Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons

TECTO-126330; No of Pages 14 Tectonophysics xxx (2014) xxx–xxx

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Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons Timothy M. Kusky a,b,c,⁎, Brian F. Windley c,d, Lu Wang a,c, Zhensheng Wang c,e, Xiaoyong Li e,f, Peimin Zhu e,f a

State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China Three Gorges Research Center for Geohazards, Ministry of Education, China University of Geosciences, Wuhan, China Center for Global Tectonics, China University of Geosciences, Wuhan, China d University of Leicester, UK e Faculty of Earth Sciences, China University of Geosciences, Wuhan, China f Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, China b c

a r t i c l e

i n f o

Article history: Received 7 March 2014 Received in revised form 19 May 2014 Accepted 24 May 2014 Available online xxxx Keywords: Craton destruction Hydroweakening Slab rollback North China craton Wyoming craton Brazil shield

a b s t r a c t The mechanisms of growth and destruction of continental lithosphere have been long debated. We define and test a unifying plate tectonic driving mechanism that explains the numerous petrological, geophysical, and geological features that characterize the destruction of cratonic lithospheric roots. Data from three Archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs, some along the mantle transition zone, beneath the cratons. During flat slab subduction dehydration reactions add water to the overlying mantle wedge. As the subducting slabs roll back, they suck in mantle material to infill the void space created by the slab roll back, and this fertile mantle becomes hydrated. The roll-back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting, generating new magmas that gradually destroy the roots of the overlying craton through melt–peridotite reactions. Calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through this mechanism, warranting a re-evaluation of continental growth rates with time. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The rate of continental lithosphere growth and destruction through time is a long-standing controversial issue in geosciences. Nd and Hf isotopic data from Archean and Hadean zircons suggest that a large volume of continental crust, about 50–60% of the current volume, was extracted by 2.5 Ga (Condie, 2005; Griffin et al., 2013; Rollinson, 2010; Tolstikhin and Kramers, 2008), yet the present volume of Archean crust is estimated to be less than 30% of the earlier extracted volume (Condie et al., 2009). Therefore, either large volumes of crust remain undetected deep in the lithosphere (Griffin et al., 2013) or continental lithosphere recycling and destruction has been a much more widespread mechanism than currently appreciated in the geosciences. Growing data suggests that much of the sub-continental lithospheric mantle (SCLM) may be Archean in age, even in places

⁎ Corresponding author at: State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. Tel.: +86 189 7157 9211. E-mail addresses: [email protected] (T.M. Kusky), [email protected] (B.F. Windley), [email protected] (L. Wang), [email protected] (Z. Wang), [email protected] (X. Li), [email protected] (P. Zhu).

where the overlying crust is much younger (Griffin et al., 2013). There is also much evidence that appreciable volumes of continental crust and mantle have been removed by various processes through Earth history. For example, subduction erosion at trenches has been widely documented for decades (e.g., Stern, 2011; von Huene and Scholl, 1991). In Japan a whole Paleozoic arc batholith has been removed (Isozaki et al., 2010), the lower crust of some island arcs (e.g. Talkeetna) has foundered into the mantle (Jagoutz and Behn, 2013, also Lallemand, 1995), the lower crust of the Eastern Pontides of Turkey was delaminated in the late Paleozoic as a result of continent–continent collision (Dokuz, 2011), the lower lithosphere of the entire Sierra Nevada mountain range in California was removed during deformation of the Cordilleran continental margin in the Pliocene (Jones et al., 2004), and plumes, some related to rifts, have locally eroded the subcontinental lithospheric mantle (e.g., Tanzania, Yellowstone, Greenland; Foley, 2008). Other mechanisms of continental lithosphere destruction are being increasingly understood. Maruyama et al. (2007b) proposed that large quantities of tonalite–trondhjemite– granodiorite (TTG) largely of early Precambrian age were subducted to the mantle transition zone, and from there sank to the core–mantle boundary, where they make-up a slab graveyard in the form of a new

http://dx.doi.org/10.1016/j.tecto.2014.05.028 0040-1951/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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continent with a high-V anomaly of calc-alkaline material. In this work we focus on large scale destruction of sub-continental lithospheric mantle beneath Archean cratons. The North China craton is the world's best example of a craton that had a thick root in the Precambrian and Paleozoic, and experienced large-scale root loss in the Mesozoic, with models for the loss ranging from large-scale delamination or density foundering, to major thermal erosion mechanisms including melt–peridotite reaction (e.g., Foley, 2008; Gao et al., 2004, 2009). Other cratons, such as the Sahara metacraton (Liegeois et al., 2013), Wyoming, the Dharwar craton in India (Griffin et al., 2009), and the Brazilian Shield have lost large parts of their lithospheric roots, but there is no consensus on how such large scale recycling of the SCLM was initiated and carried out (e.g., Zhu et al., 2012). Here we present a new comprehensive model of large-scale sub-continental lithospheric mantle destruction, using three examples, and discuss how this mechanism is important for understanding mechanisms of continental destruction and constraining models of continental growth through time. We document and model the relationships between flat slab subduction, trench suction, and craton destruction, using examples from the North China and Wyoming cratons, each of which locally lost approximately 100 km of their lithospheric roots in the Cretaceous and which show spatio-temporal relationships with episodes of flat slab subduction in the mantle transition zone associated with deep mantle hydration, coupled with slab rollback and concomitant influx of mantle fertile material to accommodate the space created by the slab rollback. A similar process has more recently operated along the western side of the Brazilian craton where it is thrust beneath the thickened crust of the Andes in an area of trench rollback. The mutual interaction between these processes may be more generally applicable than currently perceived. Together with the other processes of subduction erosion and arc subduction, larger amounts of continental lithosphere may have been subducted or otherwise returned to the sub-lithospheric mantle than previously appreciated. 2. Comprehensive model for large-scale sub-continental lithospheric mantle removal In this paper we propose a new model of how cratons and their mantle keels can be destroyed and recycled back into the convecting mantle. Conventional wisdom is that the prolonged stability of cratonic lithosphere, specifically the development of a thick, insulating lithospheric mantle keel, restricts or even prohibits its recycling into the Earth's mantle (e.g., Durrheim and Mooney, 1994; Jordan, 1988; Kaban et al., 2003; Trubitsyn et al., 2003). However, if cratonic lithosphere can be recycled, it has important implications for crust–mantle recycling and for understanding crustal growth through time (e.g., Artemieva et al., 2002; Bowring and Housh, 1995; Condie et al., 2009; Foley, 2008; Rino et al., 2004; Taylor and McLennan, 1995; Zhu et al., 2011, 2012). Cratons are structurally complex regions that attained prolonged stability (≥ 1 Ga) within the continents and so by definition cratons are Precambrian in age. Indeed, most formed in the Archean (Goodwin, 1991; Kusky and Polat, 1999; Rudnick, 1995; Windley, 1995). In general, Archean cratons are characterized by cold, thick, structurally complex lithosphere whose density is offset by its refractory composition, giving rise to chemical buoyancy (Jordan, 1975, 1981). The thickness of these cratonic keels (also termed roots, tectosphere, or sub-continental lithospheric mantle, SCLM) is generally 200–300 km (Prodehl and Mooney, 2012). The presence of these thick, refractory and anhydrous peridotite residues beneath Archean crustal regions is widely held responsible for the inherent stability of Archean cratons (Durrheim and Mooney, 1994; Griffin et al., 2003a; Kaban et al., 2003; Pollack, 1986). However, the formation mechanism of these keels has been controversial (e.g., Griffin et al., 2013) with some models suggesting that they represent stacked subducted oceanic slabs (e.g., Helmstaedt and

