North Fiji Research Articles

Mise en place de basaltes enrichis le long d'un axe d'accrétion actif en domaine arrière-arc (dorsale Sud Pandora-Tripartite, bassin Nord-Fidjien, Sud-Ouest Pacifique)

Seven dredge hauls have been performed along the active oceanic spreading ridge of the northern North Fiji Basin during the NOFI cruise of the R/V L'Atalante in 1994 (Newstarmer project). The recovered basalts do not show the typical geochemical characteristics expected in such a back-arc setting. They have compositions of enriched tholeiites lacking the Nb depletion related to subduction setting and exhibit La/Yb, La/Nb and Ba/La ratios ranging from E-MORB ( Enriched Mid-Ocean Ridge Basalts) to OIB ( Oceanic-Island Basalts). Therefore, the geochemistry of the magma erupted along this oceanic spreading system does not seem to be related to mantle convection occurring under evolved back-arc spreading ridges but should be more probably linked to the upwelling of deeper mantle, more enriched in incompatible elements than the mantle beneath the mid-ocean ridges.

Geochemical Variations in Vanuatu Arc Lavas: the Role of Subducted Material and a Variable Mantle Wedge Composition

New compositional data are presented for recent (<3.5 Ma) basaltic lavas from the Vanuatu arc and North Fiji back-arc, and for material being subducted at the Vanuatu trench. Sr, Nd and Pb isotope data for the lavas vary systematically north–south along the arc and highlight where the D'Entrecasteaux Zone on the subducting plate collides with the fore-arc. Major and trace element data show significant lateral variations in enrichment of the mantle wedge relative to an N-MORB source. New elemental and isotopic analyses of the North Loyalty Basin sediments have allowed the flux of elements in the subducting sediments to be calculated. The wedge has been variably modified by addition of slab-derived material containing a significant subducted sediment component. Merelava in the rear-arc is anomalous and is dominated by a fluid component derived from subducted oceanic crust. Most lavas originate from a Pacific-MORB-like mantle source except for those erupting close to the collision zone in central Vanuatu. Collision of the D'Entrecasteaux Zone has disrupted the arc complex and brought less depleted, isotopically distinct (Indian-MORB-like) mantle material from the back-arc into the arc magma source in this region.

Megafauna associated with hydrothermal vents in the Manus Back-Arc Basin (Bismarck Sea)

The vent fauna of Manus Back-Arc Basin were investigated during the 21st cruise of the Akademik Mstislav Keldysh in May–June 1990. Communities associated with three active hydrothermal fields were explored during a series of dives by the DSRV Mir. At least sixteen vent species were collected; some of them are new. Material for physiological and biochemical analyses was collected as well. Special attention was paid to the exploration of spatial distribution of animals. Distributional information is combined with our knowledge on the biology of animals to construct a food web of the community. Species composition of the communities investigated is similar but relative abundance varies. These differences seem to be a result of temporal changes in the communities. Early stages of the cycle of the Manus Basin vents are hypothetically dominated by vestimentiferans but mesogastropods will replace them with time. Faunal composition and community structure of the Manus Basin reinforce the likelihood that the hydrothermal vents of western Pacific spreading centres have been in communication with each other. The Manus Basin vent fauna displays close affinity with the communities of the North Fiji and Lau Basins as compared with the Mariana Trough vent region.

Multiple active spreading centres in the hot North Fiji Basin (Southwest Pacific): a possible model for Archaean seafloor dynamics?

The North Fiji Basin (NFB), in the Southwest Pacific, is one of the most evolved back-arc basins. Within the basin, active spreading is taking place along several ridges, some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates (segmented accretion centres, organized magnetic lineations and propagators, for example). However, the great cumulate length of the spreading centres of the NFB relative to the Basin's surface makes it an unique example among the world's oceanic basins. We have reviewed a comprehensive body of geological, geophysical and geochemical evidence (multiple spreading centres of various trends, high heat flow, high geoid and slow upper mantle seismic velocities) which collectively demonstrate that a hot upper mantle underlies the North Fiji Basin. Active upper mantle convection is the key process controlling the large crustal and lithospheric production in the NFB, which would be thus largely independent from the evolution of even the largest structural features, such as the Vitiaz and New Hebrides subduction zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: numerous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowing for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean seafloor dynamics.

