Constraints on Fluids in Subduction Zones from Electromagnetic Data Anne Pommier1 and Rob L

Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Constraints on fluids in subduction zones from electromagnetic data Anne Pommier1 and Rob L. Evans2 1Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA GEOSPHERE; v. 13, no. 4 2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA

doi:10.1130/GES01473.1 ABSTRACT bound in the crystallographic structure of minerals (e.g., Iwamori, 1998). These 7 figures volatiles are then released and transported through metamorphism and melt- Magnetotelluric data have been increasingly used to image subduction ing processes (e.g., Hermann et al., 2006; Iwamori et al., 2007). The hydrous CORRESPONDENCE: revans@whoi.edu zones. Models of electrical resistivity commonly show features related to the fluid–melt cycle in subduction zones can be constrained through geophysical release of fluids at several depths through the systems imaged, consistent investigations that image fluid-bearing reservoirs and channels in present-day CITATION: Pommier, A., and Evans, R.L., 2017, Con with thermal and petrologic models of dehydration of the downgoing slab. subduction zones, in particular by electromagnetic (EM) techniques (e.g., Mc- straints on fluids in subduction zones from electro magnetic data: Geosphere, v. 13, no. 4, p. 1026–1041, Imaging the release of fluids from sediments and pore space in the crust re- Gary et al., 2014; Pommier, 2014a). Because electrical conductivity is sensitive doi:10.1130/GES01473.1. quires controlled source electromagnetic techniques, which have to date only to thermal and compositional changes (e.g., Yoshino, 2010, and references been used in one setting, offshore Nicaragua. The release of fluids related to therein; Pommier, 2014b, and references therein; Tyburczy and DuFrane, 2015), Received 2 December 2016 the transition of basalt to eclogite is commonly imaged with magnetotelluric EM profiles across subduction zones can be used to constrain both time and Revision received 1 April 2017 data. Deeper fluid release signals, from the breakdown of minerals like serpen- space evolution of subduction, and in particular, locate the origin of primary Accepted 4 May 2017 Published online 20 June 2017 tine, are highly variable. We hypothesize that regions where very strong con- arc magmas, constrain the extent of melting, and identify the main chemical ductive anomalies are observed in the mantle wedge at depths of ~80–100 km fluxes from shallow slab dehydration to deeper partial melting, thus providing are related to the subduction of anomalous seafloor, either related to exces- critical constraints for petrological and geodynamic modeling. sive fracturing of the crust (e.g., fracture zones), subduction of seamounts, EM studies of subduction zones have increased significantly over the past or other ridges and areas of high relief. These features deform the seafloor decade, and numerous soundings have been completed. A compilation of prior to entering the trench, permitting more widespread serpentinization of global results suggested the widespread presence of a lower crustal–upper the mantle than would otherwise occur. An alternative explanation is that the mantle (15–30 km depth) conductor situated on the trench side of the volcanic large conductors represent melts with higher contents of crustal-derived vola- arc (Worzewski et al., 2010; Fig. 1) that is attributed to an accumulation of aque- tiles (such as C and H), suggesting in particular locally higher fluxes of carbon ous fluids. In some cases, this interpretation seems appropriate; however, into the mantle wedge, perhaps also associated with subduction of anoma- there are also instances where such conductors are probably the signature lous seafloor structures with greater degrees of hydrothermal alteration. of melt accumulation (e.g., McGary et al., 2014). Untangling these competing possibilities requires detailed EM transects. Ideal studies would be those that permit a view of an entire system from the undeformed incoming plate across INTRODUCTION the volcanic arc, as well as providing insights into along-strike variability in fluid release. There are almost no studies that meet those criteria, and so The cycle of fluids in subduction zones, especially volatile-bearing melts, here we consider the best data sets available. Two EM approaches have been is a major component of slab recycling, arc volcanism, and continent-building used in subduction settings. Controlled source electromagnetics (CSEM) is a processes. A better understanding of the role of melt and volatiles in subduc- tool used for crustal-scale studies. It has been used in one subduction zone tion, combining field and laboratory-based electrical models, is key to improv- setting, offshore Nicaragua, where it constrains the free fluid budget of the ing our knowledge of the thermal and petrologic structure of subduction zones incoming crust (Key et al., 2012; Naif et al., 2015) and identifies processes of as well as the dynamic processes at work. It can also help us better assess shallow fluid release and transport related to squeezing of pore fluids and the tangible hazards in these contexts by examining, for example, links between breakdown of clay minerals (Naif et al., 2016). EM methods are not sensitive fluid release and seismicity and properties of the megathrust that can rupture to the hydrous alteration products of the oceanic crust, and so constraints and generate tsunamis and volcanic eruptions (e.g., Polet and Kanamori, 2000; on this component of the incoming fluid budget must come from seismic Walter and Amelung, 2007; Davey and Ristau, 2011). methods (e.g., Horning et al., 2016). The magnetotelluric method (MT) is used Though fluids and volatiles efficiently subducted via pore fluids are largely for deeper imaging into the mantle where it is well suited for imaging the For permission to copy, contact Copyright expelled by compaction at shallow depth (Moore and Vrolijk, 1992; Solomon release of aqueous fluids from the slab and the generation and transport of Permissions, GSA, or [email protected]. et al., 2009), some amount of volatiles persists to greater depth (>15 km), melt to the volcanic arc. MT is not, however, well suited to mapping resistive

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that are brought to depth, causing conductive anomalies at shallow depths. Crust formed at slow-spreading ridges, where deep penetrating faults as well as low-angle detachment faults are thought to cause widespread mantle serpentinization (e.g., Tucholke et al., 1998; Blackman et al., 1998; deMartin et al., 2007; Canales et al., 2000), might result in much higher fluid inputs into a system than one where the serpentinization occurs at the trench; therefore, an understanding of the basin-wide system back to the ridge can be important, although this is not well quantified. Even though a single subduction system such as Cascadia can be reason- ably quantified by single values for the primary descriptors of a subduction system, as we carry out more detailed studies of entire systems we see sub- stantial variability along strike, reminding us that these systems are always more complex than they appear at first glance. Recent MT studies in several

