Two cruises of the French-Japanese starmer joint project carried out with the submersibles Nautile (1989) and Shinkai 6500 (1991), have been devoted to the in situ exploration of the North Fiji Basin ridge. Four areas, located near 16°20′, 17°, 18°50′ and 19°S have been explored during 42 dives. At 16°20′S, the axial zone trending N160° corresponds to a prominent graben with walls 1000 m high. Tectonic features and recent screes are present, and the hydrothermal activity is characterized by sporadic fluid discharge. At 17°S, present accretion is confined to a high dome less than 1900 m deep and cut in its axial part by a wide graben trending N15°. At this site, hydrothermal features of two types are observed: fossil chimneys of sulfides, and active vents, made of anhydrite and expelling water at 285°C. The site at 18°50′S is located on top of an axial dome, very similar to those of the East Pacific Rise. Small fissures and very fresh volcanic flows argue for a present-day activity, with weak evidence of tectonics. Hydrothermal emanations with animal colonies are found scattered over the whole area. The fourth site, at 19°S, shows old and viscous lavas and a very mature tectonic activity, compared to the preceding station which is located only 20 km northward. Large-scale variations have been detected between three segments. Different thermal regimes can explain the distinct morphologies observed. Decakilometric scale variations do occur between stations 14, with volcanic predominance, and “19°S”, where tectonic activity is dominating. Finally, small-scale variations are observed on sites 4 and 14, where volcanic features gradually give way to tectonic structures. The North Fiji Basin ridge—with an intermediate average spreading rate—shows tectonic, volcanic and hydrothermal variability, ranging from 1 km to tens of kilometres and related to a temporary fine scale organisation of the accretionary processes.

The aim of the Yokosuka 90 cruise, which was carried out between 10th January and 6th February 1991, was the geological, geophysical and geochemical study of the N160 portion of the North Fiji Basin ridge (SW Pacific). This segment is characterized by an actual accretion system which is constituted by a succession of "en echelon" grabens, which can be more than 4,000 m deep. These grabens are flanked by about 1,000 m high steep walls. These characteristics are usually found on slow spreading ridges while the rate of opening in this area is 5 cm/y. This suggests that the magmatic budget of this ridge segment has been very low during the last 200,000 years (approximative age of creation of the grabens). The main part of the accretion would be of an amagmatic type.

Du 10 janvier au 6 fevrier 1991, la campagne Yokosuka 90 a eu pour objet l'etude geologique, geophysique et geochimique de la branche de direction N160 de la dorsale du Bassin Nord-Fidjien (SW Pacifique). Ce segment est caracterise par un systeme d'accretion actuel constitue d'une succession en echelon de grabens dont certains depassent 4 000 m de profondeur. Ces grabens sont bordes par des murs abrupts d'environ 1 000 m de hauteur. Ces caracteristiques sont habituellement celles des dorsales lentes, alors que le taux d'ouverture dans cette zone est estime a 5 cm/an

The anticlinal nappe which forms the Huon Peninsula and adjacent ranges extends offshore as the Huon Ridge. The frontal thrust of the nappe is the Ramu-Markham Fault (onshore) and a deformation front along the line of the Markham Canyon (offshore). The timing and geometry of the Finisterre arc-continent collision is controversial, and the origin of the Finsch Deep is unresolved.

The area to the southeast of the Trobriand Platform contains an easterly continuation of the Oligocene to Quaternary Cape Vogel Basin (CVB). Within this area, three major seismic sequences are recognized: sea bed-A, A–B, and B–D. The latter sequence occurs within a depocenter with approximately 3,200 m of sediment. The A–B and B–D sequences were faulted and gently folded in Late Miocene times.

The western New Britain Trench contains relatively thin sediment fill in the east, compared to the west where a sequence of thick turbidites is ponded behind a basement high in the trench axis, The trench trends toward Huon Gulf, but intersects the Trobriand Trench at an acute angle at the 149° Embayment, where both trenches end. Seismic structure west of the trench is incoherent, related to incipient collision of the Indian-Australia Plate and the South Bismarck Plate. The collision suture is marked by the Markham Canyon, continuous in its upper reaches with the Ramu-Markham Fault Zone on shore.

The western Solomon Sea is bounded by the Paleogene collision complex of the Papuan Peninsula to the south, and land masses constructed by Cainozoic volcanism to the north and cast. Oblique collision of two trenches in the western Solomon Sea, and concomitant collision of upper plates, have produced structural complexities that may include the local doubling of crustal thickness, coincident with a strong negative gravity anomaly west of 149°E. Lateral flexing of the subducted plate in the New Britain Trench may have caused flexure of the upper plate; this flexure is expressed in the gravity field, faults, dip-slopes, exposure of basement, and alignment of volcanoes.