The Lansdowne Bank is a partly drowned, isolated carbonate platform of around 4000 km2 located 300 km west of New Caledonia, in the SW Pacific Ocean, in water depths of 20 to 100 m. New multibeam bathymetric data, high resolution seismic reflection profiles and sediment gravity cores have been acquired on the bank top and adjacent slopes. This dataset reveals an almost continuous 4 km wide outer reef rim located in ca. 50 m water depth, surrounding a gently deepening inner platform, reaching up to 100 m water depth. The bank is bordered by very steep slopes showing numerous erosional morphologies such as canyons, channels and gullies. Along with these bypass features, spectacular bank margin collapses and slope failures are evidenced by up to 20 kmwide bank edge and intraslope failure scars, respectively, resulting in a typical “scalloped” geometry of the bank margin. These failure scars can lead to a complete collapse of the outer reef rim and impact subsequent reef development. Bank margin collapses are evidenced by hectometer to kilometer-scale blocks and debris shed on the slope, likely emplaced by rock fall/avalanching processes originating from the brittle failure of early cemented bank edge and upper slope sediments. In turn, failures triggered on the un-cemented mud-prone middle to lower slopes likely generate more cohesive, submarine debris flows that could be at the origin of erosive morphologies within the debris fields. Estimated individual failure volumes can reach up to 3 km3. Quaternary sea-level lowstands, that would have led to platform exposure, fracturing and karstification, and the development of an erosional sea cliff, as well as subsequent rising sea-level are believed to play a significant role in mass wasting event emplacement, yet “bottom up” submarine processes such as the upslope propagation of bypass morphologies by retrogressive headward erosion cannot be ruled out. In terms of geomorphic and stratigraphic constraints, the documented bank margin collapses affect a terrace located in 70 m water depth around the bank, which, depending on its age and origin, could provide a minimum age for collapse events. Finally, considering the shallow water depth of failure headscarps, the volumes of material involved in the slides as well as their vicinity to the nearby main island of New Caledonia, numerical simulations of the tsunamigenic potential of submarine slides have been performed. They showed that these slides would have been able to produce a meter-scale wave that would reach the northern coast of the island in less than an hour.

SEDLAB

Convergence and subduction started in the Late Palaeocene, to the east of New Caledonia, in the South Loyalty Basin/Loyalty Basin, leading to the formation of the Subduction-Obduction Complex of Grande Terre. Convergence during the Eocene consumed the oceanic South Loyalty Basin, and the north easternmost margin of Zealandia (the Norfolk Ridge). The attempted subduction of the Norfolk Ridge led eventually to the end-Eocene obduction. Intra-oceanic subduction started in the South Loyalty Basin, as indicated by high-temperature amphibolite (56 Ma), boninite- and adakite-series dykes (55-50 Ma), and changes in sedimentation regime (55 Ma). The South Loyalty Basin and its margin were dragged to a maximum depth of 70 km, forming the high pressure – low temperature Pouébo Terrane and the Diahot-Panié Metamorphic Complex, before being exhumed at 38-34 Ma. The obduction complex was formed by the stacking from NE to SW of several allochthonous units over autochthonous Zealandia, which include the Montagnes Blanches Nappe (Norfolk Ridge crust), the Poya Terrane (crust of the South Loyalty Basin), and the Peridotite Nappe (mantle lithosphere of the Loyalty Basin). A model of continental subduction accepted by most authors is proposed and discussed. Offshore continuations and comparable units in Papua-New Guinea and New Zealand are presented.

KANACONO

One of the largest ophiolitic peridotite masses in the world covers a quarter of the island of Grande Terre, New Caledonia. The Peridotite Nappe was obducted during the Eocene, is weakly deformed, and corresponds to the highest of a structurally simple pile of thrust nappes. We present new marine seismic data that allow us to track the offshore continuation of the Peridotite Nappe along strike for a distance of more than 500 km south of New Caledonia and to image its preobduction, synobduction, and postobduction sedimentary records. Offshore, the Peridotite Nappe underlies a ~150 km wide and 2 km deep basin. Flat-topped horsts of peridotite are clearly bounded by major normal faults; in contrast, faults are obscure onland. To the east, the Peridotite Nappe roots along the eastern margin of the Félicité Ridge (new name), a ~300 × 25 km dome-shaped ridge, which we interpret as being the southern extension of the high-pressure/ low-temperature metamorphic core complex observed in New Caledonia. Two alternative tectonic models address the relative timing and relationships between Peridotite Nappe emplacement, uplift of a metamorphic core complex, and extensional tectonics. These models provide new ideas for the understanding the formation of the eastern margin of the Zealandia continent. Our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins.

