Morfoestructura y evolución del ramal N160 de la dorsal de la Cuenca Nor-Fidjiana (Pacífico sudoeste)

The North Fiji Basin is a complex marginal basin formed 10 Ma ago. It is located in the south-west Pacific, on the border of the Pacific and Indo-Australian crustal plates, between two subduction zones of opposed polarity: the New Hebndes trench, to the west, and the Tonga- Kennadec trench, to the east. The North Fiji Basin contains, on a small scale, many of the essential components of global plate tectonics: fracture zones, active spreading ridges, and triple junctions. In the center of the North Fiji Basin, there is a spreading axis constituted of three separate branches which can be individualized in accordance to their dominant directions. One of them, the N160 segment, is discussed in detail in this article, mainly based on recent data sets obtained during the Yokosuka 90 cruise (STARMER project, managed by the IFREMER, France, and the Science and Technology Agency, Japan). The aim of this cruise, carried out between 10th January and 6th February 1991, was the geological and geophysical study of the N160 section of the North Fiji Basin Ridge. Specific features of the N160 segment are pointed out which make it especially interesting with regard to the general knowledge and hypotheses about oceanic spreading ridges. As an example, the N160 segment shows an intermediate spreading rate of 5 cm/a and, at the same time, has a morphology which should be considered as being typical of slow-spreading centers. A succession of en échelon alternating rises and grabens exists between the two triple junctions limiting the segment, the northern one belonging to the Ridge-Ridge-Ridge (RRR) type, and the southern one to the Ridge-Ridge-Fracture Zone (RRF) type. The entire N160 segment is an extremely young morphostructural feature which, according to recorded magnetic stripes, began to be active less than one million years ago as a result of a rapid volcano-tectonic event

L'extrémité occidentale de la zone de fracture fidjienne et le point triple de 16°40S-résultats du leg III de la campagne SEAPSO du N.O. Jean-Charcot (décembre 1985) dans le bassin nord-Fidjien (SW Pacifique)

Au cours du leg III de la campagne SEAPSO du N.O. Jean-Charcot, à été réalisé un levé géophysique et bathymétrique Seabeam de l'extrémité occidentale de la fracture nord-Fidjienne, à la jonction avec la dorsale du bassin nord-Fidjien. Une étude détaillée de ce levé permet de mettre en évidence un réarrangement récent de la direction du système d'accrétion de part et d'autre de cette jonction, accompagné du fonctionnement synchrone d'un point triple centré sur 16°40S. (Résumé d'auteur

Precipitation of ordered dolomite via simultaneous dissolution of calcite and magnesite: New experimental insights into an old precipitation enigma

7International audienceIn the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200°C). The temperature and high-carbonate alkalinity have significantly co-promoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200°C from XRD patterns and FESEM observations. Conversely, higher proportion of ordered dolomite, i.e. clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100°C and two kinetic steps were identified (1) proto-dolomite formation after about five days of reaction, characterized by rounded sub-micrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2thetha on XRD patterns; (2) proto-dolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to 29 kJ/mol by using conventional Arrhenius linear-equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems