Tectonophysics 478 (2009) 175–183

Contents lists available at ScienceDirect

Tectonophysics

j ourna l homepage: www.e lsev ie r.com/ locate / tecto

Crustal structure beneath North-West Iberia imaged using receiver functions

J. Díaz a,⁎, J. Gallart a, J.A. Pulgar b, M. Ruiz a, D. Pedreira b

a Institute of Earth Sciences J. Almera, CSIC, Barcelona, Spainb Department of Geology, Univ. of Oviedo, Spain

⁎ Corresponding author. Tel.: +34 93 409 5410.E-mail address: jdiaz@ictja.csic.es (J. Díaz).

0040-1951/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tecto.2009.08.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 March 2009Received in revised form 31 July 2009Accepted 3 August 2009Available online 11 August 2009

Keywords:Crustal structureNorth-West IberiaReceiver functionsVariscan and Alpine orogenies

In the last years, the deep crustal structure of the North-Western part of the Iberian Peninsula has beenexplored using teleseismic receiver function (RF) analysis of P to S conversions at main crustal interfaces.This area has been previously investigated by seismic reflection and refraction experiments and thereforeprovides an excellent opportunity to compare the results of both approaches. The region shows the imprintof two orogenic events, the Variscan and Alpine ones that exhibit different, reverse intensities from west toeast. In a first stage, a N–S transect across the Cantabrian Mountains was instrumented to study the areaaffected by the Alpine compressional tectonics. Later on, the limit between the undisturbed Variscan unitsand the reworked Alpine zones was explored by N–S and E–W transects. Finally, an array was deployed inthe NW edge of Iberia, over the Variscan hinterland zone. The receiver functions are calculated by inversefiltering deconvolution of the L component from the Q component, and the resulting RF are processed using asimple form of migration to obtain images in depth of the lithosphere that can be compared to the 2-Dvelocity–depth models from active seismic experiments. The deep crustal structure constrained by bothtechniques is remarkably consistent, and provides further evidence on the crustal doubling and wedgingbetween the Iberian and European crusts throughout the northern part of the Iberian Peninsula affected bythe Alpine tectonics. The undisturbed Variscan domains, characterized by a clear subhorizontal Moho andfew intracrustal convertors, show up at the southern edge of the N–S transects (southward of the Alpinedeformation front) and over the Variscan hinterland in the E–W transect. The crust resulting from Alpinereworking presents a complex structure, with short and frequently dipping convertors, which in some casesseem to image preserved Variscan structures.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Active and passive seismic surveys such as multichannel reflection/refraction profiles and teleseismic receiver function (RF) analysisprovide independent and complementary results on lithosphericstructure. Multichannel vertical reflection profiles and RF pseudo-migrated sections constrain the geometry of the seismic discontinuitieswithin the crust, the former using rays coming from the top and thelatter rays reaching the discontinuity from below. The frequencycontent of the seismic signals used in each case is significantly different(frequencies of 10 Hz and above for active seismic experiments,dominant periods of few seconds for teleseismic events in RF surveys).This fact results in large differences in the vertical resolution, sig-nificantly lower for RF surveys.Wide-angle reflection/refraction profilesprovide detailed information about the velocity–depth structure of thecrust. 1-D inversion of RFs can also constrain the S-wave velocitystructure, even if their uncertainty is often very large. On the other hand,the cost of a RF survey appears much advantageous with respect to the

l rights reserved.

active experiments. Combining information from active and passivesurveys sampling a same area will certainly improve the structuralconstraints on its geodynamic evolution. One of the scarce regionswhere such comparison of both kinds of datasets and results is alreadyfeasible is the Northern part of the Iberian Peninsula.

Following the ECORS seismic surveys carried out across thePyrenees in the 80's, extensive seismic research from active methodshas been performed in North Iberia. The ESCIN seismic reflectionprogram, with inland and marine deep vertical seismic profiles,provided the first crustal images beneath the Cantabrian Mountainsand its associated continental margin (Pulgar et al., 1996; Álvarez-Marrón et al., 1996). Complementary onshore–offshore wide-anglemeasurements, as well as two consecutive networks of refractionprofiles using land shots gave accurate velocity–depth sections alongN–S and E–W transects (Fernández-Viejo et al., 1998, 2000; Pedreiraet al., 2003). Previously, the Galicia region was also explored by wide-angle profiling (Córdoba et al., 1987, 1988; Téllez et al., 1993). Morerecently, the Spanish MARCONI project investigated the deep struc-ture of the eastern half of the Bay of Biscay using both multichannelseismic profiles and wide-angle data recorded at OBS and land sta-tions to constrain the velocity structure of the area (Ruiz, 2007; Ferreret al., 2008).

176 J. Díaz et al. / Tectonophysics 478 (2009) 175–183

177J. Díaz et al. / Tectonophysics 478 (2009) 175–183

During the last decade, the deep crustal configuration of NorthIberia (Fig. 1) has also been explored using teleseismic receiverfunction (RF) analysis of P to S conversions at main crustal interfaces,in the framework of different Spanish research projects that havecollected a large amount of passive seismic data from temporary arraydeployments. In a first stage, a N–S transect was deployed across theCantabrian Mountains (Díaz et al., 2003), a region where the Variscancrust was extensively reworked during the Alpine orogeny (Pulgaret al., 1996; Pedreira et al., 2003). In this contribution we will discussthose results together with new data gathered in later deploymentscovering the north-westernmost part of Iberia (Fig. 1), which willprovide an image of the transition at depth between the Alpine andVariscan domains.

From fall 2002 to spring 2003, portable seismic stations weredeployed along a N–S transect within the West Asturian–Leonese Zoneof the Iberian VariscanMassif (a transitional zone between the forelandfold and thrust belt, that is, the Cantabrian Zone, and the most internalparts of the Variscan orogen). This transect approximately marks thelimit between the almost undisturbed Variscan units to the west, andthe zones uplifted during the Alpine orogeny to the east (Fernández-Viejo et al., 2000). An E–W transect was also implemented to connectboth domains. Finally, within a passive seismic experiment mainlyfocused to monitor local seismicity, a network was installed for a fewmonths in the Central Iberian and Galicia–Tras-os-Montes Zones, in theNW edge of the Iberian Peninsula (Díaz et al., 2007). These laterdeployments cover the most internal parts of the Variscan belt of NorthIberia, where there is no evidence of major Alpine reworking.

2. Tectonic setting

The study area experienced a complex tectonic history, beingaffected by the Variscan and Alpine compressional events, separatedby a large extensional episode during the Mesozoic. The Variscanorogeny started with the closure of the Rheic Ocean and the collisionbetween Laurentia–Baltica–Avalonia and the continental margin ofGondwana during the Carboniferous, giving rise to the building of thePangea supercontinent (Matte, 1991).

The Iberian Massif is a large outcrop of Precambrian and Paleozoicrocks that extends over most of the western half of the IberianPeninsula and constitutes one of the best exposed sections of theVariscan belt in Europe, with a well established zonation based onstructural, metamorphic and paleogeographic differences (Julivertet al., 1972; Farias et al., 1987). A complete section across the branchof this belt located in the footwall of the suture can be traced in thenorthern part of Iberia (Fig. 1B). The Cantabrian Zone corresponds tothe foreland thrust and fold belt and is characterized by thin-skinnedtectonics and a rather tight arcuate trend with a general transportdirection towards the east (Pérez-Estaún et al., 1988). The hinterlandzones are located immediately to theWest and its first unit is theWestAsturian–Leonese Zone (WALZ), interpreted as a transitional area,with westward increasing metamorphism, magmatism and internaldeformation (Martínez-Catalán et al., 1990). Finally, the most internalpart of the orogen is located in the western end of northern Iberia andis divided into the Central Iberian Zone (CIZ) and the Galicia–Tras-os-Montes Zone (GTMZ), corresponding respectively to autochthonousunits and a group of para-autochthonous and allochthonous units.The Variscan deformation ceased in this area at the end of theCarboniferous, and the Stephanian (Upper Pennsylvanian) beds areessentially post-tectonic, lying unconformably over the older Paleo-

Fig. 1. A) Simplified topographic and tectonic map of North Iberia, highlighting the effect ofcross sections reporting different amounts of Alpine shortening: A-A′, 125 km (Vergés et al., 11998); D-D′, 86 km (Pedreira, 2005); E-E′, 96 km (Gallastegui, 2000). BCB, Basque–CantabriaCatalan Coastal Ranges. B) Detail of the Variscan geology in the NW corner of Iberia, showdeployments (stars). C) Map of the MCS and wide-angle reflection/refraction profiles prevonshore–offshore active seismic experiments.

zoic and Precambrian rocks (Fig. 1B). By Permian times, the orogenicbuilding was already deeply eroded and the sedimentation wasmainly controlled by the development of normal faults and grabens ina new distensive regime.

During the Mesozoic, the situation evolved to a rifting stage inwhich several basins were individualized at the borders and in theinterior of the Iberian Peninsula. It was the beginning of the Alpinecycle, related to the divergence between the European and Americancontinental blocks, preluding the opening of the Atlantic Ocean andthe Bay of Biscay. Oceanization in the Bay of Biscay, started at chronM0 (118 Ma) and lasted until chron A33o (80 Ma) (Sibuet et al.,2004). The opening of the Bay of Biscay resulted in the formation ofthe Armorican and CantabrianMargins, with the development of largesedimentary basins and the individualization of Iberia as a subplate.One of these basins developed at the northern border of the Iberiansubplate is the Basque–Cantabrian Basin, where more than 10 km ofMesozoic sediments were deposited over an extremely thinnedcontinental crust (Pedreira, 2005; García-Mondéjar, 1989, and ref-erences therein).

Later on, from the latest Cretaceous, the convergence betweenIberia and Eurasia produced the inversion of those basins and theuplift of basement blocks and thrust sheets in the contact zone,building the Pyrenean–Cantabrian mountain belt in the framework ofthe Alpine orogeny. The northern branch of the doubly-vergentPyrenean Chain is prolonged along the Cantabrian Margin, while thesouthern branch propagated through the Mesozoic Basque–Cantab-rian Basin to the western termination of the Cantabrian Mountains(Fig. 1A). This continuous, E–W trending mountain belt was upliftedmainly in Eocene to Miocene times. In the Pyrenean part, the cal-culated amount of shortening ranges from 165 km in central Pyrenees(Beaumont et al., 2000) to 75–80 km in the west central Pyrenees(Teixell, 1998), whereas in the eastern and central Cantabrian Moun-tains and continental margin it ranges between 86 and 96 km(Gallastegui, 2000; Gallastegui, et al., 2002; Pedreira, 2005), rapidlyvanishing from the western Cantabrian Mountains to the West.

3. Summary of previous results from active seismic profiles

The northern part of the Iberian Peninsula has been extensivelyinvestigatedby active seismic experiments during the last threedecades(Fig 1C). In the early 80s several wide-angle reflection/refractionprofiles were acquired in the NW corner of Iberia along differentazimuths. The interpretation of these data depicted a 3-layered crust,even if the limit between upper and intermediate levels was oftenunconstrained (Córdoba et al., 1987, 1988; Téllez et al., 1993; Téllez andCórdoba, 1998). The Moho was located at depths close to 31 km,shallowing to 28 kmclose to the coastline. In the northernpart of theCIZ(Galicia), onshore–offshoreprofileshave showed aprogressive thinningof the crust towards the center of the Bay of Biscay (Fernández-Viejoet al., 1998). Close to the coastline, theMoho is overlain by a 6.6–6.9 km/slower crust that produces very energetic reflections.Multichannel seismicprofiles offshore (profile ESCIN-3.2) also show a deep crustal reflector atabout 6s TWT beneath this area on top the Moho interface, which wasidentified at 8s TWT (Ayarza et al., 2004).

Extensive seismic exploration carried out in the nineties overnorthern Iberia has evidenced that the areas affected by the Alpineorogeny show a crustal structure similar to that inferred for thePyrenean Chain, consistently showing crustal indentation and wedgingbetween the Iberian and the European–Cantabrian Margin crusts

the Alpine deformation. Red marks show the location of balanced along-strike crustal995); B-B′, 147–165 km (Muñoz, 1992; Beaumont et al., 2000); C-C′, 75–80 km (Teixell,n Basin (Mesozoic basin presently incorporated to the Pyrenean–Cantabrian belt); CCR,ing the structural zonation and the location of the stations active during the differentiously recorded over the area. Squares and circles stand for OBSs and land stations in

178 J. Díaz et al. / Tectonophysics 478 (2009) 175–183

(Pulgar et al., 1996; Fernández-Viejo et al., 2000; Gallastegui, 2000;Pedreira et al., 2003; 2007). A quasi-continuous crustal root can befollowed along the strike of the Pyrenean–Cantabrian range, with thetop of the Iberian lower crust defined at around 35 km depth and theMoho found at46–48 kmdepth. The lower European crust is located at amuch shallower level, withMoho depths less than 30 km. The presenceof high-velocity bodies embedded at mid-crustal levels has also beeninferred from the seismic data, and they have been interpreted as piecesof the European–Cantabrian Margin lower crust indenting southwardthe Iberian crust (Pedreira et al., 2003, 2007). The onshore–offshoreprofiles acquired in the MARCONI experiment confirm this indentation,allowing to better constraint its geometry (Ruiz, 2007). A significantthinning of the crust to the westernmost part of the Cantabrian Moun-tains, whereMoho depths are about 30 km, depicts the transition to thezones of the Iberian Massif that had not been significantly reworked bythe Alpine orogeny (Fernández-Viejo et al., 2000).

The reflection seismic profile ESCIN-1, oriented E–W beneath theCantabrian Mountains, revealed the Variscan basal detachment ofthe Cantabrian Zone gently dipping westward to reach 16 km depthbeneath the WALZ. Two deep reflectivity bands can also be identifieddipping westward to join a reflective lower crust located between 25and 29 km and forming a crocodile-like structure (Pérez-Estaún et al.,1994; Gallastegui et al., 1997). Offshore, reflection seismic profileESCIN-3.3, running parallel to the NE Galicia coast, also evidence thepresence of subhorizontal, west-dipping intermediate-depth reflec-tions interpreted as Variscan compressional structures. Two highlyreflective subhorizontal bands at 7–9 and 11–12s TWT have beeninterpreted as imaging two levels of lower crust separated by uppermantle materials (Álvarez-Marrón et al., 1996; Ayarza et al., 1998).

4. Data acquisition

Here, we analyze data acquired in different experiments carriedout in the northern part of Iberia between 1999 and 2003 (Fig. 1B).The first deployment delineated a N–S transect along the central partof the Cantabrian Mountains (CM), and was active during a 9 monthperiod. This transect was instrumented using six seismic stationsequipped with Lennartz seismometers (Le20s and Le5s), with flatvelocity response electronically broadened up to periods of 20 and 5 srespectively. Two additional transects were deployed later on, onealong a N–S transect located at the West Asturian–Leonese Zone (NS–WALZ), and the second one along an E–W profile connecting theWALZ and the CM transects. In this case a total of 12 stations, all ofthem equipped with Le20s seismometers, were installed for a periodof about 9 months between summer 2002 and spring 2003. Finally,the Central Iberian Zone (CIZ) and the Galicia–Tras-os-Montes Zone(GTMZ) at the NW edge of Iberia were investigated by a network of8 stations with Le20s seismometers for a period of 5 months duringsummer 2003.

Teleseismic events with epicentral distances between 35° and 95°and clear P arrivals were selected to analyze P to S conversions at maincrustal interfaces by means of the RF technique. Following themethoddescribed by Kosarev et al. (1999), the records are rotated to the raycomponents (L, Q, T) using back azimuth and incidence angle tominimize energy on the radial and transverse components for the Parrival. The receiver functions (RF) are then calculated by inverse filterdeconvolution of the L component from the Q component. In order toget structural information readily comparable to the wide-angle re-sults, the resulting RFs are treated similarly to crustal reflection data,using a Common Conversion Point (CCP) procedure to obtain imagesof the lithosphere in depth domain. The RF from different stations andevents are traced back along its raypath using a 1-D velocity model,the amplitudes contributing to each box of the model are stacked andthe data are projected along a profile. Therefore, the amplitude con-trasts in the final images depict the zones where incoming wavessuffer P to S conversions. 1-D velocity–depth models have also been

obtained using classical RF inversion procedures (Kind et al., 1995)for selected stations to further constrain the interpretation of theobserved convertors.

The azimuthal coverage provided by the teleseismic events finallyretained extends from N140W to N90E, which means that only onequadrant is missing. The number of useful events varies for eachdeployment. For the EW, CMZ andWALZ transects, data from 30 to 45events were used, even if the typical number of events per station isbetween 15–20 and only few events can be used in specific stations.For the Galicia deployment, which only lasted 5 months, 8 usefulevents have been retained for interpretation.

5. Receiver functions analysis

5.1. Central Iberian and Galicia–Tras-os-Montes Zones deployment

The northern part of the CIZ and the GTMZ was covered by anetwork of 8 stations that has allowed to build up two parallel N–Stransects, even if the overlap between stations is rather loose. In bothcases the crust has a simple structure, with a well defined sub-horizontal Moho close to 30 km that thins northwards to reach 28 kmnear the shoreline (Fig. 2). In the eastern transect (Fig. 2B) the pre-sence of an intracrustal convertor in the 15–18 km depth range isevidenced, although it is difficult to be correlated along the profile.Wide-angle modeling of line IAM12 (coincident with our easternsection) shows a crustal discontinuity at about 12 km depth and ahighly reflective lower crust, located between 22 and 28 km depthand overlaying the Moho (Fernández-Viejo et al., 1998). The marinereflection profile ESCIN-3.2 (oriented SW–NE, see Fig. 1C) shows a 1–2s thick band of reflectivity at 8–9s TWT, corresponding either to thelower crust or to the crust/mantle boundary (Álvarez-Marrón et al.,1996). Another reflector gently dipping towards the southwest isidentified at 5–7s TWT in that profile, and has been attributed toVariscan features. This reflector can be correlated with our intracrus-tal convertor.

5.2. West Asturian–Leonese N/S transect (WALZ)

This transect shows a very clear difference between its southernand northern parts (Fig. 3). To the south, the crustal structure is quitesimple, with a well defined, subhorizontal convertor associated to theMoho discontinuity and located at ~33 km depth. The northern part ofthis transect depicts a rather different, more complex image. At about15 km depth, we identify a convertor interpreted as the limit betweenthe upper and middle crust, in agreement with the available wide-angle models (Córdoba et al., 1988; Fernández-Viejo et al., 2000). Thetransition between crust and upper mantle is in this case also com-plex, marked by a broad convertor extending between 28 and 35 km.Consistently with wide-angle data, we interpret this feature as theunshaped image of the conversions at the top and bottom of arelatively thin lower crust overlaying the Moho. The clear differencebetween the northern and southern parts of the transect reflects theN–S extent of the area affected by Alpine reworking.

5.3. Cantabrian Mountains N/S transect (CMT)

This transect has already been published (Díaz et al., 2003), andhence we do not show here a specific figure. However, it is included inthe 3D diagram presented in Fig. 5D. The transect holds again cleardifferences between its two ends. The southern part, sampling thetransition between the Cantabrian Mountains and the Duero Basin,shows two well defined convertors, located at 15 and 35 km depth.The convertor at 15 km depth is related to the limit between upperand intermediate crust evidenced by seismic profiling in other areas ofthe Iberian Massif (Díaz et al., 1993). The convertor at 35 km depth isassociated to the Moho, and coincides with the bottom of the crust

Fig. 2. Pseudo-migrated crustal sections from RF data along the two transects over the NW Central Iberian and Galicia–Tras-os-Montes Zones; A) Western transect, B) Easterntransect. Contributing stations are indicated by grey triangles. Inverted black triangles show the crossing point of the EW transect. The two transects are aligned in latitude.

Fig. 3. Same as Fig. 2 for the West Asturian–Leonese Zone transect.

179J. Díaz et al. / Tectonophysics 478 (2009) 175–183

interpreted from active seismic profiles (see compilation in Díaz andGallart, 2009). The central part, near the intersection with the E–Wtransect, displays a different pattern, with an upper convertor that hasbeen modeled by a thin high-velocity layer and can be related to thehigh-velocity body evidenced in the E–W and N–S wide-anglereflection/refraction seismic profiles sampling this area. This featurehas been interpreted as a slab of European lower crust indenting theIberian middle crust (Pedreira et al., 2003, 2007). The crust/mantletransition is loosely imaged, with a conversion zone extending from 30to 43 kmdepth. Even if more data is needed to be conclusive, this imagemay suggest a limited subduction of Iberia northwards, in agreementwith the active seismic models. Finally, the northern end of the transectshows some complexity on the uppermost crustal levels. Deeper on,only one convertor at 25 km depth is identified and it can be related tothe top of the Cantabrian Margin (European) lower crust according tothe modeling of an N–S active seismic profile (Fernández-Viejo et al.,1998).

5.4. E/W Transect

One of the main objectives of this work has been to merge datafrom different passive seismic experiments to produce a large West–

Fig. 4. A) EW transect along the whole investigated area. In this case, the inverted black dots show the position of the available NS transects. Dotted lines mark the reflectors deducedfrom wide-angle profiles. Labels are discussed in the text. B) Interpretation of the ESCIN-1 seismic reflection profile by Pérez-Estaún et al. (1997) overlapped over the coincidentsection of the E–W receiver function transect. The vertical scale of the seismic reflection profile is in seconds (TWTT). Dotted lines mark the reflectors deduced from wide-angleprofiles. Label “M” in the seismic reflection profile denote the Moho discontinuity; other labels in blue circles denote convertors discussed in the text.

180 J. Díaz et al. / Tectonophysics 478 (2009) 175–183

East lithospheric transect depicting the crustal-scale variations troughthe different Variscan tectonic domains sampled, which have differentdegrees of Alpine reworking. Fig. 4A shows the compiled results forthis 375 km long E/W transect. The first relevant observation is themarked difference in crustal complexity between the stations locatedin the CIZ–GTMZ and those located eastwards. This difference allowsto clearly distinguish between the areas that suffered a major crustalreworking during the Alpine tectonics and those essentially undis-turbed since the Variscan orogeny.

The western section of the transect, running through the CIZ–GTMZ depicts, as in the NS transects previously discussed, a quitesimple structure, with a clearly definedMoho located between 27 and30 km and slightly deepening towards the East (labeled D in Fig. 4A).An intracrustal convertor can be identified at 14 km depth beneathstation COTA and at 18 km depth beneath station OLAS (labeled C inFig. 4A). If both convertors correspond to the same geological feature,this will depict a westward dipping discontinuity at mid-crustallevels, probably of Variscan origin.

In the eastern part, the transect crosses the WALZ and CM, areasstrongly affected by the Alpine orogeny, and the image is muchmore complex, suggesting that both the implicit hypotheses of theRF technique and the modest amount of data do not allow todocument here the present-day structure with enough resolution.However, some main features can be identified and discussed inrelation with active seismic data available in the zone. At upper

crustal levels, the convertor reported at the GTMZ and CIZ can becontinued up to 7°W. Further East, the most prominent reflector,labeled A in Fig. 4A, can be followed from 6.6°W up to the easternend of the profile and interpreted as the uplifted base of the Iberianupper crust, in agreement with the gravimetric modeling of Pedreiraet al. (2007). The B convertor, identified from 6.6°W to 5.8°W with awestward dip and followed discontinuously to the eastern end ofthe transect, can be related to the reflectivity band observed in theESCIN-1 seismic reflection profile, almost overlapping our transect(Fig. 4B). This reflectivity band has been interpreted as the western endof the Cantabrian Zone basal detachment (Pérez-Estaún et al., 1997). Inparticular, the westward dipping convertor observed in the E–Wtransect can be related to the back-limb of the Narcea antiform. Thisconvertor seems to reach the lower crust, forming a “crocodile” typestructure similar to what is observed in the MCS profile ESCIN-1. Thisterm, first proposed by Meissner (1989), is used in vertical reflectionprofiles to describe diverging reflectors that appear as branchingstructures at upper and mid-crustal layers. East of 5.8°W, aneastward-dipping convertor, labeled E in Fig. 4, can be identified from22 km depth at 5.7°W to 30 km depth at the eastern end of the profile.Previous wide-angle and gravimetric modeling have identified in thesame position the base of a lower crustal wedge from the Europeandomain indentedbetween theupper andmiddle Iberian crusts,with thetop of this wedge located immediately beneath the Variscan basaldetachment of the Cantabrian Zone (Pedreira et al, 2003; Pedreira et al,

Fig. 5. Block diagrams of the pseudo-migrated RF transects fromWest (A) to East (D). Continuous black lines show the identified convertors, discussed in the text. Dotted line along the EW transect connect the convertors associated to the Mohodiscontinuity.

181J.D

íazet

al./Tectonophysics

478(2009)

175–183

182 J. Díaz et al. / Tectonophysics 478 (2009) 175–183

2007). The pseudo-migrated RF data seems to confirm the presenceof such indentation (Figs. 4A and B).

