Résumé
In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible "Nautile". The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2-3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.