The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides Island Arc: 1. Sea Beam morphology and shallow structure

Abstract

Sea Beam bathymetric data, closely spaced single‐channel seismic reflection sections, and geopotential field data were collected aboard the R/V J. Charcot over the collision zone between the North d'Entrecasteaux Ridge (NDR) and the New Hebrides island arc. The NDR trends east at a small angle (14°) to the plate convergence direction so that the ridge creeps northward along the trench at about 2.5 cm/yr, requiring a continual structural adjustment of the accretionary complex. In this report we study the shallow structure, tectonic erosion, and consequent mass wasting of the accretionary complex, all of which were produced by the collision of the NDR in a complex deformational environment. The accretionary complex in this collision zone can be divided into northern, central, and southern parts. The northern part of the collision zone, which lies north of the leading flank of the ridge, shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and N60°W trending strike‐slip faults; a middle accretionary complex that bulges seaward and appears to be rotated 45°E from the regional arc trend; and an upper accretionary complex that is shaped by slumps and a canyon network. The central part of the collision zone, which directly overlies the crest of the ridge, forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps. Rocks forming this protrusion have been uplifted, possibly by as much as 1500–2500 m, and tilted to the north, causing northward block sliding along an extensional detachment surface. The southern part of the collision zone, which lies in the wake of the ridge, is deformed by large slumps and normal faults that trend parallel to the ridge axis. Rocks forming this southern part collapse, causing widespread mass wasting. These geophysical data indicate that normal trench convergence has apparently produced only a few trench‐parallel structures confined to the toe of the accretionary complex, whereas the along‐trench motion of the NDR has resulted in well‐developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern. The area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the NDR, and the time required to heal the disturbance may be about 0.8 m.y.