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Twenty Years of Subduction Zone Science: Subduction Top to Bottom 2 (ST2B-2)

G.E. Bebout*, Dept. of and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA; D.W. Scholl, emeritus, Geological Survey, Menlo Park, California 94025, USA, and University of Fairbanks, Fairbanks, Alaska 99775, USA; R.J. Stern, Dept. of Geosciences, University of Texas at Dallas, Richardson, Texas 75083, USA; L.M. Wallace, University of Texas Institute for , Austin, Texas 78758, USA, and GNS Science, Lower Hutt, ; and P. Agard, Institut des Sciences de la Terre de Paris, UPMC Université Paris 06, CNRS, Sorbonne Universités, Paris, France

ABSTRACT become increasingly interested in the pro- resultant publication encourages continued No other plate-tectonic setting has cess since the term was introduced by efforts along these lines. attracted such diverse, multidisciplinary White et al. (1970; also see the prescient “SUBCON” (Subduction Conference) research as convergent margins. Under­ sketch of a subduction zone by Coats, was held in Avalon, Santa Catalina , standing the dynamics of subduction is 1962) on the heels of the plate tectonic rev- California, USA, on 12–17 June 1994, particularly important for realistic assess- olution. Subduction zones are where litho- largely funded by the United States ment of associated hazards such as earth- sphere is recycled into the and they Geological Survey (USGS) but with addi- quakes, , and volcanic eruptions. provide the third dimension for the tional support from the National Science A number of recent initiatives have been ~55,000 km length of convergent plate Foundation (NSF). At this conference, successful in building communities not margins. The sinking of in sub- ~120 scientists from around the world only to investigate subduction processes, duction zones provides most of the power shared their understanding of subduction but also to convey knowledge about sub- for plate motions and is directly respon- zone dynamics as a function of depth— duction zone processes to other scientists, sible for crustal deformation and arc mag- from top to bottom—bringing together students, postdocs, and the broader pub- matism. Convergent margin processes observations and predictions from the lic. These efforts must include synthesiz- affect directly (volcanic gasses) many diverse perspectives of the group. ing and simplifying subduction-zone sci- and indirectly (producing relief and stimu- Although it is now common to see multi- ence for classroom presentations and to lating weathering), contributing impor- disciplinary groups working together to help prepare the public for subduction- tantly to maintaining Earth’s habitability, understand subduction, this was not so in related disasters. at the same time producing societal ben- the mid-1990s. To many at SUBCON, the Tremendous advances over the past efits (ore deposits and hydrocarbon-rich audience to which they revealed their work 20 years or so have been made in subduc- basins) and some of the most dangerous was unusually diverse in interests and tion zone science, with increasingly natural hazards (, tsunamis, expertise, resulting in some surprising and multidisciplinary efforts producing some and explosive ). In spite of their useful feedback. As a part of SUBCON, of the greatest insights. We have initiated importance, subduction zones are not easy the group visited selected outcrops of the a publication effort in the GSA journal to study because they are hidden deeply in Catalina Schist exposed on the island and, Geosphere, with a Themed Issue the Earth; teasing out their secrets has for many of the participants, this was a “Subduction Top to Bottom 2” (or involved years of efforts by geologists, first view of an ancient subduction zone. “ST2B-2”) aimed at showcasing the geophysicists, and geochemists from many SUBCON and its aftermath helped lead to recent advances, following up on the con- different countries working on land, at , some very productive multidisciplinary ceptually similar Subduction Top to and using observations from space. The collaborations, including the 1996 publica- Bottom published in 1996 as an American enormity of the challenge of studying sub- tion of Subduction Top to Bottom (Bebout Geophysical Union Geophysical duction zones has spurred efforts to build a et al., 1996, American Geophysical Union Monograph. The ST2B-2 Geosphere cross-disciplinary community of govern- [AGU] Geophysical Monograph 96, some- Themed Issue is accumulating papers ment, academic, and industrial geoscien- times called “Big Purple” because of the and is open to ALL wishing to contribute tists from many nations, and for members cover). Big Purple contains 39 papers of to this effort—we anticipate accepting of this community to learn how to explain either a review or case-study nature cover- manuscripts through all of 2018 and their findings to scientists with different ing all aspects of subduction zones and possibly beyond. expertise as well as to students and the their products. public. Workshops and edited volumes In the 20 or so years since SUBCON INTRODUCTION play key roles in building this community. and Big Purple, many advances have been Subduction is a uniquely powerful and An excellent early example of this commu- made in the fields of geophysics, petrol- important Earth process, so it is no sur- nity-building effort was held 23 years ago, ogy, , and geodynamics, with prise that the geoscientific community has and the success of this workshop and the the work increasingly conducted by highly

GSA Today, v. 28, doi: 10.1130/GSATG354A.1. Copyright 2017, The Geological Society of America. CC-BY-NC.

*Email: [email protected] multidisciplinary groups. Understanding AGU Geophysical Monograph 138 (J. Eiler, network comprising twelve leading inter- of subduction dynamics has greatly ben- ed., 2003). Major efforts similar to national universities and research centers efited from this approach, leading to an MARGINS and GeoPRISMS have been and nine industrial partners. The U.S. increasingly sophisticated view of subduc- undertaken by subduction communities in GeoPRISMS E-FIRE project (ExTerra tion zones and how they evolve. More Japan, Europe, New Zealand, Central and Field Institute and Research Endeavor: comparative studies have also helped to South America, and southeast Asia, result- Western Alps; http://geoprisms.org/ move the community away from thinking ing in significant investment in subduction exterra/e-fire/), funded by the NSF’s that each convergent margin segment is zone science by their respective national PIRE program (Partnerships in unique and toward seeing subduction as governments. International Research and Education), the central problem, with individual mar- This is an exciting time for those inter- builds on the success of ZIP, providing gins showing different variations—and ested in understanding subduction margins! support for eight Ph.D. students and two thus opportunities for insights—on the In addition to the GeoPRISMS initiative, postdoctoral fellows at 10 academic unifying theme. As we’ve examined indi- the U.S. subduction science community is institutions. vidual margins in greater detail and con- discussing the potential of a “Subduction trasted them with other margins, patterns Zone Observatory,” which is presented in a WHAT HAVE WE LEARNED have emerged that reveal some of the con- “SZ4D Initiative” report aimed at reveal- ABOUT SUBDUCTION IN THE trols on convergent margin behavior and ing the short- and long-term evolution of PAST TWENTY YEARS? evolution. In fact, quite a number of mod- subduction margins. That report resulted As a thought exercise for this report, ern subduction margins show dramatic from an NSF-sponsored workshop in 2016 each of the co-authors assembled a list of along-strike variations in key physical fac- that was attended by 250 scientists from what they consider to be the greatest tors influencing their thermal and mechan- the USA and 22 foreign countries (https:// advancements in subduction zone science ical evolution. A few of these factors are www.iris.edu/hq/workshops/2016/09/ in the past two decades since SUBCON/ convergence rate and obliquity; age of szo_16). The SZ4D Initiative, as presently Big Purple. Several themes were repeated incoming plate and the subduction zone configured, proposes three key compo- on our lists and we agreed on the follow- itself; physical, thermal, and chemical state nents: a modeling component, an interdis- ing. This list can of course be quibbled of the subducting oceanic lithosphere; ciplinary science program, and a commu- with, but it does capture how exciting presence of and other heteroge- nity infrastructure program (see McGuire subduction science has been and the range neities on the downgoing plate; the nature et al., 2017). Its science “net” is cast widely, of approaches that are being used and the and thickness of subducting ; with the aim of fostering integrated geo- diverse research communities that have versus erosion; and the composi- physics, , petrology, geochemistry, been involved: tion and structure of the upper plate. and geodynamic modeling. Planning for Some Advances in Subduction Zone Understanding of the dynamics of sub- future subduction zone studies