Mantle wedge anisotropy in Southern Tyrrhenian Subduction Zone (Italy), from receiver function analysis

Tectonophysics 462 (2008) 35–48

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Mantle wedge anisotropy in Southern Tyrrhenian Subduction Zone (Italy), from receiver function analysis Nicola Piana Agostinetti ⁎, Jeffrey Park, Francesco Pio Lucente Istituto Nazionale di Geofisica e Vulcanologia, Arezzo, Italy Department of Geology and Geophysics, Yale University, United States Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

a r t i c l e

i n f o

Article history: Received 31 January 2007 Received in revised form 21 February 2008 Accepted 6 March 2008 Available online 22 August 2008 Keywords: Subduction zone processes Seismic anisotropy Body waves Mantle processes

a b s t r a c t We constrain mantle wedge seismic structure in the Southern Tyrrhenian Subduction Zone (Italy) using teleseismic receiver functions (RF) recorded at station CUC of the Mednet seismographic network. Station CUC lies above the northern portion of the Calabrian slab segment, which is recognized from deep seismicity and tomographic imaging as a narrow, laterally high-arched slab fragment, extending from the surface below Calabria down to the transition zone. To better define the descending slab interface and possible shearcoupled flow in the mantle wedge above the slab, we computed receiver functions from the P-coda of 147 teleseismic events to analyze the back-azimuth dependence of Ps converted phases from interfaces beneath CUC. We stack the RF data-set with back azimuth to compute its harmonic expansion, which relates to the effects of interface dip and anisotropy at layer boundaries. The seismic structure constrained through the RF analysis is characterized in its upper part by a sub-horizontal Moho at about 25 km depth, overlying a thin isotropic layer at top of mantle. For the deeper part, back-azimuth variation suggests two alternative models, each with an anisotropic layer between two dipping interfaces near 70- and 90-km depth, with fast- and slow-symmetry axes, respectively, above the Apennines slab. Although independent evidence suggests a north-south strike for the slab beneath CUC, the trend of the inferred anisotropy is 45° clockwise from north, inconsistent with a simple downdip shear-coupled flow model in the supra-slab mantle wedge. However complexities of flow and induced rock fabric in the Tyrrhenian back arc may arise due to several concurring factors such as the arcuate shape of the Apennines slab, its retreating kinematics, or slab edge effects. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Mantle wedge seismic anisotropy has been widely recognized from seismic observations of shear-wave splitting (Fouch and Fischer, 1998; Peyton et al., 2001; Wiens and Smith, 2003; Nakajima and Hasegawa, 2004; Levin et al., 2004; Long and van der Hilst, 2005) and receiver functions (Levin et al., 2002; Park et al., 2004; Savage et al., 2007). However, the recognition of an uniform pattern of anisotropic structure in the supra-slab mantle wedge is still lacking, with no observational consensus on how traction from the descending slab is transmitted into flow and rock fabric in the wedge. Laboratory and theoretical research on peridotite deformation in a subduction zone (McKenzie, 1979; Jung and Karato, 2001; Holtzman et al., 2003) has exposed varying LPO in response to deformation in the presence of water or partial melt, 3-D geodynamic models have exposed the limitations of physical intuition gained from 2-D cornerflow models (Buttles and Olson, 1998; Hall et al., 2000) and waveform modelling has highlighted the small birefringence signals expected in

⁎ Corresponding author. Istituto Nazionale di Geofisica e Vulcanologia, Arezzo, Italy. E-mail address: [email protected] (N. Piana Agostinetti). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.03.020

the subduction-zone forearc (Levin et al., 2007), where the greatest shear-coupling to the slab might be expected. Kneller et al. (2005) and Abers et al. (2006) hypothesize a partial-slip zone for the slab beneath the forearc, which would reduce shear-coupled flow and further weaken birefringence. We study the seismic properties of the mantle wedge in the southern Tyrrhenian where the Tyrrhenian–Apennine subduction system meets the oceanic lithosphere of the Ionian Sea, i.e. in the Calabrian Arc (Fig. 1). Beneath it, intermediate-depth hypocenters (Selvaggi and Chiarabba, 1995) and tomographic images (Lucente et al., 1999; Piromallo and Morelli, 2003) reveal the presence of a narrow W dipping slab, which is the last remnant of a formerly wider subduction zone, whose late Cenozoic rollback is responsible for the opening of the Tyrrhenian basin and the shaping of the Apennines (Lucente et al., 2006). The Calabrian Benioff zone, paired with the calcalkaline volcanism of the Eolian islands, is a persuasive example of active subduction and can be regarded as the only area within the Tyrrhenian–Apennine system where subduction processes have not yet been impeded by the accretion of continental material (Rosenbaum and Lister, 2004). The Calabrian segment of the Apennines slab is laterally edged by deep tear faults and appears to be high-arched and spoon-shaped at

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Fig. 1. Schematic tectonic map of southern Tyrrhenian subduction zone, modified from Civello and Margheriti (2004). Blue area represents P-velocity anomaly 1.2% higher than the ambient mantle velocity at 100 km depth, from Lucente et al. (1999). Circles indicate well located subcrustal earthquakes recorded in the period 1981–2002 by the Italian Permanent Seismic Network (RSNC) (Castello et al., 2006).

shallow depths, as a response to its rollback motion; such a geometry and kinematics prefigure a complex mantle flow and rock fabric in the supraslab mantle wedge, as pointed out by recent studies (e.g. Kincaid and Griffiths, 2003; Funiciello et al., 2004; Civello and Margheriti, 2004).

Station CUC lies northwest of Calabria (Fig. 1), at the junction between the Calabrian Arc and the NW–SE trending southern segment of the Apennines, where subduction may have stalled due to the introduction of a thick, buoyant sequence of Apulian carbonate platform rocks into the collision zone (Rosenbaum and Lister, 2004).

Fig. 2. Epicentral distribution of teleseismic events used in this study. Grey stars indicate teleseismic events occurred during the recording period. Yellow and red stars indicate events for which we have computed RFs, and events whose RFs have been selected for our analysis, respectively. Blue dashed circles delimit the epicentral distance range of events used in the plots of RF harmonic angular analysis and back-azimuth RF sweeps. Brown dashed lines show sector used for plots of epicentral RF sweeps.

