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Lithos 113 (2009) 179–189 Contents lists available at ScienceDirect Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s Mantle wedge asymmetries and geochemical signatures along W- and E–NE-directed subduction zones Carlo Doglioni a,⁎, Sonia Tonarini b, Fabrizio Innocenti b,c a b c Dipartimento di Scienze della Terra, Università La Sapienza, Rome, Italy CNR-Istituto di Geoscienze e Georisorse, Pisa, Italy Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy a r t i c l e i n f o Article history: Received 27 March 2008 Accepted 26 January 2009 Available online 8 February 2009 Keywords: Mantle wedge Subduction zone Westward drift B and Nd isotopes a b s t r a c t Subduction zone kinematics predict that, assuming a fixed lower plate, the velocity of the subduction equals the velocity of the subduction hinge (Vs = −Vh). In all subduction zones the subduction hinge migrates toward the lower plate. However, two main types of subduction zones can be distinguished: 1) those where the upper plate converges toward the lower plate slower than the subduction hinge (mostly W-directed), and 2) those in which the upper plate converges faster than the subduction hinge (generally E- or NE-directed). Along the first type, there generally is an upward flow of the asthenosphere in the hanging wall of the slab, whereas along the opposite second type, the mantle is pushed down due to the thickening of the lithosphere. The kinematics of W-directed subduction zones predict a much thicker asthenospheric mantle wedge, larger volumes and faster rates of subduction with respect to the opposite slabs. Moreover, the larger volumes of lithospheric recycling, the thicker column of fluids-rich, hotter mantle wedge, all should favour greater volumes of magmatism per unit time. The opposite, E–NE-directed subduction zones show a thinner, if any, asthenospheric mantle wedge due to a thicker upper plate and shallower slab. Along these settings, the mantle wedge, where the percolation of slab-delivered fluids generates melting, mostly involves the cooler lithospheric mantle. The subduction rate is smaller, andesites are generally dominant, and the lithosphere thickens, there appears to be a greater contribution to the growth of the continental lithosphere. Another relevant asymmetry that can be inferred is the slab-induced corner flow in the mantle along W-directed subduction zones, and an upward suction of the mantle along the opposite E- or NNE-directed slabs. The upward suction of the mantle inferred at depth along E–NE-directed subduction zones provides a mechanism for syn-subduction alkaline magmatism in the upper plate, with or without contemporaneous rifting in the backarc. Positive δ11B and high 143Nd/144Nd characterize W-directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab. However, this observation disappears where large amounts of crustal rocks are subducted as along the W-directed Apennines subduction zone. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The mantle wedge (Fig. 1) is the triangular section of the mantle confined between the top of the slab and the base of the upper plate (e.g., van Keken, 2003; Wiens et al., 2008). It is generally considered to be composed of asthenosphere, although some authors also include the entire lithospheric mantle section above the slab. The mantle wedge filters fluids released by the slab that melt the overlying mantle (Abers et al., 2006), and feed arc magmatism (Tatsumi et al., 1983; Syracuse and Abers, 2006). The mantle wedge is usually conceived as a relatively “hot” body, where the melting feeding the magmatic arc can take place ⁎ Corresponding author. E-mail address: [email protected] (C. Doglioni). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.01.012 (N1200 °C?), bounded by lower temperatures at the inclined base (top of the slab) and the top (base of the lithosphere?). The transit and location of melting areas into the wedge have been identified by magnetotelluric or electrical conductivity studies (Brasse et al., 2002; Brasse, 2005). The mantle wedge is therefore a crucial area for plate tectonics, where relevant chemical transfer occurs and new material is produced and added to the crust. What happens in the mantle wedge can be inferred from seismic tomography, geochemistry of lavas and xenoliths, plus other indirect information such as gravimetric and geoelectrical studies. In the Tonga backarc basin, the mantle wedge has been seismically illuminated showing a series of well-bedded reflectors, indicating a form of stratified architecture (Zheng et al., 2007). Martinez and Taylor (2002) proposed an eastward flow in the mantle wedge to compensate for slab rollback, this flow being distorted by the corner flow associated with the subduction. These 180 C. Doglioni et al. / Lithos 113 (2009) 179–189 opposite subduction zones. This may be ascribed to the thinner column (if any) of continental crust percolated by melts along W-directed subduction zones. In fact, even along W-directed zones, K can be very abundant when continental lithosphere occurs both in the lower and in the upper plate (e.g., the central Apennines, Peccerillo, 2005). 2. Subduction asymmetry Fig. 1. The mantle wedge is the triangular section of mantle in the hanging wall of subduction zones. It is considered as the source for the magmatic arc, being percolated and metasomatized by the fluids delivered by the dehydration of the descending slab. Relative to the upper plate, the subduction hinge can diverge or converge. The kinematics of the hinge is a good indicator on the mantle wedge geometry. The legend in the figure indicates the range of values of the main parameters. authors suggest that this flow system can explain the geochemical asymmetries in the backarc spreading and the slab-related volcanic front. Moreover, shear wave splitting analyses often indicate trenchorthogonal direction in the backarc basin, and trench-parallel direction in the forearc (e.g., Levin et al., 2004). This has been interpreted signifying that deformation in the mantle causes lattice-preferred orientation (LPO), which in turn affects the directional dependence of seismic wave velocity (Kneller et al., 2005). Based on shear-wave splitting analysis, trench-parallel ultra-fast velocities (500 mm/year) have been measured in the Tonga mantle wedge (Conder and Wiens, 2007), consistently with the “eastward” mantle flow implicit in the net-rotation, or “westward” drift of the lithosphere (Gripp and Gordon, 2002; Scoppola et al., 2006; Doglioni