is also Science in the Past 20 Years duction is of particular importance in being undertaken by the USGS, which has assessing the associated hazards of earth- recently announced a major directive, • Improved understanding of how new quakes, tsunamis, and volcanic eruptions. “Reducing Risk Where Tectonic Plates subduction zones form (see the review The scientific community, governments, Collide—A Plan to Advance Subduction by Stern and Gerya, 2018); and the broader public increasingly recog- Zone Science” (https://www.usgs.gov/ • The importance of outer rise normal nize the need to assess hazards that sub- news/usgs-publishes-a-new-blueprint-can- faulting and deep hydration of subducted duction margins pose, especially to regions help-make-subduction-zone-areas-more- lithospheric mantle, and the connection of high population densities around the resilient; Gomberg et al., 2017). As its with deep-seated seismicity (e.g., Pacific and Indian (e.g., Japan, name implies, this initiative aims to focus Ranero et al., 2005; Van Avendonk et al., Indonesia, the Cascadia margin). Recent geological, geophysical, and petrologic/ 2011; Fig. 1); initiatives promoting this effort include geochemical investigations and modeling • Understanding the magnitude and signifi- community-led NSF initiatives such as at understanding and forecasting hazards cance of ; that most MARGINS and its successor GeoPRISMS, associated with subduction plate boundar- convergent margins lose material from in each of which subduction-zone science ies. Naturally, the Cascadia margin figures the upper plate and only a minority add has constituted a major component. The prominently in this planned endeavor material by growing accretionary prisms two initiatives have identified “focus sites” because of the large , , (e.g., Scholl and von Huene, 2007); where effort was concentrated, and these and volcanic hazards it poses to the • Exploration of submarine arc and back- included MARGINS Subduction Factory increasingly populated Pacific Northwest arc basin volcanoes and associated and SEIZE (SEIsmogenic ZonE) efforts in region. Another example of an initiative hydrothermal systems and volcaniclastic , Izu-Bonin-Mariana, and emphasizing study of subduction processes sedimentation, especially in the Izu- Nankai, and GeoPRISMS Subduction and hazards is the ZIP project (Zooming in Bonin-Mariana and Tonga Kermadec Cycles and Deformation focus sites of the between Plates), which is a collaborative systems (e.g., Baker et al., 2008; Dziak Cascadia, Aleutian/Alaska, and New research and training project funded by the et al., 2015); Zealand subduction zones. The MARGINS- European community as a European Marie • Recent major subduction earthquakes sponsored Theoretical Institute, “Inside the Curie Initial Training Network (http://www (such as the 2004 Sumatra and the 2011 Subduction Factory,” in 2000 was a par- .zip-itn.eu/). This project involves 12 Ph.D. Tohoku-Oki earthquakes) have taught us ticularly important milestone, resulting in students and two postdoctoral fellows in a important lessons about what is possible Hydration outer rise subduction zones, revealing intricacies volcanic between short- (seconds) and long-term arc (million years) deformation on plate Moho interfaces (see Figs. 4 and 5; Angiboust 1 seismogenic et al., 2012a, 2012b), the volumetric zone Serpentinization2 importance of subcrustal accretionary Moho underplating (e.g., Bassett et al., 2010) 30 versus frontal accretion, as well as pro- thrust 3 bend-faulting viding insights about chemical cycling in and above subduction zones (see the 4 60 review by Bebout, 2014, and references dehydration Melting therein); • Studies of ultrahigh-pressure metamor-

