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CITATION Arculus, R.J., M. Gurnis, O. Ishizuka, M.K. Reagan, J.A. Pearce, and R. Sutherland. 2019. How to cre- ate new subduction zones: A global perspective. Oceanography 32(1):160–174, https://doi.org/​ 10.5670/oceanog.2019.140.

DOI https://doi.org/10.5670/oceanog.2019.140

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HOW TO CREATE NEW SUBDUCTION ZONES A Global Perspective

By Richard J. Arculus, Michael Gurnis, Osamu Ishizuka, Mark K. Reagan, Julian A. Pearce, and Rupert Sutherland

Sand and gravel shed by turbidite flows from volcanoes of the nascent Izu-Bonin-Mariana island arc, recovered at Site U1438 of IODP Expedition 351, in the Amami- Sankaku Basin. Photo credit: Richard Arculus

160 Oceanography | Vol.32, No.1 ABSTRACT. The association of deep-sea trenches—steeply angled, planar zones where INTRODUCTION earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive In the late nineteenth century, surveying volcanoes had been recognized for 90 years prior to the development of plate tectonic expeditions discovered that there were theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading cen- great depths in the ocean. Their mea- ters and recycled into the mantle at subduction zones, where down-going lithospheric surements included spot soundings by plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is HMS Challenger in 1873 in the Puerto a challenge because evidence of the processes involved is typically destroyed or buried Rico Trench (7,087 m) and in 1875 in by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean the Mariana Trench (8,184 m). The deep- Discovery Program (IODP) specifically targeted these processes with three back-to- est sounding at that time (8,500 m) was back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and reported by Captain George Belknap one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were on USS Tuscarora; it marks the first rec- initiated ~52 million years ago, coincident with a proposed major change of Pacific ognition of a linear deep, the Kurile- plate motion. These expeditions explored the tectonism preceding and accompany- Kamchatka Trench. By the 1930s and ing subduction initiation and the characteristics of the earliest crust-forming magma- 1940s, it was realized that trenches are tism. Lack of compressive uplift in the overriding plate combined with voluminous associated with all of the volcanic arcs basaltic seafloor magmatism in an extensional environment indicates a large com- encircling the Pacific, plus those of ponent of spontaneous subduction initiation was involved for the IBM. Conversely, Indonesia, the Mediterranean, and the a complex range of far-field uplift and depression accompanied the birth of the TK Atlantic (Antilles and South Sandwich; system, indicative of a more distal forcing of subduction initiation. Future scientific Figure 1). The association of these volca- ocean drilling is needed to target the three-dimensional aspects of these processes nic island chains with ocean deeps, and at new converging margins. their positions parallel to one another, had

FIGURE 1. Global distribution of island arcs based on the Global Topography base (https://topex.ucsd.edu/marine_topo/). Main panel is centered on the Pacific, with inset showing the Mediterranean and the Middle East.

Oceanography | March 2019 161 previously attracted attention. For exam- Earth processes is considerable. It has even destroyed through tectonic erosion, ple, Sollas (1903) demonstrated that an been shown, for example, that most of the burial, thermal and volcanic overprint- arc of islands could represent the outcrop driving force behind global plate tecton- ing, and other processes at convergent trace of a planar fault at Earth’s spherical ics derives from subducted plate (“slab”) plate boundaries (e.g., Stern, 2004). surface. Continentward-dipping zones of pull rather than “ridge push” (Forsyth The surge in ocean floor exploration earthquake foci beneath Japan (Wadati, and Uyeda, 1975; Conrad and Lithgow- of the 1950s–1960s, concurrent with the 1931), the Andes, and Tonga-Kermadec Bertelloni, 2002). The surface of a litho- development of plate tectonic theory, was (Benioff, 1949) were recognized as huge spheric plate carries a carapace of sedi- accompanied by the realization that the thrust faults (Lake, 1931). Benioff (1954) ment and fluid into Earth’s interior that compositions of oceanic and continen- further suggested that the frictional heat reflects the interactions of rocks with the tal crust are fundamentally different. The generated along these faults was respon- atmosphere, hydrosphere, and biosphere. assemblage of rock types (chert-​basalt-​ sible for the volcanism of the island Some of these components are returned diabase-​gabbro-​variably serpentinized and continental arcs developed above to the exterior as wedges of sediment that peridotite) collectively called an ophiolite earthquake zones. have been scraped off at the trenches, became key for understanding the origin In the 1920s to 1930s, submarine-​ through forearc vents (Fryer, 2012), or and development of much more inaccessi- borne gravity measurements in the via magma genesis within or above the ble in situ oceanic crust (e.g., Gass, 1968). Indonesian region led by Dutch geo- downgoing slab. Improvements in analytical techniques physicist F. Vening Meinesz revealed All major plate boundaries are ephem- for petrological, geochemical, and isoto- major negative gravity anomalies along eral; they may lengthen or shorten, move pic studies of magma types led to the real- the Sumatra-Java-Banda trenches relative to one another, or be created and ization that subduction zone inputs result (Umbgrove, 1945) similar to those sub- destroyed through time (Dewey, 1975). in trace element and mineralogical char- sequently recognized to also exist over Subduction zones can vanish, as evident acteristics in convergent margins magmas other deep-sea trenches. Understanding in the Cenozoic demise of the trench asso- that are distinct from those of magmas at the significance of radioactive heating ciated with the subduction of the Farallon divergent margins (e.g., Pearce and Cann, of Earth, earthquake and igneous activ- Plate below North America (Atwater, 1973). Convergent margin characteris- ity, and the characteristics of arc-trench 1970). By using records of paleomag- tics derive from the input of subducted systems was critical in development of a netic reversals preserved in oceanic crust, slab-sourced fluids into the regions of arc convective hypothesis to account for con- observations of seafloor fabric, and a vari- magma generation in the wedge of man- tinental drift (Holmes, 1928). However, ety of other onshore and offshore data, tle lying between a subducted slab and it was not until the development of the these events have been reconstructed for the overriding plate. plate tectonic theory (Hess, 1962; Wilson, the geologic past, with confidence for The seminal proposition by Miyashiro 1965; McKenzie and Parker, 1967) that periods where we have records of the his- (1973) that suggested the Troodos the significance of deep-sea trenches tory of plate motions (e.g., back to the Complex of western Cyprus, an arche- and associated earthquakes and volca- Jurassic; Seton et al., 2012), and less so typal ophiolite, formed in an island nism became generally understood to for older times (Matthews et al., 2016a). arc rather than at a mid-ocean ridge mark the return (subduction) of tec- Knowledge of the processes by which prompted a torrent of criticism but also a tonic plates (portions of lithosphere) into plate boundaries initiate and die is crit- re-examination of the concepts of crustal Earth’s interior. Subduction zones can ical to gaining a fuller understanding of formation at convergent boundaries. now be defined as locations where two how plate tectonics works and how it has Miyashiro’s hypothesis was founded on plates converge, as one sinks below the shaped our planet through geologic time. the similarity of the major element geo- other into Earth’s interior. At Earth’s sur- And while we have a good understand- chemistry of the Troodos Complex with face, the interface between the two plates ing of the initiation and development island arcs rather than that of mid-ocean is typically marked by a deep-sea trench; of divergent plate boundaries (i.e., mid- ridges. In particular, the presence of with increasing depth, subduction zones ocean ridges) and straightforward obser- glassy volcanic rocks containing unusu- can be recognized and traced by dipping vational opportunities for their study, the ally high MgO (>10 wt%) with inter- and planar groupings of earthquakes that same is not true for convergent boundar- mediate (i.e., andesitic) silica (~55 wt% are associated both with rocks and sedi- ies and their associated subduction zones. SiO2) is unlike the basalts and their deriv- ment in the vicinity of the plate interface, Divergent plate boundaries preserve ative melts typical of mid-ocean ridges. and also brittle failure within the down- records of their evolution in the magnetic However, for many geologists, the occur- going, colder plate interior. reversals in the oceanic crust and in the rence on Troodos of a prominent sheeted The paramount importance of sub- seafloor fabric, but much of the record dike complex represented concur- duction zones in terms of overall solid of subduction initiation is obscured or rent magmatism and extension, and the

