PUBLICATIONS RESEARCH LETTER Key Points: Subduction zones are inherently weak The shear stresses are of the order of 1to10MPa The low stresses are related with hydration of the upper mantle Supporting Information: Text S1, Figure S1, and Movies S1 and S2 Movie S1 Movie S2 Correspondence to: J. C. Duarte, joao.duarte@monash.edu How weak is the subduction zone interface? João C. Duarte 1, Wouter P. Schellart 1, and Alexander R. Cruden 1 1 School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, Australia Abstract Several lines of evidence suggest that subduction zones are weak and that the unique availability of water on Earth is a critical factor in the weakening process. We have evaluated the strength of subduction zone interfaces using two approaches: (i) from empirical relationships between shear stress at the interface and subduction velocity, deduced from laboratory experiments; and (ii) from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength. Our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low (less than ~35 MPa). To account for this requirement, we propose that there is a feedback mechanism between subduction velocity, water released from the subducting plate, and weakening of the fore-arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally. Citation: Duarte, J. C., W. P. Schellart, and A. R. Cruden (2015), How weak is the subduction zone interface?, Geophys. Res. Lett., 42, 2664 2673, doi:. Received 17 DEC 2014 Accepted 17 MAR 2015 Accepted article online 21 MAR 2015 Published online 19 APR 2015 1. Introduction An outstanding question in geodynamics is how weak are plate boundaries when compared to their interiors? Particularly, how weak are subduction zone interfaces? The global distribution of earthquakes indicates that lithospheric deformation localizes along plate boundaries while the inner portions of the plates remain relatively undeformed over long periods of time [Isacks et al., 1968]. This implies that plate boundaries must be intrinsically weak when compared to their interiors. Some authors have proposed that there is significant variability between the strength of subduction zone interfaces. In particular, Lamb and Davis [2003] and Lamb [2006] have argued for high interfacial shear stresses (~35 50 MPa) in the central part of the South American subduction zone, compared to low stresses to the north and south (10 20 MPa). They proposed that such high stresses are responsible for the formation of the Central Andes. High shear stresses have been attributed to higher mechanical coupling due to the absence of trench sediments, resulting in a reduction in lubrication of the subduction zone interface. This hypothesis has been challenged by recent calculations, which take into account the nonneutral state of stress in the volcanic arc [e.g., Richardson and Coblentz, 1994] and show that interfacial stresses along the Andean plate interface are in the range of ~14 16 MPa [Seno, 2009]. Likewise, observations have implied low shear stresses (<20 MPa) at subduction interfaces worldwide [Bird, 1978;Magee and Zoback, 1993; Wang et al., 1995; Springer, 1999; Currie et al., 2002; Grevemeyer et al.,2003;luttrell et al., 2011]. Higher estimates of shear stress have also been suggested: 15 30 MPa [Zhong and Gurnis, 1994], 14 43 MPa [Tichelaar and Ruff, 1993], an average of ~40 ± 17 MPa with values up to 50 100 MPa [Honda, 1985;Von Herzen et al. 2001], ~100 MPa [Molnar and England, 1990; Alcock et al., 2005], and 200 300 MPa [Turcotte and Schubert, 1973; Sleep, 1975]. The latter very high values were based on the assumption that arc volcanism is a consequence of shear heating at the subduction interface. This idea was later shown to be unlikely due to consistently low heat flux measured within fore arcs, implying shear stresses of ~10 MPa or lower [e.g., Wang et al., 1995;Gutscher and Peacock, 2003]. Furthermore, rock friction experiments require shear stresses of 500 1000 MPa (with coefficients of friction around 0.6 0.8) under normal stresses of the same order of magnitude for frictional melting to occur [Byerlee, 1978]. Kneller et al. [2007] place a theoretical upper limit for interfacial viscous stresses in nature at 500 MPa, which is the value at which the viscous force balances the available slab pull force (on the order of 5 10 13 N/m). Higher shear stresses will cause the subduction to stall. However, it should be noted that a significant amount of the slab negative buoyancy force (up to ~90%) is dissipated within the slab and ambient mantle [Schellart, 2004a], leaving values on the order of 10 20% of the negative buoyancy force to overcome stresses at the subduction interface. This implies that the upper theoretical limit of 500 MPa is likely overestimated by at least 1 order of magnitude. Uyeda and Kanamori [1979] proposed that strong mechanical coupling at subduction zone interfaces is linked to the formation of Cordilleran mountain belts and giant subduction zone thrust earthquakes (Andean type subduction zones), while weak coupling is associated with back-arc basin formation and small earthquakes DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2664
