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Hydration outer rise volcaanricc subduction zones, revealing intricacies
between short- (seconds) and long-term
Moho (million years) deformation on plate
interfaces (see Figs. 4 and 5; Angiboust
1 sezisomneogenic et al., 2012a, 2012b), the volumetric
importance of subcrustal accretionary
Serpentiniz2ation Moho 30 underplating (e.g., Bassett et al., 2010)
versus frontal accretion, as well as pro-
tbhernudst-faulting 3 viding insights about chemical cycling
in and above subduction zones (see the
Sdeedhiymderanttio4n Melting 60 review by Bebout, 2014, and references
therein);
[km] • Studies of ultrahigh-pressure metamor-
phic rocks, coupled with thermome-
oESaCnfuloetdbnhmdcMeeeueTnpcttettasiuocmaotnolfonZMaripocohndiecel dCereuhcsylo+tdgraittiizoan5tion 90 chanical models, demonstrating that
120 oceanic and continental crust can be
De-serpentinization 150 subducted to >100 km depth and
returned to the surface (e.g., Gerya et al.,
6 2002; Yamato et al., 2008);
• Improved understanding of the nature of
Figure 1. Cartoon showing a conceptual model of the structure and metamorphic evolution of sub- supercritical fluids, where they exist in
ducting lithosphere formed at a fast spreading center (from Ranero et al., 2005). The topography of and above subduction zones, and their
the plate in the outer-rise/trench region has been exaggerated to better show the deformation mass transport capabilities (via experi-
associated with plate bending. Scale is otherwise approximately correct. Fault plane solutions of mental and theoretical approaches; e.g.,
earthquakes are projected into the top of the slab and the plane of the cross section. The small Manning, 2004; Hermann et al., 2006),
black-filled circles in oceanic crust indicate hydration. and appreciation of the tremendous
amounts of subducted water that could
in these great events. For example, com- • Increased appreciation of the role that be stored in the mantle transition zone;
pletely unexpected and massive near- fluids play in subduction margin • Microanalytical advances allowing mea-
trench slip (up to 50 m) in the 2011 mechanics and seismogenesis (Saffer surement of volatiles and trace element
Tohoku-Oki earthquake (Fujiwara et al., and Tobin, 2011); contents in minerals and melt inclusions,
2011) has important implications for further constraining chemical cycling
tsunamigenesis and the nature of slip • Seismological advances that better through subduction zones (Frezzotti et
near the trench; resolve earthquake structure and mecha- al., 2011) and the causes of explosive arc
• Discovery of slow-slip events in sub- nisms in the downgoing plate (e.g., volcanism (e.g., Wallace, 2005; Zellmer
duction zones, and the spectrum of Rondenay et al., 2008; Shillington et al., et al., 2015);
seismological and geodetic phenomena 2015) and improved tomography to • Improved understanding of chemical
related to slow slip (see Fig, 2, which is image the subducted slab at depths recycling via subduction of oceanic
from Gomberg et al., 2010). greater than the 670 km limit of earth- crust, sediment, and uppermost mantle
Densification of continuous GPS net- quakes (van der Hilst et al., 1997); (e.g., Plank, 2005), especially the
works above many subduction zones cycling of volatiles at convergent mar-
has been especially important in this • Accelerating exploration of the deep gins (e.g., Hilton et al., 2002; Mason et
effort (see the discussion by Gomberg oceanic trenches because of technologi- al., 2017) and technical advances
et al., 2017); cal advances in manned submersibles, enabling field measurements of arc vol-
• Statistical and observational documen- remotely operated vehicles, and autono- canic gas emissions (e.g., Fischer and
tation that high-magnitude megathrust mous undersea vehicles (e.g., Cui et al., Lopez, 2016).
earthquakes may be linked to where 2013; Okumura et al., 2016);
wide expanses of thick sediment and Now, ~20 years after SUBCON and Big
bathymetrically smooth seafloor enter • Massively increased computational Purple, we feel it is the right time to put
subduction zones (e.g., Brizzi et al., power allowing corresponding advances together another dedicated volume high-
2018), while subduction margins with in numerical modeling of subduction lighting these major breakthroughs. In this
rough incoming plates and low sedi- zone thermal structure, metamorphism, effort, we return to the philosophy of pre-
mentary thicknesses appear to be domi- rheology, and chemical budgets (e.g., vious ventures for an updated volume
nated by aseismic creep and more mod- van Keken et al., 2011; Hacker and called “Subduction Top to Bottom 2,” or
erate-sized earthquakes (Wang and Gerya, 2013; see Figs. 3 and 4); ST2B-2 for short, that is now soliciting
Bilek, 2011); manuscripts. The goal of the ST2B-2
• Greater understanding of connections Geosphere special issue is to assemble a
between studies of exhumed paleo-sub- large number of papers arranged by the sub-
duction complexes (high- and ultrahigh- duction-zone depth horizon they consider,
pressure rocks) and processes in active again, independent of the methods used.

6 GSA Today | February 2018