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Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula
Physics of the Earth and Planetary Interiors 150 (2005) 317–330
Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula I. Serrano a, ∗ , T.M. Hearn b , J. Morales a, c , F. Torcal a, d a
d
Instituto Andaluz de Geof´ısica, Universidad de Granada, 18071 Granada, Spain b Department of Physics, New Mexico State University, Las Cruces, USA c Departamento de F´ısica Te´ orica y del Cosmos, Universidad de Granada, Spain 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 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and north–south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established. © 2004 Elsevier B.V. All rights reserved. Keywords: Seismic anisotropy; Pn seismic waves; Tomography; Iberian Peninsula ∗
Corresponding author. Tel.: +34 958248912; fax: +34 958160907. E-mail address: [email protected] (I. Serrano).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.12.003
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1. Introduction The southern half of the Iberian Peninsula is made up of the Iberian massif and the Betic Cordillera (formed by the External and Internal Zones), which are connected through the Guadalquivir Basin. The Gulf of Cadiz represents the continuation of this basin on the continental and Algarve margins along the Mesozoic border of the Iberian massif. The Betic Cordillera and Moroccan Rif are connected through the Gibraltar Strait, which developed during Cenozoic convergence between Africa and Iberia (e.g. Dewey et al., 1989) making an arcuate belt. The Alboran Sea is located on the inner part of this in continuity to the east with the South Balearic Basin (Fig. 1). The Iberian massif forms part of the Variscan belt of central Europe that resulted from the late Paleozoic
suture of Gondwana and Laurasia. Its southern edge is composed of two lithostructural units from north to south: the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ). The SPZ is located in the westernmost sector of the Hercynian belt, where there are no outcrops of the deep continental crust. Its sedimentary record comprises Upper Devonian and Carboniferous sequences. However, seismic and gravity studies have revealed the presence of older crust of an unknown age (Mueller et al., 1973; Prodehl et al., 1975, etc.). Recent studies on the boundary between the SPZ and the OMZ have interpreted this as a major suture of the European Variscan Orogen (CrespoBlanc and Orozco, 1988; Quesada, 1991). The SPZ is believed to have been accreted to the rest of the Iberian massif during the orogeny, and to represent a fragment of a plate that was otherwise destroyed
Fig. 1. Simplified geological map of the study area showing the main tectonic units. The upper right-hand corner shows a small sketch of the main geological structures such as the Betic and Rif Cordilleras. SPZ: South Portuguese Zone. OMZ: Ossa Morena Zone. GB: Granada Basin, GD: Guadalquivir Depression.
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by subduction beneath the OMZ (Quesada et al., 1994). In contrast, the Betic–Rif mountain belt forms the southwestern termination of the Alpine orogen in Europe and northern Africa. Its ambivalent position straddling the plate boundary between Iberia–Europe and Africa is an orogenic artefact produced by late stage emplacement on the African foreland of secondary extensional allochthons derived from the European Alpine collisional belt (Zeck et al., 1992; Zeck, 1996, 1997, 1999; De Jong, 1991). The opening of the North Atlantic during the Late Cretaceous and Tertiary induced the rotational divergence of North America and Eurasia, which was paralleled by the counter clockwise convergence of Africa and Eurasia. The northwards drift of Africa caused the progressive closure of oceanic basins of Tethys and the rapid westward propagation of the Alpine Betic–Rif orogenic collision front in the Gulf of Cadiz, in parallel with the development of the western Mediterranean basins (Dewey et al., 1989; Garc´ıa-Due˜nas et al., 1992; Jabaloy et al., 1992; Maldonado et al., 1992). From this complicated yet intriguing geological framework, we deduce that the present study area provides a suitable opportunity to research the presence of anisotropy, a parameter used to describe a medium whose elastic properties are functions of orientation. It is now generally accepted that seismic anisotropy in the upper mantle is due primarily to the deformation-induced lattice preferred orientation of olivine crystals. This preferred orientation could have several causes. It may be related to passive motion of the lithosphere over the stationary asthenosphere, in which case, the most likely location for anisotropy would be in the transition zone between the two shells and the fast direction of anisotropy would be close to that of plate motion (Leven et al., 1981). Alternatively, the preferred orientation could be induced by flow in the asthenosphere, so the fast direction of anisotropy would coincide with the direction of flow but might differ from the direction of plate motion (Tanimoto and Anderson, 1984). Azimuthal anisotropy in the uppermost mantle may also be related to past and present deformation of the lithosphere (Fuchs, 1983). Our results suggest that anisotropy in the southern half of the Iberian Peninsula may result from a range of causes and could occur at different depths.
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The existence of upper mantle anisotropy beneath the Iberian Peninsula has been recorded by using a variety of techniques and results are not easily comparable. While Pn data measure the anisotropy within the top of the mantle, SKS data measure the vertically integrated anisotropy of the mantle; in this paper we show both results. Vinnik et al. (1989) presented the first results of the existence of upper mantle anisotropy beneath the Iberian Peninsula based on the analysis of SKS and similar phases at several broadband stations of the GEOSCOPE network and the NARS array. Their data for Europe included that from the two stations in central Spain and generally showed an E–W direction for fast velocity. This confirmed the direction previously found in central Spain by Silver and Chan (1988). However, Maupin and Cara (1992) used surface wave data and found no evidence of large scale anisotropy in the uppermost 100 km, indicating that the anisotropy observed in the shear waves could be related to deeper levels. Later, Badal et al. (1993) and Corchete et al. (1993) obtained a lateral variation in velocity at five depth intervals based on a detailed analysis of Rayleigh wave dispersion. In the same year, the models derived from the interpretation of data recorded by the Iberian Lithosphere Heterogeneity and Anisotropy experiment (ILIHA DSS Group, 1993; D´ıaz et al., 1993) showed the vertical heterogeneity of the lower lithosphere and azimuthal anisotropy at different depth levels. However, the anisotropy orientation deduced from these data is different from that obtained by Vinnik et al. (1989). To explain this discrepancy the authors suggested the existence of vertical heterogeneity in the anisotropic properties over the mantle. Later, Abalos and D´ıaz Cus´ı (1995) postulated a correlation between the lithospheric structure and seismic anisotropy from geophysical data and the major structures and tectonic history of the outcropping basement rocks in SW Iberia. According to these authors, the deviations and aerial variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust in this region indicate that the tectonic model of orogen-parallel lithospheric deformation does not appear to be linked to Hercynian surface geology and the structural geology of the subcontinental lithospheric mantle. From these data, Abalos and D´ıaz reconstructed a coherent tectonic model of polyorogenic lithospheric deformation in the southwestern Iberian massif (ibid.). Analysis
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of SKS and S phases recorded by a portable network installed in the same area of SW Iberia as that explored in the ILIHA DSS profiles (D´ıaz et al., 1996) suggests a quite consistent anisotropic structure with roughly a NE–SW to E–W fast velocity direction. The presence of anisotropy beneath the whole Iberian Peninsula was later established through the analysis of teleseismic shear-wave splitting observed in the broadband stations located over the entire peninsula (D´ıaz et al., 1998). This work was extremely significant as it provided the first anisotropy constraints beneath different tectonic domains of the Iberian Peninsula from a homogeneous seismic network. The authors postulate that regional variations in the anisotropic parameters imply that differentiated origins of the anisotropy have to be considered in some areas in relation to their particular lithospheric geodynamics. For example, within the Betic Cordillera, anisotropic results for the South Iberian domain contrast sharply with those for the Alboran crustal domain. Lastly, an inversion of Pn travel-time residuals was performed by Calvert et al. (2000) along the
Africa–Iberia plate boundary zone using a code developed by Hearn (1996) which solves for isotropic and anisotropic components of the mantle velocity structure. There is a strong correlation between results obtained in the central and western Betic Cordillera and those obtained in southernmost Iberia. The imaged fast axes have a predominant E–W trend in the Betics that becomes more NW–SW in the western internal zones before trending almost N–S beneath the Straits of Gibraltar.
2. Data set and methodology The data used in this study are Pn waveforms recorded by several networks situated in southern Spain and northern Morocco. From the Andalusian Seismic Network (RSA), which is operated by the Instituto Andaluz de Geof´ısica (IAG), we have collected data on earthquakes from 1988 to March 2003. And, the Real Observatorio de la Armada (ROA) in San Fernando (C´adiz) and the Instituto Geogr´afico Nacional (IGN)
Fig. 2. Distribution of seismicity for the study period and seismic stations located in the area. We used data from a total of 121 stations belonging to Spanish and Moroccan institutions.
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have kindly provided data from 1988 to the present. Coverage in the African continent has been obtained thanks to the collaboration of the Centre National de Coordination et de Planification de la Recherche Scientifique et Technique (CNCPRST) and the Physique du Globe at Mohamed V University (MOH V), both in Rabat (Morocco). In order to take advantage of the station distribution (Fig. 2) and provide additional data coverage in the Atlantic coast and Algerian area, those farthest from the Spanish seismic stations, we drew on data from the ISC catalogue. We used data from 121 stations in total, but a considerable imbalance exists between the number of stations on the Iberian Peninsula and those on the African continent. We define Pn as the first arrival from regional events between 200 and 1200 km. We do not keep phases for distances under 200 km to avoid misidentification errors. Data were selected with the following criteria: inversion only uses events with over 10 recorded arrivals, stations with over 10 recorded arrivals, and event depth less than 30 km. From our initial set of 39,872 traveltimes (7675 earthquakes) a total of 11,612 Pn first arrival times met our selection criteria. Some 73% of the initial data set are recorded at an epicentral distance from 200 to 500 km and about 27% are at an epicentral distance of between 500 and 1200 km. Arrivals with residuals of over 4 s were eliminated (Fig. 3).
Fig. 3. Initial travel-time residuals. We selected the earthquakes recorded at an epicentral distance of between 200 and 1200 km. Arrivals with residuals of over 4 s were eliminated.
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The final set of ray paths for the selected data is shown in Fig. 4. The ray path density is highest in the southern Iberian Peninsula, offshore from Cape San Vicente and in the Alboran Sea. The travel-times collected were inverted in the same way that was used by Hearn (1996). The Pn traveltime residuals are described as the sum of three time terms: tij = ai + bj + dijk (sk + Ak cos 2φ + Bk sin 2φ) where φ is the back azimuth angle, ai the static delay for station i, bj the static delay for event j, dijk the distance travelled by ray ij in mantle cell k, and sk the slowness perturbation (inverse velocity) of cell k. Ak and Bk are the anisotropic coefficients for cell k. (Hearn, 1984). The unknown quantities in this equation are the station and event delays ai and bi , the mantle slowness perturbation sk , and the two coefficients of anisotropy Ak and Bk . The magnitude of 1/2 the anisotropy for cell k is given by (A2k + Bk2 ) and the direction of fastest wave propagation is given by (1/2)arctan(Bk /Ak ). The tomographic method used is a preconditioned version of Paige & Saunders’ LSQR algorithm (1982). In solving the set of travel-time equations the cell size used is small and a set of Laplacian damping equations regularize the solution (Lees and Crosson, 1989). Two damping constants are separately applied to the unknown slowness and anisotropic coefficients. A proper pair of damping constants is chosen to balance the error size and the resolution width. The trade-off between velocity and anisotropy variations has been checked by using different combinations of damping parameters for both velocity and anisotropy. From our data the features of the velocity and anisotropy fields in the study are observed to be considerably stable, varying slightly for different combinations of the damping constants; the estimated standard errors of the data for 16 different combinations of the damping parameters range between 0.52 and 0.56 s. The small differences in standard errors show that all results are equally valid solutions for the inverse problem. The final damping constants were 100 for the velocity and 500 for the anisotropy, giving an estimated data standard error of 0.54 s, with a cell size of 0.25◦ × 0.25◦ which provides a significant number of hits in each cell and balances uncertainty and resolution.
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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 better coverage is obtained in the south of the Iberian Peninsula.
3. Results 3.1. Resolution Evaluations of the resolution width are made by using four test models. All models consisted of a checkerboard velocity pattern, with the first model having a square size of 3◦ by 3◦ and the second, third and fourth models having a square size of 2◦ by 2◦ , 1◦ by 1◦ and 3/4◦ by 3/4◦ . They were constructed from a synthetic data set computed using the ray paths from our database. The synthetic data was then inverted in the same way as the real data. The squares were assigned velocities that alternated between +0.3 and −0.3 km/s. Station and event delays were set to zero and no noise was included. For the first model, the 2◦ by 2◦ squares were resolvable for the entire imaged region, except southwest of the Balearic Islands. Squares as small as 1◦ were resolved for the whole of the Iberian Peninsula studied, the Alboran Sea and offshore from Cape San Vicente, and were not well resolved in northern
Africa and the southwestern Balearic Islands. In the third model, 3/4◦ by 3/4◦ squares were resolved in the south of the Iberian Peninsula and the western Alboran Sea. Results for the second (2◦ by 2◦ ), third (1◦ by 1◦ ) and fourth models (3/4◦ by 3/4◦ ) are shown in Fig. 5. 3.2. Station delays Station delays represent variations in crustal thickness and velocity relative to an initial model and they are the most robust portions of the inversion. For a mean crustal velocity of 6.2 km/s and a thickness of 33 km, a station delay of 1 s. corresponds to a change of 9.8 km in thickness or a change of 0.7 km/s in mean crustal velocity. However, it is difficult to distinguish between the crustal velocity and the crustal thickness contributions in the crustal delay times. Our results show some domains, though in general slight delays were obtained at most of the seismic stations. No delays or weakly positive delays (0.2–0.5 s)
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Fig. 5. Checkerboard test model with 2◦ by 2◦ , 1◦ by 1◦ and 3/4◦ by 3/4◦ cell size.
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were obtained in almost all the stations in the Betic Cordilleras, which is probably a consequence of an underestimated Moho depth beneath this area (the Moho being modelled by a surface at 33 km depth). The exceptions were found in the easternmost station of the Betic Cordillera (near Cabo de Gata, western Alboran Sea) and in the southeasternmost part of the Straits of Gibraltar, which showed negative delays (near 0.5 s). These negative anomalies can be interpreted as an effect of the variations in crustal thickness, which ranges from 36 km underneath the Betic and Rif Chain to <12 km beneath the easternmost part of the Alboran Sea. In the western Alboran Sea the Moho lies at a constant depth of ±18 km, deepening sharply toward the Gibraltar Strait where it reaches 30–32 km. The negative delays (nearly 0.5 s) in northern Africa could be explained in the same way. On the other hand, to the north and northwest the negative delays (from 0.2 to 0.5 s) could reflect the moderate crustal thinning toward the Iberian massif and/or would be an effect of the old and stable Hercynian crust (Judenherc et al.,
1999). One of the longest negative delays (>0.5 s) was obtained at the station on the south Portuguese coast (near Faro). Crustal thickness from the Guadalquivir Basin/Iberian massif contact to the southeastern SPZ, shows gradual lateral changes (Gonz´alez et al., 1998a, 1998b). However, near this area the same authors obtained a high apparent crustal velocity (6.4 km/s) that they interpret as a deeper level of the upper crust related to ultramafic rock outcropping. This negative delay is probably due to a combination of a thinner than average crust and an increase in mean crustal velocity. 3.3. Uppermost mantle velocities The travel-time data were fitted to a straight line as a function of distance to determine the mean Pn velocity from the inverse slope. The straight line fit gave an estimated velocity of 7.98 km/s and an intercept of 7.1 s. Lateral variations of Pn velocity are imaged as perturbations from this average velocity. The tomographic
Fig. 6. Inversion results for Pn velocity. Red and blue indicate saturated for low and high velocity, respectively. Pn velocity in the study area varies from 7.7 to 8.3 km/s.
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inversion used 25 LSQR iterations which reduced the root-mean-square error of the residuals to 0.5 s. The Pn velocity varies in the study area from 7.7 to 8.3 km/s., showing a low velocity zone parallel to the Moroccan coast on the African continent (Fig. 6). On the southeast coast of Spain, the low velocity zone coincides with the Betic Cordillera and extends towards the Balearic Islands. An extensive high-velocity zone spreads on the Iberian massif and offshore from the Moroccan Atlantic coast. We cannot determine the velocity beneath the Rif Codillera with great accuracy due to the poor coverage of the ray paths. The Alboran Sea is imaged as a region of high velocity in the north and low velocity in the south. Southwest from Cape San Vicente, a strong lowvelocity anomaly is imaged that contrasts with the mean values obtained onshore on the Iberian Peninsula. The low values reach 7.75 km/s offshore and spread across a wide area off Cape San Vicente. Otherwise, on the boundary between the Iberian Meseta and the
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Guadalquivir Basin (Fig. 1) the refracted Pn waves in the upper mantle show a mean velocity of 8.0 km/s. 3.4. Seismic anisotropy The map of Pn anisotropy shows considerable variation in both magnitude and direction beneath the study area (Figs. 7 and 8). The magnitude of anisotropy ranges 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 variation in kilometres per second and each Pn anisotropy estimate represents the average anisotropy in a region of 1◦ . Features much smaller than this do not appear in Fig. 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 trends NW–SW from the latest Tortonian to the present day (Dewey et al., 1989, etc.). However, the anisotropy in
Fig. 7. Pn anisotropy in the study area. The fast direction of Pn velocity is drawn, and the bar length is proportional to the amount of anisotropy in that direction. The magnitude of anisotropy ranges from 0 to 4% in the study area. Bottom right, we have inserted the anisotropic results of D´ıaz et al. (1998); white arrows show the fast velocity direction obtained and are proportional to the time delay; thin black arrows show the null measurements.
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Fig. 8. Image of the magnitude of Pn anisotropy. Darker shades correspond to more anisotropy.
S–SE Spain shows a rapid shift in the orientation of the fast direction, running almost parallel to the main trend of the Betic Cordillera: E–W in its central part and N–S in the curvature of the Arc of Gibraltar. The most important values, about 0.4 s are obtained in this last region. The present-day setting of the Mediterranean region is characterized by Neogene subsident basins like the Alboran Sea, surrounded by mountain belts like the Betic and Rif Cordilleras; only remaining Mesozoic oceanic crust is being subducted below the Calabrian and Hellenic arcs. The most recent studies and interpretations indicate a significant increase in the absolute velocity of Africa at 30 Ma and, a significant decrease after 20 Ma. The absolute motion of Africa toward the north is, however, always faster than the convergence velocity because the absolute motion of Eurasia is also northward with an average velocity of ∼1 cm/yr (Jolivet and Faccenna, 2000). In addition, several authors have described the slowing down of Africa as a 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 region, the resulting anisotropy would be parallel to the direction of maximum compression, as is happening in the Alboran Sea where it must be assumed that the Neogene extension had no major influence, on a lithospheric scale. However, in the uppermost mantle beneath the Betic Cordillera the pattern of Pn anisotropy appears to be strongly related to the regional tectonic trend, following the arc-structure of the belt. During the deformation process the fast axes of upper mantle anisotropic minerals tend to align with the longest axis of the ellipsoid strain, which is perpendicular to the direction of maximum compression in the pure shear deformation regime (McKenzie, 1979, etc.). Meanwhile, the slow axis tends to align parallel with the direction of maximum compression (Babuska and Cara, 1991; Silver, 1996). In orogenic belts, where pure shear deformations are likely to occur, the fastest direction of Pn velocity would thus be perpendicular to the direction of orogenic compression (Silver and Chan, 1988). Therefore the anisotropy fast orientation could be aligned
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along the Betic arc in response to regional compression across the arc. Many other convergent margins in the Mediterranean region, such as the Alps, the Apennines, and the Dinaric-Hellenic mountain chains exhibit exceptionally high arc-parallel anisotropy (Mele, 1998; Hearn, 1999). Pn anisotropy coincides with the direction of the Neogene and the present-day extension beneath the central Betic Cordilleras (for example, the Granada Basin), suggesting that this deformation might have controlled a preferred orientation of mineral in the uppermost mantle. However, contrary to what would be expected, the parallelism between the azimuths of seismic anisotropy in the southwestern Iberian lithospheric mantle and major Hercynian tectonic trends is not clear. In the Ossa Morena and SPZ the direction of anisotropic fast-Pnvelocity trends is NE–SW, very much like those obtained by D´ıaz et al. (1993), while Hercynian structures in the central and southern Ossa-Morena trend NW–SE to E–W. The present day stress state at OMZ trending N150◦ E is almost at an oblique angle to the fast propagation direction obtained. Moreover, the absolute plate motion rate of the Iberian plate is very small to be consistent with the results of the anisotropy (Abalos and D´ıaz Cus´ı, 1995). Finally, the trends of the last episode (Hercynian) of internal coherent deformation also differ from the direction obtained. Other processes should be considered in order to explain the anisotropy pattern found in the southwestern Iberian massif. It may even be the case that the mantle strain pattern has been obscured by the polyorogenic and intricate tectonic evolution of the zone (Abalos and D´ıaz Cus´ı, 1995).
4. Discussion and conclusions A principal result of this study has been to infer the velocity and anisotropy structure in the uppermost part of the mantle beneath the southern part of the Iberian Peninsula and surrounding regions. Regional variations of Pn velocity throughout the study area show good correlation with surface tectonics. According to Hearn and Ni (2001), high Pn velocity regions are stable cratonic areas which were not greatly affected by tectonic and magmatic activity for a long time resulting in a lower temperature in the mantle lid and higher Pn velocity. In Fig. 6, an extensive area of the cen-
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tral and western Iberian Peninsula is covered by the Variscan Iberian massif, a large, old and geologically stable block of continental lithosphere (Dallmeyer and Mart´ınez Garc´ıa, 1992). This area is characterized by high Pn velocity, as expected in tectonically stable regions, emphasizing the fact that they are areas that have not recently been reactivated. However, the margins of the Iberian Peninsula have undergone a number of more recent tectonic events, like the Neogene rifting of the Valencia Trough to the east and the Miocene Betic orogeny and Neogene Alboran Sea extension to the south (Vegas and Banda, 1982). Beneath these tectonically active areas, a pronouncedly low Pn velocity, with values down to 7.7 km/s is found. Our model might show the Betic crustal root is underlain by a negative anomaly beneath the southeastern Iberian Peninsula. The Alboran Sea is imaged as a region of high velocity in the north and low velocity in the south. The results of one of the refraction profiles conducted in the southern central part of the Alboran Sea (Working Group for Deep Seismic Sounding in the Alboran Sea, 1978) showed anomalously low upper mantle velocity of 7.5 km/s, coinciding with the low values obtained in this study. However, the results of the profiles carried out in the western Alboran Sea showed high velocities (about 8.4 km/s) near to the Straits of Gibraltar. On the other hand, in the Atlantic Ocean we find one of the most interesting outcomes of this study: the sharp variation in the uppermost mantle velocities in the western Straits of Gibraltar, coinciding with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Available results show that its complexity is probably due to the location and geodynamic evolution of the western Straits. Convergence processes along the plate boundary could have been influenced by passive continental margin formation processes to the north, and plate collision and post collision processes that occurred across the Gibraltar arc, to the east. Our results offshore from Cape San Vicente show a very pronounced low-velocity anomaly, while in the Gulf of Cadiz high velocities are distributed along the coast. Southwest from Cape San Vicente, a strong low-velocity anomaly is imaged. The crustal structure obtained by Gonz´alez et al. (1996) using seismic refraction and wide-angle reflection data along a 350 km NE–SW oriented transect, reveals a strong but continuous crustal thinning from 30 km onshore Iberia to less than 15 km at the southwestern end of the profile. In