(Mariana type subduction zones). This suggests that there should be a global variation in the degree of mechanical coupling along subduction zone interfaces, which should correlate with the overriding plate strain regime. The origin of such a possible variation is still debated, but trench sediment thickness and subducting plate age have been proposed as the main controlling parameters [e.g., Ruff, 1989; Ruff and Kanamori, 1980]. If there is indeed a global variation in the degree of interfacial mechanical coupling and shear stress, it should be reflected in variations in subduction velocity, with high velocities corresponding to low degrees of mechanical coupling and vice versa. Assuming that mechanical coupling is determined by trench sediment thickness and subducting plate age, then there should be a correlation between subduction velocity and trench sediment thickness, as well as between subduction velocity and subducting plate age. In this paper we explore two lines of evidence that suggest that subduction zones are inherently weak. The first derives from a new experimental approach using dynamically scaled analog models of subduction, in which the mechanical coupling at the subduction interface is varied and quantified. The second approach comes from a parametric analysis of natural subduction zones, where worldwide subduction velocities are compared to sediment thickness and plate age. The experimental results suggest that subduction zones are inherently weak, while the parametric analysis indicates that their interfacial strength does not vary significantly between subduction zones. Finally, we propose a self-regulating mechanism that can maintain weak subduction zones. 2. Physical Models of Subduction We have investigated how the degree of mechanical coupling along the subduction zone interface influences subduction velocity in a series of dynamically scaled 3-D analog experiments. The experiments were carried out in a Perspex tank and included both downgoing and overriding plates composed of high-viscosity silicone polymer + filler mixtures and a sublithospheric upper mantle comprising low-viscosity glucose syrup (Figure 1a and multimedia files in the supporting information; see Duarte et al. [2013] for complete details of the apparatus and scaling). Both the subducting and overriding plates were placed on top of the glucose syrup at a sufficient distance from the walls of the tank to ensure that the side-boundary conditions did not affect the evolution of the subduction zone. The upper surface of the downgoing plate was coated with a thin veneer of paraffin oil-petrolatum mixture, which acted as the lubricant for the subduction interface. Subduction was manually induced by bending down ~3 cm of the negatively buoyant subducting plate. After a short period the plates were in full contact and remained so for the remainder of the experiments. No kinematic control was imposed in the experiments, and consequently, all measured velocities and forces were outcomes of the dynamics of the system. Subduction was driven entirely by the negative buoyancy of the downgoing slab (the only source of energy in our system). The main resistive forces are viscous drag from the ambient viscous mantle (up to 90%) plate bending at the subduction zone hinge (up to 20%) and at the bottom of the tank [e.g., Schellart, 2004b; Capitanio et al., 2007; Irvine and Schellart, 2012], and viscous resistance at the subduction zone interface. Slab internal deformation related to plan-view slab curvature is only a minor component (<1%) [Schellart, 2010]. The only variable in the system was the postyield effective viscosity of the material used to lubricate the subduction interface. The strength of these materials (flow stress in Figure 1b) was measured with a rheometer under strain and strain rate conditions similar to those in the experiments (see Duarte et al. [2014] for a characterization of these materials). Values of interfacial strain rate and flow stress in the experiments could therefore be used as a proxy for the degree of the mechanical coupling between the downgoing and overriding plates. All other parameters (dimensions, temperature, viscosity ratios, and density differences) were kept constant. The experiments were dynamically and kinematically scaled by nondimensionalization of Stokes settling law: v s C(Δρl 2 g)/η, wherev s is the