The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins.

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

Aitchison, J.C., J.R. Ali, and A.M. Davis. 2007. When and where did India and Asia collide? Journal of Geophysical Research 112, B05423, https://doi.org/​10.1029/2006JB004706.

Alabaster, T., J.A. Pearce, and J. Malpas. 1982. The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex. Contributions to Mineralogy and Petrology 81:168–183, https://doi.org/​10.1007/BF00371294.

Arculus, R.J., J.A. Pearce, B.J. Murton, and S.R. van der Laan. 1992. Igneous stratigraphy and major element geochemistry of holes 786A and 786B. Pp. 143–169 in Proceedings of the Ocean Drilling Program, Scientific Results, vol. 125. P. Fryer, J.A. Pearce, and L.B. Stokking, eds, https://doi.org/​10.2973/odp.proc.sr.125.137.1992.

Arculus, R.J. 2003. Use and abuse of the terms calcalkaline and calcalkalic. Journal of Petrology 44:929–935, https://doi.org/10.1093/petrology/44.5.929.

Arculus, R.J. 2009. Island Arcs. Pp 481–486 in Encyclopedia of Islands. R.G. Gillespie and D.A. Clague, eds, University of California Press.

Arculus, R.J., O. Ishizuka, K.A. Bogus, M. Gurnis, R. Hickey-Vargas, M.H. Aljahdali, A.N. Bandini-Maeder, A.P. Barth, P.A. Brandl, L. Drab, and others. 2015. A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc. Nature Geoscience 8:728–733, https://doi.org/10.1038/ngeo2515.

Atwater, T. 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geological Society of America Bulletin 81:3,513–3,535, https://doi.org/​10.1130/​0016-7606​(1970)​81​[3513:IOPTFT]​2.0.CO;2.

Barnes, S.J., and P.L. Roeder. 2001. The range of spinel compositions in terrestrial, mafic and ultramafic rocks. Journal of Petrology 42:655–671, https://doi.org/​10.1093/​petrology/42.12.2279.

Benioff, H. 1949. Seismic evidence for the fault origin of oceanic deeps. Geological Society of America Bulletin 60:1,837–1,856, https://doi.org/​10.1130/​0016-7606(1949)60​[1837:SEFTFO]2.0.CO;2.

Benioff, H. 1954. Orogenesis and deep crustal structure: Additional evidence from seismology. Geological Society of America Bulletin 65:385–400, https://doi.org/​10.1130/​0016-​7606​(1954)​65​[385:OADCSE]​2.0.CO;2.

Bloomer, S.H., and R.L. Fisher. 1987. Petrology and geochemistry of igneous rocks from the Tonga Trench – A non-accreting plate boundary. Journal of Geology 95:469–495, https://doi.org/​10.1086/629144.

Bloomer, S.H., B. Taylor, C.J. MacLeod, R.J. Stern, P. Fryer, J.W. Hawkins, and L. Johnson. 1995. Early arc volcanism and the ophiolite problem: A perspective from drilling in the western Pacific. Pp 1–30 in Active Margins and Marginal Basins of the Western Pacific. B. Taylor and J. Natland, eds, Geophysical Monograph 88, American Geophysical Union, Washington, DC, https://doi.org/10.1029/GM088p0001.

Brandl, P.A., M. Hamada, R.J. Arculus, K. Johnson, K.M. Marsaglia, I.P. Savov, O. Ishizuka, and H. Li. 2017. The arc arises: The link between volcanic output, arc evolution and melt composition. Earth and Planetary Science Letters 461:73–84, https://doi.org/​10.1016/j.epsl.2016.12.027.

Buchs, D.M., R.J. Arculus, P.O. Baumgartner, C. Baumgartner-Mora, and A. Ulianov. 2010. Late Cretaceous arc development on the SW margin of the Caribbean plate: Insights from the Golfito, Costa Rica, and Azuero, Panama, complexes. Geochemistry, Geophysics, Geosystems 11, Q07S24, https://doi.org/10.1029/2009GC002901.

Cameron, W.E., E.G. Nisbet, and V.J. Dietrich. 1979. Boninites, komatiites and ophiolitic basalts. Nature 280:550–553, https://doi.org/​10.1038/​280550a0.

