Loyalty Ridge Research Articles

Detailed 40Ar/39Ar Geochronology of the Loyalty and Three Kings Ridges Clarifies the Extent and Sequential Development of Eocene to Miocene Southwest Pacific Remnant Volcanic Arcs

AbstractThe 2015 VESPA voyage (Volcanic Evolution of South Pacific Arcs) was a seismic and rock dredging expedition to the Loyalty and Three Kings Ridges and South Fiji Basin. In this paper we present 33 40Ar/39Ar, 22 micropaleontological, and two U/Pb ages for igneous and sedimentary rocks from 33 dredge sites in this little‐studied part of the southwest Pacific Ocean. Igneous rocks include basalts, dolerites, basaltic andesites, trachyandesites, and a granite. Successful Ar/Ar dating of altered and/or low‐K basalts was achieved through careful sample selection and processing, detailed petrographic and element mapping of groundmass, and incremental heating experiments on both phenocryst and groundmass separates to interpret the complex spectra produced by samples having multiple K reservoirs. The 40Ar/39Ar ages of most of the sampled lavas, irrespective of composition, are latest Oligocene to earliest Miocene (25–22 Ma); two are Eocene (39–36 Ma). The granite has a U/Pb zircon age of 23.6 ± 0.3 Ma. 40Ar/39Ar lava ages are corroborated by microfossil ages from associated sedimentary rocks. The VESPA lavas are part of a >3,000 km long disrupted belt of Eocene to Miocene subduction‐related volcanic rocks. The belt includes arc rocks in Northland New Zealand, Northland Plateau, Three Kings Ridge, and Loyalty Ridge and, speculatively, D’Entrecasteaux Ridge. This belt is the product of superimposed Eocene and Oligocene‐Miocene remnant volcanic arcs that were stranded along and near the edge of Zealandia while still‐active arc belts migrated east with the Pacific trench.

Earthquake Fingerprint of an Incipient Subduction of a Bathymetric High

AbstractThe resistance of bathymetric highs to subduction results in large‐scale morphological distortions of the outer‐rise, trench, and fore‐arc regions. Once subducted, bathymetric highs induce frictional segmentation along the plate interface that may result in increase or decrease of the plate coupling. However, the mechanics of the collision is inferred mostly from geophysical and geological surveys since earthquakes rarely illuminate finer details of the subduction of seafloor relief. A year‐long and energetic seismic sequence at the Loyalty Ridge (LR)‐Vanuatu Trench allowed us to characterize how strain is released along the collision zone. Earthquakes revealed complex fracturing in the outer‐rise and fore‐arc regions and segmentation of the interface with both limited magnitude events and aftershock productivity. The complex earthquake activity associated to the collision and subduction of the LR appears to support a frictionally segmented interface where Mw ≥ 8 megathrust earthquakes are unlikely to nucleate.

Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

Located in the South-West Pacific, at the northern extremity of the mostly submerged Zealandia continent, the New Caledonian Exclusive Economic Zone (EEZ) covers 1,470,000 km² and includes basins, ridges and seamounts where abundant ferromanganese crusts have been observed. Several investigations have been conducted since the 1970s on the nature and composition of ferromanganese crusts from New Caledonia’s seamounts and ridges, but none have covered the entire EEZ. We present data from 104 ferromanganese crusts collected in New Caledonia’s EEZ during twelve oceanographic cruises between 1974 and 2019. Samples were analysed for mineralogy, geochemical compositions, growth rates, and through a statistical approach using correlation coefficients and factor analysis. Crust thicknesses range from 1 mm to 115 mm, with growth rates between 0.45 mm/Ma and 102 mm/Ma. Based on textures, structures, discrimination plots, and growth rates, we distinguish a group of hydrogenetic crusts containing the highest mean contents of Co (0.42 wt%), Ni (0.31 wt%), and high contents of Mo, V, W, Pb, Zn, Nb, from a group of hydrothermal and/or diagenetic deposits showing high mean contents of Mn (38.17 wt%), Ba (0.56 wt%) and low contents of other trace metals. Several samples from this later group have exceptionally high content of Ni (0.7 wt%). The data shows that crusts from the southern part of the EEZ, notably seamounts of the Loyalty Ridge and the Lord Howe Rise, present high mineral potential for prospectivity owing to high contents of valuable metals, and constitute a great target for further investigation.

