Maldives Ridge Research Articles

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.

Nannofossil palaeoecology of Lower Miocene sapropels from IODP Expedition 359, the Maldives

During IODP expedition 359 multiple holes recovered Modern to Middle Miocene sediments from the Kardiva Drift which is strongly influenced by monsoonally-driven currents. Two holes continued below the drift into Lower Miocene sediments that were deposited under rather different conditions following Oligocene subsidence of the Maldives Ridge. These sediments are characterised by marked alternation between dark organic-rich sapropelic sediments and pale nannofossil oozes. This interval is also characterised by generally good to excellent nannofossil preservation. Nannofossil assemblages from the sapropel and non-sapropel intervals are broadly similar; both sediment types contain diverse normal marine assemblages and no taxa are confined to either sediment type. In the sapropels nannofossil abundance and diversity tends to be lower and Cyclicargolithus floridanus specimens are smaller. In addition Umbilicosphaera abundances are higher in the non-sapropel layers whilst Discoaster abundance is higher in most sapropels. These differences are more marked in the better developed sapropels. The nannofossil evidence supports the interpretation from geochemistry that sapropel formation was primarily driven by changes in bottom water ventilation linked to sea-level change rather than surface water productivity.

СТРОЕНИЕ ЛИТОСФЕРЫ И УСЛОВИЯ ФОРМИРОВАНИЯ ЧАГОС-ЛАККАДИВСКОГО ХРЕБТА (ПЛОТНОСТНОЕ И ФИЗИЧЕСКОЕ МОДЕЛИРОВАНИЕ)

The Chagos-Laccadiv Range is a linear-elongated structure adjacent to the passive margin of western India. The ridge consists of three segments: northern — Lakkadiv ridge, central — Maldives ridge and southern — bank (archipelago) Chagos. The ridges are separated by depressions and have different manifestations in morphology and anomalous gravitational field. Modeling of the density structure of the Chagos-Lakkadive Ridge tectonosphere showed that the Lakkadive and Maldive segments, most likely, represent submerged blocks of thinned continental crust, partially separated from the continental margin of India by a riftogenic basin. Along with the assumption that the Chagos Bank may contain fragments of the continental crust, the main factor in its formation is apparently the active magmatic activity of the Reunion hot spot, leading to an increase in the thickness of the crust due to underplating. Physical modeling showed that the formation of such a linear structure is possible in the presence of thermal (hot spot) and structural (faults and cracks) inhomogeneities in the model continental lithosphere, which within the continental margin led to a jump (jumping) of the spreading axis towards the young margin and partial separation from it narrow linearly elongated microblocks (ridges).

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.

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.

“印度板块”研究回顾及思维再完善—换个方式审视地球暨庆祝中国地质博物馆建馆100周年(1916-2016)

依据当今非洲大裂谷的发育过程可以推断出，2亿多年前由于受到裂谷系的作用，致使冈瓦纳古陆全面解体，出现了南美洲板块、非洲板块、澳大利亚板块、南极洲板块、印度-印支板块；之后裂谷系又逐渐发展成大西洋海岭、印度洋海岭、太平洋海岭。古生物学又证实，“印度板块”与“印支板块”都生存有三叠纪恐龙动物，这表明“印支板块”曾经是冈瓦纳大陆的一部分，与“印度板块”曾经也是一个相互连接的整体，即“印度-印支板块”。从冈瓦纳大陆分离出来，“印度-印支板块”于晚三叠世后期至早侏罗世初期，在赤道附近同“华南大陆板块”发生撞击，自此并入欧亚板块，并将冈瓦纳大陆的海生爬行动物鱼龙、棘皮类海百合及陆生恐龙等生物先后带入亚洲南部的贵州、云南、四川等地区。随着印度-印支板块与欧亚板块向北移动，印度-印支板块又受到来自“东经九十度海岭”的切割，新的裂谷系将其由南至北分成西部的印度板块和东部的印支板块。被分割的印度板块在“东经九十度海岭”和马尔代夫海岭合力推动下，向北移动，至喜马拉雅地区，再次与欧亚板块发生撞击，形成当今的地貌景观。两次撞击都与裂谷系相关联，因此对裂谷系的认识，有助重塑印度板块的运动轨迹，有助重新解释中生代以来众多的地质现象。 At about 200 ma ago, Gondwanaland was split by a rift system, according to the development process of present East African Rift system, into South America Plate, Africa Plate, Australia Plate, Antarctic Plate and India-Indochina Plate. The rift system itself developed to the present Mid-Atlantic Ridge, Indian Ridges, and Pacific Ridges gradually. Triassic dinosaurs, found at both Indian and Indo-China plates, showed that Indo-China Plate was also a part of Gondwanaland and that Indian Plate and Indo-China Plate were located together as Indian-Indo-China Plate. When separated from Gondwanaland, and collided with South-China Plate, it was merged in Eurasian Plate at the Equator in Late Triassic-Early Jurassic period. The collision had brought Gondwana fauna like sea reptiles Ichthyosaurus, crinoids, and terrestrial dinosaurs to South China (Guizhou, Yunnan, Sichuan, etc.). In its further movement northward, Indian-Indo-China Plate was split by the Ninety East Ridge into two parts from south to north: Indo-China Plate at its east and Indian Plate at its west. Indian Plate, driven jointly by the Ninety East Ridge and Maldive Ridge, moved northward and collided with Eurasian Plate again, at the region of present Himalaya, forming present Himalaya topography. The two collisions were caused by the forming of rift systems. So understanding the formation of the rift systems is the key to reconstruct the route of Indian Plate movement which in turns helps to interpret many geological phenomena happened since Mesozoic.

