Crustal structure beneath North-West Iberia imaged using receiver functions

Tectonophysics 478 (2009) 175–183

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

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 b

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

a r t i c l e

i n f o

Article history: Received 3 March 2009 Received in revised form 31 July 2009 Accepted 3 August 2009 Available online 11 August 2009 Keywords: Crustal structure North-West Iberia Receiver functions Variscan and Alpine orogenies

a b s t r a c t In the last years, the deep crustal structure of the North-Western part of the Iberian Peninsula has been explored 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 therefore provides an excellent opportunity to compare the results of both approaches. The region shows the imprint of two orogenic events, the Variscan and Alpine ones that exhibit different, reverse intensities from west to east. In a first stage, a N–S transect across the Cantabrian Mountains was instrumented to study the area affected by the Alpine compressional tectonics. Later on, the limit between the undisturbed Variscan units and the reworked Alpine zones was explored by N–S and E–W transects. Finally, an array was deployed in the NW edge of Iberia, over the Variscan hinterland zone. The receiver functions are calculated by inverse filtering deconvolution of the L component from the Q component, and the resulting RF are processed using a simple form of migration to obtain images in depth of the lithosphere that can be compared to the 2-D velocity–depth models from active seismic experiments. The deep crustal structure constrained by both techniques is remarkably consistent, and provides further evidence on the crustal doubling and wedging between the Iberian and European crusts throughout the northern part of the Iberian Peninsula affected by the Alpine tectonics. The undisturbed Variscan domains, characterized by a clear subhorizontal Moho and few intracrustal convertors, show up at the southern edge of the N–S transects (southward of the Alpine deformation front) and over the Variscan hinterland in the E–W transect. The crust resulting from Alpine reworking presents a complex structure, with short and frequently dipping convertors, which in some cases seem 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) analysis provide independent and complementary results on lithospheric structure. Multichannel vertical reflection profiles and RF pseudomigrated sections constrain the geometry of the seismic discontinuities within the crust, the former using rays coming from the top and the latter rays reaching the discontinuity from below. The frequency content 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, significantly lower for RF surveys. Wide-angle reflection/refraction profiles provide detailed information about the velocity–depth structure of the crust. 1-D inversion of RFs can also constrain the S-wave velocity structure, 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

⁎ Corresponding author. Tel.: +34 93 409 5410. E-mail address: [email protected] (J. Díaz). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.08.003

active experiments. Combining information from active and passive surveys sampling a same area will certainly improve the structural constraints on its geodynamic evolution. One of the scarce regions where such comparison of both kinds of datasets and results is already feasible is the Northern part of the Iberian Peninsula. Following the ECORS seismic surveys carried out across the Pyrenees in the 80's, extensive seismic research from active methods has been performed in North Iberia. The ESCIN seismic reflection program, with inland and marine deep vertical seismic profiles, provided the first crustal images beneath the Cantabrian Mountains and its associated continental margin (Pulgar et al., 1996; ÁlvarezMarrón et al., 1996). Complementary onshore–offshore wide-angle measurements, as well as two consecutive networks of refraction profiles using land shots gave accurate velocity–depth sections along N–S and E–W transects (Fernández-Viejo et al., 1998, 2000; Pedreira et al., 2003). Previously, the Galicia region was also explored by wideangle profiling (Córdoba et al., 1987, 1988; Téllez et al., 1993). More recently, the Spanish MARCONI project investigated the deep structure of the eastern half of the Bay of Biscay using both multichannel seismic profiles and wide-angle data recorded at OBS and land stations to constrain the velocity structure of the area (Ruiz, 2007; Ferrer et al., 2008).

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During the last decade, the deep crustal configuration of North Iberia (Fig. 1) has also been explored using teleseismic receiver function (RF) analysis of P to S conversions at main crustal interfaces, in the framework of different Spanish research projects that have collected a large amount of passive seismic data from temporary array deployments. In a first stage, a N–S transect was deployed across the Cantabrian Mountains (Díaz et al., 2003), a region where the Variscan crust was extensively reworked during the Alpine orogeny (Pulgar et al., 1996; Pedreira et al., 2003). In this contribution we will discuss those results together with new data gathered in later deployments covering the north-westernmost part of Iberia (Fig. 1), which will provide an image of the transition at depth between the Alpine and Variscan domains. From fall 2002 to spring 2003, portable seismic stations were deployed along a N–S transect within the West Asturian–Leonese Zone of the Iberian Variscan Massif (a transitional zone between the foreland fold and thrust belt, that is, the Cantabrian Zone, and the most internal parts of the Variscan orogen). This transect approximately marks the limit between the almost undisturbed Variscan units to the west, and the zones uplifted during the Alpine orogeny to the east (FernándezViejo et al., 2000). An E–W transect was also implemented to connect both domains. Finally, within a passive seismic experiment mainly focused to monitor local seismicity, a network was installed for a few months in the Central Iberian and Galicia–Tras-os-Montes Zones, in the NW edge of the Iberian Peninsula (Díaz et al., 2007). These later deployments cover the most internal parts of the Variscan belt of North Iberia, where there is no evidence of major Alpine reworking. 2. Tectonic setting The study area experienced a complex tectonic history, being affected by the Variscan and Alpine compressional events, separated by a large extensional episode during the Mesozoic. The Variscan orogeny started with the closure of the Rheic Ocean and the collision between Laurentia–Baltica–Avalonia and the continental margin of Gondwana during the Carboniferous, giving rise to the building of the Pangea supercontinent (Matte, 1991). The Iberian Massif is a large outcrop of Precambrian and Paleozoic rocks that extends over most of the western half of the Iberian Peninsula and constitutes one of the best exposed sections of the Variscan belt in Europe, with a well established zonation based on structural, metamorphic and paleogeographic differences (Julivert et al., 1972; Farias et al., 1987). A complete section across the branch of this belt located in the footwall of the suture can be traced in the northern part of Iberia (Fig. 1B). The Cantabrian Zone corresponds to the foreland thrust and fold belt and is characterized by thin-skinned tectonics and a rather tight arcuate trend with a general transport direction towards the east (Pérez-Estaún et al., 1988). The hinterland zones are located immediately to the West and its first unit is the West Asturian–Leonese Zone (WALZ), interpreted as a transitional area, with westward increasing metamorphism, magmatism and internal deformation (Martínez-Catalán et al., 1990). Finally, the most internal part of the orogen is located in the western end of northern Iberia and is divided into the Central Iberian Zone (CIZ) and the Galicia–Tras-osMontes Zone (GTMZ), corresponding respectively to autochthonous units and a group of para-autochthonous and allochthonous units. The Variscan deformation ceased in this area at the end of the Carboniferous, and the Stephanian (Upper Pennsylvanian) beds are essentially post-tectonic, lying unconformably over the older Paleo-

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zoic and Precambrian rocks (Fig. 1B). By Permian times, the orogenic building was already deeply eroded and the sedimentation was mainly controlled by the development of normal faults and grabens in a new distensive regime. During the Mesozoic, the situation evolved to a rifting stage in which several basins were individualized at the borders and in the interior of the Iberian Peninsula. It was the beginning of the Alpine cycle, related to the divergence between the European and American continental blocks, preluding the opening of the Atlantic Ocean and the Bay of Biscay. Oceanization in the Bay of Biscay, started at chron M0 (118 Ma) and lasted until chron A33o (80 Ma) (Sibuet et al., 2004). The opening of the Bay of Biscay resulted in the formation of the Armorican and Cantabrian Margins, with the development of large sedimentary basins and the individualization of Iberia as a subplate. One of these basins developed at the northern border of the Iberian subplate is the Basque–Cantabrian Basin, where more than 10 km of Mesozoic sediments were deposited over an extremely thinned continental crust (Pedreira, 2005; García-Mondéjar, 1989, and references therein). Later on, from the latest Cretaceous, the convergence between Iberia and Eurasia produced the inversion of those basins and the uplift of basement blocks and thrust sheets in the contact zone, building the Pyrenean–Cantabrian mountain belt in the framework of the Alpine orogeny. The northern branch of the doubly-vergent Pyrenean Chain is prolonged along the Cantabrian Margin, while the southern branch propagated through the Mesozoic Basque–Cantabrian Basin to the western termination of the Cantabrian Mountains (Fig. 1A). This continuous, E–W trending mountain belt was uplifted mainly in Eocene to Miocene times. In the Pyrenean part, the calculated 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 Mountains and continental margin it ranges between 86 and 96 km (Gallastegui, 2000; Gallastegui, et al., 2002; Pedreira, 2005), rapidly vanishing 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 extensively investigated by active seismic experiments during the last three decades (Fig 1C). In the early 80s several wide-angle reflection/refraction profiles were acquired in the NW corner of Iberia along different azimuths. The interpretation of these data depicted a 3-layered crust, even if the limit between upper and intermediate levels was often unconstrained (Córdoba et al., 1987, 1988; Téllez et al., 1993; Téllez and Córdoba, 1998). The Moho was located at depths close to 31 km, shallowing to 28 km close to the coastline. In the northern part of the CIZ (Galicia), onshore–offshore profiles have showed a progressive thinning of the crust towards the center of the Bay of Biscay (Fernández-Viejo et al., 1998). Close to the coastline, the Moho is overlain by a 6.6–6.9 km/s lower crust that produces very energetic reflections. Multichannel seismic profiles offshore (profile ESCIN-3.2) also show a deep crustal reflector at about 6s TWT beneath this area on top the Moho interface, which was identified at 8s TWT (Ayarza et al., 2004). Extensive seismic exploration carried out in the nineties over northern Iberia has evidenced that the areas affected by the Alpine orogeny show a crustal structure similar to that inferred for the Pyrenean Chain, consistently showing crustal indentation and wedging between the Iberian and the European–Cantabrian Margin crusts

Fig. 1. A) Simplified topographic and tectonic map of North Iberia, highlighting the effect of the Alpine deformation. Red marks show the location of balanced along-strike crustal cross sections reporting different amounts of Alpine shortening: A-A′, 125 km (Vergés et al., 1995); B-B′, 147–165 km (Muñoz, 1992; Beaumont et al., 2000); C-C′, 75–80 km (Teixell, 1998); D-D′, 86 km (Pedreira, 2005); E-E′, 96 km (Gallastegui, 2000). BCB, Basque–Cantabrian Basin (Mesozoic basin presently incorporated to the Pyrenean–Cantabrian belt); CCR, Catalan Coastal Ranges. B) Detail of the Variscan geology in the NW corner of Iberia, showing the structural zonation and the location of the stations active during the different deployments (stars). C) Map of the MCS and wide-angle reflection/refraction profiles previously recorded over the area. Squares and circles stand for OBSs and land stations in onshore–offshore active seismic experiments.