[km] phic rocks, coupled with thermome- 90 chanical models, demonstrating that Conceptual Model dehydration5 oceanic and can be of the Tectonic + subducted to >100 km depth and and Metamorphic Elements of a returned to the surface (e.g., Gerya et al., 120 Subduction Zone De-serpentinization 2002; Yamato et al., 2008); • Improved understanding of the nature of 6 supercritical fluids, where they exist in 150 and above subduction zones, and their mass transport capabilities (via experi- Figure 1. Cartoon showing a conceptual model of the structure and metamorphic evolution of sub- mental and theoretical approaches; e.g., ducting lithosphere formed at a fast spreading center (from Ranero et al., 2005). The topography of Manning, 2004; Hermann et al., 2006), the plate in the outer-rise/ region has been exaggerated to better show the deformation associated with plate bending. Scale is otherwise approximately correct. plane solutions of and appreciation of the tremendous earthquakes are projected into the top of the and the plane of the cross section. The small amounts of subducted that could black-filled circles in indicate hydration. be stored in the mantle ; • Microanalytical advances allowing mea- surement of volatiles and trace element in these great events. For example, com- • Increased appreciation of the role that contents in minerals and melt inclusions, pletely unexpected and massive near- fluids play in subduction margin further constraining chemical cycling trench slip (up to 50 m) in the 2011 mechanics and seismogenesis (Saffer through subduction zones (Frezzotti et Tohoku-Oki earthquake (Fujiwara et al., and Tobin, 2011); al., 2011) and the causes of explosive arc 2011) has important implications for • Seismological advances that better volcanism (e.g., Wallace, 2005; Zellmer tsunamigenesis and the nature of slip resolve earthquake structure and mecha- et al., 2015); near the trench; nisms in the downgoing plate (e.g., • Improved understanding of chemical • Discovery of slow-slip events in sub- Rondenay et al., 2008; Shillington et al., recycling via subduction of oceanic duction zones, and the spectrum of 2015) and improved tomography to crust, sediment, and uppermost mantle seismological and geodetic phenomena image the subducted slab at depths (e.g., Plank, 2005), especially the related to slow slip (see Fig, 2, which is greater than the 670 km limit of earth- cycling of volatiles at convergent mar- from Gomberg et al., 2010). quakes (van der Hilst et al., 1997); gins (e.g., Hilton et al., 2002; Mason et Densification of continuous GPS net- • Accelerating exploration of the deep al., 2017) and technical advances works above many subduction zones oceanic because of technologi- enabling field measurements of arc vol- has been especially important in this cal advances in manned submersibles, canic gas emissions (e.g., Fischer and effort (see the discussion by Gomberg remotely operated vehicles, and autono- Lopez, 2016). et al., 2017); mous undersea vehicles (e.g., Cui et al., Now, ~20 years after SUBCON and Big • Statistical and observational documen- 2013; Okumura et al., 2016); Purple, we feel it is the right time to put tation that high-magnitude megathrust • Massively increased computational together another dedicated volume high- earthquakes may be linked to where power allowing corresponding advances lighting these major breakthroughs. In this wide expanses of thick sediment and in numerical modeling of subduction effort, we return to the philosophy of pre- bathymetrically smooth seafloor enter zone thermal structure, , vious ventures for an updated volume subduction zones (e.g., Brizzi et al., rheology, and chemical budgets (e.g., called “Subduction Top to Bottom 2,” or 2018), while subduction margins with van Keken et al., 2011; Hacker and ST2B-2 for short, that is now soliciting rough incoming plates and low sedi- Gerya, 2013; see Figs. 3 and 4); manuscripts. The goal of the ST2B-2 mentary thicknesses appear to be domi- • Greater understanding of connections Geosphere special issue is to assemble a nated by aseismic creep and more mod- between studies of exhumed paleo-sub- large number of papers arranged by the sub- erate-sized earthquakes (Wang and duction complexes (high- and ultrahigh- duction-zone depth horizon they consider, Bilek, 2011); pressure rocks) and processes in active again, independent of the methods used. the rheology and mechanical failure of rocks lead to greater understanding of the relationships between devolatilization and other metamorphic reactions and observed seismicity (e.g., Incel et al., 2017)? Can we decipher the balance of material delivered through an individual subduction margin by combining knowledge of inputs derived through deep-sea drilling, heat flow measurements, thermal modeling, thermodynamic calculations, analysis of ancient metamorphic rocks, and analyses of volcanic gases?

