Chagos-Laccadive Ridge Research Articles

Population characteristics unravel occurrence of distinct stocks of yellowfin tuna (Thunnus albacares) population in the Indian seas

Fishery and biological observations of yellowfin tuna caught from seas surrounding theIndian mainland and its island territories have indicated the presence of biologically distinctstocks in the region. Those caught around the submerged mounts of Chagos-LaccadiveRidge in the Lakshadweep Sea, off Kerala-Karnataka coast exhibit early maturity, small sizeat maturity, short lifespan and a close association with sea mounts. In contrast, those caughtoff other coasts of the mainland, including Andaman waters, exhibit late maturity and largersizes at maturity indicating a fishery dominated by larger-sized individuals. These findingssuggest that the yellowfin tuna associated with the Lakshadweep Sea mounts constitutesa distinct stock, highlighting the necessity for comprehensive genetic, morphometric andpopulation investigations to confirm their stock status. Keywords:Chagos-Laccadive Ridge, Indian mainland,Lakshadweep Sea, Maturity, Stock, Sea mounts,Yellowfin tuna

Mineralogy, geochemistry and genesis of the polymetallic crusts from the Chagos-Laccadive Ridge, Arabian Sea, India

Recent offshore exploration by the Geological Survey of India (GSI) located ferromanganese crusts (FeMn) along the Chagos-Laccadive Ridge (CLR) offshore western continental margin of India. CLR FeMn crusts grew on phosphorite substrate rock (phosphorite cemented sand/silt with moderate carbonate fluorapatite (CFA) cement and goethite). Phosphorite precipitation is the product of intense upwelling and phosphorus regeneration due to organic matter degradation and fish debris dissolution. CLR FeMn crusts are of hydrogenetic origin precipitated in an oxic environment. As a result of hydrogenetic processes under oxic conditions, Fe-vernadite dominated the mineral phase of FeMn crusts. CLR FeMn crusts formed when the North Indian deep-water layer mixed with oxygen-depleted waters from the Arabian Sea, Persian Gulf, and Red Sea. Three genetic layers are identified in the FeMn crusts; Type-I, Type-II, and Type-III developed in response to fluctuation in dissolved oxygen during periods of intensified suboxic bottom waters and monsoon maxima associated with past climatic events in the Arabian Sea. Type-I layers are columnar showing low Mn/Fe ratio (∼1), reflectivity, Mg (average 1.24%), and Ni (average 0.29%). Dendritic Type-II layers show high Mn/Fe ratios (average 12.29), reflectivity, Mg (average 3.51%) and Ni (average 1.69%). Type-III layers show very low Mn/Fe ratios (average 0.21), reflectivity, Mg (average 1.4%) and Ni (average 0.01%) and contain a substantial amount of terrigenous debris of aeolian origin. Elements of economic interest such as Mn, Co, Ni, Cu, Mo, Li, Te, and REY in CLR FeMn crusts are depleted owing to terrigenous contribution.

Changes of Oceanic Conditions Drive Chagos Whale Migration Patterns in the Central Indian Ocean

Marine ecosystems are experiencing rapid shifts under climate change scenarios and baleen whales are vulnerable to environmental change, although not all impacts are yet clear. We identify how the migration behaviour of the Chagos whale, likely a pygmy blue whale (Balaenoptera musculus brevicauda), has changed in association with shifts in environmental factors. We used up to 18 years of continuous underwater acoustic recordings to analyse the relationships between whale acoustic presence and sea surface temperature (SST), chlorophyll-a concentration, El-Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). We compared these relationships between two independent sites Diego Garcia southeast (DGS) and Diego Garcia northwest (DGN) where Chagos whales are detected and are suspected to move interannually across the Chagos-Laccadive ridge. We showed that the number of whale songs detected increased on average by 7.7% and 12.6% annually at DGS and DGN respectively. At the DGS site, Chagos whales shifted their arrival time earlier by 4.2 ± 2.0 days/year ± SE and were detected for a longer period by 7.3 ± 1.2 days/year ± SE across 18 years. A larger number of songs were detected during periods of higher chlorophyll-a concentration, and with positive IOD phases. At the DGN site, we did not see an earlier shift in arrival and songs were not detected for a longer period across the 13 years. Whale presence at DGN had a weaker but opposite relationship with chlorophyll-a and IOD. The oceanic conditions in the Indian Ocean are predicted to change under future climate scenarios and this will likely influence Chagos whale migratory behaviour. Understanding how environmental factors influence whale movement patterns can help predict how whales may respond to future environmental change. We demonstrate the value of long-term acoustic monitoring of marine fauna to determine how they may be affected by changing environmental conditions.