162 Oceanography | Vol.32, No.1 absence of stratovolcanoes with aprons of protoenstatite (Smyth, 1974) that is arcs (Dietrich et al., 1978; Ishizuka et al., volcaniclastic debris typical of island arcs extremely rare in any other terrestrial 2014) revealed a stratigraphy (from deep seemed incompatible with a convergent igneous rock. Dallwitz et al. (1966) had to shallow) composed of (1) peridot- margin origin (e.g., Gass et al., 1975). previously identified clinoenstatite in ite, (2) minor gabbro, (3) sheeted dikes, Two main lines of evidence emerged glassy volcanic rocks on Cape Vogel (4) low-Ti-K tholeiitic basaltic lava flows, that have led to current acceptance that (Papua New Guinea) but had not real- (5) lavas and dikes of boninite, and most ophiolites formed in convergent ized that they inadvertently discov- (6) tholeiitic and so-called “calcalkaline” margin or so-called “supra-subduction ered boninite (Walker and Cameron, lavas (see Arculus, 2003). Reagan et al. zone” settings (e.g., Pearce, 2003; Dilek 1983). Cameron et al. (1979) reported (2010) reported that the tholeiitic lavas of and Furnes, 2014). The first, apparently boninite on the basis of glass geochem- units 3 and 4 on the trench walls are mid- arcane, line of evidence relates to the istry in the upper pillow lavas of the ocean ridge basalt (MORB)-like in terms re-examination (e.g., Kuroda et al., 1978) Troodos Complex, but noted the absence of major element and some trace element of the type locality of “boninite” (Petersen, of clinoenstatite. characteristics but with distinctively 1891) on the islands of Chichi- and The second line of evidence first lower Ti/V and Yb/V ratios than the lat- Muko-jima in the Bonin islands located came from detailed mapping of the IBM ter, interpreted to reflect a significantly in the modern Izu-Bonin-Mariana (IBM) forearc islands of Guam (Reagan and different melting environment than that forearc (Figure 2). Distinctive features Meijer, 1984) and Chichi-jima (Umino, responsible for MORB. The term forearc of the type boninite are: (1) high MgO 1985), which revealed igneous base- basalt (FAB) was applied to units 3 and 4

(>8 wt%) and low TiO2 (<0.5 wt%) with ments of near-trench-parallel, boninite on the basis of their outcrop in the IBM intermediate SiO2 (52–65 wt%) (Le Bas, sheeted dike-pillow lava complexes. In forearc (Reagan et al., 2010). Ishizuka 2000), and (2) the occurrence of clinoen- addition, submarine dredging and exam- et al. (2011) reported the FAB age range statite (Shiraki et al., 1980; Figure 3), an ination of the stratigraphy exposed on as 52–48 million years old, compared inverted polymorph of high-​temperature both trench walls of the current IBM with the oldest boninite at 48.2 million

A 30°N 25°N

B 20°N 15°N

10°N FIGURE 3. Photomicrographs of a thin section of crystal-rich boninite from a pillow lava-dike sequence at Hatsuneura, Chichi-jima. Fields of view are 2.5 mm. (A) Plane-polarized light

5°N view with two multiply-twinned clinoenstatite phenocrysts dominating the center of the field in a matrix of clear, isotropic glass and small 125°E 130°E 135°E 140°E 145°E crystals of pyroxene and olivine. (B) Crossed- FIGURE 2. Regional context of sites drilled by International Ocean Discovery polarized light view of the same thin-section. Program Expeditions (IODP) 351 (Site U1438), 352 (Sites U1439–1442), and Ocean Drilling Program (ODP) Leg 125 (Site 786).