Figure 1. Compilation of the locations of the lower crustal–upper mantle conductor observed on settings have highlighted this variability; we give examples and try to shed the trench side of the arc in several electromagnetic (EM) studies (after Worzewski et al., 2010). light on what other important structural controls or processes can influence Rectangles correspond to the trench. Data from Worzewski et al. (2010, and references therein) the release of fluids and the generation of melts in subduction settings. We and McGary et al. (2014). BC—British Columbia; OR—Oregon; WA—Washington. also highlight the role of a high-relief seafloor (i.e., containing features such as fracture zones or seamounts) on the generation of highly conductive anomalies at depths of ~80–100 km. features, and so frequently has limited ability to constrain the properties of the cool downgoing slab. In most cases it is desirable to include an a priori slab in MT inversion models to maximize the information returned from MT CONDUCTIVITY AS A PROBE OF THE HYDROGEN AND sounding data (e.g., Matsuno et al., 2010; McGary et al., 2014; Ichiki et al., CARBON CYCLES 2015). Many models in the literature that do not impose a priori structure contain resistive features that bear little resemblance to a slab. This is more than a Many of the subduction setting parameters mentioned here work together cosmetic issue: McGary et al. (2014) showed how imposing the slab improves to control the physical evolution of the downgoing slab and are tied to the the fit to the data at key sites, and, as the relaxation of regularization across volatiles cycle (in particular hydrogen and carbon). For example, a relatively the slab surface permits the inversion algorithm to fully explore model space, young (and therefore hot) incoming plate that has a slow convergence will the relaxation permits conductive features associated with fluids and melts to heat at shallow depths, releasing water more rapidly downdip (e.g., van Keken be placed adjacent to the slab where they are generated. In discussing models et al., 2011). herein, where possible, we try to use examples where slabs have been in- cluded in some manner in the inversion process. In all cases, there are limits on our ability to infer the spatial extent and size of conductivity anomalies Hydrogen Cycle and Electrical Anomalies arising from the slab, but these limits are greater where no slab has been imposed in the inversion. Water increases the electrical conductivity of the upper mantle through Subduction zones can be characterized primarily by a few key parameters the diffusion of hydrogen, which enhances the mobility of charge carriers that may be related to each other, i.e., whether the system is ocean-ocean or (e.g., Mg vacancies and polarons in olivine; e.g., Karato, 2013) and triggers ocean-continent, the age of the incoming plate, the speed of convergence, the partial melting by lowering the mantle solidus temperature (Hirschmann obliquity of convergence, and the dip angle of the slab (e.g., Jarrard, 1986). et al., 2009; Sarafian et al., 2017). Therefore, the interpretation of electrical Numerical models that quantify the effects of these parameters on subduc- conductivity models, accounting for phase transitions and dehydration re- tion structure and dynamics are well documented (e.g., Syracuse et al., 2010; actions, should reflect the cycle of water, with conductive zones caused by van Keken et al., 2008; Penniston-Dorland et al., 2015). Less well understood hydrous phases (minerals and fluids) expected at mantle depths in water-rich interactions include the role of flexure of the incoming plate and the angle subduction contexts. Furthermore, these hydrogen-related field electrical of convergence creating on reactivating faults for fluid migration through anomalies are expected to reflect the state of subduction zones, as ther- the crust and into the mantle (e.g., Naliboff et al., 2013). The role of inherited mal modeling suggests that water is brought efficiently to mantle depths fabric, formed at or close to the mid-ocean ridge, on the generation of faults in old and fast subduction zones (e.g., in the western Pacific), whereas in at the trench is also not well known. These faults may act as sinks of fluids hot subduction zones (e.g., Cascadia) a nearly complete slab dehydration is

predicted (e.g., van Keken et al., 2011). Estimates of potential fluid flux range ample of Cascadia, where a conductivity anomaly of ~0.1 S/m is observed next 2 from ~0.1 to 0.3 kg H2O/m /yr, depending on the convergence rate (Peacock, to the slab at 40 km depth (Fig. 2). The electrical conductivity of lawsonite is 1987, 1990, 1993; Gerya et al., 2002; Penniston-Dorland et al., 2015). This fluid as high as 0.1 S/m before it dehydrates and can be as high as 10 S/m at tem- flow causes a thermal perturbation, but the resulting heat transfer is thought peratures where the fluid is released as a free fluid phase (Manthilake et al., to be significant only at shallow to moderate depths (<40 km), where high 2015), and comparable conductivity values have been obtained for chlorite as shear stress and fluid fluxes enhance heat transport (e.g., Penniston-Dorland it dehydrates and coexists with magnetite (Manthilake et al., 2016). A conduc- et al., 2015), whereas at higher depths the thermal structure is mainly gov- tivity value as high as 0.1 S/m is also consistent with serpentine with as much erned by heat conduction and advection processes by subducting oceanic as 20 vol% magnetite before dehydration (Kawano et al., 2012). Although we lithosphere (e.g., Peacock, 1990). can rule out the presence of lawsonite at this depth because its stability field A suite of hydrous phases such as amphibole, brucite, serpentine, and chlo- is expected to start at higher pressure (e.g., Poli and Schmidt, 2002), it is not rite is thermodynamically stable at specific depth and temperature ranges in possible to attribute the field anomaly in the Cascades to serpentine or chlo- the incoming hydrated oceanic crust and upper mantle (e.g., Poli and Schmidt, rite without further petrological constraints. These observations point out the 1995; Hacker, 2008; Grove et al., 2012; Timm et al., 2014; Fig. 2). These phases need for combining electrical observations with phase equilibria in order to

contribute to the H2O flux in subduction zones because they can accommodate interpret conductivity anomalies in terms of chemistry. Figure 3 shows the sta-

significant amounts of water (as much as 12 wt% water) in their crystallo- bility field of hydrous phases in the basalt + 2H O system. Comparison of these graphic structure as OH groups that are eventually released at higher depth phase equilibria with the expected pressure-temperature (P-T ) paths of slabs as free fluid phases upon the breakdown reaction. In particular, the main flux from different subduction zones allows the prediction of the depth of hydrous of subducted water in the mantle may be controlled by the abundance of ser- fluid release from breakdown reactions as well as the onset of melting. As pentine (Rüpke et al., 2004). A few laboratory studies have measured a high illustrated in Figure 3, the high temperatures in Cascadia (and hot subduction conductivity for hydrous minerals, especially once the breakdown reaction zones in general) do not overlap with the stability field of lawsonite, and so it occurs, because the presence of a low-viscosity interconnected fluid phase en- is the breakdown of other stable minerals (chlorite, amphibole) that will occur hances the bulk electrical conductivity. However, the difference in conductivity in these settings, whereas metamorphosed oceanic crust in cold subduction between these hydrous minerals is often too small to be discriminated using zones (e.g., northeastern Japan) is expected to be mainly composed of law- field electrical measurements alone. To illustrate this point, we consider the ex- sonite blueschist.

Volcanic front Depth (km) Mt. Rainier Trench-close anomaly 400 Continental crust 0 +H O, CO amph Basalt-eclogite 2 2 Decompression ) transformation ration melting dehyd Forearc anomaly m 800 400 / 10 serp S arc slab ( ore- F 40 y

800 t

v 10

cld t l% melt c 1200 Fluid-assisted u Fluid-assisted d 10 5 to 10 vo melting Deep anomaly n

1200 80 c

chl l

p, a 0.1 w a ser c

l i

vol% melt r <5 t c 1

E 120

Figure 2. Detection of fluids using electromagnetic studies. Comparison between the petrological view of a subduction zone (left: after Schmidt and Poli, 1998; Grove et al., 2012; Timm et al., 2014) and an electromagnetic profile (right: Cascadia, McGary et al., 2014). Labels in the slab correspond to stable hydrous minerals: amph—amphibole; cld—chloritoid; law—lawsonite; serp—serpentine; chl—chlorite. The location of the model profile is shown on the inset map of Figure 6. Dashed lines correspond to isotherms and associated numbers refer to temperature in °C.

