IODP

doi:10.14379/iodp.sp.352.2013

Abstract

During International Ocean Discovery Program Expedition 352, we intend to drill a section through the volcanic stratigraphy of the outer fore arc of the Izu-Bonin-Mariana system in order to trace the processes of magmatism, tectonics, and crustal accretion associated with subduction initiation. This study in turn has implications for understanding the origin of the many ophiolites that are now believed to form in this setting, and the expedition provides a good opportunity to test this supra-subduction zone ophiolite model. We intend two primary sites in the Bonin fore arc (Sites BON-1A and BON-2A), which form an offset-drilling pair that together should penetrate the full ~1.25 ± 0.25 km lava section. The sites have been surveyed and surface-sampled by several diving and dredging cruises. Studies of the recovered samples have established a stratigraphy in which peridotites, gabbros, and sheeted dikes are overlain by “fore-arc basalt” (FAB) and then in turn by boninites. Drilling Sites BON-1A and BON-2A will contribute to our understanding of intra-oceanic convergent plate margins by providing (1) a high-fidelity record of magmatic evolution during subduction initiation; (2) a test of the hypothesis that FAB tholeiites lie beneath boninites; (3) a record of the chemical gradients within these units and across their transitions; (4) information on how mantle melting processes evolve during subduction initiation from early decompression melting of fertile asthenosphere to late flux melting of depleted mantle, providing key empirical constraints for realistic subduction initiation geodynamic models; and (5) a test of the hypothesis that fore-arc lithosphere created during subduction initiation is the birthplace of supra-subduction zone ophiolites. Deep Sea Drilling Project Site 459 in the Mariana fore arc provides a well-surveyed alternate site of similar age, stratigraphy, and setting that should allow coring of a similar lava sequence to Site BON-1A.