Oceanography | March 2019 163 years old. Thus, the distinctive boninite et al. (2004) pointed out that half of all as marked by a change in the orientation magma type emplaced under extension in extant subduction zones initiated in the of the Hawai’i-Emperor seamount chain the early growth of the intra-oceanic IBM Cenozoic, and concluded that forces formed by the Hawai’i mantle plume arcs is convincing evidence that some resisting subduction can be overcome in (Sharp and Clague, 2006; O’Connor ophiolites such as the Troodos Complex a diversity of settings. Two end-member et al., 2013). Assuming that plate motion might have similar origins. Whattam and situations have been identified: sponta- (at least partially) explains this Hawai’i- Stern (2011) interpreted the sequence of neous and induced (Stern, 2004; Gurnis Emperor Bend, several explanations early FAB overlain by later boninites or et al., 2004; Figure 4). The former occurs could be advanced for this change in similar lavas to be strong evidence that when large density differences exist across Pacific Plate motion from NNW to an ophiolite sequence formed during lithospheric boundaries (e.g., transform WNW, including (1) the subduction of subduction initiation. fault conversion; circum-mantle plume the Izanagi-Pacific Ridge along eastern Realization of the significance of sub- head), and the latter when pre-existing Asia (Seton et al., 2015), (2) collision of an duction zone initiation has stimulated plate motions force the development of intraoceanic arc with southern Asia prior multidisciplinary field, laboratory, and a new subduction zone (e.g., subduction to the arrival of Indian continental crust modeling studies to understand the pro- polarity switch). (Aitchison et al., 2007; Matthews et al., cess (Stern and Gerya, 2018). While One of the largest changes in global 2016b), (3) the separation of Australia McKenzie (1977) suggested that “ridges plate motions during the Cenozoic may from Antarctica (Whittaker et al., 2007), start easily, but trenches do not,” Gurnis have occurred at ~50 million years ago, and (4) the Pacific Plate began to move

FIGURE 4. Representation after Stern and Gerya (2017) of types of subduction initiation, with specific geographic examples, past and present. Locations of IODP 351 and 352 drill sites are schematically indicated under two types of settings.

164 Oceanography | Vol.32, No.1 toward another or new convergent mar- iodp-​science-​plan-​2013-2023). In 2014, distribution of the earliest magmatism, gin. It is known that subduction zones two IODP expeditions targeted the dis- and the early evolution of the arc. The developed in the western Pacific along a tal rear-arc (Expedition 351) and forearc primary goal of IODP Expedition 371 in line that extends from Japan in the north, (Expedition 352) of the Izu-Bonin system 2017 was to date and quantify the defor- southward through the IBM (Ishizuka (Figure 2) to discover the nature of the mation and uplift/subsidence of conti- et al., 2011; Reagan et al., 2013), to the arc basement, the spatial and temporal nental ribbons and intervening basins Tonga-Kermadec (TK) system, albeit with local complications (Meffre et al., 50 Ma Present 2012; Wu et al., 2016). Seismic tomog- A B raphy (subsurface images made using knowledge of seismic wave propagation) reveals an immense wall of subducted Pacific Plate along this great length of convergent boundaries (Ritsema et al., 2004; C. Li et al., 2008;). Uyeda and Ben Avraham (1972) first proposed conver- sion of pre-existing transform fault(s) to a transpressional regime at the locus 10 cm yr–1 of IBM arc initiation consequent to the change in Pacific Plate motion. Taking into account along-arc strike extension by continued arc-parallel spreading in the West Philippine Basin, Dewey and Casey C (2011) developed a model for arc initia- tion as a trench-ridge-trench triple junc- tion and for formation of a forearc ophio- lite complex as now outcrops in the IBM trench walls (Bloomer et al., 1995). The contrast between the settings at subduc- tion initiation of the IBM versus TK sys- tems is significant. For example, while many depict subduction initiation at the IBM as intra-oceanic (e.g., Hall, 2002; Whittaker et al., 2007), it developed at TK at the eastern margin of the rifted fragments of Australia (e.g., Whittaker et al., 2007; Schellart and Spakman, 2012; Meffre et al., 2012; Matthews et al., 2015, 2016b; Figure 5). A multitude of studies involving extensive geophysical surveys, coring, dredging, and data from previous sci- entific ocean drilling expeditions pro- vided the stimulus for a campaign by the

International Ocean Discovery Program FIGURE 5. Drilling sites, topography, and bathymetry in the Western Pacific. Tectonic features are (IODP) to resolve some of the major shown (A) at 50 million years ago (Ma), and (B) at the present day, using the plate tectonic recon- issues concerning subduction incep- struction of Seton et al. (2012). Velocities are in a moving hotspot frame of reference. The Izanagi- Pacific Ridge at this time had just been subducted beneath Japan and East Asia. Sites from IODP tion and arc initiation in the IBM and Expeditions 351, 352, and 371 discussed in the text are indicated with larger circles, while selected TK systems. These issues were identi- sites from earlier Deep Sea Drilling Project and ODP legs are marked with stars. This reconstruc- fied as first-order research problems tion does not account for the many discoveries from these recent IODP Expeditions. In this recon- struction, the Pacific Plate has an approximate northwest-trending motion at 50 Ma, which trends in the IODP Science Plan for 2013– to a more westerly motion at 47 Ma. (C) Locations of Expedition 371 sites (stars) along the Tasman 2023 (http://www.iodp.org/​about-iodp/​ Frontier (Sutherland et al., 2017), together with previous deep-sea drilling sites (circles).

Oceanography | March 2019 165 between Australia and the TK conver- Bloomer, 1992) and the other through Expedition 352: IBM gent margin (Figure 5, Tasman Frontier; forced convergence (Gurnis et al., 2004). Forearc Architecture and Sutherland et al., 2017) in order to The two models predicted different his- Construction Timescale test the predictions of alternative geo- tories and ages of the sediments imme- Over a distance of about 13 km, IODP dynamic models for subduction incep- diately overlying the Amami-Sankaku Expedition 352 drilled four sites in the tion. In the following sections, we pro- basement. The simple layered structure forearc orthogonal to the strike of the vide some suggestions for future drilling of the Amami-Sankaku Basin is com- Bonin Trench, and recovered a high- targets to resolve outstanding problems posed of ~1.5 km of sediment overlying fidelity record of crustal generation related to western Pacific Eocene sub- a normal oceanic crustal thickness of related to subduction initiation in the duction initiation and more recently ~6 km (Arculus et al., 2015). The seis- Bonin forearc. This was the first expe- developing systems. mic structure of the basement persists dition dedicated entirely to exploration eastward beneath the KPR and forms a of the stratigraphy of the IBM forearc. IODP EXPLORES SUBDUCTION significant portion of its total thickness. Previous drilling during Ocean Drilling INITIATION IODP Expedition 351 successfully recov- Program (ODP) Leg 125 recovered a Expedition 351: Evidence for the ered the targeted sequences—and gen- sequence of boninite and derivative rocks Earliest Evolution of the IBM erated some major surprises (Arculus (e.g., Arculus et al., 1992) at Site 786, The Kyushu-Palau Ridge (KPR) is the et al., 2015). The basement (Unit 1) is located ~270 km north and along the oldest remnant arc in the Izu-Bonin- composed of low-Ti-K tholeiitic basalts strike of the forearc from those drilled Mariana system, separated 30–25 million ~49 million years in age (Ishizuka et al., during IODP Expedition 352. A sequence years ago from the volcanic IBM front by 2018) and not Paleocene (66–56 million of FABs overlying dikes was recovered at seafloor spreading in the Parece Vela and years old) or older as hypothesized Sites U1440 and U1441, closest to the Shikoku backarc basins. At its northern pre-Expedition 351. These basalts over- trench; they were emplaced during near- end, the KPR strikes across the Amami- lap in age with the Bonin Ridge boninites trench seafloor spreading that accompa- Sankaku Basin and forms the east- (Ishizuka et al., 2006) but are younger nied subduction initiation. The basalts ern boundary of the east-west-striking than the overall basalt-to-boninite are petrologically similar to the FABs Mesozoic arc fragments of the Amami sequence recovered from the present-day retrieved by submersible, dredging, and Plateau and Daito Ridge (Figure 2). IBM forearc by dredging and submers- drilling along the entire length of the Taylor and Goodliffe (2004) emphasized ible sampling (Ishizuka et al., 2011) IBM forearc (see DeBari et al., 1999; that the KPR (and hence the inferred and by IODP Expedition 352 drilling at Reagan et al., 2010; Ishizuka et al., 2011). juvenile trench) trended at high angles Sites U1439–U1442 (Reagan et al., 2019). The FABs recovered from the drill cores, to the Amami Plateau, Daito Ridge, and Conclusions drawn from studies of together with a compositionally related bounding faults of the Amami-Sankuku the petrologic and geochemical charac- gabbroic rock collected nearby, have Basin. Accordingly, they argued against teristics (including radiogenic isotopes) ages of ~51.9–51.3 million years (Reagan the inception of the subduction zone and of the Expedition 351 basement basalts et al., 2019). Forearc basalt lavas are gen- earliest arc at a pre-existing transform (Hickey-Vargas et al., 2018; Yogodzinski erally aphyric, with rare plagioclase and fault that was linked to, for example, the et al., 2018) are: (1) they are derived from augite phenocrysts. Like the basalts from Izanagi-Pacific Ridge. mantle peridotite source(s) that were Site U1438, FABs are highly depleted in The original aims of IODP highly depleted by melt extraction prior incompatible trace elements as a result of Expedition 351 drilling at Site U1438 to IBM arc inception ~52 million years ancient melt extraction events (Shervais in the Amami-Sankaku Basin were to ago; (2) the basalts lack the fluid-mobile et al., 2018). explore the pre-arc basement underlying trace element enrichments typical of most Boninites were drilled furthest from the IBM arc and to recover the history island arc basalts, boninites, or high-Mg the trench at sites U1439 and U1442. of the early development of the arc pre- andesites (e.g., Schmidt and Jagoutz, The oldest boninites have composi- served in volcaniclastic-rich sediments 2017); (3) the compositional characteris- tions that are transitional toward FAB shed from the growing KPR that, when tics of the (micro)phenocrysts of olivine-​ (termed “low-Si boninites,” or LSB) and active, was part of the IBM arc. Prior clinopyroxene-​plagioclase-​spinel com- erupted while seafloor spreading con- to IODP Expedition 351, two hypoth- prise a globally unique data set, reflecting tinued. These lavas are beneath “high-Si eses had been put forward for the ori- generation of the magmas at relatively boninites” (HSB) that erupted atop gin of the reconstructed IBM arc, one high pressures and temperatures under the new oceanic crust and have com- in which subduction nucleated sponta- low-redox conditions, followed by rapid positions resembling the boninites on neously due to a difference in plate age transfer to the surface likely under sea- the nearby island of Chichi-jima. HSB across a former fracture zone (Stern and floor spreading conditions. ages span 51.3 to 50.3 million years old

