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CC Attribution 3.0 License. Open Access Open Access The Cryosphere The Cryosphere Discussions Introduction to the special issue on “Subduction Zones” S. J. H. Buiter1,2, F. Funiciello3, and J. van Hunen4 1Geological Survey of Norway, Geodynamics Team, Leiv Eirikssons vei 39, 7040 Trondheim, Norway 2University of Oslo, The Centre for Earth Evolution and Dynamics, 0316 Oslo, Norway 3Universita` Roma Tre, Dipartimento di Scienze, L.go S. Leonardo Murialdo 1, 00146, Rome, Italy 4Durham University, Department of Earth Sciences, Durham DH1 3LE, UK Correspondence to: S. J. H. Buiter ([email protected]) 1 The subduction process dip, nature of the overriding plate, back-arc tectonic regime, behaviour at the 660 km-discontinuity, seismicity, and vol- canic productivity of the arc (e.g., Jarrard, 1986; Pacheco et The discoveries in the previous century of continental drift, al., 1993; Heuret and Lallemand, 2005; Acocella and Funi- convection in Earth’s mantle, and the formation of oceanic ciello, 2009; Heuret et al., 2011; Hayes et al., 2012). lithosphere at mid-ocean ridges set the stage for the reali- Subduction is a complex dynamic process, which occurs sation that oceanic material is continuously recycled back over long timescales and to large depths and integrates small- into the mantle (e.g., Wegener, 1918; Holmes, 1931; Run- scale with large-scale phenomena. This process can, there- corn, 1956; Hess, 1962; Vine and Matthews, 1963; Wilson, fore, only be understood by adopting a multidisciplinary ap- 1966). This idea is also confirmed by evidence that no in situ proach integrating geological observations and geophysical oceanic lithosphere older than ca. 270 Ma exists on Earth imaging of regions of past and present subduction, with geo- (Muller¨ et al., 2008; Fig. 1a). The process of sinking of cold chemical fingerprinting of materials recycled in the subduc- oceanic lithosphere into the mantle, occurring at convergent tion system, and studies of subduction dynamics in space and margins, is called subduction. time. The direct association with devastating earthquakes and Subduction is not only responsible for mixing surface ma- active volcanism signifies a social need for an understand- terial back to the deep Earth and introducing significant ing of all aspects of this process. Decades of research have chemical variation back into the mantle (Christensen and furthered our understanding of the large-scale kinematics of Hofmann, 1994), but also for driving plate motions (Forsyth subduction processes and the close links between subducting and Ueda, 1975), mountain building and the growth of new lithosphere, mantle flow, and surface deformation. However, continental crust (Davidson and Arculus, 2006). Subduction past research has made it very clear that several fundamen- also deeply modifies the thermal structure of the mantle and tal questions remain unanswered. We therefore considered it accounts for arc volcanism, in addition to causing substan- timely to bring together a selection of papers that highlight tial surface (vertical and horizontal) deformations and re- the current state of subduction research. leasing most of the seismic energy of the Earth. The sub- duction process plays, therefore, a key role in the geody- namical and geochemical phenomena that shape our Earth 2 The Subduction Zones special issue and, for this reason, has led to a more-than-average atten- tion from the Earth Science community. But, despite these In this special issue we follow the subducting lithosphere on efforts, subduction still holds many enigmas and open ques- its journey from the surface to the lower mantle (Fig. 1b). tions. The most important are embedded in the lack of a Using tomographic imaging, numerical modelling, and field common behaviour of global subduction zones (e.g., Uyeda observations, the contributing articles outline several chal- and Kanamori, 1978). Some subduction zones are long-lived, lenges in the subduction system. In the text below, references such as the Andes subduction zone, which is over 100 million to articles in the special issue are in italics. years old, whereas others have just initiated, such as the New Subduction zones are associated with thrust earthquakes Hebrides and Puysegur subduction zones. Present-day sub- on the interplate contact and with intermediate-to-deep slab duction zones are also diverse in terms of trench motion, slab seismicity, posing a serious threat to communities living Published by Copernicus Publications on behalf of the European Geosciences Union. 