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The New Hebrides island arc strikes northward through the southwestern Pacific Ocean and parallels the trench where the Australia-India plate on the west is subducted beneath the North Fiji basin on the east. Convergence between the oceanic plate and the arc occurs in an easterly direction (N75E) as determined from earthquake focal mechanisms (Pascal et al., 1978; Isacks et al., 1981). However, the rate of convergence is poorly constrained because of plate motion beneath the North Fiji basin. Chase (1971) estimates an 8 cm/yr convergence rate at the New Hebrides trench, whereas Taylor et al. (1985) propose a convergence rate between 11 and 20 cm/yr.

The d'Entrecasteaux Zone (DEZ) is a large submarine mountain system that extends north and east from New Caledonia to where it collides with the New Hebrides arc. Near the arc the DEZ consists of two subparallel ridges that trend east-west and show considerable topographic relief. Collision of the DEZ with the arc has apparently caused abrupt transitions from a single axial line of volcanic islands south and north of the intersection to triple chains of islands at the intersection. The Aoba basin formed within the arc's summit and lies beneath 2000 to 3000 m of water. Although summit basins are present outside of the collision zone, the Aoba basin has greater bathymetric expressions than any other summit basin. The main objective of this discussion is to describe two seismic lines—-one obtained within and one obtained outside of this collision zone—-to illustrate the contrast in structure

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