Chapter 1

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

P. Maurizot1*, B. Robineau1, M. Vendé-Leclerc1 and D. Cluzel2 1Service Géologique de Nouvelle-Calédonie (New Caledonia Geological Survey), BP M2, 98 849 Nouméa, New Caledonia 2University of New Caledonia, ISEA-EA 7484, BP R4, 98 851 Nouméa, New Caledonia PM, 0000-0002-4038-6791; MV-L, 0000-0003-2586-8087; DC, 0000-0002-4362-5516 *Correspondence: [email protected]

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New Caledonia is a French overseas territory in the SW based on published and unpublished information. Except Pacific, located in the southern tropical zone between 19 and for a short paper timed for the 34th International Geological 24° S. The country consists of several islands that are the Congress (Cluzel et al. 2012b), there has been no review of emergent parts of two parallel submarine ridges: the Norfolk the geology of New Caledonia since Paris (1981) – that is, Ridge to the SW and the Loyalty Ridge to the NE (Fig. 1.1). for more than 35 years. As such, it is timely that this Memoir The total area of the country is 18 350 km2. Grande Terre, is published. the largest and main island, is mountainous, c. 400 km long This Memoir comprises a collection of ten self-contained and c. 50 km wide. The highest points are Mont Panié chapters written by 28 researchers from New Caledonia, (1628 m) in the north and Mont Humboldt (1618 m) in the France, New Zealand, Australia and Papua New Guinea. south. Grande Terre is surrounded by a 1500 km long barrier This work is not simply a summation of the factual material reef complex, the second longest in the world, with parts listed acquired since Paris (1981). The purpose of each chapter is as a UNESCO World Heritage site. By contrast, the Loyalty to present: (1) a data synthesis from which the reader can Islands have a low elevation (127 m maximum height on make his or her own interpretations and (2) an interpretation Maré). As well as Grand Terre and the Loyalty Islands, the by the authors incorporating contemporary datasets and New Caledonia archipelago includes the isolated and uninhab- geoscientific concepts. Onshore–offshore connections are ited Matthew and Hunter islands to the east and the sandy emphasized, as are the geological relations with New Zealand, Chesterfield islets to the west. The New Caledonia Exclusive Australia and Papua New Guinea, although this Memoir Economic Zone covers an area of 1 360 000 km2. certainly is not a synthesis of the geodynamics of the SW The Norfolk and Loyalty ridges are part of a complex set of Pacific. submarine continental and/or volcanic arcs and chains, This introductory chapter is a starting point for understand- mostly submerged, that developed between the Australian ing how the current state of geoscientific knowledge of New continent and the Pacific Ocean and usually referred to as Zea- Caledonia was reached. It includes a small-scale geological landia (Mortimer et al. 2017). Grande Terre is a complex map and proposes a general stratigraphic scheme for New mosaic of Phanerozoic geology, including ophiolitic units, Caledonia. It reviews the advances in knowledge of the geol- monotonous volcaniclastic sedimentary rocks, and volcanic ogy of the country since the last synthesis of Paris (1981).It and metamorphic terranes. The oldest rocks are Carbonifer- also summarizes current problems and issues and the analyses ous. The most distinctive and emblematic geological unit of strengths and weaknesses in the state of knowledge of the Grande Terre is the 5400 km2 Peridotite Nappe, which is country’s geology. one of the largest ultramafic terranes on Earth. Weathering Chapter 2 reviews the geodynamics of the SW Pacific and of the peridotites in the Late Cenozoic resulted in the forma- its relations with New Caledonian geology (Collot et al. tion of a thick lateritic regolith with important supergene 2020). The onshore geology of New Caledonia cannot be nickel deposits. The Loyalty Islands are composed largely of separated from its regional and offshore context. This chapter uplifted Neogene to Present carbonate platform deposits. identifies the major geological structures of the SW Pacific The Matthew and Hunter islands are volcanoes located on from the eastern Australian coast to the Tonga–Kermadec the southern tip of the active Vanuatu island arc, whereas Trench. This mostly submerged system of basins, ridges and the Chesterfield islets are atolls built on subsided intra-oceanic seamounts of various ages is interpreted as oceanic and volcanoes of the Lord Howe seamount chain. thinned continental crust, continental ridges, volcanic arcs, intra-plate volcanic chains and oceanic plateaus. Part of this region, including New Caledonia, is a submerged continent referred to as Zealandia (Mortimer et al. 2017). This chapter This Memoir presents and compares tectonic models that attempt to explain the formation and development of this complex area. This Memoir summarizes our current knowledge of New Chapter 3 considers the Pre-Late Cretaceous basement ter- Caledonia geology, geodynamics and mineral resources ranes of the Gondwana active margin (Maurizot et al. 2020a).