Gurney, 1995; Kusky, 1993; Stachel et al., 1998) and other models suggesting that they represent the residue from high degrees of partial melting of plumes or ambient upper mantle (Herzberg and Rudnick, 2012). While traditional models for the evolution of continental lithosphere suggest that once continents become stable or cratonized, they are indestructible and last forever (e.g. Jordan, 1975, 1981, 1988), it is becoming increasingly clear that in some cases large portions of the subcontinental lithospheric mantle can be destroyed and returned to the deep mantle long after their formation When this happens, a craton loses its cratonic characteristics, and returns to a more orogenic style of behavior, in a process that has been named the “orogen–craton– orogen” cycle (Kusky et al., 2007a). However, the processes that lead to the loss of lithospheric roots have been poorly constrained, and there are currently a wide variety of ideas about how lithospheric roots can be lost. Since the SCLM is difficult to sample directly, most models are based either on geophysical data (Cook et al., 1998), or on information from xenoliths brought up by kimberlites (e.g., Foley, 2008; Griffin et al., 2003b; Zhu et al., 2012). Although most models have assumed mechanical detachment or foundering of a lithospheric root (e.g. O'Reilly et al., 2001), some evidence suggests chemical replacement or metasomatic modification of lithospheric roots by upwelling asthenosphere by thermo-chemical processes (e.g. Griffin et al., 2003b; Xu et al., 2004; Zheng et al., 2005). We here propose a general plate tectonic and mantle dynamics model for large-scale craton destruction, based on a comparison of three cratons that have lost their roots. The model has two parts. The first is weakening of the SCLM by hydration from long-term subduction dehydration reactions beneath the cratons. This is followed by subduction roll-back during episodes of flat slab subduction along the mantle transition zone, which drives mantle flow into the region beneath the SCLM by trench suction, because the space created by slab roll back must be filled by mantle material. This new hot asthenosphere is re-hydrated by the deep dehydration reactions from the slab in the transition zone (Windley et al., 2010), then it gradually weakens and replaces the ancient SCLM by thermal erosion and by melt–peridotite reactions, replenishing the ancient lithosphere with more fertile material. 2.1. Step 1. Hydroweakening When oceanic lithosphere subducts, it hydrates the upper mantle beneath an arc from well-known dehydration reactions (e.g., Karato, 2003; Kawamoto, 2006; Peacock, 2003). However, some hydrous phases (e.g., Phase A, Phase E, and γ- and β-phase olivine) are stable to much greater depths and dehydrate even when a slab is in the mantle transition zone (e.g., Maruyama and Okamoto, 2007; Niu, 2005; Peacock, 1993; Windley et al., 2010). It is estimated that 40% of the water subducted in hydrated oceanic crust, mantle, sediments, and subducted continental material reaches the mantle transition zone between 410 and 660 km (Maruyama and Okamoto, 2007; Tonegawa et al., 2008). For instance lawsonite may contain up to 11% water, and is stable up to 11 GPa (Williams and Hemley, 2001) or about 300 km (Fig. 2a) and serpentinites can contain up to 13% water and are stable up to 7 GPa (Ulmer and Trommsdorff, 1995) and after conversion to denser hydrous phases such as β-phase olivine they can be stable up to 50 GPa (Frost, 1999; Schmidt, 1995; Williams and Hemley, 2001), well past the mantle transition zone (Fig. 2b). With increasing temperature (i.e., more time in the transition zone for deep flat slabs) these phases decompose to less hydrous wadsleyite and ringwoodite with 2.2–3.3 wt.% water, releasing water to the deep mantle, which rises and hydrates the overlying mantle (Karato, 2003; Richard et al., 2006; Smyth et al., 2003), as indicated by electrical conductivity and seismic P-wave velocity data (Ichiki et al., 2006). The water solubility of nominally anhydrous minerals in the mantle commonly ranges up to tens of thousands of parts per million H2O by weight, constituting a major

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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reservoir of water that has an important influence on mineral and bulk mantle properties such as melt relations, rheology and electrical conductivity (Bromiley et al., 2010). If the transition zone is saturated, the amount of water locked in these minerals, is potentially four times larger than all the water in the planet's hydrosphere (Smyth and Frost, 2002). Much of the water released from these phases is concentrated in the mantle transition zone between 410 and 660 km, which can lower the melting temperature, leading to the formation of magmas (Bercovici and Karato, 2003; Maruyama and Okamoto, 2007). These hydrous phases rise as hydrous magmas that weaken the overlying SCLM, setting the stage for large-scale root loss (Kusky et al., 2007a; Niu, 2005; Windley et al., 2010). In (a) the P–T trajectory is assumed based on a moderately old slab, and other paths are possible (see Maruyama and Okamoto, 2007). b) We assume an oceanic slab of 100 km thickness is subducted, and the surface geotherm follows the curve in (a) with surface heating from the overlying hot mantle wedge. The oceanic geotherm at 0 Ma

