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Zenisu Ridge: a deep intraoceanic thrust related to subduction, off southwest Japan |
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| Authors: | Siegfried Lallemant Nicolas Chamot-Rooke Xavier Le Pichon Claude Rangin |
| Institution: | 1 Département de Géologie, Ecole Normale Supérieure (C.N.R.S. Unité Associée 215), rue Lhomond, 75231, Paris Cedex 05, France 2 Département de Géotectonique, Université Pierre et Marie Curie (C.N.R.S. Unité Associée 215), Tour 16, ler étage, 4 Place Jussieu, 75005, Paris, France 3 Also at the Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan |
| Abstract: | During the French-Japanese Kaiko project, Seabeam, seismic and submersible observations were made in the eastern part of the Nankai subduction zone, close to the area of collision between the Izu-Bonin island arc and the Japan margin. The most prominent feature is the Zenisu Ridge, an elongated relief of the Philippine Sea plate running parallel to the Trench. Magnetic anomalies indicate that the crust of the Zenisu Ridge is a part of the Shikoku oceanic basin formed in the Early Miocene, 23 Ma ago and presumably uplifted at a later stage. Structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics. Mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust, both segments being of equal magnitude (vertical offset about 1 to 1.5 km). Geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth. The overall structure of the surrounding area reveals a compressive deformation zone widening toward the east, the magnitude of the compressive deformation decreasing westward as well as southward of the Zenisu Ridge. |
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