Tomographic constraints on the geodynamic ... - Wiley Online Library

JOURNALOF GEOPHYSICAL RESEARCH, VOL. 104,NO. B9,PAGES20,307-20,327, SEPTEMBER 10,1999

Tomographic constraints on the geodynamic evolution of the Italian region Francesco Pio Lucente, Claudio Chiarabba, and Giovanni B. Cimini IstitutoNazionaledi Geofisica,Rome, Italy

Domenico

Giardini

Instituteof Geophysics,Eidgenossische TechnischeHochschule,Zurich, Switzerland

Abstract. In thispaperwe presentP wave tomographicimagesof the mantlebeneathItaly obtainedby inverting-6000 teleseismicP andPKP wave arrivaltimes,accuratelyrepicked, recordedin the time period 1988-1994by the stationsof the National SeismicNetwork of the IstitutoNazionaledi Geofisica.We pay greatattentionin the dataselectionandpicking procedureof seismicphasesto obtaina very high qualitydataset.The datawere invertedwith the well-established Aki-Christofferson-Husebye tomographictechnique;differentreference modelsand residualscomputationhavebeentried to verify the stabilityof the results.The high qualityof the repickedarrivaltimesallowsus to enhancethe definitionof the deep structuresbeneathboth the Alps and the Apennines,lookingfor their lateral and vertical continuitydown to 800 km depth.The main findingof this studyis a continuoushigh-velocity body locatedbetween250 and670 km depthbeneaththe entireApenninicsystemdipping towardthe Tyrrhenianarea,whichcontinuesupwardsegmentedin two main anomaliesin the northernApenninicandthe CalabrianArcs. We interpretthis high-velocityfeatureas the subductedoceaniclithospherebetweenthe EurasianandAfrican plates,dippingdown to the upper-lowermantleboundarybeneaththe TyrrhenianSea.The retrievedimagesof the lithospheresubductingbeneathApenninesare reliablein termsof thickness(about80-90 km) andP wave velocity contrast(2-4% higherthan the normalmantle).Furthermore,our tomographicimages,which focuson the deepgeometryandcontinuityof the velocity structures, providenew keysto understanding the geodynamicevolutionof the Italian region. The segmentationof the high-velocityslabupwardsuggests a complexevolutionof the arctrenchsystemandthe initially continuoussubductionof the Ionian-Adriaticplate progressivelydevelopedin subordinate arcs,probablydueto lateralheterogeneityof the subductinglithosphere. Chiarabba,1995],the calk-alkaline volcanism in theAeolian Islands[Barberiet al., 1973], and the deephigh-velocity

1. Introduction

The geodynamicevolution of the Mediterraneanresulted from

the

relative

motion

between

the

Eurasian

and

the

African plates since the Cretaceous[McKenzie, 1972; Rebar et al., 1992]. The present-day structure of the central Mediterranean (Figure 1) is dominated by the Alpine orogenesisand by a more recent tectonic phase with the formation of the Apenninesand opening of the Tyrrhenian basinand presentsa complexpatternof surfacetectonicsand deepstructures[Pataccaand Scandone,1989]. The Apennines (Figure 1) consistof two major arcs, the northern Apenninic and the Calabrian Arcs, expressionsof subduction processesof the Ionian-Adriatic lithosphere [Patacca and Scandone, 1989]. In the southern Tyrrhenian area the NW subductingplate beneaththe CalabrianArc is confirmedby the distributionof the intermediateand deep seismicity[Ritsema, 1972; Gasparini et al., 1982; Anderson and Jackson,1987; Giardini and Velon& 1991; Selvaggiand