The <i>M</i><sub>w</sub> 7.5 Tadine (Maré, Loyalty Islands) earthquake and related tsunami of 5 December 2018: seismotectonic context and numerical modeling

Abstract. On 5 December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Islands archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty Ridge and the southern part of the Vanuatu Arc, in a tectonically complex and very active area regularly subjected to strong seismic crises and earthquakes higher than magnitude 7 and up to 8. Widely felt in New Caledonia, it was immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), and then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Two solutions of the seafloor initial deformation are considered for tsunami generation modeling, one using a non-uniform finite-source model from USGS and the other being a uniform slip model built from the Global Centroid Moment Tensor (GCMT) solution, with the geological knowledge of the region and empirical laws establishing relationships between the moment magnitude and the fault plane geometry. Both tsunami generation and propagation are simulated using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), an open-source modeling code solving the shallow-water equations on an unstructured grid allowing refinement in many critical areas. The results of numerical simulations are compared to tide gauge records, field observations and testimonials from 2018. Careful inspection of wave amplitude and wave energy maps for the two simulated scenarios shows clearly that the heterogeneous deformation model is inappropriate, while it raises the importance of the fault plane geometry and azimuth for tsunami amplitude and directivity. The arrival times, wave amplitude and polarities obtained with the uniform slip model are globally coherent, especially in far-field locations (Hienghène, Poindimié and Port Vila). Due to interactions between the tsunami waves and the numerous bathymetric structures like the Loyalty and Norfolk ridges in the neighborhood of the source, the tsunami propagating toward the south of Grande Terre and the Isle of Pines is captured by these structures acting like waveguides, allowing it to propagate to the north-northwest, especially in the Loyalty Islands and along the east coast of Grande Terre. A similar observation results from the propagation in the Vanuatu islands, from Aneityum to Efate.

Chapter 6 The Loyalty Islands and Ridge, New Caledonia

Abstract The Loyalty Ridge lies to the east and NE of the Norfolk Ridge. The three main Loyalty Islands (Maré, Lifou and Ouvéa) emerge from the ridge at the same latitude as Grande Terre. The islands are uniformly composed of carbonate deposits, except for Maré, where Middle Miocene intra-plate basalts and associated volcaniclastic rocks form restricted outcrops. Miocene rhodolith limestones constitute the bulk of the carbonate cover of the three islands. On Maré, these platform accumulations are locally topped by a dolomitic hardground, which, in turn, is covered by Pliocene–Pleistocene coral-bearing formations. These coral reef constructions are preserved as elevated rims over all three islands and define an atoll stage in their development. The Pleistocene–Holocene palaeoshoreline indicators include fringing bioconstructions and marine notches and record both eustatic sea-level changes and tectonic deformation. The ridge has been in the forebulge region in front of the active Vanuatu subduction zone since the Pliocene and each of the three islands has been uplifted and tilted to varying degrees. Offshore, the Loyalty Ridge continues northwards to the d'Entrecasteaux Zone and southwards to the Three Kings Ridge. Although typically volcanic, the nature of the deep Loyalty Ridge remains unknown.

