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Drilling results from Ocean Drilling Program (ODP) Leg 134 and multichannel seismic reflection (MCS) data show the structures that develop within the North d'Entrecasteaux Ridge (NDR)-New Hebrides Island Arc (NHIA) collisional accretionary wedge, west of Espiritu Santo Island. The NDR trends east-west on the Australian plate and collides and subducts obliquely (-14°) northward beneath the NHIA. This collision has produced a shallow fore-arc protrusion that culminates at the Wousi Bank. The summit area of the NDR comprises the ridge crest with adjacent terraces. Drilling at Site 828 on the northern ridge terrace revealed a pre-upper Eocene volcanic breccia overlain by a thin (100 m), Cenozoic, time-discontinuous, sedimentary section. Sites 827-829, drilled at the toe of the forearc slope adjacent to the ridge, demonstrated tectonic accretion of thin (28-100 m thick) thrust sheets that consist of Pleistocene trench-fill deposits and slivers of Cenozoic ridge material including volcanic fragments. The thinness of the sediment of the NDR, as well as physical properties such as sediment instability, occurrence of low strength zones with high porosity, and water content, are responsible for frontal accretion of the thin thrust sheets. MCS data show that a low-relief, imbricate, accretionary wedge has formed over the subducted ridge terrace, which contrasts with a high-relief accretionary wedge that developed where the ridge crest collides with the arc slope. These data also show the existence, in the accretionary wedge, of thick (300 m) thrust complexes that are characterized by a three-dimensional flake-like geometry and are bounded by thrusts and lateral ramps. Interpretation of drilling and MCS data suggests that the thrust complexes consist of stacked thin thrust sheets. Variation of basal friction along the décollement, possibly related to morphologic, lithologic or structural ridge complexities, may have caused the thrust complexes to develop. Although the thrusts and ramps that bound the thrust complexes may have formed during or after accretion, a sandbox experiment by Colletta et al. (1991) suggests that, during accretion, lateral variation of friction along the décollement can produce lateral ramps and thrust structures very similar in geometry to those observed in the NDR collision zone. An out-of-sequence thrust evident from drilling data and steep reverse or strike-slip faults that crosscut thrusts on MCS data indicate post-accretion deformation that develops as the accretionary wedge accommodates subduction of ridge summit asperities. This later deformation appears to extend within the subducted ridge body, suggesting that during arc-ridge collision, in addition to frontal accretion of shallow ridge sediment along the décollement, deeper parts of the ridge are being sheared off and possibly underplated. Underplating could have caused Central Espiritu Santo Island and Wousi Bank to be uplifted.

proceedings of the ocean drilling program

Tectonic Accretion and Deformation of the Accretionary Wedge in the North D'Entrecasteaux Ridge-New Hebrides Island Arc Collision Zone: Evidence from Multichannel Seismic Reflection Profiles and Leg 134 Results