Résumé
During the third leg (summer 1985) of the second part of the French-Japanese Kaïko program, ten dives at depuis reaching 6,000 m in the Japan and Kuril trenches were performed using the newly-launched submersible Nautile. The dives imply new constraints on the geodynamics ofsubduction zones due to direct observation of precisely selected sites. In the Japan and Kuril trenches, the most important resuit is the observation of huge slump scars on the inner wall of the trench. Those slumps lead to atypical active erosional morphology with vertical or even over hanging cliffs in poorly Consolidated material. At the junction between the Japan and Kuril trenches, dive observations confirm that the 20 km reentrant ; of the landward wall is prohably related to a subducted.volcano having produced this indentation. Several dives on the Kashima seamount demonstrated that this volçano has recently been cut into twoparts by normalfaults. Comparisonscan be made between the two parts of of the seamount in term lithology and paleontology. Deep sea clam colonies were observed from nearly 6,000 m up to 5,000 m on the inner wall of the trenches, in the Japan as well as Kuril trenches. It can be concluded thatthe whole margin is the site of deeply originatingfluid vehting, which is consistent with the over pressures recorded in several drilling sites on the Japan trench margin. Thefluids probably originate alongthe subduction thrust plane by dewateringof the subductingsediments and then migrate seaward andupward across the entire continentalm .
Durant la troisième campagne de la seconde partie (été 1985) du projet franco-japonaisKaïko, dix plongées utilisant le nouveau submersible Nautile apportèrent des renseignements importants sur je comportement géodynamique des marges des fosses du Japon et des Kouriles. Des observations directes ont montré la généralité des glissements en masse et l'érosion gravitaire sur les pentes continentales,tandis que l'étude des flancs du rentrant de près de 20 km, situé à lajonction dés fosses du Japon et des Kouriles, confirmait l'hypothèse de son origine par le passaged'un mont sous-marin déjà Subducté. Plusieurs plongées sur le Mont Kashima ont permis de daterle système de failles normales le décalant en 2 blocs et de prouver que la couverture calcaire étaitidentique de part et d'autre, de l'escarpement. Enfin la découverte de colonies biologiques liées àdes sorties de fluide en surpression montrent leur généralité et leur migration au travers des sédimentsde la marge continentale.