Cloos, M. 1993. Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts. Geological Society of America Bulletin 105:715–737, https://doi.org/10.1130/0016-7606(1993)105​<0715:LBACOS>2.3.CO;2.

Cluzel., D., M. Whitten, S. Meffre, J.C. Aitchison, and P. Maurizot. 2018. A reappraisal of the Poya Terrane (new Caledonia): Accreted late Cretaceous-Paleocene marginal basin upper crust, passive margin sediments, and early Eocene E-MORB sill complex. Tectonics 37:48–70, https://doi.org/​10.1002/2017TC004579.

Conrad, C.P., and C. Lithgow-Bertelloni. 2002. How mantle slabs drive plate tectonics. Science 298:207–209, https://doi.org/10.1126/science.1074161.

Dallwitz, W.B., D.H. Green, and J.E. Thompson. 1966. Clinoenstatite in a volcanic rock from the Cape Vogel area, Papua. Journal of Petrology 7:375–403, https://doi.org/10.1093/petrology/7.3.375.

DeBari, S.M., B. Taylor, K. Spencer, and K. Fujioka. 1999. A trapped Philippine Sea plate origin for MORB from the inner slope of he Izu-Bonin Trench. Earth and Planetary Science Letters 174:183–197, https://doi.org/10.1016/S0012-821X(99)00252-6.

Dewey, J.F. 1975. Plate tectonics. Reviews of Geophysics and Space Physics 13:326–332, https://doi.org/10.1029/RG013i003p00326.

Dewey, J.F., and J.F. Casey. 2011. The origin of obducted large-slab ophiolitic complexes. Pp. 431–443 in Arc-Continent Collision. D. Brown and P.D. Ryan, eds, Frontiers in Earth Sciences, Springer-Verlag.

Dietrich, V.J., R. Emmermann, R. Oberhänsli, and H. Puchelt. 1978. Geochemistry of basaltic and gabbroic rocks from the West Mariana Basin and the Mariana Trench. Earth and Planetary Science Letters 39:127–144, https://doi.org/​10.1016/​0012-821X(78)90149-8.

Dilek, Y., and H. Furnes. 2014. Ophiolites and their origins. Elements 10:93–100, https://doi.org/10.2113/gselements.10.2.93.

Forsyth, D., and S. Uyeda. 1975. On the relative importance of the driving forces of plate motion. Geophysical Journal of the Royal Astronomical Society 43:163–200, https://doi.org/10.1111/j.1365-246X.1975.tb00631.x.

Francis, D. 1986. The pyroxene paradox in MORB glasses—A signature of picritic parental magmas? Nature 319:586–589, https://doi.org/​10.1038/319586a0.

Fryer, P. 2012. Serpentinite mud volcanism: Observations, processes, and implications. 2012. Annual Review of Marine Science 4:345–373, https://doi.org/10.1146/annurev-marine-120710-100922.

Gale, A., C.A. Dalton, C.H. Langmuir, Y. Su, and J.-G. Schilling. 2013. The mean composition of ocean ridge basalts. Geochemistry, Geophysics, Geosystems 13, Q02005, https://doi.org/​10.1029/​2012GC004334.

Gass, I.G. 1968. Is the Troodos Massif of Cyprus a fragment of Mesozoic ocean floor? Nature 220:39–42, https://doi.org/​10.1038/​220039a0.

Gass, I.G., C.R. Neary, J. Plant, A.H.F. Robertson, K.O. Simonian, J.D. Smewing, E.T.C. Spooner, and R.A.M. Wilson. 1975. Comments on “The Troodos ophiolitic complex was probably formed in an island arc” by A. Miyashiro and subsequent correspondence by A. Hynes and A. Miyashiro. Earth and Planetary Science Letters 25:236–238, https://doi.org/10.1016/0012-821X(75)90202-2.

Gurnis, M., and Müller, R.D., 2003. Origin of the Australian-Antarctic Discordance from an ancient slab and mantle wedge. Pp. 417–429 in Evolution and Dynamics of the Australian Plate. R.R. Hillis and R.D. Müller, eds, Geological Society of America Special Papers, vol. 372, https://doi.org/​10.1130/​0-8137-2372-8.417.

Gurnis, M., C. Hall, and L. Lavier. 2004. Evolving force balance during incipient subduction. Geochemistry, Geophysics, Geosystems 5(7), Q07001, https://doi.org/​10.1029/​2003GC000681.