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addition, they performed 2D gravity modelling to validate the crustal structure obtained from seismic data. According to the authors, away from the continental slope, the thickness of the crust reaches 14 km (including water depth) and the upper mantle velocity is 7.8–7.9 km/s. This velocity anomaly agrees with the low values (<7.75 km/s) obtained offshore from Cape San Vicente. Otherwise, according to Gonz´alez et al. (1998), on the boundary between the Iberian Meseta and the Guadalquivir Basin (Fig. 1) the refracted Pn waves in the upper mantle emerge as a first arrival at source–receiver ranges of 120 km with an apparent velocity of 8.0 km/s. In the profiles situated in SPZ (Fig. 1), the upper mantle velocity of 8.0 km/s is derived from PMP amplitudes, critical distance and some Pn arrivals. In both cases, data agree with our own mean Pn velocities for these areas. On the other hand, Calvert et al. (2000) use two data sets of 430 and 530 events to solve for isotropic and anisotropic components of the mantle velocity structure along the Africa–Iberia plate boundary zone. Despite having less data, in the common areas the results are very similar to our own. The most significant velocity feature was a robust low-Pn-velocity anomaly imaged beneath the internal Betic and high velocities imaged beneath the Straits of Gibraltar, Iberian Meseta and Gulf of Cadiz. We have found that seismic anisotropy is an important feature in the study area. Two major anisotropic domains, characterized by Pn velocity anisotropy up to 4%, single out a part of the Betic Cordillera and southwestern Iberian massif. In the Alboran Sea and northern Morocco, the direction of fastest Pn velocity is oriented almost parallel to the Africa–Eurasia plate convergence vector (NW–SE) whereas northward, this direction is almost parallel to the main trend of the Betic Cordillera: E–W in its central part and N–S in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, in the Alboran Sea the resulting anisotropy would be parallel to the direction of maximum compression, where it must be assumed that the Neogene extension had no major influence, on a lithospheric scale. Finally, we have mapped upper mantle seismic anisotropy beneath the Iberian massif and, although an adequate coverage of the ray paths has been achieved,
we have been unable to reach definitive conclusions. Our results show a quite complex pattern of anisotropy since the relationship between the azimuths of seismic anisotropy in the SW Iberian lithospheric mantle and major Hercynian tectonic trends could not be directly established. In the OMZ, D´ıaz et al. (1998) analyzed teleseismic shear-wave splitting and obtained a fast velocity direction oriented N40◦ E which is nearly identical to that found in our study. Abalos and D´ıaz Cus´ı (1995) have argued that the Variscan orogeny may not have affected the whole OMZ, although there is evidence that the Cadomian orogeny affected the entire lithosphere; the Cadomian stretching direction is oriented N30◦ E in proximity to the fast velocity direction inferred from the present data.
Acknowledgements This work has been supported by Spanish DGI project REN2002-04198-C02-01 by UE-FEDER and by the research group RNM104 of Junta de Andaluc´ıa (Spain). The authors thanks the reviewers the valuable advices and recommends to improve this paper.
References Abalos, B., D´ıaz Cus´ı, J., 1995. Correlation between seismic anisotropy and major geological structures in SW Iberia: a case study on continental lithosphere deformation. Tectonics 14, 1021–1040. Babuska, V., Cara, M., 1991. Seismic Anisotropy in the Earth. Kluwer, Dordrecht, 217 pp. Badal, J., Corchete, V., Payo, G., Canas, J.A., Pujades, L., 1993. Shear-wave velocity structure below the Iberian Peninsula as obtained by a detailed analysis of surface-waves. Tectonophysics 225 (3), 167–190. Barley, D.K., 1992. Episodic alkaline igneous activity across Africa: implications for the causes of continental breakup. In: Storey, B.C., Alabaster, T., Pankhurst, R.J. (Eds.), Magmatism and the Causes of Continental Breakup. Geological Society, London, pp. 91–98. Burke, K., 1996. The African Plate. S. Afr. J. Geol. 99, 341–409. Calvert, A., Sandvol, E., Seber, D., Barazangi, M., Vidal, F., Alguacil, G., Jabour, N., 2000. Propagation of regional seismic phases (Lg and Sn) and Pn velocity structure along the Africa–Iberia plate boundary zone: tectonic implications. Geophys. J. Int. 142, 384–408. Corchete, V., Badal, J., Pujades, L., Canas, J.A., 1993. Shear velocity structure beneath the Iberian Massif from broad-band Rayleighwave data. Phys. Earth Planet. Interiors 79 (3–4), 349–365.
I. Serrano et al. / Physics of the Earth and Planetary Interiors 150 (2005) 317–330 Crespo-Blanc, A., Orozco, M., 1988. The southern Iberian Shear Zone: a major boundary in the Hercynian folded belt. Tectonophysics 148, 221–227. Dallmeyer, R., Mart´ınez Garc´ıa, D. (Eds.), 1992. Pre-Mesozoic Geology of Iberia. Springer-Verlag, Berlin, p. 416. De Jong, K., 1991. Tectono-metamorphic studies and radiometric dating in the Betic Cordilleras (SE Spain) – with implications for the dynamics of extension and compression in the western Mediterranean area. Ph.D. Thesis. Amsterdam Christian University, 204 pp. Dewey, J.F., Helman, M.L., Torco, E., Hutton, D.H.W., Knott, S.D., 1989. Kinematics of the western Mediterranean. In: Coward, M.P., Dietrich, D., Park, R.G. (Eds.), Alpine Tectonics, vol. 45. Geological Society Special Publication, pp. 265– 283. D´ıaz, J., Hirn, A., Gallart, J., Senos, L., 1993. Evidence for azimuthal anisotropy in southwest Iberia from deep seismic sounding data. Phys. Earth Planet. Interiors 78, 193–206. D´ıaz, J., Hirn, A., Gallart, J., Abalos, B., 1996. Upper-mantle anisotropy in SW Iberia from long-range seismic profiles and teleseismic shear-wave data. Phys. Earth Planet. Interiors 95, 153–166. D´ıaz, J., Gallart, J., Hirn, A., Paulssen, H., 1998. Anisotropy beneath the Iberian Peninsula: the contribution of the ILIHANARS Broad-band Experiment. Pure Appl. Geophys. 151, 395– 405. Fuchs, K., 1983. On the properties of deep crustal reflectors. J. Geophys. 35, 133–149. Garc´ıa-Due˜nas, V., Balany´a, J.C., Mart´ınez-Mart´ınez, J.M., 1992. Miocene extensional detachments in the outcropping basement of the northern Alboran basin (Betics) and their tectonic implications. Geo-Mar. Lett. 12, 88–95. Gonz´alez, A., Torn´e, M., C´ordoba, D., Vidal, N., Matias, L.M., D´ıaz, J., 1996. Geophys. Res. Lett. 23 (18), 2477–2480. Gonz´alez, A., C´ordoba, D., Vegas, R., Matias, L.M., 1998a. Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz. Tectonophysics 296, 317–331. Hearn, T.M., 1984. Pn travel times in Southern California. J. Geophys. Res. 89 (B3), 1843–1855. Hearn, T.M., 1996. Anisotropic Pn tomography in the western United States. J. Geophys. Res. 101 (B2), 8403–8414. Hearn, T.M., 1999. Uppermost mantle velocities and anisotropy beneath Europe. J. Geophys. Res. 104 (B7), 15123–15139. Hearn, T.M., Ni, J.F., 2001. Tomography and location problems in China using regional travel-time data. In: 23rd Seismic Research Review: Worldwide Monitoring of Nuclear Explosions, October 2–5. Gonz´alez, A., C´ordoba, D., Vegas, R., Matias, L.M., 1998b. Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz. Tectonophysics 296, 317–331. ILIHA DSS Group, 1993. A deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the Iberian Peninsula. Tectonophysics 221, 35–51. Jabaloy, A., Galindo-Zald´ıvar, J., Gonzalez-Lodeiro, F., 1992. The Mecina Extensional System – its relation with the PostAquitanian piggyback basins and the paleostresses evolution (Betic Cordilleras Spain). Geo-Mar. Lett. 12 (2-3), 96–103.
