A seismological evidence for the northwestward movement of Africa with respect to Iberia from shear-wave splitting

GEOSCIENCE FRONTIERS 3(5) (2012) 681e696

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China University of Geosciences (Beijing)

GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf

RESEARCH PAPER

A seismological evidence for the northwestward movement of Africa with respect to Iberia from shear-wave splitting Mohamed K. Salah Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt Received 10 October 2011; received in revised form 28 December 2011; accepted 28 January 2012 Available online 10 February 2012

KEYWORDS Shear-wave splitting; Seismic anisotropy; Iberian Peninsula; Northwest Africa; Western Mediterranean

Abstract Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting. Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study. Although the station coverage is not uniform in the Iberian Peninsula and northwest Africa, significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains. Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia; being consistent with the absolute plate motion in the region. In the northern part of the Iberian Peninsula, fast velocity directions are oriented nearly EeW; coincident with previous results. Few stations located slightly north of the plate boundary and to the southeast of Iberia show EeW to NE-SW fast velocity directions, which may be related to the Alpine Orogeny and the extension direction in Iberia. Delay times vary significantly between 0.2 and 1.9 s for individual measurements, reflecting a highly anisotropic structure beneath the recording stations. The relative motion between Africa and Iberia represents the main reason for the observed NW-SE orientations of the fast velocity directions. However, different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region. Many geophysical observations such as the low Pn velocity, lower lithospheric

E-mail addresses: [email protected], nada6899@ yahoo.com. 1674-9871 ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2012.01.005

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M.K. Salah / Geoscience Frontiers 3(5) (2012) 681e696 Q values, higher heat flow and the presence of high conductive features support the mantle flow in the western Mediterranean, which may contribute and even modify the splitting pattern beneath the studied region. ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction Seismic anisotropy is the directional dependence of seismic velocity within the Earth and it is a characteristic feature of the Earth’s interior structure (Huang et al., 2011). It may exist at different depth ranges in the crust, mantle and inner core (e.g., Mainprice, 2007 and references therein). Different factors dominate in producing the anisotropy at various depths, such as aligned cracks in the upper crust (e.g., Crampin, 1984), and the latticepreferred orientation of the constituting minerals in the lower crust and upper mantle (Mainprice, 2007; Karato et al., 2008). The lattice-preferred orientation in the upper mantle is generally considered to be the result of dislocation creep of the minerals (principally olivine) above a depth of w300 km (e.g., Nicolas and Christensen, 1987; Gung et al., 2003; Panning and Romanowicz, 2006). The anisotropic fabric of olivine depends on both the type and extent of strain (Savage, 1999); aligning its main crystallographic axes with respect to structural directions (lineation, pole of the foliation), thus producing a large-scale anisotropy detectable by seismic waves that is correlated with the geometry of strain in the upper mantle (W€ustefeld et al., 2009). Identifying the orientation of seismic anisotropy and its strength can thus help to constrain deformation at depth. Comparing anisotropy with other observations as topography and gravity (Simons and van der Hilst, 2003), the magnetic anomalies (Bokelmann and W€ustefeld, 2009), and the low-velocity anomalies in the mantle wedge beneath the volcanic front at subduction zones (e.g., Nakajima and Hasegawa, 2004) is helpful for understanding the tectonic processes acting within the Earth, both, ancient ones recorded within the lithosphere, as well as more recent events in the asthenosphere. The most direct evidence for the presence of seismic anisotropy is the splitting of shear waves. Anisotropic media are indeed birefringent; splitting the incoming shear waves into two perpendicularly polarized waves that propagate at different speeds. At the Earth’s surface, it is therefore possible to quantify anisotropy by measuring the two splitting parameters: dt and f. The difference in arrival times (dt) between the two split shear waves (Fig. 1) depends on both the anisotropy strength and the length of the travel path through the anisotropic medium. The azimuth (f) of the fast split shear wave polarization plane is related to the orientation of the anisotropic structure. In the beginning, the technique was limited to direct S-waves from local events (Ando et al., 1980), but is now widely employed for core-transiting phases such as SKS, SKKS, and PKS (e.g., Vinnik et al., 1989; Silver and Chan, 1991; Savage, 1999; Fouch and Rondenay, 2006). The teleseismic shear-wave splitting technique has become popular in the last few decades for performing anisotropy measurements that help revealing present or past mantle deformation. This technique has been widely applied in several geologic settings: subduction zones (e.g., Margheriti et al., 2003; Levin et al., 2004; Nakajima and Hasegawa, 2004; Salah et al., 2008), rifts and transform faults (e.g., Kendall, 1994; Gao et al., 1997; Walker et al., 2004a,b; Kendall et al., 2005; Salah, 2011), hotspots (e.g., Walker et al., 2001, 2005; Barruol and Granet, 2002),

oceanic islands (e.g., Behn et al., 2004; Fontaine et al., 2005, 2007), orogens (e.g., Barruol et al., 1998; Flesch et al., 2005) and stable continental environments (e.g., Fouch et al., 2000; Heintz and Kennett, 2005; Assumpcao et al., 2006; Fouch and Rondenay, 2006). Considering the constraints that seismic anisotropy offers on deformation within the Earth, there is much interest in understanding the origin of that anisotropy. Comparing observed fast directions with absolute plate motion directions is of interest (e.g., Vinnik et al., 1984; Silver and Chan, 1991; Montagner, 1994), since anisotropy in the asthenosphere, if caused by relative motion between plates and deeper interior of the Earth, should have fast directions parallel to plate motion. On the other hand, fossil anisotropy in the lithosphere does not need to show a correlation with current plate motion directions (W€ ustefeld et al., 2009). It is also of interest to compare seismic anisotropy with predicted seismic anisotropy from flow calculations that have become available in recent years. This comparison has been done at both regional (Griot et al., 1998; Fouch et al., 2000; Becker et al., 2006), and global (Behn et al., 2004; Becker et al., 2007; Conrad et al., 2007) scales.

Figure 1 Paths and spatial resolution associated with teleseismic (SKS) shear waves (modified from W€ ustefeld et al., 2009). Wave paths of teleseismic shear waves are nearly vertical. There is a progressive time delay between the two (split) shear waves on the path toward the station at the Earth’s surface.

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2. Seismotectonic setting Situated at the obliquely convergent African-Eurasian plate contact, the Iberia-Maghreb region, comprising Spain, Portugal and the northern parts of Morocco and Algeria, is an area of complex tectonic deformation and is under the potential threat of natural hazards induced by seismicity and tsunami generation (e.g., Zitellini et al., 2004; Dıaz et al., 2010). Present-day convergence between Africa and Iberia occurs at rates of w5e6 mm/yr, and with approximately NW-SE to WNW-ESE shortening direction (Fig. 2, DeMets et al., 1990, 1994; Calais et al., 2003; McClusky et al., 2003; Stich et al., 2006; Serpelloni et al., 2007). Extensional processes coexist with plate convergence in the westernmost

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Mediterranean sector (e.g., Platt and Vissers, 1989; Docherty and Banda, 1995; Comas et al., 1999; Meijer and Wortel, 1999; Jolivet and Faccenna, 2000), with both processes interacting to produce complex tectonic deformation over the Iberia-Maghreb region. Both surface geological data and the focal mechanisms in the western Mediterranean support the regional NE-SW extension, with triaxial to prolate stress ellipsoids (Serrano et al., 2002). As a result of the convergence and extension processes, the region is surrounded by Alpine mountain ranges (such as the Betic chain, Atlas and Maghrebides, Catalan Coastal Ranges, etc.), usually thought to accommodate the Africa-Eurasia plate convergence. Earthquakes are concentrated along regional-scale WSW-ENE lineaments in northern Algeria and easternmost Atlantic Ocean,

Figure 2 Seismicity of the western Mediterranean region during the period from April 1973 to April 2011 as derived from NEIC catalogs. Stars vary in their color according to the depth of the hypocenter. Digits refer to the station codes in Iberia and NW Africa as follow: 1. ATE; 2. ECAL; 3. POBL; 4. EBR; 5. MTE; 6. PAB; 7. PESTR; 8. PACT; 9. PMST; 10. EMUR; 11. CART; 12. SELV; 13. MELI; 14. SFS; 15. PVAQ; 16. PFVI; 17. RTC (refer to Table 1 for station coordinates). CIM: Central Iberian Massif; OMZ: Ossa Morena Zone; BC: Betic Chain; AGFZ: AzoresGibraltar Fault Zone; GC: Gulf of Cadiz; CCR: Catalan Coastal Ranges. Inset at the upper left illustrates the relative movement of Africa with respect to Eurasia (after Fernandes et al., 2003).