7 Carbon Cycle and Electrical Anomalies

e Basalt + H O t 2 Marianas face 200 soni Carbon is another key volatile in subduction mechanisms, and estimates 6 w la of the amount of C initially subducted (from sediments, crust and mantle) vary Slab sur from 40 to 114 Mt/yr, whereas the amount of C transferred from the subducting Moho NE Japan A plate to the convecting mantle ranges from 0.0001 to 52 Mt/yr (Dasgupta and 5 p

150 p Hirschmann, 2010; Kelemen and Manning, 2015). Very low estimates of carbon

o )

x fluxes in the convecting mantle suggest that most of the carbon subducted

i m P through sediments (overall annual flux of ~0.9–1.8 t mol C/yr; Evans, 2012) and

G 4

a (

t oceanic plate may be recycled in fluids (such as carbonate melts) or C-bearing

e v

r Moho e

u solid diapirs into the overlying plate (e.g., Kelemen and Manning, 2015). The s

100 d s

3 Moho e few existing electrical laboratory studies of carbonate melts observed very high

r t P face conductivities at atmospheric (Gaillard et al., 2008) and uppermost mantle con- Slab sur Slab surface h ( ditions (e.g., Yoshino et al., 2012; Sifré et al., 2014), suggesting that some field

amphibole k m

2 e electrical anomalies may be explained with a small fraction of carbonate melt. ) 50 At temperatures relevant to subduction zones, the investigated carbonatitic and lawsonit Cascadia Figure 3. Schematic petrogenetic grid extremely carbon-rich silicate melts present conductivities >10 S/m, suggesting for the basalt + H2O system (top) and the 1 e that a small melt fraction could explain field anomalies of ~1 S/m or less in CaCO3 + aqueous fluids system (bottom) showing major dehydration reactions and chlorit solidus the mantle wedge (see Fig. 2). Carbon at depth may trigger incipient melting pressure-temperature (P-T ) paths for cold and produce high-conductivity and low-viscosity melts (Sifré et al., 2014) that 0 0 and hot subduction zones. P-T paths for the should be detectable using MT surveys. The carbonatitic liquids that can be Moho and slab surface are redrawn from 400 500 600 700 800 900 1000 formed from CO release are extremely mobile and provide a potential pathway Okazaki and Hirth (2016) using Abers et al. Temperature (°C) 2 (2013) (northeastern Japan and Cascadia) for carbon recycling at shallow depths beneath arcs (especially in warm active and from Syracuse et al. (2010) (Marianas). margins) through percolation processes into the mantle wedge (<120 km depth; Top plot: field stability limits and solidus 7 Poli, 2015). However, Mann and Schmidt (2015) estimated that near the solidus, from Poli and Schmidt (2002). Bottom plot: CaCO in field stabvvility limits and fluid-saturated 3 a silicate melt may only transport 0.05–0.6 wt% bulk CO2, which should moti- face r 200 solidus for metabasalt and metasediments aqueous uids Marianas vate electrical studies of melts containing small amounts of carbon. from Kelemen and Manning (2015, and 6 references therein). Dashed line contours Further electrical laboratory investigations of solid carbon phases and in the bottom plot are parts per million Slab su ated solidus their dissolution are also needed to interpret field conductivity anomalies in Moho tur for aqueous fluids saturated in CaCO3 as a NE Japan a A terms of metamorphic processes. If the melting of sediments from the slab function of P and T. 5 uid-s p

150 p has been proposed to explain some geochemical signatures of arc magmas r

serpentine o

) (e.g., Plank, 2005), geodynamic simulations suggest that sediment melting re-

i m

P quires slab surface temperatures higher than the temperatures predicted by

G 4

a (

t thermal models (Behn et al., 2011), even in warm subduction environments. Al-

e v

r Moho

e ternative mechanisms for the melting and transport of sediments in the mantle

u s 100 d s wedge involve (1) the incorporation of sediments into mélange diapirs, expos- 3 Moho e

e Cascadia

p r

t ing sediments to temperatures higher than the slab temperature (>1000 °C; P face sur h Slab Slab surface

( e.g., Marschall and Schumacher, 2012), and (2) flux melting, with fluid being e k 2 aragonit m added from metamorphic reactions (such as water release from serpentine

e ) calcit 50 breakdown) in underlying slab lithologies (Mann and Schmidt, 2015). How- ever, as underlined by the experimental work of Mann and Schmidt (2015), 1000 1 sediment melting in a descending slab cannot be a general subduction pro- 500 cess because it causes an almost complete dehydration of the slab that would 100 lead the mantle to dry out on geological time scales. Phase equilibria sug- 0 0 gest that the release of C-rich fluids from sediments may only occur in the 400 500 600 700 800 900 1000 warmest of subduction zones (e.g., Cascadia; Fig. 3), and even in this context Temperature (°C) fluids may be expelled before the crust intersects the solidus. Fluid release and

partial melting triggered by carbon does not lead to complete decarbonation INCOMING PLATE of the slab, as calcite remains stable at temperatures above the fluid-saturated solidus (Mann and Schmidt, 2015; Fig. 3). Laboratory experiments also show Fluids are introduced into the subduction system through a variety of mech- that carbonate destabilization in subduction zones does not necessarily imply anisms. Fluids can be present as a free phase in pore space, either of sediments

carbon loss from the slab in C-bearing fluid (essentially as CO2) (Galvez et al., or oceanic crust, or they can be carried through hydrous alteration products. 2013). Galvez et al. (2013) suggested that, under reducing and low fluid flux EM methods are highly sensitive to the presence of interconnected free fluid conditions, carbon might be stored as graphite in the downgoing slab and may phases, but have almost no sensitivity to crustal alteration products. Water in be injected into the mantle under a high-P phase such as diamond, although the mantle is carried by the suite of hydrous phases discussed above. In this C in diamond form is not thought to represent a significant part of the global section we highlight the constraints that EM methods are able to place on fluid carbon inventory (Kelemen and Manning, 2015). flux in the incoming plate, which will primarily be confined to free pore fluids, as well as the limitations on our current knowledge, primarily arising from the Competing Effects of Hydrogen and Carbon limited number of soundings completed. Subduction zones are broadly categorized into either accretionary or non- Both hydrogen and carbon cycles affect subduction in different ways. It has accretionary systems describing how sediments accumulate into an accre- been suggested that in present-day Earth, carbon subduction is more efficient tionary prism (e.g., von Huene and Scholl, 1991) and are left behind as the

than H2O subduction because nearly complete dehydration of the crust is ex- slab descends, or how sediments are carried down into the mantle. The thick- pected at shallow depth for all but the coldest subduction zones (e.g., Hacker, ness of the sediment pile on the incoming plate can be mapped seismically 2008), whereas a significant amount of C can be brought to the mantle (e.g., and the bulk porosity quantified using CSEM methods. The oceanic crust has