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Judenherc, S., Granet, M., Boumbar, N., 1999. Two-dimensional anisotropic tomography of lithosphere beneath France using regional arrival times. J. Geophys. Res. 104 (B6), 13201– 13215. Jolivet, L., Faccenna, C., 2000. Mediterranean extension and the Africa–Eurasia collision. Tectonics 19 (6), 1095–1106. Lees, J.M., Crosson, R.S., 1989. Tomographic inversion for 3dimensional velocity structure at Mount St-Helens using earthquake data. J. Geophys. Res. Solid Earth Planets 94 (B5), 5716–5728. Leven, J.H., Jackson, I., Ringwood, A.E., 1981. Upper mantle seismic anisotropy and lithospheric decoupling. Nature 289, 234–239. Maldonado, A., Campillo, A.C., Mauffret, A., Alonso, B., Woodside, J., Campos, J., 1992. Alboran Sea late Cenozoic tectonic and stratigraphic evolution. Geo-Mar. Lett. 12, 179– 186. Maupin, V., Cara, M., 1992. Love–Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian Peninsula. Pure Appl. Geophys. 138 (3), 429–444. McKenzie, D., 1972. Active tectonics of the Mediterranean region. Geophys. J. Roy. Astron. Soc. 30, 109–185. Mele, G., 1998. Pn anisotropy in the northern Apennine Chain (Italy). Pure Appl. Geophys. 151 (2–4), 495–502. Mueller, S., Prodehl, C., Mendes, A.S., Sousa Moreira, V., 1973. Crustal structure in the southwestern part of the Iberian Peninsula. Tectonophysics 20, 307–318. Paige, C.C., Saunders, M.A., 1982. Algorithm-583 – LSQR – Sparse linear-equations and least-squares problems. ACM Trans. Mathematical Software 8 (2), 195–209. Prodehl, C., Moreira, V.S., Mueller, S., Mendes, A.S., 1975. Deepseismic sounding experiments in central and southern Portugal. In: General Assembly of the European Seimological Commission 14th, Berlin, DDR National-Komitee f¨ur Geod¨asie und Geophysik, Berlin, pp. 261–266. Silver, P.G., Chan, W.W., 1988. Implications for continental structure and evolution from seismic anisotropy. Nature 335 (6185), 34–39. Silver, P.G., 1996. Seismic anisotropy beneath the continents: probing the depths of geology. Annu. Rev. Earth Planet. Sci., 385– 432. Quesada, C., 1991. Geological constraints on the Paleozoic tectonic evolution of tectonostratigraphic terranes in the Iberian Massif. Tectonophysics 185, 225–245. Quesada, C., Fonseca, P.E., Munha, J., Oliveira, J.T., Ribeiro, A., 1994. The Beja–Acebuches Ophiolite (southern Iberia Variscan fold belt): geological characterization and geodynamic significance. Bol. Geol. Min. 105, 3–49. Tanimoto, T., Anderson, D.L., 1984. Mapping convection in the mantle. Geophys. Res. Lett. 11, 287–290. Vegas, R., Banda, E., 1982. Tectonic framework and alpine evolution of the Iberian Peninsula. Earth Evol. Sci. 2 (4), 320– 343. Vinnik, L.P., Farra, V., Romanowicz, B., 1989. Azimuthal anisotropy in the Earth from observations of SKS at GEOSCOPE and NARS broadband stations. Bull. Seismol. Soc. Am. 79 (5), 1542–1558.
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Working Group for Deep Seismic Sounding in the Alboran Sea, 1978. Crustal seismic profiles in the Alboran Sea – preliminary results. Pageoph 116, 167–180. Zeck, H.P., 1996. Betic–Rif orogeny: subduction of Mesozoic Tethys lithosphere under eastward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics. Tectonophysics 254, 1–16. Zeck, H.P., 1997. Mantle peridotites outlining the Gibraltar Arc – centrifugal extensional allochthons derived from the earlier
Alpine, westward subducted nappe pile. Tectonophysics 281, 195–207. Zeck, H.P., Moni´e, P., Villa, I.M., Hansen, B.T., 1992. Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras southern Spain. Geology 20, 79–82. Zeck, H.P., 1999. Alpine plate kinematics in the western Mediterranean: a westward directed subduction regime followed by slab roll-back and slab detachment. Geol. Soc. London Spec. Publ. 156, 109–120.
Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula I. Serrano a, ∗ , T.M. Hearn b , J. Morales a, c , F. Torcal a, d a
d
Instituto Andaluz de Geof´ısica, Universidad de Granada, 18071 Granada, Spain b Department of Physics, New Mexico State University, Las Cruces, USA c Departamento de F´ısica Te´ orica y del Cosmos, Universidad de Granada, Spain 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 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and north–south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established. © 2004 Elsevier B.V. All rights reserved. Keywords: Seismic anisotropy; Pn seismic waves; Tomography; Iberian Peninsula ∗
Corresponding author. Tel.: +34 958248912; fax: +34 958160907. E-mail address: [email protected] (I. Serrano).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.12.003
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1. Introduction The southern half of the Iberian Peninsula is made up of the Iberian massif and the Betic Cordillera (formed by the External and Internal Zones), which are connected through the Guadalquivir Basin. The Gulf of Cadiz represents the continuation of this basin on the continental and Algarve margins along the Mesozoic border of the Iberian massif. The Betic Cordillera and Moroccan Rif are connected through the Gibraltar Strait, which developed during Cenozoic convergence between Africa and Iberia (e.g. Dewey et al., 1989) making an arcuate belt. The Alboran Sea is located on the inner part of this in continuity to the east with the South Balearic Basin (Fig. 1). The Iberian massif forms part of the Variscan belt of central Europe that resulted from the late Paleozoic
suture of Gondwana and Laurasia. Its southern edge is composed of two lithostructural units from north to south: the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ). The SPZ is located in the westernmost sector of the Hercynian belt, where there are no outcrops of the deep continental crust. Its sedimentary record comprises Upper Devonian and Carboniferous sequences. However, seismic and gravity studies have revealed the presence of older crust of an unknown age (Mueller et al., 1973; Prodehl et al., 1975, etc.). Recent studies on the boundary between the SPZ and the OMZ have interpreted this as a major suture of the European Variscan Orogen (CrespoBlanc and Orozco, 1988; Quesada, 1991). The SPZ is believed to have been accreted to the rest of the Iberian massif during the orogeny, and to represent a fragment of a plate that was otherwise destroyed
Fig. 1. Simplified geological map of the study area showing the main tectonic units. The upper right-hand corner shows a small sketch of the main geological structures such as the Betic and Rif Cordilleras. SPZ: South Portuguese Zone. OMZ: Ossa Morena Zone. GB: Granada Basin, GD: Guadalquivir Depression.