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presumably marking two segments of the convergent AfricaEurasia plate boundary (Fig. 2). In between, at the contact of Morocco and Spain, seismicity is distributed over a more than 400 km wide zone, and is characterized by lower magnitudes (usually smaller than 5.5), compared to the adjacent sections (Buforn et al., 1995; Stich et al., 2006), with some deep earthquakes down to a depth of 640 km, which accompany the continental convergence between the two plates (e.g., Kiratzi and Papazachos, 1995; Buforn et al., 1997; Reicherter and H€ubscher, 2007). The very deep seismicity may be correlated with older subduction processes (Buforn et al., 2004). These characteristics suggest a diffuse partitioning of plate convergence strain, and complicate the definition of a plate boundary between Morocco and Spain. The Iberian Peninsula, itself, is a relatively aseismic area separated from the rest of Europe by the Pyrenees mountain range. The Pyrenees have a moderate seismicity with some instrumentally recorded earthquakes reaching magnitudes of 5; mostly located in the northwestern part of the range (Souriau and Pauchet, 1998). The shallow seismic activity, on average, does not exceed 20 km in depth; the lower crust is essentially aseismic and the seismic activity is concentrated in the upper crust between 5 and 17 km. This lower cut-off depth in the seismic activity in the crust is interpreted as a shallow brittle-ductile transition of the seismogeneic layer (Morales et al., 1997). In addition, the shallow cut-off depth in the seismic activity coincides with the presence of an intracrustal reflector detected by Banda et al. (1993) and Galindo-Zaldivar et al. (1997) and which would be layering the crust into two: a seismic part (brittle) and another aseismic (ductile) part, according to the differences observed in the style of deformation between the upper and lower crust (Galindo-Zaldivar et al., 1997). Seismicity patterns in the Iberia-Maghreb region have been treated in numerous focal mechanism studies. As the number of regional seismic broadband stations is increasing, the methodological focus turns from evaluating first motion polarities (recent studies and compilations e.g. Mezcua and Rueda, 1997; Bezzeghoud and Buforn, 1999; Borges et al., 2001; Henares et al., 2003; Buforn et al., 2004) toward regional waveform inversion, including moment tensor studies of selected events (Thio et al., 1999; Dufumier et al., 2000; Buforn and Coca, 2002; Mancilla et al., 2002), as well as systematic assembling of regional moment tensor catalogs (Braunmiller et al., 2002; Pondrelli et al., 2002; Stich et al., 2003; Rueda and Mezcua, 2005). These studies report a number of different regional faulting trends along the plate boundary zone: Northern Algeria is characterized by predominantly reverse faulting under w NNW-SSE compression. At the SW Iberian margin, reverse and strike-slip faulting occur under w NW-SE compression (Fig. 2). In the Alboran Basin and southern Spain, strike-slip and normal faulting prevail, suggesting interplay of both extensional tectonics and Africa-Eurasia plate convergence along this section of the plate contact (Mezcua and Rueda, 1997; Bezzeghoud and Buforn, 1999; Stich et al., 2003). Moreover, the stress field is heterogeneous with both spatial and temporal variations; sometimes even acting simultaneously in adjacent areas. Distinct periods of crustal deformation, fault reactivation, and halokinesis related to multiple episodes of collision between Iberia, Eurasia and Africa (Malod and Mauffret, 1990; Srivastava et al., 1990) are known to have controlled the tectono-stratigraphic evolution of parts of the Iberian Peninsula and northwest Africa since the Early Cretaceous, including their Atlantic margin (Murillas et al., 1990; Pinheiro et al., 1996; Wilson et al., 1996; Borges et al., 2001; Alves et al., 2003). However, the relatively low plate motion and strain rates in the western Mediterranean and the long recurrence time of large earthquakes make

seismological and geomorphological indicators of active deformation scarce and difficult to interpret. As a consequence, strain distribution across the Africa-Eurasia plate boundary in the western Mediterranean and strain accumulation on the major seismogenic structures are still largely unknown (Nocquet and Calais, 2004). The main aim of this study is to investigate in detail the general features of seismic anisotropy in the region by analyzing the digital waveform data from teleseismic events. The shear-wave splitting technique is a convenient and simple way to study the mantle flow processes, stress state, and the uppermost mantle deformation, which accompany the relative motion between adjacent plates. The western Mediterranean region, on the other hand, represents a convenient tectonic complex in which to study the shear-wave splitting phenomenon and its relation with the current plate motion (Dıaz et al., 1998). Previous information regarding the presence of anisotropy in the region is restricted to some specific regions. For example, using data from the permanent stations located in central Iberia, Silver and Chan (1988) and Vinnik et al. (1989) inferred a fast velocity direction oriented close to N90 E. Dıaz et al. (1993) inferred a fast velocity direction oriented roughly N10 E beneath SW Iberia. This anisotropy was restricted to well-defined depth levels between 60 and 90 km. Splitting observations on teleseismic shear waves recorded by a portable network installed in SW Iberia, suggested a NE-SW to EeW fast velocity direction (Dıaz et al., 1996); the possible origin of which has been discussed in Abalos and Dıaz (1995). At a larger spatial scale, Maupin and Cara (1992), using the surface waves recorded by the ILIHA-NARS network, have shown that anisotropy should be present beneath depths of 100 km, although the data could not constrain a fast velocity direction. Barruol and Souriau (1995) reported a fast velocity direction beneath the eastern Pyrenees, oriented N100 E from a temporary network around the ECORS profile. Dıaz et al. (1998) detected EeW fast velocity directions in central and east Iberia, and a clearly different direction of NE-SW in the Ossa-Morena zone. The mantle anisotropic features at the north of Iberia have been investigated by Dıaz et al. (2006), with an average fast velocity direction close to EeW. Finally, the inferred fast polarization directions by Dıaz et al. (2010) along the Gibraltar Arc exhibit a clear rotation following the curvature of the Rif-Betic chain. These contrasting splitting patterns detected by previous researchers in the western Mediterranean region along with the availability of many broadband stations from many seismic networks in Morocco, Algeria, Portugal, and Spain, operating for relatively long periods of time encourage us to re-measure the shear-wave splitting phenomenon and relate our observations with the current tectonics in the western Mediterranean region.

3. Data and analytical methods We collected data at 17 permanent broadband stations in Iberia and northwest Africa with a general good coverage throughout the study region except for the central northern part of Iberia (Fig. 2). Unfortunately, the high noise and the malfunctioning of some horizontal components in addition to various technical problems at some stations reduced the number of reliable anisotropy measurements, especially in south and southwest Iberia. In our study, we selected 95 teleseismic events from NEIC catalogs (Fig. 3) in the period from 2002 to 2011 with magnitudes M  5.9 at epicentral distances larger than 85 in which prominent SKS and SKKS and to a lesser extent sSKS arrivals can be clearly identified. Core-refracted teleseismic shear waves are useful for studies of the

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Figure 3 Epicentral distribution of the 95 teleseismic events (M  5.9), (solid circles) used in the present study. The solid triangle denotes the approximate center of the recording stations in the western Mediterranean.

upper mantle since they are affected only by anisotropy on their ‘receiver-side’ of their path to the recording stations. Their passage through the liquid outer core involves a conversion to compressional waves, and thus removes any “source-side” birefringence signal. We have additionally analyzed direct S phases from events at 40 e 85 epicentral distance ranges to improve the azimuthal coverage. To avoid the near-source anisotropy effect on the splitting of shear waves and to ensure that the observed splitting is mainly caused by the upper mantle anisotropy at the receiver side, all S splitting analyses are conducted only on waveform data generated from teleseismic events with focal depths >150 km (e.g., Hiramatsu and Ando, 1996; Long and van der Hilst, 2005). It seems that the azimuthal coverage from the NE and SW directions is very good, while a scarce coverage toward the south, NW and SE directions is clearly evident (Fig. 3). This uneven azimuthal distribution of events; along with the low number of measurements at many stations (as stated above), hinder the detection of anisotropy variations relative to the incoming polarization azimuth. Consequently, we assume that the medium of study possesses hexagonal symmetry with a horizontal symmetry axis and that the anisotropy is localized in a single layer beneath the receiver. Thus, measurements from many events beneath a station are statistically analyzed to deduce the dominant fast polarization direction (e.g., Crampin, 1981). Only in the case of the presence of a large enough range of arrival angles, backazimuths, and polarization directions, these assumptions can be tested (Silver and Savage, 1994). The existence of lower forms of symmetry, such as orthorhombic, rather than hexagonal, may produce large variations in splitting parameters even for rays coming in at nearvertical incidence. Most of the studied phases provide evidence for the existence of anisotropy, enabling the determination of the fast velocity direction as well as the time delay induced by the anisotropy. First, records are visually inspected for high signal/noise ratio and waveform clarity. Theoretical arrival times of the teleseismic

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shear waves are computed using the IASP91 model (Kennett and Engdahl, 1991). The waveform data were band-pass-filtered between 0.01 and 0.2 Hz to isolate the teleseismic shear wave energy, and measurements of the splitting parameters are made using the cross correlation technique (e.g., Ando, 1984; Fukao, 1984; Okada et al., 1995; Nakajima and Hasegawa, 2004, and others). In this technique, the original signal is projected in different coordinate system orientations with intervals of 3 and the fitting of the two horizontal components is analyzed at each step. The coordinate system (4) and the time difference (dt), at which the two components are closer, are identified as corresponding to the fast velocity direction and the time delay, respectively. Only the records with good fitting between the fast and slow components have been retained for further analyses. The obtained polarization direction and delay time for each seismogram are checked to exclude the noise-induced anisotropy and the poor measurements. Fig. 4 displays the NeS and EeW horizontal component seismograms generated by a teleseismic event recorded at PAB. A prominent SKS arrival at a time of 1310 s is clearly visible (Fig. 4a and b), with a delay time of w0.70 s at the NeS component. In the absence of anisotropy, the particle orbit is expected to be linear. After passing through an anisotropic medium, the particle motion would start with a movement along the polarization azimuth of the maximum velocity phase and end with a movement along the azimuth of the minimum velocity phase. The particle motion in that case is expected to be approximately elliptical for a long-lasting signal (Fig. 4c). Proper rotation and time shift will resolve the shear wave into two orthogonally polarized phases whose waveforms are similar to each other, and hence would show a linear orbit in the resultant particle motion diagram (Fig. 4d). Fig. 5 shows the horizontal component seismograms of a teleseismic event recorded at station RTC in northern Morocco. Prominent SKS and SKKS phases are clearly visible. The particle motion diagram of the anisotropy-corrected waveforms is exactly linear compared with an elliptical particle motion of the same time window before anisotropy-correction (Fig. 5c and d). Similarly, Fig. 6 displays two horizontal component seismograms recorded at ECAL with prominent SKS and S phases arriving at w1355 and 1367 s, respectively, after the origin time of the teleseismic event. Proper rotation angle and time shift results in a linear particle motion (Fig. 6d) compared with an elliptical particle motion of the same time window in the observed waveforms (Fig. 6c). In this manner, the data showing linear orbits obtained from anisotropy-corrected seismograms are used to exclude artificial results of waveforms contaminated by noise. We found that some measurements gave zero delay time (hereby we call them null measurements); and for reliable interpretation of the observed anisotropy, such data are excluded from further analysis.

4. Results Splitting results are presented in rose diagrams in Fig. 7 for some stations to define the major fast polarization directions in the western Mediterranean region. It is clear that all stations located in northern Iberia show predominantly EeW fast directions with an average dt varying from 0.46 to w0.64 s. In addition, some WNW-ESE and NW-SE fast directions are also observed. Stations ECAL, MTE, PESTR and PAB, that are located in central western Iberia, display a variety of NW-SE, ENE-WSW and WNW-ESE fast directions which are consistent with the current plate

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Figure 4 An example of horizontal component seismograms of a deep teleseismic earthquake recorded at PAB in central Iberia. Hypocentral parameters are shown above the seismograms. (a) and (b) are the observed NeS and EeW components, respectively, showing distinct SKS arrival. The horizontal scale shows the travel time in seconds of the teleseismic shear waves. (c) Original particle motion for a time window shown in (a) and (b). (d) Particle motion for the same time window corrected by removing the splitting effects. Obtained fast direction and delay time in this example are N67 E and 0.70 s, respectively.

motions between the Iberian, African and Eurasian plates in the region. Upon moving further to the southeast of Iberia; stations EMUR and CART display both ENE-WSW and WNW-ESE fast directions. On the other hand, station RTC in northwest Africa, displays predominantly NNW-SSE to NW-SE fast polarization directions. Average splitting times are generally between 0.5 and 0.7 s for the majority of the stations. This splitting pattern is geographically illustrated in Fig. 8, which displays all anisotropy measurements at the station locations excluding the null measurements. Splitting times and the fast directions vary

considerably even at the same location. However, most of the stations near the Africa-Eurasia plate boundary exhibit NW-SE fast velocity axes; while those located in northern Iberia display predominantly EeW fast directions. In order to extract the predominant fast polarization directions in a precise way from the diversified splitting pattern observed at many stations (Fig. 8), we define the main- and sub-fast velocity directions (MFD and SFD) for each rose diagram (Fig. 7) in the same way as Nakajima and Hasegawa (2004), and Salah et al. (2008). A window with an angle of 30 is set in a rose diagram and is gradually

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Figure 5 (a) and (b) are the NeS and EeW seismograms of an intermediate-depth teleseismic event recorded at RTC station in northern Morocco, showing impulsive SKS arrivals. Hypocentral parameters are shown above the seismograms. The horizontal scale shows the travel time in seconds of the teleseismic shear waves. Original particle motion for a time window shown in (a) and (b) before anisotropy-correction is elliptical (c), in contrast to the linear particle motion after anisotropy-correction (d). Obtained fast direction and delay time in this case are N127 and 0.9 s, respectively.

shifted clockwise by an interval of 15 . The frequency of fast directions in each window is estimated, and the central direction of the window having the maximum frequency of fast directions is selected as the MFD at the station. Similarly, the central direction of the window with the second maximum is selected as the SFD at that station (Table 1). The obtained MFD and SFD are indicated by thick and thin black bars, respectively, in Fig. 9 at the station locations. Except the MFD and the SFD at station PAB, all the MFD and SFD for the remaining stations show a nearly consistent pattern. These results are in general agreement with those obtained by other studies conducted beneath the western Mediterranean and are consistent with the relative plate motion as well as the extensional tectonics in

the region. The implications of these results are discussed briefly in the following paragraphs.

5. Discussion The analysis of a large data set of the splitted teleseismic shear waves recorded at 17 broadband stations in the western Mediterranean provides lots of anisotropy constraints in a tectonically heterogeneous area. Although some stations do not give reliable anisotropy measurements and the station coverage, on the whole, is not yet satisfactory, significant variations in the observed splitting pattern are clearly revealed beneath the study area. Although both

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Figure 6 (a) and (b) are the NeS and EeW seismograms of an intermediate-depth teleseismic event recorded at ECAL station in northwest Iberia, showing impulsive direct S arrival. Hypocentral parameters are shown above the seismograms. The horizontal scale shows the travel time in seconds of the teleseismic shear waves. Original particle motion for a time window shown in (a) and (b) before anisotropy-correction is elliptical (c), in contrast to the linear particle motion after anisotropy-correction (d). Obtained fast direction and delay time in this case are N109 and 1.55 s, respectively.

surface geological data and the focal mechanisms in the western Mediterranean indicate a present-day regional NE-SW extension (Serrano et al., 2002), the stress field is heterogeneous with both spatial and temporal variations, sometimes even acting simultaneously in adjacent areas. The ENE-WSW trend of fast directions observed at the easternmost part near the coast of the Mediterranean at stations EMUR and CART (Fig. 8) is consistent with the N75 E fast velocity direction detected by Dıaz et al. (1998). These two stations are located at the Eastern Betics just to the southeast of the Catalan Coastal Ranges, which are compressive structures oriented mainly NE-SW as a result of the NNE-SSW orientation of the convergence between the Eurasian and African plates in the Paleogene. Moreover, the Catalan Coastal Ranges have also been affected, since the OligoceneeMiocene transition, by extensional processes that produced a number of

Neogene normal faults, oriented ENE-WSW (Roca and Guimera, 1992). Since late Miocene, the compressive direction changed to NNW-SSE. This may account for the variety of fast polarization directions that is detected even beneath spatially very close seismic stations (Fig. 9). In general, the NE-SW fast polarization directions observed at some stations are consistent with the results of Dıaz et al. (1998). Nolet (1990) detected low shear wave velocity anomaly beneath the Iberian Peninsula south of the Pyrenees down to 180 km depth. This anomaly is defined by a shear wave velocity of the order of 4.27 km/s with a minimum velocity of 4.19  0.05 km/s at 80  50 km depth. It is remarkably different from the average velocity of 4.44 km/s which is characteristic of the northern part of Iberia and the rest of Europe. Thus, it is believed that the Iberian lithosphere-asthenosphere system exhibits a low shear velocity structure with velocity values lower than 4.6 km/s,

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Figure 7 Rose diagrams of the fast polarization directions detected at the recording stations in the western Mediterranean region. Three or four capital letters beneath each rose diagram denote the station code. Two digits between brackets to the lower right indicate the number of observations, and the average delay time in seconds, respectively.

which are the characteristic velocities at such depths, and hence an indication of partial melting (Payo et al., 1992). In addition, Rosell et al. (2011) found that the lithosphere-asthenosphere boundary under SW Iberia is deeper than under the Alboran Sea. Their

resistivity tests reveal the existence of an NeS oriented lowresistivity anomaly at lithospheric mantle depths east of the 4 W meridian. This anomaly coincides with a low-velocity area without earthquake hypocenters and is interpreted as asthenospheric

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Figure 8 Splitting vectors for all observed data plotted at the station locations (small open circles) in the western Mediterranean. The direction of a bar indicates the fast polarization direction and its length is scaled to the delay time.

material intruded by the lateral lithospheric tearing and breakingoff of the subducting Ligurian slab under the Alboran domain. These low-velocity/low-resistivity anomalies and the related partial melting zones may modify the splitting pattern and can account for the diversity of the fast velocity axes observed beneath some stations, especially EMUR and CART. Stations MTE and PAB, which are located in the Variscan domain show prominently NW-SE fast velocity directions that are probably consistent with the NW-SE Variscan main trend in the area. Station PAB shows additionally ENE-WSW fast velocity directions similar to those detected by Silver and Chan (1988), Vinnik et al. (1989) and Dıaz et al. (1998). These two fast velocity directions are related by Silver and Chan (1988) to the EeW alignment of the Variscan orogeny over the Toledo area in central Iberia. This interpretation implies that the anisotropy should be “frozen” in the lithosphere; the lattice-preferred orientation being induced by the last significant orogenic episode in the area. Although the number of reliable anisotropy measurements is not large at PESTR; fast velocity directions are oriented generally EeW to NW-SE (Figs. 7 and 8), being consistent with the Variscan main trend in the area, which is oriented roughly NW-

SE. The determinations of the stress fields by Henares et al. (2003) from the focal mechanisms support the existence of a regional stress field with a subhorizontal compression axis trending NW-SE and a tension axis in the NE-SW direction. The wide range of the fast velocity directions observed at this station as well as station PVAQ to the south may also be favored by the Variscan shearing processes affecting the region. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity detected by Serrano et al. (2005) is almost parallel to the AfricaEurasia plate convergence vector (NW-SE), which are consistent with the fast velocity directions observed at RTC in northwest Morocco and SFS near the Strait of Gibraltar. Results of Serrano et al. (2005) also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle. Contrarily, Buontempo et al. (2008) detected ENE-WSW fast directions in the Betic domain, nearly parallel to the trend of the mountain belt. Along the Gibraltar arc, they observe a smoothly varying f trend changing from ENE-WSW in the Eastern Betics to NeS in the area of Gibraltar and Ceuta, following more or less the general trend of the mountain belt around the Alboran Sea and the coastline. Fast polarization directions detected by Dıaz et al.

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Table 1 Showing the station locations in the western Mediterranean region which are used in this study. Positive and negative ‘Long.’, indicate east and west longitudes, respectively. N refers to the number of non-null anisotropy measurements. MFD and SFD are the main- and sub-fast directions as degrees from the north, respectively (see text for details). dt is the average delay time for the main fast directions. The ‘e’ means that the MFD and SFD are not determined, and consequently dt because of low number of anisotropy measurements at this particular station. Nos. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Station code ATE ECAL POBL EBR MTE PAB PESTR PACT PMST EMUR CART SELV MELI SFS PVAQ PFVI RTC

Lat. ( N) 43.0858 41.9413 41.3793 40.8206 40.3997 39.5458 38.8672 38.7680 38.7368 37.8422 37.5868 37.2383 35.2938 36.4656 37.4037 37.1328 33.9881

Long. 0.7003 6.7371 1.0847 0.4933 7.5442 4.3483 7.5902 8.8333 9.1833 1.2405 1.0012 3.7277 2.9350 6.2055 7.7173 8.8268 6.8569