The image of the crust/upper mantle transition beneath the WALZand CM sections of the transect is also complex. Coherently with theimage of the NS WALZ transect and the available wide-angle models(Fernández-Viejo et al., 2000), we interpret that the convertor atabout 27 km depth beneath stations MART and PENA corresponds tothe Moho. The converted energy observed at 38 km depth atlongitudes between 7.4°W and 6.9°W can hardly be interpreted ingeotectonic terms. As this convertor appears only from earthquakeswith back azimuths between N246E and N288W reaching the MARTstation, we regard this feature as an artifact due to the complexgeometry of the area. The Moho appears as a single convertor with ageneral east-dipping pattern, from 27 km at 6.9°W to 38 km at 5.8°W(label D in Fig. 4). This image is in agreement with the wide-angleseismicMoho (Pedreira et al., 2003). East of 5.8°W, the signal from theMoho cannot be observed in the RF section, although its position isreasonably well constrained by almost coincident seismic reflectionand refraction profiles (respectively, “M” labeled discontinuous lineand dashed line at the bottom of Fig. 4B). In any case, at the easternedge of the transect the convertor observed at about 40 km can beinterpreted again as the Moho discontinuity (Fig. 4A). At depths of30–35 km, some discontinuous converted energy is observed (labels Fin Fig. 4) and it is interpreted as resulting from conversions at the topof the lower crust. This layer shows relatively high-velocities in thewide-angle modeling, and therefore it could mask the conversions atthe Moho, explaining the lack of continuity of the Moho along thissection of the E–W transect. It must be noted that the convertorslocated beneath 50 km are in fact multiples of the mid crustdiscontinuities, as it can be verified by 1-D velocity–depth modeling.The apparent continuity of this convertor between 6.5°W and theeastern edge of the section is probably the effect of combining poordata resolution with the complex crustal structure in this region.

6. Discussion and conclusions

The compilation and analysis of all the available RF data in thenorthern part of the Iberian Peninsula from different passive seismicdeployments has resulted in an image of the crustal structure which isindependent of the one coming from previously acquired activeseismic data over the same zone. The presented RF data cover the NWcorner of the Iberian Peninsula, from the hinterland of the Variscanorogeny in the West to the areas clearly affected by the Alpineorogeny in the East, allowing to investigate the main differences incrustal structure between them. The use of pseudo-migrated, 2Dreceiver function transects has facilitated a comparison with themodels derived from MCS and wide-angle reflection profiles, and thecorresponding results are remarkably consistent. For a number ofrepresentative stations, we have also derived 1-D velocity–depthmodels which, even if often lack unicity, allow to further constrain theinterpretations. The coincidence of the main crustal structuresdeduced from active and passive techniques, in spite of their differentresolution, provides further support to their interpretation. The limit-ed amount of teleseismic data and the intrinsic hypotheses of the RFmethod make it difficult to obtain detailed images of areas withcomplex geometries, as dipping interfaces or second order disconti-nuities. However, even in those complex areas, RF data can become auseful tool to check the validity of the models derived from activeseismic experiments and to geographically extend the area investi-gated by those methods.

The first order result of this work is the clear difference in thecomplexity degree of the deep structure between the western andsouthern areas of the study area, where a quite simple crustal struc-ture is revealed, and the eastern and northern areas, where a muchmore complex crust is imaged. According to the tectonic history of theregion, those differences must be related to the imprints of the Alpine

orogeny, which affected to different degrees the tectonic units thatform the northern part of Iberia. This fact has already been establishedat surface level by geological studies and has also been evidenced byprevious seismic data, but this work provides further constraints for aregional view of the crustal-scale variations.

To better illustrate those spatial variations in crustal structure, wehave constructed block diagrams by merging the EW and the NSpseudo-migrated transects (Fig. 5). Those diagrams allow under-standing the 3D character of the crustal structure imaged by thereceiver function analysis, especially in the eastern part of the studyarea. To the West, beneath the GTMZ and CIZ, the image is rathersimple, with a well defined conversion associated to the Moho at 28–30 km depth. A mid-crustal convertor is observed in the EW transectdipping westwards and, consistently, in the eastern NS GTMZ transectslightly dipping southwards (Figs. 5A and B). This structure is notevidenced at the western end of the investigated area.

Further East, beneath theWALZ, the pseudo-migrated EW transectallows to identify the root zone of the Cantabrian basal detachment inthe Narcea Antiform (Fig. 5C). The Moho discontinuity shows up as astrong convertor around 40 km depth west of 6.0°W, shallowing tothe west to reach about 30 km beneath the NS WALZ transect. Thecomplexity observed in the northern part of this transect suggest thatsome degree of reworking affecting the whole crust during the Alpineorogeny should exist in this zone.

At the eastern end of our transect, beneath the CantabrianMountains, the image is consistent with the presence of crustalthickening and wedging (Fig. 5D), as suggested previously by activesource models. Instead of a clear Moho in this area, the EW transectshows only sparse mid-crustal conversions probably generated by thetop of the European lower crust, also observed at the northern end ofthe NS transect. The mid-crustal convertor clearly evidenced in bothtransects can be modeled with a high-velocity layer, and it is con-sistent with the interpretation derived from seismic profiling of anindentation of European lower crust in themiddle levels of the Iberiancrust. This indentation forces the northward underthrusting of thelower half of the Iberian crust, in the same sense that is observedelsewhere along the Pyrenean–Cantabrian belt (e.g. Muñoz, 1992;Pedreira et al., 2007).

Acknowledgments

This work was sponsored by Spanish Research Ministry projectsAMB98-1012-C02 and REN2001-1734-C03. M. Ruiz has benefitedfrom a SpanishMinistry “F.P.I.” Ph.D. grant. Figures were created usingGMT (Wessel and Smith, 1998). This is a contribution of the TeamConsolider-Ingenio 2010 TOPO-IBERIA (CSD2006-00041).

