Timing of Tectonic and Magmatic Events in the Philippine Sea Plate Since 50 Ma From High‐Resolution Magnetostratigraphy of IODP Site U1438

Abstract

The Philippine Sea Plate in the West Pacific is a unique natural laboratory to study subduction dynamics and the evolution of upper plate magmatism following subduction initiation. To investigate these processes, International Ocean Discovery Program Expedition 351 recovered at Site U1438, located in a rear-arc position, a complete sedimentary sequence from recent age to the early Eocene and the top of the underlying volcanic basement. The recovered cores offered the opportunity to study for the first time and in unprecedented detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu-Palau volcanic arc following the inception of the Izu-Bonin-Mariana subduction. Here, we report a magnetostratigraphy for Site U1438 based on ∼60,000 remanence directions isolated from 1,063 archive half core sections and 429 discrete specimens. We identified 142 magnetic reversals and correlated 115 of them with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, our magnetostratigraphy allowed construction of a high-resolution age model for Site U1438 and the determination of changes in sedimentation rates. We show that following subduction initiation at 52–50 Ma and the emplacement of basalts in the rear-arc at 48.7 Ma, a diffuse volcanism in the rear-arc (48.4–45.6 Ma) preceded the true emplacement of the Kyushu-Palau arc at 40.2 Ma, which then grew through four compositionally distinct eruptive phases until 28.8 Ma. Subsequent rollback of the Pacific slab triggered rifting of the arc (28.8–24.3 Ma) and ultimately back-arc spreading in the Shikoku and Parece Vela basins.