seismic anisotropy and velocity structure beneath the ...hera.ugr.es/doi/15770850.pdf · physics of...

Physics of the Earth and Planetary Interiors 150 (2005) 317–330

Seismic anisotropy and velocity structure beneaththe southern half of the Iberian Peninsula

I. Serranoa, ∗, T.M. Hearnb, J. Moralesa, c, F. Torcala, d

a Instituto Andaluz de Geof´ısica, Universidad de Granada, 18071 Granada, Spainb Department of Physics, New Mexico State University, Las Cruces, USA

c Departamento de F´ısica Teorica y del Cosmos, Universidad de Granada, Spaind Departamento de Ciencias Ambientales, Universidad Pablo de Olavide, Sevilla, Spain

Received 10 March 2004; received in revised form 1 December 2004; accepted 3 December 2004

Abstract

Travel times of 11,612Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocityand anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions.Pn phases are routinelyidentified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imagingof variations ofPn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of8.0 km/s. The peninsular area covered by the Iberian massif is characterized by highPn velocity, as expected in tectonicallystable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsulah ni might beu rp variationi oundaryi reas highv on of thef eas to then rth–southi volved int Neogenee pattern ofam©

K

0

ave undergone a great number of recent tectonic events and are characterized by a pronouncedly lowPnvelocity, as is common areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal rootnderlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sha

n the uppermost mantle velocities that coincides with the structural complexity of the European and African plate bn the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente wheelocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the directiastestPnvelocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) wherorth, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and no

n the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been inhe orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that thextension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complexnisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastestPn velocity andajor Hercynian tectonic trends cannot be directly established.2004 Elsevier B.V. All rights reserved.

eywords:Seismic anisotropy;Pnseismic waves; Tomography; Iberian Peninsula

∗ Corresponding author. Tel.: +34 958248912; fax: +34 958160907.E-mail address:[email protected] (I. Serrano).

031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.pepi.2004.12.003

318 I. Serrano et al. / Physics of the Earth and Planetary Interiors 150 (2005) 317–330

1. Introduction

The southern half of the Iberian Peninsula is madeup of the Iberian massif and the Betic Cordillera(formed by the External and Internal Zones), which areconnected through the Guadalquivir Basin. The Gulfof Cadiz represents the continuation of this basin onthe continental and Algarve margins along the Meso-zoic border of the Iberian massif. The Betic Cordilleraand Moroccan Rif are connected through the GibraltarStrait, which developed during Cenozoic convergencebetween Africa and Iberia (e.g.Dewey et al., 1989)making an arcuate belt. The Alboran Sea is located onthe inner part of this in continuity to the east with theSouth Balearic Basin (Fig. 1).

The Iberian massif forms part of the Variscan beltof central Europe that resulted from the late Paleozoic

F e main etch of themG

suture of Gondwana and Laurasia. Its southern edgeis composed of two lithostructural units from north tosouth: the Ossa Morena Zone (OMZ) and the SouthPortuguese Zone (SPZ). The SPZ is located in thewesternmost sector of the Hercynian belt, where thereare no outcrops of the deep continental crust. Its sed-imentary record comprises Upper Devonian and Car-boniferous sequences. However, seismic and gravitystudies have revealed the presence of older crust ofan unknown age (Mueller et al., 1973; Prodehl et al.,1975, etc.). Recent studies on the boundary betweenthe SPZ and the OMZ have interpreted this as a ma-jor suture of the European Variscan Orogen (Crespo-Blanc and Orozco, 1988; Quesada, 1991). The SPZis believed to have been accreted to the rest of theIberian massif during the orogeny, and to representa fragment of a plate that was otherwise destroyed

ig. 1. Simplified geological map of the study area showing th
ain geological structures such as the Betic and Rif Cordilleras. SPZD: Guadalquivir Depression.
tectonic units. The upper right-hand corner shows a small sk
: South Portuguese Zone. OMZ: Ossa Morena Zone. GB: Granada Basin,

I. Serrano et al. / Physics of the Earth and Planetary Interiors 150 (2005) 317–330 319

by subduction beneath the OMZ (Quesada et al.,1994).

In contrast, the Betic–Rif mountain belt forms thesouthwestern termination of the Alpine orogen in Eu-rope and northern Africa. Its ambivalent position strad-dling the plate boundary between Iberia–Europe andAfrica is an orogenic artefact produced by late stageemplacement on the African foreland of secondaryextensional allochthons derived from the EuropeanAlpine collisional belt (Zeck et al., 1992; Zeck, 1996,1997, 1999; De Jong, 1991). The opening of the NorthAtlantic during the Late Cretaceous and Tertiary in-duced the rotational divergence of North America andEurasia, which was paralleled by the counter clock-wise convergence of Africa and Eurasia. The north-wards drift of Africa caused the progressive closure ofoceanic basins of Tethys and the rapid westward propa-gation of the Alpine Betic–Rif orogenic collision frontin the Gulf of Cadiz, in parallel with the development ofthe western Mediterranean basins (Dewey et al., 1989;Garcıa-Duenas et al., 1992; Jabaloy et al., 1992;Maldonado et al., 1992).

From this complicated yet intriguing geologicalframework, we deduce that the present study areaprovides a suitable opportunity to research the pres-ence of anisotropy, a parameter used to describe amedium whose elastic properties are functions of ori-entation. It is now generally accepted that seismicanisotropy in the upper mantle is due primarily tothe deformation-induced lattice preferred orientationo uldh mo-t no-s fora thet uldbA in-d irec-t ono tema re-l here( int sultf rentd

The existence of upper mantle anisotropy beneaththe Iberian Peninsula has been recorded by using avariety of techniques and results are not easily com-parable. WhilePndata measure the anisotropy withinthe top of the mantle, SKS data measure the verticallyintegrated anisotropy of the mantle; in this paper weshow both results.Vinnik et al. (1989)presented thefirst results of the existence of upper mantle anisotropybeneath the Iberian Peninsula based on the analysisof SKS and similar phases at several broadband sta-tions of the GEOSCOPE network and the NARS array.Their data for Europe included that from the two sta-tions in central Spain and generally showed an E–Wdirection for fast velocity. This confirmed the directionpreviously found in central Spain bySilver and Chan(1988). However,Maupin and Cara (1992)used sur-face wave data and found no evidence of large scaleanisotropy in the uppermost 100 km, indicating thatthe anisotropy observed in the shear waves could berelated to deeper levels. Later,Badal et al. (1993)andCorchete et al. (1993)obtained a lateral variation invelocity at five depth intervals based on a detailedanalysis of Rayleigh wave dispersion. In the sameyear, the models derived from the interpretation ofdata recorded by the Iberian Lithosphere Heterogeneityand Anisotropy experiment (ILIHA DSS Group, 1993;Dıaz et al., 1993) showed the vertical heterogeneityof the lower lithosphere and azimuthal anisotropy atdifferent depth levels. However, the anisotropy orien-tation deduced from these data is different from thato -c f ver-t vertt turea them rop-p esea rvedb andt s re-g allell kedt eol-o omt entt a-t sis

f olivine crystals. This preferred orientation coave several causes. It may be related to passive

ion of the lithosphere over the stationary asthephere, in which case, the most likely locationnisotropy would be in the transition zone between

wo shells and the fast direction of anisotropy woe close to that of plate motion (Leven et al., 1981).lternatively, the preferred orientation could beuced by flow in the asthenosphere, so the fast d

ion of anisotropy would coincide with the directif flow but might differ from the direction of plaotion (Tanimoto and Anderson, 1984). Azimuthalnisotropy in the uppermost mantle may also be

ated to past and present deformation of the lithospFuchs, 1983). Our results suggest that anisotropyhe southern half of the Iberian Peninsula may rerom a range of causes and could occur at diffeepths.

btained byVinnik et al. (1989). To explain this disrepancy the authors suggested the existence oical heterogeneity in the anisotropic properties ohe mantle. Later,Abalos and Dıaz Cusı (1995)pos-ulated a correlation between the lithospheric strucnd seismic anisotropy from geophysical data andajor structures and tectonic history of the outcing basement rocks in SW Iberia. According to thuthors, the deviations and aerial variations obseetween the azimuthal anisotropy of seismic waves

he structural trends of the accessible crust in thiion indicate that the tectonic model of orogen-par

ithospheric deformation does not appear to be lino Hercynian surface geology and the structural ggy of the subcontinental lithospheric mantle. Fr

hese data, Abalos and Dıaz reconstructed a coherectonic model of polyorogenic lithospheric deformion in the southwestern Iberian massif (ibid.). Analy


