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doi:10.14379/iodp.pr.351.2015

Introduction

The formation and destruction of lithospheric plates is a fundamentally important process leading to the creation of the most important Earth surface features and a major driver of planetary physical and chemical evolution. 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 (older than ~25 Ma) ocean lithosphere is gravitationally unstable with respect to the underlying asthenospheric mantle. Given the pull of subducting plates is widely regarded as the primary driver of the kinematics of plate tectonics, the initiation of subduction zones is a geological problem of the first order.

Ignorance of subduction and arc initiation contrasts with our relatively advanced level of understanding of oceanic crust creation from initial lithospheric rifting to development of a mid-ocean ridge. There are a number of locations on the surface of the Earth where the “rift to drift” sequence can be explored, such as the Afar-Red Sea region of northeastern Africa, the northernmost extension of the Mid-Atlantic Ridge in the Arctic Ocean, or a number of back-arc basins in the western Pacific such as the Lau, North Fiji, and Manus Basins. In contradistinction, locations where the earliest stages of subduction and arc initiation can be studied are comparatively few.

Subduction at a zone of plate convergence requires resisting forces (thrust fault friction, F_f, and elastic plate bending, F_el) be overcome by the sum of driving forces (plate tectonic forces, F_pt, and the slab’s negative buoyancy, F_s; Hall et al., 2003):

On the basis of the assumed ages of the current major subduction systems bordering the Pacific and along the Alpine-Himalayan Zone, McKenzie (1977) suggests that “ridges start easily, but trenches do not.” Endorsing this view, Mueller and Phillips (1991) point out the absence of any example in the geological record of the transformation of an Atlantic-type (passive) into an Andean-type (active) margin, despite the negative buoyancy of the oldest oceanic lithosphere adjacent to the continental lithosphere at passive margins. On the other hand, Gurnis et al. (2004) point out that about half of all active subduction zones initiated in the Cenozoic in a variety of tectonic settings including old fracture zones, transform faults, extinct spreading centers, and through polarity reversals behind active subduction zones; they conclude subduction initiation is commonplace where the resisting forces are overcome through the normal evolution of plate dynamics.

A number of authors have suggested the most likely sites for subduction initiation are transform faults, primarily as a consequence of a change in relative plate motions (e.g., Uyeda and Ben-Avraham, 1972; Casey and Dewey, 1984). A localized cause such as arrival of relatively buoyant slices of continental lithosphere or an oceanic plateau at a trench are examples of triggers for such relative motion changes (e.g., Wallace et al., 2009); the subduction polarity reversal along the former Vitiaz Trench between the Pacific and Australian plates with the arrival of the Ontong Java Plateau represents the latter case (Hall, 2002; Crawford et al., 2003). But the causes of far field effects, such as the change in Pacific plate motion at ~50 Ma representing the possible trigger for inception in the Izu-Bonin-Mariana (IBM) arc system, are less readily accounted for (Whittaker et al., 2007) and highlight the 3-D aspect of the overall problem of subduction initiation.

The processes accompanying subduction initiation are identified as part of Challenge 11 of the Science Plan of the International Ocean Discovery Program (IODP; available at www.iodp.org/science-plan-for-2013-2023). Initiation may occur in a variety of ways depending on relative plate strengths and ages and the plate tectonic driving mechanism (Gurnis et al., 2004; Leng et al., 2012). Among a number of proposed hypotheses, two general mechanisms seem particularly relevant to initiation of the IBM system, one of the largest, nominally intraoceanic subduction zones in the western Pacific. These general mechanisms are induced or spontaneous (Fig. F1) (Gurnis et al., 2004; Stern, 2004). Induced subduction initiation leading to self-sustaining descent of lithosphere into the mantle results from a convergence forced by external factors such as ridge push or slab pull along strike of a given system (e.g., Gurnis et al., 2004). Outboard stepping (e.g., incipient plate boundary south of India; Sykes, 1970) or polarity reversal (e.g., Solomon Islands) may develop. Stern (2004) resuscitated the hypothesis of differential buoyancy and 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 young, buoyant lithosphere. Foundering of the old lithosphere is predicted to induce asthenospheric upwelling in an extensional regime forming boninites and eventually fore-arc ophiolites (Stern and Bloomer, 1992). The initial record on the overriding plate should be clear: in the most basic terms, induced subduction likely results in strong compression and uplift potentially shedding debris into nearby basins, whereas spontaneous subduction initiation occurs without uplift or shedding of sediment but rather basement deepening prior to rifting, spreading, and formation of magmas in an extensional setting, potentially analogous to a number of ophiolites (e.g., Gurnis et al., 2004).