From: Maurizot, P. and Mortimer, N. (eds) 2020. New Caledonia: Geology, Geodynamic Evolution and Mineral Resources. Geological Society, London, Memoirs, 51,1–12, https://doi.org/10.1144/M51-2019-33 © 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics 2 P. Maurizot et al.

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, Îles Bélep; 2, Poum–Yandé; 3, Paagoumène; 4, Pam-Ouégoa; 5, Pouébo; 6, Koumac, 7, Paimboas; 8, Hienghène; 9, Ouaco-Voh; 10, Goyéta-Pana; 11, 12, Touho-Poindimié; 13, Pouembout; 14, Paéoua; 15, Ponérihouen; 16, Baie Lebris; 17, Poya-Plaine des Gayacs; 18, Mé Maoya; 19, Houaïlou; 20, Kouaoua; 21, Bourail; 22, Moindou; 23, Canala-La Foa; 24, Thio; 26, Oua-Tom; 27, Boulouparis; 25–28, Humboldt–Port Bouquei; 29, Kouakoué; 30, La Tontouta; 31, Nouméa; 32, Saint Louis; 33, Yaté; 34, Mont Dore; 35, Prony; 36, Îles des Pins; 37, Ouvéa; 38, Lifou; 39, Maré. 1:25 000 geological and suitability to urbanization map: 40, Nouméa.

