New cruise results indicate that the New Zealand–South Fiji Basin margin comprises a volcanic plateau, separated from the continent by sedimentary basins (floored by metamorphic rocks and an ...allochthon containing Cretaceous ophiolites). Seaward lies undisturbed oceanic crust. The plateau began to form in the Early Miocene after the allochthon was obducted (in the Aquitanian). The oceanic crust of the South Fiji basin is contiguous with that of the plateau, which in turn indicates that the South Fiji Basin in this region is of Miocene rather than Oligocene age as commonly thought. The tectonic history is perforce that of a Pacific subduction zone replaced by a back-arc transform margin containing an enormous volume of volcanic material.
À la limite du Bassin sud-fidjien et de la Nouvelle-Zélande s’étend un segment de marge caractérisé par un plateau volcanique d’orientation NW–SE. Ce dernier est séparé du continent par des bassins de pente reposant, soit sur un socle métamorphique, soit sur un allochtone ophiolitique d’âge Crétacé. Le Bassin sud-fidjien se caractérise, quant à lui, par un socle océanique typique. Des données récentes indiquent que le plateau du Northland se serait mis en place au cours du Miocène inférieur, après l’obduction des ophiolites, ce qui implique un âge également miocène pour cette région du Bassin sud-fidjien. Ce segment de la marge néozélandaise, caractérisé par une énorme accumulation de matériel volcano-sédimentaire, résulte d’une histoire structurale complexe, qui a vu se succéder les effets d’une subduction et d’une obduction, puis ceux d’une ouverture arrière-arc (celle du Bassin sud-fidjien), elle-même issue de la subduction, vers l’ouest, du Pacifique.
Diving surveys were performed in the North Fiji Basin, Southwest Pacific, using deep-sea submersible Nautile and her mothership Nadir under the framework of the Japan-France Joint Research Project ...STARMER. Three diving sites were selected based on the results of previous cruises; two along the N15E central spreading axis of the basin and one at the triple junction of three rift axes. In the first diving site (16°59'S;173°55'E), an active white smoker which is called "White Lady" was found. It expels clear, low salinity, hydrothermal solution (T=285°C). We believe that the solution is the vapor condensate of boiled solution. The chimney mostly consists of anhydrite and is built on top of extinct sulfide chimneys which are composed of pyrite, marcasite, wurtzite, chalcopyrite, and covelline. Seepage of low temperature hydrothermal solution was also observed along the fault scarps of the central graben. In the second site (16°58'S; 173°55'E), which locates about 1 km north of the previous one, huge hydrothermal area, more than 1 km long, of "dead" sulfide chimney forest was observed. This hydrothermal site could be the largest one in western Pacific area. Those chimneys are about 15 m in height, 5 m in bottom diameter and are composed mainly of dendritic aggregate of marcasite. The northeast rift system in the third site (16°45'S; 174°05'E) is suggested to be tectonically active. A cross section of oceanic crust was observed on a cliff face 1000 m in height.
The GEODYNZ-SUD cruise Leg 2, carried out southwest of New Zealand, studied the structural transition zones between (from north to south): the transpressive regime of the Alpine Fault, the oblique ...subduction under the Fiordland margin and the incipient intra-oceanic subduction under the Macquarie Ridge. To the north, above the eastward subducted Australian plate, the Milford-Fiordland trough inner wall is entirely cut by at least four northeast trending (slightly oblique to the margin) strike-slip faults that converge towards the Alpine Fault transpressive system. To the south strike-slip deformation is exclusively at the Macquarie Ridge summit.
Sept dragages ont été réalisés le long du système d'accrétion actif Sud Pandora-Tripartite dans le nord du bassin Nord-Fidjien (Sud-Ouest Pacifique) lors de la campagne NOFI du N/O L'Atalante ...(programme Newstarmer), en 1994. Les laves émises sont des basaltes tholéiitiques légèrement à fortement enrichis en éléments incompatibles, ne montrant pas l'appauvrissement en Nb caractéristique d'une contamination des zones de subduction, et dont les rapports La/Yb, Ba/La et La/Nb sont intermédiaires entre ceux des MORB (
mid-ocean ridge basalts) enrichis et des OIB (
oceanic-island basalts). Ce système d'accrétion évolue ainsi de façon indépendante par rapport aux subductions avoisinantes, au moins du point de vue géochimique et cinématique. L'activité des dorsales du nord du bassin Nord-Fidjien pourrait être sous la dépendance de remontées de manteau profond, plus enrichi que celui généralement présent à l'axe des dorsales médio-océaniques.
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.
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