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ABSTRACT: Geological Framework of an Active Hydrothermal Site in the North Fiji Basin: Starmer Cruise of the Submersible Nautile

Jean-Marie Auzende

During the summer of 1989 the French submersible Nautile carried out a diving cruise on the North Fiji Basin ridge axis in the frame of the Starmer French-Japanese joint project. The diving sites were selected using the Seapso 3, Kaiyo 87, and Kaiyo 88 cruises Seabeam surveys. They are located around 17°S in the axial graben at the northern end of the N15 ridge. The axis consists of an 18 km wide, N15 elongated dome cut by a 2 km wide axial graben. The elevation of the dome with respect to adjacent oceanic floor is 500-600 m. It culminates at less than 1900 m, which is higher than a normal oceanic ridge. The axial graben width (2 km) is also unusual compared to oceanic ridge with intermediate spreading rates such as the EPR at 21°N.

Six Nautile dives have been devoted to the detailed exploration of the axial graben between 16°58^primeS and 17°00^primeS in order to locate the

hydrothermal vents in the inferred most active part of the axial graben.

A structural map has been established on the basis of dive observation. Between 17°S and 16°58^primeS, the axis shows a succession of N15-trending horsts and grabens paralleling the main orientation of the ridge. Two main lateral grabens and a central graben can be recognized. The central graben shows remarkably constant width (200 m) and depth (2000 m). It is bounded by two small horsts, few tens of meters wide. Observed tectonic features include N15 normal fault scarps and abundant open fissures with the same direction. The whole area is dusted with sediments indicating that volcanism was not active recently. Evidence of recent hydrothermal activity such as oxide staining, dead munch, dead chimney is abundant all along the central graben.

At 16°59^primeS, in the axial part of the main graben, lies an active site called "White Lady." A 13 m high white chimney made up only of anhydrite emits only shimmering transparent water by three main vents. The maximum temperature measured in the vents is 285°C. All around the chimney lives a very dense fixed fauna composed of gastropods, mussels, crabs, galatheans, and cirripeds. Seven samples of hydrothermal waters have been collected on the "White Lady" chimney. The samples give the characteristics of a hydrothermal pure sample as follows: 1, chlorinity < 0.3 mol/L; 2, pH ^sim4.5; 3, relatively high alkalinity; 4, low concentration in silica (<15 nmol/L).

North of 16°58^prime30^PrimeS the axial area with its three graben system passes into a wide area, called intersection zone of the N15 ridge with the triple junction. Two different sets of normal faults have been recognized within this domain. The first one, N15 trending, corresponds to the main orientation of the North Fiji basin ridge in this area; the second one, N140 trending, parallels the direction of the northern boundary of the ridge around the 16°40^primeS triple junction.

In the intersection zone, the ocean floor is essentially made of foundered lava lakes exhibiting typical pillars and the successive infilling levels. At the northern end of this area we have explored a very important fossil hydrothermal site. On a 1 km2 area, the hydrothermal deposits (sulfides and oxides) show a several meter thick mound overlaid by a fossil chimney (up to 20 m high) cemetery named "Pere Lachaise." Along a normal fault, a more than 30 m thick level exclusively constituted of sulfides is exposed.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990