Grande Terre comprises three accreted terranes that span the high-pressure–low-temperature metamorphic belt. The other Late Paleozoic to Early Cretaceous. They are all submarine, two components of the subduction–obduction assemblage arc-related and developed offshore from the south Gondwana are the Poya Terrane and the Montagnes Blanches Nappe. active margin during marginal basin development. This Chapter 6 describes the onshore geology of the Loyalty chapter describes the different terranes, compares them with Islands and the current state of knowledge of the possible correlative terranes in New Zealand and Australia, Loyalty Ridge (Maurizot et al. 2020d). The three main Loy- and proposes a model for the oceanwards evolution of proto- alty Islands are the emergent part of the Loyalty Ridge. They New Caledonia from the Phanerozoic subduction boundary are composed of thick Miocene to Present carbonate platform that lay along south Gondwana. deposits. Minor basalts are exposed on the island of Maré. Chapter 4 considers the Late Cretaceous to Eocene cover The geology of most of the submarine Loyalty Ridge and the transition from rifting to convergence (Maurizot remains unknown. et al. 2020b). The Late Cretaceous to Paleogene cover of sedi- Chapter 7 considers the post-obduction evolution of New mentary rocks overlies the Late Paleozoic to Early Cretaceous Caledonia (Sevin et al. 2020). The post-obduction formations basement above a pronounced unconformity and structurally in New Caledonia comprise a variety of disparate and areally underlie the allochthonous Eocene nappes. The first period restricted Neogene geological units. They include granitoid of cover bed deposition (Late Cretaceous to Late Paleocene) intrusions, regolith cover, and terrestrial and marine deposits. was controlled by plate divergence and rifting, whereas the Neogene extensional tectonic collapse of the margins of second period (latest Paleocene to Late Eocene) was domi- Grande Terre explains the meagre geological record preserved nated by plate convergence and contraction. This chapter onland and suggests that more extensive and informative describes the two-fold evolution and explains and integrates marine successions may be present offshore. The onland ter- it into the wider framework of the tectonic evolution of the restrial deposits are important, but are difficult to characterize SW Pacific. and date. The regolith on the Peridotite Nappe is a distinctive Chapter 5 discusses the Eocene subduction–obduction com- feature of New Caledonian geology. plex (Maurizot et al. 2020c). The subduction–obduction com- Chapter 8 describes New Caledonia’s geological and bio- plex of Grande Terre is the result of a period of convergence of logical evolution afresh in an up-to-date, multi-disciplinary the northern part of the Norfolk Ridge and the obduction of an scheme (Maurizot and Campbell 2020). An isolated island ophiolitic complex. This is the most extensively studied geo- today, New Caledonia was probably entirely submerged for logical topic for New Caledonia and, as such, the scientific out- significant periods of time following the break-up of Gond- put has been prolific. The two main allochthonous and wana. As a result of its isolation and widespread ultramafic metamorphic units are the obducted Peridotite Nappe and the substrate, the archipelago has inherited and/or developed a Introduction 3 unique and unusual biota with outstanding endemism. In the During this phase many important second-order features of past, incorrect geological information has fed into biogeo- New Caledonia geology were recognized. These included: graphical misinterpretations. the individual Boghen metamorphic units (Routhier 1953), Mineral resources have always been important to the econ- which were, however, mistakenly considered as a pre-Permian omy of New Caledonia. Chapter 9 (Maurizot et al. 2020e) and ‘sialic core’; the establishment of a first lithostratigraphic out- Chapter 10 (Maurizot et al. 2020f) provide an exhaustive line of Permian to Jurassic rocks (Avias 1953); the existence of review of the metallic, non-metallic, gemstone, hydrocarbon tectonic events that predated the Late Cretaceous deposits and thermal resources of the archipelago and its Exclusive (Routhier 1953); the Paleogene age of the northern meta- Economic Zone (EEZ). Both chapters deal with nickel and morphic belt (Arnould 1958); a description of the basaltic cobalt, which are actively mined, as well as resources which unit of the west coast (Routhier 1953); the overthrust nature are not currently exploited. of the Peridotite Nappe (Avias 1967); and the assignment of the Népoui carbonate formations to the Miocene (Routhier 1948). Previous work The second generation of geological mapping took place between 1970 and 1990 and was directed by the Bureau Since the 1850s, our knowledge of the geology of New Cale- de Recherches Géologiques et Minières (BRGM). New donia has progressed in step with technological advances in 1:50 000 scale topographic maps were now available and use- the geosciences and the development of the country’s infra- ful contributions by university researchers were incorporated structure. Grand Terre has restricted road and track access into the maps. Among the many geologists who