitioning of the geostrophic flow shows a persistent equatorward baroclinic velocity in the undercurrent core, which is sometimes inhibited by a stronger poleward barotropic contribution. The wavelet spectrum analysis of the transport time series displays two dominant frequency bands: (i) nearly bi-monthly (46–79days), previously observed in the surface EMC, and attributed to the forcing of barotropic waves generated in the Mascarene Basin; and (ii) nearly semi-annual (132–187days), which seems to be related to the semi-annual cycle in the equatorial winds near the Indian Ocean eastern boundary. A historical dataset of temperature–salinity Argo profiles was used to investigate the spatial variability of the thermohaline properties at intermediate levels. Lastly, Argo-derived velocities suggest an undercurrent flowing upstream until approximately 17°S.

Internal waves on the upstream side of a large sill of the Mascarene Ridge: a comprehensive view of their generation mechanisms and evolution

In this paper we aim to clarify the generation of Internal Solitary Waves (ISWs) at work to the east of the Mascarene Plateau (Indian Ocean) using Synthetic Aperture Radar (SAR) imagery and MITgcm fully nonlinear and nonhydrostatic simulations. Realistic representations of stratification and bathymetry are used with asymmetric tidal forcing (including the steady South Equatorial Current which is assumed barotropic in the model) along a 2D transect aligned with the propagation direction of the wave signatures identified in the SAR. The combined flow (i.e. steady and tidal currents) is subcritical with respect to first-mode Internal Waves (IWs), but supercritical with respect to higher wave modes. Different types of nonlinear wave trains with distinct origins (i.e. tidal phase and location) have been identified with the combined aid of model and SAR: (1) large-scale primary mode-1 ISWs evolve from the disintegration of a multimodal baroclinic structure that appears on the upstream side of the sill; (2) mode-2 ISW-like waves that evolve from this same baroclinic structure and are arrested over the sill before being released upstream at the change of flow condition; (3) a large mode-2 lee wave is generated downstream of the sill (i.e. on the west side), which is trapped there during maximum westward tidal flow and released upstream when the tide relaxes; and (4) mode-2 ISW-like waves whose length-scales are O (20km) appear some 50km upstream of the sill, after an Internal Tide (IT) beam scatters into the pycnocline, itself originating from critical topography on the leeward (i.e. westward) side of the sill. The underwater sill being investigated is in the mixed-tidal-lee wave regime, where the internal tide release mechanism, lee wave generation and IT beams can coexist. The large-scale mode-2 ISW-like waves that form far upstream from the sill are long-lived features and can be identified in the SAR due to associated short-scale mode-1 ISWs which propagate with the same phase speed, i.e. in resonance. This coupling is also seen in the model, and here it is argued that the formation of those mode-2 ISW-like waves appears to originate from the IT beam after it reflects from the sea surface and interacts with the pycnocline, a generation mechanism referred in the literature as “local generation of ISWs”. This IW generation process may be easily overlooked and could be at work in many more regions of the world than previously thought.

Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea

Microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume–ridge interactions. India separated from Seychelles at ∼64 Ma by asymmetric sea floor spreading initially when the spreading centre in the Mascarene Basin jumped towards the Indian sub-continent between magnetic chrons C29 and C28. The subsequent tectonics is difficult to comprehend since Laxmi Ridge-another microcontinent-formed during the later phase. Most of the studies considered the Laxmi Ridge as a sliver. Others considered it to be oceanic crust. High resolution, deep (∼25 km) seismic data reveals that (i) the ridge possesses > 15 km deep sea-ward dipping reflector (SDR) packages; (ii) normal faulted rift valleys devoid of syn-rift sedimentary packages; and (iii) axial magma chambers 5–7 km beneath the ridge top. Additionally, from 2D forward gravity models we deduce that the ridge most possibly comprises of high density (oceanic) crust. We conclude the Laxmi Ridge to be indeed composed of oceanic crust and a fossil spreading centre. We thus identified the ridge jumps and their relation to the Seychelles microcontinent separation.Previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating, spreading rate at the ridge, and plate ages. For repeated ridge jumps, the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating. We find that the medium spreading rates and high magmatic heating due to the Réunion plume and young plates favoured rapid and repeated ridge jumps towards the plume.

Scleractinian coral communities of the inner Seychelles 10 years after the 1998 mortality event

ABSTRACTThe size structure and taxonomic composition of coral communities in the inner (Granitic) Seychelles were studied 10 years after a thermal stress‐induced mass mortality event.A survey of the abundance, population size structure and community composition of hard corals across 21 sites from three different geomorphological reef types on the Seychelles Bank provided high resolution data for discriminating coral communities based on diversity, taxonomic composition, colony abundance, surface area and size frequency distributions.Results emphasize the severely impoverished and depauperate nature of inner Seychelles hard coral communities, which had lower generic diversity (40 genera recorded), and lower abundance (3.3 colonies m−2) of hard corals (excluding juveniles) than other coral reef regions of the Indian Ocean for which comparable data are currently available.Analysis of coral communities indicated that management had no appreciable effect on juvenile or adult coral abundance at this point in time, and that low juvenile density (9.9 colonies m−2) may severely limit recovery of many individual reefs in the inner Seychelles.While some sites were found to have appreciable coral cover (>20%), others, including long‐standing protected areas with no fishing, are now in an advanced state of erosion and framework collapse with very low juvenile coral replenishment and negligible available hard substratum suitable for coral settlement.Some of these reefs may have passed the threshold of viable recovery, now being in a self‐reinforcing, non‐coral dominated erosional phase.These findings indicate variable coral community condition, with many sites showing little sign of recovery. If persistence of live hard coral is a management goal, the existing protected areas within the Seychelles Bank may require review to ensure protection of sites with high recovery potential, while a suite of other management tools should be implemented in the remaining areas. Copyright © 2014 John Wiley & Sons, Ltd.

