Geology of New Caledonia
Currently, New Caledonia is located on the
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.[2] 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.[2] 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:
Geological unit | Rock type | Formation process/ environment | Time of emplacement | Ref. |
---|---|---|---|---|
Nepoui series,
Gwa N'doro Formation |
Clastic sedimentary | Lagoonal to fluvial deposition | Since Neogene | [2] [7] |
Granitoids | Felsic to intermediate intrusive igneous | Intrusive plutons and dykes
|
Eocene to Oligocene | |
Pouebo Terrane,
Diahot Terrane |
Foliated metamorphic
|
Subduction zone metamorphism | Eocene | |
Poya Terrane,
Peridotite Nappe |
ultramafic intrusive and extrusive igneous
|
Volcanism in the ocean, then obducted onto the island during collision between tectonic plates | ||
Montagnes Blanches Nappe,
Bourail Flysch, Koné Terrane, Koumac Terrane, Formation à charbon |
Sedimentary | Terrestrial towards marine, and back to lacustrine sedimentation | Late Cretaceous to Eocene | |
Boghen Terrane | Foliated metamorphic | Subduction zone metamorphism | Jurassic to Cretaceous | |
Teremba Terrane, Central Terrane |
Mostly sedimentary | Mainly marine sedimentation | Permian to Early Jurassic | |
Koh Ophiolite | Mafic to ultramafic intrusive and extrusive igneous | Volcanism beneath the ocean, then transported and scrapped off during collision between tectonic plates | Early Permian |
In the Permian, the Island was initially located in the southeastern margin of the huge
From the
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.[2] 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.[2] The epoch also oversaw a series of displacement of the oceanic lithosphere onto the east of the continental Norfolk Ridge.[2] One of the products of these dynamic events is the famous and economically significant Peridotite Nappe.[2][3][10][11][12][13] This is followed by intrusions of granitoids into the country rocks on Grande Terre.[14]
Since then, the Island re-emerged from the Pacific due to some minor movements.
Nowadays, the two ridges are generally parallel to each other and connect with the d'Entrecasteaux Ridge in the north.[16] 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.[17] Such plate motion makes the Loyalty often suffering from earthquakes.[18]
Notably, the French territory also administers two tiny islands on the New Hebrides Plate − Matthew Island and Hunter Island, though disputed.[19] Therefore, their geology is rather related to the archipelago of Vanuatu, which also claims the active volcanic islands with andesitic composition.[19]
Petrology
Grande Terre
Koh-Central and Teremba Terranes
In the fore-arc basin tectonic setting, the suite of Koh
The Koh Ophiolite is overlain by a series of marine sedimentary rocks starting from black
Meanwhile, the clastic and volcanic rocks of Teremba Terrane formed between the Permian and Early Jurassic.[2] The clastic rocks are mainly greywacke, which could share the same source as those in the Koh-Central Terrane;[23] the composition of the volcanics varies from rhyolitic to andesitic.[2] They are slightly deformed into very-low-grade metamorphic rocks in zeolite facies.[2]
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.[2]
Boghen Terrane
The Boghen Terrane is another group of rocks that formed in that period (Jurassic-Cretaceous).[2] It is basically composed of schists, with the grade ascending from greenschist to blueschist metamorphic facies towards the west.[24] 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.[25]
Their
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.[2]
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
Formation à charbon is overlaid by the Paleocene Koumac Terrane, which primarily consists of sedimentary black chert and also some argillite.[2][28] Collectively known as phtanites, these cherts mark the complete submergence of New Caledonia.[2] 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.[2]
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.[2] Such turbidites and coarse-grained rocks collectively form the Bourail Flysch.[29] 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.[2] Their erosion is believed to be the result of the emersion because of fore-arc bulge, which is linked to a new subduction zone.[2] The limestone is hence overlaid by a 3-to-5-km thick turbidite.[2]
"Montagnes Blanches" Nappe and Koné Terrane
The Nappe is a sedimentary rock sequence from
The Montagnes Blanches Nappe is also overlain by Koné Terre, which is a pelagic unit of siltstone, argillite and chert.[2] Its presence or exposure is limited to Koné and Koumac, and has been overthrust by the Poya Terrane.[2]
Poya Terrane and the Peridotite Nappe
The two units were oceanic lithosphere that thrust onto a continental landmass (known as Tethyan-style ophiolites).