contributed and, as in all tropical lands, the outcrops are often deeply advances were New Zealand geologists working on the weathered and masked by abundant vegetation. Unveiling Eocene high-pressure–low-temperature metamorphic belt of New Caledonia’s complex geology is therefore not an easy northeastern Grande Terre (e.g. Black 1977; Brothers and task. From an historical perspective, three phases of data Black 1973) and the Paleozoic–Mesozoic Téremba Terrane acquisition can be identified: (1) a pioneering exploration stratigraphy (Campbell 1984; Campbell et al. 1985; Grant- phase, which began shortly after the French settlement in the Mackie 1985). There were also important contributions by late nineteenth century and finished at the end of the Second Coudray (1977) on the stratigraphy of the Eocene to Neogene World War; (2) a more systematic survey phase, extending formations and the evolution of the Plio-Quaternary coastal from the post-war period to the 1980s; and (3) the current reef complex and by Gonord (1977) on the Eocene syn- period, in which the correlation of onshore and offshore data tectonic turbidites. Before the end of the BRGM geological and the integration of New Caledonia into the wider SW mapping programme, the synthesis of Paris (1981) was Pacific geodynamic context has been undertaken – and is published (again in French) and had an accompanying still in progress. 1:200 000 scale geological map. Both the monograph and map are now out of print. Although Paris (1981) was an impor- tant step, it was published when the uptake of global plate Pioneering exploration phase tectonic principles by onshore geological map-makers was at an early stage. The first phase of geological investigation was closely linked From the early 1980s, new concepts in geoscience – to the assessment of the mineral resources of the then-new including plate tectonics, terrane analysis and the recognition French colony (cf. Chapters 9 and 10). It was marked by the of ophiolites as former oceanic lithosphere – gave new per- discovery of abundant and high-grade nickel deposits hosted spectives to the geology of this region. Similarly, there were by the regolith developed on the Peridotite Nappe (Garnier important technical advances in remote sensing and geophys- 1867a, b, c), as well as other resources such as chromium, ical and analytical methods, notably isotope geochemistry coal, base metals and gold (Heurteau 1876; Pelatan 1891; and geochronology. Glasser 1904). The earliest formal scientific publication on The systematic survey period involved programmes coordi- New Caledonia’s geology was published shortly after the nated by the various geoscience institutions in New Caledonia. beginning of colonization by an artillery captain reporting In the early 1980s, the BRGM initiated a systematic Mineral on a geological excursion in the south of Grande Terre (Lom- Resources Survey programme, which had several main bardeau 1860). The first geomorphological analysis of the outputs. Notably, it led to advances in the metallogeny of country was made by Davis (1899). The first major mono- chromite deposits (Cassard et al. 1981), the platinum group graph on the general geology of Grande Terre and the first elements (Augé and Maurizot 1995; Augé et al. 1998), base synthetic geological map at the scale of 1:1 000 000 was pub- metals and gold deposits. During this programme, systematic lished by Piroutet (1917). Most of the major geological fea- sampling and geochemical analyses of stream sediments tures, notably the outlines of the ultramafic massifs, were (about three samples per square kilometre) and alluvial con- described in this document. Throughout this early phase, the centrates (about one sample per square kilometre) were petrography and mineralogy of many metamorphic, volcanic achieved across most of Grande Terre. Increasingly detailed and ultramafic rocks from the New Caledonia archipelago geological mapping of the main units of the Peridotite were described by Lacroix (1897, 1905, 1918, 1940, 1941, Nappe took place in parallel with the BRGM Mineral 1942), although he never visited the islands. Resources Survey. Much data on the mantle fabric, hypogene chromium mineralization and the kinematics of the ultramafic terrane were acquired during this period (Prinzhofer and Systematic survey phase Nicolas 1980; Prinzhofer et al. 1980; Prinzhofer 1981; Sécher 1981; Moutte 1982; Podvin 1983; Nicolas 1989). This second phase of data acquisition led to two generations of The Office de la Recherche Scientifique et Technique geological maps. The first coverage, drawn on an incomplete Outre-Mer (Overseas Scientific and Technic Research Office, topographic map at a scale of 1:100 000, was made during the ORSTOM), now the Institut de Recherche pour le Développe- 1950s. At that time, Grande Terre was subdivided into three ment (Research Institute for Development), focused efforts study areas, from which arose three major monographs (all on the ultramafic regolith and associated nickel deposits written in French): Avias (1953) for the central part; Routhier (Trescases 1975, 1985; Latham 1986) and on the regional (1953) for the NW part; and Arnould (1958) for the NE part. geodynamics of the offshore SW Pacific domain (Equipe 4 P. Maurizot et al.