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The location of CUC site, just above the northern edge of the Calabrian slab segment and Benioff zone (Fig. 1) make it a favourable candidate for highlighting the effect of mantle flow around a slab edge, and associated rock fabric, through the study of the anisotropic structure. Tomography suggests a circa north-south strike for the Apennines slab below CUC (Lucente et al., 1999). A previous study of SKS splitting (Margheriti et al., 2003) reports NNW–SSE fast-polarization in this area, which would be consistent either with flow in the sub-slab mantle of an west-dipping, eastward-retreating subduction zone or with a simple supra-slab (west-dipping) wedge corner flow in the presence of abundant volatiles (Jung and Karato, 2001) or partial melt (Holtzman et al., 2003). In this study, we explore the seismic structure under station CUC using the back-azimuth variation of receiver functions (RFs). We apply back-azimuth stacking procedures similar to those introduced by Girardin and Farra (1998), using the receiver function estimator

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described by Di Bona (1998). We investigate anisotropic layering with forward modelling similar to the strategy employed by Park et al. (2004) and Savage et al. (2007), using the synthetic seismogram code described by Frederiksen and Bostock (2000) which incorporates dipping interfaces and anisotropy. In Section 2, we present the RF data-set in both back-azimuth and epicentral distance sweeps, which highlight the first-order Ps converted phases that persist across a range of earthquakes. Qualitative analysis of RF features, such as moveouts and polarity flips with source back azimuth, highlights the probable dip and anisotropy of the crustal and mantle layers. Section 3 describes results of forward modelling using simple models to replicate the major features of the RFs. Here we identify different mantle layers with reference to previous papers on mantle wedge seismic structure, geochemistry, and mantle flow laboratory studies. Preferred models are discussed in Section 4, followed by a summary of the main results.

Fig. 3. Back-azimuth sweep of the radial and transverse components of RFs observed at CUC, using earthquakes from 80°–100° epicentral distance. The RFs are binned in 50% overlapping bins, with 10° back-azimuth width. Blue and red show positive and negative amplitude, respectively. Light green areas indicate standard deviation from stacking procedure. Roman numbers, together with vertical lines and boxes, show phases described in the text body: label “I” indicates the direct-P pulse, “II” is the Ps at the Moho, “III–V” are pulses from mantle interfaces. A horizontal grey dashed line marks the “system symmetry axis” (SSA). Black crosses on the side of both component sweeps mark back-azimuth bins used in harmonics stacking analysis.

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Fig. 4. Epicentral-distance sweep of receiver functions observed at CUC, using RFs from easterly back azimuths. Phases I–V are labeled as in Fig. 3.

Fig. 5. Epicentral-distance sweep of receiver functions observed at CUC, using RFs from westerly back azimuths. Phases I–V are labeled as in Fig. 3.

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2. Data analysis, observations and qualitative interpretation 2.1. Data and methods of analysis We analysed records of teleseismic events, in the 30°–100° epicentral distance range, with magnitude M N 5.5, recorded at seismic station CUC of the Mednet seismic network (Boschi et al., 1991), between August 2003 and December 2005. We selected 337 events with a high signal to noise ratio. Seismograms were rotated in the radial/antitransverse/vertical coordinate system, where R is computed along the great-circle path between station and event epicenter, positively-directed from event to station, T is positive 90° CW from R and Z is vertical, positive up. The convention for transverse makes the coordinate system left-handed. We computed RF using the frequency-domain deconvolution technique developed by Di Bona (1998), applying a Gaussian filter with α = 1, which excludes frequencies above roughly 0.5 Hz. After visual quality assessment we retained 147 RFs, which form our dataset (Fig. 2). RFs are stacked in 50% overlapping bins of back azimuth (10° bins) and epicentral distance (10° bins) to form back-azimuth sweeps of the RF data-set (Park et al., 2004). Fig. 3 shows a backazimuth sweep of the binned RFs for events around 90° epicentral distance, for which we have the best event distribution (see Fig. 2). Figs. 4 and 5 show two epicentral-distance sweeps from easterly and westerly back azimuths, respectively. Furthermore, to highlight the back-azimuth dependence of the pulses in the RFs, we perform a harmonic expansion of our RFs (Fig. 6), by computing sine- and cosine-weighted stacks of radial and tangential components (Girardin

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and Farra, 1998; Farra and Vinnik, 2000). Such harmonic analysis can help to separate azimuthal anisotropy effects from the effects of lateral heterogeneities, such as dipping interfaces. The radial and transverse RFs in this scheme are summed with back-azimuth phase shifts of φ = 90°, in order to capture the joint variation in Ps amplitude and polarity predicted by theory (Levin and Park, 1998). 2.2. Observations on RFs sweeps Overall, the computed RF data-set shows complex Ps-conversion patterns with strong variations both in amplitude and in timing as function of back azimuth (Fig. 3) and epicentral distance (dist) (Figs. 4,5). Such complexities indicate the presence of strong crustal and mantle heterogeneities beneath station CUC (dipping interfaces and/or anisotropy). The combined, complementary analysis performed on the binned sweeps of the RFs and on their angular harmonic expansion (Fig. 6) can be useful to define and constrain the nature and the geometry of these heterogeneities. The analysis of the back-azimuth RF sweeps at 90° epicentral distance (Fig. 3), which possesses the best back-azimuth distribution of events (Fig. 2), allows us to distinguish various pulses, on both radial and transverse RFs, which originate at depths related to their timedelay from the direct-P (t = 0 s). As candidate direct P pulse and Ps conversions from the interfaces beneath CUC, we identify the following phases on the radial RF sweep in Fig. 3: I (0 s delay time), II (2.5–3.0 s delay time), III (5 s), IV (7.5–8.5 s) and V (10.5 s). Phase-I at 0 s delay is predominantly the direct P wave, but its disappearance at

Fig. 6. Harmonic angular analysis. Panels (a) and (b), labeled k = 0, show stacked RF bins using uniform weights (i.e. simple stacking of all the RF coming from the epicentral distance range 80°–100°), for radial and transverse RFs, respectively. Panel (c) reports back-azimuth coverage. Panels (d–g), labeled k = 1, illustrate the harmonic expansions for 2π periodicity. Panels (h–k), labeled k = 2, illustrate the harmonic expansion for π periodicity. In panel (d–k), labels “R” and “T” indicate weighted stacking of radial and transverse receiver functions, respectively, while label “R + T” and “R − T” indicate the summation/subtraction of “R” and “T” weighted stacking. Phases “I–V” are labeled as in Fig. 3.

