Abstract
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.
Highlights
The western Pacific is characterized by its abundant seamounts and fracture zones with variable orientations. Wessel and Kroenke (1997) suggested that more than 8,800 seamounts exist in the Pacific, most of them in the western Pacific
We have investigated the acoustic characteristics of these seamounts on seismic profiles of the Pigafetta Basin (PB), East Mariana Basin (EMB), and Ogasawara Fracture Zone (OFZ), even though the seismic lines were too short to completely cover the basins
We identified four units in the seismic profiles taken in basins: an uppermost transparent layer with weak internal reflectors; an upper well-layered unit with a high amplitude; a lower transparent layer; a lower well-layered and thin unit with a high amplitude. 2) Unit I is exceptional in that it overlays unit III, indicating that energetic currents existed until the seamounts subsided to considerable depths
Summary
The western Pacific is characterized by its abundant seamounts and fracture zones with variable orientations. Wessel and Kroenke (1997) suggested that more than 8,800 seamounts exist in the Pacific, most of them in the western Pacific. Most of the seamounts in the western Pacific have been studied using dredged rock samples, shallow sediment cores, and geophysical data, and for only a very few have deep drill cores been taken for direct study. Of the latter, many were focused on a search for mineral resources, such as ferromanganese crusts, and the data are proprietary. The seamounts of the study area are divided into three sectors based on location: seamounts in the EMB (Ita Mai Tai and OSM3) linked to the Magellan Seamounts, the one in the OFZ trough (OSM4), and those linked to the Marshall Islands (OSM5-1, Seascan, and OSM6). The seismic data processing steps included gathering, constant velocity stacking, predictive deconvolution, amplitude recovery, and bandpass filtering
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