ORSTOM 1982). There were noteworthy publications on the have boosted scientific research. In 2007 the Centre National rooting of the Peridotite Nappe in the Loyalty Basin (Collot de Recherche et Technologie Nickel et son Environnement et al. 1987), the lithospheric bulge of the Loyalty Islands (National Centre for Research and Technology Nickel and in response to the east-dipping active Vanuatu subduction its Environment, https://www.cnrt.nc) was created. This (Dubois et al. 1974) and the petrology of arc volcanics (Monz- funding agency supports research projects (more than 50 ier et al. 1984). Several offshore boreholes were drilled in the from 2008 to 2019) devoted to the geology, natural environ- SW Pacific by the Deep Sea Drilling Project during this period ment and social sciences related to nickel. A selection of the (Burns et al. 1973). many publications of this period includes Beauvais et al. The Université de Nouvelle-Calédonie (University of New (2007), Cathelineau et al. (2016), Chardon and Chevillotte Caledonia) was founded in 1987. It included a geoscience (2006), Chevillotte et al. (2006), Dublet et al. (2012), Fritsch department from the start, which provided education, training et al. (2016), Quesnel et al. (2017), Robineau et al. (2007) and and research on the onshore geology. The date of founding of Traoré et al. (2005). the university also effectively marks the beginning of tecto- nostratigraphic terrane analysis of New Caledonia backed by the increasing use of geochronology and geochemistry (Aitch- Geological maps ison et al. 1995, 1998; Cluzel et al. 1995, 2001, 2005, 2006; Meffre 1995; Meffre et al. 1996; Cluzel and Meffre 2002). At the time of writing, the geological map coverage of New By the end of the 1990s, the terrane nomenclature of the coun- Caledonia consists of 39 geological maps at 1:50 000 scale try was effectively fixed and this nomenclature persists to the and their explanatory notes, written in French (Fig. 1.1). Cov- present day. erage is of Grande Terre and the Loyalty Islands, but not the islands of Chesterfield, d’Entrecasteaux, Matthew or Hunter. The Grande Terre maps are of variable detail and were pub- Current phase of work lished at different times (Espirat and Millon 1965, 1967, 1971; Millon 1965; Carroué and Espirat 1967; Noesmoen It is impossible to precisely define a year that marks the change 1970a, b, 1971; Carroué 1971a, b, c, 1972a, b, c; Espirat from the second to the current phase of geological investiga- 1971a, b, c; Guillon and Trescases 1972, 1976; Lozes and tion. However, attention to the offshore geology presented Yerle 1976; Lozes and Guérangé 1977; Paris 1977; Trescases a new and valuable perspective that was lacking in earlier and Guillon 1977; Guy et al. 1979; Arène et al. 1980; Faure decades. Several research and exploration programmes of and Paris 1982; Paris and Guy 1982; Faure et al. 1983; Vogt New Caledonia’s EEZ were launched from the 1970s to the and Podvin 1983; Maurizot et al. 1984, 1986, 1989; Guillon 1990s to assess the offshore biological and mineral resources, et al. 1986; Maurizot and Gasc 1986; Tessarolo et al. 1986). notably the hydrocarbon potential. These had acronyms The geological mapping of the three Loyalty Islands such as Zonéco, Extraplac, Noucaplac and Austradec (for a (https://dimenc.gouv.nc/ressources/geologie) was carried compilation, see Sutherland et al. 2012). These programmes out more recently (Maurizot and Lafoy 2003, 2004a, b). All produced essential contributions that led to the integration of these 1:50 000 geological maps were digitized, edge-matched the onshore and offshore geology of New Caledonia and a and merged in the early 2000s. The maps are still being better appreciation of its context in the broader SW Pacific improved and regularly updated, notably by regolith mapping environment (Lafoy et al. 1998, 2005; Van De Beuque on the ultramafic massifs. There is also a 1:25 000 urban geo- 1999; Auzende et al. 2000; Collot 2009). The Pleistocene– logical map of the Nouméa Peninsula (Maurizot and Gasc Holocene lagoon–reef complex of New Caledonia was thor- 1986). oughly investigated from the early 1990s to the early 2000s Figure 1.2 is a small-scale geological map synthesis based (Cabioch et al. 2008a, b). on this set of 1:50 000 scale sheets and based on the following One of the more recent institutional developments was the stratigraphic scheme. This map is provided in geographical founding in 2006 of the Service Géologique de Nouvelle- information system format in the Supplementary material Calédonie (New Caledonia Geological Survey, SGNC). The (available online). It is an updated version of the 1:500 000 SGNC was set up as a government department to create and geological map (Maurizot and Vendé-Leclerc 2009) and maintain