Timing of Tectonic and Magmatic Events in the Philippine Sea Plate since 50 Ma from High-Resolution Magnetostratigraphy of IODP Site U1438

The Izu-Bonin-Mariana (IBM) subduction system is a key natural laboratory providing fundamental insights into subduction dynamics and the evolution of associated upper plate magmatism. To investigate the processes associated with subduction initiation and the subsequent evolution of the Philippine Sea plate, International Ocean Discovery Program (IODP) Expedition 351 recovered a complete sedimentary sequence and the top of the underlying volcanic basement at Site U1438 located in a rear-arc position. This offers a unique opportunity to study for the first time and in extreme detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu-Palau volcanic arc within the IBM system. Here we report a magnetostratigraphy for Site U1438 based on ~60,000 remanence directions isolated from 1063 archive half-core sections and 429 discrete specimens. This identified 112 well-constrained magnetic reversals that may be correlated with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, this allows construction of a high-resolution age model for Site U1438 and the determination of changes in sedimentation rates during the evolution of the Kyushu-Palau arc. These age constraints show that following subduction initiation at ~52 Ma, diffuse volcanism in both the forearc and rear-arc preceded the initial emplacement of the Kyushu-Palau arc at 44.2 Ma, which then grew through four compositionally distinct eruptive phases until 29.2 Ma. Rollback of the Pacific slab then triggered rifting of the arc (29.2-24.3 Ma), leading to back-arc spreading in the Shikoku and Parece-Vela basins.