Physical and geotechnical properties of seafloor sediments from the Vanuatu Collision Zone in the Central New Hebrides Island Arc, South Pacific Ocean

Abstract

Physical properties measurements and geotechnical testing of Ocean Drilling Program Leg 134 cores from the d'Entrecasteaux Zone (DEZ) and the intra-arc North Aoba Basin (NAB) in the southwest Pacific Ocean provide a quantitative basis to further our understanding of sediment processes at active margins. Collision of the twin DEZ ridges with the Vanuatu Island arc complicates subduction in the Central New Hebrides Trench. The North d'Entrecasteaux Ridge (NDR) and Bougainville Guyot appear to clog the trench, uplifting Espiritu Santo Island at a rapid rate and helping to form the NAB, a deep depression of the Pacific plate which traps sediment and ash from the surrounding island volcanoes. Water content and porosity measurements indicate significantly less fluid exists in the Vanuatu accretionary complex than was found by previous drilling in the Barbados and Nankai prisms. This is due in part to sediments being incorporated into the complex from the DEZ which are more coarse-grained, less porous, and more permeable than the pelagic clay-dominated Barbados forearc. Tectonically induced lateral compression is taking place in the collision complex, resulting in pore volume reduction, sediment strengthening, and intense dewatering through fracture permeability. Lower matrix permeabilities in certain sedimentary units appear to channel fluids through thrust faults to more permeable, tectonically fractured lithologies. Sediments in the DEZ are primarily overconsolidated, due to intensive dewatering during the collision process. Geotechnical tests also provide evidence that each ridge of the d'Entrecasteaux zone causes a different style of forearc deformation. The denser NDR is subducting beneath Espiritu Santo and its sediment cover is being sheared off and accreted, while the more buoyant Bougainville Guyot is riding up on the arc and overconsolidating sediments in its path. Underconsolidated sediments and probable excess pore pressures on the flank of the North d'Entrecasteaux Ridge are evidence of a "squeegee effect" in front of the ridge as its scrapes material northward while obliquely subducting beneath the Vanuatu Islands. Progressively underconsolidated sediments with depth below the seafloor in the North Aoba Basin attest to rapid deposition as the result of active Quaternary volcanism. Relatively low intergranular permeabilities range from 10^-6 to 10^-8 cm/sec, and vary within lithostratigraphic horizons. Permeability values appear lower in the d'Entrecasteaux Zone than in the North Aoba Basin, possibly due to collision.