The Austral archipelago, on the western side of the South Pacific superswell, is composed of several volcanic chains, corresponding to distinct events from 35 Ma to the present, and lies on oceanic crust created between 60 and 85 Ma. In 1982, Turner and Jarrard proposed that the two distinct volcanic stages found on Rurutu Island and dated as 12 Ma and 1 Ma could be due to two different hotspots, but no evidence of any recent aerial or submarine volcanic source has ever been found. In July 1999, expedition ZEPOLYF2 aboard the R/V L'Atalante conducted a geophysical survey of the northern part of the Austral volcanic archipelago. Thirty seamounts were mapped for the first time, including a very shallow one (Ͻ27 m below sea level), located at lat 23؇26.4S, long 150؇43.8W, ϳ120 km southeast of Rurutu. A nepheline-rich scoriaceous basalt sample from pillow lavas dredged on the newly mapped seamount's western flank gave a K-Ar age of 230 ؎ 0.004 ka obtained on pure selected nepheline. We propose that this seamount, already called Arago Seamount after a French Navy ship that discovered its summit in 1993, is the missing hotspot in the Cook-Austral history. This interpretation adds a new hotspot to the already complicated geologic history of this region. We suggest that several hotspots have been active simultaneously on a region of the seafloor that does not exceed 2000 km in diameter and that each of them had a short lifetime (Ͻ20 m.y.). These short-lived and closely spaced hotspots cannot be the result of discrete deep-mantle plumes and are likely due to more local upwelling in the upper mantle strongly influenced by weaknesses in the lithosphere.

Simulation of recent northern winter climate trends by greenhouse-gas forcing, Nature, 399, [452][453][454][455]1999. Thompson, D.WJ., and J. M.Wallace,Annular modes in the extratropical circulation, Part I, month-to-month variability./ Clim., 13,1000-1016, 2000. Deep-penetration Heat Flow ProbesRaise Questions about Interpretations from Shorter Probes PAGES 317,320 More than 40% of the marine heat flow data collected since the early experiments of Sir Edward Bullard in 1949 were obtained using shallow penetration probes less than 5 m long [Louden and Wright, 1989] .The common belief that these data are reliable enough to model deep-seated thermal processes is sup ported by a few experiments in which heat flow measurements made in the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Pro gram (ODP) were compared to nearby surface heat flow measurements [e.g.,Hyndman etal, 1984]. However, thermal measurements made with 18-m penetrations recently collected on the northern flank of the South-East Indian Ridge (SEIR) bring a new perspective to this belief. In the study area, measurements of heat flow taken at the surface (0-5 m) and mea surements taken at greater depths (3-18 m) did not always concur. Investigating this lack of agreement will help address difficult ques tions about the interpretation of shallow pene tration (< 5 m) marine heat flow measurements.The data were obtained during the MD120-ANTAUS expedition carried out by R/V Marion Dufresne that was conducted from October 12 to November 7,2000 from Fremantle, Australia, to La Reunion Island.The primary objective of this cruise was to study marine heat flow vari ations along a 14-Ma isochron that parallels the South-East Indian Ridge (SEIR) between the Saint-Paul/Amsterdam hot spot and the Australian-Antarctic Discordance (AAD),an anomalously deep section of the Mid-Ocean Ridge that is often attributed to a mantle "cold spot." In the 1960s and 1970s, heat flow meas urements were obtained near the AAD as part of reconnaissance surveys [Von Herzen and Langseth, 1966; Langseth and Taylor, 1967;Anderson et al., 1977]. However, to interpret heat flow variations in this region of thin and patchy sediment cover, it is not only necessary to obtain more data, but to collect data that can be used to trace water circulation and dis criminate between the conductive and the convective components of the measured heat flow.To accomplish this, we collected long sediment cores, along with heat flow data to greater depths, to study the physical properties of the sediments and tentatively investigate the role of water circulation using helium isotopic ratios 3 He/ 4 He as tracers of hydrothermal activity. If water has circulated within the crust, then the 3 He/ 4 He isotopic signature is expected to be that of the crust and upper mantle; if water circulation has been confined to the sediment layer, it is expected to be that of the ocean and atmosphere.Despite rough weather and bad seas, a total of 25 thermal measurements was obtained using 9 autonomous digital t...

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The Southwest Pacific region in the area of study is ~ dominated by the subduction of the Australian plate beneath the New Hebrides Arc at a rate of 12 c d y , with a direction of convergence that trends WSW-ENE (Dubois et al., 1977) (Figure 1). The subduction GULWS collision between the Loyalty Ridge and the New. Hebrides Arc. West of the collision zone, three Abstract. The ZoNéCo 1 and 2 cruises of Ifremer's Research Vessel L'Afalante, collected new swath bathymetry and geophysical data over the southern and northern segments of the basins and ridges formine the Lovaltv svstem. Between the two surveyed areas, previ-_-O U~ studies found evidence for the resistance of the Loyalty Ridge to subduction beneath the New Hebrides trench near 22" 5-169" E. On the subducted plate, except for seismicity related to the down-,parallel geological units that trend W-S E north of 22030' s and N-S south of this latitude are present. from east to west (Figure 1): bending of the Australian plate, recorded shallow seismicity is sparse within the Loyalty system (Ridge and Basin) where reliable focal mechanism solutions are almost absent. Swath bathymetry, seismic reflection and magnetic data acquired during the ZoNéCo 1 and 2 cruises revealed a transverse asymmetric morphology in the Loyalty system, and an along-strike horst and graben structure on the discontinuous Loyalty Ridge. South of 23'50' S and at 20" S, the two WSW-ENE-trending fault systems, respectively, sinistral and dextral, that crosscut the southern and northern segments of the Loyalty system, are interpreted as due to the early effects of collision with the New Hebrides Arc. A NNW-SSE trend, evident along the whole Loyalty system and on the island of New Caledonia, is interpreted as an inherited structural trend that may have been reactivated through flexure of the Australian lithospheric plate at the subduction zone. Overall then, the morphology, structure and evolution of the southern and northern segments of the Loyalty system probably result from the combined effects of the Australian plate lithospheric bulge, the active Loyalty-New Hebrides collision and the overthrust of the New Caledonian ophiolite.