Hall, R. 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: Computer-based reconstructions, model and animations. Journal of Asian Earth Sciences 20:353–431, https://doi.org/10.1016/S1367-9120(01)00069-4.

Hall, R. 2018. The subduction initiation stage of the Wilson cycle. In Fifty years of the Wilson Cycle Concept in Plate Tectonics. R.W. Wilson, G.A. Houseman, K.J.W. McCaffrey, A.G. Doré, and S.J.H. Buiter, eds, Geological Society of London Special Publications, vol. 470, https://doi.org/​10.1144/SP470.3.

Herzberg, C. 2004. Partial crystallization of mid-ocean ridge basalts in the crust and mantle. Journal of Petrology 45:2,389–2,405, https://doi.org/10.1093/petrology/egh040.

Hess, H.H. 1962. History of ocean basins. Pp. 599–620 in Petrologic Studies. A.E.J. Engle, H.L. James, and B.F. Leonard, eds., Geological Society of America, https://doi.org/10.1130/Petrologic.1962.599.

Hickey-Vargas, R., G.M. Yogodzinski, O. Ishizuka, A. McCarthy, M. Bizimis, Y. Kusano, I.P. Savov, and R.J. Arculus. 2018. Origin of depleted basalts during subduction initiation and early development of the Izu-Bonin-Mariana island arc: Evidence from IODP expedition 351 site U1438, Amami-Sankaku basin. Geochimica et Cosmochimica Acta 229:85–111, https://doi.org/10.1016/j.gca.2018.03.007.

Holmes, A. 1928. Radioactivity and Earth movements. Transactions of the Geological Society of Glasgow 18:559–606.

Ishizuka, O., J. Kimura, Y. Li, R. Stern, M. Reagan, R. Taylor, Y. Ohara, S. Bloomer, T. Ishii, and U. Hargrove III. 2006. Early stages in the evolution of Izu–Bonin arc volcanism: New age, chemical, and isotopic constraints. Earth and Planetary Science Letters 250:385–401, https://doi.org/​10.1016/j.epsl.2006.08.007.

Ishizuka, O., K. Tani, and M.K. Reagan, K. Kanayama, S. Umino, Y. Harigane, I. Sakamoto, Y. Miyajima, M. Yuasa, and D.J. Dunkley. 2011. The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306:229–240, https://doi.org/10.1016/​j.epsl.2011.04.006.

Ishizuka, O., R.N. Taylor, Y. Ohara, and M. Yuasa. 2013. Upwelling, rifting, and age-progressive magmatism from the Oki-Daito mantle plume. Geology 41:1,011–1,014, https://doi.org/10.1130/G34525.1.

Ishizuka, O., K. Tani, and M.K. Reagan. 2014. Izu-Bonin-Mariana forearc crust as a modern ophiolite analogue. Elements 10:115–120, https://doi.org/​10.2113/​gselements.10.2.115.

Ishizuka, O., R. Hickey-Vargas, R.J. Arculus, G.M. Yogodzinski, I.P. Savov, Y. Kusano, A. McCarthy, P.A. Brandl, and M. Sudo. 2018. Age of Izu-Bonin-Mariana arc basement. Earth and Planetary Science Letters 481:80–90, https://doi.org/​10.1016/​j.epsl.2017.10.023.

Jenner, F.E., and H.St.C. O’Neill. 2012. Analysis of 60 elements in 616 ocean floor basaltic glasses. Geochemistry, Geophysics, Geosystems 13, Q02005, https://doi.org/10.1029/2011GC004009.

Kuroda, N., K. Shiraki, and H. Urano. 1978. Boninite as a possible calc-alkalic primary magma. Bulletin Volcanologique 41(4):563–575, https://doi.org/​10.1007/BF02597387.

Lake, P. 1931. Island arcs and mountain building. The Geographical Journal 78:149–157, https://doi.org/​10.2307/1784446.

Le Bas, M.J. 2000. IUGS reclassification of the high-Mg and picritic volcanic rocks. Journal of Petrology 41:1,467–1,470, https://doi.org/10.1093/petrology/41.10.1467.

Leng, W., and M. Gurnis. 2015. Subduction initiation at relic arcs. Geophysical Research Letters 42:7,014–7,021, https://doi.org/​10.1002/​2015GL064985.

Li, C., R.D. van der Hilst, E.R. Engdahl, and S. Burdick. 2008. A new global model for P wave speed variations in Earth’s mantle. Geochemistry, Geophysics, Geosystems 9, Q05018, https://doi.org/​10.1029/​2007GC001806.

Li, H.Y., R. Taylor, J. Prytulak, J. Shervais, J.G. Ryan, M. Godard, M.K. Reagan, and J.A. Pearce. 2017. Isotopic constraints on magma source evolution during subduction initiation; IODP 352 (Izu-Bonin Forearc). V.M. Goldschmidt Conference: Program and Abstracts, vol. 27.

Machida, S., T. Ishii, J-I. Kimura, S. Awaji, and Y. Kato. 2008. Petrology and geochemistry of cross-chains in the Izu-Bonin back arc: Three mantle components with contributions of hydrous liquids from a deeply subducted slab. Geochemistry, Geophysics, Geosystems 9, Q05002, https://doi.org/​10.1029/​2007GC001641.

Mao, X., M. Gurnis, and D.A. May. 2017. Subduction initiation with vertical lithospheric heterogeneities and new fault formation. Geophysical Research Letters 44:11,349–11,356, https://doi.org/​10.1002/2017GL075389.

Matthews, K.J., S.E. Williams, J.M. Whittaker, R.D. Müller, M. Seton, and G.L. Clarke. 2015. Geologic and kinematic constraints on Late Cretaceous to mid-Eocene plate boundaries in the southwest Pacific. Earth-Science Reviews 140:72–107, https://doi.org/10.1016/​j.earscirev.2014.10.008.

Matthews, K.J., K.T. Maloney, S. Zahirovic, S.E. Williams, M. Seton, and R.D. Müller. 2016a. Global plate boundary evolution and kinematics since the late Paleozoic. Global and Planetary Change 146:226–250, https://doi.org/10.1016/​j.gloplacha.2016.10.002.

Matthews, K.J., R.D. Müller, and D.T. Sandwell. 2016b. Oceanic microplate formation records the onset of India-Euraisa collision. Earth and Planetary Science Letters 433:206–214, https://doi.org/10.1016/j.epsl.​2015.10.040.

McKenzie, D.P., and R.L. Parker. 1967. The North Pacific: An example of tectonics on a sphere. Nature 216:1,276–1,280, https://doi.org/​10.1038/​2161276a0.

McKenzie, D.P. 1977. The initiation of trenches: A finite amplitude instability. Pp. 57–61 in Island Arcs, Deep Sea Trenches and Back-Arc Basins. M. Talwani and W.C. Pitman III, eds, Maurice Ewing Series, vol. 1, American Geophysical Union, Washington, DC, https://doi.org/10.1029/ME001p0057.

Meckel, T.A., M.F. Coffin, S. Mosher, P. Symonds, G. Bernardel, and P. Mann. 2003. Underthrusting at the Hjort Trench, Australian-Pacific plate boundary: Incipient subduction? Geochemistry, Geophysics, Geosystems 4, https://doi.org/​10.1029/​2002GC000498.

Meffre, S., T.J. Falloon, A.J. Crawford, K. Hoernle, F. Hauff, R.A. Duncan, S.H. Bloomer, and D.J. Wright. 2012. Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench. Geochemistry, Geophysics, Geosystems 13, Q12003, https://doi.org/​10.1029/​2012GC004335.

Miyashiro, A. 1973. The Troodos ophiolitic complex was probably formed in an island arc. Earth and Planetary Science Letters 19:218–224, https://doi.org/​10.1016/​0012-821X(73)90118-0.

Nakamura, K. 1977. Volcanoes as possible indicators of tectonic stress orientation – Principle and proposal. Journal of Volcanology and Geothermal Research 2:1–16, https://doi.org/​10.1016/0377-0273(77)90012-9.

O’Connor, J.M., B. Steinberger, M. Regelous, A.A. Koppers, J.R. Wijbrans, K.M. Haase, P. Stoffers, W. Jokat, and D. Garbe-Schönberg. 2013. Constraints on past plate and mantle motion from new ages for the Hawaiian-Emperor Seamount Chain. Geochemistry, Geophysics, Geosystems 14:4,564–4,584, https://doi.org/​10.1002/ggge.20267.

O’Neill, H.St.C., and F.E. Jenner. 2012. The global pattern of trace-element distributions in ocean floor basalts. Nature 491:698–705, https://doi.org/​10.1038/nature11678.

O’Neill, H.St.C. 2016. The smoothness and shapes of chondrite-normalized rare earth element patterns in basalts. Journal of Petrology 57:1,463–1,508, https://doi.org/10.1093/petrology/egw047.

Pearce, J.A., and J.R. Cann. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters 19:290–300, https://doi.org/​10.1016/0012-821X(73)90129-5.

Pearce, J.A., and D.W. Peate. 1995. Tectonic implications of the composition of volcanic arc magmas. Annual Review of Earth and Planetary Science 23:251–285, https://doi.org/10.1146/annurev.ea.23.050195.001343.

Pearce, J.A. 2003. Supra-subduction zone ophiolites: The search for modern analogues. Pp. 269–293 in Ophiolite Concept and the Evolution of Geological Thought. Geological Society of America Special Papers, vol. 373, https://doi.org/​10.1130/​0-8137-2373-6.269.

Petersen, J. 1891. Der boninit von Peel Island. Jahrbuch der Hamburgischen Wissenschaftlichen Anstalten 8:341–349.

Petterson, M.G., T. Babbs, C.R. Neal, J.J. Mahoney, A.D. Saunders, R.A. Duncan, D. Tolia, R. Magu, C. Qopoto, H. Mahoa, and D. Natogga. 1999. Geological-tectonic framework of Solomon Islands, SW Pacific: Crustal accretion and growth within an intra-oceanic setting. Tectonophysics 301:35–60, https://doi.org/10.1016/S0040-1951(98)00214-5.

Pope, E.L., M. Jutzeler, M.J.B. Cartigny, J. Shreeve, P.J. Talling, I.C. Wright, and R.J. Wysoczanski. 2018. Origin of spectacular fields of submarine sediment waves around volcanic islands. Earth and Planetary Science Letters 493:12–24, https://doi.org/10.1016/​j.epsl.2018.04.020.

Reagan, M.K., and A. Meijer. 1984. Geology and geochemistry of early arc-volcanic rocks from Guam. Geological Society of America Bulletin 95:701–713, https://doi.org/​10.1130/​0016-​7606​(1984)95​<701:​GAGOEA>​2.0.CO;2.

Reagan, M.K., O. Ishizuka, R.J. Stern, K.A. Kelley, Y. Ohara, J. Blichert-Toft, S.H. Bloomer, J. Cash, P. Fryer, B.B. Hannan, and others. 2010. Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system. Geochemistry, Geophysics, Geosystems 11, https://doi.org/​10.1029/2009GC002871.

Reagan, M.K., W.C. McClelland, G. Girard, K.R. Goff, D.W. Peate, Y. Ohara, and R.J. Stern. 2013. The geology of the southern Mariana fore-arc crust: Implications for the scale of Eocene volcanism in the western Pacific. Earth and Planetary Science Letters 380:41–51, https://doi.org/10.1016/​j.epsl.2013.08.013.

Reagan, M.K., J.A. Pearce, K. Petronotis, R.R. Almeev, A.J. Avery, C. Carvallo, T. Chapman, G.L. Christeson, E.C. Ferré, M. Godard, and others. 2017. Subduction initiation and ophiolite crust: New insights from IODP drilling. International Geology Review 59:1,439–1,450, https://doi.org/​10.1080/​00206814.​2016.1276482.

Reagan, M.K., D.E. Heaton, M.S. Schmitz, J.A. Pearce, J.W. Shervais, and A.P. Koppers. 2019. Forearc ages reveal extensive short-lived and rapid seafloor spreading following subduction initiation. Earth and Planetary Science Letters 506:520–529, https://doi.org/​10.1016/j.epsl.2018.11.020.

Ritsema, J., H.J. van Heijst, and J.H. Woodhouse. 2004. Global transition zone tomography. Journal of Geophysical Research 109(B2), https://doi.org/​10.1029/2003JB002610.

