Pierre Maurizot

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende ( 40Ar/ 39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ∼56Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ∼0.5GPa at ∼800-950C. The thermal gradient inferred from the metamorphic assemblage (∼60°Ckm -1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ∼53Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ∼56Ma.

Discovery of Early Cretaceous Rocks in New Caledonia: New Geochemical and U-Pb Zircon Age Constraints on the Transition from Subduction to Marginal Breakup in the Southwest Pacific

New U-Pb dating of detrital zircon and geochemical features of Permian-Mesozoic arc-derived volcanic rocks and volcaniclastic turbidites (graywackes), when compared with those of the volcanic rocks associated with unconformable Late Cretaceous shallow-water sediments, reveal that subduction in New Caledonia, once thought to be extinct in the Late Jurassic (ca. 150 Ma), was still active at least from ca. 130 to 95 Ma. The accumulation of volcanic arc-derived sediments during the late Early Cretaceous suggests that, as in New Zealand, active-margin activity went on for a short time in spite of the assumed subduction jamming by the Hikurangi Plateau at ca. 100 Ma. Meanwhile, the rift-related magmatic activity that preceded the marginal breakup migrated eastward from ca. 130 Ma (130–95 Ma) in eastern Australia, to 110 Ma (110–82 Ma) in New Zealand, and, finally, to ca. 89 Ma (89–83 Ma) in New Caledonia and generated large volumes of silicic magma. In contrast, marginal basins opened synchronously at ca. 83 Ma when the stretched continental crust finally broke out. In general, intraplate and volcanic arc signatures coexisted in Cretaceous syn-rift magmas. Therefore, the Australian marginal breakup appears to be the final effect of continuous southward unzipping of Gondwana that interfered with the subduction-modified mantle wedge of the Mesozoic active margin. The occurrence of lateral flow of the upper asthenospheric mantle due to the rapidly eastward-migrating Australian plate margin possibly prevented the formation of a volcanic arc at the eastern end of the system.

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.

Pre-obduction records of Eocene foreland basins in central New Caledonia: an appraisal from surface geology and Cadart-1 borehole data

The Bourail Anticline in central New Caledonia has been a target for petroleum exploration since the 1950s. It is a broad structure in which a 4-km-thick pile of Eocene turbidite (Eocene Bourail Flysch) is exposed. The Cadart-1 exploration borehole (1930 m deep) was drilled on the anticlinal axis in 1999, intersecting the Paleogene and underlying Late Cretaceous sedimentary rocks. A sedimentological, stratigraphic and provenance analysis is presented. Volcanic debris appears progressively in the upper part of the flysch, becoming predominant upwards along with the intercalation of upward-thickening and -coarsening debris flow breccia. Clinopyroxene and basalt clasts in the upper part of the flysch were derived from enriched tholeiite (E-MORB) of the Poya Terrane. The Eocene paleogeography can be interpreted as a foreland basin system with a depocentre close to the area of Bourail Anticline, an accretionary complex to the northwest and a forebulge to the southeast. This system migrated southeastward through New Caledonia from the Paleocene to the Late Eocene.

Regolith-geology mapping with support vector machine: A case study over weathered Ni-bearing peridotites, New Caledonia

Abstract Accurate maps of Earth’s geology, especially its regolith, are required for managing the sustainable exploration and development of mineral resources. This paper shows how airborne imaging hyperspectral data collected over weathered peridotite rocks in vegetated, mountainous terrane in New Caledonia were processed using a combination of methods to generate a regolith-geology map that could be used for more efficiently targeting Ni exploration. The image processing combined two usual methods, which are spectral feature extraction and support vector machine (SVM). This rationale being the spectral features extraction can rapidly reduce data complexity by both targeting only the diagnostic mineral absorptions and masking those pixels complicated by vegetation, cloud and deep shade. SVM is a supervised classification method able to generate an optimal non-linear classifier with these features that generalises well even with limited training data. Key minerals targeted are serpentine, which is considered as an indicator for hydrolysed peridotitic rock, and iron oxy-hydroxides (hematite and goethite), which are considered as diagnostic of laterite development. The final classified regolith map was assessed against interpreted regolith field sites, which yielded approximately 70% similarity for all unit types, as well as against a regolith-geology map interpreted using traditional datasets (not hyperspectral imagery). Importantly, the hyperspectral derived mineral map provided much greater detail enabling a more precise understanding of the regolith-geological architecture where there are exposed soils and rocks.