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0°–60° back azimuth in the radial RFs suggests a surface or nearsurface Ps conversion that varies with back azimuth. Phases-II and -V display positive amplitudes (blue) for the majority of the backazimuth range, indicating an upward propagation from fasterwavespeed lithology into slower-wavespeed lithology. Phase-IV is mainly negative (red), possibly representing a seismic velocity inversion. Phase-III varies from positive to negative, depending on back azimuth, suggesting a small isotropic velocity jump. The majority of these phases display a flip of polarity near 45° back azimuth on the transverse components sweep (Fig. 3), which noticeably corresponds to the maximum amplitude on the radial components for Phases-III, -IV and -V. Opposite to this direction (−135° back azimuth) the polarity of these phases flips as well, but due to the lower amplitude the effect is less evident. Also Phase-I at 0-s delay exhibits a polarity reversal near 45° back azimuth on the transverse, though its radial component amplitude reaches its minimum absolute value here. These observations identify the 45° direction as a main system symmetry axis (SSA). As general rule a cyclic variation of Ps amplitudes and polarities on radial and transverse RFs is a key indicator of anisotropic or dipping-interface effects (Owens and Crosson, 1988; Levin and Park, 1998). In our case the back-azimuth dependence of Ps amplitudes and polarities is largely 2-lobed (360° periodicity), so that the SSA can be related either to the dip direction of isotropic layering, or to the direction of a tilted anisotropy symmetry axis. A horizontal anisotropy symmetry axis would result in a 4-lobed azimuthal pattern (Levin and Park, 1998). The moveout of the Ps phases as a function of the epicentral distance can be indicative of the presence of dipping conversion interfaces (Park et al., 2004). For flat interfaces the Ps delay time is larger for smaller epicentral distances. In the case of dipping interfaces, Fermat's Principle of Least Time causes the Ps conversion point on a dipping interface to shift updip, relative to the conversion point on a horizontal interface. The dip effect increases epicentral Ps moveout on the downdip side and decreases the moveout on the updip side, even to cancel or reverse it, depending on the dip angle (Park et al., 2004). Therefore the trend of Ps moveouts with epicentral distance for opposite back-azimuth sectors can help us to discriminate the influence of dip. Earthquakes are more abundant at easterly back azimuths (Fig. 4) and Ps delay-times show no moveout for Phases-IV and -V on the transverse RFs, and reverse moveout (Ps delay time increasing with dist) on the radial RFs. At westerly back azimuths events are fewer and the Ps converted phases smaller (Fig. 5), nevertheless Phase-IV shows a moveout on the transverse RFs consistent with downdip arrivals. Additionally, for back azimuths −30°–110° (red boxed region in Fig. 3) Phase-IV shows a moveout of about 1.0 s, with smallest Ps delay time around 90° back azimuth, i.e. from the east. Phase-V displays a weaker moveout effect (~ 0.5 s) with the same back-azimuth dependence. Although updip and downdip moveout effects would affect epicentral RF sweeps that are oblique to the strike of the dipping interfaces (see Park et al., 2004 for Cascadia), in this case the moveout for both back-azimuth and epicentral distance RF sweeps (Figs. 3–5) unambiguously suggest the presence of west-dipping interfaces beneath CUC, at least for Phase-IV and Phase-V.

2000). This stacking procedure extracts the harmonic of 2π/k period in azimuth of the signals, therefore highlights back-azimuth cycles. For example, in the case of anisotropy with horizontal symmetry axis (4lobed back-azimuth pattern, see Levin and Park, 1998), the diagnostic value of k is 2 (π periodicity), while the effects of simple dipping structures, such as planar dipping interface or dipping symmetry axis, can be observed for k = 1 (Girardin and Farra, 1998). The k = 0 stack is a simple stack of all RF bins uniformly weighted: for a well-distributedin-azimuth set of RFs the k = 0 of the radial components represents the isotropic velocity structure and the k = 0 stack of the transverse components should be zero. We compute the harmonic expansion for k = 0,1,2. To avoid RFs moveout effects we used only bins of RFs coming from a narrow range of epicentral distance (around 90°, see Fig. 3). The radial k = 0 stack is positive for Phases I, II and V, negative for Phase IV, and zero for Phase III (Fig. 6), in agreement with the general characteristics of the backazimuth RF sweep for these phases (Fig. 3). The k = 0 stack of the transverse RFs is nearly zero (Fig. 6), consistent with theory (the observable small amplitude oscillations are due to the imperfect backazimuth distribution). The k = 1 and k = 2 stacks with back-azimuth angle φ enhance the 2lobed and 4-lobed variations in Ps converted-wave amplitude, respectively. Both radial and transverse RFs can be processed separately, but theory predicts that the two RF signals will add constructively when added with a 90° phase shift in φ, and destructively when subtracted (Savage, 1998; Levin and Park, 1998). The subtraction of transverse from radial RFs (“R − T” in Fig. 6) returns modest amplitudes both for k = 1 and k = 2, consistent with theory. The k = 1 RF stacks display amplitudes by far larger than the k = 2 stacks, consistent either with dipping interfaces or anisotropy with a tilted axis of symmetry (Levin and Park, 1998). The k = 2 stacks would be diagnostic of anisotropy with a horizontal axis of symmetry (Girardin and Farra, 1998; Farra and Vinnik, 2000), though in practice a composite k = 1, k = 2 RF pattern can be generated by a tilted axis of symmetry (Levin and Park, 1998), and this could also explain the nonnull amplitudes observable for the k = 2 stack. The k = 1 harmonic stacks exhibit a remarkable concordance of back-azimuth variation within the RFs among the various Ps converted phases, which all reverse their polarity from NW to SE amplitude maxima (in the 120°– 150° and 300°–330° back-azimuth intervals). This pattern confirms the observation on the back-azimuth sweep of Fig. 3 regarding a NE–SW directed main symmetry axis in the system. If caused by anisotropy, a symmetry axis with approximately 45° trend is indicated. If caused by dipping interfaces, a northeast or southwest dip would be indicated. Ps moveout favors a westward dip.

2.3. Observations on RFs harmonic expansion

2.4.1. Phase-I The amplitude pattern of the Phase-I with back azimuth (Fig. 3) can be reproduced by a dipping interface with a strong velocity contrast, which is too shallow to generate a Ps pulse separated from the direct-P pulse (Owens and Crosson, 1988; Lucente et al., 2005). Such a interface strike is possibly between −90° and −30° back azimuth (i.e. 90° CCW from the direction of minimum amplitude of the phase, see Fig. 3). The interference between the direct-P and the Ps converted phases, observed also on the transverse (Fig. 3), suggests a depth less than half wavelength (roughly 6 km) for this interface. The dip and velocity contrast at the interface cannot be resolved simply by qualitative

Harmonic analysis (Fig. 6) can help to explain some features of the RFs at CUC, allowing us to distinguish between different contributions such as lateral heterogeneity and azimuthal anisotropy (Girardin and Farra, 1998). The harmonic expansion needs to be applied on a welldistributed-in-azimuth set of RFs, and is performed through a backazimuth-weighted stack of both the radial (R) and tangential (T) components of RFs, in the way that R varies with cos(φ) and T varies with sin(φ), where φ is the back azimuth (for details on the weighted stacking procedure see Girardin and Farra, 1998; Farra and Vinnik,

2.4. Qualitative interpretation Here, we discuss some simple models which can explain the firstorder features described above. Almost all the observed Ps pulses can be explained by more than one model, and we try to discriminate as much as possible between plausible models using qualitative considerations. Remarks made here will be used as benchmarks for the following modelling stage.

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observations on the back-azimuth pattern of this phase because of the high trade-off between these parameters. 2.4.2. Phase-II The positive, low amplitude Phase-II at about 3 s can be related to the Ps conversion at a roughly 25-km depth Moho. This assumption is in agreement with Moho-depth estimates from previous receiver function studies (e.g. Steckler et al., 2008). Its low amplitude can be due to a weak velocity contrast and/or to a deterioration caused by crustal multiples. As a matter of fact, due to the short time-delay between phases I and II, we cannot exclude the presence of further intra-crustal discontinuities. The fact that this phase is not associated with a prominent phase on the transverse RFs suggests a subhorizontal Moho geometry with weak or zero anisotropy at the crustmantle transition layers. Accordingly with these observations, in the following modelling stage, we constrain the Moho dip to be b5°. 2.4.3. Phase-III This Phase inspires different hypotheses: (a) a Ps converted phase from a dipping interface at sub-crustal depth; (b) a crustal multiple; (c) a Ps converted phase from an upper mantle horizontal discontinuity separating a lower anisotropic layer with dipping symmetry axis from an upper isotropic one. The hypothesis (a) would require a very steep interface associated with a high velocity contrast (e.g. at least 45° dip for velocity contrast comparable to crust-mantle transition in IASP91 [Kennett and Engdahl, 1991]) to match the flip of the pulse polarity in the observed radial RFs (Fig. 3), and this is unrealistic, so the hypothesis (a) must be discarded. Crustal multiples from a mid-crust interface (hypothesis b) can cause a polarity flip in the radial RF at a lower dip angle (about 25°), but, in our data-set (both Figs. 3 and 6), we do not observe the direct Ps converted phase that would generate such multiples, which should be observable before the arrival of Phase III on the transverse RFs. The downward transition from an isotropic layer to an anisotropic one with a dipping symmetry axis, separated by a horizontal interface (hypothesis c), can generate a complex backazimuth pattern for the Phase-III pulse, which could match that observed in Fig. 3. Furthermore the radial k = 0 stack is zero for Phase III (Fig. 6) consistent more with a transition in rock fabric than from a transition in lithology. Anisotropy with a dipping fast-axis of symmetry is likely to match the relative amplitude on the transverse RFs, as dipping slow-axis anisotropy would produce stronger conversion from western back azimuths compared with eastern, but an inverted velocity contrast (Vs decreasing with depth) could model the polarity inversion in the radial RFs. However, the isotropic image of Phase-III (see panel a in Fig. 6) is almost null, suggesting a weak velocity contrast at the associated interface. Anisotropy symmetryaxis strike is strongly constrained by the polarity flip directions in the RFs to be NE–SW directed (Figs. 3 and 6), but plunge and velocity contrast depend on finer details of the back-azimuth amplitude pattern. Since the isotropic velocity contrast at Phase-III-generatinginterface is almost null, an anisotropy symmetry axis plunge between 30° and 60° is required in order to match the observed amplitudepolarity pattern (Levin and Park, 1998). In the modelling we parameterize the Phase-III-generating-interface as a horizontal interface separating an isotropic shallower mantle layer (beneath the Moho) and the anisotropic mantle underneath. The thickness of the isotropic mantle under the Moho can be inferred from the time-delay between Phases-II and -III. Because the RF pulses are lowpassed and not sharply defined, a conservative thickness range, between 10 and 20 km, is taken. 2.4.4. Phase-IV This phase shows both phase-degradation (i.e. it is clearly visible only in a restricted range of back-azimuth) and phase-moveout effects. A very complex model, combining both anisotropy and dipping discontinuities, would be required to qualitatively match the observed

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RFs pattern. However we can fix some benchmarks for subsequent parameterization by excluding untenable hypotheses. For example, we can exclude that Phase-IV originates from a dipping discontinuity between isotropic layers, because the directions of the Ps delay-time minimum (around 90°, Fig. 3) and of the polarity reversal on the transverse RFs (at 45°, Fig. 3) do not coincide. Therefore the presence of a further anisotropic layer, beneath the first one, is suggested. However, slow- or fast-symmetry-axis anisotropy produce similar RF back-azimuth sweeps, when strikes coincide and plunges differ by 90° (Sherrington et al., 2004), and this ambiguity would be reflected in the model parameterization. Also, the observed moveout between Phases III and IV suggests the presence of a dipping discontinuity between the upper and the lower anisotropic layer. The strike of this interface is constrained by the minimum time-delay of this phase at about 90° back azimuth to be roughly 180°, while determining its dip is more tricky. However, as will be shown in the following forward modelling section, large dip angles generate stronger Ps conversions from the downdip side, i.e. where delay time is maximum, and this would be in contrast with our observations. Thus we evaluate the dip angle to be between 10° and 20°. The isotropic velocity contrast between these two layers must be negative in order to reproduce the strong negative phase seen on panel a of Fig. 6. Finally, we evaluate the thickness of the upper anisotropic layer to be 30–40 km beneath CUC, from the mean delay times of Phases III and IV (3.5 s). 2.4.5. Phase-V This phase, at about 10.5-s, is the latest high amplitude phase we observe. It displays moveout similar to Phase-IV, but opposite polarity on the radial RFs (Fig. 3) and on the harmonic expansions (Fig. 6), and is the latest phase whose transverse RFs flip polarity at 45° back azimuth (SSA). Due to the absence of further noticeable phases on the RFs sweeps we have no ground to define the nature, if anisotropic or not, of the layer beneath the interface that generates Phase-V. We chose to parameterize it as a simple dipping interface, whose strike is constrained to be 180° (i.e. westward dipping) by the minimum delay Phase-V around 90°. Using the same considerations made for PhaseIV, the dip angle should be less than 20°. Again relative time-delay between Phases IV and V can be used to evaluate the layer thickness. Here the two Ps mean arrival times differ by about 2 s, which imply a layer thickness of 15–20 km. 2.4.6. Interpretation summary Before quantifying material properties in the forward-modelling exercises, we summarize our qualitative interpretations, trying to relate the recognized phases to the tectonic elements likely present in the study area. Based on the back-azimuth behavior of the direct P wave (Phase-I), the crust beneath CUC is cut by a shallow dipping discontinuity beneath a thin (b6 km) low-velocity layer. If we interpret Phase-II as arising from the Moho, then the crust-mantle boundary is almost flat (dip is b5°) with a weak velocity jump. The most likely phase to be the Ps conversion at the top of the slab (both for its time delay and its amplitude-polarity pattern) is Phase-V. This association is in agreement with the tomographic data indicating a west-dipping orientation of the slab beneath CUC (see Fig. 1; Lucente et al., 1999; Piromallo and Morelli, 2003). Additionally, if this is the case, then the supra-slab mantle wedge is composed of, at least, two different layers, whose interface generates Phase-III: a sub-Moho thin (10–20 km) isotropic layer and a deeper and thicker (30–40 km) anisotropic layer with positive symmetry axis plunging toward the northeast with mean S-velocity similar to the uppermost isotropic mantle layer. Also if Phase V arises from the top of the slab, Phase IV suggests that there is a further thinner (10–20 km) anisotropic layer atop the slab, characterized by an inversion of the S-velocity, and bounded by two low-angle dipping discontinuities.

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3. RF modelling In this section we examine the predicted receiver functions from simple models of crust and mantle-wedge structure beneath CUC. Such an exercise can yield results with misleading precision, due to the large tradeoffs in model parameters e.g. fast- versus slowsymmetry axes. Hypothesis-testing with forward modelling is quite useful, however, because geologically plausible mantle structure models can be eliminated to narrow the possible choices. Large trade-offs in the seismic interpretation can be constrained by geologic assumptions, but the validity of the latter must be sought in nonseismic constraints. 3.1. Forward models Mednet Station CUC lies near the transition between the northern edge of the Calabrian Benioff zone and the NW–SE trending southern portion of the Apennines (cfr. Fig. 1). We identify five P and Ps phases (I–V) to model in the receiver functions. We relate Phases IV and V with seismic structure near-at the top of the slab; their moveouts as function of both back azimuth and epicentral distance (Figs. 3–5) argue for a north-striking west-dipping geometry of the Calabrian slab segment beneath CUC, in agreement with the tomographic hints. We forward-model our observed RFs set using a simple trial and error approach, analogous to that described by Park et al. (2004). We use a model parameterization derived from the qualities retrieved in the above observations (e.g. layer thickness, dip, anisotropy). Synthetic receiver functions are computed using the anisotropic wave propagation code described by Frederiksen and Bostock (2000), which allows for anisotropic regions bounded by dipping planar interfaces. We assume hexagonal symmetry which approximates the anisotropy of real rocks and implies that wave propagation is governed by an axis of symmetry. Also, the orientation of a symmetry axis can be related to tectonic processes (Park et al., 2004). The complexity implicit in the CUC RFs requires a finely layered model (Table 1 and Fig. 7) to fit the major observed features (Figs. 3–6). We parameterize the mantle as a stack of four layers (three interfaces), which is the minimum number of layers needed to fit Phases III, IV, and V described in the previous section. The crustal structure is refined by progressively adding layers in order to achieve a better fit Table 1 Proposed models Rho

Vs

Vp/Vs

%P

%S

(Model A) C 2000. C 4000. C 4800. C 7000. C 5000. M 7300. M 36,500. ? 18,800. S Half-space

Thick

2000. 2600. 2600. 2600. 2600. 3300. 3300. 3000. 3000.

1500. 2000. 2500. 3500. 3900. 4250. 4200. 3500. 4000.

1.75 1.70 1.85 1.75 1.87 1.78 1.76 1.81 1.87

0.0 0.0 0.0 0.0 0.0 0.0 10.0 −10.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 10.0 −10.0 0.0

(Model B) C 2000. C 4000. C 4800. C 7000. C 5000. M 7900. M 37,000. ? 23,200. S Half-space

2000. 2600. 2600. 2600. 2600. 3300. 3300. 3300. 3000.

1500. 2000. 2500. 3500. 3900. 4350. 4200. 4000. 4100.

1.75 1.70 1.85 1.75 1.87 1.80 1.76 1.80 1.87

0.0 0.0 0.0 0.0 0.0 0.0 10.0 10.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 10.0 10.0 0.0

Trend

Plunge

Strike

Dip

0. 0. 0. 0. 0. 0. 45. 50. 0.

0. 0. 0. 0. 0. 0. 40. 35. 0.

0. 330. 330. 345. 0. 0. 0. 180. 180.

0. 5. 25. 10 0. 0. 0. 15. 15.

0. 0. 0. 0. 0. 0. 45. 210. 0.

0. 0. 0. 0. 0. 0. 40. 65. 0.

0. 330. 330. 345. 0. 0. 0. 175. 175.

0. 5. 25. 10. 0. 0. 0. 15. 15.

Label C, M, S refer to Crustal, Mantle wedge and Slab layers, respectively. Thick is layer thickness (m), rho is density (kg/m3), Vs is shear-wave velocity (m/s). %P and %S are percent of anisotropy for P and S waves, respectively. Angles are expressed in degrees. Trend and strike are CW from North. Plunge and Dip are positive down from horizontal.

with the data. We decide on a five-layer crust, which gives acceptable results in terms of misfit and corresponds to the layering derived from seismic exploration data interpretation (e.g. Cassinis et al., 2005). We obtain two preferred models, which substantially differ for the nature of the thin low-velocity layer atop the slab. Model A specifies an anisotropic layer for which the anisotropy symmetry axis is slow and plunges toward the northeast. Such a model might represent a layer of hydrous minerals on top of the slab surface. Model B specifies a layer where anisotropy has a fast SW-plunging symmetry axis, consistent with mantle flow that climbs the slab interface from depth to fill the space created as the Calabrian slab segment rolls back. Shearcoupling of mantle flow to the descending slab is also possible, but significant shear heating or volatile injection would be needed to depress isotropic Vs. Model A and B physical properties are given in Table 1 and Fig. 7. Receiver functions computed from synthetic P coda generated in these two models are compared with the observed data-set in the form of bin-summed composite RF sweeps (Fig. 8) and harmonic stacks (Fig. 9), using the dataset source-receiver paths in the synthetics to replicate effects of the uneven data distribution. Also the synthetics RF processing uses a frequency lowpass identical to the observed RFs. The crustal structure beneath CUC (identical for both model A and B) is formed by five layers with S-velocity that increases with depth. The presence of a steeply-dipping shallow crustal interface (at 6 km depth), bounded by 2 further low-angle-dipping interfaces, reproduce well the observed back-azimuth variation of Phase I. Phases II and III overlap in the synthetic RFs (Fig. 8), similar to the observations, distinguished by the polarity reversals of Phase III in the radial RF around −45° and 135° back azimuth. These phases are generated by structures that are similar in Models A and B (see Table 1), with Phase-II the Ps conversion at the 22.8 km depth crustmantle interface, and Phase-III the Ps conversion at the interface between the uppermost isotropic mantle layer and the anisotropic layer below it (thickness and Vs for these two layers vary slightly from Model A to Model B). The trend (NE) and plunge (40°) of the anisotropy symmetry axis in this mantle layer conflicts with simple models for supra-slab wedge anisotropy. However CUC waveforms probe anisotropic wedge structure at the edge of the slab, where 3-D complications in mantle flow motion are likely (Kincaid and Griffiths, 2003; Funiciello et al., 2004). From the comparison between Figs. 3 and 8 we notice that the different slab and supra-slab structures retrieved for Models A and B both predict well the radial-component moveout of Phase-IV observed in the data. However, there is a key distinction between models as, in Model B, the polarity flip of Phase-IV on the radial RF sweep (Fig. 8) is more evident than in Model A. This difference arises from the fact that, in Model B, Phase-IV is generated by a larger change in the orientation of the anisotropy symmetry axes between layers coupled with a small isotropic negative ΔVs, while Model A indicates a significant velocity inversion that keeps Phase-IV in the radial RFs nonpositive at almost all back azimuth values, so that the characteristic 2-lobed pattern generated by a tilted anisotropic symmetry axis is masked. Although the decisive westerly back azimuths have relatively sparse data coverage (Fig. 3) the observed data seem to favour a lowvelocity channel above the slab, and therefore Model A. The lowvelocity channel in Model A also produces stronger k = 0 slabconverted Ps for Phases IV and V, more like the observations (Fig. 9). However, even if the two models are quite different in geodynamic and petrological assumptions (Model A: layer of hydrous minerals on top of the slab surface; Model B: mantle flow that climbs the slab interface from depth), the differences in the respectively generated Pwave codas are modest (Fig. 8). The tradeoff in wave-propagation behaviour between symmetry-axis orientation and fast- and slowaxis anisotropic symmetry allows both scenarios to predict the observations in a gross sense.