the geological databases of New Caledonia and its its short explanatory note (https://dimenc.gouv.nc/ressources/ EEZ. The SGNC has promoted and participated in several la-géologie), which is available in both paper and digital form. important international marine geoscience initiatives. There An A3 size geological map of New Caledonia was also pub- have been useful collaborations with the geological surveys lished in the 2012 Research Institute for Development Atlas of New Zealand (GNS Science) and Australia (Geoscience (Maurizot and Vendé-Leclerc 2012). Australia) and Ifremer in France. Collaboration is ongoing. A regularly updated multi-scale geological map, along with This joint work took the form of several research voyages many other datasets, a catalogue, and metadata, can be viewed with the R/V Tangaroa and N/O l’Atalante oceanographic in online at the portal https://georep.nc. This online geo- ships and culminated in the International Ocean Discovery graphical information system includes a digitized version of Programme Leg 371 of research drilling in North Zealandia’s the Paris (1981) 1:200 000 geological map. The availability Tasman Frontier. At the time of writing, international projects of, and progress on, New Caledonia geological maps are are still currently ongoing and planned to explore the ‘hidden summarized in Table 1.1. continent’ of Zealandia in the areas between New Caledonia, New Zealand and Australia (Collot et al. 2015; Mortimer and Patriat 2016; Sutherland et al. 2016, 2017). A synthesis map and explanatory note of the structural provinces of the SW High-level stratigraphic scheme for New Caledonia Pacific is available at https://dimenc.gouv.nc/ressources/geo logie (Collot et al. 2011). Geological models and hypothesis are transient. However, if Surprisingly, scientific research on New Caledonia’s geological units are well defined in time and space and are nickel–cobalt deposits, which generate the main wealth of arranged in a consistent scheme, that scheme may provide a the country, was not active until the 2000s, or was not public. stable and useful framework for the earth science community However, since then, the growing global demand for nickel, as provide common terms when discussing a specific geological well as depletion of the resource and environmental issues, topic or place. The coherent classification of geological units Fig. 1.2. Geological map of New Caledonia (1:1 300 000 scale). Numerical ages after the International Chronostratigraphic Chart v2018/08 (Cohen et al. 2013; updated). Units with an asterisk are undated. Grande Terre Loyalty Islands Post-obducon Assemblage Grande Terre Sequence Loyalty Sequence Ma 0 Present Reef and Lagoon system Pleistocene -Holocene formaons Reef and Lagoon system Fluvio-lioral formaons uplied reef Plio-Pleistocene atoll stage reef system Muéo Formaon Fluvio-lacustrine Formaon Rhodolith Limestone Upper Népoui Formaon Maré Gp Rawa Limestone Nepu Member Maré Basalts Wharf Member Pindaï Conglomerate Gwa N’Doro Formaon Lower Népoui Formaon Regolith Xuudhen Limestone Member Népoui Group Operculina Green Sand Koum Chapeau Chinois Limestone Member Late Oligocene granitoids Saint Louis 34 Pre-obducon Assemblage Subducon-Obducon Assemblage Syntectonic Sequence Montagnes Blanches HP-LT Metamorphic Belt Poya Terrane Peridote Nappe Nappe Olistostrome Diahot-Panié Pouébo Volcaniclasc sandstone and breccia Mb Metamorphic Terrane Upper Bourail Red sandstone and conglomerate Mb Complex Turbidites Fm Uitoé Limestone Mb Népoui - Koumac Gp Meta-basalt Fine Grained Turbidites Mb (Pic Ougne Zone) Poya Terrane Basalts Lower Bourail Creek Aymes Limestone Mb Bourail Group Bourail Buadio Breccia Turbidites Fm Calciturbidite Mb Koné Facies Calciturbidite Mb Adio Limestone Mb Koumac Limestone Upper Micrite Mb Koumac Limestone Ri Sequence Formaon Lower Micrite and chert Mb Formaon Col de Boghen Gp Black Cherts Formaon Black Cherts Formaon Black Cherts Formaon (Post-Ri) ‘Mamelons Rouges’ Fm ‘Mamelons Rouges’ Fm ‘Mamelons Rouges’ Fm ‘Formaon à charbon’ Dumbéa Gp Pic Jacob Volcanic Rocks Khian Gp Sommet Diahot Volcanic Rocks (Syn-Ri) Congo Conglomerate 105 Basement Assemblage Mesozoic Cenozoic Téremba Terrane Koh-Central Terrane Boghen Terrane Baie de Saint Vincent Group Central Range Volcaniclasc Rocks Gondé-Nétéa sub-terranes Hierarchical level Ilot Testard Formaon Pouembout Unit Moziman Member Ponérihouen-Goipin Unit Testard Member Tani Formaon Thio Unit Assemblage Bouraké Formaon Sarraméa Unit Leprédour Shell Beds La Foa Unit Terrane, Nappe, or Sequence Ouamoui Formaon Nouméa-Dumbéa Unit Ouaraï Formaon Group Baie de Téremba Group Koh Ophiolite Formaon Moindou Formaon Cantaloupai Unit Member Mara Formaon Tarouimba-Sphynx Unit Koh Unit Major unconformity Pocquereux-Nassirah-Koua Unit