Chapter 1 Introduction to New Caledonia: geology, geodynamic evolution and mineral resources

Supplementary material: List and content of the GIS files (ESRI format) corresponding to the geological map (Fig. 1.2) and the database structure of Figure 1.3 are available at https://doi.org/10.6084/m9.figshare.c.4984385 New Caledonia is a French overseas territory in the SW Pacific, located in the southern tropical zone between 19 and 24° S. The country consists of several islands that are the emergent parts of two parallel submarine ridges: the Norfolk Ridge to the SW and the Loyalty Ridge to the NE (Fig. 1.1). The total area of the country is 18 350 km2. Grande Terre, the largest and main island, is mountainous, c. 400 km long and c. 50 km wide. The highest points are Mont Panie (1628 m) in the north and Mont Humboldt (1618 m) in the south. Grande Terre is surrounded by a 1500 km long barrier reef complex, the second longest in the world, with parts listed as a UNESCO World Heritage site. By contrast, the Loyalty Islands have a low elevation (127 m maximum height on Mare). As well as Grand Terre and the Loyalty Islands, the New Caledonia archipelago includes the isolated and uninhabited Matthew and Hunter islands to the east and the sandy Chesterfield islets to the west. The New Caledonia Exclusive Economic Zone covers an area of 1 360 000 km2. Fig. 1.1. ( a ) New Caledonia in the SW Pacific. ( b ) New Caledonia archipelago. ( c ) Map of New Caledonia with 1:50 000 scale geological map series (grey rectangle) and index numbers. 1, Iles Belep; 2, Poum–Yande; 3, Paagoumene; 4, Pam-Ouegoa; 5, Pouebo; 6, Koumac, 7, Paimboas; 8, Hienghene; 9, Ouaco-Voh; 10, Goyeta-Pana; 11, 12, Touho-Poindimie; 13, Pouembout; 14, Paeoua; 15, Ponerihouen; 16, Baie Lebris; 17, Poya-Plaine des Gayacs; 18, Me Maoya; 19, Houailou; 20, Kouaoua; 21, Bourail; 22, Moindou; 23, Canala-La …

Regional volcanism of northern Zealandia: post-Gondwana break-up magmatism on an extended, submerged continent

Abstract Volcanism of Late Cretaceous–Miocene age is more widespread across the Zealandia continent than previously recognized. New age and geochemical information from widely spaced northern Zealandia seafloor samples can be related to three volcanotectonic regimes: (1) age-progressive, hotspot-style, low-K, alkali-basalt-dominated volcanism in the Lord Howe Seamount Chain. The northern end of the chain ( c. 28 Ma) is spatially and temporally linked to the 40–28 Ma South Rennell Trough spreading centre. (2) Subalkaline, intermediate to silicic, medium-K to shoshonitic lavas of >78–42 Ma age within and near to the New Caledonia Basin. These lavas indicate that the basin and the adjacent Fairway Ridge are underlain by continental rather than oceanic crust, and are a record of Late Cretaceous–Eocene intracontinental rifting or, in some cases, speculatively subduction. (3) Spatially scattered, non-hotspot, alkali basalts of 30–18 Ma age from Loyalty Ridge, Lord Howe Rise, Aotea Basin and Reinga Basin. These lavas are part of a more extensive suite of Zealandia-wide, 97–0 Ma intraplate volcanics. Ages of northern Zealandia alkali basalts confirm that a late Cenozoic pulse of intraplate volcanism erupted across both northern and southern Zealandia. Collectively, the three groups of volcanic rocks emphasize the important role of magmatism in the geology of northern Zealandia, both during and after Gondwana break-up. There is no compelling evidence in our dataset for Late Cretaceous–Paleocene subduction beneath northern Zealandia.

Propagation of back‐arc extension into the arc lithosphere in the southern New Hebrides volcanic arc

AbstractNew geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 90° bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back‐arc domain, the active deformation is characterized by propagation of the back‐arc spreading ridge into the Hunter volcanic arc. The N‐S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike‐slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction‐related magmas has been produced including adakites, island arc tholeiites, back‐arc basin basalts, and medium‐K subduction‐related lavas.

