Geophysical investigation of seamounts near the Ogasawara Fracture Zone, western Pacific

Abstract

This paper provides an analysis of multi-channel seismic data obtained during 2000-2001 on seamounts near the Ogasawara Fracture Zone (OFZ) northwest of the Marshall Islands in the western Pacific. The OFZ is unique in that it is a wide rift zone that includes many seamounts. Seven units are delineated on the basis of acoustic characteristics and depth: three units (I, II, and III) on the summit of seamounts and four units (IV, V, VI, and VII) in basins. Acoustic characteristics of layers on the summit of guyots and dredged samples indicate that the seamounts had been built above sea level by volcanism. This was followed by reef growth along the summit margin, which enabled deposition of shallow-water carbonates on the summit, and finally by subsidence of the edifices. The subsidence depth of the seamounts, estimated from the lower boundary of unit II, ranges between 1,550 and 2,040 m. The thick unit I of the southern seamounts is correlated with proximity to the equatorial high productivity zone, whereas local currents may have strongly affected the distribution of unit I on northern seamounts. A seismic profile in the basin around the Ita Mai Tai and OSM4 seamounts shows an unconformity between units IV and V, which is widespread from the East Mariana Basin to the Pigafetta Basin.

Three-dimensional flexure modelling of seamounts near the Ogasawara Fracture Zone in the western Pacific

“…The height of the load is calculated by adding the thickness covered by the sediment to the exposed height of seamount. The total sediment thickness is determined to be 600 m in the OFZ and 500 m in the PB based on the drilled cores and the previous seismic profiles (Wedgeworth & Kellogg 1987; Abrams et al 1992; Larson et al 1992; Lee et al 2008). This thickness estimate of the sediment layer does not account for the isostatic effects of sediment loading.…”

“…Smoot (1983, 1989) suggested that the Dutton Ridge in the OFZ formed as a volcanic plateau at the extinct triple junction of the Phoenix, Farallon and Pacific plates and this once‐continuous plateau was then fractured and faulted to produce the present group of guyots and ridges. Lee et al (2005, 2008) investigated the acoustic characteristics and mass‐wasting with chirp subbottom and seismic data over the seamounts near the OFZ.…”