Benoît Quesnel

Syn-tectonic, meteoric water-derived carbonation of the New Caledonia peridotite nappe

Exceptional outcrops recently exposed in the Koniambo massif allow the study of the serpentine sole of the peridotite nappe of New Caledonia (southwest Pacific Ocean). Many magnesite veins are observed, with characteristics indicating that they were emplaced during pervasive top-to-the-southwest shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4‰ < δ18O < 29.7‰), while its carbon isotope composition varies widely (−16.7‰ < δ13C < −8.5‰). These new data document an origin of magnesite from meteoric fluids. Laterization on top of the peridotite nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which has led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

The emplacement of the Peridotite Nappe of New Caledonia and its bearing on the tectonics of obduction

The Peridotite Nappe of New Caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction. Here, we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern New Caledonia. The klippes are flat-lying and involve S/SW-vergent reverse-slip shear zones which are true compressional structures in origin. Further northeast, the nappe is folded in association with the development of a steep schistosity in low grade metasediments. This difference in structural style indicates that the Peridotite Nappe experienced compression at greater depths toward its root zone, suggesting a “push from the rear” mechanism of emplacement. This supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence. We establish a crustal-scale cross-section at the end of the obduction event, before Neogene extension. This involves a large fold nappe of high pressure rocks bounded from below by a major thrust. Furthermore we show that obduction in New Caledonia occurred through dextral oblique convergence. Oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it. This obliquity can solve the paradox of the Peridotite Nappe seemingly being emplaced at the same time the high pressure rocks were exhumed. Oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen.