Lithosphere density model in Italy: no hint for slab · PDF fileLithosphere density model in...

Lithosphere density model in Italy: no hint for slab pull

Brandmayr Enrico,1 Marson Iginio,2,3 Romanelli Fabio1,4 and Panza Giuliano Francesco1,4

1Department of Geosciences, University of Trieste, Via Weiss 4, 34127, Trieste, Italy; 2Department of Civil Engineering and Architecture,

University of Trieste, P.le Europa 1, 34127, Trieste, Italy; 3Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, OGS, Borgo Grotta

Gigante 42 ⁄C, 34010 Sgonico (TS), Italy; 4The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34014 Trieste,

Italy

Introduction

A pioneering work in density model-ling of the upper mantle in the Italicregion was performed by Marsonet al. (1995) along transects aroundthe Tyrrhenian rim, constructed with-in the geometrical constraints imposedby the results of the interpretation ofaeromagnetic, seismic and seismolog-ical data. The advantage of joininggeophysical models with geologicaland petrological data in order tounderstand the complex evolution ofItalic region has been later discussedin Panza et al. (2007a): the studydescribes, for the first time, a veryshallow crust–mantle transition and avery low S-wave velocity (VS) justbelow it, in correspondence of thesubmarine volcanic bodies (Magnaghiand Vavilov, Figs 1 and 2) indicatingthe presence of high amounts ofmagma. Panza et al. (2007b) inte-grated crustal geological and geophys-ical constraints with the VS modelsalong the TRANSMED IIIgeotraverse. As a result, a new modelof the mantle flow in a backarcsetting, which reveals an easterly ris-ing low-velocity zone (LVZ) in theactive part of the Tyrrhenian basin, isobtained. An upper mantle circulation

in the Western Mediterranean, mostlyeasterly directed, affects the boundarybetween upper asthenosphere andlower asthenosphere, which undulatesbetween about 180 and 280 km. Anexplanation for the detected shallowand very low velocity mantle in the

non-volcanic part of the Tyrrhenianregion is presented in Frezzotti et al.(2009). These anomalous layers weregenerated by the melting of sedimentsand ⁄or continental crust of the sub-ducted Adriatic–Ionian (African) lidat temperatures above 1100 �C and

ABSTRACT

The lithosphere–asthenosphere system of the Italic region interms of shear-velocity and density distribution with depth issuitable to investigate the geodynamic context of the region.The velocity structure is obtained through nonlinear inversionof dispersion curves compiled from surface wave tomographyon cells 1� · 1� and a smoothing optimization method tochoose the representative cellular model, whose layering isused as fixed (a priori) information to obtain a density model bymeans of linear inversion of gravimetric data. Seismicity andheat flow are used as independent constraints in outlining both

the crustal and the seismic lid thickness; the nonlinear momenttensor inversion of recent damaging earthquakes allows someinsight in the ongoing kinematic processes. Asymmetrybetween west-directed (Apennines) and east-directed (Alps,Dinarides) subductions is a robust feature of the velocitymodel, while density model reveals that slabs are not denserthan the ambient mantle, thus supplies no evidence for slabpull.

Terra Nova, 00, 1–8, 2011

Fig. 1 Sardinia–Balkans section: VS and density model along a SW–NE profile fromsouthern Sardinia to Sava Basin. VS distribution is given by the color scale whilecontinuous contour lines refer to density (g cm)3). Seismicity (red dots) and focalmechanisms of major events (�beach balls�plotted in map view) are shown as well.Dotted white lines delineate the lid-LVZ margin. Please refer to online version forinterpretation of references to color. This profile samples the main geological andtectonic features of the centralMediterranean: (i) the southernmost part of theCorsica–Sardinia block characterized by scarce seismicity and rather low velocities in the lid,down to about 70 km of depth; (ii) the active part of Tyrrhenian basin, characterized byan asthenospheric LVZ emerging eastward from 70 km depth in the western part of thebasin to 30 km depth below Ischia and well visible lithospheric �boudinage�; (iii) thewestward dipping Apennines, characterized by high seismicity even at intermediatedepth and high velocities; (iv) a relatively low velocity and mostly aseismic lid beneaththeAdriatic sea (as in sectionAdria42); (v) eastward dippingDinaric slab, characterizedby relatively high velocity and intermediate depth seismicity. Relatively low-densitymantle is seen beneath Corsica–Sardinia block, Apennines and Dinarides, whereas aprominent positive density anomaly is found below Tyrrhenian and Adriatic seas.

