Geology and geochemistry of a 800 m section through young upper oceanic crust in the North Fiji basin (Southwest Pacific)


Lagabrielle, Y., Auzende, J.-M., Eissen, J.-P., Janin, M.-C. and Cotten, J., 1994. Geology and geochemistry of a 800 m section through young upper oceanic crust in the North Fiji Basin (Southwest Pacific). In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STARMER French-Japanese Program. Mar. Geol., 116: 113-132. We report the results of geological and structural observations made during one dive of the French submersible Nautile during the STARMER Cruise in 1989 in the vicinity of the 16”40’S triple junction in the North Fiji Basin. This dive provided a spectacular, 800 m continuous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction. A number of volcanic facies have been encountered including massive and pillowed lavas, sheeted dikes and spectacular columnar-jointed massive lavas. Pillowed lavas have been observed only at the base and at the top of the section. The ratio between massive and pillowed basalts is high suggesting that the crust in this area grew mostly during a stage of high eruptive rate. Dikes have been observed in the middle part of the section. They probably do not represent the top of a classical “dike complex” but an isolated set of sheeted dikes, possibly the feeders of overlying flows. The abundance of vertical tectonic breccias observed during the first part of the dive confirms that the basal part of the wall can be regarded as a major tectonic boundary along which occurred significant vertical and strike-slip motions. Micropaleontological data from sedimentary rocks collected during the dive bring new constraints to the evolution of the triple junction. The oceanic crust in the surveyed area is at least as old as 1.9-1.3 Ma, based on the age of sedimentary rocks collected in talus at the base of the wall and observed at the summit of the section. The collected basalts include (1) N-type MORBs, (2) E-type MORBs and (3) BABBs. This emphasizes the heterogeneous nature of the mantle beneath the central NFB. Some portions of the depleted mantle underlying the central NFB triple junction area have recorded contamination from melts or fluids from an ancient subduction zone (probably .the New Hebrides during the opening of the NFB), whereas adjacent mantle areas show the influence from an alkali-enriched source. Such alkali-enriched characters are reported from recent basalts of the northern NFB (South-Pandora ridge-Rotuma island). Introduction extended continuous sections of very young oceanic crust have been rarely explored in major ocean as well as in marginal basins because important reliefs allowing continuous exposure within the axial domains of the oceanic ridges are uncommon. The Hess Deep in the eastern Pacific (Francheteau et 1992), the Blanco Trough in the northern Pacific (Juteau at al., 1991) and During the last IO years, in situ geological studies of cross-sections through oceanic crust and mantle have become more frequent. However, ‘Present address: Laboratoire de Géologie et Géophysique, URlF, ORSTOM, B.P. A5, Nouméa, New Caledonia 0025-3227/94/$07.00


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