Stokes settling velocity, Δρ is the density contrast between the subducting lithosphere and the sublithospheric upper mantle, g is the gravitational acceleration, C is a shape factor, and η is the dynamic shear viscosity. In order to satisfy dynamic similarity, we balance these variables such that Δρ m :l 2 m :g m η m v m ¼ Δρ p:l 2 p :g p η p v p ; (1) where subscripts m and p denote model and prototype values, respectively. This results in a scaled upper mantle viscosity of ~1.5 10 20 Pa s, which is an approximate average of the range of estimated natural upper mantle viscosities (10 19 10 21 Pa s) [Artyushkov,1983;Ranalli, 1995]. Such estimates of the sublithospheric mantle viscosity are generally derived from values integrated over depth ranges of up to ~1200 km. Our DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2665
Figure 1. (a) Line drawing of subduction experiments with low and very high mechanical coupling. In the low coupling case, after the slab arrives at the 660 km discontinuity, the subduction system reaches a steady state in which the slab rolls back while both overriding and subducting plates move trenchward (see supporting information). The plates are at 22.5 cm from the sidewall. The apparent thickening of the slab is an effect of plan-view slab curvature due to the mantle return flow. In the very high coupling experiment, subduction stalls a short time after the slab reaches the bottom of the tank due to high shear stresses at the subduction interface causing the slab to detach as a Rayleigh-Taylor instability. (b) Empirical relationship deduced from the physical models in which the subduction velocities (v S ) are plotted as function of the flow stress at the subduction interface. The values of experiments with low to extreme coupling are also provided as are the scaled values for v S and flow stress. The grey rectangular area corresponds to 90% of natural subduction zones based on analysis of subduction velocities (Figure 1c). The v S error bars correspond to the variability of subduction velocity during the phase after the slab reached the bottom of the tank. Relationships for natural sublithospheric upper mantle viscosities of 5 10 19 Pa s and 5 10 20 Pa s are also provided in the supporting information; (c) frequency distribution of subduction velocities in 226 segments of natural subduction zones. The grey area corresponds to 86% of subduction zones (data for v S are from Schellart and Rawlinson [2013]). DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2666
model sublithospheric upper mantle is Newtonian and of constant viscosity. Previous work has argued that the upper mantle is dominantly non-newtonian and strain rate softening [Billen, 2008; Cizkova and Bina, 2013], which implies that the effective viscosity ratio between a slab and the ambient upper mantle should be relatively high. Nevertheless, the effective viscosity ratio that we use in our experiments (~160) is well within the previous estimates of approximately 50 500 [e.g., Schellart, 2008;Funiciello et al., 2008;Ribe, 2010; Wu et al., 2008; Loiselet et al., 2009; Li and Ribe, 2012]. For the density difference we consider a natural upper limit of Δρ =80kgm 3, which would correspond to an old lithospheric slab in which all the oceanic crust has been altered to eclogite and with no heat diffusion into the slab. A slightly higher experimental value of Δρ =100kg/m 3 was used to compensate for the surface tension between the silicone plate and the glucose (mantle) reservoir. The velocity scale factor was 1.9 10 5 and the stresses, σ, scale with the product of the length and density difference ratios (because g m = g p ), such that σ m σ p ¼ Δρ m Δρ p l m l p : (2) The rigid bottom of the tank simulates the 660 km upper-lower mantle transition, which in nature is associated with a downward increase in viscosity of 1 to 2 orders of magnitude and thermal expansion due to the endothermic phase transition at the 660 km discontinuity [Ranalli, 1995]. Therefore, a slab that has penetrated into the lower mantle is mostly dynamically supported and has only a minor contribution to the net slab pull force [Conrad and Lithgow-Bertelloni, 2002]. Figure 1a shows two representative examples from our series of experiments, in which we systematically varied the effective viscosity of the interface material, resulting in low to very high interfacial shear stresses (here referred as low and very high mechanical coupling). In the experiment with low mechanical coupling the slab rolled back at a steady velocity after it reached the 660 km discontinuity, at the same time that both the overriding and subducting plates moved toward the trench. In the experiment with very high mechanical coupling, subduction stalled a short time after it