Crustal structure and tectonic evolution of Greater Maldive Ridge, Western Indian Ocean, in the context of plume-ridge interaction

The Greater Maldive Ridge (GMR), consisting of the Maldive Ridge (MR) and Deep Sea Channel (DSC) region, is the N-S trending, middle segment of Chagos-Laccadive Ridge system. This ridge system is widely considered to be the repository representing the interaction between Central Indian Ridge spreading centre and Reunion plume. The present study investigates in detail, spatial variations in effective elastic thickness (Te), isostasy and crustal structure of GMR using high resolution satellite derived gravity, residual geoid and bathymetry data. Estimated Te values along the GMR from 2D and 3D flexural modelling ranges from 6.5–16.5 km with comparatively lower Te values over MR (7–9 km) and slightly higher values over the DSC region (>10 km). Geoid to Topography Ratio computed for two wavelength bands shows almost similar kind of variation along the ridge with a maximum value of 1.4 m/km in the DSC region, decreasing northwards to 0.6 m/km over MR. Integrating results from the present study with crustal thickness, Moho undulations and Curie depth along the entire length of the GMR suggest that MR was formed in the vicinity of spreading centre while DSC region was under a long transform fault which has given rise to the gap zone between Chagos Bank and Maldive Ridge during plume-ridge interaction.

Generation, propagation and dissipation of internal tides on the continental shelf and slope off the west coast of India

The present study focuses on the dynamics of M2 internal tides on the shelf and slope off the west coast of India (WCI) using a high-resolution (1/48∘) numerical ocean circulation model configured for the eastern Arabian Sea. The WCI is characterized by the presence of large internal tides where most of the internal tides generated on shelf-slope regions propagate offshore as beams. Baroclinic energy budget estimates show that about 2.4 GW of barotropic tides gets converted to baroclinic tides over the region. The regions of shelf breaks and upper slope around 16–18.5∘ N and the northern part of the Chagos-Laccadive Ridge (8–14∘ N) are the two most important regions of internal tide generation along the WCI, which together contribute to more than 50% of total area-integrated M2 conversion over the domain. The diapycnal diffusivity for internal tide dissipation exceeds 10−3m2/s in these regions. The central and northern slope regions off the WCI are dominated by locally generated internal tides, whereas part of internal tides in the southern region is characterized by the presence of remotely-generated internal tides coming from the Chagos-Laccadive Islands. Internal tides experience significant seasonal variation on the shelf owing to the variation in stratification, while their variations are negligible on the slope, where the seasonal variation in stratification is not significant.

Cetacean Distribution and Diversity in Lakshadweep Waters, India, Using a Platform of Opportunity: October 2015 to April 2016

Prior stranding records suggest that at least 12 cetacean species occur within the Lakshadweep archipelago off the southwest coast of India. These islands consist of coral atolls and form the northern part of the undersea Chagos-Laccadive ridge. Distinct oceanographic features, seasonal monsoon cycles, and high productivity make this region a potentially rich cetacean habitat In this article, we report findings from the first systematic visual cetacean surveys, which were conducted from high-speed passenger ferries that sail between nine Lakshadweep islands. The surveys were carried out between October 2015 and April 2016 during both the northeast monsoon (October to December) and inter-monsoon (January to April) seasons. We used a line-transect survey framework to record sightings as well as group size estimates. We documented 139 sightings over 3,880 km of which 78 sightings were during systematic survey effort. Eight odontocete species were confirmed from these sightings: Stenella longirostris, S. attenuata, S. coeruleoalba, Tursiops spp., Globicephala macrorhynchus, Pseudorca crassidens, Grampus griseus, and Feresa attenuata. One Balaenoptera sp. was also encountered during this survey. S. longirostris was sighted the most often (n = 22) followed by Tursiops spp. (n = 18) and G. macrorhynchus (n = 13). We documented significantly higher sightings in the northeast monsoon season compared to the intermonsoon season. Along ferry routes, cetacean species differed significantly from each other with respect to their associations with seafloor slope gradients and distances to nearest landmass. We encountered mixed species assemblages of G. macrorhynchus with Tursiops sp. and S. attenuata with Tursiops sp. Given the confirmed high cetacean diversity and occurrence in this region, there is a need for in-depth, long-term studies on biogeography, ecology, and population status of cetaceans here.