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by subduction beneath the OMZ (Quesada et al., 1994). In contrast, the Betic–Rif mountain belt forms the southwestern termination of the Alpine orogen in Europe and northern Africa. Its ambivalent position straddling the plate boundary between Iberia–Europe and Africa is an orogenic artefact produced by late stage emplacement on the African foreland of secondary extensional allochthons derived from the European Alpine collisional belt (Zeck et al., 1992; Zeck, 1996, 1997, 1999; De Jong, 1991). The opening of the North Atlantic during the Late Cretaceous and Tertiary induced the rotational divergence of North America and Eurasia, which was paralleled by the counter clockwise convergence of Africa and Eurasia. The northwards drift of Africa caused the progressive closure of oceanic basins of Tethys and the rapid westward propagation of the Alpine Betic–Rif orogenic collision front in the Gulf of Cadiz, in parallel with the development of the western Mediterranean basins (Dewey et al., 1989; Garc´ıa-Due˜nas et al., 1992; Jabaloy et al., 1992; Maldonado et al., 1992). From this complicated yet intriguing geological framework, we deduce that the present study area provides a suitable opportunity to research the presence of anisotropy, a parameter used to describe a medium whose elastic properties are functions of orientation. It is now generally accepted that seismic anisotropy in the upper mantle is due primarily to the deformation-induced lattice preferred orientation of olivine crystals. This preferred orientation could have several causes. It may be related to passive motion of the lithosphere over the stationary asthenosphere, in which case, the most likely location for anisotropy would be in the transition zone between the two shells and the fast direction of anisotropy would be close to that of plate motion (Leven et al., 1981). Alternatively, the preferred orientation could be induced by flow in the asthenosphere, so the fast direction of anisotropy would coincide with the direction of flow but might differ from the direction of plate motion (Tanimoto and Anderson, 1984). Azimuthal anisotropy in the uppermost mantle may also be related to past and present deformation of the lithosphere (Fuchs, 1983). Our results suggest that anisotropy in the southern half of the Iberian Peninsula may result from a range of causes and could occur at different depths.
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The existence of upper mantle anisotropy beneath the Iberian Peninsula has been recorded by using a variety of techniques and results are not easily comparable. While Pn data measure the anisotropy within the top of the mantle, SKS data measure the vertically integrated anisotropy of the mantle; in this paper we show both results. Vinnik et al. (1989) presented the first results of the existence of upper mantle anisotropy beneath the Iberian Peninsula based on the analysis of SKS and similar phases at several broadband stations of the GEOSCOPE network and the NARS array. Their data for Europe included that from the two stations in central Spain and generally showed an E–W direction for fast velocity. This confirmed the direction previously found in central Spain by Silver and Chan (1988). However, Maupin and Cara (1992) used surface wave data and found no evidence of large scale anisotropy in the uppermost 100 km, indicating that the anisotropy observed in the shear waves could be related to deeper levels. Later, Badal et al. (1993) and Corchete et al. (1993) obtained a lateral variation in velocity at five depth intervals based on a detailed analysis of Rayleigh wave dispersion. In the same year, the models derived from the interpretation of data recorded by the Iberian Lithosphere Heterogeneity and Anisotropy experiment (ILIHA DSS Group, 1993; D´ıaz et al., 1993) showed the vertical heterogeneity of the lower lithosphere and azimuthal anisotropy at different depth levels. However, the anisotropy orientation deduced from these data is different from that obtained by Vinnik et al. (1989). To explain this discrepancy the authors suggested the existence of vertical heterogeneity in the anisotropic properties over the mantle. Later, Abalos and D´ıaz Cus´ı (1995) postulated a correlation between the lithospheric structure and seismic anisotropy from geophysical data and the major structures and tectonic history of the outcropping basement rocks in SW Iberia. According to these authors, the deviations and aerial variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust in this region indicate that the tectonic model of orogen-parallel lithospheric deformation does not appear to be linked to Hercynian surface geology and the structural geology of the subcontinental lithospheric mantle. From these data, Abalos and D´ıaz reconstructed a coherent tectonic model of polyorogenic lithospheric deformation in the southwestern Iberian massif (ibid.). Analysis
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of SKS and S phases recorded by a portable network installed in the same area of SW Iberia as that explored in the ILIHA DSS profiles (D´ıaz et al., 1996) suggests a quite consistent anisotropic structure with roughly a NE–SW to E–W fast velocity direction. The presence of anisotropy beneath the whole Iberian Peninsula was later established through the analysis of teleseismic shear-wave splitting observed in the broadband stations located over the entire peninsula (D´ıaz et al., 1998). This work was extremely significant as it provided the first anisotropy constraints beneath different tectonic domains of the Iberian Peninsula from a homogeneous seismic network. The authors postulate that regional variations in the anisotropic parameters imply that differentiated origins of the anisotropy have to be considered in some areas in relation to their particular lithospheric geodynamics. For example, within the Betic Cordillera, anisotropic results for the South Iberian domain contrast sharply with those for the Alboran crustal domain. Lastly, an inversion of Pn travel-time residuals was performed by Calvert et al. (2000) along the
Africa–Iberia plate boundary zone using a code developed by Hearn (1996) which solves for isotropic and anisotropic components of the mantle velocity structure. There is a strong correlation between results obtained in the central and western Betic Cordillera and those obtained in southernmost Iberia. The imaged fast axes have a predominant E–W trend in the Betics that becomes more NW–SW in the western internal zones before trending almost N–S beneath the Straits of Gibraltar.
2. Data set and methodology The data used in this study are Pn waveforms recorded by several networks situated in southern Spain and northern Morocco. From the Andalusian Seismic Network (RSA), which is operated by the Instituto Andaluz de Geof´ısica (IAG), we have collected data on earthquakes from 1988 to March 2003. And, the Real Observatorio de la Armada (ROA) in San Fernando (C´adiz) and the Instituto Geogr´afico Nacional (IGN)
Fig. 2. Distribution of seismicity for the study period and seismic stations located in the area. We used data from a total of 121 stations belonging to Spanish and Moroccan institutions.
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have kindly provided data from 1988 to the present. Coverage in the African continent has been obtained thanks to the collaboration of the Centre National de Coordination et de Planification de la Recherche Scientifique et Technique (CNCPRST) and the Physique du Globe at Mohamed V University (MOH V), both in Rabat (Morocco). In order to take advantage of the station distribution (Fig. 2) and provide additional data coverage in the Atlantic coast and Algerian area, those farthest from the Spanish seismic stations, we drew on data from the ISC catalogue. We used data from 121 stations in total, but a considerable imbalance exists between the number of stations on the Iberian Peninsula and those on the African continent. We define Pn as the first arrival from regional events between 200 and 1200 km. We do not keep phases for distances under 200 km to avoid misidentification errors. Data were selected with the following criteria: inversion only uses events with over 10 recorded arrivals, stations with over 10 recorded arrivals, and event depth less than 30 km. From our initial set of 39,872 traveltimes (7675 earthquakes) a total of 11,612 Pn first arrival times met our selection criteria. Some 73% of the initial data set are recorded at an epicentral distance from 200 to 500 km and about 27% are at an epicentral distance of between 500 and 1200 km. Arrivals with residuals of over 4 s were eliminated (Fig. 3).
Fig. 3. Initial travel-time residuals. We selected the earthquakes recorded at an epicentral distance of between 200 and 1200 km. Arrivals with residuals of over 4 s were eliminated.
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The final set of ray paths for the selected data is shown in Fig. 4. The ray path density is highest in the southern Iberian Peninsula, offshore from Cape San Vicente and in the Alboran Sea. The travel-times collected were inverted in the same way that was used by Hearn (1996). The Pn traveltime residuals are described as the sum of three time terms: tij = ai + bj + dijk (sk + Ak cos 2φ + Bk sin 2φ) where φ is the back azimuth angle, ai the static delay for station i, bj the static delay for event j, dijk the distance travelled by ray ij in mantle cell k, and sk the slowness perturbation (inverse velocity) of cell k. Ak and Bk are the anisotropic coefficients for cell k. (Hearn, 1984). The unknown quantities in this equation are the station and event delays ai and bi , the mantle slowness perturbation sk , and the two coefficients of anisotropy Ak and Bk . The magnitude of 1/2 the anisotropy for cell k is given by (A2k + Bk2 ) and the direction of fastest wave propagation is given by (1/2)arctan(Bk /Ak ). The tomographic method used is a preconditioned version of Paige & Saunders’ LSQR algorithm (1982). In solving the set of travel-time equations the cell size used is small and a set of Laplacian damping equations regularize the solution (Lees and Crosson, 1989). Two damping constants are separately applied to the unknown slowness and anisotropic coefficients. A proper pair of damping constants is chosen to balance the error size and the resolution width. The trade-off between velocity and anisotropy variations has been checked by using different combinations of damping parameters for both velocity and anisotropy. From our data the features of the velocity and anisotropy fields in the study are observed to be considerably stable, varying slightly for different combinations of the damping constants; the estimated standard errors of the data for 16 different combinations of the damping parameters range between 0.52 and 0.56 s. The small differences in standard errors show that all results are equally valid solutions for the inverse problem. The final damping constants were 100 for the velocity and 500 for the anisotropy, giving an estimated data standard error of 0.54 s, with a cell size of 0.25◦ × 0.25◦ which provides a significant number of hits in each cell and balances uncertainty and resolution.
