Geologic development of Daito Ridge
In 1983 and 1984, narrow multi-beam bathymetric, seismic reflection, gravity and a magnetic surveys of Daito Ridge, Oki-Daito Ridge and adjacent areas were conducted by the Hydrographic Department, MSA Japan with its survey vessel"Takuyo". The area surveyed lies north-west of the Philippine Sea,and includes three islands:Kita-Daito Zima, Minami-Daito Zima and Oki Daito Zima. In the survey the spacing of the north-south inclining tracks were 5 to 10 nautical miles. Figure 3 and 4 show the results of the topographic and geological survey. The ridges and basins, extending E-W to SE-NW lie alternately in the surveyed area. From north to south, they are Kita-Daito Basin, Daito Ridge, Minami-Daito Basin and Oki-Daito Ridge. The topographic trend of Daito Ridge is ESE on its western half, whereas the eastern half of the ridge extends ENE. The northern slope of the ridge shows horst and graben structures, probably formed in the Eocene. The southern flank of the ridge slopes down to Minam-Daito Basin with a relatively simple and steep gradient. Dredge hauls were attempted to obtain samples from outcrops of the ridge, but only manganese nodules were retrieved, Previous investigations of Daito Ridge have described green schists, serpentinites, andesites and other rocks. The Minam-Daito Basin contains a sediment-filled flat bottom and a group of seamounts. The seamount group discovered by the survey was tentatively named ”the Tyozyu Seamount Group". The basin is characterized by its negative free-air gravity anomaly of less than-50 mgal and geological structures with a northward inclination. The negative gravity anomaly of Minami-Daito Basin, probably due to its thick crust, shows remarkable contrast to the slightly positive free-air gravity anomaly of Kita-Daito Basin, where the sediment thickness and water depth are roughly the same as that of the Minami-Daito Basin. The seismic reflection profiles indicate that Minami-Daito Basin has been pluging to the north since the Eocene.' while the Tyozyu Seamount Group was formed before the Eocene. The features described above suggest that Daito Ridge may be a paleoisland arc, Kita-Daito Basin may be a marginal sea and Minami- Daito Basin may be a fore-arc basin.
| Main Authors: | , , |
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1986
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| Online Access: | http://hdl.handle.net/1834/16163 |
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