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Earth and Planetary Science Letters

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1 Subduction-triggered magmatic pulses: A new class of plumes?

2 Claudio Faccenna a,⁎, Thorsten Becker b, Serge Lallemand c, Yves Lagabrielle c, 3 Francesca Funiciello a, Claudia Piromallo d

4 a Dip. Scienze Geologiche, Università “Roma Tre” and CNR, IGAG-Rome, Italy 5 b Dep. Earth Sciences, University Southern California, Los Angeles, USA 6 c Laboratoire Geosciences Montpellier, Université Montpellier II, CNRS, France 7 d Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy 8

article info abstract 9 10 Article history: A variety of atypical plume-like structures and focused upwellings that are not rooted in the lower mantle 25 11 Received 14 December 2009 have recently been discussed, and seismological imaging has shown ubiquitous small-scale convection in the 26 12 Received in revised form 5 August 2010 uppermost mantle in regions such as the Mediterranean region, the western US, and around the western 27 13 Accepted 6 August 2010 fi fl 28 14 Paci c. We argue that the three-dimensional return ow and slab fragmentation associated with complex Available online xxxx 29 15 oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may fi 30 16 Editor: R.D. van der Hilst play a signi cant role in regional tectonics. The testable surface expressions of this process are the outside- 191718 arc alkaline volcanism, topographic swell, and low-velocity seismic anomalies associated with partial melt. 31 20 Keywords: Using three-dimensional, simplified numerical subduction models, we show that focused upwellings can be 32 21 subduction generated both ahead of the slab in the back-arc region (though ~five times further from the trench than arc- 33 22 magmatism volcanism) and around the lateral edges of the slab (in the order of 100 km away from slab edges). Vertical 34 23 upper mantle convection mass transport, and by inference the associated decompression melting, in these regions appears strongly 35 24 geodynamic modeling correlated with the interplay between relative trench motion and subduction velocities. The upward flux of 36 material from the depths is expected to be most pronounced during the first phase of slab descent into the 37 upper mantle or during slab fragmentation. We discuss representative case histories from the Pacific and the 38 Mediterranean where we find possible evidence for such slab-related volcanism. 39 © 2010 Elsevier B.V. All rights reserved. 40 44 4142 43 45 1. Introduction return flow, which may interact with a hydrated layer in the transition 63 zone (Leahy and Bercovici, 2007) to facilitate localized upwellings. 64 46 A variety of thermal plumes, including splash (Davies and Bunge, In several regions, there is evidence for volcanism that is spatially 65 Q2 47 2006), baby (e.g., Wilson and Downes, 2006) and edge (King and and temporally connected to subduction zones but not associated 66 48 Ritsema, 2000) plumes, have been recently described as focused with mantle wedge melting. Related magmas show ocean island 67 49 vertical upwellings that are not directly rooted in the lower mantle, as basalt (OIB)-type signatures developing from volcanoes located either 68 50 would be predicted by the original hotspot model (Courtillot et al., far off the arc, ahead of the trench, or at slab edges. Relationships 69 51 2003; Morgan, 1971). Moreover, high-resolution seismological between subduction and anomalous volcanism, though already 70 52 images have shown a large amount of apparent small-scale convective postulated to explain regional cases of intraplate magmatic activity 71 53 heterogeneities in the uppermost mantle of margins such as the (Changbai volcano, East Asia: Zhao et al., 2009; New Hebrides-North 72 54 western US (e.g., Sigloch et al., 2008; West et al., 2009) and the Fiji: Lagabrielle et al., 1997; Mediterranean-European: Goes et al., 73 55 Mediterranean (Faccenna and Becker, 2010). Small scale mantle flow 1999; Piromallo et al., 2008; Lustrino and Wilson, 2007), have never 74 56 could explain, for example, the velocity distribution beneath the Rio been framed and modeled in a subduction-related convecting system. 75 57 Grande Rift (van Wijk et al., 2008), the uplift and magmatism of the The purpose of this work is to illustrate that subduction within the 76 58 Colorado plateau (Roy et al., 2009) and extension beneath the Great upper mantle could generate focused, sub-lithospheric, non-thermal 77 59 basin (West et al., 2009). Subcontinental small-scale convection may mantle upwellings. The surface expressions of these small-scale 78 60 be also excited by sharp temperature gradients at a craton's edge, convective features are outside-arc alkaline volcanism, positive non- 79 61 where decompression melting may cause volcanism (King and isostatic topography and melting zones seismically recorded in the 80 62 Anderson, 1995). Here, we suggest an indirect connection to slab mantle as low-velocity anomalies. We show that focused upwellings 81 are likely generated around the slab (at the lateral edges or ahead of 82 the back-arc region) and are most pronounced during the first phase 83 84 ⁎ Corresponding author. of subduction into the upper mantle, or after the occurrence of slab E-mail address: [email protected] (C. Faccenna). fragmentation. We first illustrate the process by presenting the results 85

0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.08.012

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 2 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx

86 of simplified three-dimensional (3D) convection models and, after- (cf. Tetzlaff and Schmeling, 2000). All other mantle temperatures are 150 87 ward, we discuss several natural case studies that satisfactorily initially set to unity, the surface is kept at T=0, and the bottom 151 88 illustrate the proposed mechanism. boundary is zero heat flux (we only consider non-adiabatic processes, 152 consistent with the incompressible framework). Defined by a non- 153 89 2. Mantle circulation during subduction dimensional reference viscosity of η= , we use a Rayleigh number of 154 5.7×106, with the typical non-dimensionalization scheme and 155 90 The downwelling of cold lithospheric material into the mantle definitions (e.g., Zhong et al., 1998). The effective viscosity, η, within 156 91 triggers return flow, which has been extensively investigated in the the fluid is given by a joint rheology 157 92 context of the subduction wedge, the uppermost region confined  93 η η η = η η ð Þ between the slab and the overriding plate. Convection models have = p T p + T 1 94 evaluated the overall flow produced by a single slab. For example, 95 Garfunkel et al. (1986) modeled such circulation by means of two- where the regular, fluid-creeping viscosity ηT has a simplified tem- 158159 96 dimensional (2D) Cartesian calculations evaluating how the 660-km perature dependence as 160 97 viscosity jump and trench rollback can influence streamlines. The 2D 98 cylindrical models by Zhong and Gurnis (1995) confirmed the close η η0 ðÞðÞ− ð Þ T = T exp ET0 T 2 99 dependence of trench migration and mantle return flow patterns for a 100 self-consistently subducting lithosphere with a fault. Enns et al. using E=6.21 and T0 =1 for an equivalent viscosity contrast of 500 161162 101 (2005) analyzed the influence of slab strength, thickness and side between the slab and mantle, for consistency with earlier such 163 102 fi η0 boundary conditions on 2D mantle circulation. Some of the rst 3D models. The constant prefactor T is set to unity in the upper mantle 164 103 subduction models were performed experimentally by Buttles and and to 100 in the lower mantle to represent the probable increase in 165 104 Olson (1998) and Kincaid and Griffith (2003), both of whom viscosity due to the phase change at 660 km (e.g., Hager and Clayton, 166 105 prescribed the velocity of a rigid subducting plate by investigating 1989). 167 106 the pattern of deformation induced in the surrounding mantle. The The pseudo-plastic “viscosity” ηp is computed from a constant 168 107 5 feedback between subduction and the induced 3D mantle circulation yield stress at σy =10 and the second (shear) strain-rate invariant 169 108 has been quantified by means of laboratory models (Faccenna et al., ε η σ = ðÞε ðÞ 109 2001; Funiciello et al., 2003, 2004). These experimental results were II as p = y 2 II 3 110 later confirmed by the laboratory work of Schellart (2004) and by the 170171 111 numerical work of Stegman et al. (2006) and Di Giuseppe et al. If the yield stress is taken to be depth dependent, this 172 112 (2008). Piromallo et al. (2006) and Funiciello et al. (2006) provided approximation to plasticity is sometimes called “Byerlee” plasticity 173 113 further detailed analysis of mantle flow during subduction and, in in reference to the stress-limiting effect of brittle faulting in the cold 174 114 particular, studied the toroidal/poloidial partitioning as a function of lithosphere (e.g., the discussion in Enns et al., 2005). For simplified 175 115 slab strength. subduction models with free-slip surface boundary conditions, 176 116 Here, we explore the time-dependent evolution of subduction- yielding facilitates the formation of a properly detached slab, as 177 117 induced mantle circulation adopting a 3D self-consistent (i.e., no opposed to a Rayleigh–Taylor instability-like drip (e.g., Enns et al., 178 118 motions are prescribed) setup. These models are not meant to match 2005). In our models, the parameter choices lead to substantial 179 119 any specific subduction zone, but allow us to illustrate features whose viscosity reduction in the trench region close to the surface and the 180 120 dynamic relevance for volcanism ahead of the arc and regional formation of angular weak zones, and they eventually led to the 181 121 tectonics appear to have not been fully recognized previously. complete detachment of the oceanic plate. However, none of these 182 122 We approximate subduction dynamics by solving for incompress- rheological details matter for the general flow patterns discussed 183 123 ible Stokes flow in the infinite Prandtl number regime. We use the below as long as a relatively stronger and denser slab is able to sink 184 124 finite element code CitcomCU (e.g., Moresi and Solomatov, 1995; into the mantle. We tested various other setups, including purely 185 125 Zhong et al., 1998) as modified from the CIG (geodynamics.org) compositionally driven slabs (Enns et al., 2005), a range of different 186 126 version and compute the time-dependent flow solutions for a cold choices for the yield stress, a free “ridge” location at x=0, and 187 127 slab sinking into the mantle, which is approximated by a visco-plastic additional weak zones (“transform faults”) on the sides of the slab. 188 128 material with temperature-dependent viscosity. Our model setup is The dynamic behavior of all of these models was qualitatively similar 189 129 fairly standard and is inspired by previous subduction models (e.g., to what we discuss here. 190 130 Christensen, 1996; Enns et al., 2005; Stegman et al., 2006), but we Figure 1 shows snapshots of the slab sinking into the mantle. The 191

131 neglect complexities such as the buoyancy effect of phase transitions. subduction velocity, vs, (i.e., |vs| equal to |vt| in the fixed ridge setup) 192 132 The computational domain is meant to represent the upper mantle progressively increases during the development of subduction into 193

133 and has dimensions of 3960 km length (x), 1320 km width (y), and the upper mantle (cf. Becker et al., 1999; Fig. 1a–b). Afterward, vs 194 134 1320 km depth (z) (non-dimensional aspect-ratio: 3×1×1). The decreases once the slab reaches the upper–lower mantle discontinuity 195 135 mechanical boundary conditions are free slip. We use 288 elements and temporarily ponds at the viscosity contrast (cf. Funiciello et al., 196 136 with uniform spacing in x, 96 elements subdividing the y direction and 2003). Subsequently, the subduction process is taken up by trench 197 137 96 elements for z, 80 of which define the upper mantle. The element rollback. As discussed by Funiciello et al. (2004), the pattern of mantle 198 138 refinement reflects our focus on the upper mantle and has been circulation is strongly variable during the three aforementioned 199 139 chosen for computational convenience. The modeling details can evolutionary stages. To better describe the subduction-induced 200 140 depend on the numerical implementation of near-surface yielding mantle flow, it is instructive to perform decomposition into the 201 141 (Schmeling et al., 2007), but we have found the general system toroidal and poloidal components (e.g., Tackley, 2000; Fig. 2). The 202 142 behavior to be robust with respect to mesh resolution. poloidal component is associated with vertical mass transport, 203 143 Our initial condition for the non-dimensional temperature T whereas the toroidal corresponds to vortex-like stirring and rigid- 204 144 includes an isolated oceanic plate that is fixed at the “ridge” location body rotation. The balance between these two components is 205 145 of x=0. This is implemented by prescribing a half-space cooling representative of the vertical descent of the slab and the lateral 206 146 profile that applies 0byb0.5, corresponding to the zero age at x=0, translation of the trench. Toroidal motion is then mostly excited 207 147 and an equivalent (T=0.9) lithospheric thickness of~150 km at x=0. during the retrograde motion of the slab because the mantle material 208 148 At the old leading edge of the plate, we also prescribe a slight initial placed beneath the slab flows laterally from the sides of the slab and 209 149 dip to facilitate the initiation of subduction, though this is not critical produces vortex-like structures. 210

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 3 ab 1.0 z 0.5

1.0

y 0.5

t = 0.0000.000 t == 0.080.08 0.0 c d 1.0 z 0.5

1.0

y 0.5

t = 0.140.14 t = 0.210.21 0.0 ef 1.0 z 0.5

1.0

y 0.5

t = 0.340.34 t = 0.430.43 g h 1.0 z 0.5

1.0

y 0.5

t = 0.470.47 tt = 0.54 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 x x

vz T -2.5 0.0 2.5 0.0 0.5 1.0

Fig. 1. The eight-step evolution of the reference model. Each panel is composed of two parts. The upper part shows the lateral cross-section of the model taken through the middle of the plate. The color plot gives the magnitude of the non-dimensional lithospheric temperature, assuming the initial mantle temperature is fixed at 1. Arrows illustrate the x–z flow pattern in the mantle. The lower part shows the horizontal cross-sections taken at x km depths from the top. In this case, the color plot gives the magnitude of the vertical velocity component. Arrows illustrate the x–y flow pattern in the mantle. Non-dimensional units and unity time correspond to ~15 Ma.

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 4 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx

a 3. Mantle upwelling within the back-arc region 238 2 WM UM A number of examples of anomalous volcanism have been 239 1 described to be directly or indirectly related to the subduction 240 zones. In the following, we analyze in detail four regions, namely 241 western North America, the Central Mediterranean, the North Fiji–Lau 242 0.5 Basin, and the West Philippine basin, and we more briefly review 243 other examples. 244 toroidal / poloidal ratio 0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 3.1. Western North America 245 b 1 Very large volumes of magmatic products were emplaced in 246 WM 247 UM western North America to the east of the Juan de Fuca subduction 0.8 system during the last 20 Myrs. Three main magmatic provinces have 248 0.6 been identified: the Columbia River Basalts province, the Yellowstone 249 Hotspot Track and the High Lava Plains of Oregon, or Newberry 250 0.4 Hotspot Track (Fig. 3a). The Columbia River Basalts extend over a huge 251 0.2 area and consist of 17–14 Ma lava flows (e.g., Camp and Ross, 2004; 252 Christiansen et al., 2002) and N–S oriented dyke swarms (Fig. 3a). The 253 0 254 0 0.05 0.1 0.15 0.2 0.25 0.3 Newberry Hotspot Track consists of a sequence of volcanic domes and lava flows with north-westward age progression (Fig. 3a) (Jordan, 255 c 2005). The Yellowstone Hotspot Track is associated with the east- 256 WM P northeastward migration of silicic volcanism along the Eastern Snake 257 0.8 WM T River Plain ending at the Yellowstone Caldera (e.g., Pierce and 258 UM P Morgan, 1992; Smith and Braile, 1994; Fig. 3a). Both the Newberry 259 0.6 UM T and Yellowstone tracks show age-progressive volcanism migrating 260 0.4 away from a region near McDermitt Caldera, which was first active at 261 0.2 around 17 Ma. 262 The massive and voluminous basaltic effusion has been related to 263

poloidal or toroidal RMS 0 toroidal RMS / total 0 0.05 0.1 0.15 0.2 0.25 0.3 the impact of the head plume at around 17 Ma. In this model, the tail 264 non-dimensional time of the plume is responsible for the Yellowstone track, since the 265 absolute plate motion of the North American Plate is in a southwest- 266 Fig. 2. Temporal evolution of the a) TPR ratio, b) toroidal to total velocity field RMS ratio erly direction. This is consistent with the associated high-gravity, 267 and c) poloidal and toroidal velocities for the whole domain and the upper mantle only. high-topography, high heat flow, low-velocity anomalies (Humphreys 268 The poloidal component dominates during the whole evolution, reaching its maximum 269 weight during the first phases of the process. et al., 2000; Sigloch et al., 2008; Xue and Allen, 2007; Xue and Allen, 2010) and geochemical signature (such as the 3He/4He), which are 270 characteristics of many hotspots. Yet, despite these features, evidence 271 for an upwelling conduit through the upper mantle beneath Yellow- 272 stone remains unclear, and the debate continues as to whether a 273 211 Here, we wish to point out that the rapid vertical descent of the mantle plume is relevant as a source for volcanism (e.g., Christiansen 274 212 slab material during the initial stages of subduction, with a dominance et al., 2002; Humphreys et al., 2000; Jordan et al., 2004; Obrebski et al., 275 213 of poloidal flow, induces a convective cell ahead of the slab with 2010; Smith et al., 2009; Waite et al., 2006; Yuan and Dueker, 2005). 276 214 a wavelength on the order of the upper mantle height (Figs. 1a–b The plume model, in addition, shows discrepancies with classical 277 215 and 2). The maximum vigor of the poloidal flow is attained just before hotspot model. First, the position of the oldest center is about 200– 278 216 the slab encounters the 660 km discontinuity (Figs. 1b and 2). The 300 km west of the location where the trends of both the younger 279 217 toroidal/poloidal ratio (TPR) is less than 0.5 (for selected model hotspot track and the inferred plate motions would place the hotspot 280 218 parameters) during the slab subduction into the upper mantle and at 16–17 Ma (Fig. 3c). Possible explanations for this discrepancy is the 281 219 reaches maximum values on the order of 0.6 after the slab interaction westward deflection of the plume (Geist and Richards; 1993; Pierce 282 220 with the 660-km discontinuity (Fig. 2a). The poloidal cell involves an et al., 2000; Smith et al., 2009; Xue and Allen, 2007), or that the plume 283 221 upwelling component ahead of the slab, and, consequentially, leads to rises up along a slab tear (Sigloch et al., 2008). Second, the Newberry 284 222 a thinning of the thermal boundary layer (i.e. lithosphere) in regions track is opposite to the Yellowstone one. Third, and more important, 285 223 that would be ahead of the arc in nature. This effect, combined with recent tomographic model shows that the low-velocity anomaly 286 224 decompression melting, could explain the existence and timing of positioned below Yellowstone is not rooted at depths but is positioned 287 225 some of the anomalous volcanism we discuss below. We note that this on top of a broad, high-velocity anomaly (Burdick et al., 2008; Sigloch 288 226 upwelling component is mainly active during the transient stage of et al., 2008). Given the proximity of the slab and the proposed 289 227 slab descent into the upper mantle, while it fades away after the slab's Yellowstone plume, it is likely that the subduction and the upwelling 290 228 interaction with the 660 km discontinuity. processes, in fact, interact with one another (e.g., Geist and Richards, 291 229 Figure 1 also shows an extended region of rising material along the 1993; Obrebski et al., 2010; Pierce et al., 2000; Sigloch et al., 2008; 292 230 slab's edge that is active throughout the model. However, the vertical Smith et al., 2009; Xue and Allen, 2007, 2010). To explore the possible 293 231 flow at the slab's edges is always spatially limited, and its magnitude interaction, we reconstruct in an absolute reference frame the 294 232 correlates with the upwelling ahead of the slab recorded in the first evolution of the Juan de Fuca subduction zone and of the position of 295 233 stages only for tN0.5, when the form a blob-like downwelling the volcanic centers back over the last 17 Ma, starting from the 296 234 (Fig. 1h). This is an artifact of the visco-plastic yielding and could be distribution of the velocity anomaly as recently imaged by Burdick 297 235 suppressed by adapting the rheology. However, we show this type of et al. (2008). 298 236 flow as a proxy for the kinds of currents that might be expected to be During the last 20 Ma, the subduction rate estimate summing 299 237 enhanced during slab fragmentation. overriding plate and subducting plate velocity normal to trench is on 300

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 5

-128° -124° -120° -116° -112° 17 Ma 51° 70 100 90 North America plate 200 km 3050 10 49° mantle 400 km Columbia basalt upwelling 47° Juan de 600 km Fuca c ,6 45° plate 1 0 B 2 6 4 6 8 10 43° 13 Ma 13,8 12 11 10 11 10 12 14 Yellowstone track Newberry A 41° 200 km track MC (17) Pacific plate 400 km a 0 200 39°

600 km d

7 Ma 10 6

200 km

400 km

600 km e

3 Ma 4 1 0,6

200 km

400 km

600 km f

Present Yellowstone track (projected) A B 13,8 12 11 10 64 1 0,6

200 200 km

400 400 km depth (km) 600 600 km b g v [%] -1 0 1

Fig. 3. a) Map of the Western North America region (redrawn from Xue and Allen, 2007). Three main magmatic provinces can be recognized: the Columbia River Basalts, the Yellowstone Hotspot Track (with red showing the location of magmatic centers and related ages) and the High Lava Plains of Oregon, or Newberry Hotspot Track (with red showing the location of magmatic centers and related ages, and black thin solid lines indicating the location of magmatic centers and related ages). Dashed lines show the depth of the subducting lithosphere. MC indicates McDermitt Caldera. The A–B line indicates the cross-section of panels b–g. b) The tomographic cross-section along the Western North America margin from Li et al. (2008). c–g) Reconstruction of the evolution of the Juan de Fuca subduction zone and of the position of the volcanic centers back over the last 17 Ma, starting from the distribution of the velocity anomaly in panel b). Arrows indicate the net motions of the subducting plate, the overriding plate and the trench. The age of the volcanic centers is expressed in Ma.

301 average of ~4.5±0.5 cm/yr, with north American plate preserved its 660 km discontinuity and that the position of the volcanism lines up 318 302 southwestward rate of ~2 cm/yr (Gordon and Jurdy, 1986). The total with the slab tip (Fig. 3b). Based on seismological observation and 319 303 amount of subduction is ~900 km. This value could be underestimated plate tectonic reconstruction, we speculate that a mechanism of 320 304 as it does not consider back-arc extension, whose rate is still poorly subduction-driven upwelling, similar to the one shown in Figure 1, 321 305 defined. The tomographic model shows a widespread, high seismic- could be adapted to the Yellowstone case. We also speculate that the 322 306 velocity anomaly that broadens in the transition zone and continues massive onset of volcanism in the surrounding area could be triggered 323 307 further to the east below the Yellowstone low-velocity anomaly by a shallow upper mantle source, perhaps triggered by the separation 324 308 (Fig. 3b). The model also shows a marked low-velocity anomaly down of the oldest portion of the slab and the onset of the new subduction 325 309 to 350 km below Yellowstone and surrounding regions in the shallow cycle (Sigloch et al., 2008). 326 310 layer except for the narrow strip corresponding to the Juan de Fuca 311 plate. 3.2. Central Mediterranean 327 312 We consider the cross-section of Fig. 3b as representative of the 313 mantle structure below the Yellowstone hot-spot track, bearing in Continental Europe and the Mediterranean Sea have been affected 328 314 mind the complexity of the mantle structure (Xue and Allen, 2010). by diffuse intraplate volcanism that has been particularly active from 329 315 We restore it (Fig. 3c), considering the absolute motion of the plate the Oligocene onward (Lustrino and Wilson, 2007; Fig. 4a). Even if 330 316 system during the last 17 Ma. This exercise points out that the sparse evidence of Early Tertiary manifestation is found during late 331 317 emergence of volcanism occurred just about the slab arrived at the Cretaceous–Paleocene times, an increased surge of volcanism spread 332

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 6 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx

a 0-10 Ma 10-20 Ma 60N 20-30 Ma c

35 Ma

50N A

d A 40N B 23 Ma

mantle upwelling 30N PM0.5 TZ average 450-650 km depth

10W 0˚ 10E e 15 Ma ds -2 -1 0 1 2 P velocity variation (%)

A B 0 5101520 10 Ma b f 0 200 410 400 600 660

1000km Present g B

Fig. 4. a) Map of the Central-Western Mediterranean region showing the location of alkaline volcanic centers in the last 30 Ma. For each volcanic center, a coarse subdivision of alkaline activity in 10 Ma time intervals is indicated by the colors. The background map is a vertical average of the P-wave velocity anomalies in the 450–650 km depth range using model PM0.5 by Piromallo and Morelli (2003). The A–B line indicates the cross-section of panels b–g. b) The tomographic cross-section along the A–B section. c–g) Reconstruction (left panel→top view, right panel →lateral view) of the evolution of the Central Mediterranean subduction zone back over the last 35 Ma, starting from the distribution of the velocity anomaly in panel b). Red and yellow squares are representative of subduction-related and anorogenic volcanisms, respectively. Red arrows indicate predicted return flow triggered by retreating slabs.

333 over Europe during the Late Oligocene (including the French Massif hot-spot magmas, strengthens the plume origin hypothesis, but, on the 347 334 Central, Northern Upper Rhine Graben, Eifel, Hocheifel, Veneto, other hand, the lack of other features characteristic of plume-related 348 335 Bohemian, and Pannonian Basin), albeit discontinuously. This magma- magmatism (i.e., large-scale doming, linear space-time trends of 349 336 tism is characterized by Na-rich alkaline basalts, with minor tholeiitic volcanic centers, and large volumes of erupted magmas) has been the 350 337 volcanics, which share comparable geochemical features characterized main argument against the idea of a pure plume source. Hoernle et al. 351

338 by Na2O/K2O weight ratios ≥1, primitive mantle-normalized multi- (1995) first related the HIMU component to the sub-lithospheric mantle 352 339 element diagrams showing noticeable depletion in Pb, K and Rb reservoir imaged by an upper mantle, low seismic-velocity anomaly 353 340 contents, and high Ta and Nb contents irrespective of the diverse (Zhang and Tanimoto, 1992). Goes et al. (1999) suggested an active 354 341 tectonic regimes inferred for the whole area (Cebria and Wilson, 1995; lower-mantle upwelling under central Europe, on the basis of the 355 342 Harangi et al., 2003; Jung and Hoernes, 2000; Lustrino and Wilson, 2007; present-day correspondence between a lower mantle, low-velocity 356 343 Wilson and Bianchini, 1999; Wilson and Downes, 1991, 1992). These anomaly and the position of the Tertiary volcanic centers. In the French 357 344 signatures are typical of intra-plate magmas, namely OIB with a Massif Central (Granet et al., 1995)andEifel(Ritter et al., 2001), 358 345 significant HIMU component. Hypotheses about the source of this teleseismic tomography identified small-scale, low seismic-velocity 359 346 volcanism are controversial. The HIMU fingerprint, typical of several anomalies, but these studies are depth-limited by the seismic array 360

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 7

361 aperture, and the continuity of the structure at these depths can be only 12 Ma a 2 15ab34 6 7 362 presumed. Accounting for the NE-ward motion of the African plate VT NH 363 respect to Eurasia, Piromallo et al. (2008) envisaged the contamination NH-F arc 364 of the Mediterranean upper mantle in the Upper Cretaceous when F 365 traveling over the Cape Verde–Canary hot spots and the subsequent

366 occurrence of small-scale upwellings generated by subduction- T 660 km 367 triggered return flow mechanisms. 368 Any possible model, however, should consider that the location of 369 volcanic fields active in the last 30 Ma is grouped at the northern edge 370 of the high-velocity anomaly within the Transition Zone, where the 10 Ma VT b 371 cold material conveyed by different subductions is presently 372 accumulated (Piromallo et al., 2008; Piromallo and Faccenna, 2004; NH T 373 Fig. 4 panels a and b). Tectonic reconstructions based on kinematic NH F

374 models show that the accumulation of this material in the western 20° T 375 Mediterranean started synchronously, in the late Tertiary, behind the NC 660 km 376 Alps, the Carpathians and the central Mediterranean (Jolivet and 377 Faccenna, 2000; Piromallo and Faccenna, 2004; Wortel and Spakman, 378 2000). This suggests that volcanism could have been excited by the 7 Ma c 379 emplacement of a cold anomaly in the transition zone. VT

380 To test this hypothesis, we try to correlate the position of the NH NFB T

381 French Massif magmatism with the Calabrian subduction zone, where NH F 382 the subduction velocity has been accurately reconstructed (Faccenna Q3 383 et al., 2001, 2003; Seranne, 1999). The Calabrian slab accommodates 20° NC T 660 km 384 the slow convergence between Africa and Eurasia through the 385 subduction of the Liguro–Piedmont first and the Ionian ocean 386 afterward. The matching of the convergence rates, trench motions 3 Ma 387 and tomographic images reveals that the Calabrian trench rolled back VT d 388 for more than 800 km, inducing the opening of the Liguro–Provencal 389 (30–16 Ma) and Tyrrhenian back-arc basin (12 Ma-present; Cherchi NH NFB LBT

390 and Montandert, 1982; Burrus, 1984; van der Voo, 1993; Gorini et al., NH F Q4 391 1994; Seranne, 1999; Speranza et al., 1999). Correcting the high- 392 velocity anomaly in the upper mantle for the amount of back-arc NC T 660 km 393 extension (Fig. 4c–g), we find that the intensive phase of volcanism 394 matches the back-arc spreading phase and that the renewal of 395 volcanism in the Oligocene was initiated once the slab approached the 1.5 Ma e 396 660 km discontinuity. Such mantle circulation, linking the western- VT 397 central Mediterranean mantle, is reflected in the seismic anisotropy NH NFB LBT 398 (Barruol et al., 2004; Lucente et al., 2006). The good correspondence of NH 399 the phases of speed-up subduction (20–15 Ma and 12 Ma–1 Ma), as F 400 detected by back-arc spreading (Faccenna et al., 2001), and the main 20° 401 eruptive phases also corroborates a scenario where the two processes T 660 km NC 402 are related to one another. The OIB products can be further justified as 403 originating by the previous mixing between deep sources. Moreover, 404 Present the occurrence of such volcanism on isolated relatively small and VT f 405 short lived volcanic centers scattered around the Mediterranean rule

NH 406 out any possible direct connection with a deep, hot-spot like NH NFB LBT 407 upwelling mantle source (Lustrino and Wilson, 2007). F 408 Being aware of the complexity of the region and of the presence of 409 a crustal rifting process and deformation over all of Europe (Ziegler, T 20° 660 km 410 1992), we consider the French Massif Central to also be representative NC 411 of other volcanic centers spread around other zones of downwelling 412 (Fig. 3a), such as the Alps and Carpathian area. However, we 413 acknowledge that more specific studies and reconstructions are Fig. 5. a–f) Reconstruction (left panel→top view, right panel→lateral view) of the evolution of the North Fiji and Lau Basins region back over the last 12 Ma, starting from 414 needed to extend this simple scenario to other surrounding regions. the distribution of the velocity anomaly of Hall and Spakman (2002). VT: Vitiaz Trench, Q1 NH: New Hebrides arc, F: Fiji Platform, T. Tonga arc, NC: New Caledonia ridge, NFB: 415 3.3. North Fiji and Lau Basins North Fiji Basin, LBT: Lau basin-Tonga arc.

416 The North Fiji and Lau basins are currently opening between the now forms the basement of the active Vanuatu arc and the 425 417 New Hebrides- and the Tonga-facing subduction zones (Fig. 5f; volcanically inactive Fiji islands. The Lau Basin started to open at 426 418 Falvey, 1975; Weissel, 1977; Auzende et al., 1988, 1994, 1995; 5 Ma, after a long period of the crustal stretching of the former Tonga 427 419 Lagabrielle et al., 1997; Pelletier et al., 1998, 2001; Schellart et al., arc. The oceanic ridge system now propagates following the migration 428 420 2006). These basins represent both the most active and the largest of the eastern tip of the Louisville Ridge along the Tonga trench 429 421 present-day back-arc spreading system on Earth. Oceanic spreading in (Parson and Wright, 1996; Ruellan et al., 2003; Taylor et al., 1996; 430 422 the North Fiji Basin initiated around 12 Ma, in relation with a shift in Zellmer and Taylor, 2001). The compilation of bathymetric and 431 423 the sense of subduction at the Australia–Pacific converging boundary, geophysical surveys shows that the cumulative length of active 432 424 followed by rapid rotations of fragments of the Vitiaz paleo-arc, which spreading ridges, which can be traced within the region comprising 433

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434 the North Fiji Basin (NFB) and the Northern part of the Lau Basin (LB), to flatten at the 660 km discontinuity below the North Fiji, over a 499 435 reaches 5000 km. Therefore, this domain is characterized by an rather long distance (Hall and Spakman, 2002; van der Hilst, 1995). 500 436 extremely high anomalous ratio of active spreading ridges versus the The N–S Central Spreading Ridge (CSR) of the NFB originated about 501 437 surface of recently created oceanic lithosphere that is not observed in 3 Ma ago, possibly above the tip of the flat-lying Pacific slab, and it still 502 438 any back-arc context worldwide (Lagabrielle et al., 1997). This may remains in about the same position today. The E–W branch of the 503 439 reflect an actively convecting mantle beneath these regions, but the spreading center that propagated to the east of the CSR, as well as the 504 440 mechanisms that enhance this convection are not fully understood Lau spreading centers that developed above the same flat-lying slab, 505 441 (Pysklywec et al., 2003). In particular, the spreading system of the prevent any deep origin for the magma source. 506 442 North Fiji and Lau basins shows a number of striking features in For these reasons, the hyper-spreading activity and strong thermal 507 443 relation with a complex convecting system in the upper mantle anomaly in the region (Garel et al., 2003; Lagabrielle et al., 1997) are 508 444 underlying the region: better explained by enhanced convection in the upper mantle (which 509 generates MORB-lavas) rather than by the presence of a plume head 510 445 1. In most of the surface of the NFB and in the northern LB, the beneath the basins. Starting from the tomographic images by Hall and 511 446 spreading ridges are not parallel to the subduction boundaries, in Spakman (2002), we reconstruct the evolution of the upper mantle 512 447 contrast to recent back-arc environments, such as the Mariana or slabs beneath the region during the Neogene (Fig. 5). Our recon- 513 448 the East Scotia systems. This indicates that the overall pattern of struction shows that, at about 12 Ma, the New Hebrides subduction 514 449 the trench–arc system at a lithospheric scale does not control the zone was just initiating and that most of the production rate of the 515 450 geometry of the ridge axis far inside the central part of the back-arc back-arc areas was achieved during the fall of the Australian slab 516 451 oceanic spreading system. Moreover, some spreading ridges are inside the upper mantle. Our reconstruction also points out that the 517 452 perpendicular to the subduction zone and propagate into the arc, presence of two facing, retreating subduction zones could have 518 453 triggering intra-arc extensional deformation parallel to the arc favored strong upwelling currents in the upper mantle and the origin 519 454 direction (e.g., the South Pandora–Hazel Holmes ridge, Lagabrielle of multiple spreading centers. 520 455 et al., 1997). 456 2. The spreading system is characterized by the presence of numerous 3.4. West Philippine Basin 521 457 triple junctions and relay zones of various types. These features are 458 highly unstable, with repeated ridge jumps and local kinematic The origin of the Philippine Sea Plate (PSP) has been debated. It is 522 459 reorganization at a frequency on the order of 1 Ma. presently composed of four different oceanic basins that are younger 523 460 3. In the central domain of the NFB, the spreading ridges are from west to east. There is agreement that the three youngest basins, 524 461 separated by a distance on the order of 400 km and define a series Shikoku, Parace-Vela and Mariana, opened behind the Izu–Bonin– 525 462 of oceanic microplates. Assuming that the convecting cells in the Mariana (IBM) trench system from 30 Ma to the present, and there are 526 463 upper mantle are roughly as wide as they are deep, the size of the disagreements (Hilde and Lee, 1984; Lewis et al., 1982; Mrozowski et 527 464 microplates might be a proxy to the size of the convecting cells. al., 1982; Seno and Maruyama, 1984; Uyeda and Ben-Avraham, 1972) 528 465 4. Spreading rates in the North Fiji basins are not very high (max regarding the largest one, i.e., the West Philippine Basin (WPB). Based 529 466 8 cm/yr), but those in the LB are extremely high in the northern on a new compilation of geological and geophysical data, Deschamps 530 467 part of the basin (up to 16 cm/yr full rate; Bevis et al., 1995). The and Lallemand (2002) demonstrated that the WPB also has a back-arc 531 468 spreading axis of the lowest spreading ridges often exhibits origin with an initial rift sub-parallel to the “proto-W-Philippine” 532 469 morphological and tectonic characteristics typical of regions with trench (around 53 Ma, see Fig. 6b) and a further multi-rift evolution. 533 470 high rates of magma production, constructing an oceanic crust with Figure 6b–f gives six characteristic steps of the evolution of the WPB, 534 471 abnormal thickness and abnormal elevation. which combine the paleomagnetic constraints on paleolatitudes and 535 536 472 rotations from Hall (2002) with the geological and geophysical 473 To account for a high oceanic spreading activity, it has been first constraints from Deschamps and Lallemand (2002). The “proto-W- 537 474 suggested that a lower mantle plume is currently upwelling beneath Philippine” trench was active prior to 55 Ma, whereas the “proto-Izu– 538 475 the North Fiji and Lau basins (Lagabrielle et al., 1997). Based on high- Bonin–Mariana” (proto-IBM) trench was probably initiated at around 539 476 resolution seismic tomographic images, Pysklywec et al. (2003) 53 Ma (Fig. 6a–c). Some studies (e.g., Hilde and Lee, 1984; Uyeda and 540 477 propose that an ongoing avalanche of cold, dense slab material into Ben-Avraham, 1972) suggest that a former transform fault, separating 541 478 the lower mantle causes the development of multiple flows of a hypothetical North Guinea Plate to the south from the main Pacific 542 479 upwelling asthenosphere material below this region. Although they Plate to the north, evolved into a subduction zone and gave birth to 543 480 involve lower and upper mantles in different proportions, both the PSP (Fig. 6a). Then both trenches underwent a roll-back at that 544 481 models point to extremely abnormal convection. In addition, the high period that triggered the rifting of the trapped microplate. The initial 545 482 variability of axial morphologies and the short wave segmentation of spreading between 53 and 50 Ma occurred along a single axis parallel 546 483 the spreading axis on the order of 50 to 100 km have been viewed as to and in back of the E–W-trending “proto-Philippine” arc (Fig. 6a). 547 484 reflecting the presence of small-scale convecting cells in the upper Based on the detailed reconstruction of the basin (Deschamps and 548 485 mantle that develop over the head of a major plume (Garel et al., Lallemand, 2002), it has been shown that secondary spreading axes 549 486 2003). developed oblique to the main one at 50 Ma (Fig. 6b). Then 550 487 Therefore, the geochemical signature of basalts emplaced in the rearrangements occurred at 47.5 Ma with the abandonment of some 551 488 central region of the North Fiji Basin is consistent with an upper branches and the birth of a new spreading axis perpendicular to and 552 489 mantle, MORB-type source. Basalts emplaced along the Lau spreading south of the main one (Fig. 6d–e). Excess magmatism along that new 553 490 ridges have similarities with a mantle source that had, or still has, branch resulted in the Benham Rise (BR). The final spreading stage 554 491 interactions with the Tonga arc. Basalts emplaced along the ridges of occurred between 35 and 30 Ma along a single axis again (Fig. 6e–f). 555 492 the northern regions of the Lau and North Fiji basins are melt products We thus observe that the “classical” way of back-arc spreading, i.e., 556 493 from a mantle showing similarity with the Samoan mantle. This with a single axis subparallel to the trench, was realized only during 557 494 suggests horizontal rather than vertical mantle fluxes and a fast the first and last millions of years of the evolution of the WPB, whereas 558 495 migration rate of the Samoan mantle through a slab tear window most of its growth (during 15 Myrs) was accommodated through 559 496 opened north of the Tonga slab (Fischer et al., 2000; Harta et al., 2004; multiple spreading axes with RRR triple junctions and even ridge–arc 560 497 Turner and Hawkesworth, 1998). Tomographic models of the mantle intersections that probably promoted boninites emplacements 561 498 beneath the NFB and Lau basin region show that the Tonga slab tends (Deschamps and Lallemand, 2002). It is difficult to estimate with 562

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 9

130°E 140°E 150°E 160°E spreading center, sites 291, 292 and 293 in the western part of the 573 53 Ma 152346 7 WPB, one of them being on the BR, and sites 294 and 295 in the 574 a northern part of the WPB just south of the Oki-Daito Ridge (Hickey- 575 10°N 576 ? Vargas, 1998). All the geochemical analyses done on these cores concluded that they are basalts derived from tholeiitic initial melts, 577 0° ? with no evidence for OIB-type basalts such as those found on the Daito 578 660 km Ridge or the Daito Basin to the north (e.g., Hickey-Vargas, 1998; 579 Louden, 1976; McKee and Klock, 1974; Ozima et al., 1977; Watts et al., 580 Q5 10°S 1977; Zakariadze et al., 1980). Some samples were collected using the 581 50 Ma Shinkai 6500 submersible along the fossil-spreading ridge, attesting 582 b for the last spreading episode of the basin. Based on the Nd/Zr and Ba/ 583 proto proto 10°N -PH -IBM Zr values, these were also classified as MORBs (Fujioka et al., 1999). 584 pr WPB oto- IBM 585 t This story offers many similarities with those of the North-Fiji re n c ? 586 h 0° ? Basin. 660 km pro to-W 587 -Philip 3.5. Other regions pine tre nch 10°S 588 45 Ma In several other regions, intraplate volcanism has been related to c subduction zones and described as due to the combined effect of 589 fl 590 10°N corner ow in the mantle wedge and slab dehydration. One of the BRWPB IBM most impressive intraplate volcanic regions is located in northeastern 591 China. Beneath this province, a prominent low seismic velocity 592 0° anomaly extends down to 400-km depths, but at further depths the 593 660 km Pacific slab is stagnant in the mantle transition zone (Lei and Zhao, 594 595 10°S 2005; Li and van der Hilst, 2010). Zhao (2004) emphasized the role of the large-scale return flow (‘big mantle wedge’) above the stagnant 596 40 Ma ? Pacific slab in the formation of the intraplate volcanism in NE Asia. The 597

? d corner flow in the big mantle wedge and deep slab dehydration may 598 10°N BRWPB IBM have caused the active intraplate volcanoes in NE Asia, similar to 599 the process generating the arc volcanoes above the regular mantle 600 0° wedge. Recent results of S-wave splitting (Liu et al., 2008), electrical 601 660 km conductivity (Ichiki et al., 2006), and geochemical analysis (Chen 602 et al., 2007) all suggest a hot and wet upper-mantle wedge above 603 10°S ? the stagnant Pacific slab and a close relationship to the intraplate 604

? 35 Ma volcanism in NE China. 605 ? e Another example is represented by the Tengchong intraplate 606 10°N volcanoes in southwest China. Tomographic images beneath the 607 BRWPB IBM volcanoes show a low-velocity anomaly in the shallower region of 608 400 km on top of a high-velocity anomaly dominating the transition 609 0° zone and related to the Indian plate subduction (Lei et al., 2009; Li and 610 660 km van der Hilst, 2010). Lei et al. (2009) additionally suggest that the 611 10°S upwelling flow under the Tengchong area may originate at about a 612 400-km depth and be related to slab dehydration at this depth. 613 30 Ma Intraplate back-arc-volcanism is also known in southern South 614 f America, where very large volumes of basalts were erupted in the 615 10°N proto-PH WPB rifting PV piedmont domain of the Andean Cordillera during the Cenozoic. The 616 Meseta de Somuncura forms one of the largest exposures of back-arc 617 0° lavas in Argentina and contains tholeitic-to-alkaline basalts that were 618 660 km extruded during the late Oligocene. According to de Ignacio et al. 619 (2001), the origin of this volcanism is strongly linked to circulations of 620 10°S the upper mantle in relation with slab dynamics. The formation of a 621 first mantle melting anomaly within the lithosphere below the South 622 Fig. 6. a–f) Reconstruction (left panel→top view, right panel→lateral view) of the American Plate is interpreted as the result of a vigorous adiabadic 623 evolution of the N West Philippine basin region between 53 Ma and 30 Ma. Symbols are as in Figure 5. upwelling of hot mantle in relation with rapid changes in slab 624 geometries due to a major plate reorganization at 25 Ma (de Ignacio et 625 al., 2001). 626 563 confidence the location of slabs tips beneath the WPB at each stage 564 because the evolution of the PSP is poorly constrained. We thus have 4. Subduction plumes at slab edges 627 565 proposed reasonable lengths based on the periods of rollback and the 566 supposed great obliquity of the convergence along the proto-IBM Several worldwide sites show anomalous volcanism developed at 628 567 trench. What is quite clear is that secondary spreading centers slab edges. This volcanism is usually abundant and does not present 629 568 appeared above the slabs. the typical geochemical characteristics of arc-volcanism. Two exam- 630 569 All of the basement rocks drilled, dredged or sampled using ples are briefly reported here. 631 570 submersibles in the West-Philippine Basin are tholeiitic basalts, i.e., Mt. Etna is the largest European volcano, with a large, though 632 571 typical MORBs. There were seven sites drilled during DSDP legs: holes pulsating, production rate. Its construction started at about 500 ka, 633 572 290 and 447 in the eastern part of the WPB north of the fossil with a fissure-type tholeiitic magma shifting to a more central-style 634

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 10 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx

635 eruption of Na-alkaline-type magma. The overall geochemical an upper mantle low-velocity anomaly at the edge of the Calabrian 648 636 signature of Mt. Etna basalts is akin to that of within-plate (HIMU- slab, extending laterally and at significant depths (Fig. 7a, c; Lucente et 649 637 type) oceanic island basalt (OIB) (Schiano, 2003), with differences al., 1999; Piromallo and Morelli, 2003; Montelli et al., 2004; Montuori 650 638 that often contrast a depleted asthenospheric source model with an et al., 2007). Our modeling results show that the contradiction 651 639 enriched source/component model (Tonarini et al., 2001). Silicate between these classes of models, slab-induced asthenospheric flow 652 640 melt inclusion has been interpreted to show a progressive transition and hot-spot, is only just apparent. We propose that Mt. Etna can be 653 641 from a predominantly mantle–plume to an island-arc magmatic considered an upwelling structure confined in the upper mantle as a 654 642 source (Schiano, 2003). consequence of the complex 3-D mantle circulation triggered by the 655 643 Mt. Etna is positioned just at the southern edge of the Calabrian subducting lithosphere. 656 644 Wadati-Benioff zone (Fig. 7a). Gvirtzman and Nur (1999) proposed The topography and the Holocene and Upper Pleistocene marine 657 645 that the Mt. Etna magmatic source derives from the lateral inflow of markers all agree in showing that Mt. Etna developed on top of a 658 646 asthenospheric material from the slab side, produced by slab rollback. morphological bulge (Fig. 7b) with an uplift rate higher than 1–2mm/yr 659 647 However, tomographic images show that Mt. Etna evolved on top of (e.g., Ferranti et al., 2006) started around the Middle Pleistocene (about 660

A’A’ 40 N 40 N

Mt. Etna 35 N 35 N

a A 150 Km 250 Km 10 E 20 E 10 E 20 E MIS 5.5 marker 200 elevation (m) 150 100 50

b Messina Mt. Etna elevation (km) fault Sila 2 Malta fault 1 system Puglia 0

-1

900 km 700 500 300 100 A Etna A’ Dinaric slab

Calabrian slab

c 660 km P velocity variation (%)

-2 -1 0 1 2

Fig. 7. Mantle structure beneath and around Mount Etna from PM01 model (Piromallo and Morelli, 2003). Note the low velocioty anomalies located around the Calabria slab. a) Sections at 150 and 250 km depth. b) Topography and elevation of the MIS 5.5 marine terrace along the section (from Ferranti et al., 2006) illustrating uplift in correspondence of Mount Etna. c) AA' cross section shown in a) of the seismic tomography model showing marked low-velocity anomaly around slab.

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 11

661 700 kyr ago). The volcanism, uplift and the seismic velocity structure all 5. Discussion 727 662 evolved during the last million years in the region. The development of 663 Mt. Etna occurred during a period of quiescent activity of the Volcanic activity away from the plate boundaries occurs in a 728 664 Tyrrhennian basin, indicating that rollback, if any, was strongly reduced variety of settings. Linear, age-progressive volcanic chains have been 729 665 during the last 700 kyr (Faccenna et al., 2005). This pattern of uplift and typically explained as surface manifestations of hot plumes. However, 730 666 magmatism positioned on top of the low seismic-velocity anomaly at a number of intraplate volcanoes over both oceanic and continental 731 667 the slab's edges indicates upwelling material, and it appears similarly to plates violate the age progression expected for hot spot tracks and 732 668 the rising material active in our model along the slab's edges since the need alternative mechanisms. The small-scale, upper mantle convec- 733 669 beginning of the subduction process. This mechanism may also enhance tion associated with slab retreat has been found as a viable alternative 734 670 the mixing between deep sources, producing magma with affinities to mechanism to explain volcanism at a craton edge (King and Ritsema, 735 671 OIB. 2000), as in complex mobile settings such as the Western US (Sigloch 736 672 Other, although inactive, volcanoes are located at the edges of et al., 2008) or Mediterranean (e.g., Faccenna et al., 2005), as well as 737 673 slabs inside the Mediterranean, including Trois Fourches, found in inside continental plates such as Europe (Wilson and Downes, 2006). 738 674 Morocco at the eastern edge of the Gibraltar narrow subduction zone A melting in mantle upwellings that results from decompression is 739 675 (Duggen et al., 2003; Faccenna et al., 2004). Those volcanoes show considered as a mechanism of intraplate volcanism (Raddick et al., 740 676 alkaline, OIB-to-HIMU composition and are, again, positioned on top 2002). This mechanism can easily triggered by upwelling if the mantle 741 677 of low seismic-velocity anomalies. is at, or sufficiently near, its solidus. 742 678 Volcanism at the slab edges has been described in the northwest Here, we suggest that the decompression melting produced by 3D 743 679 Pacific Ocean where the Pacific plate subduction zone is interrupted slab-induced mantle circulation could produce intraplate off-volcanic 744 680 along the southern half of the Kamchatka Peninsula. The Shiveluc area arc-volcanism. Since the melting temperature of the mantle increases 745 681 of volcanism in Kamchatka presents distinct geochemical features with pressure or depth more rapidly than the adiabatic temperature, 746 682 with respect to the normal island arc volcanoes of the Kluchevskoy any small upward displacement of a parcel of upper mantle at or very 747 683 group (the world's most productive island arc volcano) and the near its melting temperature will cause decompression melting. This 748 684 Aleutian adakites. The strong adakite geochemical imprinting of these melting creates buoyancy because the presence of melt reduces the 749 685 groups has been interpreted as a result of little fractionation and overall density and because the residual mantle, after melt is 750 686 mixing with arc magmas related to mantle circulation around the extracted, is less dense than the undepleted mantle. 751 687 edge of a subducting lithosphere (Yogodzinski et al., 2001). Yet, Buoyancy forces due to melting cause additional upwelling, 752 688 tomographic studies suggest that the absence of a clear Wadati- which further induce melting, creating a process that may become 753 689 Benioff zone at the Pacific corner is indeed related to a strong, low- self-sustaining. 754 690 velocity anomaly created by episodes of slab break during the opening The examples shown here from North America, the Central 755 691 of a large slab window (Levin et al., 2002). This anomaly probably Mediterranean and North Fiji illustrate that volcanism develops on 756 692 extends all the way down into the upper mantle, following the top of regions of low-velocity anomalies in the uppermost mantle, 757 693 progressive lateral shoaling of deep seismicity (Davaille and Lees, floored in the transition zone by high-velocity anomalies. Decompres- 758

694 2004; Gorbatov et al., 2000). Further evidence for such upward flow, sion melting could produce high reductions of Vp and Vs,upto3%and 759 695 together with some material ascent towards the surface beneath 7% per percent melt, respectively (Hammond and Humpreys, 2000). 760 696 northwestern Kamchatka, comes from seismic anisotropy (Peyton et A more quantitative estimate is necessary to calculate the fraction 761

697 al., 2001) and from an exceptionally low VS anomaly to the northwest of melt production during return flow. Petrological models and 762 698 of the Aleutian–Kamchatka Junction (Levin et al., 2002). seismic velocity variations show that the back-arc basin mantle is 763 699 The rejuvenation of volcanism along some Samoa hotpot volca- about 100 °C hotter than expected, depending on the intensity of the 764 700 noes (Lalla Rookh, Sava'i) has also been related to the interaction of subduction-induced flow (Wiens et al., 2006). The heat flow 765 701 the hotspot with the northern edge of the retreating Tonga trench distribution on non-extending back-arc region confirms that the 766 702 during the spreading of the Lau basin. This could have not only lithosphere is thinner than expected (Currie and Hyndman, 2006). 767 703 trapped and deviated the pre-existing hotspot conduit but also The 2D numerical models indicate that slab-induced mantle flow may 768 704 enforced and fed the upwelling process (Harta et al., 2004; Hawkins induce melting structures not only in the arc but also in the back-arc 769 705 and Natland, 1975; Natland, 1980). environments (Conder et al., 2002). Petrological models also indicate 770 706 The narrow South Sandwich arc also presents anomalous features that back-arc magmas are characterized by a variable but high 771

707 at the slab edges. Geochemical analysis on the Kemp and Nelson concentration of H2O, presumably derived from the dehydratation of 772 708 seamounts at the southern edge, plus investigation of ridge segments the subducted plate (Kelley et al., 2006). A concomitant mechanism 773 709 near the northern edge, shows enhanced subduction fluxes when that could feed volcanism is the release of water by hydrous Mg–Si 774 710 compared to the rest of the arc. Moreover, Leat et al. (2004) described minerals in the subducting lithosphere at the depths of the mantle 775 711 sediment melt as an active process in the same areas. So, although transition zone (e.g., Fyfe, 1997; Maruyama and Okamoto, 2007). 776 712 there is no evidence of basaltic crust melting in Southern Sandwich Recent mineral physics experiments also show that the mantle 777 713 (i.e., adakites; Yogodzinski et al., 2001), the possible sediment melting transition zone contains several times more water than the other 778 714 reveals anomalous thermal gradient action at slab edges related to portions of the mantle, and the transition zone could be an important 779 715 mantle flux. A similar situation can be found at Grenada at the water reservoir in the Earth's interior (Inoue et al., 2004; Santosh and 780 716 southern end of the Lesser Antilles arc. Additionally, this case Omori, 2008). This process can be particularly efficient for fast slabs, 781 Q6 717 recognized an exceptional enrichment in Th, which may be attributed for which reactions may not fully complete at shallow mantle depths 782 718 to sediment melting, although Grenada has high Na and is alkalic in (100–200 km). Deep dehydration reactions have also been found in 783 719 composition whereas Nelson is not. the Tonga subduction zone (Zhao et al., 1997) and in the Mediter- 784 720 Near Taiwan, the southernmost Ryukyu slab underwent an ranean transition zone (van der Meijde et al., 2003). This process has 785 721 episode of fast rollback during the Pleistocene (Lallemand et al., been proposed as the driving one for the Changbai volcanism in China 786 722 2001), as revealed by the 80 km of extension recorded in the (Zhao et al., 2009). Numerical modeling indeed confirm that the 787 723 southernmost Okinawa Trough (Sibuet et al., 1998). Contemporane- presence of water could induce Rayleigh–Taylor type instabilities, wt- 788 724 ously, alkali basalts and tholeiites have erupted near the edge (and plumes rapidly transporting fluid from stagnant slab to the base of the 789 725 out) of the slab to form the Penghu islands plateau (Chen, 1973; lithosphere and triggering off-axis volcanism (Richard and Iwamori, 790 726 Greenough and Fryer, 2008). 2010). 791

Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012 12 C. Faccenna et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx

792 The impact of fluid release in the upper mantle has been tested in – Volcanism also remains active during the following phases of 831 793 Nicaragua, where wet volcanism is considered as fed by slab-derived subduction (Fig. 8b). In several cases, we observe that the locus of 832 794 fluids, and the subducted crust exhibits 2–3 times the hydration volcanism can shift accordingly towards the subduction zones. In 833 795 inferred for other slabs (Abers et al., 2003). S-wave velocity models other cases it is rather stable. Due to its initial position, the 834 796 imaged an upper mantle low-velocity (hot) body beneath the North upwelling is often rooted onto the edge of the high-velocity 835 797 China Basin, rooted at the edge of the stagnant Pacific subducted anomaly stagnating in the transition zone (Fig. 8b). It is possible 836 798 lithosphere and generated in the overlying asthenospheric mantle that, during this phase, slab dehydration could also contribute to 837 799 due to the infiltration of fluids/melts from the slab itself (An et al., melt asthenosphere and feed volcanism (Richard and Iwamori, 838 800 2009). 2010). Petrological and numerical modeling are needed to better 839 801 Our model is based on numerical simulations showing that a large- constraints this mechanism. 840 802 scale return flow, associated with subduction and particularly – Slabs can also show peculiar volcanism positioned at slab edges. 841 803 vigorous during infant subduction stages, can produce decompression Examples of such kinds of volcanoes are frequent, including South 842 804 melting far from the trench. We also present some natural examples Sandwich, Kamchatka, Taiwan, Tonga and Mt. Etna, the latter of 843 805 of ambiguous Na-alkaline magmatism that show neither linear which is probably one of the most evident cases. The mechanism 844 806 geographic age progression, nor anchorage to deep mantle plumes. propelling melting is still decompression, due to the vertical 845 807 These volcanic provinces, instead, are connected and placed nearby component of the upper mantle circulation around the slab edge 846 808 convergent and subducting margins. With the awareness that each being active since the beginning of the subduction process. However, 847 809 volcanic province can have a more complex history, we relate the it cannot be excluded that slab dewatering could be another efficient 848 810 origin of this volcanism to the subduction process. Combining the mechanism active in this peculiar position. Some of the cited natural 849 811 physical rules extracted from modeling with the case history, we examples, in fact, exhibit a complex geochemical signature that 850 812 deduce physical relationships between subduction and volcanism that could reveal mixing between deep components and shallow ones. 851 813 can also be used in validating this model in other regions. Shear heating between the slab and the surrounding mantle could be 852 invoked as an alternative interpretation to localized low-velocity 853 814 – Numerical modeling and tectonic reconstructions reveal that the zones at slab edges, but its contribution has been quantified as 854 815 initiation of strong volcanic activity is expected during peaks of extremely weak (Rupke et al., 2004). 855 816 poloidal mantle circulation. This condition is reached when the 817 subducting lithosphere approaches the transition zone (Fig. 8a). 6. Conclusion 856 818 In this phase the slab attains its peak velocity (Funiciello et al., 819 2006). The case of the Massif Central or Yellowstone shows this We address the possible subduction-related origin of focused 857 820 correlation. Volcanism is likely positioned between 600 and upwellings, not rooted in the lower mantle, by exploring the 3D time- 858 821 700 km from the trench (Fig. 8a). During this phase, the source dependent numerical models of mantle circulation induced by self- 859 822 of magmatism is completely related to decompression melting. At consistent subduction. Our models are simplified and not meant to 860 823 the surface this process is accompanied by a dynamic topography match any specific subduction zone, but they allow us to illustrate 861 824 signal, such as broad uplift, elevated heat flow, and normal general features that appear relevant for volcanism and regional 862 825 faulting. For the case of opposite subduction zones, such as the tectonics. 863 826 Tonga and West Philippine basins, the mechanism is similar but We show that focused mantle upwellings can be generated both 864 827 the kinematics of the process need further investigation. As for the ahead of the slab in the back-arc region (~five times further from the 865 828 case of Yellowstone, it is plausible that the renewal phase of trench than arc-volcanism) and around the lateral edges of the slab. 866 829 subduction and, in turn, the intense poloidal flow followed old The upward flux of material from significant depths and the 867 830 subductions. associated decompression melting is more vigorous during the first 868 phase of slab descent into the upper mantle, or during slab 869 fragmentation. The vertical mass transport in these regions appears 870 topographic bulge, to be strongly correlated with the interplay between relative trench 871 arc volcanism off-axis volcanism a motion and subduction velocities. 872

7. Uncited reference 873 Q7

Li et al., 2007 874 decompression melting 660 km discontinuity Acknowledgements 875

We wish to thank the editor Rob van der Hilst, Massimo Coltorti, 876 Luigi Beccaluva, Jeroen van Hunen for discussions and insights. 877 trench rollback arc volcanism rifting Support of C.F. and S.L. from a joint CNR–CNRS project. 878 b backarc extension off-axis volcanism References 879

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Please cite this article as: Faccenna, C., et al., Subduction-triggered magmatic pulses: A new class of plumes? Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.012