Dasgupta and Hirschmann, 2010). From a geodynamic viewpoint, H2O may an intrinsic porosity that can be as high as ~20% near the seafloor in young dominate deep subduction mechanisms because it significantly decreases the seafloor and decreases with depth (Evans et al., 1991, 1994). Alteration and viscosity and density of mantle minerals (e.g., Mei and Kohlstedt, 2000; Nakao sedimentary infill with age act to reduce these values (e.g., Becker et al., 1982). et al., 2016), whereas C is mostly hosted in accessory mantle minerals with a There is only one data set of crustal conductivity structure around a trench,

smaller effect than H2O on physical properties. However, C influences the on- collected by the SERPENT (serpentinite, extension and regional porosity) ex- set of partial melting (e.g., Dasgupta et al., 2007a, 2007b), and thus indirectly periment offshore Nicaragua, across the Nicaragua trench (Key et al., 2012; affects geophysical (electrical, seismic) measurements. Naif et al., 2015) (Fig. 4). These data show the development of electrical anisot- How hydrogen-bearing and carbon-bearing fluids hydrate the overlying ropy associated with the generation of bending faults and the increase in po- mantle is not completely understood and their effects on an electromagnetic rosity as these faults open and allow seawater to enter through the crust and profile can be intertwined. In particular, bulk conductivity alone cannot easily into the upper mantle. Porosity values throughout the crust essentially double constrain melt chemistry (carbon-rich versus hydrogen-rich melt) and requires due to the creation of plate-bending faults. These observations also suggest input from petrology studies. For example, the deep conductor in Cascadia that, in the absence of other vehicles for fluid penetration, most serpentiniza- starts at ~60 km depth and has a conductivity ranging from ~0.1 to 1 S/m, and tion of the upper mantle, a key component of the water carried deeper into has been interpreted as partial melt (McGary et al., 2014) (Fig. 2). At 1200 °C, the subduction system, occurs at the trench. Nicaragua has been noted as an a conductivity value of 0.1 S/m is compatible with a dry or low water content end-member system as arc-lava compositions suggest a high flux of water basalt (extrapolating the data from Ni et al., 2011), an andesitic melt containing into the system (e.g., Patino et al., 2000), which has been related to the nearly

~2 wt% H2O (Laumonier et al., 2017), or a carbonate-bearing melt containing parallel orientation of plate faults with the trench. We do not have comple-

<10 wt% CO2 (Yoshino et al., 2012). It might also be compatible with a melt mentary data from systems where the fault orientation is oblique to the trench

containing both H2O and CO2 (Sifré et al., 2014). Because hydrous melting is to test the hypothesis that preexisting fault fabric influences the fault density expected in the mantle wedge and because the first melts formed are expected and fluid content of the crust. to be basaltic (e.g., Grove et al., 2012), an anhydrous basaltic melt and an ande sitic melt are less likely to explain the field electrical anomaly than a basaltic melt containing both carbon and hydrogen, assuming complete miscibility of SHALLOW FLUID RELEASE (<~15 km DEPTH) the system. As detailed in the following, the location of electrically conductive anom- In the shallow part of subduction (<15 km), there is rapid fluid loss from alies at depths where the breakdown of volatile-bearing minerals is expected pore spaces in both the sedimentary layer and upper crust. This release results underlines the relevance of relationships between metamorphism and electri- in aqueous fluid migration along channels atop the décollement and back to- cal conductivity, particularly through carbonation and hydration reactions and ward the trench, and through interconnected cracks and faults in the overlying decarbonation and dehydration reactions of the downgoing slab. plate (Lauer and Saffer, 2015). These channels are preferred transport paths for

−89 −88 −87 −86 A 12 12

11 11 Latitude

10 10

Figure 4. Electrical structure of the incoming Cocos plate (Naif et al., 2015, 2016), highlighting the relationship between 9 9 topography and conductive fluid-bearing −89 −88 −87 −86 faults. (A) The map shows the location Longitude of the profile (black line) that featured deployment of 54 seafloor instruments and transmission using a deep-towed B controlled source electromagnetic (CSEM) transmitter. (B) The resistivity model derived from two-dimensional inversion of the CSEM data as discussed in the text (model from Naif et al., 2016).

significant amounts of aqueous fluids and thus their presence and orientation depth and migrating toward the continental Moho (e.g., Evans et al., 2014; Mc- in the slab can be imaged using EM techniques (e.g., Naif et al., 2016). Gary et al., 2014; Wannamaker et al., 2014; Fig. 2). The depth of fluid release Large amounts of aqueous fluids with a wide range of salinities are re- is consistent with thermodynamic models of basalt-eclogite transition for this leased at shallow depth. These fluids can be a source of high conductivity (e.g., hot, young end-member system (Hacker et al., 2003b; van Keken et al., 2002) Quist and Marshall, 1968; Nesbitt, 1993; Reynard et al., 2011; Sakuma and Ichiki, and with seismically imaged increases in velocity (e.g., Bostock et al., 2002; 2016; Manthilake et al., 2016). Depending on the salinity of the free fluid phase, Rondenay et al., 2008; Abers et al., 2009; McGary et al., 2014). Constraining fluid-rock mixtures in the shallow part of a subduction system can explain con- the volume fraction of fluid is difficult without knowing the composition (pri- ductivities in the range of 10–4 to 10–2 S/m (Reynard et al., 2011). However, only marily the salinity) of the fluid released from the slab as well as the thermal few quantitative electrical constraints can be placed on aqueous fluids because structure. Using numerical models of thermal structure, molecular dynamics of a lack of laboratory measurements on free fluid phases under pressures and calculations of fluid conductivity suggest that the Cascadia anomalies are temperatures relevant to subduction. consistent with a low-volume fraction of fluid with a modest salinity (about Shallow fluid release involves the breakdown of clay minerals in the down- 0.5 wt% NaCl) (Sakuma and Ichiki, 2016). going sediments (if they are carried down with the plate and not accreted) (Spi- Similar fluid release is seen in many other MT profiles spanning the forearc nelli et al., 2006; Lauer and Saffer, 2015). These fluids are typically fresh and and into the arc; this led Worzewski et al. (2010) to identify a conductive feature released at temperatures of ~60–150 °C. They can either pool at the décolle present in all considered locations at 20–30 km depth, and situated between ment where they might play a role in plate slip, or can find their way through the arc and the trench, ~30 km from the arc (Fig. 1). In addition to the Costa faults to the seafloor where they exist through seeps. Rica system imaged by Worzewski et al. (2010), another example of this feature We have a single image of the conductivity structure of a forearc (Nicara- includes that imaged across the Marlborough strike-slip system in New Zea- gua; Naif et al., 2016) through CSEM imaging. This setting is largely erosional, land (Wannamaker et al., 2009), where the fluids released from the downgoing with sediments subducting beneath a forearc interpreted to largely consist of slab in the 40–60 km depth range rise through buoyancy and interact with a crystalline basement (e.g., von Huene et al., 2004). The CSEM data show the sequence of strike-slip faults, possibly lubricating the faults. However, while high-porosity décollement, and a reduction in porosity with depth broadly con- most features imaged at these depths are likely to be aqueous fluids released sistent with compaction models (Lauer and Saffer, 2015; Spinelli et al., 2006). from the slab, there is the likelihood for some to be ponded melts. Distinguish- The data permit estimation of a total free water budget for the system. A large ing the two requires tracing the conductor back to the slab, and that requires conductor is observed at a depth broadly consistent with clay breakdown as- good data density as well as imposition of the slab in the inversion model for suming the hotter end-member model for the system, although other hypothe- reasons discussed herein. ses for this conductor include the accumulation of sediments (Naif et al., 2016). Electromagnetic and seismic signals in the crust and mantle depend on We do not have a data set from an accretionary sedimentary forearc system. temperature, fluid fraction and interconnection in differing degrees. In particular, links between fluid release and transport and seismicity are intriguing. Heise et al. (2012, 2013) showed how, on the North Island of New Zealand, the INTERMEDIATE DEPTH FLUID RELEASE (15–50 km DEPTH onset of seismicity above the slab corresponds with a transition from conduc- IN WARM SUBDUCTION ZONES, ~100 km DEPTH IN COLD tive sediments to a region of more resistive material at ~10 km depth (Fig. 5). SUBDUCTION ZONES) This band of seismicity continues down the slab, where it overlaps with a conductor that can be interpreted as due to fluid release processes. The densest As basaltic crust descends into a subduction zone, it undergoes a transi- seismic activity appears closely related to the connection between the conduction to eclogite with a resultant release of water (e.g., Peacock, 1993; Poli and tor and the slab. The depth of this conductor is shallower (~20 km) than ex- Schmidt, 1995). This process is thought to be important for subduction initi- pected for basalt eclogite, and Heise et al. (2012, 2013) suggested that instead ation (e.g., Gurnis et al., 2004) as eclogite is significantly denser than basalt. this feature is related to underplated sediments. In Cascadia, seismicity ex- The role of fluids in weakening the plate interface can result in decoupling of tends along the slab through the region of fluid release, but there is a decrease the mantle wedge from the slab, allowing a stagnant corner to form that is in seismicity once the transition to eclogite is complete (McGary et al., 2014); in conducive to serpentinization (Wada et al., 2008). There are also potential links general, through the system, there is a reasonable correlation between areas between fluid release at these depth and seismicity (e.g., Hacker et al., 2003a) of slow slip events and steady plate sliding and fluid release from eclogitiza- and aseismic transients (e.g., Liu and Rice, 2007). tion (Wannamaker et al., 2014). Scattered teleseismic data have been used to image changes in shear ve- The subduction of the incoming plate triggers high degree of deformation locity along downgoing slabs; the velocity increase is inferred to represent the in the crust and the mantle, forming zones of high strain. Electrical experi- basalt-eclogite transition (e.g., Bostock et al., 2002; Rondenay et al., 2008). Data ments on sheared partially molten rocks showed that conductivity and electri- sets from Cascadia show conductive anomalies leaving the slab at ~40 km cal anisotropy are enhanced by shear (Caricchi et al., 2011; Zhang et al., 2014;