et al., 2007), although the super fast velocities of Tonga would favour the faster net rotation inferred in the shallow hotspot reference frame (Crespi et al., 2007). The mantle wedge is considered as a section with higher mantle temperature (anomalies up to 400–600 °C, Koper et al., 1999), rich in fluids released by the downgoing slab (Billen and Gurnis, 2001; Abers, 2005; Grove et al., 2006; Panza et al., 2007a,b; Peccerillo et al., 2008), and marked by low velocity of the seismic waves (Conder and Wiens, 2006), and lower viscosity. In the literature, the mantle wedge is mostly undifferentiated, with variations related to the thickness and composition of the upper and lower plates. However, profound differences occur, for example when comparing the mantle wedge of the western versus the eastern Pacific subduction zones (Plank and Langmuir, 1988), or when comparing the Apennines (W-directed slab) and the Alps (SEdirected slab), (e.g., Peccerillo, 2005; Panza et al., 2007a,b). Multiple subduction components even within a single mantle wedge have been proposed in the arc magmatism of the central and southern America subduction zone (Hickey-Vargas et al., 2002; Tonarini et al., 2007). It has also been noted that backarc spreading must be part of a mantle flow associated with the mantle wedge. Therefore, significant differences in the mantle wedge should occur as a function of whether or not there is active backarc spreading (Ribe, 1989; Conder et al., 2002; Wiens et al., 2008). Since backarc spreading generally forms along Wdirected subduction zones (e.g., Doglioni et al., 2007), we contribute in this article some kinematic and geochemical ideas that support the concept of an asymmetry in the mantle wedge as a function of the subduction polarity. Based on the data compilation in Winter (2001), the W-directed subduction-related volcanic arcs have in general lower K, Na, Al2O3 content, and higher FeO, MgO, CaO/MgO with respect to the Subduction zones can be analyzed in terms of a wide range of parameters, such as convergence rate, topographic and structural elevation of the related orogen, subsidence rate in the trench or foredeep, erosion rate, metamorphic evolution, magmatism, dip of the foreland monocline, depth and geometry of the decollement planes that generate the accretionary prism and the belt of the upper plate, the thickness and composition of the upper and lower plates, gravity, magnetic and heat flow anomalies, seismicity and slab dip. Therefore, there is a long list of parameters, which are relevant to the geometry and evolution of each particular subduction zone. However, since the lithosphere has a netrotation relative to the mantle (the so-called “westward” drift, e.g., Le Pichon, 1968; Bostrom, 1971), subduction zones appear to be sensitive to this polarization, that is not E–W, but along an undulated flow that has the pole of rotation displaced about 30° with respect to the Earth's rotation pole (Crespi et al., 2007). Therefore, two main different classes can be distinguished as a function of the subduction polarity, i.e., in favour or against the westerly polarized tectonic mainstream that depicts the predominant direction of plate motion (Doglioni et al., 1999, 2007). Indeed, subduction zones directed to the west (Barbados, Apennines, Marianas, Tonga) show a number of common characteristics, such as low topography and low structural elevation, a deep trench or foredeep with high subsidence rates, generally a steep slab, an accretionary prism mostly composed by the shallow rocks of the lower plate and a conjugate backarc basin. In contrast, subduction zones directed to the east (e.g., Andes) or north-east (Himalayas, Zagros) exhibit opposite signatures such as high structural and morphological elevation, generally no backarc basin, shallower trench or foredeep with lower subsidence rate, deeply rooted thrust planes affecting the whole crust and lithospheric mantle, ultra-high pressure rocks and wide outcrops of metamorphic rocks, and dominantly shallower dip of the slab. Basalts and less evolved lavas are more typical along W-directed subduction zones, whereas andesites are abundant along the opposite E- or NEdirected subduction zones (Andes, Indonesia arc). All these asymmetries have generally been interpreted as related to the older age of the subducting lithosphere along W-directed subduction zones; however, they occur regardless the age and composition of the subducting slab, being more sensitive to the geographic polarity of the subduction (Doglioni et al.,1999; Cruciani et al., 2005; Lenci and Doglioni, 2007). A study on the consequences of these two end members on the mantle wedge has not yet been carried out. The westward drift of the lithosphere should affect the nature and geometry of the mantle wedge, such as the tensional or compressional tectonic regime in the upper plate, the dip of the slab, and the composition and thickness of the upper plate, all parameters that seem chiefly dictated by the subduction polarity. 3. Slab–mantle kinematics The subduction hinge is a helpful indicator of the kinematics and nature of subduction zones. The behaviour of the subduction hinge can be studied either relative to the upper plate (Fig. 1), or the lower plate (Fig. 2), or relative to the mantle. It has been shown that subduction zones have rates faster or slower than the convergence rate as a function of whether the subduction hinge migrates away or toward the upper plate (Doglioni et al., 2007). When the subduction hinge moves toward the upper plate a double verging orogen forms, whereas if the subduction moves away from the upper plate, a single verging, low-elevation prism and a backarc basin form. We present here a further simple kinematic analysis of the subduction system C. Doglioni et al. / Lithos 113 (2009) 179–189 Fig. 2. Kinematics of subduction zones assuming fixed the lower plate L. The upper plate U converges at Vu = 80 mm/year in both cases. The transient location of the subduction hinge H moves at Vh = 100 mm/year and Vh = 20 mm/year respectively. The resulting subduction S is given by Vs = −Vh and therefore is 100 mm/year in the upper panel, and 20 mm/year in the lower panel. The shortening in the orogen (lower panel), is Vu − Vh. Values are only as an example. S increases when H diverges relative to the upper plate (above), whereas S decreases if H converges relative to the lower plate (below). The upper example is accompanied by backarc spreading and mantle upwelling, a low prism, and it is typical of W-directed subduction zones. The