SCIENCE REVIEWS Subduction initiation Michael Gurnis (California Institute of Technology), Harm Van Avendonk (University of Texas Institute for Geophysics), and Mark K. Reagan (University of Iowa) he initiation of new subduction zones is a key component Te growth and stability of island arcs is a factor in the global mass of the plate tectonic cycle and evolution of continental crust balance of continental crust. When a young slab descends into the (Stern and Scholl, 2010). Subducted slabs are the primary mantle, magmatism may be enhanced, especially if subduction was Tforce driving plate motions and formation of new subduction zones initiated by a phase of extension and mantle decompression (Stern and demise of existing ones is associated with the largest changes and Bloomer, 1992). More mature subduction systems may continue in the force balance on tectonic plates (Gurnis et al., 2004). Since to add igneous material, but an equal amount of lower arc crust subduction zones are also a primary location for volatile cycling, may be lost by gravitational instability (Behn and Kelemen, 2006). major temporal changes in subduction are expected to lead to Te continental crust mass balance may therefore depend on the changes in outgassing and reinjection of volatiles into the mantle diference between magmatic additions at arcs versus forearc loss (van Keken et al., 2011). Despite the importance of subduction due to tectonic erosion. Subduction initiation may be a mechanism initiation, our understanding of the associated geological record, to maintain a positive or net balance over time. geophysical and geochemical character, and geodynamical causes Major episodes of subduction could play an important role in and consequences remain poorly understood. Addressing the cycling CO₂ and other volatiles between the atmosphere and initiation of subduction was a major focus of the MARGINS and ocean and mantle. Paleogeographic reconstructions have most GeoPRISMS programs through collection of key observations at volcanism occurring along continental-arcs, where subduction primary sites and in exhumed terranes from ancient subduction zone magmatism can react with carbonate-rich crust and generate zones. high concentrations of atmospheric CO₂. Initiation of ocean-ocean Although slab pull is the primary driving force for plate motions, subduction zones around the Pacifc during the early Eocene likely key features of this force balance remain a research focus. How impacted CO₂ fuxes to the atmosphere (Lee et al., 2013; Reagan et much of the negative buoyancy from cold slabs is transmitted to the al., 2013), although the direction of this shif and degree it impacted converging oceanic plate depends on several factors, notably on plate global climate remains uncertain (Kirtland-Turner et al., 2014). bending at oceanic trenches, interplate resistance, and drag around Wilson (1966) recognized that the Atlantic Ocean opened near the edges of slabs and base of plates (Conrad and Lithgow-Bertelloni, continental sutures in Paleozoic orogenic belts, with older ocean 2002; Stadler et al., 2010). Given the concentration of driving and basins opening and closing there in the past. Tis Wilson cycle resisting forces within the slab, the initiation and termination of requires failure of passive margins, but theoretical studies suggest subduction likely determine changes in the rates and direction of the forces required to fail thick lithosphere of a passive margin are plate motions. too great, even with a sediment load (Mueller and Phillips, 1991). Figure 1. Ages for subducton initaton events centered on the Pacifc determined from isotopic ages of arc initaton and stratgraphic, structural, plate tectonic and seismic indicators of subducton initaton. The colors of the trenches represent the latest initaton (reinitaton) age, while those of the underlying thick line represent the long-term age. Plate boundaries and absolute plate motons from Müller et al. (2016). Reprint from Hu and Gurnis (2020) by permission of John Wiley and Sons. 10 cm/yr 0 100 200 Age (Ma) 8 • GeoPRISMS Newsletter Issue No. 43 Final Volume 35° 0 A B Bonin Trench 10 30° ASB U1439- U1442 U1438 Spreading in Shikoku Bonin Ridge Shikoku & Parece DR 20 Arc Tholeites & Basin Vela Basins 25° Calc alkaline vol. ODR 30 Spreading Kyushu Palau (Ma) Age Guagua Ridge 20° Ridge 40 forearc Parece Vela forearc Basin Boninites 15°15˚ Central Basin 50 Spreading Center MarianaTrench U1438 Guam Gabbros & Basalts Basalts 10°10˚ 60 120° 125° 130° 135° 140° 145° 150° CBSC ASB KPR SPV Bonin Mar Topo (m) -5000 -2500 0 2500 5000 Figure 2. A. Locaton Map and sites for ages for key events discussed in the text associated with initaton of the IBM system. Uncertaintes on the ages of forearc basalts, gabbros and boninites denoted with lighter color shading. Abbreviatons are: DR for Daito Ridge; ODR for Oki-Daito Ridge, ASB for Amami Sankaku Basin, CBSC for Central Basin Spreading Center, KPR for Kyushu Palau Ridge, SPV for the Shikoku and Parece Vela Basins, and Mar for the Marianas. Recent plate reconstructions have continent-ocean plate boundaries Izu-Bonin-Mariana (IBM) accommodating thousands of kilometers of strike-slip during The Izu-Bonin-Mariana system with its present-day high opening and closing of oceans (Dalziel and Dewey, 2019). Actively convergence rate, simple geometry, and intact geological record shearing plate boundaries are weaker than passive margins, so strike- has been recognized as an outstanding region for studying the slip motion may precede subduction initiation. “subduction factory” (Fig. 2). Decades of geophysical research on The key reason that consensus on a unified description of IBM illustrated its structure and state (Kodaira et al., 2007). Te subduction initiation has been slow to develop is that initiation is a inputs through plate convergence (Kelley et al., 2003; Takahashi transient process whose record is generally obscured by subsequent et al., 2008) and outputs across the arc and back-arc have been subduction zone processes, most notably burial, overprinting, assessed through on-land feld work and recovery of samples from uplif, and compression and over-thrusting. In contract to rifing, the sea foor by diving, dredging, and drilling (Pearce et al., 2005; subduction initiation is the seed for the destruction of its own record. Kelley, et al., 2010; Heywood et al., 2020). Subduction initiation has Despite these limitations, there is a growing acceptance of a diverse been well-studied at IBM because the earliest record of volcanism record of Cenozoic subduction initiation events (Fig. 1). Much of the related to this process is preserved (Stern and Bloomer, 1992; Pearce recent work on subduction initiation has exploited the geological, et al. 1992; Reagan et al., 2010; Li et al., 2019), and the tectonic confguration during the time of initiation has been determined geochemical and geophysical record of these subduction initiation through plate tectonic reconstruction (Hall, 2002; Leng and Gurnis, events which represent over half of subduction zones existing today 2015; Wu et al., 2016). (Gurnis, et al., 2004). Te IBM subduction zone is on the eastern edge of the Philippine Sea Te GeoPRISMS and MARGINS programs targeted primary sites for Plate (Fig. 2) and based on the age of igneous rocks in the forearc, interdisciplinary research as a means to make accelerated progress. the subduction zone initiated at about 52.5 Ma (Reagan et al., 2019). Following the success of MARGINS and the Ocean Drilling Program To the west of the present trench, there are several active and rifed (ODP), the GeoPRISMS and International Ocean Discovery Program arcs, active and relict back arc basins, extinct spreading centers (IODP) 2013-2023 Science Plans highlighted subduction initiation as and other tectonic features. Te active arc and forearc reconstruct a key question that would beneft from this concerted, site-focused to the position of the Kyushu-Palau Ridge (KPR) at about 33 Ma approach. Four IODP Expeditions and a major seismic experiment based on the magnetic lineations and fracture zones within the all in the western Pacifc followed to address these questions. At the Shikoku and Parece Vela Basins. KPR, a long N-S feature, is a relict same time, these programs have been complemented by expanded arc of IBM subduction (Ishizuka, et al., 2011a). West of KPR is the focus on other sites, geological synthesis, geochemical analysis, and West Philippine Sea Basin with features predating- and post-dating modelling, which are reviewed here subduction initiation. Final Volume Issue No. 43 GeoPRISMS Newsletter • 9 Most spreading of the generally E-W Central Basin Spreading Center (Yogodzinsky et al., 2018). With rare exception, fresh pillow-rind postdates subduction initiation, but initially it was thought to predate basalt glasses sampled during Expedition 352 lack evidence for the it, leading to the hypothesis that subduction initiated along an old involvement of subducted fuids in their genesis (Coulthard et al., fracture zone (Uyeda and Ben-Avraham, 1972). 2017). Te high SiO2 concentrations of the forearc suggest generation IODP Expeditions 351 and 352 were mounted to constrain patterns from mantle with a relatively steep geothermal gradient (Shervais in volcanism and lithosphere generation diagnostic of subduction et al., 2019). Afer less than 1.2 million years and while seafoor initiation. Expedition 352 drilled four sites (U1439-U1442) in the spreading was ongoing, water-rich melts became involved in magma forearc to obtain a volcanic record of subduction initiation and with genesis, resulting in a transition to low-Si boninite (see Pearce and earlier diving showed that earliest volcanism produced distinctive Reagan, 2019). About the time that seafoor spreading ceased (c. basaltic to boninitic crust with identical ages at locations that are 51.3 Ma) and the arc began to be established, high-Si boninites currently 1,700 km apart (Cosca et al., 1998; Reagan, et al., 2010; began erupting (Reagan et al., 2019). Eruption of these boninites Ishizuka, et al., 2011b), but may have been about 500 km when they migrated away from the trench and continued to about 44Ma, when formed (Leng and Gurnis, 2015).