Copyright1999by theAmericanGeophysical Union. Papernumber1999JB900147. 0148-0227/99/1999JB900147509.00

bodyin theuppermantlerevealed by tomographic imaging [Amatoet al., 1993;Spakmanet al., 1993;Selvaggiand Chiarabba, 1995; Piromallo and Morelli, 1997]. In the

northernApennines, subcrustal seismicity occursdownto 90 km depth[Selvaggiand Amato,1992], and a SW dipping high-velocityanomaly has been imaged by seismic tomography[Amato et al., 1993; Spakmanet al., 1993; Piromallo and Morelli, 1997], suggestingthe westward subduction of the Adriaticplate,in agreement with geologic observations. In the centralsouthernApennines,betweenthe two main arcs, no subcrustalseismicityis recorded,and no

significanthigh-velocityfeatures appear in published tomograms[Amatoet al., 1993; Piromalloand Morelli, 1997]. Thus there is evidencethat the subductionsystemin the centralMediterraneanpresentscomplexitiesand lateral heterogeneities in onlya few hundreds of kilometers. The resultsof previoustomographic studiesoften show

significantdisagreement, both in the geometryand in the intensityof the velocityanomalies, with ensuingdifferences in geodynamic interpretations. For example,Wortel and Spakmart [1992]andSpakmart et al. [1993]proposed a model of geodynamic evolution of theareabasedonthemigration of 20,307

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the detachmentof the subductinglithospherebeneath the Apenninic Arc, which has not been observedin other studies [Areate et al., 1993]. The purposeof this study is to improve the definition of the deep velocity structuresin the uppermantleof the central Mediterranean

and to delineate

the lateral

and vertical

extent

pickingto obtainbotha homogeneous anddense raycoverage of themodeledvolumeandhigh-quality P wavearrivaltimes. 2. Data and Method

We selected148 teleseisms (Figure2) with epicentral

and the continuityof subductedlithosphere.We seekanswers distances greaterthan25ø (126 eventswithP phasesand22 to some of the open questions: Can we confirm slab eventswith PKP phases)recordedby the Italian National

detachment alongthe Apennines? Is therea •eparationat depth between subductionin the northern Apennines and CalabrianArcs? Is there a connectionbetweenthe deep roots of the Apenninesand of the Alps? Can we provide evidence to help understandingthe anomalousdepth distributionand stresspattern of deep seismicityin the Calabrian Benioff zone? Where is the lithosphere subductedin the central Mediterraneanin the last 30 Myr? In orderto resolvethesedetailsof the deepstructureandto add further constraintsto build a coherentgeedynamicmodel for the central Mediterranean, as we first employ an establishedapproachin teleseismictomography(the AkiChristofferson-Husebye (ACH) methodof Aki et al. [1977]) and focus our attention on the data selection and digital

Seismic Network(RSNC,Figure3) duringtheperiodJanuary 1988to January 1994.Teleseisms withat least35 recordings with a high signal-noise ratio and samplingthe volume denselyand homogeneously from all azimuthsand distances have been selected.

The P or PKP arrivaltimesarereadon the digitalrecords acquiredwitha commonbaseclockby applyingdigitalbandpassfilters and visual phasecorrelation[Cimini and Amato,

1993], achievinga timingaccuracy of up to 0.05 s for each reading.We discardall the travel times with a picking accuracy0.6 (more details are given in the section

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We also conducttestsselectinggeographicalsubsetsof our data. An experiment using only data recorded in southern Italy (about half of the coverage) results in a model that Figure 6. Trade-off curve used to select the "optimal" damping parameter for the inverted model. The selected agreeswell down to depthsof 400-500 km with the images produced here, using the whole network, but runs out of damping is 200 s2 (0.0200S2/%2). resolutionin the transitionzone, as expectedby the crossingray geometry.With respectto the resultsobtainedby Amato et al. [1993], the improved data set allows us to enhance the thegridforhalfoftheblock sizeinx andy directions •see definition of the deep images and to increasethe depth of Evans and Zucca, 1988]. The damping parameter 0 is investigationto the whole uppermantle. selected by compromisingbetween the average diagonal resolution and the average standarderrors (Figure 6 ). The 4.1. Horizontal Maps

diagonal of resolution

finalmodel, witha 0 2=200, achieves a variance reduction of 88%, correspondingto a final rms residualvalue of 0.35 s. This value is only marginally larger than the target rms residual of 0.3 s determined by the scatter of the relative residualspopulation (see above), allowing removal of the effect of small singular values in R without pushingthe data fit beyondthe systematicerror threshold. Our goal here is the mantle structure.Given the uncertain knowledgeof the crustalstructurein many areasof the central Mediterranean, we opt for not imposing a priori crustal corrections and static station corrections, and we model the

crustal layer (0-35 km) with conic volumes underneatheach station, which absorb the effects related to both the crustal structure

and the static station

corrections.

