Abstract
Both pillow and sheet lava flows are distributed in the Mariana Trough back-arc basin axis at 17°N. These flows suggest different eruption rates, and they may indicate age variation in the lava. In this report we present observational results of sediments using a subbottom profiler (SBP) installed on the Shinkai 6500 during the YK08-08 Leg1 cruise. Observations from three dives (#1088 and #1089 on the western slope, #1090 on the eastern slope) with the SBP with visual observations and camera photos taken by the Shinkai 6500 were conducted to observe sediment on the lava flows. We estimate that the lava flow is new in dive #1088 because the SBP and visual observation cannot find sediment cover. In dive #1089 the sedimentary layer can be observed distinctly; moreover we can find a reflection within the sedimentary layer parallel to the basement. In dive #1090 we can find sediment cover by visual observation, but cannot determine sediment thickness by SBP observation. We consider the sediment coating in the dive #1090 survey area to be thinner than that in dive #1089. Therefore, it is possible to consider that these lava flows formed in the order #1089, #1090, and #1088.