166 Oceanography | Vol.32, No.1 (Reagan et al., 2019), placing eruption of inception along the proto-TK Arc may wherein new basaltic ocean crust forms the LSB near the age of the nearby FAB. have involved elements of both sponta- by seafloor spreading. For many arcs, All boninites are formed by flux melting neous and induced elements. there are also isolated edifices or chains of depleted mantle that involves fluids of rear-arc volcanoes not formed through derived from dehydration and melting of DISCUSSION seafloor spreading, such as the Chokai subducted materials. LSB were generated Complementing the past multidisci- Zone of Japan (Tatsumi, 1989; Tamura in the presence of melts/fluids from sub- plinary and collaborative approaches et al., 2002), or the cross-chains of the ducting altered Pacific MORB, whereas to the study of the IBM and TK arcs, northern IBM arc (Machida et al., 2008). melts/fluids from subducting sediments the recent IODP scientific ocean drill- The volcanic front is dominated by sub- were involved with the genesis of HSB ing expeditions have confirmed some aerial and submarine stratovolcanoes, (H.Y. Li et al., 2017). hypotheses regarding crustal architec- with extensive aprons of volcaniclastic ture, but also produced novel and sur- debris (e.g., Pope et al., 2018). Expedition 371: Exploration of prising results. For example, one of the A persistent chain of stratovolcanoes, TK Subduction Initiation most important conclusions of the col- geometrically stationary with respect to IODP Expedition 371 drilled six sites lective studies of the IBM system is that an adjacent trench and likely fed from across some of the major structural ele- the structure and magmatic output of a plugs or radial dikes (Nakamura, 1977), ments of the Tasman Frontier, includ- nascent arc is unlike that of established nevertheless seems incompatible with the ing the northern and southern New systems. The latter can be defined as presence of an underlying sheeted dike Caledonia Trough, Reinga Basin, Lord “a chain of concurrently or potentially complex. This observation, together with Howe Rise, and Tasman Abyssal Plain active volcanic islands, consistently asso- the absence of aprons of volcanic debris, (Figure 5). The variable but persistently ciated but displaced spatially more than were prime factors in the initial rejection moderate bathymetry of the Tasman 100 km from a deep-sea trench. Much of of Miyashiro’s (1973) hypothesis that the Frontier has ensured preservation of the eruptive activity is typically explosive. Troodos Complex formed in an island depth-sensitive, fossil-rich records both Adjacent to many island arcs in the west- arc. In fact, the earliest stages of “supra-​ pre- and post-subduction initiation at ern Pacific are backarc basins, floored by subduction zone” magmatism might well ~50 million years ago along the TK arc. crustal spreading centers. Some arcs have be unrecognized in the absence of tec- Seismic reflection data across the Tasman associated non-volcanic islands between tonic and geophysical constraints. It turns Frontier reveal episodes of compression, the volcanic arc and trench compris- out that both Miyashiro and his critics uplift, and subsidence (Sutherland et al., ing uplifted basement or trench-accreted were right: the Troodos Complex formed 2017). Results from IODP Expedition 371 sediments” (Arculus, 2009; Figure 6). by seafloor spreading but at a conver- show regionally dramatic, Eocene- Implicit in this definition is the notion of gent plate margin; whether this spreading age vertical motions, with evidence for a volcanic front marking the trenchward was in a near-trench setting during sub- Eocene-Oligocene faulting and fold- limit of volcanic edifices, a forearc region duction initiation remains controversial ing (Sutherland et al., 2018). Subduction lacking volcanism, and backarc basins (Woelki et al., 2018). The accumulating

FIGURE 6. Schematic cross section of an island arc from trench to backarc basin. Orange stars rep- resent great earthquakes in the seismogenic por- tion of the Wadati-Benioff zone. Blue arrows repre- sent movement of hydrous fluids/melts.