Late Quaternary productivity changes in the equatorial Indian Ocean (ODP Hole 716A)

The equatorial Indian Ocean is swept by the Indian Ocean equatorial westerlies (IEW) which intensify in monsoon transitions during April–May and October–November, driving Eastward Equatorial Current (EEC) to the upper ocean. The EEC brings significant changes in the surface productivity in the equatorial region influencing the life style of the surface and deep-sea fauna. This study is aimed at understanding ~300Kyr record (~450–150Kyrs ago) of productivity changes linked to variations in IEW–EEC in the equatorial Indian Ocean using high resolution microfaunal record from Integrated Ocean Drilling Program (IODP) Hole 716A, Maldives Ridge. In total, 451 core samples of 10cm3 volume from the ~300Kyr long sequence (ranging ~450–150Kyrs ago) were analyzed at every 1cm interval to generate census data of the benthic foraminiferal fauna. Factor and cluster analyses using percentages of 53 highest-ranked benthic foraminiferal species were run that enabled us to identify five biofacies, indicating the varied nature of deep-sea environments during the late Quaternary. The results show a major shift roughly 300Kyr ago, across Marine Isotope Stages 9 and 8 (MIS 9/8), recorded in this study as mid-Brunhes transition at Hole 716A. The present study also combines published planktic foraminiferal fauna and pteropod records from Hole 716A to understand if the surface to shallow and intermediate water paleoenvironments were coupled with deep-water changes during the late Quaternary in the Maldivian Archipelago. Biofacies Robulus nicobarensis–Trifarina reussi (Rn–Tr), Uvigerina porrecta–Reussella simplex (Upo–Rs) and Cymbaloporetta squammosa–Bolivinita sp. (Cs–Bsp) dominate before the mid-Brunhes transition and document high organic flux with relatively low oxygen paleoenvironment, similar to the planktic foraminifer Globigerina bulloides population, while benthic foraminiferal biofacies Hoeglundina elegans–Miliolinella subrotunda (He–Ms) and Uvigerina peregrina–Quinqueloculina seminulum (Upe–Qs) record high seasonality in food supply with well-oxygenated deep water following the mid-Brunhes climatic transition (~300Kyr ago at Hole 716A). These changes are also recorded in the population of planktic foraminiferal species Globigerinoides ruber and Neogloboquadrina dutertrei as well as pteropods. Organic carbon flux linked to changes in the IEW–EEC strength appears to be the most plausible factor controlling faunal assemblages and hence paleoproductivity dynamics in the Maldivian region. Our results suggest that the strength of the IEW is inversely related with the Indian Ocean Dipole (IOD). The IOD strengthened across MIS 9/8 during which time the IEW weakened, leading to major changes at Hole 716A.

From monsoons to mantas: seasonal distribution of Manta alfredi in the Maldives

The Republic of Maldives in the central Indian Ocean is home to large numbers of manta rays, Manta alfredi. They are known to undertake seasonal migrations within the Maldives, but these movements have not been well documented. The aims of this study were to map the seasonal distribution of manta rays within the Maldives, and to provide some indications of the physical and biological oceanographic processes affecting their distribution. The seasonal distribution of mantas was determined from a national survey of fishermen, interviews with experienced divers and personal observations. The data demonstrate that the distribution of mantas is strongly influenced by the seasonally reversing monsoon currents. Mantas occur on the downstream sides of the atolls, and are rare on the upstream sides, switching sides biannually as the monsoon currents change direction. These seasonally alternating currents are driven by monsoon winds which also alternate according to the season, and bring clear oceanic water to the upstream sides of the atolls. As the currents pass over the Maldives ridge, nutrient-rich waters are lifted to the surface, promoting phytoplankton blooms (as demonstrated by the distribution of chlorophyll-a) on the downstream sides of the atolls. This manifestation of the island mass effect supports an abundance of zooplankton, which in turn supports the manta rays.

