Tectonic Setting of the Danger Islands Troughs in the Manihiki Plateau

The Manihiki Plateau, which lies in the western equatorial Pacific Ocean, is considered to be one of the Cretaceous Large Igneous Provinces, which formed at or near a triple junction (Winterer et al., 1974). Three major geomorphic plateaus, High, North, and Western plateaus, are discernible within the plateau itself. The depth of the Western Plateau, about 4000 m, is deeper than those of the other plateaus, about 3000 m. The High Plateau is separated by linear deep narrow depressions named the Danger Islands Troughs (Mammerickx et al., 1974). The Danger Islands Troughs is thought to be a trace of the plate boundaries (Winterer et al., 1974). Geophysical and geological investigations were conducted by R/V Hakuho-maru, Ocean Research Institute, the University of Tokyo, in August 2003. The bathymetric survey by the SEA BEAM 2120 multi-narrow beam echo sounders was carried out along the Danger Islands Troughs (DIT) between 9__deg40'S and 6_{_deg}30'S. The interval of the track lines is about 5 km. Geomagnetic and gravity fields were measured during the bathymetric survey. Volcanic rocks were dredged at the four sites of the slopes of the troughs. Our bathymetric survey exposes the detailed topographic expression of the DIT. The topographic expression of the DIT is a trough bordered by ridges with a height of 2500 m above the floor of the troughs. The depth of the northern part, north of 7__deg$30_fS, of the DIT is 5900 m. That of the southern part of the DIT is 4800 m. There is a seamount which high is 2500 m between the northern and southern parts of the DIT. O investigations indicates that the DIT is not an extinct normal spreading system, but a fracture zone which is a trace of an oblique spreading system or a leaky transform fault.