Circulation around La Réunion and Mauritius islands in the south-western Indian Ocean: A modeling perspective

The objective of this study is to document the circulation in the vicinity of La Reunion and Mauritius islands, i.e., within 500 km offshore, on the intraseasonal time scale, using a high-resolution realistic modeling strategy. The simulated sea level anomalies, water mass properties, and large-scale circulation compare favorably with satellite and in situ observations. Our high-resolution simulation suggests that the currents around the islands are maximal locally, oriented southwestward, to the southeast of both islands which is not visible in low-resolution satellite observations. It also highlights the high degree of variability of the circulation, which is dominated by westward propagating features. The predominant time scale of variability is 60 days. This coincides with the period of a barotropic mode of variability confined to the Mascarene Basin. The characteristics of the westward propagating anomalies are related to baroclinic Rossby waves crossing the Indian Ocean but only in the long-wave resting ocean limit. Tracking those anomalies as eddies shows that they also have a meridional tendency in their trajectory, northward for cyclones and southward for anticyclones, which is consistent with previous studies. Sensitivity experiments suggest that they are predominantly advected from the east, but there is also local generation in the lee of the islands, due to interaction between the circulation and topography.

Cretaceous to present kinematics of the Indian, African and Seychelles plates

An iterative inverse model of seafloor spreading data from the Mascarene and Madagascar basins and the flanks of the Carlsberg Ridge describes a continuous history of Indian–African Plate divergence since 84 Ma. Visual-fit modelling of conjugate magnetic anomaly data from near the Seychelles platform and Laxmi Ridge documents rapid rotation of a Seychelles Plate about a nearby Euler pole in Palaeocene times. As the Euler pole migrated during this rotation, the Amirante Trench on the western side of the plate accommodated first convergence and later divergence with the African Plate. The unusual present-day morphology of the Amirante Trench and neighbouring Amirante Banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase. The model fits FZ trends in the north Arabian and east Somali basins, suggesting that they formed in India–Africa Plate divergence. Seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71–69 Ma, and that upon arrival of the Deccan–Réunion plume and an increase to faster plate divergence rates in the period 69–65 Ma, segments of the ridge lengthened and propagated. Ridge propagation into the Indian continental margin led first to the formation of the Laxmi Basin, which accompanied extensive volcanism onshore at the Deccan Traps and offshore at the Saurashtra High and Somnath Ridge. A second propagation episode initiated the ancestral Carlsberg Ridge at which Seychelles–India and India–Africa Plate motions were accommodated. With the completion of this propagation, the plate boundaries in the Mascarene Basin were abandoned. Seafloor spreading between this time and the present has been accommodated solely at the Carlsberg Ridge.

The extent of continental crust beneath the Seychelles

The granitic islands of the Seychelles Plateau have long been recognised to overlie continental crust, isolated from Madagascar and India during the formation of the Indian Ocean. However, to date the extent of continental crust beneath the Seychelles region remains unknown. This is particularly true beneath the Mascarene Basin between the Seychelles Plateau and Madagascar and beneath the Amirante Arc. Constraining the size and shape of the Seychelles continental fragment is needed for accurate plate reconstructions of the breakup of Gondwana and has implications for the processes of continental breakup in general. Here we present new estimates of crustal thickness and VP/VS from H–κ stacking of receiver functions from a year long deployment of seismic stations across the Seychelles covering the topographic plateau, the Amirante Ridge and the northern Mascarene Basin. These results, combined with gravity modelling of historical ship track data, confirm that continental crust is present beneath the Seychelles Plateau. This is ∼30–33 km thick, but with a relatively high velocity lower crustal layer. This layer thins southwards from ∼10 km to ∼1 km over a distance of ∼50 km, which is consistent with the Seychelles being at the edge of the Deccan plume prior to its separation from India. In contrast, the majority of the Seychelles Islands away from the topographic plateau show no direct evidence for continental crust. The exception to this is the island of Desroche on the northern Amirante Ridge, where thicker low density crust, consistent with a block of continental material is present. We suggest that the northern Amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup.

Ridge push, mantle plumes and the speed of the Indian plate

SUMMARY The buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics. In late Cretaceous and early Paleogene times, the Indian plate moved more rapidly over the mantle than freely subducting slabs sink within it. This signal event has been attributed to arrival of the Deccan-Rmantle plume beneath the plate, but itisunknowninwhichproportionstheplumeactedtoalterthebalanceofexistingplatedriving torques and to introduce torques of its own. Our plate kinematic analysis of the Mascarene Basin yields a detailed Indian plate motion history for the period 89-60Ma. Plate speed initially increases steadily until a pronounced acceleration in the period 68-64Ma, after which it abruptly returns to values much like those beforehand. This pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate. A simple analysis of the gravitational force related to the Indian plate's thickening away from its boundary with the African plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head. In this instance, torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion.