In the southwest Pacific, a discontinuous series of narrow and elongated troughs separates the New Hebrides island arc from the adjacent active marginal basin, the North Fiji Basin. This chapter reviews the structural, geophysical, geochronological, and petrological data available for the New Hebrides backarc troughs (NHBAT) and discusses the significance of these structures. A diffuse horst-and-graben morphology, partly obscured in some places by recent volcanic complexes, characterizes the northern Jean-Charcot troughs (JCT). By contrast, the southern Coriolis troughs (CT) show well-developed flat-bottomed grabens. Moreover, no backarc troughs are observed in the central backarc area, adjacent where the d'Entre- casteaux zone collides with the arc. Volcanic rocks dredged in the NHBAT show a wide range of SiO, contents, with high- Alzo3 and low-Tio, contents, features typical of their archackarc environments. Trace element analyses indicate a much stronger subduction component in the volcanics of the southern CT than in those of the northern JCT. However, large-ionic-radius-lithophile- element (LILE) (Ba, Rb, Sr) enrichments and high-field-strength-elements (HFSE) (Ta, Nb, Zr, Ti,-Y, Yb) depletions, relative to N-MORB (mid-ocean ridge basalts), are generally observed in most NHBAT volcanics and are features characteristic of island-arc basic and

Rifting that produced the Jean Charcot Trough (JCT) in the Vanuatu back-arc region commenced 2 3 Ma, leading to SiO2-bimodal magmatism in the northern half of the trough. Dacite with high Na2O (> 6.0%) and low K2O (< 1.0%) was recovered from the northern termination of the JCT, accompanied by thioleiitic N-MORB-type back-arc basin basalt similar to that erupted at the central spreading axis of the North Fiji Basin (NFB). Incompatible element ratios are very similar in the dacite and associated basalt, and also in the NFB back-arc basin basalts (Zr/Rb > 10, Nb/Rb > 0.5, Y/Rb > 2.0, Nb/La > 0.5, and Ba/Zr < 1.5), but are quite different to lavas from the active Vanuatu volcanic arc.The dacites are compositionally close to oceanic plagiogranite produced during fractional crystallization of an N-MORB basaltic magma, and mass balance calculations show that they may be similarly derived from the associated basalts. We show that JCT basalts are derived from a mantle source compositionally very close to that beneath the central spreading axis of the NFB, with minimal contamination by a subduction component. Westward injection of diapiritic source mantle similar to that of the NFB back-arc basin basalts may have caused rifting of the JCT. During incipient back-arc rifting, high Na/K felsic magmas may have evolved from parental low-K basalts via side-wall crystallization within a large lower crustal magma chamber.

Two cruises of the French-Japanese starmer joint project carried out with the submersibles Nautile (1989) and Shinkai 6500 (1991), have been devoted to the in situ exploration of the North Fiji Basin ridge. Four areas, located near 16°20′, 17°, 18°50′ and 19°S have been explored during 42 dives. At 16°20′S, the axial zone trending N160° corresponds to a prominent graben with walls 1000 m high. Tectonic features and recent screes are present, and the hydrothermal activity is characterized by sporadic fluid discharge. At 17°S, present accretion is confined to a high dome less than 1900 m deep and cut in its axial part by a wide graben trending N15°. At this site, hydrothermal features of two types are observed: fossil chimneys of sulfides, and active vents, made of anhydrite and expelling water at 285°C. The site at 18°50′S is located on top of an axial dome, very similar to those of the East Pacific Rise. Small fissures and very fresh volcanic flows argue for a present-day activity, with weak evidence of tectonics. Hydrothermal emanations with animal colonies are found scattered over the whole area. The fourth site, at 19°S, shows old and viscous lavas and a very mature tectonic activity, compared to the preceding station which is located only 20 km northward. Large-scale variations have been detected between three segments. Different thermal regimes can explain the distinct morphologies observed. Decakilometric scale variations do occur between stations 14, with volcanic predominance, and “19°S”, where tectonic activity is dominating. Finally, small-scale variations are observed on sites 4 and 14, where volcanic features gradually give way to tectonic structures. The North Fiji Basin ridge—with an intermediate average spreading rate—shows tectonic, volcanic and hydrothermal variability, ranging from 1 km to tens of kilometres and related to a temporary fine scale organisation of the accretionary processes.

The chronological, petrological and geochemical studies of lavas dredged from the New Hebrides back-arc troughs allow a new interpretation of the origin of these troughs. In every area, volcanism from the troughs precedes that of the adjacent islands. Four main periods of volcanic activity have been defined: 6.5 to 4.8, 4.1 to 2, 2 to 1 and 1 to 0 Ma. The volcanic affinity is generally orogenic. But some variation exists and two geochemical types (Mg-IAT basalts and hyper-K acid lavas) seem to mark the trough structuration. The succession of the different geochemical types reveals a polyphased and diachronous formation of the troughs from south to north on an arc substratum. Only the Vanikoro area (the most northern one) shows basalts with geochemical characteristics intermediate between MORB and island-arc tholeiites and acid lavas near primitive island-arc lavas, which illustrate the initiation of the arc in this area. So, the New Hebrides back-arc troughs must be considered as intra-arc troughs and are back-arc structures only because of their location at the rear of the active emerging arc.

The Wallis Islands are located on the western extension of the Samoan seamount chain and are composed largely of basaltic flows and pyroclastics with ages that are less than 0.5 Ma. The youngest eruptives are tholeiitic in composition and are relatively depleted in large ion lithophile and high field strength trace and minor elements. In contrast, the older basaltic flows and pyroclastics, which make up the bulk of exposed volcanic rocks, have transitional alkalic chemistry and are relatively enriched in incompatible trace and minor elements. The younger tholeiites have lower Sr-87/Sr-86 (0.7045-0.7048) and tend to have higher Nd-143/Nd-144 (0.512844-0.512910) than the older alkalic rocks (Sr-87/Sr-86: 0.7049-0.7052; Nd-143/Nd-144: 0.512800-0.512895). Major- and trace-element modelling based on relationships inferred from the isotopic and trace-element compositions of stratigraphically related samples supports the hypothesis that some of the evolved basalts could have been generated by crystal fractionation from less evolved parental basalts.The Quaternary volcanism on the Wallis Islands is too young to be directly related to passage of the Pacific Plate over the Samoan hot spot, but isotopically Wallis basalts are very similar to shield lavas of the Samoan Islands. This suggests that Wallis basaltic magmas were generated in the lithosphere by a later thermal disturbance that was probably related to deformation along the transform boundary between the Pacific and Indo-Australian plates. A model is proposed whereby the basalts derived from a complex, heterogeneous, lithospheric mantle by progressive melting which initially extracted relatively alkalic melts from mantle domains dominated by an enriched component. With time, as melting proceeded, the enriched component was diluted by a more depleted, and more refractory component. The relationship of the mantle plume, that generated the Samoan seamount chain, to the volcanism on the chain may be largely thermal with very little geochemical input from the plume into the erupted magmas. The northern Melanesian Borderland may be a region where unusual lithospheric mantle has existed for some time and the shield lavas of the Samoan chain, along with the Wallis basalts, may have been extracted largely from this lithosphere.

The North Fiji Basin (NFB) is a 12 m.y. old back-arc basin that has a complex multi-stage history. The presently active spreading system can be divided into four segments between 16°S and 22°S, which from north to south trend N160, N15 and N-S (the fourth segment is the N-S trending segment located near 174E). The main N-S segment is morphologically similar to other medium-rate oceanic ridges, whereas the other segments have rougher morphologies which have been severely disturbed by a triple junction at 16°45′S and several instability features such as overlapping spreading centres (OSCs) and propagating rifts. The spreading rate seems to diminish from 7.8 cm/yr near 20°S to 4.6 cm/yr near 18°S. Mineralogical, pertological and geochemical data were obtained on 24 new stations located along all the four segments. The petrogenesis of the basalts collected is essentially controlled by low-pressure crystal fractionation of plagioclase±olivine±clinopyroxene (plagioclase>olivine>clinopyroxene) with 52% of the NFB basalts reaching the four-phase cotectic. Locally, some magma mixing occurs, but this is limited to magma batches of closely related composition, as might be expected to occur inside a magma reservoir. The N-S segment, which, since 3 m.y., is the only steady-state segment, is also petrologically and geochemically very comparable to other medium-rate oceanic spreading centres, producing moderately evolved LILE and LREE-depleted N-MORB. In contrast, the three other segments produce basalts of much more variable petrology and geochemistry characterized by LILE and slightly LREE-enriched magmas cf back-arc basin basalt (BABB) affinity (but not as enriched as, for example, the Mariana BABB); MORB is, however, also found on the N160, N15 and 174E segments. Diagrams using Ba, Rb, K/P and (K/Ti)N (Normalized to the chondrites) plotted against latitude clearly show along-strike variations. Beneath the recently formed segments, the mantle source is heterogeneous, and locally has some BABB affinities, whereas beneath the more steady-state N-S segment the magma source is more homogeneous, being generally depleted in LILE and REE as is the case for classical N-MORB mantle sources. Simple evolution from an early stage of BABB production to MORB described in the Lau basin and proposed for the NFB does not seem to occur. Present-day activity still produces large amounts of BABB along the less-stable and more recently created segments, and MORB was produced in the earlier stages of the development of the NFB.

For the first time, an upper Pliocene transitional basaltic volcanism is described on Futuna and Alofi islands. On these islands, the orogenic fingerprint existing in the former tholeiitic volcanism disappears with this new eruptive stage. The transitional basalts seem to be in accordance with the general pattern of a regional alteration in the volcanism, beginning in Fiji about 3 Ma ago, and extending to numerous sites of the North Tonga region. This modification is related to a general plate reorganization of the South-West Pacific marked, in the North Tonga termination, by the transition from a convergent (Vitiaz-Tonga subduction) to a transform regime (North Fiji fracture zone).

Structure, age and origin of the New Hebrides back-arc troughs (southwest Pacific) The New Hebrides back-arc troughs are restricted to the southem and northern rear of the arc. Lack of trough in the central part is related to the compression induced by the collision of the d'Entrecasteaux ridge and the New Hebrides arc. The southern troughs are 50-60 km wide and are composed of one or two grabens striking N130-140", while the northem troughs extend over 70 km wide and are composed of a succession of N-S trending grabens, horsts and half-grabens. Although the location and the main trend (N150") of the troughs indicate that they are largely guided by the subduction, the direction of extension (N40-45') is clearly oblique to the volcanic line and thus does not seem related to the subduction process. Lack of island arc tholeiitic lavas in the volcanoclastic series outcropping along the eastern scarps of the different troughs since 2.6-2.2 Ma, can be considered as an evidence of the initiation of the troughs structuration, which is not necessary synchronous all along the back-arc domain. The geometry of the sedimentary filling indicates that the back arc trough tectonics has been continuous with accelerated phases underlined by unconformities (at least two after the initiation of the troughs formation); it is still active. The thickness of the sedimentary filling inside the half grabens of the northern troughs is in good accordance with the Upper Pliocene (2.6-2 Ma) age proposed for the onset of the extensional tectonics. Geochemical characteristics of the lavas of the troughs indicate that this volcanism is more closely related to different geochemical provinces of the arc rather than affiliated with the troughs tectonics. However, a low-K acid volcanism emplaced since 1.8 Ma could be associated with punctiform initiation of opening in the northern trough. Formation of the New Hebrides back-arc results from extensional tectonics induced by a major reorganization at 3 Ma of the seafloor spreading of the North Fiji basin, which appears since this time to be developped independently of the subduction process. Oceanologica Acta, 1990, volume spécial 10, Actes du colloque Tour du Monde Jean Charcot, 2-3 mars 1989, Paris, 43-60.

