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.

The Southwest Pacific region in the area of study is ~ dominated by the subduction of the Australian plate beneath the New Hebrides Arc at a rate of 12 c d y , with a direction of convergence that trends WSW-ENE (Dubois et al., 1977) (Figure 1). The subduction GULWS collision between the Loyalty Ridge and the New. Hebrides Arc. West of the collision zone, three Abstract. The ZoNéCo 1 and 2 cruises of Ifremer's Research Vessel L'Afalante, collected new swath bathymetry and geophysical data over the southern and northern segments of the basins and ridges formine the Lovaltv svstem. Between the two surveyed areas, previ-_-O U~ studies found evidence for the resistance of the Loyalty Ridge to subduction beneath the New Hebrides trench near 22" 5-169" E. On the subducted plate, except for seismicity related to the down-,parallel geological units that trend W-S E north of 22030' s and N-S south of this latitude are present. from east to west (Figure 1): bending of the Australian plate, recorded shallow seismicity is sparse within the Loyalty system (Ridge and Basin) where reliable focal mechanism solutions are almost absent. Swath bathymetry, seismic reflection and magnetic data acquired during the ZoNéCo 1 and 2 cruises revealed a transverse asymmetric morphology in the Loyalty system, and an along-strike horst and graben structure on the discontinuous Loyalty Ridge. South of 23'50' S and at 20" S, the two WSW-ENE-trending fault systems, respectively, sinistral and dextral, that crosscut the southern and northern segments of the Loyalty system, are interpreted as due to the early effects of collision with the New Hebrides Arc. A NNW-SSE trend, evident along the whole Loyalty system and on the island of New Caledonia, is interpreted as an inherited structural trend that may have been reactivated through flexure of the Australian lithospheric plate at the subduction zone. Overall then, the morphology, structure and evolution of the southern and northern segments of the Loyalty system probably result from the combined effects of the Australian plate lithospheric bulge, the active Loyalty-New Hebrides collision and the overthrust of the New Caledonian ophiolite.

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Four successive spreading phases are distinguished in the northwestern part of the North Fiji Basin. After an initial NE‐SW opening, a N‐S spreading phase took place, up to the northwesternmost tip of the basin, along the South Pandora, Tikopia and 9°30 Ridges. The N‐S spreading phase in the northern North Fiji Basin was followed by an E‐W opening phase along the central North Fiji Basin axis. A triple junction was probably active during an intermediate stage between the two phases. E‐W spreading underwent a reorganisation that induced the functioning of the 16°40′S triple junction and the development of the E‐W trending Hazel Holme Extensional Zone from the active central spreading axis to the southern tip of the New Hebrides Back‐Arc Troughs. Active extension also occurs along the E‐W Santa Cruz Trough which crosscuts the arc platform at the northern end of the N‐S trending Back‐Arc Troughs. The existence of the Back‐Arc Troughs is mainly due to the construction of the 400 km‐long volcanic Duff Ridge which trapped a piece of the old North Fiji Basin oceanic crust.

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