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Fig. 7. Forward models A and B. (Top) S-velocity profile (solid lines) and anisotropy (dotted lines) for the two models, together with the Vp/Vs profile (dashed lines). (Bottom) Southern hemisphere projection of the 3D parameters for the two models. In both models, the thick gray arrow and gray dashed line indicate dip vector and strike direction of the supra-slab interface, respectively. Gray circle and star indicate trend and plunge of the upper and lower anisotropic layers, respectively. The light gray star indicates positive anisotropy and the dark gray star indicates negative anisotropy. See Table 1 for details on the two models. Roman number “I–V” are labeled as in Fig. 3 and associate interfaces to phases.

3.2. Modelling tests To validate some of the qualitative inferences used to choose the model parameterization we perform two forward modelling tests, one for understanding the effect of variations in the strike of interfaces at depth (slab related interfaces), the second to ascertain that anisotropy and dipping interfaces are both needed to explain the observed features in our RFs set. All the tests are carried out by modifying Model A, as we assume that the resulting inferences hold using both Model A and Model B as background model. We change one parameter at a time (more if correlated), such as the strike of the slab interface, or the anisotropy percentage, in order to highlight the first-order dependence of the RF from the different parameters. In the first set of tests, we change the strike of the 2 interfaces at depth, testing a 135° (i.e. SW dipping) and a 225° (i.e. NW dipping) striking slab (Fig. 10). In fact, while the depth of the slab is constrained by the time delay of Phase-V (between 70 and 100 km for 10.5 s delay, at standard mantle velocities values), and its dip is estimated to be less than 20° from observations on the pulses amplitudes and their backazimuth variations (Section 2.4.4, Fig. 3), the features in the RFs which help to constrain the slab strike are more subtle, being (a) the backazimuth dependence of Phases-IV and -V time-delay (minimum at about 90°), (b) the polarity flip of Phase-V on the transverse component (at about 60°). In both our proposed Models A and B, the strike of the slab and of the dipping mantle layer above it is 180° (Table 1), but the

symmetry axes in the two anisotropic layers are roughly NE–SW directed (45° and 50° in Model A; 45° and 210° in Model B), implying complex geometric relationships between the elements of the system, which conflict with simple subduction wedge anisotropy models. The model with slab-related interfaces strike = 135° would be parallel to the southern Apennines trend and normal (dip parallel) to the anisotropic fast-axis direction, consistent with simple shearcoupled corner flow in the supra-slab wedge. The strike = 225° case represents the complementary end-member of the symmetry system, i.e. slab parallel (dip normal) to the anisotropy symmetry axes. Synthetic RFs for the strike = 135° model (Fig. 10a) can match the polarity reversals of the transverse RFs and the maximal radial RF amplitudes for 0°–90° back azimuth. However they fail to correctly reproduce the move-out of the Phase-IV and -V, which show minimum time-delays in the synthetic RFs at 45° back azimuth instead of 90° (Fig. 10a). For the 225° strike direction the moveout effect in the synthetic RFs is too strong (Fig. 10b) if compared with the data (Fig. 3) and with our preferred models (Fig. 8), moreover the polarity reversal of Phase V on transverse RFs at 0° back azimuth (Fig. 10b) should be at back azimuth N45° (Figs. 3, 8). Summing up, the fit to the data is slightly worsened using a model with dip azimuth parallel to anisotropy symmetry axes (strike = 135°, Fig. 10a), and the match to data deteriorates for dip perpendicular to the symmetry axes in the anisotropic layers (strike = 225°, Fig. 10b). Though slab geometry cannot be precisely resolved from our data-set,

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Fig. 8. Back-azimuth-sweeps (radial and transverse components) of the synthetic RF dataset using both Models A and B. Phases “I–V” are labeled as in Fig. 3.

we conclude that the 180° strike of our preferred models is better than the two above hypotheses that align slab dip with the anisotropic symmetry axis.

In the second set of tests, we modify the mantle structure of Model A, by using flat horizontal interfaces only (Fig. 11a) or by removing anisotropy (Fig. 11b,c). In absence of dipping interfaces in the mantle,

Fig. 9. Comparison between observed (black) and synthetic (red) RF datasets for models A and B using harmonic expansion of the RFs. The R-component RFs for k = 0 and the “R + T” summed harmonic for k = 1 are shown.

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Fig. 10. Synthetic RF data-set using two different test models, carried out by modifying Model A. Panels “a” and “b” show back-azimuth sweeps for SW-dipping mantle interfaces. Similar parameters, but for NW-dipping interfaces, are used for Panels “c” and “d”.

anisotropy with a tilted symmetry axis can replicate first-order features observed for Phases III, -IV and -V, such as the amplitude and polarity of the pulses, but it does not reproduce the observed backazimuth moveout (Fig. 11a). In a purely isotropic mantle the transverse

RFs are generated only from out-of-plane Ps refraction from dipping interfaces, and variations in amplitude, polarity and time-delay of the phases, both on radial and tangential components, depend on the dip of the interfaces (for the same epicentral distances, as in Fig. 3). Fig. 11b

Fig. 11. Synthetic RF data-set using three different test models based on Model A. Panels “a” and “b” show results obtained for a test model without dipping mantle interfaces. In the second test model (panels “c” and “d”) we remove anisotropy from Model A. In the third test (panels “e” and “f”) we remove anisotropy and increase the dip angle of the mantle interfaces to 45°.