THE MAKEUP OF ST2B-2 The ST2B-2 endeavor is intended to generate a large, online themed issue in the Geological Society of America jour- nal Geosphere. With this online format— in our view a clear example of the future of scientific communication—we are unencumbered by page limits imposed by a physical book and by costs of color fig- Figure 2. Illustration of northern 2007 (ETS) event in Cascadia (from Gom- berg et al., 2010). The oceanic subducts beneath the North America plate at ures; furthermore, this format encourages ~4 cm/yr roughly perpendicular to the coast (white arrow). The plates are coupled for part of their use of interactive graphics and online interface (tan-colored surface) such that relative motion is inhibited or “locked” to a varying degree. data sets. Individual papers are published Uncertain are the location and mechanism by which the locking changes to a freely slipping inter- face. The fraction of relative plate motion is portrayed as continuous aseismic slip that increases soon after manuscripts are accepted; there down-dip from 40% to 80% (dashed contours). Inland of the locked zone, tremor epicenters pro- is no waiting for the slowest author(s). jected onto the plate interface (circles) overlie the area that experienced slow slip (gray area on plate interface) during the last two weeks of January 2007. Color shading of tremor epicenters Published works in Geosphere are land- shows its temporal migration. scape-format and so more amenable to

We particularly encourage manuscripts 50 employing diverse observations and meth- British Columbia ods to identify problems where differing 70 Mexico disciplines examine similar processes. Cascadia To raise awareness of the new volume, we 90 have been holding “Subduction Top to 110 Bottom 2” sessions (Fall 2016 AGU and N Peru gap 2017 GSA Annual and Fall AGU meetings). 130 C Peru gap Izu As examples of where multidisciplinary Nankai New Zealand Solomon Colombia-Ecuador pursuits could be particularly fruitful, 150 C Chile gap

C Sumatra N-C Chile S Chile forearc seismic events commonly originate N Sumatra

depth (km) 170 Scotia Alaska along the active subduction interface or in S-C Chile Bonin N Chile accreted sediments experiencing pressure- New Britain 190 C Chile N Kurile Costa Rica E S Kurile temperature conditions preserved in Kamchatka AK Peninsula Peru S Marianas Java N Vanuatu Ryukyu forearc metasedimentary suites represent- Bali-Lombok C Honshu 210 N Honshu S Vanuatu Kyushu S Lesser Antilles N -El Salvador Hokkaido Tonga Aegean S Sumatra ing similar but ancient processes. Might W Banda Sea Calabria Kermadec S Philippines E Aleutians N Lesser Antilles examination of the 230 Sunda Strait W Aleutians N Marianas C Aleutians record better tell us how slow slip events 250 and related seismic phenomena (such as 015 015202530 tremor) happen (see Fig. 2) and, more spe- slab H2O loss (Tg/Myr/m) cifically, the roles of fluids in generating such events? Could highly brecciated Figure 3. Diverse H2O loss calculated as a function of depth for oceanic lithosphere and sediment zones of from ancient subduction subducting into each of Earth’s modern subduction zones (the “Tokyo Subway Map” from van Keken et al., 2011). The warmest subduction zones lose most of their H2O beneath the forearc. All zones be the products of catastrophic subducting slabs lose significant water when the slab comes into contact with the hot overlying energy release along the interface that gen- (in these models, at 80 km depth). For many slabs (e.g., Kamchatka, Calabria) further dehydration is minor. Other slabs (e.g., Chile) continue to dehydrate significantly with increasing erated ancient earthquakes (see Fig. 5)? depth principally due to the dehydration of the uppermost mantle. A few slabs (e.g., Marianas) are

Could laboratory experiments regarding very cool, and far less H2O is lost to even 230 km depths. Figure 4. Schematic view of a subduction zone between 35 and 85 km depth based on numerical model results (and on study of exhumed/exposed ophiolitic ter- ranes) showing inferred morphol- ogies and the detachment of large folded slices of oceanic lithosphere, accreted along the plate interface (from Angiboust et al., 2012b). This figure also illustrates the main deformation- enhanced fluid pathways (associ- ated with deep serpentinite producing/consuming reactions), dominantly at the boundary between materials with marked rheological contrasts.