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is from Faul and Jackson (2005). We neglect any changes in the slab thickness from heating, and ignore heating from below for this simple analysis. The red lines show the change in the geotherm through the subducting slab with time, assuming a 45° dip angle and a subduction rate of 3 cm/yr (as we use below). At 7 Ma the slab top is 7 Ma ∗ 3 cm/y ∗ sin (45°) km (148.49 km) deep, so the slab bottom is 248.49 km deep. Therefore, the isochronic geotherm at 7 Ma can be approximated as shown. Similarly, the isochronic geotherms at 14 Ma and 18 Ma can be drawn roughly. When the slab reaches the transition zone (we assume at 440 km for the top of the slab) it flattens out, so the pressure no longer increases, but the temperature continues to rise with time by conductive heat transfer, which releases water from hydrous phases to the transition zone and overlying mantle by the timedependent dehydration reactions. Note that the crustal (MORB + H2O) section becomes anhydrous at about 300 km, but the upper section of the mantle (peridotite + H 2 0) remains hydrous all the way to the transition zone.

Fig. 1. (a) 3D view of the North China craton, and its position over the Pacific flat slab situated in the mantle transition zone. The slab is dehydrating and generating melts in the mantle that rise to alter the base of the SCLM (inspired by Zhu et al., 2012). (b) Two perpendicular vertical cross-sections of whole-mantle P-wave tomography beneath eastern Asia showing slow velocities in red, and fast in blue. The fast velocities outline the flat-lying slabs beneath eastern Asia (modified after Zhao and Ohtani, 2009). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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2.2. Step 2. Slab flattening and rollback causes influx of fertile mantle beneath SCLM It is well established that the portion of the NCC that experienced root destruction is underlain by a horizontal stagnant slab in the mantle transition zone (Fig. 1; Huang and Zhao, 2006; Zhao et al., 2007), and several authors have speculated on the causal relationship between this coincidence (e.g., Zhu et al., 2011, 2012). We here investigate the physical and geometrical relationships between placing a slab in the transition zone, slab rollback, lithospheric extension, mantle replacement, and root loss. Slab rollback associated with flat slab subduction in the transition zone is different from slab rollback associated with a normal steeplydipping and deeply-penetrating slab at a trench. When a slab becomes horizontal, that section of the slab no longer exerts slab pull forces on the overlying slab, and encounters more and more resistance to further penetration as the flat section grows longer, which will eventually result in it becoming incapable of further penetration (Fig. 3). When this happens, the rollback rate must become equal to the subduction velocity, unless the geometry of the slab changes. There are three end-member cases to consider (Fig. 3). To understand this geometrically and kinematically we first must define some parameters. Fig. 3a (modified and expanded from a steep-slab subduction system modeled by Becker and Faccenna, 2009) shows a simple flat-slab subduction system with an overriding plate and fore-arc sliver. The convergence velocity at the trench (VC) is given by the velocity of the oceanic plate (VP, positive towards trench) plus the velocity of the overriding plate (VOP, positive towards trench),

whereas the velocity of the trench (VT, positive for rollback) is given by VOP + the velocity of back arc deformation (VB, positive for extension) if there is no subduction erosion or accretion at the trench (Lallemand, 1995). The velocity of subduction (VS) is given by VP + VOP + VB (Fig. 2). Becker and Faccenna (2009) have shown that trench migration rates (VT) globally are typically less than 50% of the convergence rates (VC). This has important implications as discussed below. Trenches may advance or rollback relative to the overriding plate, and in this work we only consider the cases of slab rollback. Fig. 3b,c,d shows possible geometric consequences of what happens when a slab has a very long flat segment ponded in the transition zone and the trench is retreating or rolling back. Weak slabs have a more difficult time than strong slabs to penetrate a viscosity or density contrast at depth (e.g., Christensen, 1996; Davies, 1995; Kincaid and Olson, 1987) and thus tend to “pond” or accumulate along the transition zone. For these ponded slabs, there will come some length where the force needed to make it continue to penetrate further along the transition zone exceeds the forces pushing it along the boundary (e.g., van Hunen et al., 2000), and its forward motion will stop (VPEN then = 0), whence it can be considered anchored. For slabs that lie flat along the transition zone, the force of slab pull contributing to VP will only consist of gravitational sinking of the part of the slab above the 600 km discontinuity, which will cause the bend in the lower part of the slab to have a complimentary “anti-rollback” or flattening (VFLAT; Fig. 3) at the transition from the steep to flat-slab segments (Fig. 3). If the resistive forces in the transition zone cause VPEN to become zero and the slab is anchored, then the slab will be forced to retreat and VFLAT (anti-rollback) should equal VT (rollback), and the trench

Fig. 2. Phase diagrams for MORB + H2O (a) and peridotite + H2O systems (b) simulating a subducted slab reaching pressures equivalent to the mantle transition zone, showing how hydrous phases (e.g. lawsonite) are stable in the MORB section until approximately 300 km, and stable in the peridotite section into the transition zone (modified with re-calculations after Maruyama and Okamoto, 2007). In (a) the P–T trajectory is assumed based on a moderately old slab, and other paths are possible (see Maruyama and Okamoto, 2007). b) We assume that an oceanic slab of 100 km thickness is subducted, and the surface geotherm follows the curve in (a) with surface heating from the overlying hot mantle wedge. The oceanic geotherm at 0 Ma is from Faul and Jackson (2005). We neglect any changes in the slab thickness from heating, and ignore heating from below for this simple analysis. The red lines show the change in the geotherm through the subducting slab with time, assuming a 45 degree dip angle and a subduction rate of 3 cm/yr (as we use below). At 7 Ma the slab top is 7 Ma ∗ 3 cm/yr ∗ sin (45°) km (148.49 km) deep, so the slab bottom is 248.49 km deep. Therefore, the isochronic geotherm at 7 Ma can be approximated as shown. Similarly, the isochronic geotherms at 14 Ma and 18 Ma can be drawn roughly. When the slab reaches the transition zone (we assume at 440 km for the top of the slab) it flattens out, so the pressure no longer increases, but the temperature continues to rise with time by conductive heat transfer, which releases water from hydrous phases to the transition zone and overlying mantle by the time-dependent dehydration reactions. Note that the crustal (MORB + H2O) section becomes anhydrous at about 300 km, but the upper section of the mantle (peridotite + H20) remains hydrous all the way to the transition zone. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Fig. 3. (a) Kinematic framework of flat slab subduction associated with slab rollback. VC = convergence rate at trench, VOP = velocity of overriding plate (+towards trench), VB = backarc deformation rate (+for extension), VT = velocity of trench (+for rollback), VP = velocity of oceanic plate (+to trench), VS = sinking velocity of slab, δ = slab dip angle, R = bending radius at trench, VPEN = velocity of slab penetration, VFLAT = velocity of slab flattening (+for rollback, as with VT), fluxm = amount of new mantle material needed to fill space created by slab rollback. Some vectors after Becker and Faccenna (2009). (b–c–d) show how the geometry of the slab changes with variations between VT and VFLAT.

