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doi:10.14379/iodp.sp.352.2013

Introduction

Expedition 352 was designed to decode the earliest evolution of arc crust at the Izu-Bonin-Mariana (IBM) arc-trench system. The expedition was based on achieving a full volcanic stratigraphy for the IBM fore arc that will provide a basis for determining the petrogenetic evolution of the magmas that immediately postdate subduction initiation. This in turn will enable us to test hypotheses for the geodynamics of subduction initiation.

The lavas drilled will be the first products of the subduction factory, the crust produced when subduction begins. Crustal production rates at this time are much (perhaps an order of magnitude) greater than those estimated for mature arcs. The mode of crustal production during the initial stages of arc development appears to be the result of extension and seafloor spreading accompanying lithospheric collapse and asthenospheric upwelling (e.g., Stern, 2004) and is quite different from the focused magmatism that characterizes mature magmatic arcs. The early, voluminous volcanism associated with subduction initiation is also responsible for many, perhaps most, ophiolites, themselves key indicators of Earth’s changing tectonics and the magmatic, hydrothermal, and tectonic processes that accompany seafloor spreading. The IBM fore arc is an excellent, probably the best, modern analog for supra-subduction zone (SSZ) ophiolites and is the ideal place to probe the structure of infant arc crust. The area has already been studied by drilling, including the highly successful Deep Sea Drilling Project (DSDP) Sites 458 and 459 in the Mariana fore arc (e.g., Natland and Tarney, 1982) and Ocean Drilling Program (ODP) Site 786 in the Izu-Bonin fore arc (e.g., Pearce et al., 1992). However, these sites were drilled as relatively minor parts of drilling legs; there has been no dedicated fore-arc drilling leg, and hence there is no full lava stratigraphy of the detail needed to interpret subduction initiation and make the ophiolite link.

Expedition 352 provides an opportunity to investigate oceanic crustal accretion following the initiation of subduction, the proposed setting of SSZ ophiolites, the most common ophiolite type (Pearce, 2003). The origin of SSZ ophiolites is still debated, however. The original Miyashiro (1973) contention, that ophiolites originated as the roots of island arc volcanoes, was contested in print but never tested. The subduction initiation/infant arc model of Stern and Bloomer (1992) provides a way for near-trench seafloor spreading to produce a SSZ ophiolite, frozen in place to become fore-arc lithosphere, ready to be obducted when buoyant crust enters the trench. The discovery of voluminous basalts in the IBM fore arc overlain by boninites (Reagan et al., 2010) is clear support for the fore-arc origin of SSZ ophiolites, but these basalts have not yet been investigated by IODP. The IBM fore-arc stratigraphy makes it a particularly good place for IODP to realize the important Initial Science Plan objective: “…the validity of the ophiolite model, will only be addressed by direct, in situ sampling of the lower oceanic crust and Moho by drilling. A high priority is to recover intact and tectonically undisrupted sections…” (Bickle et al., 2011). The chosen sites will provide a vertical observatory, enabling study of subduction initiation and comparison of subduction initiation and ophiolite lava sections.

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