These

corrections

are of the order of _+5% on average, indicating that the contribution

of

shallow

and

crustal

anomalies

is

-0.7

s

(roughly 15% of the overall signal present in the data). Following the procedureproposedby Dawson et al. [1990], we have also performed an inversion only for the mantle structureby strippingthe crust, finding that the anomaliesin the mantle

are stable and the shallow

and crustal structure

is

coherently removed. We do not discussthe crustal results here.

The deeper part of the model is the most difficult to constrain.We run different trials with varying thicknessand bottomingdepthof the deepestlayer. Our resultsindicatethat the data coverage offers sufficient resolution to image velocity structuredown to and even below the upper-lower mantle boundary;we loose significantresolutionbelow 800 km depth, owing to insufficient volume samplingrelated to the receiver-arrayaperture[see Evans and Achauer, 1993]. Our targetvolume depthis thus800 km; inversionsextending deeperobtain the sameupper mantle images,while models

In the 35-100 km depth range (layer 2, Plate 1 ) the velocitypatternimagesthe main tectonicunits,asexpected.A stronghigh-velocityperturbation(up to +4%) is seenbeneath the central westernAlps, whereasa less perturbedmantle is found beneath the eastern Alps. A striking, well-defined curvedlow-velocity extendsfrom the Po Plain to centralItaly, marking the external front of the whole northernApenninic Arc, while a positive anomaly (up to +4%) is located in the internal part of the northernApenninic Arc, again following its curvature, and is bordered to the west by a low-velocity zone correspondingto a high heat flow area. Subcrustal seismicityis located inside the easternlow-velocity anomaly. The mantle region beneath the central southernApennines appears rather homogeneous, while a strongly positive velocity anomaly (+4%) marks the Tyrrhenian margin of Calabria, the Ionian Sea, and the Sicily-Calabria Strait, in good agreementwith the pattern of the seismicity in this depth range. A wide low-velocity zone characterizescentral western Sicily, following the geographicalextent of the African promontory. In the 100-250 km depth range (layers 3-4, Plate 1) we observethree main high-velocityzones (with perturbationas high as +4%) beneath the southern Tyrrhenian area, the northernApennines,and the Alps. The northernand southern Apennineshave a differentsignatureat thesedepths,as only a weaklypositiveanomalyis foundbeneaththe centralsouthern Apennines.We observea slight migrationwith depth of the anomaliesunderneaththe Apenninestoward the Tyrrhenian basin, indicating that the velocity anomalieshave an almost vertical geometry; in the southern Tyrrhenian this pattern agreeswell with the seismicitydistribution.The widespread low-velocity anomaly found at shallowerdepthsstill marks

20,314

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ON GEODYNAMIC

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the Adriaticmarginof the northernApennines,the northern 250 km (+1.5% on average). Below250 km thehigh-velocity partof theAdriaticsea,andthePo Plain.A linearsequence anomalybecomesstrongeranddipssteeplydownto 600 km, of smalllow-velocityperturbations (0.6 arecloseto zero,indicatingreducedsmearingof the anomaly. In Figure 7 (top) we contour all the diagonalelementsof R for layer 5 and cross section AA'. The horizontal resolution image is very flat, with most of the model cells locatedwithin the volume sampledby the rays having resolution>0.6; for only a few cells doesthe resolutionreach0.8, while it drops rapidly to 0 in the peripheral cells. The overall resolution level is dictatedby the selectionof the dampingparameter 2

deepseismicity is associated onlyto theshallower partof this structure. The thickness of the high-velocity anomalyis -80 0 ; for lower dampingvaluesthe overall picture remains km, with minor changeswith depth.The largestvelocity similar, but the resolution level increases. perturbations (upto +4%) areconfined to theupper250 km, Figure 7 (bottom) shows the standard errors for each in agreement withobservation in othersubduction areas[Lay, parameterin layer 5 and cross section AA'. Standarderrors 1994].MovingdeeperandtowardtheNW, thehigh-velocity reach maximum values of 1.1% and in the best resolved anomalylosesgeometrical definitionandvelocitycontrast in portionof the model are generally