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• Scientific Objectives

International Ocean Drilling Programme (IODP³) Expedition 502 plans to explore the nature of the acoustic basement in the outer rise area of the NW Pacific subduction system, where layer-1 pelagic sediment of the subducting old (120–130 Ma) Pacific Plate is exceptionally thin. The hypothesis is that the acoustically thin sediment cover can be attributed to basalt sill intrusions or sheet lava extrusions in and on the pelagic sediment package by basalts fed by petit-spot magmatism. This hypothesis will be tested to see if petit-spot magmatism at the outer rise is more widely distributed than previously thought. The existence of widely-distributed petit-spots would strongly impact the subduction system, including rupture nucleation and slip propagation of plate boundary megathrust earthquakes, as well as effect changes to the geochemical cycle of arc magmatism and the global volatile cycle due to the differing materials associated with the subducted oceanic plate. Testing this hypothesis will shed light on the impacts of subduction inputs and help to determine the global role of petit-spot magmatism, both of which are important elements of the scientific objectives of the International Ocean Drilling Programme (IODP³).

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• Science Plan and Operation overview

At Site TPC-01A, wireline logging will be conducted first, using tools equipped with multiple sensors to collect geological data and to identify the boundary between sediments and volcanic rocks as well as the detailed sub-seafloor structure. Following this, coring operations will recover continuous 9.5-meter-long samples from the sediments down to the volcanic rocks at the target depth. These cores will then be observed and analyzed on board by science team.