Jean-Marie Auzende

Emplacement of deep crustal and mantle rocks on the west median valley wall of the MARK area (MAR, 23°N)

Abstract Exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow-spreading Mid-Atlantic Ridge. Nine dives of the submersible Nautile explored the western axial valley wall in the northern cell of the MARK area (Mid-Atlantic Ridge/Kane fracture zone), where peridotite and gabbro outcrops had been previously reported, in order to constrain the structure and determine emplacement mechanisms. The ridge/transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl, locally overlain by metabasalts and metadolerites, capped by slightly weathered basalts. The morphology is controlled by ridge-parallel faults, dipping moderately (40–65°) to the east. Transform-parallel scarps, present in the northernmost dives, become rare toward the south. Brittle deformation, along moderately dipping fault scarps, produced a dense microcrack network filled with greenschist facies minerals (chlorite, actinolite, epidote, quartz), locally overprinting high-temperature ductile deformation fabrics. The small hill located at 23°20′ in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl. Above 3100 mbsl, the summit of the hill is composed of pillow-basalts and sediments. The peridotite outerops display a strong schistosity dipping 20–40° to the east, parallel to striated normal fault planes. Some steeper east-facing fault scarps truncate the lower-dipping fault surfaces. The serpentinites, deriving from a harzburgitic protolith, are cut by rare dikelets of highly differentiated gabbro. These new data, combined with previous results of Alvin dives, are used to draw a generalized geological map of the western axial valley wall. This map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the MARK area: the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets. Low-angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south.

Acidic and sulfate-rich hydrothermal fluids from the Manus back-arc basin, Papua New Guinea

Hot (≥ 88-120 °C) and acidic (pH ≤ 2.1) hydrothermal fluids rich in sulfate were discovered venting in the DESMOS caldera (depth = 1926 m), eastern Manus back-arc basin, Bismarck Sea, surrounded by Papua New Guinea. The abundant sulfate (≥ 32.8 m M , higher than the seawater value of 28 m M ) with elemental sulfur deposition around the vents, and remarkably low δD(H 2 O) and δ 34 S(H 2 S) values (−8.1‰ and −5.6‰, respectively), are suggestive of the incorporation of a magmatic fluid and the disproportionation of the exsolved SO 2 from a magma body. The DESMOS fluid may be similar in origin to the acidic sulfate-chloride hot springs associated with subaerial volcanic activity. In contrast to the typical hydrothermal end member Mg concentration of 0, the DESMOS fluids are rich in Mg (46–52 m M ), probably because of Mg dissolution by acid attack upon magnesium silicate minerals.

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.

Recent geodynamic evolution of the north Fiji basin (southwest Pacific)

A cruise of the R/V Jean Charcot (Seapso III, December 1985) in the north Fiji basin between 16° and 22°S obtained new structural and magnetic data concerning the central part of the basin. These have been used for the reinterpretation of the National Oceanic and Atmospheric Administration and U.S. Naval Research Project aeromagnetic survey (1979). The structural, surface-ship magnetic, and aeromagnetic data allow us to present a sketch of the evolution in four stages of the north Fiji basin for the past 10 m.y.

La Mediterranee occidentale depuis l'Oligocene Schema d'evolution

-The evolution of the Western Mediterranean basin during Tertiary time is discussed. It is proposed that it was created by a continental drift process during Middle Oligocene time, synchronously with the creation of the grabens over the continents. The significance of the Messinian evaporitic sedimentary episode is discussed in liaison with the problem of large scale subsidence which has been at least six kilometers since Oligocene time.

Detailed geological mapping by submersible of the East Pacific Rise axial graben near 13°N

An intensive exploration of the East Pacific Rise spreading center was conducted in two areas of restricted size near 13°N: site A (0.75 km2) and site B (0.15 km2). The two sites were chosen for their different general appearance based on data acquired previously. A large number of submersible dives (“Cyana”) at site A (21) and site B (6), has permitted the construction of morpho-structural maps at a scale of 1/1000. This paper compares the two areas on the basis of their structure, morphology, lava types, hydrothermal activity and mineral associations. The observed variations are interpreted as the expression of different stages of a cyclic evolution of the axial zone of mid-ocean ridges in contrast to the steady-state model. In this evolution, a tectonic phase leads to the creation of the axial graben by progressive collapse of slabs separated by fissuring of the rise. After extension of the graben, the tectonic phase is followed by a volcanic phase which partially or locally fills in the central graben with volcanic effusions. Ultimately, a dome replaces the central graben with lava flowing down the flanks of the rise. Hydrothermal activity does not appear to be preferentially linked to a given stage of this cyclic evolution. This evolution of alternate tectonic and volcanic cycles best explains observations at sites A and B, which are only 5.5 km apart. Possible magmatic processes responsible for these cycles are discussed.