NW SE AB 177 178 179 10 20 Vp 30

Depth (km) 40 −38 −38

Latitude 50 Latitude

C 10 km

10 −39 −39 177 178 179 20 Vp/Vs Longitude 30

Figure 5. (A) Magnetotelluric (MT) model from the North Island of New Zealand that Depth (km) 40 shows how the onset of seismicity above the slab corresponds with a transition from conductive sediments to a region of more resistive material at ~10 km depth, as well as 50 the start of a large conductor thought to represent sediments or perhaps the release of fluids. (B) Contours show coincident Vp seismic velocities. (C) Contours show Vp/Vs (P-wave to S-wave velocity ratios), which also show reductions in velocities with a greater drop in Vs, as expected for a fluid-rich region (from Heise et al., 2012). The location of the profile is shown by the line in A. Resistivity (Ohm-m)

10 30 100 300

Pommier et al., 2015). In particular, experiments on deformed polycrystalline high strains are expected (such as the slab–mantle wedge interface) may be olivine showed that grain-boundary conduction can explain high conductivi- explained by smaller amounts of fluids than estimates made when consider- ties (8 × 10–2 S/m at 1300 °C for a shear strain of 7.3) and electrical anisotropy ing an undeformed solid matrix. Constraining the amount of fluids from elec- as high as 4.5 without the presence of volatiles or melt, due to a noticeable trical conductivity requires an understanding of the degree of interconnectivity effect of grain boundary orientation distribution (Pommier et al., 2016). These of the liquid phase, because its geometry affects bulk conductivity (e.g., ten experimental results suggest that the interpretation of EM data requires con- Grotenhuis et al., 2005, and references therein; Miller et al., 2015). However, sideration of the effect of rock deformation on the bulk electrical response, the current electrical database does not allow us to place tight constraints on instead of attributing high electrical conductivities solely to rock chemistry melt or aqueous fluid fraction at conditions P( -T composition rheology) rele- (presence of aqueous fluids and/or melt). Highly conductive anomalies where vant to subduction zones.

DEEP FLUID RELEASE AND MELT GENERATION tor associated with the release of fluids from the transition of basalt to eclogite (>~50 km DEPTH) at ~40 km depth (as discussed herein). It is important to note that dehydration of chlorite could also contribute to electrical anomalies at ~40–50 km depth (Fig. 3) The presence of significant quantities of aqueous fluids and partial melt can and lead to higher conductivities at the slab–mantle wedge interface. The sec- lower the viscosity of the upper mantle (e.g., Dixon et al., 2004). This lowered ond key feature of the model is a deeper conductor associated with serpentinite viscosity, along with fluid processes involved in plate-mantle coupling, results breakdown that releases fluids and triggers melting above the slab. This melt is in convection in the mantle wedge above the subducting plate and elevated imaged migrating upward toward the arc volcano (McGary et al., 2014; Fig. 2). surface heat flow (e.g., Wada and Wang, 2009). Existing EM profiles across Partial melting is expected to start at ~60 km depth, where the slab surface tem- subduction zones reveal different electrical structures, highlighting the com- perature intersects the solidus curve, and extends to higher depths (Fig. 3). plexity of subduction mechanisms at work and the dependence on geological Other profiles across Cascadia and 3D inversions of Earthscope TA data setting. Differences in electrical profiles between cold versus hot subduction show variability in the along-strike structure, particularly at depths associated zones can be explained by differences in the thermal evolution of the slab and with serpentinite breakdown (~80 km) and generation of melt (Fig. 6) (Wanna- surrounding mantle and the location of major metamorphic reactions, influ- maker et al., 2014; Meqbel et al., 2014; Bedrosian and Feucht, 2014). In particu- encing the extent of partial melting in the mantle wedge. For example, the ther- lar, the 3D model of Bedrosian and Feucht (2014) shows the largest conductive mal structure in warm subduction zones will cause the top of the subducted anomaly at 75 km roughly in the same region as the Café line, although in oceanic crust to intersect several dehydration reactions of hydrous minerals at this inversion (with no slab imposed) the conductor is widespread and shifted shallow depth as well as the possible partial melting of hydrous basalt, leading to the west of the arc. There are other smaller conductors along strike at the to the formation of high-conductivity fluid-bearing zones at the slab-mantle same depth, but these are also all shifted westward of the arc. Putting aside interface and in the mantle wedge. In colder subduction zones (e.g., north- the geometry of the conductors, the variability in conductivity at these depths eastern Japan), the slab can transport surface water in the form of hydrous suggests primary differences in the amounts of water released from the slab minerals (serpentine) into the deep mantle, resulting in the release of fluids at and in the generation of melts along strike. greater depths than in warm subduction environments such as Cascadia (Toh Similarly, in the Kyushu subduction zone, which has featured 3D electro- et al., 2006; Iwamori and Zhao, 2000). magnetic imaging using both network MT and geomagnetic transfer function The onset and extent of partial melting are directly related to the abun- approaches (Hata et al., 2015, 2017), there is significant variability in the mantle dance of volatiles and the thermal structure (the presence of water decreas- wedge conductivity, with only one region, beneath Kirishima volcano, associ- ing the solidus temperature). Very hydrous melts are expected in subduction ated with a significant melt-related conductor (Fig. 7). In this system the Philip zones and may coexist with free aqueous fluid phases. These partially molten pine Sea plate subducts, carrying with it the relict Kyushu-Palau Ridge. The zones are expected to be very conductive, in agreement with laboratory stud- Kyushu-Palau Ridge was formed as part of the backarc system and features

