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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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