The geology of New Caledonia includes all major rock types (igneous, sedimentary, and metamorphic), which here range in age from ~290 million years old (Ma) to recent. Their formation is driven by alternate plate collisions and rifting. The mantle-derived Eocene Peridotite Nappe is the most significant and widespread unit (labelled as "Ophiolites" and coloured in bright green in Fig. 1). The igneous unit consists of ore-rich ultramafic rocks thrust onto the main island. Mining of valuable metals from this unit has been an economical pillar of New Caledonia for more than a century.
New Caledonia is located on the Indo-Australian Plate and the largely submerged continent of Zealandia. After New Zealand, it is the second-largest subaerial landmass, and the northernmost part of this continent. As shown on the map, the landmass is elongated in a NW-SE orientation, which is similar to the distribution patterns of most of the geological units. Thrusting, exhumation and folding of some of the units have contributed to the rugged topography of the elongated main island, with Mont Panié and Mount Humboldt being the highest points (over 1,600 m).
Overview of the regional geology
The geology of the Collectivity (autonomous region) is divided into two relatively separated parts: the larger Grande Terre in the West and the Loyalty Islands in the East. The smaller islands are younger and are believed to have emerged as a volcanic island arc in the Eocene, but are currently covered by coral reefs and carbonate rocks. For the main island of Grande Terre, there are diverse geological units. Before illustrating each of them and their implications, here is the table to summarize the units in a reversed chronological order, as the older rocks usually occupy deeper locations:
+Brief summary of the rock units on Grande TerreGeological unitRock typeFormation process/ environmentTime of emplacementRef.Nepoui series,
Gwa N'doro FormationClastic sedimentaryLagoonal to fluvial deposition Since Neogene GranitoidsFelsic to intermediate intrusive igneousIntrusive volcanism to form plutons and dykesEocene to OligocenePouebo Terrane,
Diahot TerraneFoliated metamorphicSubduction zone metamorphismEocenePoya Terrane, Peridotite NappeMafic to ultramafic intrusive and extrusive igneousVolcanism in the ocean, then obducted onto the island during collision between tectonic platesMontagnes Blanches Nappe, Bourail Flysch,
Koné Terrane,
Koumac Terrane,
Formation à charbonSedimentaryTerrestrial towards marine, and back to lacustrine sedimentationLate Cretaceous to EoceneBoghen TerraneFoliated metamorphicSubduction zone metamorphismJurassic to CretaceousTeremba Terrane, Central TerraneMostly sedimentaryMainly marine sedimentationPermian to Early JurassicKoh OphioliteMafic to ultramafic intrusive and extrusive igneousVolcanism beneath the ocean, then transported and scrapped off during collision between tectonic platesEarly Permian
In the Permian, the Island was initially located in the southeastern margin of the huge supercontinent of Pangea, and then in the united southern continent of Gondwana. Rocks in Koh-Central, Teremba and Boghen Terranes on the island suggest the units were emplaced and compressed between a volcanic arc and the associated subduction zone (known as a forearc basin along a collisional plate boundary).
From the Upper Cretaceous to Eocene, proto-New Caledonia transformed into an extensional tectonic setting. It is evidenced by a series of marine sedimentary rocks, and can be regarded as a miniature of rifting within Zealandia, and even the partition of Zealandia from Gondwana. Northern Zealandia separated into the Lord Howe Rise in the west and the narrow strip of Norfolk Ridge in the east, of which New Caledonia is the northernmost tip. The New Caledonia Basin hence formed between the two blocks, but is still believed to be continental, though highly extended. Just like the Lord Howe Rise, the Norfolk Ridge extends towards New Zealand in the South via its namesake, the Australian Norfolk Island.
However, these were followed by a new collisional event. The abyssal lithosphere east of Grande Terre subducted under an oceanic plate in the east in the South Loyalty Basin. Hence, the associated volcanism during the Eocene is believed to have created the sinuous volcanic island arc of Loyalty Islands/Ridge as the smaller part of the territory. The epoch also oversaw a series of displacement of the oceanic lithosphere onto the east of the continental Norfolk Ridge. One of the products of these dynamic events is the famous and economically significant Peridotite Nappe. This is followed by intrusions of granitoids into the country rocks on Grande Terre.
Since then, the Island re-emerged from the Pacific due to some minor movements. Processes like exhumation, erosion and shallow-sea sedimentation are recorded by series of fossil-bearing lagoonal and fluvial sedimentary rocks. Currently, Grande Terre is surrounded by a barrier reef with a length of 1600 km, thus forming the largest lagoon globally.
Nowadays, the two ridges are generally parallel to each other and connect with the d'Entrecasteaux Ridge in the north. Meanwhile, the North Loyalty Basin is sandwiched by the eastern side of the Loyalty Ridge, as well as the New Hebrides Trench, where the Australian Plate subducts under the micro New Hebrides Plate. Such plate motion makes the Loyalty often suffering from earthquakes.
