Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence

摘要

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