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Lithos 113 (2009) 179–189
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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
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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
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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).
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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. We will miss his widespread
culture, his fine sense of humour, his generosity, and his enthusiasm
for life, science and company. Many thanks to Yildirim Dilek for
inviting us to contribute to this volume, and to Andrew Kerr for his
editorial and linguistic help. Thanks also to Françoise Chalot-Prat and
an anonymous referee for the critical reading and criticisms.
Discussions with Samuele Agostini, Enrico Bonatti, Eugenio Carminati,
Marco Cuffaro, Giuliano Panza and Federica Riguzzi were very fruitful.
This research was supported by the Sapienza University.
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