Oceanic lithosphere created at oceanic ridges is returned to Earth’s interior at sutures marked by deep-sea trenches in a process called subduction. The formation and destruction of lithospheric plates is a fundamentally important process leading to the creation of most important surface features and a major driver of the physical and chemical evolution of Earth. Although we have a relatively good understanding of the processes accompanying the formation of lithosphere in the so-called “rift-to-drift” cycle, we have minimal direct evidence of how subduction is initiated. This essential component of the global plate tectonic cycle is targeted as one of the major challenges of the new science plan for the International Ocean Discovery Program (IODP).

As an initial step in addressing this challenge, during IODP Expedition 351 we will core and log a prime site in the Amami Sankaku Basin, located <100 km west of the northern portion of the Kyushu-Palau Ridge (KPR), a remnant arc of the intraoceanic Izu-Bonin-Mariana (IBM) arc in the western Pacific on the northern part of the Philippine Sea plate. Over the past several decades, multidisciplinary efforts, including deep-sea drilling, have been made to understand the crustal characteristics and structure, tectonic and temporal evolution, and magma origins of the IBM system since its inception 52 m.y. ago. Subsequently, Site IBM-1 has been identified in the Amami Sankaku Basin where samples of the pre-arc basement can be recovered. We infer a Cretaceous to Paleogene age for the basement, overlain by a pre-arc-inception sediment section and an upper sedimentary sequence recording regional tectonic events accompanying formation of the KPR as a proto-arc at 52 Ma. Additional sedimentary cover recorded the Paleogene volcaniclastic and pyroclastic evolution of the Izu-Bonin arc with diminishing completeness, as the KPR was abandoned as a remnant arc at ~25 Ma, accompanying formation of the Shikoku back-arc basin.

The objectives of Expedition 351 involve the study of both the sediment (oceanic crust Layer 1) and igneous basement (Layer 2) and were established to address a number of fundamental aims. The primary objectives include

  1. Determining the nature of the preexisting crust and mantle prior to subduction onset in the middle Eocene,

  2. Identifying and modeling the subduction initiation process and initial arc crust formation,

  3. Determining the Paleogene compositional evolution of the IBM arc, and

  4. Establishing the geophysical properties of the Amami Sankaku Basin.

We also have a number of secondary objectives based on recovering sedimentary records of (1) early Tertiary and possibly Late Cretaceous paleooceanographic conditions in the eastern Tethys–western Pacific, (2) onset and persistence of the East Asia Monsoon and other climate-modulated terrestrial inputs, and (3) an ash record of the evolution of the Ryukyu-Kyushu arc, located west of the Amami Sankaku Basin.

Expedition 351 is conceptually straightforward, targeting a single site involving penetration of a thick sedimentary section overlying oceanic crust of normal thickness. Nevertheless, the water depth (4720 m), sediment thickness (1300 m), and consequent depth into basement (~150 m) impose technical challenges. Relatively short transit times to and from the Amami Sankaku Basin will maximize the time available for scientific drilling and logging operations.