Oceanography | March 2019 167 evidence clearly supports the model ini- usual basalts associated with strato- IODP expeditions, in combination with tially proposed by Stern and Bloomer volcanoes (so-called island arc tholeiitic previous studies (see references cited ear- (1992) and recently extended by Stern and calcalkalic suites; Ishizuka et al., lier), we emphasize five issues. and Gerya (2018) for the earliest devel- 2011; Reagan et al., 2019). We note that 1. Inception of the IBM system was opment of the IBM arc. Simply stated, a slab dipping at ~45° and subducting adjacent to a series of Mesozoic-aged the tholeiitic basalts forming the earliest at 60 mm yr–1 (Whittaker et al., 2007; arc-basin systems (Arculus et al., 2015; crust in the IBM arc, sampled at the pres- Figure 5) takes ~2.4 million years to reach Leng and Gurnis, 2015). Inception of ent-day forearc (Sites U1440 and U1441), a depth of 100 km below a volcanic front the TK system was adjacent to a welt of were emplaced at one or more spreading that is characteristic of mature arcs. This continental crustal ribbons, interven- ridges underlain by sheeted dike swarms. is a relatively short period of time com- ing basins, and older arcs (Meffre et al., The architecture of this oceanic crust sug- pared with a duration of ~7 million years 2012; Sutherland et al., 2017). These gests the basalts formed at a ridge on the from the earliest tholeiitic to boninitic two systems were not intra-oceanic​ in overriding plate during subduction initi- magmatism in the case of drill recovery the sense of birth surrounded solely by ation (Ishizuka et al., 2014). At this stage, from the present-day forearc to the out- oceanic lithosphere. there was no chain of stratovolcanoes, crops on the Bonin Ridge. The processes 2. The earliest lavas accompanying sub- and the concept of a forearc, rear-arc, and that drive the transition from seafloor duction initiation were basalts erupted backarc for the earliest situation as delim- spreading during the nascent arc stages to in a seafloor spreading environ- ited by a volcanic front cannot be applied. establishment of the archetypal magmatic ment, above sites of asthenospheric Seen in this context, the basalts drilled at arc with a forearc-volcanic front-rear/ upwelling. U1438 are younger than FAB and erupted backarc geometry are not yet understood. 3. These early basalts have extremely low at the same time as boninites along the However, the processes must reflect the abundances of the light (L) relative to Bonin Ridge. Reagan et al. (2017, 2019) transition from hinged subsidence of the the heavy (H) rare earth elements (REE) propose the Amami-Sankaku Basin base- old seafloor, with asthenospheric upwell- and small to insignificant enrichment ment formed through trench-distal mag- ing into the space created, to true down- in the most subduction-mobile ele- matism, possibly as a type of later backarc dip subduction and a reversal of astheno- ments; hence, they lack the large neg- activity, after trench-proximal seafloor pheric flow (Stern and Bloomer, 1992). A ative Nb anomalies that characterize spreading ceased, rather than as a contin- corollary is that the strike of the KPR is almost all arc lavas. uation of the seafloor spreading that pro- related to the establishment of the volca- 4. These early basalts are strikingly radio- duced the FAB. nic front of a mature magmatic arc, and genic 176Hf/177Hf (εHf ≤ 22.1) at given Restoring the geometry of the initial does not necessarily bear any simple rela- 143Nd/144Nd compared with ocean stages of magmatism in the proto-IBM tionship to preexisting ridge-transform floor basalts from the Pacific Plate. arc by closing the backarc basins and segments and their orientations. 5. Clinopyroxene-bearing phenocryst/ forearc rifts places the Amami-Sankaku Comparison of petrological and geo- microphenocryst assemblages in the Basin ~250 km across strike from the chemical characteristics of the tholeiitic early basalts reflect rapid transfer from Bonin Ridge adjacent to Expedition 352 basalts erupted during the initial stages mantle source depths (~30 km) to the Sites U1439–1442 (Figure 2). The dura- of convergent margin formation with surface without the extensive staging tion of the earliest period of tholeiitic those from mid-ocean ridges and back- that is characteristic of sub-mid-ocean basalt eruption was less than 1.2 mil- arc basins is critical for understanding ridge chambers and mush zones. lion years at the present location of the the respective petrogenetic processes Bonin forearc (FAB, 52–48 million years involved. We require distinguishing Elaborating on the significance of the ago; Ishizuka et al., 2011; Reagan et al., compositional parameters to recog- set of geochemical and petrologic char- 2019), with perhaps two additional mil- nize tholeiitic magmatism associated acteristics, Figure 7 plots the slopes and lion years in the Amami-Sankaku Basin with subduction inception events in the curvatures of the chondrite-normalized (49–47 million years ago; Ishizuka et al., Mesozoic and older eras (e.g., Buchs REE abundances for global MORBs (gray 2018). The period of boninitic magma- et al., 2010). For comparative purposes, or white circles; O’Neill, 2016) compared tism endured at least from 51 to 44 mil- we need to know the absolute abun- with the nascent arc tholeiitic basalts (col- lion years ago on the Bonin Ridge, pos- dances of major and trace elements allied ored symbols). Quantification of the total sibly associated, during the later stages of with isotopic characteristics and con- shapes of the usual chondrite-normalized​ high-silica boninite eruption, with indi- stituent mineral compositions of these abundance plots (see the three insets) is vidual topographically prominent volca- nascent subduction-related basalts. In the great advantage of this type of fig- nic edifices (Reagan et al., 2017), before this framework, abstracting the most sig- ure. It shows the Amami-Sankaku Basin transitioning to eruption of the more nificant results published to date from the basalts from IODP Site U1438 define

168 Oceanography | Vol.32, No.1 a LREE-depleted limit of the global anomalies for Nb and Ta in the case of the so-called “clinopyroxene paradox” is in MORB array. While also highly depleted, Amami-Sankaku Basin basalts and FABs fact consistent with phase relationships FABs are distinct compared with the can therefore be interpreted as precluding of effectively anhydrous MORB magma. Amami-Sankaku Basin basalts in hav- slab-derived additions of the LREE to the Decompression of a melt saturated with ing less concave-​downward curvatures mantle sources, therefore ruling out mod- olivine, plagioclase, and clinopyrox- of the chondrite-normalized LREE abun- ification of the intrinsic, pre-subduction ene from a crustal magma (melt-crystal dances and more diverse REE patterns inception Nd and Hf isotopic characteris- mush) chamber at ~2–4 km depth below (Shervais et al., 2018). Nevertheless, both tics of this sample set. the seafloor will eliminate clinopyroxene the Amami-Sankaku Basin basalts and The mineralogy of IBM FABs and as a near-liquidus phase. The persistence FABs have Hf-Nd isotopic characteristics Amami-Sankaku basalts is also distinc- of clinopyroxene in the nascent IBM requiring mantle source(s) that were more tive compared with the overwhelming basalts might be accounted for by more depleted by prior melting than any equiv- majority of MORBs. The presence of phe- rapid ascent from such depths allied with alents tapped beneath the global network nocrystic clinopyroxene, for example, is higher water contents. Clinopyroxenes in of mid-ocean ridges. Development of highly unusual compared with the per- the Amami-Sankaku Basin basalts and very high 176Hf/177Hf requires an increase sistent lack of this phase as a phenocryst FABs have lower Na and Ti compared in the Lu/Hf ratio of the mantle sources in primitive (Mg-rich) MORBs (Francis, with the rare phenocrysts in MORBs above that of the MORB source(s) and 1986; Herzberg, 2004). We know that and the abundant compositional data must result from basalt melt extraction clinopyroxene is involved in the genera- for MOR gabbros; the Amami-Sankaku combined with the passage of time. There tion of the global MORB compositional Basin clinopyroxenes range to more can be a trade-off between the extent array (O’Neill and Jenner, 2012) and is aluminous compositions, whereas the of Lu/Hf fractionation and the melting a major constituent of gabbros recov- FAB clinopyroxenes are less aluminous events subsequently associated with sub- ered by scientific ocean drilling, dredg- than MORB. Spinel compositions in the duction, but development of radiogenic ing, and submersible sampling of the oce- Amami-Sankaku Basin basalts range Hf must precede the subduction initi- anic crust. Herzberg (2004) notes the from Fe3+-poor, aluminous-chromian ation event (Reagan et al. 2010; H.Y. Li et al., 2017; Yogodzinski et al., 2018). After careful filtering for post-eruptive​ seawater alteration, Hickey-Vargas et al. (2018) show the abundances of the fluid-​ mobile trace elements (alkali, alkaline earth, Pb, U, and Th) in the Amami- Sankaku Basin basalts are typical of the most highly depleted MORBs with no evidence of subducted-slab derived inputs. After similar filtering, Shervais et al. (2018) conclude that most FABs lack a subduction influence in their gene- sis, although some FABs from Site U1441 with elevated Sr (fluid-mobile)/Zr(fluid immobile) (~3.2) likely require some subducted slab-derived water inputs to their mantle sources (Reagan et al., 2017). The presence of negative high-​field-​ strength element anomalies in island arc basalts is commonly interpreted to result from enrichments in La and other LREE of the mantle wedge sources by slab-​ derived fluids (e.g., Pearce and Peate, FIGURE 7. Comparison of the curvature (lambda 2) vs. slopes (lambda 1 values) of