reached the bottom of the tank and the slab detached as a Rayleigh-Taylor instability. Figure 1b plots values of subduction velocity (v S ) during the steady state subduction phase of five different experiments as a function of the shear stress calculated for the subduction interface. The diagram shows that subduction velocity decreases exponentially with an increasing degree of shear stress or mechanical coupling along the subduction interface. In the cases of high, very high, and extreme mechanical coupling, subduction stopped prematurely due to high resistive forces along the interface (Figure 1a). Only the low and medium mechanical coupling models had significant scaled subduction velocities (more than ~2 cm/yr; Figure 1b) and showed single-sided subduction zone geometries comparable to those observed in nature, in particular the low-coupling model. In all other experiments, the scaled velocities were always < 2 cm/yr. Even though these values are not unrealistic, only ~10% of natural subduction zones have velocities < 2 cm/yr. Furthermore, these models quickly developed into subvertical blob-like double-sided downwelling geometries, unlike subduction zones observed in nature. We have calculated the available slab negative buoyancy force in our experiments (the only driving force in the system) and the shear resistance force at the subduction interface. In the experiments with low mechanical coupling the shear resistance is ~18% of the driving force, while in the experiments with medium and very high mechanical coupling the shear resistance is ~55% and ~68% of the driving force, respectively. The negative buoyancy forceisalsousedtodrivemantle flow (up to 90%), plate bending (2 20%), and internal deformation of the plate (e.g., plan-view curvature <1%). This explains why subduction slows down and eventually stalls when interfacial stresses are high, because there is insufficient driving force available to overcome the shear resistance at the interface. In our low-coupling experiment the low shear stresses scale to values in nature of approximately 4 MPa. Note that we used a slab negative buoyancy close to the upper limit in nature, corresponding to a 80 km thick oceanic plate with a high-density contrast of 80 100 kg/m 3 and an effective viscosity ratio between the slab and ambient mantle that is considered representative of the natural prototype. Therefore, the velocities and shear stresses obtained in our models should be regarded as close to the upper limits in nature. 3. Subduction Velocities in Nature Figure 1c plots the frequency distribution of subduction velocities for 226 subduction zone segments, each 200 km in trench-parallel extent (data from Schellart and Rawlinson [2013]). For this segment length, the DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2667
distribution is slightly bimodal with an average value of 5.6 cm/yr 90% of subduction zone segments have values higher than 2 cm/yr and 86% are between 2 cm/yr and 12 cm/yr (grey area in Figure 1c). From the empirical relationship in Figure 1b, this suggests that 90% of subduction zone segments have interfacial shear stress lower than ~35 MPa (grey area in Figure 1b). These combined observations of the natural prototypes and the results of our physical experiments imply that natural subduction zones have relatively low values of interplate shear stress, which are indicative of low degrees of mechanical coupling. A large range in the subduction velocity values should be observed if there was a significant variation in the strength of natural subduction zones. It could be inferred that the degree of mechanical coupling must be high for the remaining 10% of subduction segments with v S < 2 cm/yr. However, from these 23 segments, 17 have a slab tip that has subducted to depths of only ~100 300 km, indicating that the low subduction velocity can be ascribed to a short slab length and consequent weak slab pull force. Of the remaining six, four are located near a slab edge, where slow velocities are likely a consequence of adjacent collisional tectonics or resistance to slab tearing at the edges. Conversely, subduction zones with very high subduction velocities (4% of the total) either correspond to cases in which the interface is extremely weak or in which the ambient mantle viscosity is highly non-newtonian and strain rate softening with a much lower effective viscosity. We note that our velocity scaling is sensitive to the value assumed for the sublithospheric upper mantle viscosity (see equation (1)), which in our case is 1.5 10 20 Pa s. The sensitivity of the relationship in Figure 1b to sublithospheric mantle viscosity is evaluated in the supporting information (Figure S1). For example, when the sublithospheric mantle viscosity is 5 10 