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shows that removing anisotropy from Model A, the predicted Ps converted phases from the dipping interfaces are far too small to match observations. We then increase the dip angle to 45°, which is ~ 10° higher than the mean value of dip in the shallow part of oceanic slabs around the world (Lallemand et al., 2005). Also in this case (Fig. 11c), an isotropic-dipping-interface model is unable to match the high amplitudes observed in the RF sweeps (Fig. 3), moreover Ps conversions on radial RFs are stronger from the down-dip side (between −60° and −120°, Fig. 11c), while our observed RFs display the maximum amplitudes of the mantle-interface-generated-phases from northeasterly back azimuths (Fig. 3). In other words, an isotropic model would require a NE dipping slab to match the radial-RF amplitude variation with back azimuth, and conflict with the moveout evident in the back-azimuth and epicentral distance sweeps (Figs. 3–5). Results of these basic tests suggest that the contribution of both anisotropy and dipping interfaces is needed to explain the pattern of the observed Ps conversion beneath the CUC station. Although a simple forward-modelling approach leaves room for different hypotheses, our preferred Models A and B can be suitable to represent of the main characteristics of mantle wedge seismic structure beneath CUC (Fig. 12). 4. Tectonic interpretations Analysis of data from a single station, no matter how sophisticated the analysis, offers a limited perspective on the outstanding questions regarding the complicated subduction process beneath Italy. Nevertheless, some hypotheses can be tested that address issues that extend beyond the small patch of mantle beneath station CUC. The tectonics of the Italian peninsula is conventionally attributed to the subduction of the retreating Apennines slab (Lucente et al., 1999; Faccenna et al., 2001; Rosenbaum and Lister, 2004), but only the Calabria region exhibits all the characteristics of a typical subduction zone e.g. calcalkaline volcanism above the slab (in the Aeolian Islands offshore Calabria) and deep earthquakes (Selvaggi and Chiarabba, 1995). Lateral variations in volcanism and in the tomographic expression of the Apennines slab have motivated alternate tectonic scenarios, such as small-scale lithospheric detachment (Wortel and Spakman, 2000), episodic retreat of isolated slab segments (Lucente and Speranza, 2001) and even mantle-plume dynamics (Lavecchia et al., 2003).

In this context, the results of our study can be read and interpreted with increasing levels of detail and complexity. At first order, the receiver functions beneath CUC reinforce the conventional view of Italian geodynamics. They verify that, in Calabria at least, the uppermost mantle displays structures similar to those observed at active subduction zones worldwide (Park et al., 2004; Savage et al., 2007; see also Helffrich and Abers, 1997; Abers, 2005). A strongly anisotropic low velocity layer characterizes both our proposed Models A and B for the mantle beneath CUC (cfr. Fig. 7 and 12), consistent either with hydrated oceanic crust, and perhaps metasediments, atop the downgoing slab (Model A), or with elevated temperature, elevated volatile content in the mantle wedge just above the slab (Model B). Volatile compounds released by the slab are thought to induce islandarc volcanism in subduction zones, and our study adds evidence for this process. It is interesting to speculate whether similar anisotropic low velocity zones exist in regions of Italy where the Apennines slab is imaged by seismic tomography but arc volcanism is absent or weak. Our study suggests that such structures should be resolvable with two years of teleseismic data recorded at a broadband seismic station. At a greater level of detail, the mantle beneath CUC diverges from simple subduction models, in which kinematic velocities, thermal evolution, petrologic character and surface tectonics follow a 2-D geometry related, in the simplest cases, to corner-flow geodynamics (Ida, 1983; Peacock and Wang, 1999; Willett, 1999; Kneller et al., 2005, Abers et al., 2006). In such models mantle-wedge strain develops in alignment with downdip slab motion via shear coupling at the top of the slab. Under dry conditions the olivine fast axis would align with the flow (trench normal), though sufficiently wet conditions could induce a B-type olivine fabric that would align trench-parallel (Jung and Karato, 2001). At CUC the layers atop the subducting slab have strong anisotropy consistent with strong shear, but, in the proposed Models A and B, the symmetry axis is oblique to the predictions of both wet and dry 2-D flow scenarios. 3-D mantle flow beneath CUC is likely, owing to its position near the northern edge of the Calabrian Benioff zone, likely a slab fragment edge (cfr. Fig. 1; Lucente et al., 2006). More curious is the lack of anisotropic signature in the layer beneath the interface that generates Phase-II in our RF sweeps at 25km depth, which we infer to be the Moho. Steady-state shear-coupled mantle-flow models predict strong shear at the upper surface of the mantle wedge as well as at the slab interface, and evidence for strong

Fig. 12. Models interpretation. The cross-sections, cut through the rays-sampled area beneath CUC, are N45°E oriented, i.e. roughly parallel to anisotropy axes trend. Gray shades are representative of the different S-velocity in each layer. White arrows indicate symmetry axis of anisotropy.