References

Álvarez-Marrón, J., Pérez-Estaún, A., Dañobeitia, J.J., Pulgar, J.A., Martínez-Catalán, J.R.,Marcos, A., Bastida, F., Ayarza, P., Aller, J., Gallart, J., González-Lodeiro, F., Banda, E.,Comas, M.C., Córdoba, D., 1996. Seismic structure of the northern continentalmargin of Spain from ESCIN deep seismic profiles. Tectonophysics 264, 153–174.

Ayarza, P., Martínez-Catalán, J.R., Gallart, J., Pulgar, J.A., Dañobeitia, J.J., 1998. ESCIN 3.3:a seismic image of the Variscan crust in the hinterland of the NW Iberian Massif.Tectonics 17 (2), 171–186.

Ayarza, P.,Martínez-Catalán, J.R., Zeyen, H., Juhlin, C., AlvarezMarrón, J., 2004. Geophysicalconstraints on the deep structure of a limited ocean–continent subduction zone at theNorth Iberian Margin. Tectonics 23, TC1010. doi:10.1029/2002TC001487.

Beaumont, C., Muñoz, J.A., Hamilton, J., Fullsack, P., 2000. Factors controlling the Alpineevolution of the central Pyrenees inferred from a comparison of observations andgeodynamical models. J. Geophys. Res. 105 (B4), 8121–8145.

Córdoba, D., Banda, E.and Ansorge, J., (Reporters) 1987. The Hercynian crust in north-western Spain: a seismic survey, Tectonophysics, 132: 321–333.

Córdoba, D., Banda, E., Ansorge, J., 1988. P-wave velocity–depth distribution in theHercynian crust of Northwest Spain. Phys. Earth Planet. Inter. 51, 235–248.

Díaz, J., Gallart, J., Córdoba, D., Senos, L., Matias, L., Suriñach, E., Hirn, A., Maguire, P.,ILIHA DSS Group, 1993. A deep seismic sounding investigation of lithosphericheterogeneity and anisotropy beneath the Iberian Peninsula. Tectonophysics 221,35–51.

183J. Díaz et al. / Tectonophysics 478 (2009) 175–183

Díaz, J., Gallart, J., Pedreira, D., Pulgar, J.A., Ruiz, M., Lopez, C., González-Cortina, J.M.,2003. Teleseismic imaging of alpine crustal underthrusting beneath N-Iberia.Geophys. Res. Let. 30, 11. doi:10.1029/2003GL017073.

Díaz, J., Gallart, J., Gaspà, O., Ruiz, M., Córdoba, D., 2007. Seismicity analysis at the‘Prestige’ oil-tanker wreck area (Galicia Margin, NW of Iberia). Mar. Geol. 249, 1–2150–165 (2008).

Díaz, J., Gallart, J., 2009. Crustal structure beneath the Iberian Peninsula and surroundingwaters: a new compilation of deep seismic sounding results. Phys. Earth Planet. Int.173, 181–190. doi:10.1016/j.pepi.2008.11.008.

Farias, P., Gallastegui, G., González-Lodeiro, F., Marquínez, J., Martín-Parra, L.M.,Martínez-Catalán, J.R., Pablo Maciá, J.G., Rodríguez Fernández, R., 1987. Aporta-ciones al conocimiento de la litoestratigrafía y estructura de Galicia Central. Mem.Fac. Cienc. Porto 1, 411–431.

Fernández-Viejo, G., Gallart, J., Pulgar, J.A., Gallastegui, J., Dañobeitia, J.J., Córdoba, D.,1998. Crustal transition between continental and oceanic domains along the NorthIberian Margin from wide-angle seismic and gravity data. Geophys.Res. Lett. 25,4249–4252.

Fernández-Viejo, G., Gallart, J., Pulgar, J.A., Córdoba, D., Dañobeitia, J.J., 2000. Seismicsignatures of Variscan and Alpine tectonics in NW Iberia: crustal structure of theCantabrian Mountains and Duero basin. J. Geophys. Res. 105, 3001–3018.

Ferrer, O., Roca, E., Benjumea, B.,Muñoz, J.A., Ellouz,N., theMARCONI Team, 2008. Thedeepseismic reflection MARCONI-3 Profile: role of extensional Mesozoic structure duringthe Pyrenean contractional deformation at the eastern part of the Bay of Biscay. Mar.and Pet. Geol. 25, 714–730. doi:10.1016/j.marpetgeo.2008.06.002.

Gallastegui, J., Pulgar, J.A., Alvarez-Marrón, J., 1997. 2-D seismic modeling of theVariscan Foreland Thrust and Fold Belt crust in NW Spain from ESCIN-1 deepseismic reflection data. Tectonophysics 269, 21–32.

Gallastegui, J., 2000. Estructura cortical de la Cordillera yMargen Continental Cantábricos:Perfiles ESCI-N. Trabajos Geol. Univ. Oviedo 22, 9–234.

Gallastegui, J. Pulgar, J. and Gallart, J., 2002. Initiation of an active margin at NorthIberian continent-ocean transition. Tectonics, 21 (4), 15-1 a 15-14.

García-Mondéjar, J. 1989. Strike-slip subsidence of the Basque–Cantabrian basin ofnorthern Spain and its relationship to Aptian–Albian opening of Bay of Biscay. In:Extensional tectonics and stratigraphy of the North Atlantic margins (A. J. Tankardy H. R. Balkwill, Eds.), AAPG Mem. 46, p. 395–409.