The upper
.The two units sandwich another two formations: the Nepoui Flysch and Koumac
Their presence on high altitude is extraordinary already, as oceanic lithospheres normally subduct rather than thrust onto less dense continental basements.[12] Moreover, since their relative positions are reversed compared to the other ophiolitic suites, their genetic relationships are disputed.[35][36] One of the explanations is that during the uplift by compression, the top mafic terrane slid off by faulting and gravity onto the Norfolk Ridge, followed by the ultramafic Nappe.[36] This is based on a lack of obduction-induced thickening or deformation in the Nappe but the presence of the olistostrome.[36]
Eocene metamorphic complex
The complex consists of two parts: the Pouébo and Diahot Terranes.[2] They both have experienced clockwise P/T paths of metamorphism with similar age (44–34 Ma), which is typical for metamorphic rocks that formed along a subducting slab.[2] With the forming conditions of high pressure but low temperature, it is therefore believed to be formed along the detached Australian slab of the oceanic lithosphere, right before the obduction of the Eocene ophiolites.[2]
Sharing the same sedimentary sequence, the Diahot Terrane is regarded as the metamorphosed layer of the Montagnes Blanches Nappe and the Koné Terrane.[2] The terrane itself consists of blueschist and eclogite in albite–epidote–omphacite, as well as ferroglaucophane–lawsonite mineral zones.[37] Such mineral assemblages help the estimation of the burial depth to be 50 km.[2]
As the Pouebo Terrane is formed deeper in the mantle (80 km), the rocks have a higher-grade garnet-bearing greenish eclogite and glaucophanite.[2][28] It also contains eclogitic mélange that is believed to be originated from the dragged-down and the subducted part of the Poya Terrane.[2][38]
Models suggest the complex have isostatically rebounded to form the Mount Panie Antiform during an extensional event with Diahot in the west, and Pouebo in the east, after obduction later on.[2][11] This could indicate a renewed compression afterwards.[39]
Eocene dykes and Oligocene plutonic rocks
After the obduction event, there were several intrusions of
Rock formations since Miocene
Sedimentary rocks of the Nepoui Series are spotted in places like Nepoui Bay, which include lagoonal limestone, calcareous sand to slightly unconformable conglomerate and sands again.[2] Re-emergence of plant fossils after obduction is spotted in the latter layer.[7] The regolith pebbles in the conglomerate are well-rounded and cover and consist of pre-existing local rocks like peridotite.[2][40] Similar conglomerates and breccias are also found in the Gwa (or Goa) N'doro Formation near Houaïlou, and represents the remnants of a river network developed since Miocene.[2][40][28] These all witness the uplift, erosion, then deposition of the island since Oligocene.[2][15]
Entering the Quaternary, wind-transported
Loyalty Islands
Compared to Grande Terre, the islands are the younger part of the territory with no common geological units between them. The islands and the submerged parts of the Loyalty Ridge are overlain by Miocene carbonate sedimentary rocks, as well as modern coral reefs.[2] These makes investigations on its formation more complicated, but geophysical and bathymetric data found the seamounts are just similar to the other island arcs.[2] Moreover, Eocene andesitic rocks have been drilled there, while ocean island basalt (OIB) on Mare Island hints the Islands could represent a north–south trending hotspot.[2] Combining the features of the subduction-obduction complex on Grande Terre, the explanation of the island arc is more likely. Although their formation is disputed, it is agreed their recent uplift and emergence are associated with subduction of the Australian Plate, in which the section of the crust nearby is bulge.[2]
Tectonic evolution
By observing the properties and also the age of the rocks in different terranes, the tectonic evolution of the region can be reconstructed into three phases, with each phase illustrated in the sections below.[2]
Pre-Early Cretaceous subduction of Phoenix Plate
The first phase starts from the
Rifting from Upper Cretaceous to Eocene
In the Late Cretaceous, marginal rifting occurred on the island, leading to the complete submergence of the island.
Subductions and obduction in Eocene and Oligocene
Entering the Eocene, the tectonic setting shifted back to a collision one, but the mechanisms are not well understood.[45] One of the explanations is the reactivation of the former backarc spreading ridge into a trench by the convection current, causing subduction of the Australian Plate from 55 Ma.[45]
The presence of the subduction-obduction complex suggests a sequence of a new intra-oceanic subduction zone with
After obduction, the uplift and exhumation of the metamorphic core complex between
Re-emergence of New Caledonia after Oligocene
Nepoui Series witnessed the uplift of Grande Terre in the Miocene, possibly by the forearc bulge in response to the subduction nearby to Vanuatu.[2] It was followed by erosion with deposition happening again, along with the development of fluvial network(s).[2][40] Nowadays, the tropical climate on the island facilitates heavy erosion, while the coral reef surrounding the islands has started developing only in the last 400,000 years.[2]
Meanwhile, as the Loyalty Ridge approaches the New Hebrides Trench in the east, current seismic activity is triggered with potential orogeny in the future.[47]
Earth resources
Due to the obduction of the Peridotite Nappe, origininating in the metal-enriched mantle, metal ores are concentrated as the main earth resources in the region.[2]
Nickel and cobalt
Being the most important earth resource in the territory, the
Cobalt mining was also very important, and was mined in different sites from that of nickel.[8] Being first mined between 1876 and 1906, the territory was the monopoly of the cobalt exportation globally, but is no longer operational since the 1920s.[8]
Chromium
The mantle-derived Peridotite Nappe also consists of
Over 3 million tonnes of ore had been mined since the 1880s, but the last mine closed in the 1990s.[2][53]
Note that the podiform chromitites are also enriched in platinum group elements like osmium, iridium and ruthenium relatively, but the concentrations are still not high enough for extraction.[8]
Iron
Sharing the same source as nickel, the iron ore of ferricrete is extensively found on the top of the laterite profiles.[8] It was mined and exported until 1960, when its high impurities of nickel and chromium make the ore no longer suitable for the new smelting industry.[8]
Other metals
Gold was extracted in the Fern Hill Mine in the late 19th century, while its occurrence in other places on the island lacks economic importance.[2] Copper, lead and silver were also found in the Diahot Terrane at the same time, with discontinued mining undertaken in the mid 20th Century.[2][28]
Manganese ore, in the form of small pods, was extracted from the Poya basalts and the altered Peridotite Nappe, from 1918 to 1922 and again from 1949 to 1953.[28]
Non-metals
There are also some minor non-metal resources. For instance, phosphate was mined on the small Walpole Island of the Loyalty Islands from 1888 to 1914.[55]
Limestone formations, except for those which were formed in the Paleocene, have been mined for the local cement industry, as well as for nickel hydrometallurgy.[55]
See also
References
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