Isostasy and crustal structure of the Chagos–Laccadive Ridge, Western Indian Ocean: Geodynamic implications

The Chagos–Laccadive Ridge (CLR), a prominent linear aseismic ridge in the Western Indian Ocean is believed to be a trace of the Réunion hotspot. In order to understand the mode of emplacement of this ridge and the nature of the underlying crust, we carried out three-dimensional (3D) flexural modelling and coherence analysis of satellite-derived gravity and bathymetry data along the ridge. The analysis revealed variations in Effective Elastic Thickness ( $$T_{\mathrm{e}}$$ ) along the CLR. While the northernmost part of CLR is associated with low $$T_{\mathrm{e}}$$ of 3 km with a subsurface to surface loading ratio (f) of 1, towards the south, the Maldive Ridge and the Chagos Bank have a fairly uniform $$T_{\mathrm{e}}$$ of 8–10 km with a very low loading ratio f of 0.1–0.2. We consider the Laccadive Ridge as a continental sliver possessing underplated magmatic rocks caused by the Réunion hotspot volcanism. The Maldive Ridge and the Chagos Bank appear to have emplaced on a lithosphere of intermediate strength possibly on the flanks of the Central Indian Ridge.

An integrated geophysical study east of the southern Chagos Laccadive Ridge Complex, Central Indian Ocean Basin: Implications for the Rodriguez Triple Junction dynamics in the Late Cretaceous

The Central Indian Ocean Basin, east of the southern Chagos Laccadive Ridge complex has evolved due to seafloor spreading in the N-S direction since the Late Cretaceous. The trace of the Rodriguez Triple Junction (RTJ) has been inferred in this basin, but its exact location and dynamics prior to C29o are not known. About 20,000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the RTJ in the Late Cretaceous.Synthetic seafloor spreading models endorse the presence of Late Cretaceous to early Palaeogene magnetic anomalies C34y to C24o in the study area. Half-spreading rates are initially low (32 mm/yr) between C34y to C33y, and increase up to 78 mm/yr by C28y. Thereafter a decreasing trend is seen (57 mm/yr). Towards the west in the study area, higher spreading rates (120 mm/yr) between C33y and C32ay, and distorted anomalies between chrons C32ay and C30y are observed. Detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust, and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation.Plate reconstruction models suggest that the RTJ at C34y time lay southwest of Comorin Ridge which coincided with the northern edge of the Conrad Rise. Until C33y, the RTJ left a linear trace parallel to the fracture zone trend on both the Indian and Antarctic plates, while it gradually moved eastwards on the African plate. The RTJ migrated westwards until C31y causing the transfer of a considerable amount of crust from the African plate to both the Indian and Antarctic plates. Thereafter, the RTJ rapidly reversed its migration direction towards the east until C29o causing the formation and rotation of a microplate on the Indian plate and lengthening of the Southwest Indian Ridge.The present study suggests that this episodic migration of the RTJ may be due to ridge propagation under the influence of the Marion hotspot which was in the vicinity during that period. This RTJ migration resulted in microplate formation, lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the Pacific Ocean. Thus the origin of anomalous structures such as the Crozet Bank may be related to triple junction dynamics under the influence of Marion hotspot. Further, the counter clockwise rotation of the Southwest Indian Ridge between C33y and C29y may have a causal link with the formation and rotation of the microplate, which occurred during the same period. The present study thus facilitated the inference of the dynamics of the RTJ trace between C34y and C29o on the three plates.