Ultrastructural characteristics of spermatogenesis in three species of deep-sea hydrothermal vent mytilids

To enhance our understanding of the reproductive biology of deep-sea hydrothermal vent mytilids, the histology of the male gonad and the ultrastructure of its gametes were studied in Bathymodiolus thermophilus, B. puteoserpentis, and B. elongatus. Specimens of B. thermophilus were collected at the 13°N site on the East Pacific ridge, while B. puteoserpentis were sampled from the Snake Pit site of the mid-Atlantic ridge and B. elongatus were obtained from the North Fiji Basin. Gonad histology conformed to the typical bivalve profile; the differences in the proportions of acinal and interacinal tissue, as well as differences in acinal fullness in B. puteoserpentis, indicate that gametogenesis is discontinuous in these deep-sea mytilids. Evidence of protandric hermaphroditism was observed in B. elongatus, which exhibited acini containing both maturing and residual male gametes and immature oocytes. The ultrastructural characteristics of the male gametes conform to those described for littoral bivalve species, and the spermatozoon is of the primitive type. No species-specific differences in spermatozoon ultrastructure were discerned. No evidence of bacterial inclusions was found in either the gametes or the associated gonad cells in any of the species examined. The male gametes are thus probably not vectors for the endosymbiotic bacteria that characterize the nutritional biology of the adults in this genus.

EOCHIONELASMUS PAQUENSIS, NEW SPECIES (CIRRIPEDIA: BALANOMORPHA), FROM 17?25'S, NORTH OF EASTER ISLAND: FIRST RECORD OF A SESSILE HYDROTHERMAL BARNACLE FROM THE EAST PACIFIC RISE

ABSTRACT The first sessile barnacle from a deep-sea hydrothermal vent of the East Pacific Rise (EPR) has been recovered by the French submersible Nautile from 17°25'S, north of Easter Island. It is similar in general appearance to the most primitive living balanomorph barnacle Eochionelasmus ohtai known from the North Fiji, Lau, and Manus Basins, in the Southwest Pacific (Yamaguchi and Newman, 1990; Galkin, 1992), but the morphology of its opercular plates, mouthparts, and cirri is distinct. Therefore, it is recognized here as a new species of the previously monotypic genus Eochionelasmus. However, it is anticipated that even greater differences will be found between the two species when fully developed individuals of the Easter Island form are found. If so, distinction at the genus-group level will need to be evaluated. The new species is not only the first record of a sessile vent barnacle from the East Pacific Rise, but it is also the first from a midocean ridge. This discovery corroborates the hypothesis that biotic exchange between midocean ridges and back-arc basins occurs relatively infrequently or was curtailed a long time ago.

The hydrothermal vent barnacles Eochionelasmus (Cirripedia, Balanomorpha) from the North Fiji, Lau and Manus Basins, South-West Pacific

The hydrothermal vent barnacles Eochionelasmus (Cirripedia, Balanomorpha) from the North Fiji, Lau and Manus Basins, South-West Pacific

Gravity Anomaly and Tectonics in the Northern Part of the North Fiji Basin - Preliminary Result of the 1994 NOFI Cruise under the NEW STARMER Programme

A surface ship gravity survey was carried out in the northern part of the North Fiji Basin during the NOFI cruise by the R/V l'Atalante in August-September, 1994. The two ridges inside the study area, the South Pandora Ridge and the Tripartite Ridge, present different structures and states of isostatic equilibrium in terms of gravity anomaly and its tectonic implications. The former is supported by a restoring force of an imaginary elastic plate in the crust and the latter by the Airy type isostasy. These characteristics can be derived from the difference in magmatic activity, as influenced by the difference in lithospheric structure. The latter is characterised by greater active magmatism and hydrothermalism underneath the ridge than the former. Such a difference in the magmatic activity and the horizontal scale of the shallow subsurface structure is derived from the difference in the stiffness or viscosity of the lithosphere beneath the two ridges.

Thermococcus fumicolans sp. nov., a new hyperthermophilic archaeon isolated from a deep-sea hydrothermal vent in the north Fiji Basin.