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(Pulgar et al., 1996; Fernández-Viejo et al., 2000; Gallastegui, 2000; Pedreira et al., 2003; 2007). A quasi-continuous crustal root can be followed along the strike of the Pyrenean–Cantabrian range, with the top of the Iberian lower crust defined at around 35 km depth and the Moho found at 46–48 km depth. The lower European crust is located at a much shallower level, with Moho depths less than 30 km. The presence of high-velocity bodies embedded at mid-crustal levels has also been inferred from the seismic data, and they have been interpreted as pieces of the European–Cantabrian Margin lower crust indenting southward the Iberian crust (Pedreira et al., 2003, 2007). The onshore–offshore profiles acquired in the MARCONI experiment confirm this indentation, allowing to better constraint its geometry (Ruiz, 2007). A significant thinning of the crust to the westernmost part of the Cantabrian Mountains, where Moho depths are about 30 km, depicts the transition to the zones of the Iberian Massif that had not been significantly reworked by the Alpine orogeny (Fernández-Viejo et al., 2000). The reflection seismic profile ESCIN-1, oriented E–W beneath the Cantabrian Mountains, revealed the Variscan basal detachment of the Cantabrian Zone gently dipping westward to reach 16 km depth beneath the WALZ. Two deep reflectivity bands can also be identified dipping westward to join a reflective lower crust located between 25 and 29 km and forming a crocodile-like structure (Pérez-Estaún et al., 1994; Gallastegui et al., 1997). Offshore, reflection seismic profile ESCIN-3.3, running parallel to the NE Galicia coast, also evidence the presence of subhorizontal, west-dipping intermediate-depth reflections interpreted as Variscan compressional structures. Two highly reflective subhorizontal bands at 7–9 and 11–12s TWT have been interpreted as imaging two levels of lower crust separated by upper mantle materials (Álvarez-Marrón et al., 1996; Ayarza et al., 1998). 4. Data acquisition Here, we analyze data acquired in different experiments carried out in the northern part of Iberia between 1999 and 2003 (Fig. 1B). The first deployment delineated a N–S transect along the central part of the Cantabrian Mountains (CM), and was active during a 9 month period. This transect was instrumented using six seismic stations equipped with Lennartz seismometers (Le20s and Le5s), with flat velocity response electronically broadened up to periods of 20 and 5 s respectively. Two additional transects were deployed later on, one along a N–S transect located at the West Asturian–Leonese Zone (NS– WALZ), and the second one along an E–W profile connecting the WALZ and the CM transects. In this case a total of 12 stations, all of them equipped with Le20s seismometers, were installed for a period of 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 of 8 stations with Le20s seismometers for a period of 5 months during summer 2003. Teleseismic events with epicentral distances between 35° and 95° and clear P arrivals were selected to analyze P to S conversions at main crustal interfaces by means of the RF technique. Following the method described by Kosarev et al. (1999), the records are rotated to the ray components (L, Q, T) using back azimuth and incidence angle to minimize energy on the radial and transverse components for the P arrival. The receiver functions (RF) are then calculated by inverse filter deconvolution of the L component from the Q component. In order to get structural information readily comparable to the wide-angle results, the resulting RFs are treated similarly to crustal reflection data, using a Common Conversion Point (CCP) procedure to obtain images of the lithosphere in depth domain. The RF from different stations and events are traced back along its raypath using a 1-D velocity model, the amplitudes contributing to each box of the model are stacked and the data are projected along a profile. Therefore, the amplitude contrasts in the final images depict the zones where incoming waves suffer 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 the observed convertors. The azimuthal coverage provided by the teleseismic events finally retained extends from N140W to N90E, which means that only one quadrant is missing. The number of useful events varies for each deployment. For the EW, CMZ and WALZ transects, data from 30 to 45 events were used, even if the typical number of events per station is between 15–20 and only few events can be used in specific stations. For the Galicia deployment, which only lasted 5 months, 8 useful events 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 a network of 8 stations that has allowed to build up two parallel N–S transects, even if the overlap between stations is rather loose. In both cases the crust has a simple structure, with a well defined subhorizontal Moho close to 30 km that thins northwards to reach 28 km near the shoreline (Fig. 2). In the eastern transect (Fig. 2B) the presence of an intracrustal convertor in the 15–18 km depth range is evidenced, although it is difficult to be correlated along the profile. Wide-angle modeling of line IAM12 (coincident with our eastern section) shows a crustal discontinuity at about 12 km depth and a highly reflective lower crust, located between 22 and 28 km depth and overlaying the Moho (Fernández-Viejo et al., 1998). The marine reflection 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 the lower crust or to the crust/mantle boundary (Álvarez-Marrón et al., 1996). Another reflector gently dipping towards the southwest is identified at 5–7s TWT in that profile, and has been attributed to Variscan features. This reflector can be correlated with our intracrustal convertor. 5.2. West Asturian–Leonese N/S transect (WALZ) This transect shows a very clear difference between its southern and northern parts (Fig. 3). To the south, the crustal structure is quite simple, with a well defined, subhorizontal convertor associated to the Moho discontinuity and located at ~ 33 km depth. The northern part of this transect depicts a rather different, more complex image. At about 15 km depth, we identify a convertor interpreted as the limit between the upper and middle crust, in agreement with the available wideangle models (Córdoba et al., 1988; Fernández-Viejo et al., 2000). The transition between crust and upper mantle is in this case also complex, marked by a broad convertor extending between 28 and 35 km. Consistently with wide-angle data, we interpret this feature as the unshaped image of the conversions at the top and bottom of a relatively thin lower crust overlaying the Moho. The clear difference between the northern and southern parts of the transect reflects the N–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), and hence we do not show here a specific figure. However, it is included in the 3D diagram presented in Fig. 5D. The transect holds again clear differences between its two ends. The southern part, sampling the transition 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 upper and intermediate crust evidenced by seismic profiling in other areas of the Iberian Massif (Díaz et al., 1993). The convertor at 35 km depth is associated to the Moho, and coincides with the bottom of the crust