Petrology and geochemistry of dolerite and lamprophyre sills in Mesozoic successions of Khanozai–Muslim Bagh area, northwestern Pakistan

In Khanozai–Muslim Bagh area, NW Pakistan, the sills are intruded into mainly Triassic–Jurassic successions of Indian platform sediments. They are petrographically identified as dolerite and lamprophyre. The dolerite is a normal dolerite and olivine dolerite and is altered compared with lamprophyre. Lamprophyre is classified as sannaite, camptonite, minette, and damtjernite. The geochemical signature of dolerite, olivine dolerite, and lamprophyre suggests that these rocks belong to alkali series by classification and may be alkaline in nature. Normal mid-oceanic ridge basalts (NMORB)–normalized plots of dolerite, olivine dolerite, and lamprophyre show higher enrichment of large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and marked positive anomalies on Nb which confirm their origin from an enriched mantle source. The ocean island basalt (OIB)–normalized plots of these rocks exhibit patterns almost alike to those of OIB suggesting a source similar to OIB. The tectono-magmatic discrimination plots of dolerite, olivine dolerite, and lamprophyres plot them in the OIB field and indicate that they are alkaline rocks in nature. The petrogenesis and tectonic setting of these rocks suggest that they are OIB in nature and may represent a Late Cretaceous magmatic activity that erupted as hotspot fluid through the crust of Indian Plate during the Late Cretaceous. It is much similar to the other hotspot-related rocks that intruded into the Parh Group and Bela ophiolite melange. Much like the magmatism of Deccan Traps and the Chagos-Laccadive Ridge, these rocks may be the melt of a hotspot possibly Reunion that intruded into the Indian Plate margin when it had passed over it during the Late Cretaceous.

Qualitative appraisal of high resolution satellite derived free air gravity anomalies over the Maldive Ridge and adjoining ocean basins, western Indian Ocean

High resolution satellite derived Free Air Gravity (FAG) anomalies over the Maldive Ridge segment of the Chagos-Laccadive Ridge and adjoining areas of the Arabian Basin and Central Indian Basin was analyzed to understand their structure. Indian Ocean Geoidal Low corrected FAG depict the N-S trending relative highs associated with the Maldive Ridge, the lows associated with the Vishnu Fracture Zone, and the seafloor fabrics associated with spreading Central Indian Ridge in the Arabian Basin. Transformation operations for enhancing the short, intermediate and long wavelength anomalies were carried out. The northward continuation of the Vishnu Fracture Zone, the eastward extension of the fracture zones of the Arabian Basin into the Deep Sea Channel (DSC) region, between the Maldive Ridge and the Chagos Bank, and the signatures related to north-south trending coral reefs have been deciphered from the enhancement of short wavelength anomalies. The long wavelength anomalies show N-S highs along the axis of the Maldive Ridge and the DSC, which on comparison with previous studies, is inferred to be the signature related with the underplated material from the passage of the Indian plate over the Reunion Plume. Depth to top of two interfaces over the Maldive Ridge and the DSC were computed using the power spectral method by assuming a window of 110 km dimension and 50 km overlap. Depths were thus calculated for 44 blocks along the ridge axis. The average depth to the top of the shallow interface was found to be 5.5 km while that of the deeper was 11.0 km. These depths and interfaces were confirmed by undertaking 2D forward modelling along a 300 km long N-S profile along the western segment of the Maldive Ridge. Integrating with the available previous geophysical studies, it is inferred that the shallow interface represents the depth to the top of the acoustic basement underlying 1–1.5 km thick lava flow unit while the deeper is associated with the interface between the crust and initial Moho before the underplating took place (presently top of the underplated material). From the present study, it has been inferred that the Deep Sea Channel between the Maldive Ridge and Chagos Bank can be purely oceanic in nature.