(2010) along the Gibraltar Arc, on the other hand, exhibit a remarkable rotation following the curvature of the Rif-Betic chain. They shift from N60 E at the northern Betic domain, to approximately NeS around the Strait of Gibraltar, then to N50 W at the Moroccan side of the arc. Focal mechanisms of shallow earthquakes (Buforn et al., 2004) show thrust faulting in the Gulf of Cadiz and Algeria with horizontal NNW-SSE compression, and normal faulting in the Alboran Sea with EeW extension. Focal mechanisms of intermediate-depth earthquakes in the Alboran Sea display vertical motions, with a predominant plane trending EeW. Fault plane solutions for very deep shocks correspond to vertical dip-slip along NeS trends. The stress pattern of intermediatedepth and very deep earthquakes have different directions: vertical extension in the NW-SE direction for intermediate depth earthquakes, and tension and pressure axes dipping about 45 for very deep earthquakes. Regional stress pattern may result from the collision between the African plate and Iberia, with extension and subduction of lithospheric material in the Alboran Sea at intermediate depths (Buforn et al., 2004). According to the elastic structures obtained by Lana et al. (1997), a low shear velocity channel, associated with the asthenosphere, is detected at depths ranging from 80 to 180 km. The anelastic structures obtained by Caselles et al. (1997) seem to partially confirm these features. Heat flow determination by Fernandez et al. (1998) suggests higher heat production in the southern Variscan Iberian massif compared with other parts in the Iberian Peninsula. Moreover, the presence of high conductive features at the middle-lower levels of the crust (e.g., Santos et al., 2002), may locally affect the splitting pattern beneath these stations. Another feature worth bearing in mind is that the lithospheric Q values under the Iberian Peninsula (Canas et al., 1988) are lower than those observed in other large continental areas like North America, South America, the Indian shield, and the Himalayas (Hwang and Mitchell, 1987). In addition, Calvert et al. (2000) detected a low Pn velocity (7.6e7.8 km/s) and significant Sn attenuation in the uppermost

N 22 76 18 70 69 20 18 5 5 57 42 2 8 42 20 15 41

MFD 

N90 N90 N75 N105 N120 N120 N90 e e N75 N120 e N165 N150 N90 N90 N165

SFD 

N75 N75 N60 N120 N135 N60 N75 e e N60 N105 e N50 N165 N75 N105 N150

dt average 0.78 0.42 0.45 0.69 0.85 0.75 0.15 e e 0.62 0.49 e 0.11 0.65 0.34 0.14 0.61

mantle beneath the Betics in southern Spain, in sharp contrast to the relatively normal Pn velocity (8.0e8.1 km/s) and efficient Sn imaged beneath the Alboran Sea. Slow Pn velocity anomalies are also imaged beneath the Rif and Middle Atlas in Morocco. The low velocity and attenuating regions beneath the Betics and possibly the Rif are interpreted as indicating the presence of partial melt in the uppermost mantle which may be underlain by faster and less attenuating mantle. Panza et al. (2007) confirmed the presence of a well stratified upper mantle beneath the older African continent, with a marked low-velocity layer between 130 and 200 km of depth; suggesting upper mantle circulation in the western Mediterranean, mostly easterly-directed and affecting the boundary between the upper asthenosphere and lower asthenosphere. These observations may explain the wide range of the fast polarization directions observed along the northern strip of the Africa-Iberia plate boundary. Geodetic estimates agree on a N45 W  20 convergence between Africa and Eurasia at the longitude of Sicily, transitioning progressively to a more EeW convergence direction toward the Gibraltar Strait. Estimates of convergence rates range between 3 and 7 mm/yr at the longitude of Sicily, decreasing westward to 2e5 mm/yr at the longitude of the Gibraltar Strait (Nocquet and Calais, 2004). The northwestward motion of the African plate with respect to stable Europe, imply that the Africa-stable Europe motion in the western Mediterranean is essentially accommodated by deformation in northern Africa and southern Iberia. The NW-SE oblique convergence between Africa and Iberia is consistent with the earthquake focal mechanisms that show mostly reverse motion on NE-SW trending, and usually NW-dipping faults, often combined with a strike-slip component. The extension direction in the Iberian Peninsula is generally normal to the regional NW-SE convergence (DeMets et al., 1990). In contrast to the unexpected NeS fast velocity directions detected by Dıaz et al. (1998), station ATE, which is located on the western edge of the Pyrenees displays predominantly EeW to

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Figure 9 Distribution of the main- and sub-fast directions (MFD and SFD) at each station shown by thick and thin black bars, respectively (see text for details). Length of each thin bar is normalized relative to the frequency of MFD.

WNW-ESE fast velocity directions (Fig. 7) which are consistent with many previous results and can be interpreted as due to the formation of the E eW Pyrenean chain and the NW-SE Celtiberian chain (Dıaz et al., 1998). The presence of partial melting zones results generally from rising temperatures and is usually accompanied by low-velocity/low-resistivity anomalies. These anomalous zones facilitate the flow processes in the crust and upper mantle giving rise to a more versatile splitting pattern. In accordance with this notion, long period magnetotelluric data acquired by Campanya et al. (2011) in the central Pyrenees constrain a lowresistivity structure in the Iberian subducted lower crust, plotting their upper limit 9 km deeper. This new upper limit reinforces the hypothesis of partial melting. Recent studies of seismic tomography (Souriau et al., 2008; Koulakov et al., 2009) reveal the presence of a minimum of P-wave velocity anomaly at lower crustal/upper mantle depths beneath central Pyrenees. However, the absence of a major S-wave velocity anomaly (Koulakov et al., 2009) could be related to the small amount of partial melting and to the absence of a thermal anomaly in this region. The mantle anisotropic features at the NW part of the Iberian Peninsula were investigated by Dıaz et al.