Description of a new species of Typhinellus (Gastropoda: Muricidae: Typhinae) from the Western Atlantic

There are seven known living species of Typhinellus. Five species occur in the Indo-West Pacific: T. amoenus Houart, 1994 ranges from Natal, South Africa to Somalia with other records off Quissico, southern Mozambique (MNHN); T. androyensis Bozzetti, 2007, described from south Madagascar and also present in the Gulf of Aden (RH coll.); T. bicolor Bozzetti, 2007 described from and presently restricted to southern Madagascar; T. insolitus Houart, 1991 from the Loyalty Ridge and T. occlusus (Garrard, 1963) described from Queensland, Australia and also recorded from southern Philippine Islands. A single species, Typhinellus labiatus (Cristofori & Jan, 1832), occurs throughout the Mediterranean and off the Atlantic coasts of Morocco and southern Spain (Houart, 2001). This species was also recorded from the Canary Islands (Nordsieck & Garcia Talavera, 1979) and off Dakar, Senegal (MNHN and Marche Marchad, 1958). The specimen recorded from the Gulf of Oman in Houart (2001: 114) turned out to be T. androyensis (see above) while the one from Somalia, also in Houart (2001: 114) probably also refers to this species. Another species, Typhinellus lamyi Garrigues & Merle, 2014, was recently described from Guadeloupe. It seems that only one fossil species, T. chipolanus (Gertman, 1969) from late lower Miocene (Burdigalian) of Florida occurs in the Western Atlantic. Two additional species will be described from south Madagascar (Houart & Héros, in press).

Perotrochus caledonicus (Gastropoda: Pleurotomariidae) revisited: descriptions of new species from the South-West Pacific

Morphological (shell) and molecular examination of a large suite of specimens of pleurotomariids from around New Caledonia and the Coral Sea reveals the existence of four species in the complex of Perotrochus caledonicus: Perotrochus deforgesi Métivier, 1990 and P. pseudogranulosus sp. nov. live allopatrically on the plateaus and guyots of the Coral Sea; Perotrochus caledonicus Bouchet & Métivier, 1982 and Perotrochus wareni sp. nov. live sympatrically - but essentially not syntopically - on the slopes of New Caledonia, Norfolk Ridge and the Loyalty Ridge. All species live in the 300–500 m interval, and together form a significant component of the mollusc fauna living on hard bottoms in the SW Pacific, with individual dredge hauls containing up to 25 specimens of Perotrochus.

An inter‐ocean comparison of coral endemism on seamounts: the case of Chrysogorgia

AbstractAimThe biogeography of seamount fauna remains poorly known, with less than 1% of the world's seamounts having been investigated. Here, we report data on the geographical isolation of species in the octocoral genus Chrysogorgia from south‐west Pacific seamounts and slopes, and contrast the results with patterns observed in the north‐western Atlantic.LocationSeamounts of the Norfolk Ridge (NR) and Loyalty Ridge (LR), the slope of New Caledonia, and the Matthew and Hunter Islands, south‐west Pacific Ocean, with comparative material from the Pacific and Atlantic oceans.MethodsThe mitochondrial gene mtMutS was used to measure diversity within Chrysogorgia. Community structure was analysed using rarefaction, multivariate analyses, parsimony analysis of endemicity and analysis of molecular variance. The impact of underestimating species richness when using mitochondrial haplotypes was tested using simulations.ResultsSix hundred and thirty‐four colonies and 31 haplotypes were sampled from New Caledonia. Contrary to what was observed in the north‐western Atlantic, seamount‐scale endemism of south‐west Pacific Chrysogorgia was substantial (23% and 39% for haplotypes with n ≥ 20 and n ≥ 2, respectively). LR sheltered 64% of the New Caledonian haplotype diversity. Assemblages were structured less by habitat type (slope versus seamounts) than by depth. Rarefaction analyses suggested that LR and NR seamounts hold more species than the New Caledonian slope, but additional sampling in the south‐western Pacific (133 colonies) revealed that some seemingly geographically restricted haplotypes from New Caledonia have wide geographical distributions, reaching as far as Taiwan.Main conclusionsThe distribution of Pacific Chrysogorgia is characterized by high levels of rarity, patchiness and diversity, with the levels of seamount‐scale and seamount‐chain‐scale endemism higher than in the Atlantic. We hypothesize that the contrast between the wide geographical distribution of Atlantic Chrysogorgia haplotypes and the higher proportion of endemics in the Pacific is largely explained by differences in depth between the seamounts of these two regions.

