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Abstract

Cenozoic history of the Daito Ridge and two associated abyssal basins is preliminarily discussed, based on a shipboard near-trace monitor record section from a multichannel seismic reflection survey across the region and on dredge-sample data from the ridge area.

On the Daito Ridge, middle Eocene Nummulites-bearing deposits, shown by a dipping high-frequency pattern on the profile, accumulated in a shallow sea environment on a basement that included the Daito Metamorphic Rocks. Volcanic activity around the time of deposition is suggested by dredged rocks from the Daito north peak. The Daito Ridge has undergone a change in environmental conditions from neritic to pelagic and has subsided to a depth of 1.5 km below sea level from the middle Eocene through the Quaternary, resulting in the deposition of a transparent veneer on the main peak.

The acoustic sequence in the Minami-Daito Basin is divided into Units A through E, in descending order. Of them, Units C and D are partly correlative with the middle Eocene deposits on the ridge and include the earlier deposits. They represent archipelagic apron sedimentation on the slope and basin boundary area when the top of the ridge was extensively uplifted. Possibly related to the submergence of the Daito Ridge, a new sedimentary cycle characterized by turbiditic sedimentation (Unit B) started after the cease of apron sedimentation, and was followed by pelagic deposition (Unit A). A similar feature is observed in the Kita-Daito Basin and in the valley on the Daito Ridge.

Two types of post-opening intrusive activity are observed in the abyssal basins. One is indicated by Unit E, characterized by stratified layers and diffractions and having a smooth upper surface. It represents the anomalously smooth oceanic crust of the basin and resulted from repeated intrusion of sills after or during the deposition of Units C and D. Another is indicated by the acoustic basement with rugged upper surface and dense diffractions in the Kita-Daito Basin. It represents the rugged oceanic crust of the basin, which was likely formed by extensive intrusion of dikes after or during the deposition of Unit B.

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