Paleozoic 300

Fig. 1.3. Formal nomenclature of geological units of New Caledonia. Excludes the Hunter–Matthew and Chesterfield island groups. Numerical ages after the International Chronostratigraphic Chart v2018/08 (Cohen et al. 2013; updated). Introduction 5 using consistently applied biostratigraphic, lithostratigraphic, recognized in the 1950s as a geological horizon of regional chronostratigraphic or other criteria is not an easy task in a first-order importance, equivalent with that of New Zealand country where the geology is highly complex and where (Avias 1953; Routhier 1953; Laird 1981). Ubiquitous base- most stratigraphic names, to date, are informal. There is no ment rift structures have been traced throughout Zealandia specific committee to approve New Caledonian geological in seismic profiles (Bache et al. 2012; Rouillard et al. 2014, map units or stratotypes. By convention, the names of geolog- 2015; Mortimer et al. 2017). Although the syn-tectonic nature ical units are generated in published papers and a test of their of most of the Eocene deposits was recognized in the 1980s usefulness is simply how well, or if, they are cited. (Gonord 1977), a coherent interpretation for the whole Eocene Here, we formalize a basic stratigraphic hierarchy of the flysch was still missing. The Bourail Group is now interpreted New Caledonia geological units that are used in the different as having been deposited in a typical foreland basin, synchro- chapters of this Memoir, in the legend of the geological nous with the Eocene convergence and subduction (Cluzel map (Fig. 1.2) and in the attribute table of the digital geo- et al. 1998; Maurizot 2011, 2013, 2014; Maurizot and Cluzel logical map provided in the Supplementary material for this 2014). The Late Eocene Népoui–Koumac Flysch, formerly chapter. The basis of the hierarchy is tectonostratigraphic regarded as post-dating the obduction of the Peridotite because such schemes have remained stable and have been Nappe (Paris et al. 1979; Paris 1981), is now interpreted applied usefully in New Caledonia since the end of the as a pre-obduction syn-tectonic infill of piggy-back basins 1990s (Aitchison and Meffre 1992; Cluzel et al. 1994), even transported on the Poya Terrane during its forearc accretion if some issues still remain. A summary of the units is presented (Cluzel 1998). in Figure 1.3. The scheme is hierarchically organized into Three separate allochthonous units in the subduction– four high-level ‘Assemblages’ (which match the individual obduction complex (Chapter 5) are now clearly defined in chapters of this Memoir) then, in increasing detail, ‘Terrane, their successive stacking order – namely, the Montagnes Nappe or Sequence’, ‘Group’, ‘Formation’ and ‘Member’. Blanches Nappe (Maurizot 2011), the Poya Terrane (Cluzel The lower three ranks are only populated by coherent sedi- et al. 2001), including the Koné facies (Cluzel et al. 2017), mentary successions. The general terms ‘unit’ and ‘com- and the Peridotite Nappe (Avias 1967). Numerous radiomet- plex’ are not included in this new formal hierarchy, but are ric ages constrain (1) the beginning of convergence at the still useful synonyms. So are the terms ‘basement’ and Paleocene–Eocene boundary (Cluzel et al. 2006, 2012a, b, ‘cover’, which are in common use in New Zealand (Mortimer 2016; Maurizot 2013), (2) subduction spanning the whole et al. 2014); these are also occasionally used in New Caledonia Eocene (Ghent et al. 1994; Rawling 1998; Cluzel et al. (instead of Basement Assemblage and Pre-Obduction 2001; Spandler et al. 2005; Baldwin et al. 2007; Pirard Assemblage). and Spandler 2017) and (3) the final obduction and exhuma- The four major tectostratigraphic assemblages of New Cal- tion at the end of the Eocene (Ghent et al. 1994; Rawling edonia are: (1) the Basement Assemblage (Chapter 3), which 1998; Baldwin et al. 2007). The Pouébo Terrrane has been consists of three amalgamated terranes lying beneath the Late recognized in the high-pressure–low-temperature metamor- Cretaceous unconformity; (2) the Pre-Obduction Assemblage phic belt as the metamorphosed equivalent of the Poya Ter- (Chapter 4), Late Cretaceous to Paleogene sedimentary rane (Cluzel et al. 2001). A number of radiometric ages and successions that unconformably overlie the basement and geothermobarometry on various mineral assemblages has structurally underlie the allochthonous terranes; (3) The Sub- constrained the P–T–t metamorphic path (Black 1974; duction–Obduction Assemblage (Chapter 5), consisting of Clarke et al. 1997; Carson et al. 1999; Fitzherbert 2002; mostly allochthonous units emplaced during the Eocene Fitzherbert et al. 2003, 2004; Agard and Vitale-Brovarone period of convergence, which involved subduction followed 2013; Vitale Brovarone and Agard 2013, Taetz et al. by obduction; and (4) the Post-Obduction Assemblage, 2016). The gabbronorite cumulates of the Massif du Sud, which consists of a variety of Late Oligocene to Holocene geo- which were originally thought to be the products of a mid- logical units on both Grande Terre and the Loyalty Islands oceanic spreading ridge (Prinzhofer et al. 1980; Prinzhofer (Chapters 6 and 7). and Allegre 1983), are now reassessed as a supra-subduction forearc crust (Marchesi et al. 2009; Pirard et al. 2013; Cluzel et al. 2016; Secchiari et al. 2018). Thirty-nine years of advances Many advances have also been made in the Post-Obduction Assemblage (Chapters 6 and 7). The Loyalty Islands were A comparison of the last synthesis of New Caledonia geology almost ignored in 1980s publications. Geological mapping by Paris (1981) with the present Memoir highlights several of the islands was carried out in the 2000s (Maurizot and major advances in knowledge that have been made during Lafoy 2003, 2004a, b, 2006) and the Loyalty Ridge is now the last 39 years. integrated in the geology of New Caledonia (Chapter 6). Three terranes are now defined in the basement (Chapter The Koum and Saint Louis granitoids, which intrude the Peri- 3) and constitute a stable and much-used scheme. The Bog- dotite Nappe and its substrate, have been dated at 27–24 Ma hen Terrane was formerly termed the ‘pre-Permian sialic (Cluzel et al. 2005). Systematic mapping of the regolith is core’ of Grande Terre on the basis of its metamorphic still in progress, while indirect dating of the main ferricrete nature and a putative unconformity below unmetamor- of the Goro and Tiébaghi ultramafic massifs by palaeomag- phosed Permian rocks (Avias and Gonord 1973). The proto- netic methods has been interpreted at c. 25 Ma (Sevin et al. lith of the unit was proven to be Mesozoic by U–Pb dating 2012). The discovery by drilling of a new c. 100 m thick reefal on zircons (Cluzel and Meffre 2002; Cluzel et al. 2010) and limestone underlying the so-called ‘basal conglomerate’ of the is now interpreted as a Mesozoic subduction complex. The Miocene Népoui Formation led to the reinterpretation of the two other terranes, the Téremba and Koh-Central terranes, latter as a consequence of short-lived uplift due to post- are interpreted to be proximal and distal forearc basin obduction slab break-off (Sevin et al. 2014). The siliciclastic deposits, respectively. They are related to an intra-oceanic and carbonate-dominated Népoui Group is now dated as volcanic arc located to the west, offshore from the eastern Aquitanian–Burdigalian by its foram content and strontium Gondwana margin, and probably separated from it by a isotope stratigraphy (Maurizot et al. 2016). The older exposed marginal basin. sequence of the carbonate platform of the Loyalty Islands has In what is now the Pre-Obduction Assemblage (Chapter 4), been dated in Maré as Mid-Miocene (Maurizot and Lafoy the Late Cretaceous basal unconformity was already 2003). 6 P. Maurizot et al.

