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 an off-axis 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. (Résumé d'auteur

Segmentation d'une dorsale océanique en domaine arrière-arc : l'axe d'accrétion Sud Pandora - Tripartite (bassin Nord-Fidjien) : résultats de la campagne NOFI de l'Atalante

La campagne NOFI du NIO L'Atalante (Programme franco-japonais NEW STARMER, 25 août - 19 septembre, 1994), avait pour objectif l'étude bathymétrique et géophysique des axes d'accrétion dans la partie septentrionale du bassin arrière-arc nord-fidjien. Les dorsales actives "Sud Pandora" et "Tripartite", orientées de N75 à N110, ont été cartographiées à l'aide du sondeur multifaisceaux SIMRAD EM12-dual, de 171°E à 178°E et de 13°S à 15°S. L'axe actif des dorsales est découpé en segments de 80 à 120 km de long, présentant des caractères morphologiques très variés; il s'agit soit d'un graben parfois occupé par une ride néovolcanique ou obstrué par un volcan, soit d'un puissant dôme volcanique allongé. Les segments sont décalés de 10 à 30 km par des zones de relais de natures variées (décalages simples, "OSC", propagateurs, ...). La dorsale Sud Pandora fonctionne au moins depuis 3 Ma, tandis que la dorsale Tripartite est une structure très récente se propageant au sein d'une croûte plus ancienne jusqu'à 178°E, où elle disparaît. Bien que ressemblant aux centres d'accrétion des grands océans, ce système actif se distingue par l'absence de véritables failles transformantes et par sa position orthogonale à la fois au système central N-S d'accrétion du bassin nord-fidjien et à la frontière convergente des Nouvelles Hébrides. (Résumé d'auteur

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 an off-axis 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. (Résumé d'auteur

Propagation "en échelon" de la dorsale du Bassin Nord-Fidjien entre 16°40 et 14°50'S

International audienceDu 10 janvier au 6 février 1991, la campagne Yokosuka 90 a eu pour objet l'étude géologique, géophysique et géochimique de la branche de direction N10 de la dorsale du Bassin nord-fidjien (SW Pacifique). Ce segment est caractérisé par un système d'accrétion actuel constitué d'une succession "en échelon" de grabens dont certains dépassent 4000 m de profondeur. Ces grabens sont bordés par des murs abrupts d'environ 1000 m de hauteur. Ces caractéristiques sont habituellement celle des dorsales lentes, alors que le taux d'ouverture dans cette zone est estimé à 5 cm/an. (Résumé d'auteur

Japanese submersible explores the north Fiji basin

Since 1987, Japanese and French geologists, geophysicists, and biologists have been studying the North Fiji Basin Ridge within the framework of a joint project named STARMER (Science and Technology Agency of Japan—IFREMER of France). This ridge was first geologically, geophysically, and geochemically surveyed during the 1985 SEAPSO 3 cruise of the R/V Jean Charcot [Auzende et al., 1988]. At that time, water sampling and morphotectonic analysis indicated that the North Fiji Basin Ridge was technically and hydrothermally active. Within the STARMER project, four surface ship cruises have been conducted {Kaiyo 87‐88‐89 and Yokosuka 90). One significant result of these surveys is the complete mapping of the entire ridge between 14°S and 22°S (Figure 1), an area approximately 900 km long and more than 50 km wide (Sea Beam and Furuno multibeam swath‐mapping systems were used). During the Kaiyo 87 cruise, the presence of active hydrothermalism (sulfide deposits, chimneys expelling shimmering water, and associated living animal colonies) was discovered through water sampling and video deep towing

Propagation "en échelon" de la dorsale du Bassin Nord-Fidjien entre 16°40 et 14°50'S (Yokosuka 90-Starmer)

Du 10 janvier au 6 février 1991, la campagne Yokosuka 90 a eu pour objet l'étude géologique, géophysique et géochimique de la branche de direction N10 de la dorsale du Bassin nord-fidjien (SW Pacifique). Ce segment est caractérisé par un système d'accrétion actuel constitué d'une succession "en échelon" de grabens dont certains dépassent 4000 m de profondeur. Ces grabens sont bordés par des murs abrupts d'environ 1000 m de hauteur. Ces caractéristiques sont habituellement celle des dorsales lentes, alors que le taux d'ouverture dans cette zone est estimé à 5 cm/an. (Résumé d'auteur