will rapidly retreat away from the overriding plate. In this case, if VOP is zero, then back-arc extension (VB) will also equal the rollback velocity (Fig. 3b). However, Becker and Faccenna (2009) have shown that VT never exceeds VP, and is typically less than 50% of VP. This implies that if VPEN has gone to zero, that VFLAT should be greater than Vt, and the slab will become steeper (δ will decrease) during flat slab anchoring (Fig. 2c). This can only continue until the slab is vertical, since there are no known slabs that are overturned above the transition zone. If VT were greater than VFLAT, then the slab would become flatter above the transition zone (Fig. 3d). It is likely that flat-lying slabs do not actually penetrate and move along the transition zone, but simply hit the density and rheological contrast there, and cannot penetrate if they are weak. This is likely because there is no slab pull force driving the slab forward, and the ridge push and basal traction forces on the slab at this point are much smaller (approximately 30% of slab pull) according to LithgowBertelloni and Richards (1998). This would induce slab rollback (increase VT) and a simple flattening of the slab with concomitant extension of the overriding plate (VB). A simple geometric analysis shows that if a 45°-dipping slab flattens out along the transition zone at 600 km, and stops without moving horizontally (i.e., VPEN is zero) then if VT is zero, the slab can flatten out leaving a 600 km long flat slab along the transition zone before the overlying slab becomes vertical (note: we use 600 km for these simple calculations since the base of the transition zone is about 660 km, the slab is 50–100 km thick, and the base of the slab can flatten out anywhere between 410 and 660 km). Since VT rarely exceeds 50% of VP, then another 300 km of the subducting slab can be flattened out without penetration along the transition zone before the slab is vertical. This length could be increased, if limited true slab penetration along the transition zone is allowed, perhaps at a rate of up to 30% of VP, assuming that only the forces not driven by slab pull (gravity) are pushing the slab. This could allow another 180 km of slab penetration for a slab hitting the 600 km discontinuity, resulting in a flat slab section 1100 km long. Interestingly, the flat part of the Pacific slab underlying the area of cratonic root loss in eastern Asia is approximately 1500 km long (Huang and Zhao, 2006).

The crux of why this is related to craton destruction is that geometrically, if the slab has moved away from the original site of the trench by 600–900 km, it has carried the mantle above the slab with it (since voids cannot be created in the mantle), causing new mantle material to flow sideways below the overriding plate to replace the moving mantle; we call this flow the mantle flux (fluxm in Figs. 3, 4). The mantle flux can be huge — if we take a typical example of a 1000 km-wide (measured parallel to the trench) subduction zone, that has rolled back by 900 km with a flat slab section in the transition zone at 600 km, then 540 million cubic kilometers of mantle need to flow into the space between the overriding plate and transition zone to fill the space created by the retreating slab (Fig. 4). This new mantle is then hydrated by the dehydration reactions from the slowly heating and dehydrating wadslyeite and ringwoodite in the slab, inducing partial melting (Fig. 4). The extension of the overriding plate (VB) induced by the slab rollback has caused lithospheric thinning, so the new mantle material must also rise in addition to flowing sideways, causing additional adiabatic melting (e.g., de Smet et al., 1999; Vlaar, 1983). Together with this mantle hydration, the movement of new material above the flat slab induced by the slab suction of the rolling back slab, and adiabatic decompression forms enough melts that can cause a melt–peridotite reaction to thermochemically erode the base of a craton, resulting in large-scale craton destruction (e.g., Foley, 2008; Gao et al., 2004, 2009). 3. North China Craton The North China craton is the world's best example of a craton that had a thick root in the Archean, and lost the eastern half of the root during Mesozoic tectonomagmatic events. There are numerous reviews on the current geometry of the root beneath the craton (S.L. Li et al., 2011; Tian and Zhao, 2013; Wang et al., 2013; Xu et al., 2011; Y.Y. Li et al., 2011), and the evidence for its prior existence and loss (e.g., Griffin et al., 1998; Kusky et al., 2007a, 2007b; Menzies et al., 1993; Wilde et al., 2003; Wu et al., 2003; Yang, 2003; Zhu et al., 2011, 2012); we only briefly summarize this evidence here.

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Fig. 4. New comprehensive model for craton destruction through flat slab dehydration, slab rollback, mantle influx, melt-generation, and melt SCLM peridotite reaction. See text for discussion.

The North China craton is divisible into the Eastern and Western blocks (Fig. 1), separated by the Central Orogenic belt that represents an Archean–Paleoproterozoic collisional orogen (Kusky, 2011; Zhao and Zhai, 2013). The Central Orogenic belt also roughly corresponds to a topographic and gravity gradient that separates the Eastern block with thin lithosphere from the Western block with thick lithosphere, although the NS-trending gravity lineament extends far beyond the borders of the NCC lying parallel to the Pacific margin and outboard subduction zone (e.g., Niu, 2005). The Western block (also known as the Ordos platform) of the NCC is a stable craton with a thick mantle root (up to 215 km deep), few earthquakes, low heat flow, and has experienced little deformation since the Precambrian (Yuan, 1996; Zhai and Liu, 2003; Zhu et al., 2012). The Eastern block is different — it has high heat flow, numerous sometimes large earthquakes, active volcanoes, and has undergone significant deformation especially since the Mesozoic (J.L. Liu et al., 2011; Zhang et al., 2011). Geophysical data (Zhu et al., 2012) show that the eastern half of the NCC now has a thin lithosphere (as thin as 60–65 km) and no preserved lithospheric root (Yuan, 1996; Zhu et al., 2012). However, data from xenoliths in kimberlites and lavas erupted in the Paleozoic and Mesozoic show that the Eastern block once had a thick root developed in the Archean, but lost it sometime in the Mesozoic (Fan and Menzies, 1992; Gao et al., 1998, 2002, 2004, 2009; Griffin et al., 1998; Menzies et al., 1993; Wu et al., 2003, 2008; Zheng and Wu, 2009; Zhu et al., 2011, 2012). The best constraint on the age of the root loss is 132–117 Ma (Zhang et al., 2014) with a peak at about 125 Ma (e.g., Zhu et al., 2011). The loss of the eastern part of the lithospheric root of the NCC took place during two stages.

