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During the JAMSTEC R/V Yokosuka YK08-08 Leg-1 cruise, the submersible Shinkai 6500 survey of the central Mariana Trough at 17°N was carried out to study a formation process of oceanic lithosphere in the active back-arc spreading center. Three dives were devoted to the median valley in the 17°N segment center. Visual observation of geology, lava flow morphology, and fault structure provides ground references for the backscattering imagery obtained from the deep-towed side-scan sonar survey conducted in 2003 to discuss volcanotectonic features. Smooth surfaces of side-scan imagery with high backscattering intensities are found to be jumbled-wrinkled, folded sheet, or lobate lava flows. The visual observation proves that the 17°N segment is covered with sheet lava flows of high effusion rates. Quantifiable degrees of sedimentation superposed on the lavas suggest relative age differences of formation. Sheet lava flows in the axial portion of the median valley are considered to be the youngest among the three dive observations. Sedimentation more or less gradually increases toward the western margin of the median valley. The observed eastern margin of the median valley is covered with sedimentary layers of ∼0.5-2 m thickness, thus the area is regarded as the oldest in our dive observations. North-south trending tectonic structures (i.e. faults, fissures), oblique to the median valley direction, could be younger than valley-parallel NNW-SSE trending structures because they show relatively little accumulation of sediment, and they cut the other structures in some places. Basaltic rock samples were collected at a total of 22 stations where several types of lava flow morphology are shown. Spatial variations in the rock faces, suggesting variations of chemical characteristics and rock ages, were found (phyric/aphyric, manganese coating, vesiculation). Hemipelagic sediment samples including nanofossils were also collected at 6 stations.

A submersible study of the Mariana Trough back-arc spreading center at 17.DEG.N