An extremely thermophilic archaeon, strain ST557T (T = type strain), was isolated from a deep-sea hydrothermal vent in the North Fiji Basin. This strain is a strictly anaerbic coccus whose cells are about 0.8 to 2 microns in diameter. The optimum temperature, pH and sea salt concentration for growth are 85 degrees C, 8.5, and 20 to 40 g/liter, respectively. Strain ST557T grows preferentially in the presence of elemental sulfur on proteinaceous substrates and on a mixture of 20 amino acids. It grows slowly on pyruvate and maltose. Growth is inhibited by rifampin. The DNA G + C content is 54 to 55 mol%. Sequencing of the 16S rRNA gene revealed that strain ST557T belongs to the genus Thermococcus. We propose that this organism should be placed in a new species, Thermococcus fumicolans.

Study yields surprises about seafloor spreading in back‐arc basins

A study of the seafloor in the North Fiji Basin is leading to a better understanding of the geometry and evolution of spreading in back‐arc basins. Two of the most important features of the area, the South Pandora Ridge and the Tripartite Ridge, belong to a presently active spreading system that began forming at least 7.2 m.y.a. The ridges spread at an average rate of about 1.6 cm/yr. By studying the segments of the ridges in greater detail, researchers found that each was either a “tectonic type,” with a more or less complex deep axial valley, or a “volcanic type,” with a high, rifted volcanic massif occupying the axial domain.

Compositional variations in submarine volcanic ashes from the vicinity of the Vanuatu Island Arc: a response to ridge—arc collision?

The late Tertiary-Recent collision of the d'Entrecasteaux Zone with the Vanuatu (New Hebrides) Island Arc appears to have been associated with a shift to generally more potassic compositions. Glass shards analysed from Quaternary volcanic ashes recovered from ODP Leg 134 drill sites follow distinct shoshonitic and calc-alkaline trends, consistent with those previously recognised onshore. In the Aoba Basin (Site 832) a marked compositional change is recognised at 37 m below sea floor (mbsf). From estimates of sedimentation rates this is equivalent to an age of about 100 ka and may be associated with the development of the Central Basin volcanoes of Santa Maria, Aoba and Ambrym on fracture zones transverse to the island arc. Deeper than 37 mbsf differentiated compositions fall on a medium-K calc-alkaline trend but the associated basaltic glasses are strongly potassic. Conversely, above this level the differentiated glasses follow a shoshonitic trend but the basaltic glasses are medium-K calc-alkaline. Separate sources are envisaged, associated with subduction and rifting. Episodic subduction of the d'Entrecasteaux Zone exercises tectonic control over the release of magma from different depths. In contrast, shoshonitic glasses are sparsely represented in the Vanuatu forearc where relatively low-K calc-alkaline compositions probably correspond to the ashes in the lower part of the Aoba Basin sequence. This is consistent with their age (> 100 ka) estimated from lower sedimentation rates at these sites. The scarcity of younger shoshonitic ashes at the forearc sites is attributed to the influence of high-level westerly winds transporting ash from the Central Chain volcanoes eastwards across the North Fiji Basin.

Mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge: East Pacific Rise, 17° to 19°S (Naudur cruise, 1993) phase separation processes controlled by volcanic and tectonic activity

During the Naudur cruise in December 1993, 23 dives using the French submersible Nautile were conducted on the axis of the East Pacific Rise between 17° and 19°S where the spreading rate is among the fastest in the ocean (14 to 16 cm/yr). Twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered, respectively, at 17°25′S, 18°15′S, and 18°26′S. The fluids exhibit a very wide range of temperature, chemical, and gas compositions. On the 17°25′S and 18°26′S segments, fluids have quite uniform compositions, low chlorinities, are gas‐enriched and are low in dissolved metals relative to fluids from the 18°15′S segment which show high chlorinities, are less gas‐enriched and show high‐metal concentrations. Chloride and metal depletion associated with gas enrichment is consistent with phase separation. Whereas CH4 endmembers show large variations between sites, the 13CH4 data are very similar, with 13C values in a narrow range −22.0 to −23.9‰ versus pee‐dee belemnite (PDB). δ13CO2 measured in fluids within the 18°15′S and 18°26′S segments are, respectively, −7.9 and −5.8‰ versus PDB, similar to 13C of CO2 trapped in mid oceanic ridge basalts, suggesting a magmatic origin. The variability in fluid composition is linked to the variability of the accretion process observed on the three segments. The uniform venting of low‐chlorinity fluids in the 17°25′S and 18°26′S segments is connected with volcanic activity which causes boiling with preferential venting of vapor‐enriched fluids. High‐salinity fluids are emitted on the 18°15′S segment where the ridge is tectonics‐dominated and subseafloor circulation controlled by faults. Phase‐separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area, as previously observed on the Juan de Fuca Ridge or in the North Fiji Basin Ridge.