Tectonic stresses seaward of an aseismic ridge—Trench collision zone. A remote sensing approach on the Loyalty Islands, SW Pacific

The Loyalty Islands are a series of limestone karstified islands on the Australian Plate that are presently approaching the Vanuatu subduction zone (SW Pacific). They are deformed due to the combined effects of the bulging of the subducting lithosphere and the beginning collision between the Loyalty Ridge and the Vanuatu subduction zone. Therefore, they constitute a series of markers for early phases of ridge arc collision. Lineaments deduced from remote sensing images (aerial photos, SPOT3, SPOT4 and ENVISAT data), termed here as fractures, are analyzed by comparison with planar structures measured during field studies and termed as geological data. Fracture data indicate a stable main N110 direction with a large variance of nearly 15° in rms and two minor directions 45° apart which may constitute shear directions associated to the major one. A scale dependent analysis shows that fractures longer than 2000 m are close to the N110 direction and that their orientation shifts progressively to reach the N125 direction for L < 400 m. Geological data which are generally measured on 1–10 m long structures, reveal a main N135 direction in Lifou, which may be considered as the continuation of the trend of fracture data for decreasing lengths, and are dispersed in Maré. Since the shortest scale structures are being subjected to local disturbances, the longest ones are considered as indicative of the present stress state of the Loyalty Islands. They are modeled as tension cracks resulting from the elastic bulging of the Australian lithosphere before its subduction at the Vanuatu Trench and from a single force corresponding to the beginning collision. It is suggested that the stress field deduced from this model may help constraining general models of ridge arc collision and that the Vanuatu–New Caledonia region could be a valuable natural example to calibrate such models.

Panbiogeography of New Caledonia, south‐west Pacific: basal angiosperms on basement terranes, ultramafic endemics inherited from volcanic island arcs and old taxa endemic to young islands

AbstractAim To investigate areas of endemism in New Caledonia and their relationship with tectonic history.Location New Caledonia, south‐west Pacific.Methods Panbiogeographical analysis.Results Biogeographical patterns within New Caledonia are described and illustrated with reference to eight terranes and ten centres of endemism. The basement terranes make up a centre of endemism for taxa includingAmborella, the basal angiosperm. Three of the terranes that accreted to the basement in the Eocene (high‐pressure metamorphic terrane, ultramafic nappe and Loyalty Ridge) have their own endemics.Main conclusions New Caledonia is not simply a fragment of Gondwana but, like New Zealand and New Guinea, is a complex mosaic of allochthonous terranes. The four New Caledonian basement terranes were all formed from island arc‐derived and arc‐associated material (including ophiolites) which accumulated in the pre‐Pacific Ocean, not in Gondwana. They amalgamated and were accreted to Gondwana (eastern Australia) in the Late Jurassic/Early Cretaceous, but in the Late Cretaceous they separated from Australia with the opening of the Tasman Sea and break‐up of Gondwana. An Eocene collision of the basement terranes with an island arc to the north‐east – possibly the Loyalty Ridge – is of special biogeographical interest in connection with New Caledonia–central Pacific affinities. The Loyalty–Three Kings Ridge has had a separate history from that of the Norfolk Ridge/New Caledonia, although both now run in parallel between Vanuatu and New Zealand. The South Loyalty Basin opened between Grande Terre and the Loyalty Ridge in the Cretaceous and attained a width of 750 km. However, it was almost completely destroyed by subduction in the Eocene which brought the Loyalty Ridge and Grande Terre together again, after 30 Myr of separation. The tectonic history is reflected in the strong biogeographical differences between Grande Terre and the Loyalty Islands. Many Loyalty Islands taxa are widespread in the Pacific but do not occur on Grande Terre, and many Grande Terre/Australian groups are not on the Loyalty Islands. The Loyalty Islands are young (2 Myr old) but they are merely the currently emergent parts of the Loyalty Ridge whose ancestor arcs have a history of volcanism dating back to the Cretaceous. Old taxa endemic to the young Loyalty Ridge islands persist over geological time as a dynamic metapopulation survivingin situon the individually ephemeral islands and atolls found around subduction zones. The current Loyalty Islands, like the Grande Terre terranes, have inherited their biota from previous islands. On Grande Terre, the ultramafic terrane was emplaced on Grande Terre in the Eocene (about the same time as the collision with the island arc). The very diverse endemic flora on the ultramafics may have been inherited by the obducting nappe from prior base‐rich habitat in the region, including the mafic Poya terrane and the limestones typical of arc and intraplate volcanic islands.