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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) Eastern transect. 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.

interpreted from active seismic profiles (see compilation in Díaz and Gallart, 2009). The central part, near the intersection with the E–W transect, displays a different pattern, with an upper convertor that has been modeled by a thin high-velocity layer and can be related to the high-velocity body evidenced in the E–W and N–S wide-angle reflection/refraction seismic profiles sampling this area. This feature has been interpreted as a slab of European lower crust indenting the Iberian middle crust (Pedreira et al., 2003, 2007). The crust/mantle transition is loosely imaged, with a conversion zone extending from 30 to 43 km depth. Even if more data is needed to be conclusive, this image may suggest a limited subduction of Iberia northwards, in agreement with the active seismic models. Finally, the northern end of the transect shows some complexity on the uppermost crustal levels. Deeper on, only one convertor at 25 km depth is identified and it can be related to the top of the Cantabrian Margin (European) lower crust according to the modeling of an N–S active seismic profile (Fernández-Viejo et al., 1998). 5.4. E/W Transect

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

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

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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 deduced from 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 coincident section 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-angle profiles. Label “M” in the seismic reflection profile denote the Moho discontinuity; other labels in blue circles denote convertors discussed in the text.

East lithospheric transect depicting the crustal-scale variations trough the different Variscan tectonic domains sampled, which have different degrees of Alpine reworking. Fig. 4A shows the compiled results for this 375 km long E/W transect. The first relevant observation is the marked difference in crustal complexity between the stations located in the CIZ–GTMZ and those located eastwards. This difference allows to clearly distinguish between the areas that suffered a major crustal reworking during the Alpine tectonics and those essentially undisturbed since the Variscan orogeny. The western section of the transect, running through the CIZ– GTMZ depicts, as in the NS transects previously discussed, a quite simple structure, with a clearly defined Moho located between 27 and 30 km and slightly deepening towards the East (labeled D in Fig. 4A). An intracrustal convertor can be identified at 14 km depth beneath station COTA and at 18 km depth beneath station OLAS (labeled C in Fig. 4A). If both convertors correspond to the same geological feature, this will depict a westward dipping discontinuity at mid-crustal levels, probably of Variscan origin. In the eastern part, the transect crosses the WALZ and CM, areas strongly affected by the Alpine orogeny, and the image is much more complex, suggesting that both the implicit hypotheses of the RF technique and the modest amount of data do not allow to document here the present-day structure with enough resolution. However, some main features can be identified and discussed in relation with active seismic data available in the zone. At upper