ies on hydrous melts containing several weight percent H2O and carbon-bear- anomalously thick oceanic crust (Nishizawa et al., 2007). The projection of this ing silicate melts (Gaillard, 2004; Pommier et al., 2008; Ni et al., 2011; Yoshino ridge in the direction of plate convergence is roughly in the region of high et al., 2012; Sifré et al., 2014; Laumonier et al., 2014, 2017; Guo et al., 2016). mantle conductivity, as discussed in Hata et al. (2017). The central Mariana trench is an ocean-ocean subduction system and is Variability in the Presence of a Deep Conductor another example of where a strong conductor comes from the subducting plate at depths of 100 km and greater. The incoming Pacific plate in this section of the Considerable work has been conducted in Cascadia (Meqbel et al., 2014; Mariana is very old (ca. 150–140 Ma), but also contains abundant seamounts. Bedrosian and Feucht, 2014; McGary et al., 2014; Soyer and Unsworth, 2006; The central Mariana trench is host to a series of diapiric serpentinite seamounts Wannamaker et al., 2014). Earthscope Transportable Array data (www.usarray (Maekawa et al., 2001), although the source of serpentinite is thought to come .org), provide a good three-dimensional (3D) framework for Cascadia, and MT from the mantle wedge rather than the downgoing mantle, and is related to models derived from the ~70 km spaced transportable array (TA) coverage high- the impact of the subduction of seamounts on the incoming plate (Fryer et al., light the variability across the system, although with this station spacing, many 1985). A seafloor MT profile across the system from incoming old Pacific plate details of the subduction system are not resolved. More densely spaced wide- through to the backarc was discussed in Matsuno et al. (2010). Final models of band and long-period MT data were collected on the Café line through Washing- the measured data contain a highly conductive mantle above the slab over a ton State (McGary et al., 2014; Fig. 2), coincident with a large seismic deployment wide range of depths, suggesting that a significant amount of volatiles is carried (Abers et al., 2009). This permitted use of the seismic imaging to guide the MT into the system with subsequent melt generation. However, there is no signal inversion. In this case, an a priori model containing a slab as well as relaxation of conductive melt in the mantle associated with the backarc spreading system. of regularization across the slab interface was implemented in inverting the MT Matsuno et al. (2012) examined the 3D geometries of melt features that could data. The resulting model shows two key features. The first feature is a conduc- exist beneath the backarc and would be undetected by the data collected.

C −126 −124 −122 −120 A

48 JdF-G A 48

Cafe C 46 46 B B

JdF-G 44 44 Latitude C C 42 42

) 0 C 50 JdF-G 100 40 40

−126 −124 −122 −120 depth (km 150 –600 Longitude –300 0

Figure 6. A series of cross sections through the three-dimensional resistivity model of the Cascadia distance (km) subduction zone (Bedrosian and Feucht, 2014). These sections, derived from Earthscope Transportable Array data (www.usarray.org), highlight the variability in mantle wedge structure through the Cascadia system. The locations of the profiles (A, B, and C) are as labeled in the inset. The location of the café-line resistivity (Ωm) model section shown in Figure 1 (McGary et al., 2014) is also shown. The coast line is denoted by the C in each section, and the top of the Juan de Fuca–Gorda (JdF-G) plate is shown by the dashed line. Modified from Bedrosian and Feucht (2014). 310 100300

LINKAGES BETWEEN INCOMING PLATE PROPERTIES AND subduction system through more extensive serpentinization of the incoming FLUID RELEASE SIGNALS mantle (e.g., Singer et al., 2007). Supporting this model, it has been shown that the presence of faults and fractures in the oceanic crust and uppermost Large conductors, like that seen in the Café MT line (McGary et al., 2014), mantle contributes to expose peridotite to seawater hydrothermal alteration, have been seen elsewhere (e.g., Hata et al., 2015, 2017), but they are not ubiqui leading to the formation of serpentine at or near the plate surface (Kerrick, tous features of subduction zone conductivity models. Understanding why this 2002). Because serpentine is stable to depths of 120–200 km in subducted is the case requires an understanding of the along-strike variability inherent oceanic crust and upper mantle rocks (e.g., Ulmer and Trommsdorf, 1995; Poli in a system and relating this back to the controlling properties of the subduc- and Schmidt, 2002; Rüpke et al., 2004), it may be an important vector of fluids tion process. that helps release large volumes of water deep into the mantle (e.g., Singer Interpretations of geochemical data have suggested that the subduction et al., 2007), thus enhancing mantle conductivity. However, the conductivity of fracture zones results in the passage of greater amounts of water into the of serpentine is unlikely to explain high conductivity values (Reynard et al.,

A 130 131 132 B 200 34 34 45–50 km

100

33 33 Aso Unzen

Aso 0 Latitude

Unzen Distance (km) 32 32 Kirishima Figure 7. (A) Slices through a three-dimen −100 sional (3D) resistivity model derived from Kirishima network magnetotelluric (MT) data collected on Kyushu, Japan. The model reveals significant variability in mantle wedge 6050 40 30 10 structure, with a large conductor seen only 20 31 31 −200 beneath Kirishima volcano (from Hata 130 131 132 et al., 2017). (B) Plan view of the 3D model −1000100 at a depth of 45–50 km. (C) Cross sections at Longitude Distance (km) locations as shown in A and on the model 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 plan view in B. Resistivity (log10 Ohm-m) C

2011), even if it contains magnetite (Kawano et al., 2012), unless the magnetite seamount loading of the plate creates additional pathways for fluids through forms a well-connected network. the crust and into the upper mantle, resulting in more extensive serpentini- The presence of seamounts is another potential vector for high water zation of the mantle than would otherwise be the case. This high degree of flux into the deeper parts of the subduction system. Seamounts are known serpentinization could be responsible for the large fluid release and melting to cause flexure in the crust where sediments accumulate around the vol triggered above the downgoing slab. canic edifices (e.g., Bassett and Watts, 2015). This extra amount of sediments If we accept that the fluids released to create these large conductors come brought to depth (compared to an incoming plate that does not contain sea- from serpentinite, then we can conceive of two ways in which variability can be mounts) may enhance dehydration processes at shallow depth. The influence created along strike: (1) there is variability in the degree of mantle serpentini- of subducted seamounts on slab dehydration processes is supported by iso- zation beneath the incoming plate, before the creation of bending faults, or topic measurements on fluids from the Mariana arc (Ishikawa and Tera, 1999). (2) the process of creating bending faults and the associated fluid penetration In this study, local differences in isotopic composition of fluids along the arc into the mantle create variability along strike. In this model, the large conduc- are likely to be explained by variable amounts of subducted sediments and tors seen would represent larger amounts of released fluids and, presumably, are attributed to the subduction of seamounts. We further suggest that the higher melt generation above the slab.