A model for fracturation in the Loyalty Islands

The Loyalty Islands are a series of limestone karstified islands that are currently uplifted and deformed on the elastic bulge of the Australian plate before its subduction at the Vanuatu trench (SW Pacific). These islands have been extensively surveyed for geology and hydrogeology, and fracturation maps have been produced which indicate a major direction N 110 ± 35 ° . This fracture orientation is analytically modeled as resulting from the elastic deformation of the Australian lithosphere before its subduction. First, the vertical deflection around a circular subduction zone is determined. Second, a point force is introduced which accounts for the first stages of a collision between the Loyalty ridge and this circular subduction zone. This model yields principal stress orientations and elevations of the islands in fair agreement with real data.

New insights on the tectonics along the New Hebrides subduction zone based on GPS results

At the New Hebrides (NH) subduction zone, ridges born by the subducting Australia plate enter the trench and collide with the overriding margin. Results from GPS surveys conducted on both sides of the trench and new bathymetry maps of the NH archipelago bring new light on the complex tectonics of this area. Convergence vectors present large variations that are not explained by Australia/Pacific (A/P) poles and that define four segments. Vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors. Slow convergence (i.e., 30–40 mm/yr) is found at the central segment facing the D'Entrecasteaux Ridge. The southern segment moves faster than A/P motion predicts (89 to 124 mm/yr). Relatively to a western North Fiji basin (WNFB) reference, the northern and southern segments rotate in opposite directions, consistently with the extension observed in the troughs east of both segments. Both rotations combine in Central Vanuatu into an eastward translation that “bulldozes” the central segment into the WNFB at ∼55 mm/yr. That model suggests that the motion of the central segment, forced by the subduction/collision of the D'Entrecasteaux ridge, influences the motion of the adjoining segments. The New Caledonia archipelago is motionless with respect to the rest of the Australia plate despite the incipient interaction between the Loyalty ridge and the NH margin. Southeast of the interaction area, convergence is partitioned into a ∼50 mm/yr trench‐normal component accommodated at the trench and a ∼90 mm/yr trench‐parallel component, close to the A/P convergence, and presumably accommodated by a transform boundary at the rear of the NH arc.

Discordance de l'Éocène supérieur et événements pré-obduction en Nouvelle-Calédonie

By comparison with the Bougainville seamount and the Loyalty Ridge that emerged in the Quaternary as a consequence of the fore-arc bulge of the Australian plate off the New Hebrides subduction zone, the uncorformity at the base of the Upper Eocene pre-obduction sequence in New Caledonia may be due to the emersion and tectonic distorsion of the northern part of the Norfolk/New Caledonia Ridge when it obliquely crossed the fore-arc bulge zone of the Loyalty island arc at the Middle Eocene/Upper Eocene boundary. The unconformable bioclastic limestone is overlain by an upwardly coarsening clastic sequence that ended with the Uppermost Eocene obduction. These sedimentary and tectonic events were contemporaneous and genetically related to the tectonic unroofing of the high-pressure metamorphic core complex of Mt Panié (northern New Caledonia).