Correspondence: Enrico Brandmayr,

Department of Geosciences, University of

Trieste, Via Weiss 4, 34127, Trieste, Italy.

Tel:+390405582128;fax:+390405582111;

e-mail: [email protected]

� 2011 Blackwell Publishing Ltd 1

doi: 10.1111/j.1365-3121.2011.01012.x

pressure greater than 4 GPa (130 km).The resulting low fractions of carbon-ate-rich melts have low density andviscosity and can migrate upwardforming a carbonated partially moltenCO2-rich mantle in the depth rangefrom 130 to 60 km. Carbonate-richmelts upwelling to depth less than 60–70 km induce massive outgassing ofCO2, that can migrate and be accu-mulated beneath the Moho and withinthe lower crust. Brandmayr et al.(2010) contributed to the debate onthe geodynamical evolution of theItalic region with an improvement ofthe existing cellular model of thelithosphere–asthenosphere system ofthe area. The VS-depth model consid-ered in this work represents a furtheroptimization of previous models con-strained with independent studies con-cerning Moho depth (Dezes andZiegler, 2001; Grad and Tiira, 2008;Tesauro et al., 2008), seismicity-depthdistribution (ISC, 2007) and P-wavevelocity (VP) tomographic data (Piro-mallo and Morelli, 2003). The layeringof each representative cellular modelis used as fixed (a priori) informationto obtain a three-dimensional densitymodel by means of linear inversion ofgravimetric data (ISPRA, ENI andOGS, 2009). The results reveal someunexpected features of the mantle insubduction zones: in particular, noevidence of negative buoyancy of thesubducting slabs with respect to thesurrounding mantle is found. Thisfinding agrees with recent xenolitesstudies (Kelly et al., 2003) as wellwith the LLAMA model proposedby Anderson (2010). Therefore, thecommon concept of �slab pull�, usu-ally evoked as leading force in sub-duction dynamics, but criticized insome detail by Doglioni et al. (2007),is hardly in agreement with ourresults.

Methodology

A set of VS models for cells sized1� · 1� in the Italic region (Fig. 3),using the cellular dispersion curvescompiled from surface wavetomography, has been obtained withthe nonlinear �hedgehog� inversionmethod (Panza et al., 2007a,b), whereVP is a dependent parameter withrespect to VS (in general VP ⁄VS =31 ⁄ 2) while density is fixed at popularvalues increasing with depth. These

cellular models extend to about350 km of depth. In order to selectthe representative solution among this

set, all the solutions for each cell havebeen simultaneously processed withan optimized smoothing method with

Fig. 2 Tyrrhenian II section: VS and density model along a NW–SE profile fromsouthern Corsica to Ionian Sea. The labelling is the same as Fig. 1. Prominent featureof the section is the high velocity body representing subducting lid beneath CalabrianArc, seismically active at all depths. To the west, the active part of the Tyrrhenian ismarked by a lid at about 60–70 km thick, with some very low velocity soft mantle lidbeneath volcanic areas. Lid is slightly thicker beneath Sardinia–Corsica block andreaches about 90 km of thickness. To the east of the Calabrian Arc, the Ionian lid isabout 130 km thick, and the underlying mantle is relatively faster than the Tyrrhenianone. Generally, the mantle velocity is higher in the Ionian plate than in theTyrrhenian basin. A prominent negative density anomaly characterizes the subduct-ing slab and the Sardinia–Corsica block, whereas a positive anomaly characterizesboth the active Tyrrhenian, with a 3.3 g cm)3 density of about 60–70 km of depth,and the Ionian mantle, with a 3.3 g cm)3 density reaching about 80 km of depth anda 3.4 g cm)3 density at about 170 km of depth.

50°N

45°N

40°N

35°N8°E 12°E 16°E 20°E 24°E

1

8

Ionian

Sea

7

6

2

3

4

Dinarides

Calabrian

Arc

Julian AlpsWestern

Alps

5

Po Plain

Apennines

Sard

inia

-Cor

sica

bl

ock

AdriaTyrrhenian Sea

Fig. 3 Study area: Map of the studied area, showing the geographical location of theconsidered sections, is enclosed by the dashed bold line. The cellular grid is omittedfor sake of clarity. Main tectonic lineaments are shown, comb lines indicatingcompressive fronts and single lines indicating transfer zones. The sections arenumbered as follows: (1) Ecors (Fig. 6); (2) Transalp (Fig. 9); (3) Adria43-46 (Fig. 7);(4) Crop03 (Fig. 10); (5) Adria42 (Fig. 8); (6) Tyrrhenian I (Fig. 5); (7) Sardinia–Balkans (Fig. 1); (8) Tyrrhenian II (Fig. 2).

Lithosphere density model in Italy: no hint for slab pull • B. Enrico et al. Terra Nova, Vol 00, No. 0, 1–8

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2 � 2011 Blackwell Publishing Ltd

the aim to define a smooth three-dimensional model of the lithosphere–asthenosphere system. The optimiza-tion method, that according to theSmoothness and Flatness criteriahelps to choose, for each cell, asrepresentative solution the one thatminimizes the local lateral velocitygradient, consists of three algorithms:LSO (Local Smoothness Optimiza-tion) technique, that searches itera-tively within five neighbour cells thesolution for which the lateral gradientin shear wave velocity is minimized,GSO (Global Smoothness Optimiza-tion) and GFO (Global Flatness Opti-mization), which, respectively, searchfor the minimizing solution along arow of cells and through the wholestudy area. The three optimizationalgorithms (Boyadzhiev et al., 2008;Brandmayr et al., 2010) have beenapplied hierarchically, i.e. commonsolutions to LSO and GSO methodsare fixed and then GFO is applied.The resulting representative cellular

models are appraised and constrainedusing independent studies concerningMoho depth (Dezes and Ziegler, 2001;Grad and Tiira, 2008; Tesauro et al.,2008), seismicity-depth distribution(ISC, 2007), VP tomographic data(Piromallo and Morelli, 2003) andother independent information as heatflow (Hurting et al., 1991; DellaVedova et al., 2001). The data from

Dezes and Ziegler (2001), Tesauroet al. (2008) and Grad and Tiira(2008) are used to appraise the range

of Moho depth for each cell, by com-paring it to the Moho depth, takenwith its uncertainties, of the selected

mGal

Cou

nt300

250

200

150

100

0–10 –8 –6 –4 –2 0 2 4 6 8

50

Fig. 4 Density model uncertainties:Histogram of the differences betweenobserved and model predicted gravimet-ric anomalies. The maximum value is8 mGal, consistent with the quality andthe noise level of the data.

Fig. 5 Tyrrhenian I section: VS and density model along a NW–SE profile fromLigurian Sea to Gulf of Salerno. The labelling is the same as Fig. 1. This profilesamples the Tyrrhenian Sea along its supposed direction of opening. It is clearlycharacterized by a south-eastward emerging LVZ from about 100 km of depthbeneath Ligurian Sea and northern Corsica to about 30 km in the Ischia volcaniczone. The VS model, the seismicity distribution and the so inferred lid–LVZ boundaryfairly agree with extensive �boudinage�, at different scales (Ramsay and Huber, 1983,and references therein) that characterized the evolution of the basin still going on inpresent days. The northern part of the basin is characterized by relatively low-densitymantle, in contrast with the southern active part, where density increases, as seen inother sections. Again, a relatively low density mantle is found east of Ischia, in thevicinity of Apennines subduction.

Fig. 6 Ecors section: VS and density model along a W–E profile from Flumet (westernAlps) to Euganei Hills. The labelling is the same as Fig. 1. Striking feature of thesection is the subduction of the European plate below the Adriatic plate, with deep�lithospheric roots� below Milan, represented by high-velocity lid, well in agreementwith what was shown by Mueller and Panza (1986). In contrast to Apenninessubduction, no intermediate depth seismicity is observed in western Alps. To the Westthe LVZ is found at a depth of about 180 km. To the East, a prominent LVZ marksthe Euganei Hills magmatic zone below a depth of about 130 km. The subducting lidis characterized by negative density anomaly, whereas a positive density anomaly isseen beneath Euganei Hills, as shallow as 30 km.

Terra Nova, Vol 00, No. 0, 1–8 B. Enrico et al. • Lithosphere density model in Italy: no hint for slab pull

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model. The depth distribution of seis-micity is used as an additional criterionin outlining both Moho depth and lidthickness. The revised ISC (2007) cat-alogue for the period 1904–June 2005is used and for each cell several histo-grams are computed, grouping hypo-centres in depth intervals consistentwith hypocentre depth uncertainties,4 km for crustal seismicity and 10 kmfor mantle seismicity (Panza andRaykova, 2008; Brandmayr et al.,2010). The analysis of focal mecha-nisms is a complementary informationwith respect to seismicity-depth distri-bution, even if statistically poorerthan the latest, and it allows someinsight in the kinematic processesacting in different tectonic provincesproviding further constraints on thecurrent geometries and stress field ofthe study region. Therefore, the data-set of the seismic moment tensor forthe earthquakes with Mw ‡ 4.8 in theItalian region (Guidarelli and Panza,2006; Brandmayr et al., 2010),obtained using an advanced wave-form inversion technique namedINPAR (Sileny et al., 1992; Sileny,1998), has been updated to the mostrecent events.The VP data are taken from Piro-

mallo and Morelli (2003), who pro-vide a P-wave tomography study wellresolving the upper mantle propertiesof the whole Mediterranean region.The calculated VP value for each layerin our cellular models is used toreduce the uncertainty ranges of VS,as far as the VP ⁄VS ratio in the mantleis kept as close as possible to 1.82(Kennett et al., 1995).The layering of each representative

cellular model is used here as fixed(a priori) information to obtain athree-dimensional density model bymeans of linear inversion of gravimet-ric data. The starting density model isan upgrading of that given by Farina(2006) along a set of two-dimensionalprofiles in the Italic region and sur-roundings. We use the inversion soft-ware GRAV3D developed by theUniversity of British Columbia-Geo-physical Inversion facility (Li andOldenburg, 1998). The inverse prob-lem is formulated as an optimizationproblem where the following objectivefunction of the density model is min-imized, subject to the constraints thatthe data is reproduced within a givenerror tolerance:

Fig. 7 Adria43-46 section: VS and density model along a SW–NE profile fromCapraia Island to Julian Alps. The labelling is the same as Fig. 1. The pronouncedhigh-velocity, almost vertical, slab represents Apennines subduction consistent withsignificant seismicity at intermediate depth. To the West, a low-velocity astheno-spheric mantle wedge, mainly aseismic, is present. To the East, a high velocity lidabout 100 km thick extends towards the NE-dipping Alpine subduction, marked bythe shallow to intermediate depth seismicity in the right part of the section. Thebottom of the lid is well marked by a LVZ. Striking features of density model are thenegative density anomaly beneath Apennines and Julian Alps, with the exception ofthe relatively high-density value found beneath Tuscany, probably related to mantlewedge. Po plain and Northern Adriatic are instead characterized by relatively high-density mantle at depths ranging from 60 to 200 km.

Fig. 8 Adria42 section: VS and density model along a W–E profile from centralApennines to Dinarides. The labelling is the same as Fig. 1. In this section, as insection Crop03, Apennines slab is marked by a high velocity almost vertical body,with some intermediate depth seismicity reaching about 60 km of depth. To the West,the low velocity at a depth of about 50–100 km, just above the sinking slab, isprobably related to mantle wedge dynamics (e.g. dehydration). To the east, a relativelow velocity, mainly aseismic, mantle lid extends for about 100 km just beneath theAdriatic crust. In the easternmost portion of the section, an eastward thickening lid iswell marked by high velocity and intense intermediate depth seismicity, in fairagreement with a low angle Dinaric slab. Prominent negative density anomaliescharacterize both the Apennines and Dinarides at depth between 100 and 200 km,whereas high-density material seems to be related to the Adriatic plate, both in the lidand in the asthenospheric LVZ.


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/mðqÞ¼as

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where the functions ws, wx, wy and wz

are spatially dependent, whereas as,aX, aY and aZ are coefficients whichaffect the relative importance of thedifferent components of the objectivefunction. The greater the ratio aX ⁄aSthe smoother the recovered model isalong that axis direction. The objec-tive function defined by Eq. (1) hasthe flexibility to allow many differentmodels to be constructed. The relativecloseness of the final model to thestarting model at any location iscontrolled by the function ws. Theweighting functions wx, wy and wz canbe designed to enhance or attenuatestructures in various regions in themodel domain. The function w(z) inEq. (1) is a depth weighting thatdepends upon the model discretizationand observation location. The weight-ing function is used to counteract thedecay of the kernel function withdepth. It is assumed that the data arecontaminated by Gaussian noise withzero mean and, to define a measure ofthe misfit, a two-norm measure isused. The inverse problem is solvedby finding a density that minimizes themeasure and misfits the data accord-ing to the noise level. As the densitycontrast is limited to a small range forany practical problems, and oftenthere are well-defined bounds on thedensity contrast based on other geo-logical information, it is possible toimpose constraints to restrict the solu-tion to lie between a lower and upperbound. The data used for the inver-sion are a subset of those used toproduce the Digital Gravity Maps ofItaly (ISPRA, ENI and OGS, 2009).A Gaussian noise with amplitude of1.5 mGal has been added to the grav-ity anomaly data. The input meshconsists of 32 · 22 cells (each0.5� · 0.5�) along the x and y direc-tions, where the density is defined vs.depth in 12 layers of different thick-ness, fixed on the base of the VS

layering. As the observed and inverteddata are Bouguer anomalies, in thewater layer, when present, the Bou-guer density has been used andallowed to vary only within a verylimited range of density ()0.01,+0.01 g cm)3), i.e. fixed for anypractical reason. The differences be-tween the observed and predictedanomaly values show a Gaussianbehaviour with a standard deviationof 1.8 mGal, consistent with the qual-ity and level of the noise (Fig. 4).

Discussion

The lid and the LVZ are differentconcepts from the lithosphere and theasthenosphere (e.g. see p. 12 inBrandmayr et al., 2010), although theterms are often confused in literature,as clearly evidenced by Anderson(2010). In the following discussion,we use the terms lid and LVZ whenreferring to strictly seismological fea-tures. Velocity features of most of thesections, discussed in detail in

Brandmayr et al. (2010), are substan-tially confirmed and can be summa-rized as follows: high velocity lidcharacterizes subducting slabs(VS > 4.5 km s)1); the bottom of thelid is generally marked by a velocityinversion, or at least by a null gradient,with VS lower than 4.4 km s)1; verylow mantle velocities (soft mantle lid,VS even lower than 3.6 km s)1) arefound in the active part of the Tyrrhe-nian basin (Figs 5, 1 and 2), in agree-ment with a high percentage of meltsand magmas (Panza et al., 2007a).Asymmetry between west-directed

(Apennines) and east-directed (Alps,Dinarides) subductions is a robustfeature of the model: low angle sub-duction is seen in western Alps, alongsection Ecors (Fig. 6) and in JulianAlps and Dinarides (Figs 7, 8 and 1).Very low angle collision process(<7�) is found in Eastern Alps, alongsection Transalp (Fig. 9). Subductionbelow Apennines (Fig. 10a) is steeperthan below Alps and Dinarides in allanalysed sections. This feature is

Fig. 9 Transalp section: VS and density model along an N–S profile from BavarianAlps to Northern Apennines. The labelling is the same as Fig. 1. The low-anglesubduction of the European plate below the Adriatic plate is fairly well evidenced bygently N–S dipping high-velocity lid and in accordance with scarce shallow seismicityand almost absent intermediate depth seismicity, as seen in Ecors section. A relativelylow velocity lid is found beneath Dolomites, just below the Moho. The bottom of thelid is found at about 120–140 km of depth and overlies a well-marked LVZ in theasthenosphere. In the southernmost part of the section, Apennines subduction isdelineated by high seismicity that reaches intermediate depth. The prominent high-density body, as shallow as 40 km of depth, below Venetian plain, may be a signatureof the Euganei Hills magmatic activity.


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much more clear-cut in a sectionwhere the distribution of the shearmodulus (l) is shown (e.g. Fig. 10b);as a rule, in the Alpine and Adriatic

sections, the dotted white line corre-sponds to the isoline of 65 GPa. Theasymmetric feature, as well the east-ward emerging LVZ in the Tyrrhe-

nian asthenosphere (Fig. 5) is inagreement with an east-directed man-tle flow (Doglioni et al., 1999). In allthe sections but one, the velocity inthe mantle on the west side of Apen-nines is lower than in the east side,both in the Ionian plate and in theAdriatic plate, mainly in the depthrange from 100 to 200 km (astheno-sphere), where the eastward flow canreach its maximum. The exception isgiven by section Adria43-46 (Fig. 7),across northern Apennines, where amarked LVZ is found below and tothe north-east of the slab (VS about4.2 km s)1), whereas mantle velocitieson the western flank are about4.35 km s)1. Similarly, asymmetricfeatures in the velocity model can beobserved between northern andsouthern Tyrrhenian basin, with alongitudinal axis roughly correspond-ing to the 41�N parallel. The southern(active) part of the basin is generallycharacterized by mantle velocities thatare lower than those in its northernpart (Figs 5 and 2); this difference canbe attributed to several causes, e.g.different extension rates, difference inthe intensity of mantle flow, presenceof lithospheric fragments, metamor-phism and ⁄or compositional differ-ences.In the density model, a dominant

feature in all the sections is that thesubducting slabs are not denser thanthe ambient mantle, but they areslightly lighter. This feature persists,regardless of the orientation or type(i.e. Alps or Apennines) of colli-sion ⁄ subduction.Western Alps (Fig. 6) and Sardinia–

