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      Site U1438: Expedition 351

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          Understanding how subduction zones are initiated and continental crust forms in intraoceanic arcs requires knowledge of the inception and evolution of a representative intraoceanic arc, such as the Izu-Bonin-Mariana (IBM) arc system. An intraoceanic setting is mandatory to avoid the obscuring geochemical, geophysical, and structural veils of preexisting continental crust and the practicality of recovery depth of basement cores by drilling. The IBM satisfies these criteria. Understanding the evolution of the IBM system, particularly in the more recent half of its 50 My history, has improved considerably over the past three decades, not the least from studies of ash and other pyroclastic material recovered by ocean drilling. However, we have poorer records for the nature of arc development in the first half of the system’s history and very limited understanding of how this (or any other) arc was initiated. IODP Expedition 351 targeted, in particular, evidence for the earliest evolution of the IBM system following inception. Site U1438 is located in the Amami Sankaku Basin (ASB), <100 km west of the northern portion of the Kyushu-Palau Ridge (KPR). The KPR is a remnant arc of the intraoceanic IBM arc in the western Pacific on the northern part of the Philippine Sea plate. Site U1438 is located at the intersection of two seismic reflection profiles obtained by the Japan Oil, Gas, and Metals National Corporation. Site U1438 was chosen as an ideal location because it fulfilled the following criteria: (1) there are remnants of drillable oceanic crust that existed in the region immediately before arc inception, (2) the initial IBM magmatic record is preserved and includes geological evidence of the tectonic setting at subduction initiation, (3) temporal variations of magmatism in the rear IBM arc are preserved in a sequence of volcaniclastic sediments and tephra, and (4) the effects of along-strike variation are minimized or well understood, which highlights the temporal evolution of the IBM arc crust. There were two primary targets during Expedition 351: the basement and the overlying sedimentary sequence of Site U1438. Recovering oceanic basement samples allows us to determine the petrological, geochemical, and age characteristics of the pre-KPR (IBM) crust and mantle in the region prior to subduction initiation in the middle Eocene. Overlying sediments preserve a volcanic and geologic record spanning the pre-arc, arc initiation, and remnant arc stages of the IBM. These sediments, combined with basement samples, identify and help model the subduction initiation process and initial arc formation. They also help constrain the geophysical properties of the ASB. Sediment samples permit determination of the Paleogene compositional evolution of the IBM arc and the evolution of the Ryukyu-Kyushu arc, located west of the ASB, through an extensive ash record. The sedimentary record further aids in environmental reconstructions in the western Pacific, including the onset and persistence of the East Asian Monsoon.

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                Proceedings of the International Ocean Discovery Program
                International Ocean Discovery Program
                25 August 2015

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