KANACONO

La majorité des informations disponibles sur les minéralisations marines profondes au sein de la ZEE de Nouvelle-Calédonie provient des travaux entrepris par l’ORSTOM (IRD) entre 1970 et 1980. L’objectif de ce projet, mené conjointement entre l’Institut Français pour l'Exploitation de la Mer (IFREMER), le Service Géologique de Nouvelle-Calédonie (SGNC) et l'Agence de Développement Économique de Nouvelle- Calédonie (ADECAL) est de mettre à jour les connaissances sur ces dépôts en utilisant des méthodes analytiques modernes. L'inventaire de tous les échantillons de dragages existants en Nouvelle-Calédonie depuis 1970 a permis de mettre en évidence 104 échantillons présentant des encroûtements au sein et à proximité de la ZEE. Ces échantillons sont principalement localisés au Sud-Est de la Nouvelle-Calédonie, mais également au Nord dans la continuité de la Ride des Loyauté et à l'Ouest sur les monts sous-marins de la Ride de Lord Howe. Une caractérisation chimique et minéralogique de ces minéralisations (XRD/XRF, MEB et ICP-MS) a permis de confirmer qu'elles sont pour la plupart des encroûtements ferromanganésifères hydrogénés, et a également mis en évidence l’originalité de certaines minéralisations associées à une activité hydrothermale et/ou diagénétique, dont la description et l'analyse sont une première en Nouvelle-Calédonie. Les encroûtements ferromanganésifères hydrogénés sont enrichis en métaux de base, traces et terres rares (17,4% Mn, 20,8% Fe, 0,42% Co, 0,31% Ni et 1 307 ppm ΣREE), alors que les minéralisations hydrothermales et/ou diagénétiques sont appauvries en métaux traces et terre rares, mais présentent pour quelques-unes des concentrations fortes en Mn (plus de 50%) et Ni (jusqu'à 0,7%). La comparaison de ces résultats avec ceux issus d'autres océans du globe montre que la Nouvelle-Calédonie se situe dans la moyenne des concentrations en métaux. Les fortes concentrations en Co, Ni et Cu de certains échantillons, ainsi que la physiographie particulière de la zone Sud des rides de Loyauté et de Norfolk, en font une des zones les plus propice pour des investigations supplémentaires. Une première approche, couplant les données de prélèvements, de réflectivité du fond et de bathymétrie haute-résolution, a permis de tester une méthode de prédectivité de présence d'encroûtement sur les sites les plus intéressants et potentiellement minéralisés. La robustesse de ces cartes a été testée en comparant les zones potentielles avec des données vidéo ROV de fonds marins acquises lors de la campagne KANADEEP II. Cette étude a permis de répertorier pour la première fois les minéralisations Fe-Mn de la ZEE de Nouvelle- Calédonie, et d'en caractériser leurs grandes diversités, compositions et morphologies. L'effort d'échantillonnage reste malgré tout insuffisant pour poursuivre l'évaluation du potentiel économique. La poursuite de cette évaluation nécessite la réalisation de campagnes d'exploration dédiées avec des méthodes d'échantillonnage et d'acquisition de données propres à l'étude des minéralisations profondes.

BATHUS 3, EBISCO, KANADEEP, KANADEEP 2

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.

BATHUS 3, EBISCO, KANADEEP, KANADEEP 2

Neogene and Quaternary shallow-water carbonate records surrounding New Caledonia main island, Grande Terre, provide a good example for understanding the stratigraphic architecture of tropical mixed carbonatesiliciclastic systems. Due to a southeastern tilt of the eastern margin, the eastern shelf of Grande Terre has been better preserved from erosion than the western part, favouring the development and preservation of shallow-water carbonates. Based on the integration of bathymetric and seismic data, along with paleoenvir­ onmental and biostratigraphic constraints derived from dredged carbonate rocks, a comprehensive geomor­ phological and architectural characterization of the offshore eastern margin of Grande Terre has been made. During the Mio-Pliocene, a wide, up to 750 m-thick carbonate build-up developed and extended over at least 350 km from north to south. This Mio-Pliocene build-up, currently lying at 300 to 600 m water depths, is overlain by a Pleistocene-Holocene barrier reef-lagoon complex and associated slope deposits. The switch from aggrading Neogene carbonate banks to backstepping Quaternary platforms likely reflects an increase in accommodation due to a high subsidence rate or to relative sea-level rise, and/or results from a switch in carbonate producers associated with global environmental changes. The internal architecture of the Quaternary barrier reef-lagoon complex is highlighted, especially the development of lowstand siliciclastic prisms alternating with trans­ gressive shallow-water carbonate sequences. This pattern agrees with the reciprocal sedimentation model typically invoked for mixed sedimentary systems. This stratigraphic pattern is well developed in front of the Cap Bayes inlet in the north of our study area, yet it is not observed southward along the eastern margin. This dif­ ference suggests that other factors than relative sea-level variations directed the architecture of the margin, such as low terrigenous inputs, lagoon paleo-drainage networks or sediment by-pass towards deep basins.

KANACONO

BIOCAL, EBISCO, EXBODI, KANACONO, KANADEEP, KANADEEP 2, NORFOLK 2, SEDLAB, SPANBIOS