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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 better coverage is obtained in the south of the Iberian Peninsula.
3. Results 3.1. Resolution Evaluations of the resolution width are made by using four test models. All models consisted of a checkerboard velocity pattern, with the first model having a square size of 3◦ by 3◦ and the second, third and fourth models having a square size of 2◦ by 2◦ , 1◦ by 1◦ and 3/4◦ by 3/4◦ . They were constructed from a synthetic data set computed using the ray paths from our database. The synthetic data was then inverted in the same way as the real data. The squares were assigned velocities that alternated between +0.3 and −0.3 km/s. Station and event delays were set to zero and no noise was included. For the first model, the 2◦ by 2◦ squares were resolvable for the entire imaged region, except southwest of the Balearic Islands. Squares as small as 1◦ were resolved for the whole of the Iberian Peninsula studied, the Alboran Sea and offshore from Cape San Vicente, and were not well resolved in northern
Africa and the southwestern Balearic Islands. In the third model, 3/4◦ by 3/4◦ squares were resolved in the south of the Iberian Peninsula and the western Alboran Sea. Results for the second (2◦ by 2◦ ), third (1◦ by 1◦ ) and fourth models (3/4◦ by 3/4◦ ) are shown in Fig. 5. 3.2. Station delays Station delays represent variations in crustal thickness and velocity relative to an initial model and they are the most robust portions of the inversion. For a mean crustal velocity of 6.2 km/s and a thickness of 33 km, a station delay of 1 s. corresponds to a change of 9.8 km in thickness or a change of 0.7 km/s in mean crustal velocity. However, it is difficult to distinguish between the crustal velocity and the crustal thickness contributions in the crustal delay times. Our results show some domains, though in general slight delays were obtained at most of the seismic stations. No delays or weakly positive delays (0.2–0.5 s)
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Fig. 5. Checkerboard test model with 2◦ by 2◦ , 1◦ by 1◦ and 3/4◦ by 3/4◦ cell size.
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were obtained in almost all the stations in the Betic Cordilleras, which is probably a consequence of an underestimated Moho depth beneath this area (the Moho being modelled by a surface at 33 km depth). The exceptions were found in the easternmost station of the Betic Cordillera (near Cabo de Gata, western Alboran Sea) and in the southeasternmost part of the Straits of Gibraltar, which showed negative delays (near 0.5 s). These negative anomalies can be interpreted as an effect of the variations in crustal thickness, which ranges from 36 km underneath the Betic and Rif Chain to <12 km beneath the easternmost part of the Alboran Sea. In the western Alboran Sea the Moho lies at a constant depth of ±18 km, deepening sharply toward the Gibraltar Strait where it reaches 30–32 km. The negative delays (nearly 0.5 s) in northern Africa could be explained in the same way. On the other hand, to the north and northwest the negative delays (from 0.2 to 0.5 s) could reflect the moderate crustal thinning toward the Iberian massif and/or would be an effect of the old and stable Hercynian crust (Judenherc et al.,
1999). One of the longest negative delays (>0.5 s) was obtained at the station on the south Portuguese coast (near Faro). Crustal thickness from the Guadalquivir Basin/Iberian massif contact to the southeastern SPZ, shows gradual lateral changes (Gonz´alez et al., 1998a, 1998b). However, near this area the same authors obtained a high apparent crustal velocity (6.4 km/s) that they interpret as a deeper level of the upper crust related to ultramafic rock outcropping. This negative delay is probably due to a combination of a thinner than average crust and an increase in mean crustal velocity. 3.3. Uppermost mantle velocities The travel-time data were fitted to a straight line as a function of distance to determine the mean Pn velocity from the inverse slope. The straight line fit gave an estimated velocity of 7.98 km/s and an intercept of 7.1 s. Lateral variations of Pn velocity are imaged as perturbations from this average velocity. The tomographic
Fig. 6. Inversion results for Pn velocity. Red and blue indicate saturated for low and high velocity, respectively. Pn velocity in the study area varies from 7.7 to 8.3 km/s.
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inversion used 25 LSQR iterations which reduced the root-mean-square error of the residuals to 0.5 s. The Pn velocity varies in the study area from 7.7 to 8.3 km/s., showing a low velocity zone parallel to the Moroccan coast on the African continent (Fig. 6). On the southeast coast of Spain, the low velocity zone coincides with the Betic Cordillera and extends towards the Balearic Islands. An extensive high-velocity zone spreads on the Iberian massif and offshore from the Moroccan Atlantic coast. We cannot determine the velocity beneath the Rif Codillera with great accuracy due to the poor coverage of the ray paths. The Alboran Sea is imaged as a region of high velocity in the north and low velocity in the south. Southwest from Cape San Vicente, a strong lowvelocity anomaly is imaged that contrasts with the mean values obtained onshore on the Iberian Peninsula. The low values reach 7.75 km/s offshore and spread across a wide area off Cape San Vicente. Otherwise, on the boundary between the Iberian Meseta and the
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Guadalquivir Basin (Fig. 1) the refracted Pn waves in the upper mantle show a mean velocity of 8.0 km/s. 3.4. Seismic anisotropy The map of Pn anisotropy shows considerable variation in both magnitude and direction beneath the study area (Figs. 7 and 8). The magnitude of anisotropy ranges 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 variation in kilometres per second and each Pn anisotropy estimate represents the average anisotropy in a region of 1◦ . Features much smaller than this do not appear in Fig. 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 trends NW–SW from the latest Tortonian to the present day (Dewey et al., 1989, etc.). However, the anisotropy in
Fig. 7. Pn anisotropy in the study area. The fast direction of Pn velocity is drawn, and the bar length is proportional to the amount of anisotropy in that direction. The magnitude of anisotropy ranges from 0 to 4% in the study area. Bottom right, we have inserted the anisotropic results of D´ıaz et al. (1998); white arrows show the fast velocity direction obtained and are proportional to the time delay; thin black arrows show the null measurements.
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Fig. 8. Image of the magnitude of Pn anisotropy. Darker shades correspond to more anisotropy.