19 Pa s we obtain subduction velocities > 2 cm/yr for interplate zone shear stresses 54 MPa (Figure S1a), and subduction velocities > 2 cm/yr are estimated for shear stresses 7 MPa when the viscosity is 5 10 20 Pa s (Figure S1b). This implies that a higher effective viscosity for the sublithospheric mantle in nature would further promote a weak subduction interface and vice versa. However, comparison of our subduction models with natural subduction zones suggests that the effective viscosity of the sublithospheric upper mantle should not be as low as 5 10 19 Pa s because it requires correspondingly high interfacial shear stresses of 54 MPa for v S = 2 cm/yr. For the majority of subduction zones in nature with v S =2 4cm/yr (Figure 1c) such values of interfacial shear stress would correspond to our high-very high coupling experiments, which predict unrealistic subduction zone behavior (e.g., premature stalling and development of Rayleigh-Taylor instabilities). 4. Sediment Thickness and Plate Age If there were a causal relationship between the degree of mechanical coupling at the plate interface and the subduction velocity, then a correlation should be observed between v S and the parameters that might control mechanical coupling, such as sediment thickness or subducting plate age. These two parameters tend to be related because older plates generally have a thicker sedimentary cover [Divins, 2003]. Moreover, older oceanic plates are expected to be more hydrated [e.g., Korenaga, 2007]. Weak, hydrated rocks entering the subduction channel could act as a lubricant, potentially decreasing the shear stresses along the interface. Figure 2a indicates that there is no statistical relationship between subduction velocity and either sediment thickness or subducting plate age. Indeed, there is a slight anticorrelation (although insignificant) with sediment thickness and no correlation with subducting plate age. One explanation for these observations is that there is no causal relationship between the degree of lubrication of the subducting plate due to sediments or plate hydration and mechanical coupling at the subduction zone interface. Alternatively, we suggest that the observations indicate that all subduction zones are weak, with low values of mechanical coupling and consequently that a weak subduction zone interface is always present. 5. Are Subduction Zones Inherently Weak? Our analysis indicates that the mean subduction velocity of all subduction zone segments is 5.6 cm/yr and for 90% it exceeds 2 cm/yr. The empirical relationship determined by our laboratory experiments implies that such velocities are characteristic of relatively low degrees of interplate mechanical coupling (shear stresses < 35 MPa). For the remaining 10% of subduction zone segments that have lower subduction velocities it is also likely that the magnitude of mechanical coupling is similarly low. Other factors most likely control low subduction velocities in those locations, such as subduction of a short slab (e.g., South Shetland, southernmost Chile, and Puysegur), oblique subduction, or high resistance to slab tearing at a subduction DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2668
Figure 2. (a) Charts plotting the correlation between trench sediment thickness and subduction velocity (v S ) and subducting plate age at the trench and subduction velocity. Black lines are least squares linear best fits and R 2 is the coefficient of determination (data from Schellart and Rawlinson [2013]); (b) conceptual model showing the role of water in maintaining the subduction zone interface at low shear stresses and similar subduction rates. zone edge (e.g., the southernmost Mariana subduction segment and the southernmost Lesser Antilles). The 4% of subduction segments with subduction velocities higher than 12 cm/yr can possibly be attributed to locally lower values of mantle viscosity. The absence of correlations between subduction velocity and both plate age and sediment thickness suggests that subduction zones are intrinsically weak with uniform low interfacial shear stress values. It is reasonable to assume that our arguments for low subduction interface shear stresses require all of the resistive forces to be proportional to the driving force (the negative buoyancy force of the slab). It follows that a doubling of the driving force (for example, by doubling the plate thickness due to an increase in plate age from 50 km at ~20 Myr to 100 km at ~80 Myr) should result in a doubling of the subduction zone interface shear force and shear stress. If shear stresses at the interface are globally relatively large, for example, 30 MPa for a ~20 Myr old subducting plate, then this would require a 60 MPa shear stress for an ~80 Myr old subducting