Tectonic processes at the collision of the D'Entrecasteaux Zone and the New Hebrides Island Arc

Abstract

The d'Entrecasteaux zone (DEZ), a large submarine mountain chain composed of two east trending, subparallel ridges, collides with the north trending New Hebrides island arc. The direction of convergence of the oceanic plate and arc suggests that the DEZ is being subducted at a slightly oblique angle (10°–15°), so that the DEZ creeps slowly north, parallel to the trench. One possible consequence of this northward creep is that slope strata south of the DEZ should bear a structural overprint caused by ridge collision, whereas strata north of the DEZ may show no such overprint. Migrated multichannel seismic data presented in this report reveal the probable form of this overprint: Within the collision zone, strata forming the arc's accretionary wedge dip toward the trench, possibly recording progradation of slope rocks across the ocean floor, and neither outer arc ridge nor slope basin is present. Seismic data from south of the DEZ, however, show no evidence for this overprint because both an outer arc ridge and slope basin are present and the structure of slope strata on opposite sides of the DEZ appears broadly similar, involving a nearly invariant stratal dip toward the arc. Nevertheless, the slope's morphology shows that collision has modified the structure of rocks below the arc's slope and that the arc responded differently to collision of the DEZ's two ridges. The north ridge is nearly flat‐topped, and mid‐ocean ridge basalt has been dredged from its flanks. Isobaths of the slope bulge westward where the ridge is being subducted, suggesting that the slope has been uplifted by the ridge. In contrast, the south ridge shows sharp relief and is made up of stratified, possibly sedimentary, rocks, and the Bougainville Spur, a sharp peak, projects above the arc's lowermost slope along the trend of this ridge. The spur may be a peak, similar to one of those surmounting the south ridge, that rammed the arc's slope.