Table 1.1. Information on geological maps of New Caledonia

Paper* Online digital†

Scale Date of publication Map Explanatory note Map Explanatory note

1:50 000 1967–86 Yes Yes Yes No 1:200 000‡ 1981 Out of print Out of print Yes No 1:500 000 (A1)§ 2009 Yes Yes Yes Yes 1:1 300 000 (A3)¶ 2019 Yes Yes Yes Yes

*Paper versions are available at Direction de l’Industrie, des Mines, et de l’Energie de Nouvelle-Calédonie, Geological Survey of New Caledonia, 1 ter rue Unger, Vallée du Tir, B.P. M2, 98849 Nouméa CEDEX, New Caledonia. †Online digital version downloadable at the portal https://georep.nc ‡Paris (1981). §Maurizot and Vendé-Leclerc (2009). ¶This Memoir.

Remaining problems • The crustal sequence of the Peridotite Nappe is still undated. • Based on its morphology and geophysical anomalies, the Although many advances have been made in the last 39 years, deep part of the Loyalty Ridge near New Caledonia is there remain many shortcomings in our knowledge of New regarded as an Eocene volcanic arc (e.g. Cluzel et al. Caledonia geology. 2001). However, no sample has ever been obtained and the age and nature of the Loyalty Ridge basement remains • Refining the stratigraphy of all the Paleozoic–Early one of the main unknowns of SW Pacific tectonics. Deep Mesozoic basement terranes of Grande Terre is hampered dredging or drilling of this part of the ridge could test the by the rarity of fossils and detrital zircons. Both are Eocene arc hypothesis. scarce because of the dominantly volcaniclastic prove- • Reconstituting the spatio-temporal development of the nance. The Koh-Central Terrane in particular lacks good Post-Obduction Assemblage evolution of Grande Terre lithostratigraphic subdivision and the protolith age of the is complicated by the fact that many of its formations areally extensive metamorphic Boghen Terrane relies only derive from the weathering of the Peridotite Nappe, on a few samples (Cluzel and Meffre 2002; Cluzel et al. which defy accurate and precise dating by any method 2010). Knowledge of the Basement Assemblage could be tried so far. Direct dating by palynomorphs could be a sol- greatly improved by more detailed cross-sections, geologi- ution in the Fluvio-lacustrine Formation and/or in the cal mapping, systematic sampling and detrital zircon Népoui Group. analysis. • The development of the Pleistocene–Holocene lagoon–reef • The P–T–t path of the Boghen Terrane is still poorly complex is a potentially rich field of research that has been defined. largely abandoned since the premature passing away of its • Although progress has been made in establishing a general leader G. Cabioch (e.g. Cabioch et al. 2008a, b). This stratigraphy of the Eocene flysch (Maurizot 2011, 2013, research theme should be taken forward as a result of its 2014; Maurizot and Cluzel 2014), dating has been based importance regarding climate change baselines. only on benthic and planktonic forams. This needs to be refined in key sections by integrated nannofossil, radiolar- From a more general standpoint, the geoscientific data infra- ian, foram and magnetostratigraphy work (e.g. Dallanave structure of New Caledonia still have some weaknesses. et al. 2018). The oldest geological maps (1:50 000) date back to the • A better knowledge of the Subduction–Obduction Assem- 1970s and are largely obsolete. A new generation of geo- blage of New Caledonia is certainly desirable in terms of logical maps is desirable. Systematic geophysical surveys the scientific, economic and environmental benefits. Initia- (e.g. airborne electromagnetic, gravimetric and radiometric tives such as the New Caledonia Drilling Project to promote coverage) are seriously lacking in a country that is dependent a proposal (NCDP 2019) to international drilling organiza- on its mineral resources and has many difficult to access tions (e.g. the International Ocean Discovery Program or remote areas. The deep structure and nature of the crust and the European Consortium for Ocean Research Drilling) upper mantle underlying the New Caledonia area remains to should be encouraged. be investigated using seismic methods, including refraction, • The subduction–obduction complex of New Caledonia reflection and tomography. Such studies, combined with is one of the major occurrences of high-pressure–low- onshore and offshore investigations, will lead to a better temperature metamorphic rocks worldwide, but contradic- understanding of the geology of New Caledonia in the context tory tectonic models exist (Cluzel et al. 2001; Rawling of the continent of Zealandia. and Lister 2002; Lagabrielle et al. 2013; Gautier et al. 2016). The high-pressure–low-temperature metamorphic Acknowledgements The original idea of this Memoir was the ini- belt is under-investigated in the remote area of Mont tiative of the Geological Survey of New Caledonia in partnership with Panié. As explained in this Chapter 5, the nature of the the Earth Science department of the University of New Caledonia. boundary between the Diahot Terrane and the Montagne However, it could not have been realized without the long-term Blanche Nappe is poorly characterized. • co-operation and collaboration of many geologists from the wider The study of Peridotite Nappe petrofabrics performed by SW Pacific region, mainly from New Zealand and Australia. These lattice preferred orientation analysis with the universal joint efforts are both long-standing and valued. This Memoir is ded- stage dates back to the 1980s (Prinzhofer et al. 1980). A icated to all those involved. We gratefully acknowledge the Govern- more accurate and areally extensive appraisal of high- ment of New Caledonia for their support of this work, and its temperature mantle deformation could be obtained by sys- appreciation that the importance of natural resources to the economy tematic back-scattered electron diffraction or anisotropy of the country was a major motivation of the project. We acknow- of magnetic susceptibility methods. ledge the role played by editor Nick Mortimer in reviewing all the Introduction 7 submissions, co-ordinating referee comments and providing guidance southwest Pacific. Geological Society of America, Special on revisions. Papers, 419, 117–134. Beauvais, A., Parisot, J.C. and Savin, C. 2007. Ultramafic rock weathering and slope erosion processes in a South West Pacific Funding This research was funded by the Geological Survey of tropical environment. Geomorphology, 83,1–13, https://doi. 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