Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence

Abstract

Geophysical studies support inferences from outcrop geology that during the Late Eocene an ophiolite sheet exposed on New Caledonia was thrust southward over a rock complex consisting of sedimentary, volcanic, and metamorphic rocks that range in age from Pre‐Permian to Eocene. The outcropping ultramafic complex consists of a layered sequence, approximately 3000 m thick, including harzburgite, dunite, wehrlite, serpentinite, and gabbro. The absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting, these units were removed either by thrust faulting or by erosion. Seismic refraction profiles, collected over the adjacent Loyalty Basin, show that the lower crust and upper mantle are characterized by velocities of 4.7, 5.8, 6.8, and 8.1–8.4 km/s, suggesting the presence of a complete oceanic section that includes sediment, basalt, gabbro, and peridotite. The lower crust is probably 6.5 km thick beneath the Loyalty Basin. A seismic reflection profile that extends across the Loyalty Basin is interpreted to show the oceanic crust rising toward New Caledonia. We suggest that the ophiolite on New Caledonia is continuous with this rising oceanic crust. Studies of gravity anomalies observed both on land and offshore indicate the presence of a short‐wavelength, high‐amplitude (+180 mGal), asymmetrical, free‐air gravity anomaly along the northeastern coast of New Caledonia. A gravity anomaly profile, calculated for a geological model characterized by a 10‐km‐thick slab of oceanic crust and mantle material extending continuously from the ophiolite on New Caledonia to the oceanic crust of the Loyalty Basin, successfully matches the observed gravity data. We interpret the extension of the high‐gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex. A tentative geodynamic reconstruction suggests that the proto‐Loyalty Basin crust formed a marginal basin along the eastern margin of Gondwanaland prior to Upper Cretaceous time; after the opening of the New Caledonia Basin during Upper Cretaceous to early Paleocene time a subduction zone developed along the western margin of New Caledonia. While this zone was active, the Loyalty Basin lithosphere overthrust New Caledonia in a southeasterly direction and locally rotated clockwise. This compressional tectonic event was terminated by upper Eocene time.

120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

“…Kroenke (1984) and Bitoun and Recy (1982) suggested that the South Loyalty Basin formed in the Eocene. In contrast, Dubois et al (1973) and Collot et al (1987) argued that the South Loyalty Basin formed as a marginal basin prior to 80 Ma above an east-dipping subduction zone, with a trench west of New Caledonia, implying subduction of oceanic crust of the New Caledonia Basin. New seismic interpretations of South Loyalty Basin sediment fill from the southern part of the basin away from the zone of Eocene emplacement of the New Caledonia ophiolite suggest that the South Loyalty Basin formed by extension and later ocean crust accretion around 120 Ma in the Early Cretaceous (Auzende et al 2000), perhaps contemporaneous with the opening of the New Caledonia Basin.…”

“…Collot et al (1987) have invoked east-dipping subduction west of New Caledonia from 80 Ma to Early Paleocene (ca 64 Ma), although there are presently no known rocks of this age with subductionrelated signatures to support this model. Sdrolias et al (2002) also argued for a long period of east-directed subduction east of the Lord Howe Rise during the period from around 90 Ma until about 45 Ma.…”

“…This led to emplacement of the New Caledonia ophiolite. Seismic transects reported by Collot et al (1987) show oceanic crust of the South Loyalty Basin to the west of the Loyalty Ridge rising towards, and being continuous with, the New Caledonian ophiolite. The highly refractory nature of the mantle section of the New Caledonian ophiolite (Prinzhofer 1981;Leblanc 1995), and the occurrence of low-Ca boninites in fault slices beneath the mantle section of the ophiolite (Cameron 1989), strongly support a forearc origin of the main ophiolite on New Caledonia.…”

“…A number of workers (Dubois et al 1973;Collot et al 1987) proposed that east-directed subduction of New Caledonia Basin oceanic crust occurred before 80 Ma, with the trench occurring along the western edge of the New Caledonia-Norfolk Ridge. However, this model demands the presence of a Late Cretaceous-Paleocene island arc in the immediate New Caledonia-Norfolk Ridge region for which there is still no evidence (Regnier 1988).…”

Multiple melting stages and refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite

“…In that sense, they provide a unique opportunity to study the physical and chemical processes that affect the upper mantle in mid oceanic and subduction environments. The Cretaceous to Eocene New Caledonia ophiolite is an extensive and very well-exposed mafic-ultramafic complex ( [Avias, 1967], [Collot et al, 1987] and [Prinzhofer, 1981]) that is still partially connected to the modern oceanic floor of the South Loyalty Basin to the east of the island (Fig. 1).…”