of SKS and S phases recorded by a portable networkinstalled in the same area of SW Iberia as that exploredin the ILIHA DSS profiles (Dıaz et al., 1996) suggestsa quite consistent anisotropic structure with roughlya NE–SW to E–W fast velocity direction. The pres-ence of anisotropy beneath the whole Iberian Peninsulawas later established through the analysis of teleseismicshear-wave splitting observed in the broadband stationslocated over the entire peninsula (Dıaz et al., 1998).This work was extremely significant as it provided thefirst anisotropy constraints beneath different tectonicdomains of the Iberian Peninsula from a homogeneousseismic network. The authors postulate that regionalvariations in the anisotropic parameters imply that dif-ferentiated origins of the anisotropy have to be consid-ered in some areas in relation to their particular litho-spheric geodynamics. For example, within the BeticCordillera, anisotropic results for the South Iberian do-main contrast sharply with those for the Alboran crustaldomain.

Lastly, an inversion ofPn travel-time residualswas performed byCalvert et al. (2000)along the

Africa–Iberia plate boundary zone using a code devel-oped byHearn (1996)which solves for isotropic andanisotropic components of the mantle velocity struc-ture. There is a strong correlation between results ob-tained in the central and western Betic Cordillera andthose obtained in southernmost Iberia. The imagedfast axes have a predominant E–W trend in the Bet-ics that becomes more NW–SW in the western internalzones before trending almost N–S beneath the Straitsof Gibraltar.

2. Data set and methodology

The data used in this study arePn waveformsrecorded by several networks situated in southern Spainand northern Morocco. From theAndalusian SeismicNetwork(RSA), which is operated by theInstituto An-daluz de Geof´ısica (IAG), we have collected data onearthquakes from 1988 to March 2003. And, theRealObservatorio de la Armada(ROA) in San Fernando(Cadiz) and theInstituto Geogr´afico Nacional(IGN)

F c statio belongingt

ig. 2. Distribution of seismicity for the study period and seismio Spanish and Moroccan institutions.

ns located in the area. We used data from a total of 121 stations


have kindly provided data from 1988 to the present.Coverage in the African continent has been obtainedthanks to the collaboration of theCentre National deCoordination et de Planification de la Recherche Sci-entifique et Technique (CNCPRST)and thePhysiquedu Globe at Mohamed V University(MOH V), bothin Rabat (Morocco). In order to take advantage ofthe station distribution (Fig. 2) and provide addi-tional data coverage in the Atlantic coast and Alge-rian area, those farthest from the Spanish seismic sta-tions, we drew on data from theISC catalogue. Weused data from 121 stations in total, but a consid-erable imbalance exists between the number of sta-tions on the Iberian Peninsula and those on the Africancontinent.

We definePnas the first arrival from regional eventsbetween 200 and 1200 km. We do not keep phases fordistances under 200 km to avoid misidentification er-rors. Data were selected with the following criteria: in-version only uses events with over 10 recorded arrivals,stations with over 10 recorded arrivals, and event depthless than 30 km. From our initial set of 39,872 travel-times (7675 earthquakes) a total of 11,612Pn first ar-rival times met our selection criteria. Some 73% of theinitial data set are recorded at an epicentral distancefrom 200 to 500 km and about 27% are at an epicentraldistance of between 500 and 1200 km. Arrivals withresiduals of over 4 s were eliminated (Fig. 3).

F akesr . Ar-r

The final set of ray paths for the selected data isshown inFig. 4. The ray path density is highest in thesouthern Iberian Peninsula, offshore from Cape SanVicente and in the Alboran Sea.

The travel-times collected were inverted in the sameway that was used byHearn (1996). ThePn travel-time residuals are described as the sum of three timeterms:

tij = ai + bj +∑

dijk(sk + Ak cos 2φ + Bk sin 2φ)

whereφ is the back azimuth angle,ai the static de-lay for station i, bj the static delay for eventj, dijkthe distance travelled by rayij in mantle cellk, andsk the slowness perturbation (inverse velocity) of cellk. Ak andBk are the anisotropic coefficients for cellk.(Hearn, 1984). The unknown quantities in this equa-tion are the station and event delaysai and bi , themantle slowness perturbationsk, and the two coef-ficients of anisotropyAk andBk. The magnitude of

the anisotropy for cellk is given by (A2k + B2

k)1/2

andthe direction of fastest wave propagation is given by(1/2)arctan(Bk/Ak). The tomographic method used isa preconditioned version ofPaige & Saunders’ LSQRalgorithm (1982). In solving the set of travel-time equa-tions the cell size used is small and a set of Laplaciandamping equations regularize the solution (Lees andCrosson, 1989). Two damping constants are separatelyapplied to the unknown slowness and anisotropic co-efficients. A proper pair of damping constants is cho-s idth.T ari-a mbi-n anda citya o bec m-b stan-d onso and0 howt rsep

loc-i atasw cella

ig. 3. Initial travel-time residuals. We selected the earthquecorded at an epicentral distance of between 200 and 1200 kmivals with residuals of over 4 s were eliminated.

en to balance the error size and the resolution whe trade-off between velocity and anisotropy vtions has been checked by using different coations of damping parameters for both velocitynisotropy. From our data the features of the velond anisotropy fields in the study are observed tonsiderably stable, varying slightly for different coinations of the damping constants; the estimatedard errors of the data for 16 different combinatif the damping parameters range between 0.52.56 s. The small differences in standard errors s

hat all results are equally valid solutions for the inveroblem.

The final damping constants were 100 for the vety and 500 for the anisotropy, giving an estimated dtandard error of 0.54 s, with a cell size of 0.25◦ × 0.25◦hich provides a significant number of hits in eachnd balances uncertainty and resolution.


Fig. 4. Ray paths for the 11,612 travel-times used in the inversion. Only the mantle portions of the ray paths are shown. We can see that bettercoverage is obtained in the south of the Iberian Peninsula.

3. Results

3.1. Resolution

Evaluations of the resolution width are made by us-ing four test models. All models consisted of a checker-board velocity pattern, with the first model having asquare size of 3◦ by 3◦ and the second, third and fourthmodels having a square size of 2◦ by 2◦, 1◦ by 1◦and 3/4◦ by 3/4◦. They were constructed from a syn-thetic data set computed using the ray paths from ourdatabase. The synthetic data was then inverted in thesame way as the real data. The squares were assignedvelocities that alternated between +0.3 and−0.3 km/s.Station and event delays were set to zero and no noisewas included. For the first model, the 2◦ by 2◦ squareswere resolvable for the entire imaged region, exceptsouthwest of the Balearic Islands. Squares as small as1◦ were resolved for the whole of the Iberian Penin-sula studied, the Alboran Sea and offshore from CapeSan Vicente, and were not well resolved in northern

Africa and the southwestern Balearic Islands. In thethird model, 3/4◦ by 3/4◦ squares were resolved in thesouth of the Iberian Peninsula and the western Albo-ran Sea. Results for the second (2◦ by 2◦), third (1◦by 1◦) and fourth models (3/4◦ by 3/4◦) are shownin Fig. 5.

3.2. Station delays

Station delays represent variations in crustalthickness and velocity relative to an initial model andthey are the most robust portions of the inversion. Fora mean crustal velocity of 6.2 km/s and a thicknessof 33 km, a station delay of 1 s. corresponds to achange of 9.8 km in thickness or a change of 0.7 km/sin mean crustal velocity. However, it is difficult todistinguish between the crustal velocity and the crustalthickness contributions in the crustal delay times. Ourresults show some domains, though in general slightdelays were obtained at most of the seismic stations.No delays or weakly positive delays (0.2–0.5 s)


Fig. 5. Checkerboard test model with 2◦ by 2◦, 1◦ by 1◦ and 3/4◦ by 3/4◦ cell size.


were obtained in almost all the stations in the BeticCordilleras, which is probably a consequence of an un-derestimated Moho depth beneath this area (the Mohobeing modelled by a surface at 33 km depth). Theexceptions were found in the easternmost station of theBetic Cordillera (near Cabo de Gata, western AlboranSea) and in the southeasternmost part of the Straits ofGibraltar, which showed negative delays (near 0.5 s).These negative anomalies can be interpreted as aneffect of the variations in crustal thickness, whichranges from 36 km underneath the Betic and Rif Chainto <12 km beneath the easternmost part of the AlboranSea. In the western Alboran Sea the Moho lies at aconstant depth of±18 km, deepening sharply towardthe Gibraltar Strait where it reaches 30–32 km. Thenegative delays (nearly 0.5 s) in northern Africa couldbe explained in the same way. On the other hand, tothe north and northwest the negative delays (from 0.2to 0.5 s) could reflect the moderate crustal thinningtoward the Iberian massif and/or would be an effect ofthe old and stable Hercynian crust (Judenherc et al.,

