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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
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