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active, in the sense that they are under the (Collot et al., 1987), which was emplaced erosion, a deep weathering profile may
influence of an active plate boundary. onto the continental Norfolk Ridge at the be preserved. Conversely, it is destroyed.
The striking asymmetrical distribution end of the Eocene (Cluzel et al., 2012) Mantle rocks (dunite, harzburgite,
of the marginal basins-arc-trench system and was mentioned as a typical example lherzolite), which have a very low
at Earth’s surface, which was remarked of the obduction concept defined by hydraulic conductivity, are, however,
upon by some authors in the early stages Coleman (1971). densely fractured as a result of their long
of global tectonic theory (e.g., Uyeda and The terrane was exposed to emergence, evolution in oceanic domains and their
Kanamori, 1979), is out of the scope of dismantling, weathering, and erosion eventual obduction. Serpentinization of
this paper, but the coincidence of (i) the during the Neogene post-obduction the protolith, diffuse or closely associ-
large-scale structure of the Earth, where period. Early Miocene conglomerate of ated with fracturing, is a common altera-
important portions of oceanic crust and the Népoui area reworks abundant weath- tion, which, on the whole, corresponds to
mantle have been thrusted on land, and of ered products of the peridotite, including hydration and cooling of the lithospheric
(ii) a warm and wet intertropical climate supergene nickel ore (Coudray, 1971; mantle rocks. Once exposed on land,
belt, illustrates perfectly how the overlap- Maurizot et al., 2016). The oldest weath- above sea level, the fracture network
ping of two independent factors may be ering profiles have been dated indirectly allows pervasive underground water cir-
determinant in the formation and location by paleomagnetism methods at 25 Ma culation and setting up of a water table.
of a mineral resource. (Sevin et al., 2012). The nickel ore– Through internal water fluctuations and
In a comparable accretionary context, forming process was therefore already circulation, fluids in the ultramafic pro-
New Caledonia is the emerged part of active at the end of Oligocene, ~10 m.y. tolith propagate weathering, leading to a
the Norfolk Ridge in the northern part of after obduction, and is likely ongoing, general saprolitization, karstification
what has been recently referred to as as attested to by dissolution activity and with sinkholes, underground conduits,
Zealandia, a mostly submerged continent youngest paleomagnetic ages. and lapies. A complete and differentiated
(Mortimer et al., 2017). In this area of weathering profile may reach 50 m in
interaction between the Australian and THE NICKEL FACTORY thickness. The youngest and active front
Pacific plates, the Eastern Gondwana Worldwide, Ni-Co–laterite deposits is at the base, the oldest at the top. It
margin has been fragmented into several are controlled by a wide range of inter- comprises, from top to base (Eggleton,
thinned continental ribbons and exten- acting factors (Freyssinet et al., 2005; 2001; Fig. 2):
sional basins, some of which are oceanic Butt and Cluzel, 2013): lithology (includ- • Lateritic residuum divided into:
(e.g., the Tasman Sea and Loyalty ing serpentinization) of the ultramafic - The topmost duricrust (or ferricrete
basins). The most emblematic and prom- rock, fracturing, permeability, climate or iron cap), which corresponds to
inent geological unit of Grande Terre, change, tectonic activity, and morpho- the zone of oscillation of the sub-
the “main island” of New Caledonia, is logic evolution. Weathering tends to surface water table, where alternat-
the Peridotite Nappe (Avias, 1967; Fig. accumulate laterite and thus nickel; ing saturated and unsaturated condi-
3), which covers about a third of the sur- tectonic activity or eustatic variations tions allow precipitating of massive
face area of the country. The ultramafic tend to erode and destroy, or cover up iron oxy-hydroxides;
terrane is interpreted as the lithospheric and conceal, these accumulations. If the - A plasmic zone (or red limonite),
mantle of the Late Cretaceous to Paleo- rate of chemical weathering of a perido- in which all primary structures
gene South Loyalty Marginal Basin tite is greater than the rate of physical are erased;

Figure 3. Simplified geological map of Grande Terre and section along Grande Terre showing the general shape of the ultramafic terrane.

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