Schellart, W.P., and W. Spakman. 2012. Mantle constraints on the plate tectonic evolution of the Tonga-Kermadec-Hikurangi subduction zone and the South Fiji Basin region. Australian Journal of Earth Sciences 59:933–952, https://doi.org/​10.1080/​08120099.​2012.679692.

Schmidt, M.W., and O. Jagoutz. 2017. The global systematics of primitive arc melts. Geochemistry, Geophysics, Geosystems 18:2,817–2,854, https://doi.org/​10.1002/2016GC006699.

Seton, M., R.D. Müller, S. Zahirovic, C. Gaina, T. Torsvik, G. Shephard, A. Talsma, M. Gurnis, M. Turner, S. Maus, and M. Chandler. 2012. Global continental and ocean basin reconstructions since 200 Ma. Earth-Science Reviews 113:212–270, https://doi.org/10.1016/j.earscirev.2012.03.002.

Seton, M., N. Flament, J. Whittaker, R.D. Müller, M. Gurnis, and D.J. Bower. 2015. Ridge subduction sparked reorganization of the Pacific plate-mantle system 60-50 million years ago. Geophysical Research Letters 42:1,732–1,740, https://doi.org/​10.1002/2015GL063057.

Sharp, W.D., and D.A. Clague. 2006. 50-Ma initiation of Hawaiian-Emperor Bend records major change in Pacific plate motion. Science 313:1,281–1,284, https://doi.org/10.1126/science.1128489.

Shervais, J.W., M.K. Reagan, E. Haugen, R. Almeev, J.A. Pearce, J. Prytulak, J.G. Ryan, S.A. Whattam, M. Godard, T. Chapman, and others. 2018. Magmatic response to subduction initiation: Part I. Fore-arc basalts of the Izu-Bonin Arc from IODP Expedition 352. Geochemistry, Geophysics, Geosystems, https://doi.org/​10.1029/​2018GC007731.

Shiraki, K., N. Kuroda, H. Urano, and S. Maruyama. 1980. Clinoenstatite in boninites from the Bonin Islands, Japan. Nature 285:31–32, https://doi.org/​10.1038/285031a0.

Sigurdsson, H., and J.-G. Schilling. 1976. Spinels in mid-Atlantic Ridge basalts: Chemistry and occurrence. Earth and Planetary Science Letters 29:7–20, https://doi.org/​10.1016/​0012-​821X(76)90021-2.

Smyth, J.R. 1974. Experimental study on the polymorphism of enstatite. American Mineralogist 59:345–352.

Sollas, W.J. 1903. The figure of the Earth. Quarterly Journal of the Geological Society 59:180–188, https://doi.org/10.1144/GSL.JGS.1903.059.01-04.18.

Stern, R.J., and S.H. Bloomer. 1992. Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California arcs. Geological Society of America Bulletin 104:1,621–1,636, https://doi.org/​10.1130/​0016-7606​(1992)104​<1621:SZIEFT>​2.3.CO;2.

Stern, R.J. 2004. Subduction initiation: Spontaneous and induced. Earth and Planetary Science Letters 226:275–292, https://doi.org/10.1016/​j.epsl.2004.08.007.

Stern, R.J., and T. Gerya. 2018. Subduction initiation in nature and models: A review. Tectonophysics 746:173–198, https://doi.org/​10.1016/​j.tecto.2017.10.014.

Straub, S.M., J.D. Woodhead, and R.J. Arculus. 2015. Temporal evolution of the Mariana arc: Wedge and subducted slab controls revealed with a tephra perspective. Journal of Petrology 56:409–439, https://doi.org/10.1093/petrology/egv005.

Sutherland, R., J. Collot, F. Bache, S. Henrys, D. Barker, G.H. Browne, M.J.F. Lawrence, H.E.G. Morgans, C.J. Hollis, C. Clowes, and others. 2017. Widespread compression associated with Eocene Tonga-Kermadec subduction initiation. Geology 45:355–358, https://doi.org/10.1130/G38617.1.

Sutherland, R., G.R. Dickens, P. Blum, and Expedition 371 Scientists. 2018. Expedition 371 Preliminary Report: Tasman Frontier Subduction Initiation and Paleogene Climate. International Ocean Discovery Program, https://doi.org/10.14379/iodp.pr.371.2018.