3.1. Stage 1. Hydroweakening associated with long-term sub-craton subduction The North China craton has experienced a long and complex evolution, including an arc/continent and maybe continent/continent collisions in the Archean, leading to the formation of a thick Archean mantle root (see reviews by Kusky et al., 2007a,b; Kusky, 2011; J.G. Liu et al., 2011; J.L. Liu et al.,2011; Zhai and Santosh, 2011). Paradoxically, even though the mantle root formed in the Archean, many workers suggest that the craton did not amalgamate until about 1.8 Ga in the Paleoproterozoic (e.g., Zhao and Zhai, 2013; Zhao et al., 2001). Alternatively, Kusky and Li (2003), Kusky et al. (2007a), and Kusky and Santosh (2009) suggested that the 2.3 to 1.9–1.8 Ga tectonic events in the NCC were focused along the northern margin of the craton during its life as an old passive margin converted to an Andean margin after an arc/continent collision, then a major continent–continent collision during amalgamation with the Columbia (Nuna) supercontinent at 1.9–1.8 Ga. This model also explains how the Archean mantle root along the north margin of the craton was replaced by fertile Paleoproterozoic mantle at 1.85 Ga (J.G. Liu et al., 2011). In any case the NCC experienced a long period of subduction beneath the craton in the Proterozoic, potentially adding water and hydroweakening the mantle above the subducting slabs, pre–setting the stage for the generation of melts to interact with and destroy the overlying SCLM. After 1.8 Ga, the NCC underwent a period of relative calm until ~700 Ma with subduction under the craton until ~250 Ma (Maruyama et al., 2007a, 2007b) from Dabie Shan in the south from 500 to

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Fig. 5. Geometry and location of the North China craton above subducting slabs from the Early Paleozoic to Tertiary. From Windley et al., 2010, with permissions.

250 Ma, from subduction under the craton from the Solonker suture in the north, and from 200 Ma to present with subduction from the Pacific and paleo-Pacific margin (see Fig. 5 and reviews by Kusky et al., 2007a, b; Windley et al., 2010). It is estimated that more than 18,000 km of oceanic lithosphere were subducted beneath the NCC, perhaps more than any other craton on Earth (Kusky et al., 2007a,; Windley et al., 2010). This prolonged subduction is suggested to have led to pronounced hydroweakening, as described in the model above. 3.2. Stage 2. Flat slab subduction, roll back, and influx of new mantle material Plate reconstructions of the paleo-Pacific realm or Panthalassic Ocean (e.g., Engebretson et al., 1985; Lithgow-Bertelloni and Richards, 1998; Norton, 2007; Seton et al., 2012; Smith, 2003; Utsonomiya et al., 2007; van der Meer et al., 2012; Yang, 2013) show that the continental margin of East Asia has experienced several episodes of subduction of oceanic lithosphere since the early Paleozoic (Maruyama et al., 2007a, 2007b), as well as several ridge subduction events (e.g., Isozaki et al., 2010). Van der Meer et al. (2012) presented a new reconstruction for Triassic–Jurassic times and suggested that the Panthalassic Ocean was divided into distinct paleo-oceanic plate systems, the Pontus in the

west and the Thalassa in the east, separated by the Telkhinia system of subduction zones. By the Late Triassic–Early Jurassic the Thalassa Ocean (Fig. 6) was divided into the Izanagi, Farallon, and Phoenix plates whose motion carried several exotic terranes across the ocean to collide with Asia and North America (van der Meer et al., 2012; Yang, 2013). The Pontus Ocean gradually closed by subduction in the Telkhinia subduction zones as the Izanagi, Farallon, and Phoenix plates grew. By the Early Cretaceous (150–140 Ma) the Izanagi plate was subducting beneath the entire East Asian margin, as the Pacific plate began to grow from the triple junction between the three oceanic plates. The Izanagi plate continued to subduct beneath East Asia until about 90 Ma, when the Izanagi–Pacific triple junction migrated northwards along the margin that was subducting this ridge, and this led to the Pacific plate being subducted beneath East Asia since 90 Ma. Thus, the slabs visible in the mantle beneath Asia (Fig. 1b; Huang and Zhao, 2006; Zhao et al., 2007) should contain a record of 60 Ma of subduction of the Izanagi plate from 150 to 90 Ma, and 90 Ma of subduction of the Pacific plate to the present day. If we assume rates of trench-perpendicular subduction of 3 cm/yr, (a reasonable lower-end estimate, see Seton et al., 2012) there should be beneath Asia a 1800 km-long slab of the Izanagi plate, and a 2700 km-long slab of the Pacific plate.

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Fig. 6. Plate reconstructions of the paleo-Pacific (Panthalasia) Ocean and adjacent continents from the Late Triassic to Late Cretaceous. Modified from Yang, 2013.