Variability of the axial morphology and of the gravity structure along the Central Spreading Ridge (North Fiji Basin): evidence for contrasting thermal regimes

The Central Spreading Ridge (CSR) is located in the central part of the North Fiji Basin, a complex back-arc basin created 12 Ma ago between the Pacific and Indo-Australian plates. The 3.5 Ma old CSR is the best developed, for both structure and magmatism, of all the spreading centers identified in the basin, and may be one of the largest spreading systems of the west Pacific back-arc basins. It is more than 800 km long and 50–60 km wide, and has been intensively explored during the French-Japanese STARMER project (1987–1991).

Geometry and structure of the Vitiaz Trench Lineament (SW Pacific)

Swath bathymetric, sonar imagery and seismic reflection data collected during the SOPACMAPS cruise Leg 3 over segments of the Vitiaz Trench Lineament and adjacent areas provide new insights on the geometry and the stuctural evolution of this seismically inactive lineament. The Vitiaz Trench Lineament, although largely unknown, is one of the most important tectonic feature in the SW Pacific because it separates the Cretaceous crust of the Pacific Plate to the north from the Cenozoic lithosphere of the North Fiji and Lau Basins to the south. The lineament is considered to be the convergent plate boundary between the Pacific and Australian Plates during midde to late Tertiary time when the Vitiaz Arc was a continuous east-facing are from the Tonga to the Solomon Islands before the development of the North Fiji and Lau Basins. Progressive reversal and cessation of subduction from west to east in the Late Miocene-Lower Plioene have been also proposed. However, precise structures and age of initiation and cessation of deformation along the Vitiaz Trench Lineament are unknown.

Gravity field derived from the altimetric geoid and its implications for the origin, driving force and evolution of microplate-type marginal basins in the southwestern Pacific

The thickness anomaly of lithosphere in the North Fiji Basin, Lau Basin and the surrounding area was obtained from gravity data derived by satellite altimetry, using crustal thickness determined by available seismic refraction studies. It was revealed that the North Fiji Basin is an isolated asthenospheric dome and that the lithospheric thickness is about 50km less than that of the surrounding area. Such a lithospheric structure and surface topographic feature suggest a plume-like mantle upwelling beneath these basins. The result of numerical simulation shows the possibility of opening of the back-are area and change of subduction polarity which occurred at the first stage of the evolution of the North Fiji Basin. As the lifetime of the southwestern Pacific marginal basins is limited to some 10 million years in most cases, the heat source (plume) which initiates these marginal basins might be sporadic, and their expansion will end when the heat source is wholly consumed.

Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40′ E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.

Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia

The Central Chain ophiolites in New Caledonia are fragments of a supra‐subduction zone (SSZ) ophiolite, now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick Middle Triassic to Upper Jurassic volcaniclastic sequence. Most of the fragments were formed by a single tholeiitic magmatic episode, but one of these, the Koh ophiolite, was formed by two tholeiitic magmatic episodes separated by boninites. The first event in the Koh ophiolite formed cumulate gabbros, dolerites, plagiogranites, and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements (LREE) and abnormally low TiO2 (0.5% at Mg#=60). Shortly after their eruption, these tholeiitic lavas were overlain by a high‐Ca boninitic unit with a basal section of boninite pillows, flows, and breccias and an upper section of boninitic dacites and tuffs. The last magmatic phase involved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite. This second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts (BABB) erupted during the early rifting history of modern back arc basins. The boninitic volcanics belong to a high‐Ca series with slightly lower SiO2, Al2O3, and TiO2 compared to those from modern island arc systems, and they lack the positive Zr spike relative to adjacent rare earth elements (REE) in normalised element variation patterns. These boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40–60% extension of the initial basin crust and eruption of the upper tholeiites. The dominance of BABB‐like tholeiites throughout the Central Chain ophiolites in New Caledonia, the restricted occurrence of boninites, and the stratigraphy and chemistry of the Koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation. We suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites, during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre. The occurrence of a thick blanket of calc‐alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the Koh boninites were not associated with the initiation of subduction. A close modern analogy for the Koh ophiolite exists on the Hunter Ridge protoisland arc between southernmost Vanuatu (New Hebrides island arc) and the Fijian islands; there, high‐Ca boninites lacking positive Zr spikes occur together with low‐Ti tholeiites and more typical BABB tholeiites where the southern spreading centre of the North Fiji Basin is propagating into the protoarc crust of the Hunter Ridge.

