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doi:10.2204/iodp.sp.351.2013

Introduction

The formation and destruction of lithospheric plates is a fundamentally important process leading to the creation of the most important surface features and a major driver of the physical and chemical evolution of Earth. Subduction zones marking sites of plate destruction are unique to Earth among the terrestrial planets, but as yet, we do not have a good understanding of how they are initiated beyond the recognition that old (more than ~25 million years) ocean lithosphere is gravitationally unstable with respect to the underlying asthenospheric mantle. Ignorance of this aspect of the plate tectonic cycle contrasts with our relatively advanced level of understanding of oceanic crust creation from initial rifting to development of a mid-ocean ridge. The processes accompanying subduction zone initiation are identified as part of Challenge 11 of the Science Plan of IODP (available at www.iodp.org/science-plan-for-2013-2023/). Among a number of hypotheses that have been proposed, two general mechanisms seem particularly relevant to initiation of one of the largest, nominally intraoceanic subduction zones in the western Pacific, that of the Izu-Bonin-Mariana (IBM) system. These mechanisms are induced or spontaneous (Fig. F1) (Gurnis et al., 2004; Stern, 2004).

Induced subduction initiation results from continued convergence forced by subducted lithospheric plate (slab) pull along strike of a given system, despite local jamming of the subduction zone by buoyant continental or thickened oceanic lithosphere. Outboard stepping (e.g., incipient plate boundary south of India) or polarity reversal (e.g., Solomon Islands consequent to jamming of the Vitiaz Trench by the Ontong Java Plateau) may develop. Spontaneous subduction initiation occurs when a change in plate motion allows the gravitationally unstable lithosphere to founder along an existing plate boundary; Stern (2004) suggested the IBM system represents an example of spontaneous initiation wherein subsidence of relatively old Pacific lithosphere commenced along a system of transform faults/fracture zones adjacent to relatively buoyant lithosphere. Foundering of the old lithosphere is predicted to induce asthenospheric upwelling in an extensional regime forming boninites and eventual fore-arc ophiolites. The initial record on the overriding plate should be clear: induced subduction likely results in strong compression and uplift shedding debris into nearby basins, whereas spontaneous subduction commences with rifting, spreading, and formation of magmas such as highly depleted, low-K tholeiites and boninites.

Following subduction initiation, further igneous development is fundamental to the creation of an island arc, which in turn is essential to the formation and evolution of continental crust, at least through the Phanerozoic (Davidson and Arculus, 2006). Accretion of arc welts to continental margins with accompanying structural, metamorphic, and magmatic modifications are arguably the most important processes for continuing continental crust evolution. For many trace and minor elements, the continental crust is quantitatively important despite its volumetric insignificance on a planetary scale. Among all the terrestrial magma types, those of island arcs are uniquely similar to the continental crust in terms of trace element abundance characteristics (Taylor, 1967). Furthermore, specific overlap in terms of absolute abundances of the common intermediate (~52–63 wt% SiO₂) rock type of arcs (andesite) and the bulk intermediate silica composition of the continental crust, led Taylor (1967) to propose the “andesite” model for formation of this crustal type.

Testing models of subduction initiation and subsequent arc evolution requires identification and exploration of regions adjacent to an arc, where unequivocally pre-arc crust (basement) overlain by undisturbed arc-derived materials can be recovered. An essential boundary condition for understanding arc evolution and continental crust formation is to know the composition, structure, and age of the crust and mantle that existed before subduction began. This condition derives from the fact that besides slab-derived components (e.g., volatiles and fluid-mobile trace elements), the mantle wedge and overriding plate are important and in some cases volumetrically dominant contributors to the magmas that form the arc. Determining the various net contributions from the mantle wedge and subducted and overriding plates to the magmas forming the arc through time requires that we know the geochemical characteristics of these individual components.

The IBM system is globally important because we have clear evidence for the age (~52 Ma; Ishizuka et al., 2011a) and exact site (Kyushu-Palau Ridge [KPR]) of inception, duration of arc activity, and changes in magmatic composition through time through extensive drilling (ash and pyroclast records) and dredging. We also have intervals where back-arc magmatism accompanied arc activity but at other times overwhelmed that of an isolated volcanic front and vice versa, and less directly, the nature of the sum product of magmatic activity in the form of seismically determined crustal structure. It is possible to identify the oceanic basement on and in which the initial arc products following subduction inception were emplaced. For most arc systems, the age of inception is unknown and the basement is obscured and/or deeply buried. The majority of currently active intraoceanic arcs are located in the western Pacific. For the IBM system, a region in the Amami Sankaku Basin (ASB), where the foundations of the nascent arc can be investigated, has been identified (Fig. F2), and the overlying sediment will preserve evidence of the inception and subsequent evolution of the arc, particularly through the first ~25 m.y. of its history.

There are two primary targets for Expedition 351: (1) the basement and (2) the overlying sedimentary sequence. Recovering oceanic basement samples will allow us to determine the petrological, geochemical, and age characteristics of the preKPR crust in the region, from which we can determine the geochemical composition of the mantle prior to IBM arc inception and growth. The Amami Sankaku basement is likely to be the easternmost fragment of Neotethys (Early Cretaceous or older) crust preserved in the oceans. Overlying the basement is 1300 m of sediment, in which evidence will be preserved for early uplift (compression and unconformities; erosion) or subsidence (basin deepening) associated with subduction initiation. These different responses of the overriding plate during the initial stages of subduction initiation are predicted to result from either forced convergence (uplift) or spontaneous nucleation of the subduction zone. Above the subduction initiation horizon, the explosive ash and pyroclastic fragmental records for at least the first 25 m.y. of the developing arc will be preserved. This record is likely to diminish in intensity and volume following the formation of the Shikoku Basin and eastward migration of the active arc volcanic front away from the KPR. To date, the geochemical data available for these kinds of materials recovered from the fore-arc regions of the IBM system are concentrated in the Neogene (e.g., Straub, 2003), so the record from the ASB will be complementary and significant for resolving the Paleogene history of the arc in greater detail. In addition, the Neogene volcanic history of the adjacent Ryukyu arc will continue to be preserved in the sediment of the ASB. Depending on the age of the basement, there is a possibility that Cretaceous ocean anoxic events (OAEs) (e.g., OAE 3 at 85.8 Ma, OAE 2 at 93–94 Ma, and possibly OAEs 1a–1d between 121 and 98 Ma) could be recovered from the older sediment, providing an important link between the classic European and Pacific locations (Schlanger and Jenkyns, 1976).

The IBM system is an ideal location for tackling the global challenges of subduction inception and evolution of an arc given the wealth of scientific data collected for the system and the potential preservation of critical sites, where by drilling alone, critical records of these fundamental Earth processes can be recovered. We know that a vast number of intraoceanic subduction systems were initiated at ~50 Ma in the western and northern Pacific, likely accompanying global plate reorganization at this time. In addition to the IBM, the Solomons-New Hebrides-Tonga-Kermadec and Aleutians arc systems appear also to have commenced at this time. Although the current geochemical characteristics of the Tonga arc are similar to those of IBM, others, such as the Aleutians, seem persistently geochemically and geophysically distinct. Given this diversity, we know the investigation of arc inception and evolution is a burgeoning field of study. The IBM is a well-constrained, representative end-member of the diversity of arc systems where we can commence this effort.

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