plate. However, we are unaware of any evidence for such a large measurable difference in nature. On the other hand, if shear stresses are globally relatively low, for example, 5 MPa for a ~20 Myr old subducting plate, then a shear stress of ~10 MPa for a ~80 Myr old subducting plate would still be low and the relatively small difference would likely not be measurable in nature. The opposite assumption that some resisting forces may not be proportional to the driving force can be assessed by investigating its implications. For this purpose, we assume that the driving force increases with slab age while the resistive viscous mantle drag force, bending force, and the slab internal deformation either increase at a slower rate, remain constant, or decrease. For example, laboratory models that investigated DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2669
the role of subducting plate thickness indicate that the subducting plate bending dissipation rate increases at a rate that is slower than the rate of slab potential energy release with increasing plate thickness [Irvine and Schellart, 2012], although the bending dissipation rate is only 4 10% of the potential energy release rate in these experiments. In this scenario, to match the observed age-independent subduction velocity in nature (Figure 2a), the subduction zone interfacial shear stresses must increase with age in order to counteract the increasing available driving force. However, such an age-dependent and increasing interfacial shear stress contradicts worldwide observations of heat flux, focal mechanisms, GPS, and in situ stress measurements, which all indicate very low values of interfacial shear stresses (<20 MPa) regardless of plate age [Bird, 1978; Magee and Zoback, 1993; Wang et al., 1995; Springer, 1999; Currie et al., 2002; Grevemeyer et al., 2003;Luttrell et al., 2011]. The notion of an age-dependent and increasing interfacial shear stress also disagrees with numerical modeling results that imply the opposite scenario, in which interfacial stresses should decrease with age due to strain weakening [Gerya et al., 2002;Gerya and Meilick, 2011]. Conversely, it could also be assumed that the driving force increases with slab age while one or more of the three resistive forces increase at a faster rate. To match the observed age-independent subduction velocity (Figure 2a) in this scenario, the subduction zone interfacial shear stresses must decrease with age in order to counteract the decrease in the available driving force. This implies that the interfacial shear stresses could decrease with age, which is also not supported by observations. What observations consistently show is that interfacial shear stresses are low and therefore that subduction zones are inherently weak. 6. The Role of Water in Plate Interface Weakening Even though the mechanism responsible for maintaining low shear stresses at plate interfaces is still under debate, we suggest that the answer lies with the abundance of liquid water on Earth. Indeed, it has long been recognized that plate tectonics requires the presence of large amounts of liquid water [e.g., Fyfe, 1988; Mian and Tozer, 1990]. The upper portions of the oceanic plates are thought to be strongly hydrated [e.g., Korenaga, 2007; Faccenda et al., 2009] and hydrated rocks are known to be relatively weak (low effective strength) [Kohlstedt et al., 1995]. However, the absence of a correlation between both the amount of sediment and plate age with subduction velocities (a proxy for the subduction interface strength) implies that subducting plate hydration alone does not control weakening of the interface; otherwise, its effect should be reflected in the subduction velocities. More importantly, this implies that whatever the controlling factor is for maintaining low interfacial shear stresses, it is not fundamentally linked to weakening of the subducting plate itself. We propose that the degree of fore-arc mantle (overriding plate) hydration exerts a key control, which is dependent on water released from the subducting plate, as suggested by Gerya et al. [2008] and Gerya and Meilick [2011]. A wide range of geophysical and geological data indicates that extensive serpentinization of the fore-arc mantle lithosphere is expected and observed [Hyndman and Peacock, 2003; Wada et al., 2008]. This serpentinization is caused by the upward release of fluids (mostly water) trapped in the downgoing plate (Figure 2b). The presence of the subducting plate also cools the overlying fore arc to temperatures at which hydrous serpentine minerals and talc are stable [Hyndman and Peacock, 2003] at the same time that high pore fluid pressure contributes to a reduction in the effective strength of fore-arc rocks closer to the surface [Kohlstedt et al., 1995]. From this perspective, the lower part of the fore-arc region can be considered as the effective subduction interface [Gerya et al., 2002] (Figure 2b). Due to overpressure and serpentinization-related volume increase, a significant portion of the fore-arc rocks and water migrate toward the surface where they extrude within mud and serpentine volcanoes [Fryer, 2012] or are eroded and dragged down into the mantle wedge [Tonarini et al., 2011]. This is consistent with the observation that subduction zones do not become increasingly weaker with time. Thereby, the amount of water in the fore arc is likely to progressively decrease if a continuous inflow is not maintained. The loss of water will also reduce pore fluid pressure, contributing to an increase in effective strength. A steady release of water into the fore arc could therefore play a crucial role in maintaining a weak interface and subduction velocities at a characteristic rate (~5.6 cm/yr). A self-regulated feedback mechanism capable of maintaining a weak subduction interface could work as follows. In the case of low subduction rates, and thereby a hotter slab, dehydration reactions will be pulled up to shallower levels resulting in a higher degree of water release into the fore-arc mantle, weakening the DUARTE ET AL. 2015. American Geophysical Union. All Rights Reserved. 2670
fore-arc rocks and allowing faster subduction rates. Conversely, faster subduction rates produce colder slabs, which have the opposite effect by pushing dehydration reactions to deeper levels, thereby causing the release of most of the water into the mantle wedge [Wada et al., 2008; Gerya et al., 2002]. This will result in a decrease in the amount of fore-arc hydration, increasing its strength and decreasing the subduction velocity. With the resulting decrease in subduction velocity, more water will again be released into the fore arc, because dehydration reactions will be pulled back up, enhancing serpentinization reactions and increasing pore fluid pressure, resulting in a decrease in fore-arc rock strength. Such a self-regulating feedback mechanism can potentially explain how low shear stresses are maintained at subducting plate interfaces and consequently help to explain why global subduction velocities are so uniform. It should be noted that our experiments and conceptual model are limited by a number of simplifications. For example, the models are isothermal and isoviscous. Our rigorous scaling procedure and choice of boundary conditions guarantee that the main driving and resisting forces are approximately scaled to nature and thus the results provide a robust first-order approximation. Finally, an important simplification is that our experiments do not incorporate a brittle crust. It could be argued that the shallow frictional part of the interface also contributes to the overall subduction interface strength in a significant way, even overshadowing the contribution from the mantle wedge. However, observations and modeling studies have shown that the strength of crustal faults (including megathrusts) is very low, with coefficients of friction lower than 0.15 or even as low as 0.02 0.08 [Cattin et al., 1997;Magee and Zoback, 1993;Wang et al., 1995; Wang and He, 1999; Buiter et al., 2001; Provost et al., 2003; Townend and Zoback, 2004; Vernant and Chéry, 2006]. In the case of subduction zones, such behavior is attributed to the overpressure of pore fluid and porosity collapse of subducted sediments, which is expected to occur at shallow depths (<20 40 km; in the case of subduction under continental margins) [Magee and Zoback, 1993, and references therein]. Acknowledgments The authors acknowledge financial support from a Discovery grant (DP110103387) from the Australian Research Council. J.C.D. is financed by a DECRA Fellowship (DE150100326); W.P.S. is financed by a Future Fellowship (FT110100560) both from the Australian Research Council. Marc-André Gutscher and Filipe Rosas are thanked for reading a first version of the manuscript and for inspiring discussions and constructive suggestions. Taras Gerya, and two anonymous reviewers, and the Editor Michael Wysession are sincerely thanked for helpful and constructive comments and suggestions that substantially improved the manuscript. All data and methods necessary to understand, evaluate, replicate, and build upon the reported research are presented in the manuscript. The Editor thanks an anonymous reviewer for assisting in evaluating this paper. 7. Conclusions Our combined laboratory modeling and observational results suggest that interplate mechanical coupling in subduction zones is always relatively low and associated with shear stresses < 35 MPa. 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