Subduction initiation as recorded in the Izu-Bonin-Mariana forearc

Abstract

IODP Expedition 352 drilled four sites in the Bonin outer forearc, providing a detailed record of lava eruption and dike emplacement during a short-lived (<2 Ma) seafloor spreading event that took place immediately after the initiation of Pacific Plate subduction at c. 52.5 Ma. This spreading produced up to 120 km of oceanic lithosphere with a structure analogous to that of a supra-subduction zone ophiolite. Two groups of sites were drilled: Sites U1440 and U1441 are oldest and nearest to the trench and principally recovered forearc basalt (FAB); Sites U1439 and U1442 are younger and farther from the trench and recovered boninites. Holes 1440B and 1439C are both rooted in dike complexes, confirming a likely seafloor spreading origin for most of the drilled crust. Geochemical evidence points to the shoaling of a mantle plume beneath a Mesozoic ridge and basin terrane as a likely trigger for the Pacific plate to begin to subduct beneath that terrane. The timing and nature of the collision between the Indian and Asian continents allows it also to have played a role in triggering this event. Seafloor spreading began due to extension related to slab rollback immediately following the start of subduction. Mantle entering this new extensional régime was highly depleted both by melting and by incorporation of ancient, depleted mantle associated with the plume. Initially, slab-derived aqueous fluids made negligible to small contributions to the mantle source, which then underwent decompression melting producing oceanic crust with a FAB composition that resembles the most incompatible element-depleted basalts in present-day seafloor spreading settings. The same, or a renewed, ridge system produced low-Si boninite (LSB) magmas once hydrous melts from the subducting Pacific oceanic crust fluxed the now highly depleted, but still hot, mantle. High-Si boninite (HSB) volcanism began at c. 51.3 Ma, about the time when seafloor spreading slowed and more focused volcanism began in response to increased subduction relative to slab rollback. The subsequent history of the drilled area was marked by several kilometers of uplift and erosion, perhaps related to serpentinization of its depleted mantle foundation, followed by extension to produce small sedimentary basins.