Kasuga, S., Y. Kato, S. Kimura, K. Okino et al.

Characteristics of arc-trench systems and back-arc basins in the southern waters of Japan - Outline of the geophysical survey by the Hydrographic Department of Japan -

Rept. Hydrogrp. Res., 28, 19-53, 1992 (Japanese with English abbstract)


The hydrographic department of Japan has been conducted detailed bathymetric, geological and geophysical survey under the Continental Shelf Surveys Project since 1983. The purpose of this project is to collect basic information on the exclusive economic zone and the continental shelf of Japan and to prepare bathymetric, geological structure, geomagnetic anomaly and gravity anomaly charts at a scale of 1 to 500,000.Bathymetric survey by a multi-narrow beam echo-sounder (SeaBeam), seismic reflection profiling, magnetic and gravity measurements were carried out in the survey boxes each covering a 3 latitude by 2.5 longitude. Three boxes of them have been surveyed each year. The average spacing between survey track lines was 5 or 6 nautical miles and areas of special interest were covered by closer track lines. Twenty-three survey boxes in the southern waters of Japan were covered by the end of Fiscal Year 1990. This area covers from west to east, the Okinawa Trough, Ryukyu arc-trench system, Daito Ridge Group, Kyushyu-Palau ridge, Shikoku Basin, Izu-Ogasawara arc-trench system and Ogasawara plateau.These systematic surveys have prepared homogeneous and high quality and quantity data in the northern Philippine Sea bordered by the two arc-trench systems. The survey results were compiled in the detailed bathymetric, total magnetic intensity anomaly, Free-air and Bouguer gravity anomaly charts at a scale of 1 to 6,000,000. This paper summarizes the geological and geophysical features revealed mainly by the survey under the Continental Shelf Surveys Project in the southern waters of Japan.Some of the distinct features are:
1) prominent magnetic anomalies along the Quaternary volcanic front of the Ryukyu arc and Izu-Ogasawara arc, and lineated magnetic anomalies characterized by en echelon pattern in the back arc region of the these arcs,
2) Low Bouguer gravity anomalies on the Daito Ridge Group and the Kyusyu-Palau Ridge , indicating thick crustal structures below these ridges compared to those below the surrounding basins,
3) Steep scarps and several dipole-type magnetic anomalies just below the scarps along the eastern edge of the Kyushu-Palau Ridge, probably caused by the rifting of the arc with some volcanic intrusion prior to the opening of the Shikoku Basin.
4) Complicated pattern of lineated magnetic anomalies and well-developed small scale minor ridges and troughs showing three evolutionary stages of sea-floor spreading of the Shikoku Basin,
5) Uplifting and deformation, magnetic anomalies and low Bouguer anomaly zones on the landward slope of the Nanseisyoto Trench and Izu-Ogasawara Trench, caused by the subduction of the western extension of the Amami Plateau and Daito Ridge under the Ryukyu arc, and the western extension of the Ogasawara Plateau under the Izu-Ogasawara arc respectively.

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