lower example is rather characterized by lowering of the mantle, double verging and elevated orogen. It forms frequently along E- to NNE-directed subduction zones. The velocity of the hinge equals the velocity of the subduction in both cases. In the upper case the subduction is independent of the upper plate velocity, whereas in the lower case it is a function of it. These opposite kinematic settings indicate different dynamic origin of the subduction, i.e., slab/mantle interaction for the upper section, and upper/lower plates interaction for the lower case. The two cases support two end members of the mantle wedge, very thick for the upper case and very thin for the lower case. assuming fixed the lower plate L (Fig. 2). Assuming the upper plate U converging at a fixed rate (Vu = 80 mm/year), the subduction hinge H is moving faster (Vh = −100 mm/year) or slower (Vh = −20 mm/year) than the upper plate. In both cases the subduction hinge converges or rolls back toward the lower plate. In this reference system, the subduction rate Vs is equal to −Vh, i.e., the velocity of the subduction hinge H relative to L. In the first case, the subduction rate is not controlled by the convergence of the upper plate. In the second case, the subduction rate is instead function of the convergence rate, decreased by the amount of shortening of the upper plate, which is controlled by the viscosity of the lithosphere (Doglioni et al., 2006). The two cases show that 1) subduction zones have two different origins, 2) the mantle in the hanging wall is either uplifted or pushed down as a function of the prevailing mechanism, 3) the subduction rate can be either faster or slower than the convergence rate, 4) the different behaviour and origin of the two end members suggest the passive role of subduction zones in plate tectonics, 5) where the subduction hinge migrates faster than the upper plate toward the lower plate, the subduction is controlled only by the slab–mantle flow relationship (W-directed subduction zones); in case the hinge migrates slower than the upper plate toward the lower plate (E- or NE-directed subduction zones), the subduction is controlled by the upper–lower plates convergence rate, plus density, thickness and viscosity of the upper and lower plates. Another relevant kinematic observation relates to the motion of the slabs in the mantle reference frame. In this reference, the subduction hinge should be fixed relative to the mantle along W-directed subduction zones, whereas it should be W- or SW-ward retreating along E- or NE-directed subduction zones. However, this reference frame remains problematic because some authors consider the opening of 181 the backarc basin as a reliable indicator of the subduction hinge speed (e.g., Lallemand et al., 2005), while other separate the speed of the backarc spreading from the subduction hinge velocity as a function of the accretion in the prism or the asthenospheric intrusion at the subduction hinge (Doglioni, 2008). However, assuming the hotspot reference frame as valid (Gripp and Gordon, 2002), regardless if the volcanism is sourced by the deep or shallow mantle (Cuffaro and Doglioni, 2007), the E- or NE-directed subduction zones are remounting in the mantle, i.e., moving toward the direction opposite to that of the subduction. In this case the subduction occurs because the upper plate overrides faster the lower plate (Doglioni et al., 2007). The fastest global plate velocity is along the Tonga subduction zone (Bevis et al., 1995), where the subduction hinge converges relative to the lower plate at 240 mm/year as a minimum. This high speed implies that 1) the subduction can reach the base of the upper mantle in about 2.5 Ma; 2) the fast slab retreat implies a similar velocity in the mantle compensating the volume left by the slab. This is consistent with the shear-wave splitting analysis of Conder and Wiens (2007) who propose a WNW–ESE mantle flow in the backarc, shifting to a direction about NNE–SSW parallel to the Tonga subduction direction when approaching the slab, and having a super fast rate of about 500 mm/year. According to the aforementioned kinematics, W-directed subduction zones have the fastest descending slabs, providing i) about three times larger volumes of lithospheric recycling into the mantle than the opposite subduction zones and ii) the larger amount of fluids released into the mantle wedge. Moreover, along W-directed subduction zones, the accretionary prisms are smaller because the basal decollement plane is travelling at shallow level on top the lower plate (Lenci and Doglioni, 2007) and the upper plate in mostly in extensional regime. Along the opposite E- or NEdirected subduction zones, the basal decollement layers are deeper and affecting the entire lithosphere. The accretionary prism is rather composed by shortening of the upper plate, whereas the lower plate is more widely affected during the collision stage. These orogenic systems provide minor loss of material due to subduction, but more accretion due to the more deeply rooted thrust plates. Moreover they exhibit large plutonic emplacement. All this generates a larger flux of material for the growth of the continental crust and related lithospheric mantle. In summary, the analysis of the subduction hinge both in the upper and in the lower plate reference frame confirms the existence of two end members of subduction style and related orogen, regardless if the mantle reference frame is accepted or not. 4. Mantle wedge geometry The source of arc magmatism is mainly within the mantle wedge, triggered by fluids released by the downgoing slab. The estimated depth of the top of the slab beneath volcanic arcs ranges between 72 and 173 km, being the mean depth projection at around 105 km (Syracuse and Abers, 2006). We have seen that the steeper the slab and thinner the upper plate, the thicker is the resulting mantle wedge. The shallower the slab and the thicker the upper plate, the thinner is the mantle wedge. These two end-members are in general associated with subduction zones in which the slab hinge migrates away from the upper plate, or toward the upper plate, respectively. In the lower plate reference frame, the classification applies to cases where the slab hinge migrates toward the lower plate faster or slower than the upper plate. The two end members typically apply to W-directed and the E- or NE-directed subduction zones. As noted in the previous section, the W-directed subduction zones are generally faster because they have the subduction hinge generally moving away with respect to the upper plate, and converging relative to the lower plate faster than the upper plate. Therefore these subduction zones should supply much larger volumes to mantle recycling than the opposite subduction zones (Fig. 3). The total budget of fluids released by the slab should then be also greater. Moreover, the mantle wedge of 182 C. Doglioni et al. / Lithos 113 (2009) 179–189 Fig. 3. The main differences between orogens are a function of the subduction polarity along the tectonic mainstream reported in the map above, which appears polarized due to the relative “eastward” mantle counterflow. The volumes recycled along W-directed subduction zones are about 2–3 times higher than along the opposite settings due to the aforementioned kinematic constraints. Moreover, the asthenospheric wedge above slabs is much thicker along W-directed subduction zones (AW) with respect to the E–NE directed subductions, if any (AE), modified after Doglioni et al. (2007). W-directed subduction should be hotter because it involves a thicker section of asthenosphere that is generally assumed N1300 °C. This can explain why it has lower P and S seismic velocity and very low Q factor with respect to the E- or NNE-directed subduction zones (Fig. 4). W-directed subduction zones show peculiar gravimetric signatures in which there is a much larger positive anomaly in the upper plate with respect to the opposite E- or NE-directed subduction settings. Free-air gravity anomaly is in average about 75 mgal in the hanging wall of W-directed subduction zones, whereas it is about 50 mgal along the E- or NE-directed subduction zones (Harabaglia and Doglioni, 1998). This can be explained by the shallower presence of the asthenosphere in the hanging wall of W-directed slabs, and is also supported by the higher heat flow. For example, the heat flow (Hurtig et al., 1992) in the Tyrrhenian Sea is much higher (N200–250 mW) than in the Aegean rift (b100–120 mW), or in the western Pacific backarc basins than in the Andean Cordillera (Pollack and Chapman, 1977). Along the Chilean Andes, the top of the slab beneath the volcanic arc is between 90 and 110 km (England et al., 2004; Syracuse and Abers, 2006). However, the thickness of the continental lithosphere along the same belt is estimated between 100 and 125 km (Artemieva and Mooney, 2001; Gung et al., 2003). These two values prevent the existence of an appreciable thickness of the mantle wedge of asthenospheric origin along this section of the Cordillera. Therefore the so-called mantle wedge will be primarily composed by the upper plate continental lithospheric mantle, possibly with an average temperature lower than 1300 °C (Fig. 5). There are few transitional cases between the two end members, i.e., those in which a rifting occurs in the hanging wall of E- or NNE-directed subduction zones (e.g., the Aegean, Andaman, and Basin and Range). The first two are syn-subduction rifts, whereas the third is post-subduction. However there are profound differences between backarc spreading associated with W-directed or to E–NE-directed subduction zones (e.g., Doglioni, 1995). The first type occurs when the retreat of the lithosphere leaves an empty volume, which is replaced by the asthenosphere. The upper plate extension along the opposite E- or NE-directed subduction zones has another origin and cannot be compared to the W-directed settings. The Aegean and Andaman rifts have for example a number of differences from the classic backarc setting. The rift is concentrated only in limited areas in the hanging wall of the subduction, whereas along the W-directed subduction zones the backarc rift is ubiquitous in the upper plate. The rift (e.g., Aegean, Innocenti et al., 2005) forms where the hanging wall lithosphere is split into two independently advancing plates, like the Andaman rift located at the transition between the slower Himalayas convergence (34–38 mm/year) and the faster Indonesian subduction zone arc (around 60–64 mm/year). Moreover, this type of rift has a slower rate of spreading with respect to the opposite setting. The W-directed subduction rift-related backarc is pervasively distributed throughout the whole upper plate, and is fast (usually N0.6 km/Ma subsidence rate, N10 km/Ma spreading rate), arriving at oceanization in few million years. It may develop with or without convergence between the upper and lower plates, and it can simply be explained as being related to slab retreat relative to the upper plate. The retreat occurs regardless its origin, i.e., the slab pull or the eastward mantle flow implicit in the westward drift of the lithosphere. Summing up the differences in polarity of the subduction with respect to the undulate tectonic mainstream of plate motion (Doglioni Fig. 4. Comparison between the Vp tomography of the Tonga and Andean subduction zones. Note the much slower velocities in the mantle wedge of the Tonga subduction (Conder and Wiens, 2006) with respect to the opposite Andean setting (Heit, 2005). C. Doglioni et al. / Lithos 113 (2009) 179–189 183 Fig. 5. The four cases represent the possible settings of W-directed and E- or NE-directed subduction zones as a function of the composition of the lower plate (oceanic or continental). The magmatism volume should be controlled by the slab dehydration, the asthenospheric wedge thickness and the subduction rate. The asthenospheric wedge thickness increases with the slab dip and decreases with the upper plate thickness. The thickest mantle wedge of asthenosphere is along the W-oceanic case, where the slab is steeper and the upper plate is young oceanic lithosphere. A slightly thinner asthenospheric wedge occurs in W-continental, where it is expected a shallower melting of the lower plate. The steep slab along the W-cases is controlled by the negative buoyancy (if any) and the advancing mantle flow. Along the E-oceanic, widespread volcanism forms; the upper plate along the E-oceanic can also be oceanic (usually older than the lower plate). The thinnest asthenosphere should occur along the E- or NE-continental example, where in fact there is the lowest amount of volcanism. The shallow slab dip of the E-cases is controlled by low negative buoyancy (if any), and the sustaining mantle flow. AOC, altered oceanic crust. et al., 1999; Crespi et al., 2007) and the “westward” drift of the lithosphere relative to the mantle (Scoppola et al., 2006), the mantle wedge varies as a function of the polarity of the subduction, plus the thickness and composition of the upper and lower plates (Fig. 5). The upward flow of the mantle in the hanging wall of a subduction zone is controlled by the amount of rifting in the upper plate. However, the backarc width and its spreading rate are determined by few concomitant parameters, i.e., the migration rate of the subduction hinge away from the upper plate, minus the amount of accretionary prism growth and/or the mantle intrusion at the subduction hinge (Doglioni, 2008). Therefore, backarc spreading forms in two settings: 1) along W-directed subduction zones where the basin opens as the asthenosphere replaces the retreated lithosphere, and 2) along the E–NEdirected subduction in which the upper lithosphere is split into two subplates that have a differential advancement velocity relative to the lower plate. Along W-directed subduction zones, the hinge diverges relative to the upper plate, and the backarc spreading is given by the rate of hinge retreat, minus the volume of the accretionary prism, or, in case of scarce or no accretion, minus the volume of the asthenospheric intrusion at the subduction hinge. Since the volume of the accretionary prism is proportional to the depth of the decollement plane, the backarc rifting is inversely proportional to the depth of the decollement (Doglioni et al., 2007). Fig. 5 shows four simplified cartoons of the possible settings of W-directed and E- or NE-directed subduction zones as a function of the composition of the lower plate. The volume of magmatism should be controlled by slab dehydration, asthenospheric wedge thickness and subduction rate. The asthenospheric wedge thickness decreases with the shallowing of slab dip and with the increase of the upper plate thickness. The thickest wedge of asthenosphere is along the W-directed subduction zones where the slab tends to be very steep, and the lower and upper plates are oceanic. The steep slab along the W-cases is controlled by the negative buoyancy (if any), and the advancing mantle flow. Along the E-directed subduction zones, where the lower plate is oceanic, and the convergence rate is high (N3 cm/ year?) widespread volcanism occurs. The thinnest asthenosphere should be present along the E- or NE-continental example, where in fact there is the lowest amount of volcanism (e.g., Himalayas). The shallow slab dip of the E-cases is controlled by low negative buoyancy (if any), and the E–NE-ward relative mantle flow, which should rather sustain the slab (Fig. 5). 184 C. Doglioni et al. / Lithos 113 (2009) 179–189 5. Chemical fluxing in subduction zones: boron isotope evidence The differences between W-ward and E–NE-ward directed subduction zones is likely to be apparent in the geochemistry of the associated magmas. Plank and Langmuir (1988) identified two end members of mantle wedge, thick and thin. They suggested the two end members represent a short and a long melting column, respectively, plus a number of differing geochemical signatures such as the Na6 content, increasing with the short column. Metallogenesis also appears to be controlled by subduction style (Mitchell and Garson, 1981). Porphyry copper deposits are concentrated in collisional settings and Chilean type subduction zones. Mariana type subduction is instead characterized by Kuroko or similar volcanogenic sulphide deposits (Nishiwaki and Uyeda, 1983). Understanding the relations between slab outputs (e.g., aqueous fluid, silicate melt, supercritic fluid; see Abers, 2005; Kessel et al., 2005; Hermann et al., 2006), their ascending into the mantle wedge and the physical controls of the “subduction factory” are complicated by the large number of parameters involved (see above). For example, the addition of slab-derived fluids and possibly melts to the mantle wedge explains many geochemical characteristics of arc magmas. However, the geochemical variability of subduction related magmas (regionally, and sometimes within single volcanoes) suggests a complex interplay between lithologic heterogeneities of subducted materials, metamorphic reactions, and element mobility during devolatilization and concomitant mass transfer. Hydrothermal fluids, derived from metamorphic dehydration reactions may account for the mobilization of FME (Fluid-Mobile Elements) into arc magma source regions (Noll et al., 1996). In cross-arc transects, the decline of FME/ fluid immobile element ratios from the volcanic arc towards the backarc regions, was interpreted as due to the increase of the slab depth and thus to the increment of the metamorphic grade in subduction associations (Ryan and Langmuir, 1993; Ryan et al., 1996). On the other hand, based on a compilation of published data and detailed studies of oceanic magmatic and mantle rocks, Scambelluri and Philippot (2001) and Chalot-Prat et al. (2003) have shown that metamorphism up to the eclogite facies is isochemical and that, contrary to what it is usually assumed, the fluids are not extracted during HP-LT metamorphism, even within the eclogite facies, up to 13 kbar. Bebout (2007) suggests that H2O-rich fluids are the dominant fluids phase released by dehydration of sediments and basalts in forearc regions, whereas supercritical fluids and hydrous silicate melts appear to be more important at higher P and T beneath the volcanic fronts and across the arcs. Table 1 Boron isotopic variations in the major geochemical reservoirs. Material δ11B‰ Reference Range − 10.5 −7 − 7.6 −15 − 14 −6 −3 −7.6 −13 −1 −1 −1.8 −1 −6 0 −1 −2 −7 Altered oceanic crust −4 17 Pelagic sediments Metasediments Forearc serpentinites Serpentinites −15 −10 5 5 5 −5 21 15 MORB OIB Continental crust Roy-Barman et al., 1998 Chaussidon and Marty, 1995 LeRoux et al., 2005 Chaussidon and Marty, 1995 Gurenko and Chaussidon, 1997 Tanaka and Nakamura, 2005 Tonarini et al., 2005 Turner et al., 2007 Chaussidon and Albarède, 1992 Kasemann et al., 2000 Spivack and Edmont, 1987 Smith et al., 1995 Ishikawa and Nakamura, 1993 You et al., 1995 Benton et al., 2001 Bonatti et al., 1984 Boschi et al., 2007 Fig. 6. Schematic representation showing the 143Nd/144Nd and δ11B evolution of hydrous fluids; isotope variations associated to crustal contamination are also shown. AOC = altered oceanic crust. Boron and B isotopes are, perhaps, the most successful geochemical tracers of hydrous fluids migrating from slab into the overlying mantle wedge. Boron is released from downgoing slabs earlier and more efficiently than other elements (e.g., Rb, K, Ba) and its abundance suggests a close relationship to H2O-rich fluids. It is high soluble in aqueous fluids, thus it is easily decoupled from less soluble elements (REE, Rare Earth Elements, and HFSE, High Field Strength Elements) via separation of fluid phases (Leeman and Sisson, 1996 and reference therein). Boron isotopes exhibit a large range in isotopic compositions (δ11B, permil variation with respect to the 11B/10B ratio of NIST-SRM 951 standard, varies between −30 to +50‰, Palmer and Swihart, 1996). Table 1 summarizes B isotope characteristics of major geochemical reservoirs at subduction zones. The δ11B shows large variability in the mantle products (with MORB, mid-ocean ridge basalts in the range −1 to −10.5‰ and OIB, ocean island basalts, between −1 and −15‰; the continental crust has an average δ11B of about −10.5‰, Chaussidon and Albarède, 1992; Kasemann et al., 2000). Altered oceanic crust (AOC) is enriched in seawater derived B and its isotopic composition is between +17 and −4‰ (Smith et al., 1995). In marine sediments B is present in two components: a desorbable one, (δ11B = +15‰, Spivack and Edmont, 1987) largely lost at relatively low temperature (You et al., 1995); and a structurally bonded one (from + 2 to − 15‰). Another important boron reservoir is represented by serpentinized peridotites of the oceanic lithosphere (δ11B ≈ + 5/+ 15‰, Boschi et al., 2007), where B is hosted by serpentine. Serpentinites, generated in the forearc position and sampled in the Mariana forearc, are characterized by an average of B and δ11B values of 27 ppm and + 14‰, respectively (Benton et al., 2001). δ11B exhibits large cross-arc and inter-arc variations in volcanic arcs, like the other fluid mobile elements. Redistribution of boron and fractionation of its isotope species is temperature dependent, hence shallow dehydration of the slab yields fluids with isotopically heavy boron, since 11B is preferentially partitioned in the fluid phase, leaving the residual slab progressively more 11B-depleted (e.g., Benton et al., 2001). Nevertheless, studies on trace element mobility stress the impact on the boron budget of some metamorphic minerals such as phengite in the subducting slabs (Marschall et al., 2007). B is very efficiently absorbed by phengite-free assemblage, whereas its concentration in fluids released from phengite-bearing rocks is significantly lower. Moreover, the δ11B of fluids released during the first steps of dehydration are positive, while fluids generated at greater depth, as slab dehydration proceeds, rapidly reach negative values (Marschall et al., 2007). The B isotope composition appears to be relatively insensitive to crustal contamination processes during magma ascent. Studies on C. Doglioni et al. / Lithos 113 (2009) 179–189 Fig. 7. Worldwide distribution of 143Nd/144Nd vs. δ11B in volcanic lavas from the main subduction zones. W-directed subduction zones have more positive δ11B and higher 143 Nd/144Nd, apart the Apennines subduction zone, which is strongly contaminated by crustal rocks. Data sources are reported in Table 2. continental arc-volcanism (in the Central Andes, Western Anatolia and Phlegrean Volcanic District, Rosner et al., 2003; Tonarini et al., 2004, 2005) where crustal contamination is well documented, indicate a small variation in Sr and Nd isotope compositions, suggesting a relatively constant degree of contamination. By contrast, δ11B shows a wide range of values that is plausibly linked to slab components, suggesting that B can be used as tracer for subduction processes not only in oceanic settings but also in continental environments. Also, interaction of calc-alkaline magmas with lower continental crust is unlikely to modify their δ11B because boron is 185 systematically mobilized during prograde metamorphism, thus generally the lower crust appears to be significantly depleted in B (Leeman et al., 1992). Summing up, geochemical behaviour of B is related to fluid phases so that it is decoupled with respect to elements barely mobile in the fluid but sensitive to crustal contamination like Nd (Fig. 6). Generally speaking, we may expect that in warm subducting slabs such as in the E-directed slabs, the higher heat produces shallow release of aqueous fluids rich in 11B and high 143Nd/144Nd, whereas at greater depth (beneath the volcanic arcs), it is likely that hydrous silicate melts and/ or supercritical liquids (Kessel et al., 2005) with δ11B negative and low 143 Nd/144Nd ratio, are released (Fig. 6). On the other hand, in colder subducting slabs, the lower heat allows deeper slab dehydration and probably large volatile flux below the arcs (Fig. 6), although the lower temperature favours larger shear heating. In these cases, the δ11B for arc magma suites are consistent with progressive loss of B and specifically 11B (low δ11B) whereas Nd again maintains almost the same isotopic compositions of its source (altered oceanic crust and lithosphere). One caveat is that the forearc regions (metasomatized by shallow aqueous fluids strongly enriched in 11B) may be involved in magma genesis. Such forearc materials, dragged downward by convective mantle flow may release large volume of water (as well as dissolved FME) to the mantle beneath the volcanic arcs (90–130 km depths; Hattori and Guillot, 2003) giving to the arc magmas a heavy boron signature (Straub and Layne, 2002; Tonarini et al., 2007). Leeman et al. (2002) suggest that boron enrichment and isotope signature in arc magmas generally show strong correlations with physical parameters such as subduction zone geometry, convergence rate, slab dip, slab thermal state and lithosphere thickness, but they are poorly correlated with the estimates of sediment flux given by Rea and Ruff (1996). B and Nd isotope data from arc lavas are plotted in Fig. 7; the mean and the extreme isotope values in W- and E-directed subduction zones are reported in Table 2. Most of arc lavas from Pacific and Atlantic W-directed subduction zone are characterized by high Nd Table 2 Mean and extreme B and Nd isotope compositions in arc lavas. 143 Nd/144Nd Rock types δ11B‰ Mean SD n Max value Min value Mean SD n 0.51308 0.51306 0.51306 0.51299 0.51291 0.00003 0.00005 0.00004 0.00002 0.00010 6 27 9 14 14 0.51312 0.51315 0.51312 0.51301 0.51305 0.51305 0.51299 0.51299 0.51296 0.51269 1.5 3.1 4.9 5.0 0.6 2.7 2.6 2.3 0.7 2.1 12 25 9 19 8 5.6 5.9 7.3 6.2 3.5 −3.7 − 3.8 1.2 4 − 2.3 1 2 3 4 5 0.51285 0.51273 0.51225 0.51243 0.51245 0.51205 − 2.4 − 7.6 −6.3 2.4 1.6 1.7 22 23 4 2.3 − 3.6 − 4.3 − 6.1 − 10.6 −6 6 7 8 15 8 0.51304 0.51302 0.51297 0.51667 15.1 1.0 1.6 3.0 15 7 17.6 5.8 12 −3.2 9 10 0.00005 0.00002 0.00003 9 17 17 0.51300 0.51301 0.51251 0.51286 0.51295 0.51225 − 6.3 3.5 −0.4 1.7 2.6 2.1 10 17 17 − 0.4 6.29 4.1 −9.8 − 2.7 − 7.2 11 12 13 0.00004 10 0.51249 0.51237 − 8.5 5.0 10 − 0.1 − 14.6 14 Western-directed Pacific arcs Kamchatka Kurili Izu Marianna Halmahera C-A C-A C-A C-A C-A Mediterranean arcs Aeolian Phlegrean Volcanic District North Apennines C-A basalts to dacites C-A ktrachybasalts to latites C-A rhyolites 0.51267 0.51254 0.51211 0.00011 0.00001 0.00010 22 22 4 Atlantic arcs SSI Martiniquea LK tholeiites to dacites C-A basalts to dacites 0.51302 0.51287 0.00004 0.00012 Eastern-directed Pacific arcs Cascades El Salvador, Central America Andes LK tholeiites-HKCA basalts C-A basalts to dacites C-A basaltic andesites to dacites 0.51291 0.51299 0.51242 Mediterranean arcs Western Anatolia C-A bas-andesites to latites 0.51244 basalts basalts basalts basalts basalts to dacites to dacites to rhyolites; HMg andesites to dacites to dacites Max value Min value Reference 1: Ishikawa et al., 2001 and reference therein; 2: Ishikawa and Tera, 1997 and reference therein; 3: Ishikawa and Nakamura, 1994 and reference therein; 4: Ishikawa and Tera, 1999 and reference therein; 5: Palmer, 1991; 6: Tonarini et al., 2001; 7: D'Antonio et al., 2007, 8: Tonarini et al., 2003 and reference therein; 9: Tonarini and Leeman, unpublished data; 10: Smith et al., 1997 and reference therein; 11: Leeman et al., 2004; 12: Tonarini et al., 2007; 13: Rosner et al., 2003; 14: Tonarini et al., 2005. SD = standard deviation; n = number of samples. a Exclused strongly crustal contaminated samples. 186 C. Doglioni et al. / Lithos 113 (2009) 179–189 isotope compositions whereas the δ11B values show a wide range from +18‰ (SSI, Tonarini et al., 2006) to − 2.3 (Halmahera arc, Palmer, 1991). Other negative values are found in back-arc lavas from Kurili and Kamtchacka (Ishikawa and Tera, 1997, 1999; Ishikawa et al., 2001). However, the negative δ11B values are few and the average δ11B is positive (see Table 2). E-directed Pacific subduction zones are characterized by products with a wide range in both Nd and B isotope compositions. In particular, the higher δ11B values are found in the El Salvador (maximum value 6.3‰, Tonarini et al., 2007), whereas negative values are common in the Cascades (minimum values −9.8‰, Leeman et al., 2004); values lower than −5‰ were also found in Central Andes and Southern Andes (Rosner et al., 2003; Tonarini et al., 2006). The W-directed Mediterranean arcs show Nd and B isotope ratios significantly lower with respect to their Pacific counterparts, suggesting involvement of sediments and crustal materials (Fig. 7). The two isotope ratios show large variations and are positively correlated. The western Anatolia is the only NE-directed subduction zone of the Mediterranean area for which δ11B values are published; δ11B values are as low as −15‰ whereas the Nd isotope data is almost constant. The W-directed subduction zone of the southern and northern Apennines is characterized by negative δ11B (Table 2); Nd isotope ratios clearly support the presence of crustal components as expected for the subduction of the Adriatic continental lithosphere and/or acquired through crustal contamination during magma upraise. Comparing the δ11B against 87Sr/86Sr (not shown), the scenario is similar to that illustrated in Fig. 7, however some differences can be pointed out. Sr is a relatively mobile element in the aqueous fluid, thus a weak positive correlation between δ11B and 87Sr/86Sr is sometimes observed in arc lavas (e.g. El Salvador, Tonarini et al., 2007). When crustal materials are involved in arc magma genesis, a negative correlation between δ11B and Sr isotope ratios is observed, as for example in W-directed Mediterranean arcs (D'Antonio et al., 2007). 6. Discussion Independent of the westward drift, there are at least two basic reasons why the E- or NE-directed subduction zones (e.g., central America, Andes, Alps, Dinarides, Zagros, Himalayas, Indonesia) have a thinner mantle wedge: 1) they commonly have a thick continental upper plate (e.g., Panza, 1980; Panza et al., 1982; Artemieva and Mooney, 2001; Panza et al., 2003; Manea et al., 2004), and 2) the slab is on average less inclined (e.g., Cruciani et al., 2005). In the Andes the slab depth beneath the volcanic arc can be even thinner than the thickness of the upper plate continental lithosphere. This suggests either a very thin asthenosphere between the slab and the upper plate, or even the absence of the asthenosphere, being the mantle wedge composed only by lithospheric mantle. Moreover, from the kinematics of subduction zones and the behaviour of the slab hinge, the E- or NE-directed slabs have slower sinking velocity than the opposite W-directed subduction zones. Tatsumi and Eggins (1995) have shown a correlation between convergence rate and volumes of magmatism along subduction zones. The larger volumes of subduction predicted along W-directed slabs should favour the formation of a greater amount of arc-related magma, and generation of large volumes of oceanic crust in the backarc setting for a number of reasons: 1) the larger subduction rate should generate also more abundant slab dehydration, lowering the melting temperature; 2) the thicker and hotter (asthenospheric) mantle wedge should have a thicker column of potential melting; 3) faster slab entering means also larger shear heating. Subduction zones trigger a corner flow in the host mantle (e.g., Turcotte and Schubert, 2002). However, the aforementioned asymmetric kinematics of subduction zones would predict that this mechanism might be valid only for W-directed subduction, whereas along the opposite settings, the mantle should be flowing upward, or sucked by the “westward” motion of the lithosphere relative to the mantle (Fig. 8). In this reconstruction, at a global scale the mantle would be flowing eastward relatively to the lithosphere, generating a first order flow. Subduction and rift zones are then a second order turbulence disturbing the main flow. Along W-directed subduction zones, slab retreat is compensated by the asthenosphere in the backarc, but it determines a downgoing corner flow in the host mantle. Conversely, along E- or NE-directed subduction zones, in general there is no void to fill in the backarc setting. However, the slab is “remounting” relative to the mantle, and a suction flow from below is expected (Fig. 8). In fact, relative to the mantle, the lower plate is moving westward out of the mantle, in the direction opposed to the dip of the slab. The subduction occurs because the upper plate is moving westward faster than the lower plate. It is noteworthy that a number of subduction zones are characterized by alkaline magmatism in the foreland of the retrobelt or within the orogen itself (e.g., Mineralnie Vodi in the northern Caucasus; Euganei Hills, in the Southern Alps; Patagonia; Aegean Sea, etc.). This magmatism may be related to an upward suction of the mantle due to the opposite slab motion, as illustrated in Fig. 8. In this model, the high velocity body recorded by tomography along E-directed subduction zones (e.g., see the Andean “slab” of Fig. 4) could be interpreted not as subducted lithosphere (in Fig. 8. The subduction zones disturb or deviate the general “eastward” flow of the mantle relative to the lithosphere. W-directed slabs produce a corner flow, whereas the opposite slab should rather generate an upward suction flow from the underlying mantle. Such suction could trigger “fertile” mantle from below, and its decompression may locally generate OIB-type magmatism (e.g. Patagonia back-arc basalts, Bruni et al., 2008). The fluids released by both W- and E-directed slabs (e.g., the white lenses in the hanging wall of the subduction) decrease the viscosity at the top of the asthenosphere, speeding up the upper plate. The fluids are sheared by the lithospheric decoupling and determine a migration away from the lower plate along W-directed subduction zones, facilitating the backarc spreading. It triggers an opposite behaviour along the E- or NE-directed subduction zones where the westward increase of the upper plate velocity rather favours the convergence between the upper and lower plates, i.e., determining a double verging Andean-Alpine type orogen. BABB: back-arc basin basalts; IAT: island-arc tholeiites; CA, SHO: calc-alkaline and shoshonitic series; OIB-type: basalts with ocean island or intraplate affinity. H, subduction hinge; L, lower plate; U, upper plate. The arrows of L and H refer to fixed U, not to the mantle. C. Doglioni et al. / Lithos 113 (2009) 179–189 fact it is mostly aseismic), but as deeper mantle upraised by the suction mechanism. The deeper, more viscous and rigid mantle has a more compacted crystallographic structure and higher seismic velocities. The fluids released from the slab into the overlying mantle should trigger a decrease of viscosity in the hanging wall of the subduction, at the bottom of the upper plate. The asthenosphere top is the main decollement surface of the lithosphere, and the decrease of the viscosity can accelerate the relative decoupling. Therefore the upper plate increases its velocity moving away from the lower plate along the W-directed subduction zones. This facilitates the formation of the backarc spreading (Fig. 8). Along the E- or NE-directed subduction zones, the upper plate is converging faster with respect to the lower plate, facilitating the generation of double verging orogens such as the Andes or Himalayas. 7. Conclusions The kinematics and the resulting geometries along subduction zones would indicate that: 1) the mantle wedge is different as a function of the subduction polarity, being thicker and mostly asthenospheric in the hanging wall of W-directed slabs; conversely, it is thinner and mostly composed by lithospheric mantle along the E- or NE-directed subduction zones; 2) the subduction rate is dependent on the slab–mantle interaction along W-directed subduction zones, independent of the movement of the upper plate; conversely, the subduction rate is controlled by the far field velocity of plates and the velocity of the hinge along E- or NE-directed subduction zones; this observation indicates a different origin of the opposite subduction systems as a function of the geographic polarity (Fig. 2). Moreover it supports a passive behaviour of the slab, rather than it being the primary force in driving plate tectonics. This information contains a genetic and geographic asymmetry in the origin of subduction zones, regardless the mantle (hotspot) reference frame. The B and Nd isotopes seem to confirm the asymmetry of subduction zones (Fig. 7). Hotter and thicker asthenosphere in the hanging wall of W-directed subduction zones is generally accompanied by positive and higher δ11B and 143Nd/144Nd, except where there is a significant crustal contribution (e.g., Apennines). We suggest that mantle wedge thickness, composition and temperature are affected by the asymmetries imposed by the westward drift of the lithosphere (Fig. 3) and the consequent differences among subduction zones (Fig. 5). This flow can be interpreted as the first order flow between lithosphere and mantle, whereas in this kinematic interpretation, the subduction and rift zones are secondary turbulences (Fig. 8). While it has been widely accepted a corner flow in mantle around subduction zones, it is here proposed that this mechanism occurs dominantly along W-directed subduction systems, whereas it is the opposite along the E- or NE-directed subduction zones (Fig. 8), where plates move W-ward or SW-ward relative to the mantle, in a direction opposite to that of the subduction (Cuffaro and Doglioni, 2007). If this is true, the motion of the slab should generate an upward flow of the mantle beneath the subduction zone, sucking up the mantle and depressurising it (Fig. 8). This mechanism upraises the mantle from deeper levels, which has faster seismic velocities with respect to the mantle at shallower depths. This could explain the ghost of a slab beneath E–NE-directed subduction zones even without the presence of a real slab, and the absence of continuous seismicity. This could in turn facilitate partial melting and alkaline magmatism in “backarc” settings, with or without extensional tectonics, along the retrobelt of orogens associated with E- or NE-directed subduction zones. Release of fluids from the slab to the asthenosphere speeds up the decoupling at the base of the upper plate. However, this generates opposite tectonic consequences in the relationship between upper and lower plates along subduction zones. In fact, due to the W-ward drift of the lithosphere, it facilitates the widening between the upper and the lower plates along W-directed subduction zones, whereas it 187 promotes the convergence along the opposite E–NE-directed subduction systems (Fig. 8). Acknowledgements The day after this article was accepted, Fabrizio Innocenti passed away. Fabrizio has been a precious mentor and a close friend of us during the years. We are very grateful to him for the endless number of constructive discussions together. 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