Moho depth variations over the Maldive Ridge and adjoining Arabian and Central Indian Basins, Western Indian Ocean, from three dimensional inversion of gravity anomalies

Analysis of high resolution satellite derived free air gravity data has been undertaken in the Greater Maldive Ridge (GMR) (Maldive Ridge, Deep Sea Channel, northern limit of Chagos Bank) segment of the Chagos Laccadive Ridge and the adjoining Arabian and Central Indian Basins. A Complete Bouguer Anomaly (CBA) map was generated from the Indian Ocean Geoidal Low removed Free Air Gravity (hereinafter referred to as “FAG-IOGL”) data by incorporating Bullard A, B and C corrections. Using the Parker method, Moho topography was initially computed by inverting the CBA data. From the CBA the Mantle Residual Gravity Anomalies (MRGA) were computed by incorporating gravity effects of sediments and lithospheric temperature and pressure induced anomalies. Further, the MRGA was inverted to get Moho undulations from which the crustal thickness was also estimated. It was found that incorporating the lithospheric thermal and pressure anomaly correction has provided substantial improvement in the computed Moho depths especially in the oceanic areas. But along the GMR, there was not much variation in the Moho thickness computed with and without the thermal and pressure gravity correction implying that the crustal thickness of the ridge does not depend on the oceanic isochrones used for the thermal corrections. The estimated Moho depths in the study area ranges from 7 km to 28 km and the crustal thickness from 2 km to 27 km. The Moho depths are shallower in regions closer to Central Indian Ridge in the Arabian Basin i.e., the region to the west of the GMR is thinner compared to the region in the east (Central Indian Basin). The thickest crust and the deepest Moho are found below the N-S trending GMR segment of the Chagos-Laccadive Ridge. Along the GMR the crustal thickness decreases from north to south with thickness of 27 km below the Maldives Ridge reducing to ∼9 km at 3°S and further increasing towards Chagos Bank. Even though there are similarities in crustal thickness between Maldive Ridge and other regions like Mascarene Plateau which was recently interpreted as underlain by continental crust, much more geoscientific work including drilling has to be undertaken to finally confirm the exact nature of the ridge.

Sedimentary sources and processes in the eastern Arabian Sea: Insights from environmental magnetism, geochemistry and clay mineralogy

The spatial distribution patterns of surficial sediment samples from different sedimentary domains (shallow to deep-sea regions) of the eastern Arabian Sea were studied using sediment proxies viz. environmental magnetism, geochemistry, particle size and clay mineralogy. Higher concentrations of magnetic minerals (high χlf) were recorded in the deep-water sediments when compared with the shallow water sediments. The magnetic mineralogy of one of the shallow water samples is influenced by the presence of bacterial magnetite as evidenced from the χARM/χlfvs. χARM/χfd biplot. However, the other samples are catchment-derived. The high correlation documented for χlf, anhysteretic remanent magnetisation (χARM) and isothermal remanent magnetisation (IRM) with Al indicates that the deep-sea surficial sediments are influenced by terrigenous fluxes which have been probably derived from the southern Indian rivers, the Sindhu (the Indus) and the Narmada-Tapti rivers. A lower Mn concentration is recorded in the upper slope sediments from the oxygen minimum zone (OMZ) but a higher Mn/Al ratio is documented in the lower slope and deep-sea sediments. Clay minerals such as illite (24–48.5%), chlorite (14.1–34.9%), smectite (10.6–28.7%) and kaolinite (11.9–27.5%) dominate the sediments of shallow and deep-sea regions and may have been derived from different sources and transported by fluvial and aeolian agents. Organic carbon (OC) data indicate a low concentration in the shallow/shelf region (well oxygenated water conditions) and deeper basins (increased bottom-water oxygen concentration and low sedimentation rate). High OC concentrations were documented in the OMZ (very low bottom-water oxygen concentration with high sedimentation rate). The calcium carbonate concentration of the surface sediments from the continental shelf and slope regions (<1800 m) up to the Chagos-Laccadive Ridge show higher concentrations (average = 58%) when compared to deep basin sediments (average = 44%). Our study demonstrates that particle size as well as magnetic grain size, magnetic minerals and elemental variations are good indicators to distinguish terrigenous from biogenic sediments and to identify sediment provenance.

Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition

Abstract. In this paper we examine time-series measurements of near-surface chlorophyll concentration from a mooring that was deployed at 80.5°E on the equator in the Indian Ocean in 2010. These data reveal at least six striking spikes in chlorophyll from October through December, at approximately 2-week intervals, that coincide with the development of the fall Wyrtki jets during the transition between the summer and winter monsoons. Concurrent meteorological and in situ physical measurements from the mooring reveal that the chlorophyll pulses are associated with the intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as they occur in the Atlantic and Pacific oceans, since eastward winds that force Wyrtki jet intensification should drive downwelling. The chlorophyll spikes could be explained by two alternative mechanisms: (1) turbulent entrainment of nutrients and/or chlorophyll from across the base of the mixed layer by wind stirring or Wyrtki jet-induced shear instability or (2) enhanced southward advection of high chlorophyll concentrations into the equatorial zone. The first mechanism is supported by the phasing and amplitude of the relationship between wind stress and chlorophyll, which suggests that the chlorophyll spikes are the result of turbulent entrainment driven by synoptic zonal wind events. The second mechanism is supported by the observation of eastward flows over the Chagos–Laccadive Ridge, generating high chlorophyll to the north of the equator. Occasional southward advection can then produce the chlorophyll spikes that are observed in the mooring record. Wind-forced biweekly mixed Rossby gravity waves are a ubiquitous feature of the ocean circulation in this region, and we examine the possibility that they may play a role in chlorophyll variability. Statistical analyses and results from the OFAM3 (Ocean Forecasting Australia Model, version 3) eddy-resolving model provide support for both mechanisms. However, the model does not reproduce the observed spikes in chlorophyll. Climatological satellite chlorophyll data show that the elevated chlorophyll concentrations in this region are consistently observed year after year and so are reflective of recurring large-scale wind- and circulation-induced productivity enhancement in the central equatorial Indian Ocean.

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.

Heavy mineral variation in the deep sea sediment of southeastern Arabian Sea during the past 32 kyr

The present study is based on heavy mineral assemblages (HM) of top 104-cm thick section of gravity core SK 221 (Lat. 8∘7.12′N; Long. 73∘16.38′E and water depth – 2188 m) located near the Chagos–Laccadive Ridge in the southeastern Arabian Sea to evaluate the provenance and paleoenvironmental changes during the last 32 kyr. The biogenic carbonate, acid insoluble residue, magnetic susceptibility, total organic carbon (TOC) and clay based humidity index, i.e., kaolinite/illite ratio are also utilized to correlate with the above paleoenvironmental changes. Ilmenite, garnet, staurolite, pyroxenes, andalusite and zircon are the dominant HM with moderate to low ZTR (zircon-tourmaline-rutile) index indicating instability of the sediments and rapid erosion in the source region. The characteristic HM suggest their mixed sources from the basic igneous, gneisses/granites, high grade metamorphic rocks and sandstones occurring mainly in the western and southwestern India. The temporal variations of HM, AIR (acid insoluble residue), MS (magnetic susceptibility), biogenic carbonate and Corg (TOC) during preglacial and early Holocene suggest intensive weathering, erosion, and transportation of terrigenous detritus from continental region by fluvial processes and summer monsoon led high biogenic productivity, respectively. The convective mixing of waters due to intense winter monsoon resulted in very high biogenic carbonate content during the early stages of glacial period. The HM and associated proxies indicated that the winter monsoons of Heinrich (H3, H2, and H1) and Younger Dryas (YD) events and summer monsoons of Bølling/Allerød (BA) event were not strong enough to bring drastic changes in the above parameters.

Tectonic framework of Laccadive Ridge in Western Continental Margin of India

From the analysis of the satellite derived high resolution free air gravity data it is inferred that the evolutionary history of the Western Continental Margin of India to north and south of Goa appears to be totally different . The shelf edge is continuous and not disturbed in the northern part whereas to the south (south of 16°N) it has been affected by the onshore tectonics. It is hotly debated if the Laccadive Ridge (northern part of the Chagos-Laccadive Ridge) is continental or oceanic in nature or just of volcanic origin. From the filtered maps of the free air gravity anomalies, sources at different depths can be deciphered that help build an evolutionary model of the much debated Laccadive Ridge. The low pass filtered maps depicting deeper features, show a persistence of NW–SE to NS Dharwarian trends and it appears that the Laccadive Ridge to the north of around 8.5°N is of continental origin being bounded to the south and north by the offshore extension of the Bhavani Shear and Chapporo lineament respectively. It may have separated from India along with Madagascar during Cretaceous. At the Cretaceous Tertiary boundary as Laccadive-India passed over the Reunion hotspot, the pre-existing faults on the Laccadive Ridge were re-activated and the hotspot trace was left behind in the intermediate/shallow wavelength of the anomalies, as intrusives in the Laccadive Ridge. These are reflected in the High Pass filtered maps, depicting shallow to intermediate wavelength anomalies, which show NE–SW structures. The crustal structure derived from the 2D model of the satellite derived free air gravity data and a representative gravity-magnetic profile from ship borne data also depict continental nature of the Laccadive Ridge with emplacement of volcanic intrusives as it passed over the Reunion hotspot.