crustal levels, the convertor reported at the GTMZ and CIZ can be continued 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 eastern end of the profile and interpreted as the uplifted base of the Iberian upper crust, in agreement with the gravimetric modeling of Pedreira et al. (2007). The B convertor, identified from 6.6°W to 5.8°W with a westward dip and followed discontinuously to the eastern end of the transect, can be related to the reflectivity band observed in the ESCIN-1 seismic reflection profile, almost overlapping our transect (Fig. 4B). This reflectivity band has been interpreted as the western end of the Cantabrian Zone basal detachment (Pérez-Estaún et al., 1997). In particular, the westward dipping convertor observed in the E–W transect can be related to the back-limb of the Narcea antiform. This convertor seems to reach the lower crust, forming a “crocodile” type structure similar to what is observed in the MCS profile ESCIN-1. This term, first proposed by Meissner (1989), is used in vertical reflection profiles to describe diverging reflectors that appear as branching structures at upper and mid-crustal layers. East of 5.8°W, an eastward-dipping convertor, labeled E in Fig. 4, can be identified from 22 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 the same position the base of a lower crustal wedge from the European domain indented between the upper and middle Iberian crusts, with the top of this wedge located immediately beneath the Variscan basal detachment of the Cantabrian Zone (Pedreira et al, 2003; Pedreira et al,

J. Díaz et al. / Tectonophysics 478 (2009) 175–183 181 Fig. 5. Block diagrams of the pseudo-migrated RF transects from West (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 Moho discontinuity.

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2007). The pseudo-migrated RF data seems to confirm the presence of such indentation (Figs. 4A and B). The image of the crust/upper mantle transition beneath the WALZ and CM sections of the transect is also complex. Coherently with the image of the NS WALZ transect and the available wide-angle models (Fernández-Viejo et al., 2000), we interpret that the convertor at about 27 km depth beneath stations MART and PENA corresponds to the Moho. The converted energy observed at 38 km depth at longitudes between 7.4°W and 6.9°W can hardly be interpreted in geotectonic terms. As this convertor appears only from earthquakes with back azimuths between N246E and N288W reaching the MART station, we regard this feature as an artifact due to the complex geometry of the area. The Moho appears as a single convertor with a general 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-angle seismic Moho (Pedreira et al., 2003). East of 5.8°W, the signal from the Moho cannot be observed in the RF section, although its position is reasonably well constrained by almost coincident seismic reflection and refraction profiles (respectively, “M” labeled discontinuous line and dashed line at the bottom of Fig. 4B). In any case, at the eastern edge of the transect the convertor observed at about 40 km can be interpreted again as the Moho discontinuity (Fig. 4A). At depths of 30–35 km, some discontinuous converted energy is observed (labels F in Fig. 4) and it is interpreted as resulting from conversions at the top of the lower crust. This layer shows relatively high-velocities in the wide-angle modeling, and therefore it could mask the conversions at the Moho, explaining the lack of continuity of the Moho along this section of the E–W transect. It must be noted that the convertors located beneath 50 km are in fact multiples of the mid crust discontinuities, as it can be verified by 1-D velocity–depth modeling. The apparent continuity of this convertor between 6.5°W and the eastern edge of the section is probably the effect of combining poor data resolution with the complex crustal structure in this region. 6. Discussion and conclusions The compilation and analysis of all the available RF data in the northern part of the Iberian Peninsula from different passive seismic deployments has resulted in an image of the crustal structure which is independent of the one coming from previously acquired active seismic data over the same zone. The presented RF data cover the NW corner of the Iberian Peninsula, from the hinterland of the Variscan orogeny in the West to the areas clearly affected by the Alpine orogeny in the East, allowing to investigate the main differences in crustal structure between them. The use of pseudo-migrated, 2D receiver function transects has facilitated a comparison with the models derived from MCS and wide-angle reflection profiles, and the corresponding results are remarkably consistent. For a number of representative stations, we have also derived 1-D velocity–depth models which, even if often lack unicity, allow to further constrain the interpretations. The coincidence of the main crustal structures deduced from active and passive techniques, in spite of their different resolution, provides further support to their interpretation. The limited amount of teleseismic data and the intrinsic hypotheses of the RF method make it difficult to obtain detailed images of areas with complex geometries, as dipping interfaces or second order discontinuities. However, even in those complex areas, RF data can become a useful tool to check the validity of the models derived from active seismic experiments and to geographically extend the area investigated by those methods. The first order result of this work is the clear difference in the complexity degree of the deep structure between the western and southern areas of the study area, where a quite simple crustal structure is revealed, and the eastern and northern areas, where a much more complex crust is imaged. According to the tectonic history of the region, those differences must be related to the imprints of the Alpine