Notably, the French territory also administers two tiny islands on the New Hebrides Plate − Matthew Island and Hunter Island, though disputed. Therefore, their geology is rather related to the archipelago of Vanuatu, which also claims the active volcanic islands with andesitic composition.
Petrology Grande Terre Koh-Central and Teremba Terranes In the fore-arc basin tectonic setting, the suite of Koh Ophiolite is known to be the earliest terrane to be formed on the island in the early Permian. It is unusual that the unit actually was created by two tholeiitic magmatic episodes, and is divided by a layer of boninite. The older and lower layers are therefore intruded by the younger evolved tholeiite as dykes and sills. The rock suite reveals the once oceanic lithosphere was scraped off and retained in the basin during the start of subduction (i.e., a supra-subduction setting). By the U/Pb SHRIMP analysis, its own formation can be traced back to as early as the Carboniferous.
The Koh Ophiolite is overlain by a series of marine sedimentary rocks starting from black shale. This is followed by greywacke of the Central Terrane that bears some pyroclastic characteristics, as well as siltstone and chert.
Meanwhile, the clastic and volcanic rocks of Teremba Terrane formed between the Permian and Early Jurassic. The clastic rocks are mainly greywacke, which could share the same source as those in the Koh-Central Terrane; the composition of the volcanics varies from rhyolitic to andesitic. They are slightly deformed into very-low-grade metamorphic rocks in zeolite facies.
Through basin analysis, it is believed that the two terranes are a pair of "twins": they formed simultaneously along a forearc, with the former one being formed offshore and the latter one in an onshore setting.
Boghen Terrane The Boghen Terrane is another group of rocks that formed in that period (Jurassic-Cretaceous). It is basically composed of schists, with the grade ascending from greenschist to blueschist metamorphic facies towards the west. With the protoliths being rocks like tholeiitic basalt and chert, these hint at an oceanic origin of the rock, but materials from land or a volcanic arc are also mixed into the rocks.
Their regoliths contain both oceanic and terrigenous material, including volcanic arc detritus, deposited as an accretionary complex. Likely due to a westward plate subduction, the complex further metamorphosed under high pressure but low temperature in the Late Jurassic to form blueschist.
Nowadays, alongside Koh-Central and Teremba Terranes, the three terranes are found to be lying on the island in a sub-parallel and elongated pattern.
Sedimentary rocks from Upper Cretaceous to Eocene Sediments settled during this period tends to become finer-grained over time, indicating the sedimentary environment was subsiding (i.e., a retrogradational parasequence). Sandstone, coal-bearing siltstone and tuff as the record of minor volcanism, are among the first rocks formed in the setting. This group of rocks is called Formation à charbon, and shares an angular unconformity with the underlying older terranes. Zircon analysis of the formation reveals that the sources of the clasts are the three pre-Late Cretaceous Terranes. The presence of angiosperm fossils and coal proved that at least some parts of the island had already emerged from the sea by the Late Cretaceous. However, a shift of fossil records to marine bivalves and gastropods indicate the subsidence of the landmass turned the sedimentary environment into an estuarine one.
Formation à charbon is overlaid by the Paleocene Koumac Terrane, which primarily consists of sedimentary black chert and also some argillite. Collectively known as phtanites, these cherts mark the complete submergence of New Caledonia. On the other hand, the gradual replacement of siliceous content by calcitic micrite signals a northward drift of the continent, where a warmer climate favours deposition of calcite.
In Eocene, on one hand there is an abrupt change of white micrite into pink marl, followed by the deposition of limestone breccia and conglomerates in the North of the Island. Such turbidites and coarse-grained rocks collectively form the Bourail Flysch. These all indicate an uplift of the crust, by a shift of the marine sedimentary records into a lacustrine environment, while the presence of the shallow-water limestone with the same age represents a constant marine environment. Their erosion is believed to be the result of the emersion because of fore-arc bulge, which is linked to a new subduction zone. The limestone is hence overlaid by a 3-to-5-km thick turbidite.
"Montagnes Blanches" Nappe and Koné Terrane The Nappe is a sedimentary rock sequence from argillite, chert, micrite and calci-turbidite, and was formed between the Late Cretaceous and Eocene. Pierre Maurizot, the namer of the Nappe, modelled the younger flysch thrust over the native flysch southward to form this displaced unit (allochthon). It is therefore systematically intercalated between the sedimentary rocks mentioned above, and the overlying obducted ophiolite units across the Island.
The Montagnes Blanches Nappe is also overlain by Koné Terre, which is a pelagic unit of siltstone, argillite and chert. Its presence or exposure is limited to Koné and Koumac, and has been overthrust by the Poya Terrane.
Poya Terrane and the Peridotite Nappe The two units were oceanic lithosphere that thrust onto a continental landmass (known as Tethyan-style ophiolites). The underlying Poya Terrane on the western coast consists of pillow and massive undepleted mid-oceanic ridge basalts (MORB) with uplifts of m- to km- scale of thickness. K-Ar dating suggest their age to be 61-38 Ma.