130 S. J. H. Buiter et al.: Introduction “Subduction Zones” Steinberger et al.; a) Warren van Keken et al. Ramachandran Tavani & Hyndman Bottrill et al. Tong et al. Jaxybulatov et al. Subduction zone Age of ocean crust Continent Continental shelf 0 50 100 150 200 250 Ma b) Trench 1 Arcay (2012); Ramachandran & Hyndman (2012) 1 5 2 Tong et al. (2012); 2 van Keken et al. (2012) 4 3 3 Bentson & van Keken (2012) 4 Warren (2013) 410 km 5 Li & Hampel (2012); Tavani (2012) 6 Bottrill et al. (2012); 660 km 6 Magni et al. (2012); Jaxybulatov et al. (2013) Slab 7 Chertova et al. (2012); 7 Steinberger et al. (2012) Fig. 1. (a) Map of present-day subduction systems, and (b) schematic subduction zone cross-section with areas of study of the Subduction Zones special issue indicated as dashed boxes. Age of ocean crust from Muller¨ et al. (2008). close to these convergent regions (Fig. 1a). Previous stud- elling results with observations, it suggests a moderate-to- ies have found that faults in the upper part of a subduc- strong viscosity reduction of the asthenosphere due to meta- tion zone are susceptible to changes in ice and water loads somatism of the mantle wedge. (e.g., Chochran et al. 2004; Luttrell and Sandwell, 2010). Li The thermal structure of a subduction system is important and Hampel (2012) use 2-D finite element models to show for understanding subduction seismicity, slab dehydration, that glacial–interglacial sea level variations can be responsi- arc volcanism and geochemistry. Bengtson and van Keken ble for significant stress variations within convergent plates (2012) investigate to which extent the common approach of and, in turn, for the earthquake cycle of subduction thrust using 2-D cross-sections provides a reasonable image of the faults. Earthquakes are promoted during sea-level fall and 3-D system. 3-D numerical models of the thermal structure delayed during sea-level rise. Arcay (2012) finds that sub- and the flow regime in the mantle wedge show that 2-D cross- duction interplate dynamics are strongly dependent on both sections are reasonable for arcs that have minor along-strike ductile and brittle strength parameters. The evolution of the variations. However, a full 3-D model is unavoidable to sim- depth of the brittle-to-ductile transition and the kinematic de- ulate strongly curved subduction systems, such as the Mar- coupling depth along the subduction interface are both in- ianas, where significant 3-D mantle flow is induced in the fluenced by the interplate friction coefficient and the mantle mantle wedge by the subduction process. wedge strength. When comparing the thermochemical mod- Solid Earth, 4, 129–133, 2013 www.solid-earth.net/4/129/2013/ S. J. H. Buiter et al.: Introduction “Subduction Zones” 131 When an oceanic basin closes and continental lithosphere raphy to constrain fluids released from the young and hot enters a subduction zone, the positive buoyancy of the conti- Cascadia subducting lithosphere. The seismic velocities indi- nental crust will resist subduction, leading to a slow-down, cate substantial amounts of serpentinite in the forearc mantle, and ultimately to the end of subduction (e.g., McKenzie, lending itself towards weakness, which has important conse- 1969). Continental collision is accompanied by severe crustal quences for earthquake potential and collision tectonics. Sig- and lithospheric deformation, mountain building, slab break- nificant silica deposition at the base of the lower crust makes off, metamorphism, and the exhumation of high and ultra- it a primary contributor to the average composition of the high pressure [(U)HP] rocks. Warren (2013) reviews mech- continental crust. The Pacific slab subducting under north- anisms to subduct continental crust to (U)HP conditions and east Japan is an example of a cold subduction system due exhume the rocks back to Earth’s surface. The record of ex- to the high convergence rate and old age of the subducting humation is complicated by the fact that exhumation mecha- lithosphere. Tong et al. (2012) present P- and S-wave seis- nisms may change in time and space within the same subduc- mic tomography that image fluids released from dehydration tion zone. Buoyancy and external tectonic forces drive ex- of the subducting Pacific lithosphere under the 2011 Iwaki humation, but the changing spatial and temporal dominance earthquake area and Fukushima nuclear power plant location.