Another possibility is that the regions where large conductors are present et al., 2014). Convergence of the Juan de Fuca and Gorda plates beneath Cas- represent loci of larger carbon release from the slab, perhaps also related to cadia is highly oblique, so that examination of seafloor morphology offshore the subduction of anomalous seafloor, although in this case the anomalous needs to be extrapolated back to a likely point of origin that is much further structure could be related to thicker crust where most of the carbon carried into south than the profile. Examination of the seafloor geology of the subduction the system is thought to reside through alteration products (Alt and Teagle, Juan de Fuca and Gorda plates shows evidence for a region of damage (Wilson, 1999). In this model, the strong conductors do not necessarily represent higher 1988), referred to as a propagator wake, that projects to the location of the large

melt fractions, but rather CO2-enriched melts that are more electrically conduc- conductor beneath the Café line. We infer that this damage zone, when at the tive (Yoshino et al., 2012; Sifré et al., 2014). Melting processes involving carbon seafloor, would have allowed a greater addition of fluid into the mantle, cooling might be important in a hot system like Cascadia and thus may explain some both the crust and mantle, and possibly a greater degree of serpentinization. of the electrically conductive anomalies, as the expected P-T conditions of the Subduction into the western side of the South American continent features slab cross the solidus of fluid-saturated metabasalt and metasediments (Fig. 3). many transform faults and ridges. For example, a series of fracture zones and In the following we discuss some of the evidence to support these mod- areas of anomalously high seafloor was observed in the Chile-Peru region of els, but neither model can be conclusively proven at present. There are likely the South American subduction system by Contreras-Reyes and Carrizo (2011), geochemical analyses of lavas, as well as geophysical surveys in ideally cho- who showed evidence linking the subduction of seafloor that has high topo- sen locations that would help discriminate between the water and carbon graphic relief (e.g., Iquique Ridge) with areas where earthquake rupture has models of melt generation. occurred along the Chile-Peru margin. Contreras-Reyes and Carrizo (2011) hy- pothesized that the excess topography is related to thicker crust, but also to Evidence for Faulting and Fracture Zones as Agents of Fluid higher degrees of mantle serpentinization related to faulting that permits more Transport (Cascadia, Kyushu, and South America) fluids to enter the mantle. It can be difficult to distinguish between thickened crust and serpentinized peridotite on the basis of seismic velocity, because The topography and faulting of the incoming plate control the amount of there is a similarity in physical properties of gabbro and peridotite at low de- fluid and sediment transported to depth, and thus may explain partly the dif- grees of serpentinization (Carlson and Miller, 1997; Hyndman and Peacock, ferences in the conductivity profiles observed in several subduction zones. The 2003). The number of seismic surveys around seamounts or ridges with the crustal porosity and the density of conductive faults progressively increase ability to constrain serpentinization is limited. In the Kyushu subduction zone, with proximity to the trench and represent efficient fluids pathways to the shal- the high conductivity zone in the mantle wedge, which is beneath Kirishima low part of a subduction system (Fig. 4). If these faults extend sufficiently deep, volcano, coincides with the projection of this Kyushu-Palau Ridge in the direc- through the crust and into the mantle, then serpentinization at the trench can tion of plate convergence (Hata et al., 2015, 2017). A working hypothesis is that occur. Nicaragua is cited as an end-member case for trench serpentinization, a large release of water and hence melting are related to the subduction of this where the formation of deep faults is permitted by the alignment of preexisting anomalous seafloor. Seismic refraction data across the ridge show anoma- seafloor fabric and the trench. However, to date, there are limited land-based lously thick crust ranging from 8 to 20 km (Nishizawa et al., 2007). P-wave ve- MT data that would identify the presence of a large conductor associated with locities <8 km/s are seen in the uppermost mantle, but this would only support the breakdown of this serpentinite. We do not have a good contrasting profile modest serpentinization (Hyndman and Peacock, 2003), unless some of the from a system where fault orientation is oblique to the trench, nor have we low velocities ascribed to the lower crust are due to serpentinite. constrained the porosity structure of the incoming plate to see if this really A number of MT surveys in South America have looked at subduction zone makes a difference in the fluid content of the crust and serpentinization of the processes (e.g., Echternacht et al., 1997; Brasse and Soyer, 2001; Brasse et al., mantle. Numerical models suggest that plate bending is able to generate deep 2002, 2009; Brasse and Eydam, 2008; Kühn et al., 2014). Although some of these faults regardless of preexisting fabric (e.g., Naliboff et al., 2013); this in turn surveys see zones of high conductivity (e.g., Brasse et al., 2002; Brasse and would suggest a degree of mantle serpentinization at all subduction systems, Soyer, 2001), the thick continental crust contains high-conductivity anomalies as long as thermal conditions permit serpentinite stability. An ideal place to that mask deeper anomalies and the models do not generally image the slab test variability in serpentinization associated with preexisting fabric is in the and conductors associated with fluid release and melting. One exception is the eastern Aleutians, where adjacent segments of seafloor subduct with very dif- region around Pocho volcano shown by Booker et al. (2004); here the arc is ferent fabric alignments. much further inland than the rest of the system, as the slab geometry is flat for Offshore Cascadia, detailed seismic imaging points out significant along- a long distance beneath the continent before finally descending. A conductive strike variability in the density of mantle penetrating faults, suggesting that signal associated with melt release is present to depths of at least 250 km, al- there are likely to be variations in the degree of mantle serpentinization (Han though the association of the conductor with the slab is not well resolved and et al., 2016), although an analysis of seismic velocities also predicts a thermal the strength of the conductor in the mantle wedge is not as strong as some of regime that is too hot to permit serpentine stability in the upper mantle (Canales the other anomalies we have discussed. Although there is a line of seamounts