Neogene terrestrial sediments: a record of the post-obduction history of New Caledonia

The poorly studied iron-rich terrestrial sediments, referred to as the Fluvio-lacustrine Formation, that crop out in the southern part of the Grande Terre of New Caledonia document the last 25 Ma of the geological history of the island. The age of this formation, which is mainly derived from the erosion of an ultramafic regolith, is not tightly constrained yet; however, it has recorded several episodes of post-obduction erosion and sedimentary infill preceded and followed by weathering and reactivated erosion. A correlation with early Miocene slab break-off, which may have triggered a first stage of erosion marked by coarse conglomerate, is suggested. Thereafter, sediments filled topographic lows and were in turn weathered during an interval of tectonic quiescence. Finally, Holocene sea-level drop and southward tilt of southern New Caledonia, owing to the involvement of eastern Australian Plate in the New Hebrides (Vanuatu) subduction zone, locally changed the drainage pattern and deeply eroded the sediments.

Magneto-biostratigraphic constraints of the Eocene micrite–calciturbidite transition in New Caledonia: tectonic implications

ABSTRACT We conducted an integrated magneto-biostratigraphic study of a 37 m-thick composite section exposed at two sites near Nouméa (New Caledonia). The section contains a transition from pelagic micrite to terrigenous-rich calciturbidites. This transition, observed regionally in coeval records of New Caledonia, marks a shift from pelagic sedimentation on a stable continental submarine plateau to turbidite deposition indicating development of a slope in a convergent tectonic regime. The studied section spans magnetic polarity Chrons C22r to C20r, calcareous nannofossil zones CNE5 to CNE10, and radiolarian zones RP9 to RP11 (49.5 to c. 44 Ma), and the micrite–turbidite transition occurred around 45.3 Ma (early middle Eocene). This transition could be the onshore correlative of a regional switch from tectonic extension to compression, which has been inferred from analysis of new seismic profiles acquired for the Tasman–northern Zealandia area, and that has been interpreted as precursor of the Tonga–Kermadec subduction initiation.

Deepwater fold-and-thrust belt along New Caledonia's western margin : relation to post-obduction vertical motions

Classically, deepwater fold-and-thrust belts are classified in two main types, depending if they result from near- or far-field stresses and the understanding of their driving and triggering mechanism is poorly known. We present a geophysical dataset off the western margin of New Caledonia (SW Pacific) that reveals deformed structures of a deepwater fold-and-thrust belt that we interpret as a near-field gravity-driven system, which is not located at a rifted passive margin. The main factor triggering deformation is inferred to be oversteepening of the margin slope by post-obduction isostatic rebound. Onshore erosion of abnormally-dense obducted material, combined with sediment loading in the adjacent basin, has induced vertical motions that have caused oversteepening of the margin. Detailed morpho-bathymetric, seismic stratigraphic and structural analysis reveals that the fold-and-thrust belt extends 200 km along the margin, and 50 km into the New Caledonia Trough. Deformation is rooted at depths greater than 5 km beneath the seafloor, affects an area of 3500 km2, and involves a sediment volume of approximately 13 000 km3. This deformed belt is organized into an imbricate fan system of faults, and one out-of-sequence thrust fault affects the seabed. The thrust faults are deeply rooted in the basin along a low-angle floor thrust and connected to New Caledonia Island along a major detachment. This study not only provides a better knowledge of the New Caledonia margin, but also provides new insight into the mechanisms that trigger deepwater fold-and-thrust belts.