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anisotropy near the Moho was reported for multiple locations in the Kamchatka forearc (Levin et al., 2002). Slab rollback should induce extension in the overriding plate, on which CUC sits. Either extension of shallow lithosphere beneath CUC did not occur, extension occurred in a manner that did not lead to anisotropy, or an extensional texture was erased by a subsequent process. Magmatic underplating of the crust, for instance, could disrupt or replace a mantle deformation fabric. Evidence from other locations is needed to determine whether this feature of the CUC RFs has broader significance. We identify the lowermost dipping discontinuity in our models as the Moho of the subducting plate, at about 90 km depth. The slab Moho would correspond to the slab “top” as imaged in Vp tomography and has been referred to as the slab top in the preceding text. Tomographic images of the Calabrian subduction zone show a fast Vp anomaly in this depth range (Lucente et al., 1999). Calabria comprises the forearc of the subduction zone, between the volcanic arc (Aeolian Islands) and the inferred plate boundary in the Ionian Sea, so a slab depth b100-km is expected. Selvaggi and Chiarabba (1995) show from seismicity and Vp tomography that the Apennines slab bends steeply beneath the volcanic arc to near-vertical dip. The sharp bend conflicts with a simple “conveyor-belt” kinematic condition for the slab, and might help explain why the anisotropic expression of a simple cornerflow geometry is missing from the mantle forearc beneath CUC. Finally, focussing on the mantle forearc beneath CUC, our forward modelling demonstrates that the receiver functions support the existence of an anisotropic low-velocity layer above the slab, but do not discriminate well between a slow or fast axis of symmetry. In Model A, we propose an anisotropic layer for which the anisotropy symmetry axis is slow and plunges toward the northeast. Such a model might represent a layer of hydrated oceanic crust, perhaps with subducted sediments atop the slab Moho. For Model B, we specify a layer where anisotropy has a fast SW-plunging symmetry axis, consistent with mantle flow that climbs the slab interface from depth to fill the space created as the Apennines slab rolls back. In the Model B scenario the low wavespeed could imply elevated temperature, elevated volatile content or both. Moreover, in the Model B scenario, there is no subducted oceanic crust or metasediments, implying an atypical subduction process e.g., no steady supply of volatiles for calc-alkaline volcanism in the subduction arc. We favor Model A, however, because both Models A and B have a second anisotropic layer above the low-velocity zone, needed to explain the back-azimuth-dependent Phase-III Ps converted phase at 5-s delay. We argue that this second layer is easier to explain as a mantle-wedge shear zone above the subducted oceanic crust, rather than a second sheared layer within the wedge (Fig. 12). 5. Summary Teleseismic receiver functions (RFs), from permanent broadband station CUC in the southern Apennines, reveal and constrain the presence of anisotropy in the mantle wedge and supra-slab layers. We analysed the back-azimuth variation of the Ps converted phases using RFs computed from 147 teleseismic events. Harmonic angular analysis of the RFs dataset shows two-lobed back-azimuth dependence, consistent with a dipping interface and/or anisotropic layers with a dipping symmetry axis. We modelled the Ps converted phases using a stack of anisotropic layers with dipping symmetry axes bounded by dipping discontinuities. Stable features of our models include: (1) a thin isotropic mantle lid under the crust; (2) an anisotropic low-velocity zone in the mantle wedge, with plunging fast symmetry axis oblique with respect to the trench-normal direction; (3) an anisotropic layer above the slab with depressed velocity; and (4) an isotropic mantle lithosphere (i.e. the subducting slab). The presence of a low velocity layer above the Apennines slab beneath Calabria agrees with observations from several subduction zones (Helffrich and Abers, 1997; Ferris et al., 2003; Park et al., 2004;

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Savage et al., 2007). Such layers can be related to hydrated oceanic crust. However, our model tests do not exclude all other lithologies, e.g. the hydration of the supra-slab mantle from volatiles released from the downgoing slab (Abers, 2005). Serpentinite could generate the slow-axis anisotropy that characterizes our Model A (Kern et al., 1997). Because CUC lies above the northern edge of the active Calabrian Benioff zone, which retreats eastward as it subducts, the source of the mantle wedge anisotropy beneath CUC may be asthenospheric flow around the edge of the slab segment, from the shrinking Ionian Sea toward the growing Tyrrhenian Sea. This hypothesis would be consistent with complex mantle circulation patterns triggered by subduction rollback of laterally finite slabs, similar to those retrieved both through experimental (e.g. Kincaid and Griffiths, 2003; Funiciello et al., 2004) and observational (e.g. Civello and Margheriti, 2004) approaches. SKS splitting does not detect these complexities beneath CUC, in contrast to splitting observed at the southern edge of the Apennines slab (Civello and Margheriti, 2004). This could mean that (1) beneath CUC a separation of the Calabrian slab segment from the rest of the Apennines slab is more recent, (2) much of the surrounding mantle volume sampled by SKS-waves has retained its pre-existing fabric or (3) a more complex mantle circulation has developed only in the supra-slab wedge, implying that the SKS birefringence is influenced mainly by anisotropy deeper than 100 km. In conclusion, station CUC offers a RF data set that is large enough to capture the back-azimuth variation clearly, providing some ground to interpret the seismic structure of the mantle wedge in the region, and on its geodynamic and petrological implications. However it offers only one geographical sample of a presumably complicated wedge structure. Analysis of further broadband data in the region is required to understand better the geometric and petrologic characteristics of the underlying mantle flow. Acknowledgements This work was supported by the Istituto Nazionale di Geofisica e Vulcanologia. J. Park was supported by NSF Grant EAR-0208652. References Abers, G.A., 2005. Seismic low-velocity layer at the top of subducting slabs; observations, predictions, and systematics. Phys. Earth Plan. Inter. vol.149 (no.1– 2), 7–29. Abers, G.A., van Keken, P.E., Kneller, E.A., Ferris, A., Stachnik, J.C., 2006. The thermal structure of subduction zones constrained by seismic imaging; implications for slab dehydration and wedge flow. Earth Plan. Sci. Lett. 241, 387–397. Boschi, E., Giardini, D., Morelli, A., 1991. MedNet: the very broad-band seismic network for the Mediterranean. Nuovo Cimento Soc. Ital. Fis., Sezione C. 14, 79–99. Buttles, J., Olson, P., 1998. Laboratory model of subduction zone anisotropy. Earth Pl. Sc. Let., 164 1, 245–262. Cassinis, R., Scarascia, S., Lozej, A., 2005. Review of Seismic Wide-Angle ReflectionRefraction (WARR) results in the Italian region (1956–1987). In: Finetti, I.R. (Ed.), CROP PROJECT: Deep Seismic Exploration of the Central Mediterranean and Italy, pp. 31–55. Castello, B., Selvaggi, G., Chiarabba, C., Amato, A., 2006. CSI Catalogo della sismicità italiana 1981-2002, versione 1.1. INGV-CNT, Roma, http://www.ingv.it/CSI/. Civello, S., Margheriti, L., 2004. Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting. Geophys. Res. Lett. 31. doi:10.1029/2004GL019607. Di Bona, 1998. Variance estimate in frequency-domain deconvolution for teleseismic receiver function computation. Geophys. J. Int. 134, 634–646. Faccenna, C., Becker, T.W., Lucente, F.P., Jolivet, L., Rossetti, F., 2001. History of subduction and back-arc extension in the Central Mediterranean. Geophys. J. Int. 145, 809–820. Farra, V., Vinnik, L., 2000. Upper mantle stratification by P and S receiver functions. Geophys. J. Int. 141 (3), 699–712. doi:10.1046/j.1365-246x.2000.00118.x. Ferris, A., Abers, G.A., Christensen, D.H., Veenstra, E., 2003. High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50–150 km depth. Earth Plan. Sci. Lett. vol.214 (3–4), 575–588. Fouch, M.J., Fischer, K.M., 1998. Shear wave anisotropy in the Mariana subduction zone. Geophys. Res. Lett. 25, 1221–1224. Frederiksen, A.W., Bostock, M.G., 2000. Modelling teleseismic waves in dipping anisotropic structures. Geophys. J. Int. 141, 401–412. Funiciello, F., Faccenna, C., Giardini, D., 2004. Role of lateral mantle flow in the evolution of subduction system: insights from 3-D laboratory experiments. Geophys. J. Int. 157, 1393–1406.

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