offshore that could potentially line up with the location of the arc, the age of the Abers, G.A., Nakajima, J., van Keken, P.E., Kita, S., and Hacker, B.R., 2013, Thermal-petrological slab at the point where melt is being generated makes any inference of the ef- controls on the location of earthquakes within subducting plates: Earth and Planetary Sci- ence Letters, v. 369–370, p. 178–187, doi:10.1016/j.epsl.2013.03.022. fects of anomalous seafloor tenuous at best. Unfortunately, although there are Alt, J.C., and Teagle, D.A., 1999, The uptake of carbon during alteration of ocean crust: Geo potentially other locations along the South American system that could identify chimica et Cosmochimica Acta, v. 63, p. 1527–1535, doi:10.1016/S0016-7037(99)00123-4. the impacts of subduction of transform faults or ridges on the generation of Bassett, D., and Watts, A.B., 2015, Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief: Geochemistry, Geophysics, Geosys- melts, there are currently no suitable MT profiles that can accomplish this. tems, v. 16, p. 1508–1540, doi:10.1002/2014GC005684. Becker, K., Von Herzen, R.P., Francis, T.J.G., Anderson, R.N., Honnorez, J., Adamson, A.C., and Alt, J.C., 1982, In situ electrical resistivity and bulk porosity of the oceanic crust Costa Rica Rift: Nature, v. 300, p. 594–598, doi:10.1038/300594a0. CONCLUDING REMARKS Bedrosian, P.A., and Feucht, D.W., 2014, Structure and tectonics of the northwestern United States from EarthScope USArray magnetotelluric data: Earth and Planetary Science Letters, EM surveys have great potential to improve constraints on fluid fluxes at v. 402, p. 275–289, doi:10.1016/j.epsl.2013.07.035. subduction zones, both in terms of inputs from the incoming plate and outputs Behn, M.D., Kelemen, P.B., Hirth, G., Hacker, B.R., and Massonne, H.J., 2011, Diapirs as the source of the sediment signature in arc lavas: Nature Geoscience, v. 4, p. 641–646, doi:10 .1038 /ngeo1214 . related to volcanism. However, we have not, for example, attempted to place Blackman, D.K., Cann, J.R., Janssen, B., and Smith, D.K., 1998, Origin of extensional core com- constraints on melt fractions in the mantle wedge based on conductivity mod- plexes: Evidence from the Mid-Atlantic Ridge at Atlantis fracture zone: Journal of Geophysi els. Some of the publications reporting the original results attempt to do this cal Research, v. 103, no. B9, p. 21,315–21,333, doi:10.1029/98JB01756. Booker, J.R., Favetto, A., and Pomposiello, M.C., 2004, Low electrical resistivity associated with at varying degrees of sophistication. Putting aside issues of model resolution, plunging of the Nazca flat slab beneath Argentina: Nature, v. 429, p. 399–403, doi:10 .1038 some of which are tied to the slab geometry as discussed herein, a significant /nature02565. challenge to estimating melt fractions is knowing the interactions between Bostock, M.G., Hyndman, R.D., Rondenay, S., and Peacock, S.M., 2002, An inverted continental Moho and serpentinization of the forearc mantle: Nature, v. 417, p. 536–538, doi:10 .1038 aqueous fluids and the melt. As fluid is released from the slab, it has a com- /417536a. position and temperature that control its conductivity. These compositions Brasse, H., and Eydam, D., 2008, Electrical conductivity beneath the Bolivian orocline and its are not well known and there is also some uncertainty on the fluid conduc- relation to subduction processes at the South American continental margin: Journal of Geo- tivity as a function of temperature and salinity (e.g., Sakuma and Ichiki, 2016). physical Research, v. 113, no. B7, doi:10.1029/2007JB005142. Brasse, H., and Soyer, W., 2001, A magnetotelluric study in the Southern Chilean Andes: Geo- The fluids trigger melting, and on melting, some of the water is partitioned physical Research Letters, v. 28, p. 3757–3760, doi:10.1029/2001GL013224. into the melt (e.g., Gaetani and Grove, 1998). The initial and subsequent melt Brasse, H., Lezaeta, P., Rath, V., Schwalenberg, K., Soyer, W., and Haak, V., 2002, The Bolivian composition are a function of temperature and volatile (especially water and Altiplano conductivity anomaly: Journal of Geophysical Research, v. 107, no. B5, p. EPM 4-1–EPM 4-14, doi:10.1029/2001JB000391. carbon dioxide) content (e.g., Dasgupta et al., 2013) and the bulk conductivity Brasse, H., Kapinos, G., Li, Y., Muetschard, L., Soyer, W., and Eydam, D., 2009, Structural electri- depends on the amounts of volatiles (e.g., Pommier et al., 2008; Ni et al., 2011; cal anisotropy in the crust at the south-central Chilean continental margin as inferred from Laumonier et al., 2014; Sifré et al., 2014) and melt present as well as the melt geomagnetic transfer functions: Physics of the Earth and Planetary Interiors, v. 173, p. 7–16, doi:10.1016/j.pepi.2008.10.017. network and the nature of the melt connectivity (e.g., Miller et al., 2015). This is Canales, J.P., Detrick, R.S., Lin, J., Collins, J.A., and Toomey, D.R., 2000, Crustal and upper a highly nonlinear and complex system; we do not have sufficient understand- mantle seismic structure beneath the rift mountains and across a non-transform offset at ing to place tight constraints at present, but it offers a significant challenge for the Mid-Atlantic Ridge (35 N): Journal of Geophysical Research, v. 105, no. B2, p. 2699–2719, future research, including critical laboratory measurements and EM profiles in doi:10.1029/1999JB900379. Canales, J.P., Carbotte, S.M., Carton, H.D., and Nedimovic, M.R., 2014, State of hydration of the well-chosen settings. Juan de Fuca Plate along the Cascadia deformation front from controlled-source wide-angle There are clearly significant primary variations in electrical structure, even seismic data: Eos (Transactions, American Geophysical Union), abs. T51B-4615. within a given subduction system, that point to variability in inputs and out- Caricchi, L., Gaillard, F., Mecklenburgh, J., and Le Trong, E., 2011, Experimental determination of electrical conductivity during deformation of melt-bearing olivine aggregates and electrical puts. We have suggested two possibilities for this variability that can be tested anisotropy in the oceanic low velocity zone: Earth and Planetary Science Letters, v. 302, by field observations, including petrology and geochemistry. p. 81–94, doi:10.1016/j.epsl.2010.11.041. Carlson, R.L., and Miller, D.J., 1997, A new assessment of the abundance of serpentinite in the oceanic crust: Geophysical Research Letters, v. 24, p. 457–460, doi:10.1029/97GL00144. ACKNOWLEDGMENTS Contreras-Reyes, E., and Carrizo, D., 2011, Control of high oceanic features and subduction chan- We would like to thank Maki Hata, Paul Bedrosian, and Samer Naif for providing us figures for nel on earthquake ruptures along the Chile-Peru subduction zone: Physics of the Earth and the paper. Two reviewers, Guest Associate Editor Gray Bebout, and Science Editor Shan de Silva Planetary Interiors, v. 186, p. 49–58, doi:10.1016/j.pepi.2011.03.002. are thanked for their comments on the first draft of the manuscript. Jasmine Zhu is thanked for Dasgupta, R., and Hirschmann, M.M., 2010, The deep carbon cycle and melting in Earth’s interior: assistance with generating maps. AP acknowledges support from NSF-EAR (award #1551200). Earth and Planetary Science Letters, v. 298, p. 1–13, doi:10.1016/j.epsl.2010.06.039.

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