The Loyalty—New Hebrides Arc collision: Effects on the Loyalty Ridge and basin system, Southwest Pacific (first results of the ZoNéCo programme)

I. ' I' ' , ,. , - * ' 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 cdy, 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 ZoNeCo 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- - - _- OU~ 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-SE 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 ZoNeCo 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.

Magnetostratigraphic dating of the uplifted atoll of Maré: Geodynamics of the Loyalty Ridge, SW Pacific

The Loyalty Islands (SW Pacific) are uplifted as they are progressively affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Arc. These geodynamic changes are constrained by magnetostratigraphically dating two sections from Maré Island, where mineral extractions coupled with rock‐magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single‐domain to multidomain magnetite/maghemite. With the help of faunal determinations and radiometric dating, the sequences of polarity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chron (late Pliocene). This new chronostratigraphy refines the timing of two distinctive carbonate units (rhodolith platform/coral reefs) separated by a hardground whose transition is known to coincide approximately with a regional event around the Miocene/Pliocene boundary. The magnetostratigraphic dating indicates that the hardground represents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging from 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolution of Maré Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm/kyr.

High-Mg andesites from the southern termination of the New Hebrides island arc (SW Pacific)

In the southern New Hebrides arc, magmatic and tectonic processes are closely linked. Between 21°S and 22°S, a “normal” broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc, whereas south of 22°S, at the southern termination of the arc, a high-Mg andesite suite appears, with the more mafic endmembers having mineralogical and compositional affinities to high-Ca boninites. During the last 2 Ma, this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the N-S spreading axis of the North Fiji Basin. In this region, which has been dominated for several million years by fast transform motions, the low magma production rates in this section of the arc, and the unusual boninite-related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones, abnormally small depth-to-slab distances beneath volcanoes, and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back-arc spreading axis. The on-going collision between the Loyalty Ridge and the arc may also contribute to these unusual magmatic characteristics, as presently only a very small amount of convergence occurs along the southernmost segment of the trench. In such a complex tectonic setting, with a spreading axis propagating into an intra-oceanic arc, and major transform plate motions, generation of high-Ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the Hunter Ridge, the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis. The high-Mg andesite suite probably results via a combination of fractional crystallization from such high-Ca boninitic parental magmas and subsequent magma mixing processes, operative all along the southern termination of the arc.

Present and recent sedimentation in the Loyalty Basin along the Thio-Lifou profile (New Caledonia, Southwest Pacific)

The Loyalty Basin, northeast of New Caledonia, is about 100 km wide and 2350 m deep in the transect between Thio (New Caledonia) and the island of Lifou. It is bounded to the southwest by New Caledonia and its surrounding barrier reef and to the northeast by the barrier reef of the Loyalty Ridge. Lifou Island, a raised atoll, is an emerged part of the Loyalty Ridge. The sediments cored in the Loyalty Basin during the BIOCAL mission in 1985 are bioturbated and composed of an alternation of turbidites and hemipelagites. The hemipelagites are brown-yellowish muddy oozes to calcareous muds; the turbidites are composed of sandy and silty oozes capped transitionally with muddy oozes. The hemipelagite and the T e division of the Bouma sequence have different characteristics but are not located in the core. However, they can be distinguished by carbonate composition and grain size. All the sediments originate from five sources: (1) New Caledonia (quartz, mud and heavy minerals), (2) the barrier reef (fragments of algae, madreporarian corals, foraminifera and mud of the periplatform, (3) plankton (foraminifera, coccoliths and pteropoda), (4) the bathyal community and (5) pyroclastic fragments. Turbidites, distributed in cores between Thio and Lifou, constitute a low-profile submarine fan fed by the main canyons of the slope opposite the Thio Pass. The fan developed from a rim of slumps and mud flows located near the base of the slope. The fan extends 50 km toward the northeast as far as the tectonic Lévi Ridge. The hemipelagites are predominant beyond this ridge. The modern sedimentation rate is 60 m/Ma on the proximal fan. It is 4–6 times less than the average sedimentation rate calculated for the period since the origin of the basin in the late Eocene.