Structure and evolution of the backstop in the eastern Nankai Trough area (Japan): Implications for the soon-to-come Tokai earthquake |
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Authors: | X Le Pichon,S. Wmant,,H. Tokuyama, F. Thoué , P. Huchon, P. Henry |
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Affiliation: | Laboratoire de Géologie (CNRS URA 1316), Ecole Norrnale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France;Ocean Research Institute, Tokyo University, 1-15-1 Minami Dai, Nakano Ku, Tokyo 164, Japan;Laboratoire de Pétrologie et Tectonique —URA 726, Bat. 402, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France |
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Abstract: | Abstract We present data showing that the intra-oceanic shortening now occurring south of the eastern Nankai Trough that has produced the Zenisu Ridge has also been responsible for the formation of a previous ridge now buried below the continental margin. This ridge, that we refer to as Paleo-Zenisu, is presently adjacent to the backstop and its location coincides with the outer limit of the seismogenic decollement. The subduction of the paleo-Zenisu ridge below the wedge has led to its complete reorganization and has given its identity to the Great Tokai earthquake rupture zone. The formation of paleo-Zenisu and its consequent subduction has induced the tilting of the backstop toward the northwest since ca 2 Ma. This model suggests that the backstop and possibly the wedge are dextrally sheared because they are extruded southwestward in relation to the collision of the Izu-Bonin Ridge with Japan. We use the finite motion from Zenisu to paleo-Zenisu to derive both the subduction vectors along the Nankai Trough and the shortening vectors within Zenisu-Izu. The amount of shortening absorbed within Zenisu-Izu increases toward the northeast. The corresponding subduction vectors of the Zenisu platelet below the wedge decrease accordingly to the northeast from 50 to less than 20 mm/year and the Zenisu body rotates clockwise with a pole near 36° North, 139° East. This might explain the apparent longer repetition time of great earthquakes in the Tokai area. On the other hand, the 25-35 mm/year obtained for the rate of shortening along the Zenisu thrust indicates a high seismic potential there. |
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Keywords: | eastern Nankai Trough area |
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