1995). The abundances of elements such chondrite-normalized (CN), rare earth element abundances (O’Neill, 2016) for sub- as Nb and Ta are then assumed to reflect duction-inception related magmas compared with mid-ocean ridge basalts. Insets show where representative types of patterns project in the main figure. D = Depleted. the pre-​enrichment values of the unmod- E = Enriched. N = Normal. Gea = From Gale et al. (2013). J&O’N = Jenner and O’Neill, ified mantle. The absence of negative 2012. Tea = Todd et al. (2012). Uea = Umino et al. (2015).

Oceanography | March 2019 169 to ferrian (magnetite), exceeding the generating FAB magma with minimal first few million years approached that of range known in MORBs (Sigurdsson or no material transfers from the slab. mid-ocean ridges (~1,000 km3 per million and Schilling, 1976; Barnes and Roeder, Progression to slab-derived flux melt- years per kilometer of ridge), but dimin- 2001), and consistent with higher pres- ing of the wedge proceeds through the ished once the mature arc, backarc spread- sures (or greater depths) of derivation of generation of the most depleted FAB to ing, and the KPR became established. the Amami-Sankaku Basin basalts than boninite over less than 1.2 million years The development of the TK system dif- for most MORBs. and must reflect changes in the motion of fered from that of the IBM in a number of Based on the results of Expedition 352, the sinking lithosphere and the advecting ways. Crustal elements developed above Reagan et al. (2017, 2019) outlined the mantle above it. This early and fast igne- an eastward–dipping subduction zone progression from subduction inception ous outpouring of FAB, LSB, and HSB in the 57–35 million year old Loyalty- at a transform margin through seafloor forms the basement of the present-day Three Kings arc were thrust over the spreading and the development of the IBM arc and forearc, and a portion of the Norfolk Ridge and are now exposed in first chain of individual stratovolcanoes welts of remnant arcs, such as the Kyushu- New Caledonia (e.g., Cluzel et al., 2018). or “typical” arc (Figure 8). Rapid col- Palau and West Mariana ridges. In terms Meffre et al. (2012) argue that the oldest lapse or sinking of the subducting slab of crustal growth rates in arcs, this conclu- (~52–48 million years old) igneous rocks leads to decompression of asthenosphere, sion is important because the flux in the (plagiogranites) now in the TK forearc were originally created in a backarc-arc setting. Subduction polarity reversal and collision of the Loyalty-Three Kings arc with New Caledonia placed the 52 mil- lion year-aged rocks into the forearc of the TK system. Todd et al. (2012) reported the earliest subaerially exposed magmatic products of the Fiji-TK arc, including island arc tholeiite, boninite, and “early-​ arc tholeiite” interbedded in outcrops on Viti Levu and Eua (Fiji). The authors interpret these to be products of decom- pression and flux-enhanced melting of a proto-mantle wedge following sub- duction inception. For comparison with the forearc and Amami-Sankaku Basin basalts of the IBM system, Figure 7 shows the shapes of the chondrite-​normalized abundance patterns of the early arc tholeiites from Fiji. A number of these samples extend into the more LREE- depleted range of global MORB, but are not as extreme as those from the Amami- Sankaku Basin or the IBM forearc.