1999). One of the longest negative delays (>0.5 s) wasobtained at the station on the south Portuguese coast(near Faro). Crustal thickness from the GuadalquivirBasin/Iberian massif contact to the southeastern SPZ,shows gradual lateral changes (Gonzalez et al., 1998a,1998b). However, near this area the same authorsobtained a high apparent crustal velocity (6.4 km/s)that they interpret as a deeper level of the upper crustrelated to ultramafic rock outcropping. This negativedelay is probably due to a combination of a thinnerthan average crust and an increase in mean crustalvelocity.

3.3. Uppermost mantle velocities

The travel-time data were fitted to a straight line as afunction of distance to determine the meanPnvelocityfrom the inverse slope. The straight line fit gave an es-timated velocity of 7.98 km/s and an intercept of 7.1 s.Lateral variations ofPnvelocity are imaged as pertur-bations from this average velocity. The tomographic

F aturate av

ig. 6. Inversion results forPn velocity. Red and blue indicate saries from 7.7 to 8.3 km/s.

d for low and high velocity, respectively.Pn velocity in the study are


inversion used 25 LSQR iterations which reduced theroot-mean-square error of the residuals to 0.5 s. ThePnvelocity varies in the study area from 7.7 to 8.3 km/s.,showing a low velocity zone parallel to the Moroccancoast on the African continent (Fig. 6). On the south-east coast of Spain, the low velocity zone coincides withthe Betic Cordillera and extends towards the BalearicIslands. An extensive high-velocity zone spreads onthe Iberian massif and offshore from the Moroccan At-lantic coast. We cannot determine the velocity beneaththe Rif Codillera with great accuracy due to the poorcoverage of the ray paths. The Alboran Sea is imaged asa region of high velocity in the north and low velocityin the south.

Southwest from Cape San Vicente, a strong low-velocity anomaly is imaged that contrasts with themean values obtained onshore on the Iberian Peninsula.The low values reach 7.75 km/s offshore and spreadacross a wide area off Cape San Vicente. Otherwise,on the boundary between the Iberian Meseta and the

Guadalquivir Basin (Fig. 1) the refractedPnwaves inthe upper mantle show a mean velocity of 8.0 km/s.

3.4. Seismic anisotropy

The map ofPnanisotropy shows considerable vari-ation in both magnitude and direction beneath the studyarea (Figs. 7 and 8). The magnitude of anisotropyranges with maximum values of near±0.40 km/s,and only cells with more than 10 arrivals are plotted.Anisotropy is half the total anisotropic velocity varia-tion in kilometres per second and eachPn anisotropyestimate represents the average anisotropy in a regionof 1◦. Features much smaller than this do not appear inFig. 7, which shows a range of between 0 and 4%.

In the Alboran Sea and northern Morocco,anisotropy is oriented almost parallel to the Africa–Eurasia plate convergence vector which trendsNW–SW from the latest Tortonian to the present day(Dewey et al., 1989, etc.). However, the anisotropy in

F city is d tropyi 4% in c results ofD btaine the nullm

ig. 7. Pnanisotropy in the study area. The fast direction ofPnvelon that direction. The magnitude of anisotropy ranges from 0 toıaz et al. (1998); white arrows show the fast velocity direction oeasurements.

rawn, and the bar length is proportional to the amount of anisothe study area. Bottom right, we have inserted the anisotropi

d and are proportional to the time delay; thin black arrows show


Fig. 8. Image of the magnitude ofPnanisotropy. Darker shades correspond to more anisotropy.

S–SE Spain shows a rapid shift in the orientation of thefast direction, running almost parallel to the main trendof the Betic Cordillera: E–W in its central part and N–Sin the curvature of the Arc of Gibraltar. The most impor-tant values, about 0.4 s are obtained in this last region.The present-day setting of the Mediterranean regionis characterized by Neogene subsident basins like theAlboran Sea, surrounded by mountain belts like theBetic and Rif Cordilleras; only remaining Mesozoicoceanic crust is being subducted below the Calabrianand Hellenic arcs. The most recent studies and inter-pretations indicate a significant increase in the absolutevelocity of Africa at 30 Ma and, a significant decreaseafter 20 Ma. The absolute motion of Africa towardthe north is, however, always faster than the conver-gence velocity because the absolute motion of Eurasiais also northward with an average velocity of∼1 cm/yr(Jolivet and Faccenna, 2000). In addition, several au-thors have described the slowing down of Africa asa consequence of the collision with Eurasia (Barley,1992; Burke, 1996). So, if the olivine orientation is di-

rectly related to mantle strain in the Mediterranean re-gion, the resulting anisotropy would be parallel to thedirection of maximum compression, as is happeningin the Alboran Sea where it must be assumed that theNeogene extension had no major influence, on a litho-spheric scale. However, in the uppermost mantle be-neath the Betic Cordillera the pattern ofPnanisotropyappears to be strongly related to the regional tectonictrend, following the arc-structure of the belt. Duringthe deformation process the fast axes of upper mantleanisotropic minerals tend to align with the longest axisof the ellipsoid strain, which is perpendicular to the di-rection of maximum compression in the pure shear de-formation regime (McKenzie, 1979, etc.). Meanwhile,the slow axis tends to align parallel with the directionof maximum compression (Babuska and Cara, 1991;Silver, 1996). In orogenic belts, where pure shear de-formations are likely to occur, the fastest direction ofPnvelocity would thus be perpendicular to the direction oforogenic compression (Silver and Chan, 1988). There-fore the anisotropy fast orientation could be aligned


along the Betic arc in response to regional compres-sion across the arc. Many other convergent margins inthe Mediterranean region, such as the Alps, the Apen-nines, and the Dinaric-Hellenic mountain chains ex-hibit exceptionally high arc-parallel anisotropy (Mele,1998; Hearn, 1999). Pnanisotropy coincides with thedirection of the Neogene and the present-day extensionbeneath the central Betic Cordilleras (for example, theGranada Basin), suggesting that this deformation mighthave controlled a preferred orientation of mineral in theuppermost mantle.

However, contrary to what would be expected, theparallelism between the azimuths of seismic anisotropyin the southwestern Iberian lithospheric mantle and ma-jor Hercynian tectonic trends is not clear. In the OssaMorena and SPZ the direction of anisotropic fast-Pn-velocity trends is NE–SW, very much like those ob-tained byDıaz et al. (1993), while Hercynian struc-tures in the central and southern Ossa-Morena trendNW–SE to E–W. The present day stress state at OMZtrending N150◦E is almost at an oblique angle to thefast propagation direction obtained. Moreover, the ab-solute plate motion rate of the Iberian plate is verysmall to be consistent with the results of the anisotropy(Abalos and Dıaz Cusı, 1995). Finally, the trends ofthe last episode (Hercynian) of internal coherent defor-mation also differ from the direction obtained. Otherprocesses should be considered in order to explain theanisotropy pattern found in the southwestern Iberianmassif. It may even be the case that the mantle strainp d in-tC

4

thev parto rianP tionso odca a-t tec-t ingi herP n-

tral and western Iberian Peninsula is covered by theVariscan Iberian massif, a large, old and geologicallystable block of continental lithosphere (Dallmeyer andMartınez Garcıa, 1992). This area is characterized byhighPnvelocity, as expected in tectonically stable re-gions, emphasizing the fact that they are areas that havenot recently been reactivated. However, the marginsof the Iberian Peninsula have undergone a number ofmore recent tectonic events, like the Neogene riftingof the Valencia Trough to the east and the MioceneBetic orogeny and Neogene Alboran Sea extension tothe south (Vegas and Banda, 1982). Beneath these tec-tonically active areas, a pronouncedly lowPnvelocity,with values down to 7.7 km/s is found. Our model mightshow the Betic crustal root is underlain by a negativeanomaly beneath the southeastern Iberian Peninsula.The Alboran Sea is imaged as a region of high ve-locity in the north and low velocity in the south. Theresults of one of the refraction profiles conducted inthe southern central part of the Alboran Sea (WorkingGroup for Deep Seismic Sounding in the Alboran Sea,1978) showed anomalously low upper mantle velocityof 7.5 km/s, coinciding with the low values obtained inthis study. However, the results of the profiles carriedout in the western Alboran Sea showed high velocities(about 8.4 km/s) near to the Straits of Gibraltar. On theother hand, in the Atlantic Ocean we find one of themost interesting outcomes of this study: the sharp vari-ation in the uppermost mantle velocities in the west-ern Straits of Gibraltar, coinciding with the structuralc nd-a hati eo-d ncep en in-fl pro-c olli-s r arc,t entes ilei ngt trongl ureo -f kmN tinu-o lesst . In

attern has been obscured by the polyorogenic anricate tectonic evolution of the zone (Abalos and Dıazusı, 1995).