Michel MONZIERa, Jacques DANIEV, Patrick MAILLET' ORSTOM, BP AS. Noumea, Nouvelle-Caledonie b QRSTOM, Dcpartmcnt o f Geology. La Trobe Universily, Bundoora. Victoria, Australia 3083 a et b GOR «Genese et Evolution des Domaines Oceaniques», Brest, France. Recu le 15,00/89, revise le 17,{)5!90, accepte le 31,{)S,9(1 La collision entre le saillant fonne a 22°S par la ride des Loyaute, appartenant a la plaque indo-australienne, et la temlinaison meridionale de l'arc des Nouvelle-Hebrides a debute il y a environ 300 000 ans. Toutefois. par son action sur le bombement de la plaque plongeante, la ride influcncc probablement, depuis 1 a 2 Ma, le fonctionnement de la terminaison meridionale arquee de la zone de subduction des Nouvelles-Hebrides. L'enmcinement el la floltabi li te de cette ride massive contrarient la flex ure de la plaque plongeante ct rendent difficile sa subduction. Ainsi, les failles normales qui decoupent le saillant sont peu actives; la zone de contact entre les plaques, defin ie par la sismicite superficielle, s'est rapprochee de la fosse; le mur interne, localement souleve, montre un debut d'accret ion et le front des deformations compressives a migre sur le mur externe. L'approche de la ride des Loyaute, puis la collision avec la terminaison meridionale arquee de la zone de subduction des Nouvelles Hebrides, ont provoque une evolution differente, de part et d 'autre de 22°S, du dispositif de convergence. Vers 22°S, un decrochement estouest senestre, d 'amplitude regionale, decoupe l'arc et affecte le bord de la plaque plongeante. Au nord de cct accident, la coll ision, bien qu'intense, ne suffit pas, pour l'instant, a contrecarrer totalement la convergence, nonnale a la fosse, car l 'effet d 'entrainement lateral induit par la subduction de la lithosphere oceanique du bassin nord-Loyaute est dominant. Au sud de cet accident, la convergence oblique sous la terminaison arquee est arretee, et une microplaque, fortement couplee a la plaque ind()-australienne, est apparue. Au droit de la zone de coll ision, la faib le longueur de la zone de 8 enioff, le developpement limite voire l'absence de la chaine volcanique sommitale, la terminaison des fosses intra-are du Coriolis, ainsi que le decrochement important de l'axe de la dorsale active du bassin nord-fidjien ne sont probablement que des consequences de l'approche de la ride et

Petrology and geochemistry of the central North Fiji Basin spreading center (SW Pacifique) Abstract - The axial accretion zone of the central North Fiji Basin has been sampled quite regularly between 16s aiid 2030'S. Collected basalts are mainly MORB, even if BABB are present. Observed mineralogical, petrological and geocheinical variations show that this baclc-arc spreading center has reached a mature state very siinilar to any mid-oceanic ridge of the siinilar medium spreading rate (6- 8 cinly). However each of the three r~iorplio-structurally defined accretion segments presents its own chemical and petrological signature, espacially along the segment situated between 1640'S and 1820'S, which is very heterogeneous, reflecting the recent reorganization of the triple junction of 1640'S and probably a geoclieniical heterogeneity of the mantel.

Detailed surveys (bathymetry, magnetism, seismicity and focal mechanism solutions) recorded on the junction between the southern New Hebrides Arc and the North Fiji Basin enlighten the geodynamic complexity of the area.

Matthew and Hunter, the two southernmost active volcanoes of the New Hebrides island arc (southwest Pacific) differ markedly from the other (mainly tholeiitic Quaternary volcanoes of this arc.