viewing on a computer screen. For com- aseismic ridges). AGEs: Mike 13. Geochemical and seismological parison, Big Purple faced strict manuscript Underwood and Andy Fisher; expressions of deep subducted slabs. length guidelines, authors did their own 3. Forces driving subduction—thermal AGEs: Catherine Chauvel and formatting (following the old style of AGU and geodynamic modeling. AGE: Stéphane Rondenay; journals), resulting in a fairly unpolished Taras Gerya; 14. Backarc basins, cross chains, and appearance, publication of color graphics 4. Getting started (subduction initiation). -and-thrust belts. AGEs: Fernando was expensive (and thus few authors pub- AGE: Mark Reagan; Martinez and Ron Hyndman; lished color graphics), and we had a strict 5. Outer rise (slab bending, deep hydra- 15. Resource implications. AGEs: Gray time deadline in order to keep the book tion of slabs). AGEs: Doug Wiens, Bebout, Bob Stern, and Dave Scholl; publication project on schedule (resulting Cesar Ranero; 16. Crust formation at convergent mar- in some authors ultimately not submitting 6. Shallow forearc dynamics (initial gins. AGEs: Kiyoshi Suyehiro and manuscripts). In the end, Big Purple con- dewatering and diagenesis, fluids, Kent Condie; and tained 39 papers, varying greatly in length, accretion, erosion). AGE: Nathan 17. Convergent margin education and with uneven coverage of the full range of Bangs; outreach. AGE: Bob Stern. top-to-bottom subduction science. 7. Deformation of and physical condi- The Geosphere ST2B-2 themed issue can For the all-electronic Geosphere themed tions in the subduction interface from be accessed at https://pubs.geoscienceworld issue, we have identified 17 subduction- the seismogenic zone through the .org/geosphere/pages/st2b2. It is open to zone science categories, arrayed as a func- source of episodic slow slip and ALL wishing to contribute to this effort. tion of increasing depth in a subduction tremor. AGEs: Shuichi Kodaira, Sue Ideally, papers in the issue will cover each zone, beginning with “What Goes In” and Bilek, and Samuel Angiboust; of the 17 topics, and we are optimistic that “Forces Driving Subduction.” Each of 8. Upper plate deformation over varying more than 100 papers will ultimately be these science categories has one to three timescales. AGEs: Frédérique Leclerc published. We anticipate that submissions assistant guest editors (AGEs) assigned to and Nathalie Feuillet; for the ST2B-2 themed issue will be identifying authors invited to contribute 9. Into the pressure cooker (metamor- accepted at the least through the end of manuscripts and be the contact individuals phism, fluid-rock interactions, records 2018, and we encourage anyone interested for those wanting to submit manuscripts. of deep underplating and , in contributing to contact either one of the The science categories (and the associated nature of deep subduction interface; five guest editors (one of the five of us) or AGEs) are as follows—at the time of also including arc delamination and the AGE(s) associated with the science acceptance of this paper, Geosphere drips). AGEs: Sarah Penniston- category into which you envision your con- ST2B-2 had amassed ~40 manuscripts in Dorland and Ake Fagereng; tribution fitting. various stages of review, revision, produc- 10. Forearc to subarc mantle wedge. AGEs: tion, and publication. Maureen Long and Marco Scambelluri; ACKNOWLEDGMENTS 11. Subduction zone magmatism (models We thank Shan de Silva, science editor of Outline of ST2B-2 Geosphere Themed for evolution, petrology, geochemis- Geosphere, for working with us to plan and Issue try, and isotopes, including batho- coordinate the assembly of this themed issue. Many thanks to reviewer Peter Kelemen for his 1. Introduction; liths). AGE: Paul Wallace; constructive input, particularly in the assembly of 2. What goes in (seafloor lithosphere 12. Explosive volcanism hazards. AGE: the list of “Some Advances in Subduction Zone and sediment, seamounts, and Bob Tilling; Science in the Past 20 Years,” to the other, anonymous reviewer, and to Steve Whitmeyer for Letters, v. 42, p. 1480–1487, https://doi.org/ his efforts as GSA Today editor. 10.1002/2014GL062603. 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