. Discussion and conclusions

A principal result of this study has been to inferelocity and anisotropy structure in the uppermostf the mantle beneath the southern part of the Ibeeninsula and surrounding regions. Regional variaf Pn velocity throughout the study area show goorrelation with surface tectonics. According toHearnnd Ni (2001), highPnvelocity regions are stable cr

onic areas which were not greatly affected byonic and magmatic activity for a long time resultn a lower temperature in the mantle lid and hign velocity. In Fig. 6, an extensive area of the ce

omplexity of the European and African plate boury in the Gulf of Cadiz. Available results show t

ts complexity is probably due to the location and gynamic evolution of the western Straits. Convergerocesses along the plate boundary could have beuenced by passive continental margin formationesses to the north, and plate collision and post cion processes that occurred across the Gibraltao the east. Our results offshore from Cape San Vichow a very pronounced low-velocity anomaly, whn the Gulf of Cadiz high velocities are distributed alohe coast. Southwest from Cape San Vicente, a sow-velocity anomaly is imaged. The crustal structbtained byGonzalez et al. (1996)using seismic re

raction and wide-angle reflection data along a 350E–SW oriented transect, reveals a strong but conus crustal thinning from 30 km onshore Iberia to

han 15 km at the southwestern end of the profile


addition, they performed 2D gravity modelling to val-idate the crustal structure obtained from seismic data.According to the authors, away from the continentalslope, the thickness of the crust reaches 14 km (in-cluding water depth) and the upper mantle velocity is7.8–7.9 km/s. This velocity anomaly agrees with thelow values (<7.75 km/s) obtained offshore from CapeSan Vicente. Otherwise, according toGonzalez et al.(1998), on the boundary between the Iberian Mesetaand the Guadalquivir Basin (Fig. 1) the refractedPnwaves in the upper mantle emerge as a first arrivalat source–receiver ranges of 120 km with an appar-ent velocity of 8.0 km/s. In the profiles situated in SPZ(Fig. 1), the upper mantle velocity of 8.0 km/s is derivedfrom PMP amplitudes, critical distance and somePnarrivals. In both cases, data agree with our own meanPnvelocities for these areas.

On the other hand,Calvert et al. (2000)use two datasets of 430 and 530 events to solve for isotropic andanisotropic components of the mantle velocity struc-ture along the Africa–Iberia plate boundary zone. De-spite having less data, in the common areas the resultsare very similar to our own. The most significant ve-locity feature was a robust low-Pn-velocity anomalyimaged beneath the internal Betic and high velocitiesimaged beneath the Straits of Gibraltar, Iberian Mesetaand Gulf of Cadiz.

We have found that seismic anisotropy is an impor-tant feature in the study area. Two major anisotropicdomains, characterized byPn velocity anisotropy upt nds andni latec , thisd theB int thata eeni thea , int par-a erei ad nom

mica h ana ieved

we have been unable to reach definitive conclusions.Our results show a quite complex pattern of anisotropysince the relationship between the azimuths of seismicanisotropy in the SW Iberian lithospheric mantle andmajor Hercynian tectonic trends could not be directlyestablished. In the OMZ,Dıaz et al. (1998)analyzedteleseismic shear-wave splitting and obtained a fast ve-locity direction oriented N40◦E which is nearly iden-tical to that found in our study.Abalos and Dıaz Cusı(1995)have argued that the Variscan orogeny may nothave affected the whole OMZ, although there is evi-dence that the Cadomian orogeny affected the entirelithosphere; the Cadomian stretching direction is ori-ented N30◦E in proximity to the fast velocity directioninferred from the present data.

Acknowledgements

This work has been supported by Spanish DGIproject REN2002-04198-C02-01 by UE-FEDER andby the research group RNM104 of Junta de Andalucıa(Spain). The authors thanks the reviewers the valuableadvices and recommends to improve this paper.

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