Sedimentation and structure of the Izu-Ogasawara (Bonin) Trench off Tokyo: new lights on the results of a diving campaign with the bathyscape “Archimede”

Direct observations from submersible prove that in the explored areas, the Izu-Ogasawara Trench is devoid of any significant sediment cover. The effects of the tensional forces affecting the oceanic crust have been observed in the outer wall of this trench, especially near its foot between 8200 and 8500 m depth. They lead to a staircase morphology (with rock exposures) corresponding probably with vertical faulting parallel to the trend of the trench axis. The inner wall, at about 5000 m depth, shows large rock exposures that have been sampled. These rocks are volcanic lavas of the tholeiitic series (icelandites). The staircase morphology observed in this part of the overriding plate seems to prove the occurrence of distensive forces induced probably by the underthrusting of the oceanic crust. No indication of compressive stress has been registered in the whole investigated area.