Tomographic data (Fig. 1b; Huang and Zhao, 2006; Zhao et al., 2007) show a high-velocity anomaly interpreted to be the Pacific (and perhaps older) slab dipping beneath Asia, and flattening out into and along the mantle transition zone, and extending almost to the NS gravity lineament and western extent of root loss of the NCC (Fig. 1). The flat part of the slab is abnormally thick (it could be thicker representing two stacked slabs, or it could be an artifact of the data) and is about 1500 km long, whereas the dipping part is about 1200 km long (measured parallel to dip). Thus, the 2700 km of preserved slabs beneath Asia accord well with the idea that they represent both the Izanagi and Pacific slabs, especially if the flat section has a doubled thickness or is stacked (Fig. 1b). This concept of the mantle home of this long-lived subduction slab system was well portrayed by Maruyama et al. (Fig. 5 in. 2007a) as the Gondwana slab graveyard under the present Pacific Ocean. If the Izanagi plate started subducting at 150 Ma at a rate of 3 cm/yr along the same trajectory (dip, or δ in Fig. 2) with a 1200 km-long dipping segment, then it would take 40 Ma for the slab to reach the transition zone at 110 Ma. If the slab dip was vertical it would reach the transition zone with a length of 600 km at 20 Ma after subduction initiation at 130 Ma, shortly before the peak time of root loss for the NCC at 125 Ma. These times would be less if the slabs flattened out at 400 km instead of 600 km, or if the subduction rates were faster. We assume that the Izanagi plate was a weak slab and unable to penetrate the transition zone (as shown by the fact that it is currently flat lying within the transition zone), and that it flattened out at this time. This would happen if the penetration velocity (VPEN) of the Izanagi plate was zero, and further subduction (VS) was accommodated by VFLAT or flattening of the slab at depth, with concomitant rollback of the slab on the surface. This dynamic situation set up the lateral flow of mantle material to beneath the craton (Fig. 4), to fill in the space above the slab created by the slab rollback as described above, bringing in new fertile mantle, that was hydrated by the water released from dehydrating

minerals in the flat slab, and as this new hydrated fertile mantle rose it partially melted, reacted with the lithospheric root, and thermochemically eroded the root (e.g., Foley, 2008; Gao et al., 2004, 2009; Xu et al., 2004; Zheng et al., 2005). We calculate the mantle flux (flux m) associated with this case by approximating a 1500 km-long flattened slab segment at 600 km, for a 3000 km slab width (measured parallel to the trench), and for a mantle flux of 2.7 trillion cubic km of new mantle that moved in beneath East Asia due to trench rollback and slab flattening. Schellart et al. (2008) made global estimates of the mantle flux (flux m in our terminology) of 456–539 km 3 per year, which if extended for the time period equivalent for the approximately 30 Ma of slab rollback beneath Asia (and assuming 500 km3 /yr), would yield 15 trillion cubic km of slab-rollback induced mantle flux globally, meaning that the rollback of the Pacific slab beneath Asia accounted for about 20% of the total global rollback-related mantle flux in that period. This model has another fundamental difference from other perceptions of how flat slab subduction operates. Many researchers assume that the subduction zone and slab have stayed essentially in place, that the slab subducted to the transition zone, then became flat and continued to penetrate horizontally by moving along the transition zone. In our model, we use the opposite end-member, and assume that the slab subducted to the transition zone, then flattened out because it was too weak to penetrate the rheological/phase boundary, then it progressively rolled back along the transition zone and the surface (Fig. 3b–d). Thus, the trench at 150–120 Ma ago was located much closer to the Asian mainland, and moved away as the slab flattened and rolled back, opening a “gap” 1500 km wide and 400–600 km deep that had to be filled by new mantle material moving in laterally from the west (fluxm in Fig. 4). This process has the potential to generate enormous amounts of melt, explaining not only the loss of the root of the NCC, but also much of the magmatism in eastern China (e.g., Niu, 2005).

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Fig. 7. (a) Map of the Wyoming craton and surrounding orogens. Part of the root of the western part of the craton has been lost. Map redrawn after Foster et al., 2006. (b) Geodynamic model showing the position of the Farallon slab beneath North America at 70 Ma and present. From Steinberger (2008), with data from Liu et al. (2008) (with permissions). Mantle temperatures are shown along the vertical section (drawn along the 41 N latitude), clearly showing the position of the Farallon plate. The surface shows dynamic topography, with blues representing depressions and green to yellow higher topography.(c) Map of South America showing locations of present and past flat slab subduction segments, and Precambrian basement. Modified from Beck and Zandt, 2002, and Kusky, 2010. (d) Cross section across the Altiplano showing the roots of the Brazilian shield being eroded from mantle influx during slab rollback and steepening (modified from Beck and Zandt, 2002). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

An additional aspect of slab rollback is that as the subduction zone retreats from the continental margin, the overriding plate will experience extension if the velocity of the overriding plate (VOP) is not greater than the velocity of rollback (VT). Upper plate extension

is described by VB, and it is intriguing to note that during this interval of slab flattening and trench rollback, the upper crust of the NCC underwent massive extension and the formation of metamorphic core complexes (e.g., Darby et al., 2004; J.G. Liu et al., 2011; J.L. Liu et al.,