orogeny, which affected to different degrees the tectonic units that form the northern part of Iberia. This fact has already been established at surface level by geological studies and has also been evidenced by previous seismic data, but this work provides further constraints for a regional view of the crustal-scale variations. To better illustrate those spatial variations in crustal structure, we have constructed block diagrams by merging the EW and the NS pseudo-migrated transects (Fig. 5). Those diagrams allow understanding the 3D character of the crustal structure imaged by the receiver function analysis, especially in the eastern part of the study area. To the West, beneath the GTMZ and CIZ, the image is rather simple, with a well defined conversion associated to the Moho at 28– 30 km depth. A mid-crustal convertor is observed in the EW transect dipping westwards and, consistently, in the eastern NS GTMZ transect slightly dipping southwards (Figs. 5A and B). This structure is not evidenced at the western end of the investigated area. Further East, beneath the WALZ, the pseudo-migrated EW transect allows to identify the root zone of the Cantabrian basal detachment in the Narcea Antiform (Fig. 5C). The Moho discontinuity shows up as a strong convertor around 40 km depth west of 6.0°W, shallowing to the west to reach about 30 km beneath the NS WALZ transect. The complexity observed in the northern part of this transect suggest that some degree of reworking affecting the whole crust during the Alpine orogeny should exist in this zone. At the eastern end of our transect, beneath the Cantabrian Mountains, the image is consistent with the presence of crustal thickening and wedging (Fig. 5D), as suggested previously by active source models. Instead of a clear Moho in this area, the EW transect shows only sparse mid-crustal conversions probably generated by the top of the European lower crust, also observed at the northern end of the NS transect. The mid-crustal convertor clearly evidenced in both transects can be modeled with a high-velocity layer, and it is consistent with the interpretation derived from seismic profiling of an indentation of European lower crust in the middle levels of the Iberian crust. This indentation forces the northward underthrusting of the lower half of the Iberian crust, in the same sense that is observed elsewhere along the Pyrenean–Cantabrian belt (e.g. Muñoz, 1992; Pedreira et al., 2007). Acknowledgments This work was sponsored by Spanish Research Ministry projects AMB98-1012-C02 and REN2001-1734-C03. M. Ruiz has benefited from a Spanish Ministry “F.P.I.” Ph.D. grant. Figures were created using GMT (Wessel and Smith, 1998). This is a contribution of the Team Consolider-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 continental margin 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., Alvarez Marrón, J., 2004. Geophysical constraints on the deep structure of a limited ocean–continent subduction zone at the North Iberian Margin. Tectonics 23, TC1010. doi:10.1029/2002TC001487. Beaumont, C., Muñoz, J.A., Hamilton, J., Fullsack, P., 2000. Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models. J. Geophys. Res. 105 (B4), 8121–8145. Córdoba, D., Banda, E.and Ansorge, J., (Reporters) 1987. The Hercynian crust in northwestern Spain: a seismic survey, Tectonophysics, 132: 321–333. Córdoba, D., Banda, E., Ansorge, J., 1988. P-wave velocity–depth distribution in the Hercynian 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 lithospheric heterogeneity and anisotropy beneath the Iberian Peninsula. Tectonophysics 221, 35–51.

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