Orbital and suborbital variability in the equatorial Indian Ocean as recorded in sediments of the Maldives Ridge (ODP Hole 716A) during the past 444 ka

Abstract This study is aimed at understanding past 444 ka record of climate variability in the equatorial Indian Ocean using high resolution records of planktic and benthic foraminifera and pteropods from Ocean Drilling Program Hole 716A, Maldives Ridge, southeastern Arabian Sea. In total, 892 samples of 10 cm 3 volume from 444 ka old sequence were analysed at 1 cm intervals to generate census data of the foraminiferal fauna and pteropods. The percent and detrended time series of mixed-layer species Globigerinoides ruber and Globigerinoides sacculifer and thermocline species Neogloboquadrina dutertrei , benthic foraminifera Cymbaloporetta squammosa , Sphaeroidina bulloides and Uvigerina proboscidea , and pteropods from ODP Hole 716A reveal significant changes in wind intensity during the past 444 ka. An abrupt decrease in the Cymbaloporetta squammosa population at c. 300 ka (across MIS 8/9) suggests a weakening of equatorial wind intensity, which could be linked to Indian monsoon and may have driven pronounced changes in the oxygen minimum zone in the Maldivian region. These changes were contemporaneous with the Mid-Brunhes Climatic Event, the beginning of aridity in the Indonesian-Australian region and the onset of a humid phase in equatorial East Africa as observed in several oceanic and continental records. This strengthens a connection between equatorial Indian Ocean wind intensity, the Indian monsoon and Indonesian-Australian-African climates. Supplementary material: Percentages of benthic and planktic foraminifera and pteropods used in the present study are available at http://www.geolsoc.org.uk/SUP18413

Late Quaternary benthic foraminifera from Ocean Drilling Program Hole 716A, Maldives Ridge, southeastern Arabian Sea - PART 1

A study on deep-sea benthic foraminifera from the interval ~445 ka BP to the Present of the Ocean Drilling Program (ODP) Hole 716A (4°56.0′N, 73°17.0′E; present water depth 533.3m), Maldives Ridge, southeastern Arabian Sea, documented 201 species belonging to 105 genera. These taxa were examined under scanning electron microscope (SEM) to illustrate their interspecificmorphological variations. Several of these species are dominant showing significant down core fluctuations in their abundances whereas some are rare and sporadic.

Late Quaternary benthic foraminifera from Ocean Drilling Program Hole 716A, Maldives Ridge, southeastern Arabian Sea - PART 2

A study on deep-sea benthic foraminifera from the interval ~445 ka BP to the Present of the Ocean Drilling Program (ODP) Hole 716A (4°56.0′N, 73°17.0′E; present water depth 533.3m), Maldives Ridge, southeastern Arabian Sea, documented 201 species belonging to 105 genera. These taxa were examined under scanning electron microscope (SEM) to illustrate their interspecific morphological variations. Several of these species are dominant showing significant down core fluctuations in their abundances whereas some are rare and sporadic.

Five new species of benthic foraminifera from upper Pleistocene sequence in ODP Hole 716A, Maldives Ridge, equatorial Indian Ocean

Study of benthic foraminifera from late Pleistocene (∼444 Kyr to 150 Kyr BP) samples from Ocean Drilling Program (ODP) Hole 716A (4°56.0′N, 73°17.0′E; present water depth 533.3 m), Maldives Ridge, equatorial Indian Ocean, led to the discovery of one new genus Acostinella n.gen. and five new species of benthic foraminifera including Adelosina bhallai n.sp., Siphonaperta bhallai n.sp., Triloculinella bhallai n.sp., Hofkeruva sahaii n.sp., Acostinella sahaii n.sp. These taxa were examined under the scanning electron microscope, which has helpedto explain their morphological descriptions and assign them a taxonomic classification. The taxa have rare and sporadic occurrences at Hole 716A and thus we do not know their potential in paleoceanographic and paleoclimatic reconstructions, which requires further studies from other parts of the Indian Ocean.

Seismic refraction measurements in the northwest Indian Ocean

Fifteen seismic refraction profiles in the northwest Indian Ocean are described. The results suggest a connection between the linear Maldive ridge and the Deccan traps of the Indian mainland. No anomalous upper mantle velocities have been found near the crest of the Carlsberg ridge. A great thickness of material with velocity 6.03 km/sec west of the Saya de Malha bank has been interpreted as an extension of the Seychelles granitic block to the south. At two stations between the Chagos archipelago and the Seychelles bank the Mohorovicic discontinuity appears to be less than 8 km below sea level.