Twenty Years of Subduction Zone Science: Subduction Top to Bottom 2 (ST2B-2) G.E. Bebout*, Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA; D.W. Scholl, emeritus, United States Geological Survey, Menlo Park, California 94025, USA, and University of Alaska 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 Geophysics, Austin, Texas 78758, USA, and GNS Science, Lower Hutt, New Zealand; 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 Island, 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 mantle and they Geological Survey (USGS) but with addi- quakes, tsunamis, 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 lithosphere 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 climate 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 (earthquakes, tsunamis, expertise, resulting in some surprising and multidisciplinary efforts producing some and explosive volcanism). 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 sea, 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, geochemistry, 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 seamounts 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 sediments; with the aim of fostering integrated geo- diverse research communities that have accretion versus erosion; and the composi- physics, geology, 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