Tamura, Y., Y. Tatsumi, D. Zhao, Y. Kido, and H. Shukuno. 2002. Hot fingers in the mantle wedge: New insights into magma genesis in subduction zones. Earth and Planetary Science Letters 197:105–116, https://doi.org/10.1016/S0012-​821X(02)00465-X.

Tatsumi, Y. 1989. Migration of fluid phases and genesis of basalt magmas in subduction zones. Journal of Geophysical Research 94:4,697–4,707, https://doi.org/10.1029/JB094iB04p04697.

Taylor, B., and A.M. Goodliffe. 2004. The West Philippine Basin and the initiation of subduction, revisited. Geophysical Research Letters 31, L12602, https://doi.org/10.1029/2004GL020136.

Todd, E., J.B. Gill, and J.A. Pearce. 2012. A variably enriched mantle wedge and contrasting melt types during arc stages following subduction initiation in Fiji and Tonga, southwest Pacific. Earth and Planetary Science Letters 335–336:180–194, https://doi.org/10.1016/j.epsl.2012.05.006.

Umbgrove, J.H.F. 1945. Different types of island-arcs in the Pacific. The Geographical Journal 106:198–209, https://doi.org/​10.2307/​1788957.

Umino, S. 1985. Volcanic geology of Chichijima, the Bonin islands (Ogasawara islands). Journal of the Geological Society of Japan 91:505–523, https://doi.org/10.5575/geosoc.91.505.

Umino, S., K. Kitamura, K. Kanayama, A. Tamura, N. Sakamoto, O. Ishizuka, and S. Arai. 2015. Thermal and chemical evolution of the subarc mantle revealed by spinel-hosted melt inclusions in boninite from the Ogasawara (Bonin) Archipelago. Geology 43(2):151–154, https://doi.org/10.1130/G36191.1.

Uyeda, S., and Z. Ben-Avraham. 1972. Origin and development of the Philippine Sea. Nature Physical Science 240:176–178, https://doi.org/10.1038/physci240176a0.

Wadati, K. 1931. Shallow and deep earthquakes. Geophysical Magazine (Tokyo) 4:231–285.

Walker, D.A., and W.E. Cameron. 1983. Boninite primary magmas: Evidence from the Cape Vogel Peninsula, PNG. Contributions to Mineralogy and Petrology 83:150–158, https://doi.org/10.1007/BF00373088.

Whattam, S.A., and R.J. Stern. 2011. The ‘subduction initiation rule’: A key for linking ophiolites, intra-oceanic forearcs and subduction initiation. Contributions to Mineralogy and Petrology 162:1,031–1,045, https://doi.org/10.1007/s00410-011-0638-z.

Whittaker, J.M., R.D. Müller, G. Leitchenkov, H. Stagg, M. Sdrolias, C. Gaina, and A. Goncharov. 2007. Major Australian-Antarctic plate reorganization at Hawaii-Emperor bend time. Science 318:83–86, https://doi.org/10.1126/science.1143769.

Wilson, J.T. 1965. A new class of faults and their bearing on continental drift. Nature 207:343–347, https://doi.org/10.1038/207343a0.

Woelki, D., M. Regelous, K.M. Haase, R.H.W. Romer, and C. Beier. 2018. Petrogenesis of boninitic lavas from the Troodos Ophiolite, and comparison with Izu-Bonin-Mariana fore-arc crust. Earth and Planetary Science Letters 498:203–214, https://doi.org/​10.1016/j.epsl.2018.06.041.

Wu, J., J. Suppe, R. Lu, and R. Kanda. 2016. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods. Journal of Geophysical Research 121:4,670–4,741, https://doi.org/​10.1002/2016JB012923.

Yogodzinski, G.M., M. Bizimis, R. Hickey-Vargas, A. McCarthy, B.D. Hocking, I.P. Savov, O. Ishizuka, R. Arculus. 2018. Implications of Eocene-age Philippine Sea and forearc basalts for initiation and early history of the Izu-Bonin-Mariana arc. Geochimica et Cosmochimca Acta 228:136–156, https://doi.org/10.1016/j.gca.2018.02.047.

Zhou, X., Z.-H. Li, T.V. Gerya, R.J. Stern, Z. Xu, and J. Zhang. 2018. Subduction initiation dynamics along a transform fault control trench curvature and ophiolite ages. Geology 46:607–610, https://doi.org/​10.1130/G40154.1.

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