Seafloor spreading in the North Fiji Basin (Southwest Pacific)

Abstract The Seapso Leg 3 cruise (December, 1985) was carried out in the North Fiji Basin aboard the IFREMER R.V. “Jean Charcot”. The main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16° S and 22°S. To this end. bathymetric profiles (with the help of the Seabeam multichannel echosounder), single-channel seismic reflection profiles and magnetic and gravimetric profiles were made. Moreover, geological samples were taken (by dredging and piston coring) as well as water samples (with the help of a multiprobe). One of the aims of the cruise was to explore the present-day spreading axis of the North Fiji Basin. We were able to show that accretion (the rate of which is about 7 cm/yr) in the North Fiji Basin has been constrained in a N-S direction for 3 Ma. Very recently, between 0.6 and 0.7 Ma ago, the accretion process changed, resulting in the creation of a ridge-ridge-trench triple junction centered at 173°30′E and 16°40′S. Water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese, which are probably linked to hydrothermal events. In its morphological characteristics (transversal as well as longitudinal) the ridge of the North Fiji Basin shows striking similarities to the East Pacific Rise. It presents the same differences in level, the same sizes of structures and the same particular features (overlapping spreading centers, offsets, etc.). Another aim of the SEAPSO cruise was to study the complex deformation area located immediately to the west of the Fiji Islands, which we interpret as a distensive type deformation in a strike-slip system.

The North Fiji Basin Geology, Structure, and Geodynamic Evolution

As the result of intensive studies conducted by U.S., French, and Japanese scientific teams, the North Fiji Basin ridge, poorly known 10 years ago, is one of the most exhaustively investigated ridge axes of the world’s oceans. Today, a ridge segment more than 800 km long and 100 km wide has been fully mapped with the Sea Beam and Furono echo sounders. This ridge axis shows four main segments characterized by the same morphostructural aspect and limits that characterize mid-oceanic ridges. Along the whole length of the axis, a water-column sample has been taken every 20 km and rock samples every 10 km. Different types of hydrothermal activity have been discovered and explored either during the Nautile cruise in 1989 or during the Shinkai 6500 cruise in 1992. The most famous site is the “White Lady,” located around 17°S; it is characterized by 285°C shimmering hot water, which is very poor in metallic elements, expelled through an anhydrite chimney. This water probably represents the low salinity end-member resulting from phase separation in the deep levels of the oceanic crust. Other active sites have been observed all along the axis showing different characteristics such as low-temperature diffusion zones. Even though some parts of the North Fiji Basin remain poorly investigated, the newly acquired data from the ridge axis and from the eastern and northwestern parts allow us to develop a new tectonic model of basin evolution since its creation 12 m.y. ago.

Multiple active spreading centres in the hot North Fiji Basin (Southwest Pacific): a possible model for Archaean seafloor dynamics?

Abstract The North Fiji Basin (NFB), in the Southwest Pacific, is one of the most evolved back-arc basins. Within the basin, active spreading is taking place along several ridges, some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates (segmented accretion centres, organized magnetic lineations and propagators, for example). However, the great cumulate length of the spreading centres of the NFB relative to the Basins surface makes it an unique example among the worlds oceanic basins. We have reviewed a comprehensive body of geological, geophysical and geochemical evidence (multiple spreading centres of various trends, high heat flow, high geoid and slow upper mantle seismic velocities) which collectively demonstrate that a hot upper mantle underlies the North Fiji Basin. Active upper mantle convection is the key process controlling the large crustal and lithospheric production in the NFB, which would be thus largely independent from the evolution of even the largest structural features, such as the Vitiaz and New Hebrides subduction zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: numerous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowing for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean seafloor dynamics.

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank, North Atlantic ocean

Abstract Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast. We applied the 40 Ar/ 39 Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma). Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events. The basement rocks of Gorringe Bank reveal distrubed 40 Ar/ 39 Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.