Résumé
The results of the 1989 Kaiko-Nankai diving cruise to the eastern Nankai accretionary wedge show that a narrow corridor on the top of a 2000 m deep ridge is characterized by concentrated surface manifestations of fluid seepage and chaotic outcrop orientations, suggesting active faulting along a right-lateral shear zone. Modern biological indicators of fluid venting include bivalve vesicomyid species and vestimentiferan tube worms. Ancient manifestations of fluid venting consist of similar vesicomyid species found in 20'and 150 ka concretions. This suggests relatively constant biotypes. Carbonate deposits including fossil cemented shells were observed near active sites, either as "paving stones" (diffuse seepage and cementation) cut by channels or as pinnacles (local seepage and cementation). Despite its proximity (less than 15 km), a nearby 3800 m deep site, where substantial fluid discharge occurs, seems to be completely separate from the 2000 m deep ridge site. The 3800 m deep site differs from its shallower counterpart in terms of its seafloor manifestations and the composition of its fluids. The sediment at the top of the ridge ranges in age around 500 + 300 ka, which attests that the transition from a depositional to an erosional stage was younger than 800 ka but older than 200 Ka. This transition can be due to either the incorporation of this unit from the trench fill into the accretionary wedge front or the uplift of a slope basin. The first hypothesis leads to a 2.5 cm/yr minimum progradation rate associated with a 2.5 mm/yr minimum uplift rate from the paleotrench to the present ridge. The second hypothesis implies a recent uplift of a "piggy-back" basin at a minimum rate of 1 mm/yr.