OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy 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 PERMISSIONS Oceanography (ISSN 1042-8275) is published by The Oceanography Society, 1 Research Court, Suite 450, Rockville, MD 20850 USA. ©2019 The Oceanography Society, Inc. Permission is granted for individuals to read, download, copy, distribute, print, search, and link to the full texts of Oceanography articles. Figures, tables, and short quotes from the magazine may be republished in scientific books and journals, on websites, and in PhD dissertations at no charge, but the materi- als must be cited appropriately (e.g., authors, Oceanography, volume number, issue number, page number[s], figure number[s], and DOI for the article). Republication, systemic reproduction, or collective redistribution of any material in Oceanography is permitted only with the approval of The Oceanography Society. Please contact Jennifer Ramarui at [email protected]. Permission is granted to authors to post their final pdfs, provided byOceanography , on their personal or institutional websites, to deposit those files in their institutional archives, and to share the pdfs on open-access research sharing sites such as ResearchGate and Academia.edu. DOWNLOADED FROM HTTPS://TOS.ORG/OCEANOGRAPHY SPECIAL ISSUE ON SCIENTIFIC OCEAN DRILLING: LOOKING TO THE FUTURE 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.