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Completion of the ST2B-2 (Subduction Top

to Bottom 2) Themed Issue in Geosphere

Gray E. Bebout, Dept. of Earth and Environmental Sciences, Particularly in the larger sessions
Lehigh University, Bethlehem, Pennsylvania 18015, USA; David (in both oral and poster venues),
W. Scholl, Emeritus, U.S. Geological Survey, Menlo Park, one could figuratively traverse a
California 94025, USA, and University of Alaska Fairbanks, subduction zone system, from top
Fairbanks, Alaska 99775, USA; Robert J. Stern, Dept. of to bottom, beginning on the seafloor,
Geosciences, University of Texas at Dallas, Richardson, Texas continuing through the forearc and
75083, USA; Laura M. Wallace, University of Texas Institute for subarc, and, in some cases, through to
Geophysics, Austin, Texas 78758, USA, and GNS Science, Lower the base of the mantle where ancient
Hutt, New Zealand; and Philippe Agard, Sorbonne Université, subducted lithospheric slabs reside.
CNRS-INSU, Institut des Sciences de la Terre Paris, ISTeP UMR Presentations were arranged by depth
7193, 75005 Paris, France and process, not by approach or meth-
ods, and presenters often found them-
Over the past 25 years, tremendous advances have been made selves presenting adjacent to someone
in our understanding of convergent plate margins. These advances with a quite different but stimulating per-
have increasingly been made by highly multidisciplinary groups spective. In recent years, ST2B has shifted focus to highlight par-
involving the fields of geophysics, petrology, geochemistry, and ticular depth-horizons or products of subduction margins, with a
geodynamics, to a large extent made possible by NSF-funded initia- 2021 session “Focus on the Forearc” and a 2022 session “Origin
tives like MARGINS, GeoPRISMS, and SZ4D. Understanding of and Evolution of Magmatic Arcs.”
the dynamics of subduction is particularly important for assessing This brief note is to call attention to the now-completed
earthquake, tsunami, and volcanic hazards. The scientific commu- Subduction Top to Bottom 2 (ST2B-2) Themed Issue in the all-
nity, governments and the broader public increasingly recognize the online GSA journal Geosphere. This issue contains 74 papers,
need to assess hazards that subduction margins pose, especially to more than twice as many papers as GM96, and is published only
regions of high population densities around the Pacific and Indian electronically, thus with fewer constraints on length and format
Oceans (e.g., Japan, Indonesia, the Cascadia margin). As we’ve and the inclusion of color graphics and supplementary materials.
examined individual margins in greater detail and contrasted them Unlike the GM96 book project, these 74 papers appeared when-
with other margins, patterns have emerged that reveal some of the ready, without the need to await the full assembly of the volume,
controls on convergent margin behavior and evolution. These and some of the papers have been among the most-cited in
include convergence rate and obliquity; age of incoming plate and Geosphere for several years running. The issue contains assess-
the subduction zone itself; physical, thermal, and chemical state of ment of subduction from all perspectives, in many cases in multi-
the subducting oceanic lithosphere; presence of seamounts and disciplinary combinations of geological, geophysical, geochemi-
other heterogeneities on the downgoing plate; the nature and thick- cal, and theoretical approaches.
ness of subducting sediments; accretion versus erosion; and the Naturally, we begin on the seafloor, evaluating sedimentary and
composition and structure of the upper plate. lithospheric inputs, outer-rise plate bending and related hydration,
The “Subduction Top to Bottom” endeavor has fostered and and incipient diagenesis and fluid release and flow. Papers on
highlighted multidisciplinary research on modern and ancient sub- forearc regions provide syntheses of knowledge of what leads to
duction margins, in part through sponsoring of sessions at recent rupture, including consideration of the roles of fluids and the pro-
conferences (AGU, GSA) but also through large publication proj- cesses leading to some particularly damaging earthquakes. For
ects. The first publication was inspired by the 1994 SUBCON greater depths, an array of papers considers processes leading to the
held in Avalon, Santa Catalina Island, California, USA. The generation of magmatic arcs, including merging knowledge of arcs
SUBCON meeting resulted in the publication of AGU Geophysical with that of metamorphic processes in deeply subducting oceanic
Monograph v. 96 (GM96), in which 35 papers provided a top-to- slabs and sediments. Related to this, another set of papers considers
bottom survey of the state of knowledge of subduction dynamics, the make-up of the subduction interface, in part as viewed from
as of 1996. Much in the way of multidisciplinary research on sub- high- and ultrahigh-pressure metamorphic exposures.
duction has since happened and, over the last five or so years, We encourage you to check out the ST2B-2 Themed Issue in
we sponsored an update, Subduction Top to Bottom 2 (ST2B-2). Geosphere and take a virtual spin through Earth’s unique subduc-
Many will have noticed the ST2B-2 sessions run at AGU and tion zones, top-to-bottom!
GSA meetings over these years—together, these sessions have
attracted 526 abstracts, and one session had 136 presentations! https://pubs.geoscienceworld.org/geosphere/pages/st2b2

32 GSA TODAY | November 2022