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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2011; Lin et al., 2011). These extend across much of eastern Asia, and correspond in time to the loss of the lithospheric root. Extension also has the possibility of creating fault-related pathways for magmas to migrate upwards through the crust, aiding the thinning processes, and explaining much of the surface magmatism during this interval. 4. Wyoming Craton The Wyoming craton in northwestern USA (Fig. 7a) consists predominantly of Late Archean rocks, with an older division in the NW (The Montana Metasedimentary Terrane) comprised of 3.2–3.5 Ga gneisses intercalated with metasedimentary rocks that underwent a major tectonic event at 2.8 Ga (Chamberlain et al., 2003; Foster et al., 2006; Wooden and Mueller, 1988). The southern part of the craton is known as the Southern Accreted Terranes with ages of 2.65–2.63 Ga, reflecting the accretion of juvenile arc and other terranes to the craton (Frost et al., 2006; Mueller and Frost, 2006). It is bounded by the 1.92– 1.77 Ga Trans-Hudson orogen on the east, the 1.86–1.77 Ga Great Falls tectonic zone on the north, the ~2.4–1.6 Ga Selway and Farmington zones (and the younger Belt Basin and Idaho Batholith) in the west, and the 2.0–1.7 Ga Mojave and Yavapai orogens on the south (Fig. 7a). Miocene to Recent volcanic rocks of the Snake River Plain and intrusives related to the Yellowstone plume affected the western part of the craton. The Wyoming craton was amalgamated into the cratonic core of Laurentia by the collisions of arcs and continental fragments along the Trans-Hudson orogen at 1.85–1.78 Ga (Hoffman, 1988; Whitmeyer and Karlstrom, 2007). In contrast to the North China craton, the Wyoming craton seems to have been dominated by subduction away from the craton during the Proterozoic, with accretion of arcs and other blocks along its margins (Whitmeyer and Karlstrom, 2007). Thus, mantle hydration and early root loss of the Wyoming craton did not start until much later, during the Laramide orogeny in the Cretaceous when the craton was underlain by flat slabs that hydrated the sub-continental lithospheric root (e.g., Humphreys et al., 2003). In general, the Wyoming craton is characterized by thick and strong lithosphere with a distinct velocity structure (Dueker et al., 2001), but there is some indication that the Wyoming craton has been gradually decratonized since the mid-Cretaceous at about 100 Ma. Results from the DEEP PROBE (Snelson et al., 1998) and CD-ROM (Yuan and Dueker, 2005, 2010) seismic profiles, and xenolith data, reveal that the crust of the Wyoming craton is 50–60 km thick, with an unusually thick lower mafic crust that was underplated at circa 1.8 Ga (Keller, 2008). The Wyoming craton was stable from 1.8 Ga until the Laramide orogeny in the Cretaceous, generally attributed to flat-slab subduction of the Kula and Farallon plates (e.g., English and Johnston, 2004). During the Laramide, the Wyoming craton saw the formation of widespread basins separated by basement uplifts (Fig. 7a) bounded by steep thrust faults, and deep tomographic images show that the typical thick North American cratonal root is not fully preserved beneath the craton (Keller, 2008; Pavlis, 2011; Snelson et al., 1998). Currently, magmatism from the Yellowstone plume and associated Snake River Plain lavas are further eroding the root of the Wyoming craton. The Wyoming craton is therefore very similar to the NCC, in that it formed in the Archean, had additional terranes accreted to its margins, saw a major event with new mantle underplating at 1.8 Ga (compare with J.G. Liu et al., 2011) during incorporation into a larger continent, was relatively stable (excepting some rifting along the margins) until the Cretaceous when flat slab subduction (shallow in this case) started, and since then the craton has been progressively decratonized and acts like an orogen again, repeating the orogen–craton–orogen cycle (Kusky et al., 2007a). The latest phase of decratonization is related to impingement of the Yellowstone plume on the base of the craton (Stachnik et al., 2008). The relict, flat, subducted Kula–Farallon slab is still visible tomographically beneath western North America and under the Wyoming craton (Fig. 7b). It is interesting that the slab is presently in

the mantle transition zone between 410 and 600 km beneath the Wyoming craton, but farther east it steepens and drops into the lower mantle beneath the Superior craton, which preserves its thick root. Calculating the mantle flux (fluxm) of the Wyoming craton is not as straightforward as for the NCC. The Farallon slab was flat at shallow levels beneath the Wyoming craton at 70 Ma, and has since sunk and flattened out into the transition zone, before plunging deeper into the mantle farther to the east (Fig. 7b). Thus we estimate that a 1600 km-long slab (Fig. 7b) with a width of 1500 km (parallel to the paleo-trench) has flattened and sunk 400 km to the transition zone, yielding a mantle flux of 960 million cubic km of mantle material that had to flow sideways into the region above the sinking slab, explaining the partial root loss of the Wyoming craton. 5. Destruction of the Brazilian Craton beneath the Andes In a somewhat similar process to the slab rollback and lateral influx of fertile mantle beneath the NCC and Wyoming Cratons described above, Beck and Zandt (2002) and Ramos and Folguera (2009) described delamination of the subcrustal lithospheric mantle of the Brazilian craton beneath the Altiplano, and similar processes elsewhere in the crustally-thickened Andes (Fig. 7c). The Andes presently are segmented into regions of flat slab subduction, and regions of steep subduction (Barazangi and Isacks, 1976), and some of these such as the Peru and Chile flat slabs have been shown to be actively retreating (Manea et al., 2012), even though South America is moving towards the trench (van Hunen et al., 2004). The trench outboard of the Chilean flat slab has been actively retreating for the past 25 Ma (Schellart et al., 2007). Over the history of the Andes, different subduction segments have changed from steep to flat, and flat to steep. Ramos and Folguera (2009) found that when subducting slabs change from being flat to being steep, if the overriding plate has a thick crust, they respond with delamination of the root of overlying crustal material, with basaltic underplating, crustal extension (Allmendinger et al., 1997), higher heat flow and thermal uplift (Allmendinger et al., 1997), and felsic; (rhyolitic) volcanism (Kay and Kay, 1993). If slab steepening affects an overlying thin crust, then flood basalts erupt in areas where the lithosphere has already been thinned (Kay et al., 1987). Although the slabs beneath the Andes are flat at relatively shallow levels (i.e., not in the mantle transition zone), the processes may be similar to those in the NCC. As the slab steepens, fertile mantle material must flow in laterally from the Atlantic or deep beneath South America, and then flows into a region hydrated by slab dehydration, partially melts, and erodes the lithospheric root. Beck and Zandt (2002) showed that the SCLM of the Brazilian craton is thrust beneath the Eastern Cordillera of the Andes on the edge of the Aliplano (Fig. 7c). The root of the western part of the craton has been thinned from a normal thickness of about 200 km to about 100 km in this zone where the subcrustal lithosphere is delaminating piece by piece (Fig. 7c). The lower crust is flowing in beneath the strong upper crust towards the trench, aiding the delamination. The reason that the destruction of the root of the Brazilian craton is much more limited in scope than the NCC is because the flat slab beneath it is shallow, and as it rolls back into a steep attitude, much less fertile mantle material or ductile lower crust flows in to replace the space created by the slab steepening/rollback. 6. Discussion 6.1. Relationships between flat slab subduction, trench suction and craton destruction and the Cretaceous superplume, mantle avalanches, and hot mantle temperatures Above we described the partial destruction of the North China, Wyoming and Brazilian cratons during slab rollback events in the Cretaceous and Cenozoic. The early to middle Cretaceous was a period of upwelling of the mid-Pacific superplume, and mantle avalanches