Draft Genome Sequence of Bhargavaea cecembensis Strain DSE10 T , Isolated from a Deep-Sea Sediment Sample Collected at a Depth of 5,904 m from the Chagos-Laccadive Ridge System in the Indian Ocean

Here, we report the 3.2-Mbp draft genome sequence of Bhargavaea cecembensis strain DSE10T, isolated from a sediment sample collected from the Chagos-Laccadive ridge system in the Indian Ocean at a depth of 5,904 m.

Three dimensional lithospheric structure of the western continental margin of India constrained from gravity modelling: implication for tectonic evolution

This paper describes a 3-D lithospheric density model of the Western Continental Margin of India (WCMI) based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the Indian peninsula. The model covers the north-eastern Arabian Sea and the western part of the Indian Peninsula and incorporates constraints from a wide variety of geophysical and geological information. Salient features of the density model include: (1) the Moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior; (2) the lithosphere–asthenosphere boundary (LAB) located at depths between 70 km in the southwestern corner (under oceanic crust) and about 165 km below the continental region; (3) thickening of the crust under the Chagos–Laccadive and Laxmi Ridges and (4) a revised definition of the continent–ocean boundary. The 3-D density structure of the region enables us to propose an evolutionary model of the WCMI that revisits earlier views of passive rifting. The first stage of continental-scale rifting of Madagascar from India at about 90 Ma is marked by relatively small amounts of magmatism. A second episode of rifting and large-scale magmatism was possibly initiated around 70 Ma with the opening of the Gop Rift. Subsequently at around 68 Ma, the drifting away of the Seychelles and formation of the Laxmi Ridge was a consequence of the down-faulting of the northern margin. During this second episode of rifting, the northern part of the WCMI witnessed massive volcanism attributed to interaction with the Reunion hotspot at around 65 Ma. Subsequent stretching of the transitional crust between about 65 and 62 Ma formed the Laxmi Basin, the southward extension of the failed Gop Rift. As the interaction between plume and lithosphere continued, the Chagos–Laccadive Ridge was emplaced on the edge of the nascent oceanic crust/rifted continental margin in the south as the Indian Plate was moving northwards.

Seaward dipping reflectors along the SW continental margin of India: Evidence for volcanic passive margin

Multi-channel seismic reflection profiles across the southwest continental margin of India (SWCMI) show presence of westerly dipping seismic reflectors beneath sedimentary strata along the western flank of the Laccadive Ridge — northernmost part of the Chagos-Laccadive Ridge system. Velocity structure, seismic character, 2D gravity model and geographic locations of the dipping reflectors suggest that these reflectors are volcanic in origin, which are interpreted as Seaward Dipping Reflectors (SDRs).The SDRs; 15 to 27 km wide overlain by ∼1 km thick sediment; are observed at three locations and characterized by stack of laterally continuous, divergent and off-lapping reflectors. Occurrence of SDRs along western flank of the Laccadive Ridge adjacent to oceanic crust of the Arabian Basin and 2D crustal model deduced from free-air gravity anomaly suggest that they are genetically related to incipient volcanism during separation of Madagascar from India.We suggest that (i) SWCMI is a volcanic passive margin developed during India-Madagascar breakup in the Late Cretaceous, and (ii) continent-ocean transition lies at western margin of the Laccadive Ridge, west of feather edge of the SDRs. Occurrence of SDRs on western flank of the Laccadive Ridge and inferred zone of transition from continent to ocean further suggest continental nature of crust of the Laccadive Ridge.