Julivert, M., Fontboté, J. M., Ribeiro, A. and Conde, L., 1972. Mapa Tectónico de laPenínsula Ibérica y Baleares, 1:1.000.000. Inst. Geol. Min. Esp. Madrid, 113 p.

Kind, R., Kosarev, G.L., Petersen, N.V., 1995. Receiver functions at the stations of theGerman regional seismic network (GRSN). Geophys. J. Int. 121, 191–202.

Kosarev, G., Kind, R., Sobolev, S.V., Yuan, X., Hanka,W., Oreshin, S., 1999. Seismic evidencefor a detached Indian Lithospheric mantle beneath Tibet. Science 283, 1306–1309.

Martínez-Catalán, J.R., Pérez-Estaún, A., Bastida, F., Pulgar, J.A., Marcos, A., 1990. WestAsturian–Leonese Zone: Structure. In: Dallmeyer, R.D., Martínez, E. (Eds.), Pre-Mesozoic Geology of Iberia. Springer-Verlag, Berlin, pp. 103–114.

Matte, Ph., 1991. Accretionary history and crustal evolution of the Variscan belt inWestern Europe. Tectonophysics 196, 309–337.

Meissner, R., 1989. Rupture, creep, lamellae and crocodiles: happenings in the continentalcrust. Terra Nova 1, 17–28.

Muñoz, J.A., 1992. Evolution of a continental collision belt: ECORS-Pyrenees cristalbalanced cross section. In: McClay, K. (Ed.), Thrust Tectonics. Chapman and Hall,New York, pp. 235–246.

Pedreira, D. (2005). Estructura cortical de la zona de transición entre los Pirineos y laCordillera Cantábrica. Tesis Doctoral, Universidad de Oviedo (2004). Ed. Univ. deOviedo (Ediuno), CD-ROM, ISBN: 84-8317-528.2, 343 pp.

Pedreira, D., Pulgar, J.A., Gallart, J., Díaz, J., 2003. Seismic evidence of Alpine crustalthickening and wedging from the western Pyrenees to the Cantabrian Mountains(north Iberia). J. Geophys. Res. 108 (B4), 2204. doi:10.1029/2001JB001667.

Pedreira, D., Pulgar, J.A., Gallart, J., Torné, M., 2007. Three-dimensional gravity andmagnetic modeling of crustal indentation and wedging in the western Pyrenees–Cantabrian Mountains. J. Geophys. Res. 112, B12405. doi:10.1029/2007JB005021.

Pérez-Estaún, A., Bastida, F., Alonso, J.L., Marquínez, J., Aller, J., Alvarez-Marrón, J.,Marcos, A., Pulgar, J.A., 1988. A thin-skinned tectonics model for an arcuate fold andthrust belt: the Cantabrian zone (Variscan Ibero-Armorican fold). Tectonics 7 (3),517–537.

Pérez-Estaún, A., Pulgar, J.A., Banda, E., Álvarez-Marrón, J., ESCI-N Research Group,1994. Crustal structure of the external variscides in northwest Spain from deepseismic reflection profiling. Tectonophysics 232, 91–118.

Pérez-Estaún, A., Pulgar, J.A., Alvarez-Marrón, J., ESCI-N Group, 1997. Crustal structureof the Cantrabrian Zone: seismic image of a Variscan foreland thrust and fold belt(NW Spain). Revista Sociedad Geológica España 8 (4), 307–320 1995.

Pulgar, J.A., Gallart, J., Fernández-Viejo, G., Pérez-Estaún, A., Álvarez-Marrón, J., ESCINGroup, 1996. Seismic image of the Cantabrian Mountains in the western extensionof the Pyrenean belt from integrated reflection and refraction data. Tectonophysics264, 1–19.

Ruiz, M., (2007). Caracterització estructural i sismotectònica de la litosfera en el dominiPirenaico–Cantàbric a partir de métodes de sísmica activa i pasiva, Ph. Thesis, Univ.Barcelona. 353 pp.

Sibuet, J-Cl, Srivastava, S.P., Spakman, W., 2004. Pyrenean orogeny and plate kinematics.J. Geophys. Res. 109, B08104. doi:10.1029/2003JB002514.

Teixell, A., 1998. Crustal structure and orogenic material budget in best centralPyrenees. Tectonics 17, 395–406.

Téllez, J., Matias, L.M., Cordoba, D., Mendes-Victor, L.A., 1993. Structure of the crust inthe schistose domain of Galicia–Tras-os-Montes (NW Iberia Peninsula). Tectono-physics 221, 81–93.

Téllez, J., Córdoba, D., 1998. Crustal shear-wave velocity and Poisson's ratio distributionin Northwest Spain. J. Geodynamics 25 (1), 35–45.

Vergés, J., Millán, H., Roca, E., Muñoz, J.A., Marzo, M., Cirés, J., Den Bezemer, T.,Zoetemeijer, R., Cloetingh, S., 1995. Eastern Pyrenees and related foreland basins:pre-, syn- and post-collisional crustal-scale cross-section. Mar. Petrol. Geol. 12 (8),893–915.

Wessel, P., W.H.F. Smith 1998. New, improved version of the generic mapping toolreleased, EOS Trans., AGU, 79, 579.