Fatty acid characteristics in two symbiotic gastropods from a deep hydrothermal vent of the West Pacific

Two deep-sea gastropods, Ifremeria nautilei and Alviniconcha hessleri, collected on a hydrothermal site of the North Fiji Basin (Southwestern Pacific) were analysed for polar and neutral lipids using gas-liquid chromatography/mass spectrometry. A high level of monounsaturated fatty acids (MUFAs) and a low level of omega 3 series polyunsaturated fatty acids (PUFAs) indicated that nutrition of both gastropods was related to a food web based mainly on bacterial supply. From differences in MUFA distribution between the 2 gastropods, it appeared that most of the energy requirements of A. hessleri were supplied by sulfur-oxidizing endobacteria whereas I. nautilei probably had a mixotrophic diet based on endogenous as well as exogenous bacteria. Given the relatively high level of linoleic acid, which represented from 2 to 8% of the phospholipid fatty acids, hydrothermal gastropods did not appear to be depleted in omega 6 PUFAs. It was hypothesized that they obtain linoleic acid from a pathway different to that in heterotrophic marine molluscs. In contrast to omega 6 PUFAs, both hydrothermal gastropods appeared to be depleted in omega 3 PUFAs, indicating the limited importance of photosynthesis-based food supplies. Some non-methylene-interrupted dienes, particularly 20:2 omega 9,15 which represented from 9 to 18% of the phospholipid fatty acids, may be synthesized by deep-sea symbiotic molluscs in order to restore the depleted omega 3 PUFAs considered as essential for animals. Gills of both gastropods had high levels of neutral lipids, mainly MUFAs that may have originated from degradation of endobacterial phospholipids.

Systematic position and relationship of Phymorhynchus hyfifluxi n. sp., a further new turrid gastropod species associated with hydrothermal vent sites in the North Fiji Basin (Gastropoda: Prosobranchia: Turridae)

Systematic position and relationship of Phymorhynchus hyfifluxi n. sp., a further new turrid gastropod species associated with hydrothermal vent sites in the North Fiji Basin (Gastropoda: Prosobranchia: Turridae)

Dike complexes in American Samoa

Coherent dike complexes in which dikes make up more than 50% of the total rock occur in American Samoa in two volcanoes on Tutuila and in Ofu-Olosega volcano in the Manu'a Islands. The dikes are remarkably narrow (median width 0.24–0.36 m; average width 0.46–0.56 m, based on 860 measured dikes), significantly narrower than dikes in the Koolau volcano (Oahu, Hawaii), the most closely comparable volcano for which quantitative data are available. The vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface: depths of 200–900 m are inferred from the topography. Many dikes have prominent bands of vesicles parallel with their margins. They tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another. Examples are found of center injection repeated up to eight times. Local breccias, often composed largely of dike fragments evidently brecciated in situ, occur where irregular dikes were accommodated by deforming the host rocks. We see evidence for two dike facies: (1) narrow and vesicle banded, early formed and shallow-injected “clustered” dikes in the dike complexes, which are cut by (2) scattered broad and almost nonvesicular deep-injected “solitary” dikes. Surface eruptions were fed by solitary dikes. The ENE-trending dikes and elongation of Tutuila Island contrast with the WNW trend of rift zones and island elongation elsewhere in Samoa. Tutuila is nearly in line with the North Fiji transform fracture zone suggesting that at 1.5-1.0 Ma, when Tutuila was formed, the volcanism of Tutuila was guided by a temporary extension of this fault zone into the Pacific Plate. Fault movement offset the Samoan hotspot trace by 45 km. The en-échelon arrangement of dike exposures on Tutuila is consistent with left-lateral transcurrent motion above this concealed fracture zone.