S–SE Spain shows a rapid shift in the orientation of the fast direction, running almost parallel to the main trend of the Betic Cordillera: E–W in its central part and N–S in the curvature of the Arc of Gibraltar. The most important values, about 0.4 s are obtained in this last region. The present-day setting of the Mediterranean region is characterized by Neogene subsident basins like the Alboran Sea, surrounded by mountain belts like the Betic and Rif Cordilleras; only remaining Mesozoic oceanic crust is being subducted below the Calabrian and Hellenic arcs. The most recent studies and interpretations indicate a significant increase in the absolute velocity of Africa at 30 Ma and, a significant decrease after 20 Ma. The absolute motion of Africa toward the north is, however, always faster than the convergence velocity because the absolute motion of Eurasia is also northward with an average velocity of ∼1 cm/yr (Jolivet and Faccenna, 2000). In addition, several authors have described the slowing down of Africa as a 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 region, the resulting anisotropy would be parallel to the direction of maximum compression, as is happening in the Alboran Sea where it must be assumed that the Neogene extension had no major influence, on a lithospheric scale. However, in the uppermost mantle beneath the Betic Cordillera the pattern of Pn anisotropy appears to be strongly related to the regional tectonic trend, following the arc-structure of the belt. During the deformation process the fast axes of upper mantle anisotropic minerals tend to align with the longest axis of the ellipsoid strain, which is perpendicular to the direction of maximum compression in the pure shear deformation regime (McKenzie, 1979, etc.). Meanwhile, the slow axis tends to align parallel with the direction of maximum compression (Babuska and Cara, 1991; Silver, 1996). In orogenic belts, where pure shear deformations are likely to occur, the fastest direction of Pn velocity would thus be perpendicular to the direction of orogenic compression (Silver and Chan, 1988). Therefore the anisotropy fast orientation could be aligned
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along the Betic arc in response to regional compression across the arc. Many other convergent margins in the Mediterranean region, such as the Alps, the Apennines, and the Dinaric-Hellenic mountain chains exhibit exceptionally high arc-parallel anisotropy (Mele, 1998; Hearn, 1999). Pn anisotropy coincides with the direction of the Neogene and the present-day extension beneath the central Betic Cordilleras (for example, the Granada Basin), suggesting that this deformation might have controlled a preferred orientation of mineral in the uppermost mantle. However, contrary to what would be expected, the parallelism between the azimuths of seismic anisotropy in the southwestern Iberian lithospheric mantle and major Hercynian tectonic trends is not clear. In the Ossa Morena and SPZ the direction of anisotropic fast-Pnvelocity trends is NE–SW, very much like those obtained by D´ıaz et al. (1993), while Hercynian structures in the central and southern Ossa-Morena trend NW–SE to E–W. The present day stress state at OMZ trending N150◦ E is almost at an oblique angle to the fast propagation direction obtained. Moreover, the absolute plate motion rate of the Iberian plate is very small to be consistent with the results of the anisotropy (Abalos and D´ıaz Cus´ı, 1995). Finally, the trends of the last episode (Hercynian) of internal coherent deformation also differ from the direction obtained. Other processes should be considered in order to explain the anisotropy pattern found in the southwestern Iberian massif. It may even be the case that the mantle strain pattern has been obscured by the polyorogenic and intricate tectonic evolution of the zone (Abalos and D´ıaz Cus´ı, 1995).
4. Discussion and conclusions A principal result of this study has been to infer the velocity and anisotropy structure in the uppermost part of the mantle beneath the southern part of the Iberian Peninsula and surrounding regions. Regional variations of Pn velocity throughout the study area show good correlation with surface tectonics. According to Hearn and Ni (2001), high Pn velocity regions are stable cratonic areas which were not greatly affected by tectonic and magmatic activity for a long time resulting in a lower temperature in the mantle lid and higher Pn velocity. In Fig. 6, an extensive area of the cen-
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tral and western Iberian Peninsula is covered by the Variscan Iberian massif, a large, old and geologically stable block of continental lithosphere (Dallmeyer and Mart´ınez Garc´ıa, 1992). This area is characterized by high Pn velocity, as expected in tectonically stable regions, emphasizing the fact that they are areas that have not recently been reactivated. However, the margins of the Iberian Peninsula have undergone a number of more recent tectonic events, like the Neogene rifting of the Valencia Trough to the east and the Miocene Betic orogeny and Neogene Alboran Sea extension to the south (Vegas and Banda, 1982). Beneath these tectonically active areas, a pronouncedly low Pn velocity, with values down to 7.7 km/s is found. Our model might show the Betic crustal root is underlain by a negative anomaly beneath the southeastern Iberian Peninsula. The Alboran Sea is imaged as a region of high velocity in the north and low velocity in the south. The results of one of the refraction profiles conducted in the southern central part of the Alboran Sea (Working Group for Deep Seismic Sounding in the Alboran Sea, 1978) showed anomalously low upper mantle velocity of 7.5 km/s, coinciding with the low values obtained in this study. However, the results of the profiles carried out in the western Alboran Sea showed high velocities (about 8.4 km/s) near to the Straits of Gibraltar. On the other hand, in the Atlantic Ocean we find one of the most interesting outcomes of this study: the sharp variation in the uppermost mantle velocities in the western Straits of Gibraltar, coinciding with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Available results show that its complexity is probably due to the location and geodynamic evolution of the western Straits. Convergence processes along the plate boundary could have been influenced by passive continental margin formation processes to the north, and plate collision and post collision processes that occurred across the Gibraltar arc, to the east. Our results offshore from Cape San Vicente show a very pronounced low-velocity anomaly, while in the Gulf of Cadiz high velocities are distributed along the coast. Southwest from Cape San Vicente, a strong low-velocity anomaly is imaged. The crustal structure obtained by Gonz´alez et al. (1996) using seismic refraction and wide-angle reflection data along a 350 km NE–SW oriented transect, reveals a strong but continuous crustal thinning from 30 km onshore Iberia to less than 15 km at the southwestern end of the profile. In
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addition, they performed 2D gravity modelling to validate the crustal structure obtained from seismic data. According to the authors, away from the continental slope, the thickness of the crust reaches 14 km (including water depth) and the upper mantle velocity is 7.8–7.9 km/s. This velocity anomaly agrees with the low values (<7.75 km/s) obtained offshore from Cape San Vicente. Otherwise, according to Gonz´alez et al. (1998), on the boundary between the Iberian Meseta and the Guadalquivir Basin (Fig. 1) the refracted Pn waves in the upper mantle emerge as a first arrival at source–receiver ranges of 120 km with an apparent velocity of 8.0 km/s. In the profiles situated in SPZ (Fig. 1), the upper mantle velocity of 8.0 km/s is derived from PMP amplitudes, critical distance and some Pn arrivals. In both cases, data agree with our own mean Pn velocities for these areas. On the other hand, Calvert et al. (2000) use two data sets of 430 and 530 events to solve for isotropic and anisotropic components of the mantle velocity structure along the Africa–Iberia plate boundary zone. Despite having less data, in the common areas the results are very similar to our own. The most significant velocity feature was a robust low-Pn-velocity anomaly imaged beneath the internal Betic and high velocities imaged beneath the Straits of Gibraltar, Iberian Meseta and Gulf of Cadiz. We have found that seismic anisotropy is an important feature in the study area. Two major anisotropic domains, characterized by Pn velocity anisotropy up to 4%, single out a part of the Betic Cordillera and southwestern Iberian massif. In the Alboran Sea and northern Morocco, the direction of fastest Pn velocity is oriented almost parallel to the Africa–Eurasia plate convergence vector (NW–SE) whereas northward, this direction is almost parallel to the main trend of the Betic Cordillera: E–W in its central part and N–S in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, in the Alboran Sea the resulting anisotropy would be parallel to the direction of maximum compression, where it must be assumed that the Neogene extension had no major influence, on a lithospheric scale. Finally, we have mapped upper mantle seismic anisotropy beneath the Iberian massif and, although an adequate coverage of the ray paths has been achieved,
we have been unable to reach definitive conclusions. Our results show a quite complex pattern of anisotropy since the relationship between the azimuths of seismic anisotropy in the SW Iberian lithospheric mantle and major Hercynian tectonic trends could not be directly established. In the OMZ, D´ıaz et al. (1998) analyzed teleseismic shear-wave splitting and obtained a fast velocity direction oriented N40◦ E which is nearly identical to that found in our study. Abalos and D´ıaz Cus´ı (1995) have argued that the Variscan orogeny may not have affected the whole OMZ, although there is evidence that the Cadomian orogeny affected the entire lithosphere; the Cadomian stretching direction is oriented N30◦ E in proximity to the fast velocity direction inferred from the present data.
Acknowledgements This work has been supported by Spanish DGI project REN2002-04198-C02-01 by UE-FEDER and by the research group RNM104 of Junta de Andaluc´ıa (Spain). The authors thanks the reviewers the valuable advices and recommends to improve this paper.
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