(2006) from different temporary array deployments. A remarkable consistency is found in the retrieved anisotropic parameters throughout the study area, with an average fast velocity direction close to EeW; a result that is consistent with our observations at station ECAL (Fig. 9). It may correspond to an anisotropic imprint around the lithosphereeasthenosphere transition related to the eastward displacement of the Iberian plate due to the Mesozoic extensional processes during the opening of the North Atlantic and the Bay of Biscay domains (Dıaz et al., 2006). Thus, the complex splitting pattern observed at the northern part of Iberia (at ECAL and ATE, Fig. 8) could be associated either with crustal anisotropic materials, or more likely, to an additional anisotropic signature within the lithosphere led by major Variscan and Alpine orogenic processes. Some of the stations where multiple measurements are available (e.g., stations ECAL, MTE, CART, and EMUR; Figs. 7 and 8) show some dispersion in the fast velocity direction, which is difficult to be related to the resolution of the data set. This feature has also been reported in other regional scale anisotropic studies (e.g., Barruol and Souriau, 1995; Dıaz et al., 1998), and is usually

M.K. Salah / Geoscience Frontiers 3(5) (2012) 681e696 obliterated by statistical analyses. This fact has to be interpreted as reflecting the presence of a more complex anisotropic structure, probably including dipping axis of anisotropy and/or a symmetry system other than the hexagonal one. Anisotropy is usually thought to result from aligned cracks in the upper crust (e.g., Crampin, 1984), and the lattice-preferred orientation of the constituting minerals in the lower crust and upper mantle (Mainprice, 2007; Karato et al., 2008). A simple approach to explain the origin of the lattice-preferred orientation of the olivine minerals in the upper mantle, assumes that it is due to the progressive simple shear induced by the motion of the overriding plate over a stationary mantle or, alternatively, of the drag of the asthenospheric flow on the base of the lithosphere (e.g., Vinnik et al., 1989). According to this hypothesis, the fast velocity direction is expected to be aligned with the absolute plate motion and should remain stable along large regions. The motion direction between the African plate and the Iberian Peninsula is to the northwest (DeMets et al., 1990), and therefore this hypothesis can explain the NW-SE fast velocity directions observed in NW Africa, southern and central Iberia (Figs. 7 and 9). Some recent calculations of the absolute plate motion in the Eurasian plate show a motion that is oriented roughly N75 E in the Iberian Peninsula. This is consistent with the general ENE-WSW trend for the fast velocity direction in large regions of Iberia (stations POBL and EMUR) but it cannot explain the results at some specific domains (Betic Chain, Ossa-Morena zone). Alsina and Snieder (1995) proposed that present-day flow in the mantle, at smaller scale than the absolute plate motion, can also contribute to the observed anisotropic pattern and could explain regional differences. Another way is to relate the origin of the anisotropy with the last main tectonic event in the area. According to this hypothesis, the major trends of the Variscan orogeny could explain the inferred fast velocity direction for some locations at the central Iberian Massif, the Catalan Coastal Ranges, and in the Pyrenees. More recent tectonic episodes can also be invoked as responsible for the anisotropy pattern; for example the Alpine orogeny for the Pyrenees and the Paleogene compression in the Catalan Coastal Ranges. The observed delay times vary significantly in most cases between 0.2 up to 2 s, with averages that are slightly lower than 1.0 s (Figs. 7 and 8). This variation in the delay time measurements is observed even at single stations (Fig. 8). This fact cannot be explained by the simple hypotheses of the origin of anisotropy and may be attributed to the existence of more complex anisotropic patterns. In addition, the regional variations in the anisotropic parameters documented in this study imply that various origins of the anisotropy have to be considered in some areas, in relation to their particular lithospheric geodynamics. This is because a strong contrast often appears in the anisotropic results between spatially close tectonic domains.

6. Conclusions Shear waves approaching recording stations nearly vertically are useful to investigate seismic anisotropy in the upper mantle. Thus, teleseismic shear-wave splitting measurements are conducted on waveform data generated by 95 distant events recorded at 17 broadband stations in the western Mediterranean region. The majority of the fast polarization directions around the Africa-Iberia plate boundary have NW-SE orientations, being consistent with the relative convergence motion between the two plates. Some stations exhibit additionally NE-SW fast directions; parallel to the extension

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direction accompanying the complex tectonic episodes affecting the western Mediterranean. Stations located in northern Iberia show predominantly EeW fast directions, which may be associated with the eastward motion of Iberia with respect to the Eurasian plate. The complex splitting pattern observed at some stations can be explained by active dynamic flow in the asthenosphere. Results of low Pn velocity, significant Sn attenuation, lower lithospheric Q values, low shear wave velocity in the upper mantle, high heat flow, stress alignment, and the presence of high conductive features beneath the Iberian Peninsula and the northwest of Africa support the assumed upper mantle directed flow. Finally, additional shear wave splitting observations at much denser seismic networks beneath the western Mediterranean region are necessary for a better understanding of the detailed anisotropy structure and its relation with the current tectonic activity, stress state and the induced flow in the upper mantle.

Acknowledgements The author thanks the IRIS Consortium (http://www.iris. washington.edu), and the ORFEUS (http://www.orfeus-eu.org) for the availability of the digital waveform data on the world-wide web. Comments of two anonymous reviewers improved the manuscript. Most figures in this article are produced with GMT (Generic Mapping Tools) software, which is written by Wessel and Smith (1998).

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