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