Corsica block (Fig. 1) present a com-mon distribution of density withdepth, with a relative low-densitymantle with respect to surroundings(3.3 g cm)3 at about 230–250 km).This similarity may be due to theircommon origin, as Sardinia–Corsicablock is a fragment of the EuropeanPlate (Lustrino et al., 2009 and refer-ences therein). On the contrary, adensity higher than the surroundingsis found in the LVZ, with the3.3 g cm)3 density reaching depths ofabout 70 km (Figs 5, 1 and 2), inparticular, in the active part of theTyrrhenian basin, where the astheno-sphere is very shallow. Similarly, highdensity mantle is found beneath PoPlain and northern Adriatic (Figs 9and 7).

(a)

(b)

Fig. 10 Crop03 section: (a) VS and density model along a SW–NE profile fromnorthern Corsica to Adriatic Sea. The labelling is the same as Fig. 1. The pronouncedhigh-velocity, almost vertical, slab represents Apennines subduction. To the West, anasthenospheric LVZ emerging from about 100 km depth beneath Corsica to about40 km depth beneath Tuscany clearly marks the bottom of the lid. To the East of thesubduction, a high-velocity lid extends beneath Adria down to about 120 km ofdepth, lying on a well marked LVZ. A high density body is seen below Tuscany and inthe subducting slab, whereas lower density characterizes both Corsica block andAdriatic mantle. (b) The shear modulus (l), in GPa, along the same section, gives amore clear-cut picture of the subducting slab.


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The north–south asymmetry ob-served in the mantle velocities of theTyrrhenian basin persists in the den-sity model, with lower mantle densi-ties in the northern part of the basinthan in the southern (Fig. 5), the axisof symmetry being the 41�N parallel.Therefore, higher densities in themantle seem to be strictly related tothe eastward flow itself and to itsascent beneath the back-arc basin.All these findings suggest that ma-

terial coming from a deep source, inspite of depressurisation, persists to bedenser than ambient mantle. There-fore, the common concept of �slabpull�, usually evoked to be one of theleading forces in subduction dynam-ics, is hard to be endorsed in thiscontext, as no evidence of negativebuoyancy of the slab itself withrespect to the ambient mantle isfound.

Conclusions

The asymmetry between E-vergingand W-verging subduction zones,clearly evidenced by VS model, sup-ports the hypothesis of an eastwardmantle flow, especially in the LVZ,between 120 and 200 km of depth,that is likely to represent the decou-pling between the lithosphere and theunderlying mantle at global scale(Panza et al., 2010). The flow is veryshallow in the active Tyrrhenian basindue to mantle compensation inducedby the eastward migration of theApennines subduction. High densities(>3.3 g cm)3) in the mantle seem tobe strictly related to the eastward flowitself and to its ascent beneath theback-arc basin or to other extensionaltectonics or to volcanism. On thecontrary, slabs are not denser thanthe ambient mantle, but they appearto be slightly lighter; this evidenceconflicts with the concept of �slab pull�and thus calls for different actors insubduction dynamics (e.g. Doglioniet al., 2007).

Acknowledgements

We thank A. Levshin and Don L. Ander-son for thoughtful and constructivereviews. This research has benefited of thegrant �Studio della struttura della crosta edel mantello superiore dell�area mediterra-nea mediante metodologie sismologichedi inversione non lineare� by Dipartimento

di Geoscienze dell�Universita degli Studi diTrieste, funded by the Italian Ministerodell�Istruzione, dell�Universita e della Ric-erca PRIN 2008 and Italian PNRA(2004 ⁄2.7-2.8) ‘‘Sismologia a larga banda,struttura della litosfera e geodinamica nellaregione del Mare di Scotia’’. This researchhas been partly developed in the frame-work of the ASI-Pilot Project �SISMA:SISMA-Information System for Monitor-ing and Alert�. Gravimetric Inversion isperformed with the program Grav3D,developed by University of British Colum-bia (‘‘UBC’’), of 2075 Westbrook Mall,Vancouver, British Columbia V6T 1Z3 andlicensed by UBC. Part of figures have beenplotted using GMT (Generic MappingTools; Wessel and Smith, 1995).

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Received 8 May 2011; revised versionaccepted 17 June 2011


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