Following subduction initiation, evolving magmatism 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). The earliest magmatic stages of island arcs have recently been identified in the current fore arcs of the IBM arc system (Mariana: Reagan et al., 2010; Izu-Bonin: Ishizuka et al., 2011a) and the Central American arc (Buchs et al., 2010). In the case of the former, a suite of low-K, mid-ocean-ridge basalt (MORB)-like tholeiitic magmas were erupted for a period of ~4 My followed by boninites for a similar duration. The tholeiitic suite has been termed “fore-arc basalt” (FAB) by Reagan et al. (2010). Subsequent development of the IBM system apparently reverted to eruptive tholeiitic activity focused at a volcanic front, with possibly more K-rich magmas behind the volcanic front. In the case of Central America, Buchs et al. (2010) identified a MORB-like, low-K tholeiitic suite with minimal large ion lithophile element enrichments as the initial magmatic stage of the arc system in the Late Cretaceous; they termed this suite “proto-arc.” A normal arc tholeiitic suite was among the volcanic products subsequently erupted, but no boninites have been recognized.

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 relatively enriched island arcs are uniquely similar to continental crust in terms of lithophile trace element abundances (Taylor, 1967). Specific overlap in terms of absolute abundances of intermediate arc andesites (~52–63 wt% SiO₂) and the bulk intermediate silica content 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 pre-arc and/or 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 in addition to 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 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 there is 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. The nature of the sum product of prolonged magmatic activity has also been restrained by seismically determined crustal structure. It is possible to identify the oceanic basement on and into which the initial arc products following subduction initiation were emplaced. For most arc systems, the age of initiation is unknown and the basement is obscured and/or deeply buried. For the IBM system, a region has been identified in the Amami Sankaku Basin (ASB), where the pre-arc basement foundations of the nascent arc could be investigated (Fig. F2); the overlying sediment preserves evidence of the initiation and subsequent evolution of the arc, particularly through the first ~25 My of its history.

There were two primary targets for IODP Expedition 351: the oceanic basement and the overlying sedimentary sequence. Recovering oceanic basement samples allowed us to determine the age and petrological and geochemical characteristics of the pre-KPR crust, from which the geochemical composition of the mantle prior to IBM arc initiation can be determined. Overlying the basement is ~1460 m of sediment, in which evidence is preserved for the timing and processes associated with subduction initiation. Different responses of the overriding plate during the initial stages of subduction initiation are predicted to result from either induced (uplift) or spontaneous initiation of the subduction zone. Shallower in the sedimentary section, the explosive ash and pyroclastic fragmental records for at least the first 25 My of the developing KPR are preserved. This record diminishes in intensity and volume following the formation of the Shikoku Basin and eastward migration of the active arc volcanic front. 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 thick Paleogene record from the ASB is complementary and significant for resolving the early history of the arc.

The IBM system is an ideal location for tackling the global challenges of understanding subduction initiation and early 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 an immense strike length of intraoceanic subduction systems was initiated at ~50 Ma in the western and northern Pacific, likely accompanying global plate reorganization at this time (Sharp and Clague 2006; Seton et al., 2012). In addition to the IBM system, the Solomons-New Hebrides-Tonga-Kermadec and Aleutian arc systems appear also to have formed 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 initiation and evolution is a burgeoning field of study. The IBM system is a well-constrained, representative end-member of the diversity of arc systems where we can commence this effort.

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