Expedition 351 summary

The intraoceanic Izu-Bonin-Mariana (IBM) arc in the western Pacific has been intensively examined over the past few decades, and the outlines of its overall tectonic and magmatic history have been revealed. Arc inception occurred at ~52 Ma, concurrent with a major change in the motion of the Pacific plate. Rifting of the active volcanic axis took place at ~25 Ma, with accompanying seafloor spreading and eastward migration of the active volcanic front forming a volcanically inactive remnant arc (Kyushu-Palau Ridge; KPR). The Amami Sankaku Basin (ASB) flanks the northern KPR; the ASB seafloor has a simple structure comprising ~1.5 km of sediment overlying igneous oceanic crust. International Ocean Discovery Program Expedition 351 targeted the ASB, anticipating recovery of the sedimentary record of the earliest stages of arc inception and evolution of the northern IBM arc. Igneous basement samples would permit determination of the petrological, geochemical, and age characteristics of the pre-KPR crust in the region, from which the geochemical composition of the mantle prior to IBM arc inception and growth could be inferred. The expedition successfully accomplished its primary and most of its secondary objectives at Site U1438 in 4700 m water depth. Drilling penetrated 1461 m of sediment and 150 m of variably altered and veined aphyric to sparsely phyric tholeiitic basalt lava flows, which form the uppermost igneous oceanic basement.