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