OUTSTANDING QUESTIONS We have learned much concerning the FIGURE 8. Cartoon with time slices after Reagan et al. (2019) of cre- creation of new subduction zones over ation and evolution of the Izu-Bonin-Mariana (IBM) crust after sub- duction initiation. (A) Philippine Sea Plate (PPP)-Pacific Plate in con- the past five years from a variety of multi- tact prior to subduction initiation along a transform fault/fracture zone disciplinary approaches and focused (TF/FZ). (B) Subduction inception between 52.5 and 51.9 million years IODP scientific ocean drilling, but ago (Ma), with asthenospheric upwelling and decompression melting with trench-proximal seafloor spreading and forearc basalt (FAB) gen- what are the outstanding problems, and eration. (C) Continued Pacific Plate subduction and seafloor spread- what future drilling projects might be ing generating FAB followed by slab-derived fluid fluxing generating undertaken to solve them? low-silica boninite (LSB). (D) Embryonic arc (chain of stratovolcanoes, advecting wedge, and possibly early trench-distal [backarc] seafloor Noting that oceanic lithosphere spreading). IODP Expedition 351 and 352 sites indicated. >10 million years old is unstable grav-

170 Oceanography | Vol.32, No.1 itationally compared with the under- of subduction initiation along trans- those drilled by Expedition 352 and lying mantle (Cloos, 1993), there is broad form faults bordering preexisting arc ter- shown schematically in Figure 8 are the agreement that Stern’s (2004) funda- ranes and stretched continental ribbons obvious locations. mental division of subduction initiation followed. This event can potentially be The early IBM basalts are a global end into spontaneous and induced types is viewed as a hybrid that included induced member with respect to the degree to a useful way to approach the issues sur- processes exploiting along-strike varia- which they are depleted in highly incom- rounding creation of convergent mar- tions in local buoyancy forces. patible elements, and isotopic composi- gins. Nevertheless, individual examples In the specific case of the IBM system, tions convincingly show that this deple- may well involve a combination of these end members, and it is abundantly clear that subduction inception is intrinsically a three-dimensional process (Zhou et al., 2018). Building on regional geophysical Knowledge of the processes by which surveys and tectonic syntheses, localized plate boundaries initiate and die is critical to but extensive rock and sediment cores have been recovered that record the oper- gaining a fuller understanding of how plate ation of fundamental structural and mag- “ tectonics works and how it has shaped our matic processes. Identifying the localized as opposed to regional-scale factors then planet through geologic time. . becomes important in overall under- standing of these processes. Leng and Gurnis (2015) point out that exten- sional events in the relatively buoyant exploration of the foundations and earli- tion occurred in their mantle sources Cretaceous-aged arcs (Amami Plateau- est magmatic products is demonstrably long before subduction initiation” Daito Ridge; Figure 2) and, hence, ther- incomplete. We have minimally explored (Yogodzinsky et al., 2018). This opens mal rejuvenation of the region, were the three-dimensional aspects of the the possibility that subduction initiation important factors leading to subduction Amami-Sankaku Basin, both in terms of occurred along some sort of geochem- initiation along an adjacent transform probing the lateral and vertical structure ical boundary in the mantle, like that fault bounding the Mesozoic arcs and and composition of the basement, as well represented by the Australia-Antarctic comparatively dense Pacific oceanic lith- as through exploring the early stage of arc Discordance to the south (Gurnis and osphere. There is considerable disagree- growth via the KPR output as recorded Müller, 2003). However, basalts from ment over whether spontaneous subduc- in the overlying sediments (Straub et al., ophiolites generally are not unusually tion initiation can occur at an old fracture 2015; Brandl et al., 2017). Combining depleted in incompatible trace elements zone absent any other factors (Leng and sample recovery from Deep Sea Drilling (e.g., the Samail Ophiolite; Alabaster Gurnis, 2015; Stern and Gerya, 2018). Project Leg 60, ODP Leg 125, and IODP et al., 1982). Thus, if our conclusion that However, the juxtaposition of old Pacific Expedition 352 with those from diving the IBM forearc is the in situ equivalent of Plate to reheated buoyant relic arcs pro- and dredging along the IBM forearc, we an ophiolite is correct, then strong prior vides sufficient driving forces for spon- have the beginnings of an along-strike depletion of the mantle is not required for taneous initiation (Leng and Gurnis, recovery of crustal sequences associated subduction initiation to proceed. 2015). In broad terms, the same situa- with subduction initiation. The crust For the TK system, ODP Leg 135 pri- tion prevailed in the TK system. Other clearly has all of the lithologies found in marily targeted the evolution of the Lau complexities of regional geology in the ophiolites, but the structural and compo- Basin. It is clear, however, from the suc- case of the IBM system concern the sitional relationships between the units cess of Expedition 352, in combination impact of the Oki-Daito plume and initi- remain poorly known. Penetration to the with the dredging recoveries from the ation of spreading in the West Philippine gabbros that must underlie the sheeted Tonga Trench wall (Bloomer and Fisher, Basin (Ishizuka et al., 2013). There is dikes, and ultimately to the underlying 1987; Meffre et al., 2012), that deep-sea general agreement that the potential mantle remains a priority for understand- drilling of the TK forearc is of fundamen- change in azimuth of Pacific Plate motion ing the development of the nascent crust tal importance to further understanding ~52–50 million years ago coincided with after subduction initiation and the precise of the development of the system. initiation of subduction of the IBM and link between the results of drilling forearc Globally, there is much interest in fur- the TK systems, but the ultimate causal crust and the genesis of supra-subduction​ ther exploring the current tectonism factors are not yet known. Localization ophiolites. Sites closer to the trench than and geophysical characteristics of the

Oceanography | March 2019 171 Puysegur-Fiordland boundary between multiplicity of global targets for drilling Brandl, P.A., M. Hamada, R.J. Arculus, K. Johnson, K.M. Marsaglia, I.P. Savov, O. Ishizuka, and H. Li. the Australian and Pacific Plates south of and a potentially coherent ensemble of 2017. The arc arises: The link between volca- New Zealand (e.g., Meckel et al., 2003). different examples and stages of subduc- nic output, arc evolution and melt composition. Earth and Planetary Science Letters 461:73–84, This transpressional margin appears to tion initiation. Scientific ocean drilling https://doi.org/​10.1016/j.epsl.2016.12.027. be transitioning from induced (forced) is the best way to recover the records of Buchs, D.M., R.J. Arculus, P.O. Baumgartner, C. Baumgartner-Mora, and A. Ulianov. 2010. 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