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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related to the closure of Tethys (e.g., Larson, 1991; Maruyama et al., 2007a; Utsonomiya et al., 2007). This superplume probably followed the major mantle avalanche events that started at about 180 Ma, whose thermal effects heated the mantle to about 50 K above its normal temperature, peaking at 125 Ma, and that lasted for about 30 Ma (Machetel and Humler, 2003), which was also the peak time of destruction of the NCC. Also, during this time interval and the Cretaceous Normal Superchron (125–84 Ma), plate motions greatly accelerated (Seton et al, 2012), as shown by the time (132–117 Ma) of superfast-spreading of the Pacific plate (Zhang et al., 2014), suggesting that all these phenomena may be somehow inter-related. We speculate that the higher mantle temperatures and faster plate motions aided craton destruction simply by making the slab rollback and mantle influx more efficient. Using Fig. 3, if plate velocities VS increase, and slab penetration (VPEN) is still held at zero, then the slab rollback (VT) and slab flattening (VFLAT) will increase and the slab will rollback and flatten at the rate of the increased plate velocity. To balance the volume, then the new fertile mantle will have to flow into the newly created space above the rolling-back slab at a faster rate (fluxm), made easier by the higher temperature-buffered mantle viscosities, enhancing the rise of melts to interact with and thermochemically erode the roots of cratons. 6.2. Implications for models of crustal and lithospheric growth Crustal growth models and curves have long been important to produce, but contentious in their interpretations, largely because they so often fail to take account of all the multitudes of variables that inevitably relate to the extremely complex make-up and development of the continental crust. The concept of subduction erosion and crustal recycling at subduction zones has only been topical in the last decade, when it became apparent that more material was removed from the margins of continents or accreted arcs than previously supposed (e.g. Clift et al., 2009; Scholl and von Huene, 2010). Thus, it is essentially difficult to estimate the true continental or crustal growth rate, because of the poorly known volume of the recycled materials (Rino et al., 2004). The concept of subduction zone recycling does not take account of the delamination and removal of the lithospheric roots of thickened cratons and orogens. If large volumes of thickened roots of cratonic lithospheric can be recycled into the mantle, as we propose here, this has important implications for crust– mantle recycling and for understanding crustal growth through time (e.g., Artemieva et al., 2002; Bowring and Housh, 1995; Condie et al., 2009; Foley, 2008; Rino et al., 2004; Taylor and McLennan, 1995; Zhu et al., 2011, 2012). O'Reilly et al. (2009) pointed out that high-resolution global seismic tomography (Vs) models reveal high-velocity domains in Africa that are interpreted as Archean depleted buoyant continental roots that remain attached to overlying thinned continental crust. Such high-velocity buoyant roots and overlying thinned crust locally extend, often as fragments, well out under the deep Atlantic Ocean, and interpreted by O'Reilly et al. (2009) as remnant fragments of lithosphere that were separated from main continental regions by episodes of rifting. If this is the case, then thickened continental roots were more extensive than normally perceived, and also disruption and removal of the roots to the mantle has likewise been more pervasive. Clearly crustal growth curves are in need of recalculation and re-assessment. 6.3. Growth of the “second” and “third” continents through subduction erosion, arc subduction, and craton destruction Most traditional models for the evolution of continental lithosphere are based on the premise that once continents form, they are forever permanent features that remain on the surface, but can be broken up, re-arranged in the supercontinent cycle, and remelted forming more evolved magmas. Continental crust in particular is largely regarded as unsubductable because of its low density and buoyancy relative to the underlying mantle. Over the past two decades this view has been gradually

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changing. It is now recognized that approximately the same volume of material that is added to continental crust at subduction zones is brought back into the mantle by gradual subduction erosion (e.g., von Huene and Scholl, 1991), and in some cases entire crustal sections of immature arcs are subducted and returned to the mantle (e.g., Kawai et al., 2013). The reason the crustal material can accumulate there is that phase transitions below 270 km forming jadeite-bearing assemblages make the continental crust more dense than surrounding mantle between 270 and 660 km (Kawai et al., 2009) Kawai et al. (2009, 2013) suggested that much of this material has accumulated along the mantle transition zone between 660 and 410 km, and that the volume of felsic material in this zone could be six times the volume of continental crust. They accordingly name this region the “second continent.” In this work (and the many previous works cited herein) we have shown that not only the continental crust can be recycled, but large portions of the sub-continental lithospheric mantle can be returned to the convecting mantle. Thus, our concepts of continental and lithospheric stability through time have changed, and are undergoing a stage of metamorphosis to a more dynamic Earth model in which much more continental material has been extracted from the mantle than previously thought, but most of it has been taken back, and is now lying on the mantle transition zone as lost continents. The Earth's earliest crust may have been anorthositic, but virtually none of this material is left on the surface. Kawai et al. (2009) speculated that much of this early crust may now reside along the core-mantle boundary forming the D″ layer, since anorthosite would have a similar density to mantle at the transition zone, but with phase changes it is denser than the lower mantle and could founder, sinking to form a “third continent”, lost along the core–mantle boundary. This presents a very different picture of the Earth than geoscientists realized a decade ago. Three of the major boundaries on the planet (atmosphere-crust, mantle transition zone, and D″) are marked by large regions of felsic or continental crust, with large density contrasts across the boundaries. We know that plate tectonics operates on the surface, giving rise to the first continent, but have not yet explored whether or not some form of plate tectonics may operate on the second and third continents. Just as it took centuries to document plate tectonics on the first continent, it will be a challenge for the next generation of Earth scientists to determine if plate tectonics operates along all three of the major Earth interfaces. 7. Conclusions Analysis of three cratons that have lost parts of their roots leads to a general model for loss and recycling of sub-continental lithospheric mantle. 1. Dehydration reactions from flat-lying slabs can significantly hydrate the overlying mantle, generating melts. 2. Flat-lying slabs in the mantle transition zone are generated largely by rollback of the subduction zone, not lateral penetration of the slabs. 3. Rollback causes significant extension and thinning of the overlying lithosphere, and a rise in the base of the SCLM boundary. 4. Rollback of flat-lying slabs causes huge influxes of fertile mantle to move into the void created by the rollback. 5. The new fertile and hydrated mantle rises into the space created by the slab rollback and lithosphere thinning, causing adiabatic melting. The melts rise to the base of the SCLM causing melt–peridotite reactions that destroy the roots of cratons. 6. Cratons are not forever; they can be destroyed and recycled back to the mantle in the orogen–craton–orogen cycle. 7. Models of crustal growth need to be modified with the new recognition that much more continental lithosphere has been recycled to the mantle than previously thought. 8. Estimates of mantle composition need to take into consideration a much larger flux of recycled crustal and SCLM material.

Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028

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Please cite this article as: Kusky, T.M., et al., Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.028