Abstract
During the Kaiko-Nankai cruise, several dives were devoted to the exploration of a linear 2000m deep ridge (yukie ridge) located at the trench-slope break of the Eastern Nankai accretionary wedge, off southwest Japan. This ridge is locally one of the two most active venting sites discovery across the prism. Based on the interpretation of seismic lines together with in-situ structural observations, whe show that this ridge is basically an asymmetrical anticline related to the motion along a prominent backthrust (landward verging thurst fault). This backthrust evolves north-eastward to a 5 km wide deformation zone with dominant landward vergence. This backthrust system vanished westward as Zenizu ridge vanishes too and is thus present only opposite the Zenisu intra-oceanic deformed zone. We thus propose to related the development of the backthrust to the presence of an older compressional oceanic ridge buried below the prism. Surprisingly, no significant fluid venting is associated with the outcrop of the main backthrust whereas spectacular seeps (carbonate sementation and biloogical activity) were observed in a strike-slip zone crossing the Yukie anticlinal ridge but none on the northern slope.