Mesozoic extension in the Basque–Cantabrian basin (N Spain):Contributions from AMS and brittle mesostructures
Ruth Soto a,⁎, Antonio M. Casas-Sainz b, Juan J. Villalaín a, Belén Oliva-Urcia c
a Dpto. Física, Univ. Burgos, Av. Cantabria s/n, 09006 Burgos, Spainb Dpto. Ciencias de la Tierra, Univ. Zaragoza, C/ Pedro Cerbuna, 50009, Spain
c Geological Sciences, University of Michigan,1100 N. Univ. Ave. 48109 Ann Arbor, MI, USA (Now at University of Karlsruhe,Hertzstrasse 16, 6.36. 76187 Karlsruhe, Germany)
Received 23 March 2007; received in revised form 24 September 2007; accepted 28 September 2007Available online 6 October 2007
Abstract
In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison withpalaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in thewestern end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic andLower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequencelinked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabricsare typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems tobe related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineationthat is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording ofthe Tertiary compressional events, except for sites with compression-related cleavage.
The study of Anisotropy of Magnetic Susceptibility(AMS) is proved to be a useful tool in the study ofdeformation and flow in sedimentary, metamorphic andigneous rocks (see e.g. Hrouda, 1982; Jackson and Tauxe,1991; Tarling and Hrouda, 1993), in rocks lacking other
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visible markers of deformation and/or sedimentation. Inweakly deformed sediments, the magnetic lineation isusually parallel to the fold axes or the strike of thrustsin compressional scenarios (e.g. Borradaile and Tarling,1981; Hrouda, 1982; Sagnotti et al., 1998), whereas itcoincides with the stretching direction in extensionalbasins (e.g. Mattei et al., 1997; Cifelli et al., 2005).Among methods applied to the determination of the tec-tonic setting of basin development when structural ele-ments are present, the analysis of brittle mesostructureshas also been demonstrated to represent a useful tool todetermine the stress regime during rock formation, pro-vided that relative chronologies can be established (e.g.Hancock, 1985). In this work we study and check AMSdata by means of a comparison with the deformationpattern obtained from the analysis of brittle mesostruc-tures. Previousworks have already compared successfullyAMS results with brittle mesostructures to unravel thetectonic histories in weakly deformed basins with simpletectonic evolution (e.g. Kissel et al., 1986; Lee et al.,1990; Sagnotti et al., 1994, 1999; Mattei et al., 1997,1999; Faccenna et al., 2002; Borradaile and Halminton,2004; Cifelli et al., 2004, 2005).
The interpretation of AMS depends on the lithology(i.e. mainly in function of the paramagnetic and fer-romagnetic minerals) and the tectonic history of rocks(e.g. Evans et al., 2003). The Cabuérniga basin in NorthSpain constitutes an excellent scenario to study howdifferent rock types record the magnetic fabric, since itdisplays a wide range of different sedimentary sequencesboth in age (Lower Triassic to Lower Cretaceous) andlithology (red beds, shales and marly marine and la-custrine limestones). Moreover, its overall tectonic evo-lution is relatively simple and brittle mesostructuresappearing in the syn-rift sequence allow for bothmethodologies to be compared.
The Cabuérniga basin is located in the Basque–Cantabrian basin, one of the most important Cretaceousbasins in western Europe, with an evolution closelylinked to the history of the northern Iberian margin. It ischaracterised by an extensional stage, from the Triassicto the Early Cretaceous (e.g. Pujalte, 1982), followed bya strike–slip or transtensional stage in the transitionbetween the Early and Late Cretaceous (e.g. García-Móndejar et al., 1996). Finally, a compressional stage,with inversion of previous basins, linked to the Iberia–Europe convergence occurred (e.g. Gómez et al., 2002).Its extensional and the strike–slip stages are not yet fullyunderstood, especially concerning the orientation of theprincipal stress or deformation axes. The geometry of theCabuérniga basin cannot be defined straightforwardbecause of the deformational structures imprinted by the
inversion stage. The complexity of the extensional faultpattern, with several directions for major faults, E–W,NW–SE and N–S, most of them inherited from the Late-Variscan fracturing, also precludes a direct interpretationof extension directions. The combination of palaeostressanalysis from brittle mesostructures and AMS offers thepossibility of better defining the kinematic history of thispart of the northern Iberian plate margin. Furthermore,this work shows the potential in using AMS analysisin inverted basins to unravel its previous extensionalhistory when the magnetic fabric has not been modifiedin subsequent deformation events.
2. Geological setting
The studied area is located in the westernmost sectorof the Basque–Cantabrian basin (Western Pyrenees,North Spain) (Fig. 1). The Basque–Cantabrian basindeveloped as a result of Mesozoic extension associatedwith the opening of the North Atlantic and Bay of Biscay(Le Pichon and Sibuet, 1971; García-Mondéjar, 1996).The opening of the Bay of Biscay caused a counter-clockwise rotation of Iberia of about 35° with respect tostable Europe due to the creation of oceanic crust and itspropagation of extension towards the East. The timing ofthis rotation can be constrained between Early Aptianand Santonian (118–84Ma) considering both sea-floormagnetic anomalies (Williams, 1975; Montadert et al.,1979; Srivastava et al., 1990; Sibuet and Collette, 1991),paleomagnetic data of stable Iberia (van der Voo, 1969;Juárez et al., 1998) and paleogeographic reconstructions(García-Mondéjar, 1996). Associated with this rotation,a sinistral translation of Iberia with respect to Europeoccurred (Malod and Mauffret, 1990; Choukroune,1992) and different structures developed in the westernPyrenees along WNW–ESE minor sinistral shear zones(Martínez-Torres, 1989; García-Móndejar et al., 1996).From the Late Cretaceous, the African plate began todrift northwards, conditioning the convergence betweenIberia and Europe. The Pyrenean orogeny began duringthe late Santonian and persisted until the end of theOligocene, with maximum convergence rates occurringduring early Eocene times (Vergés et al., 1995). This ledto the inversion of the thick Early Cretaceous basins inthe Basque–Cantabrian basin (e.g. Alonso et al., 1996;Vergés and García-Senz, 2001).
We focused our study in the Cabuérniga basin, anarea that accumulated at least 2500 meters of Mesozoicsedimentation (present-day preserved thickness) andrepresents one of the maximum depocenters of theBasque–Cantabrian basin during the Jurassic and LowerCretaceous rifting event (Pujalte, 1982). The pre-rift series
Fig. 1. Geological map of the studied area with the location of sites of measurement of brittle mesostructures and sites of sampling for AMS analysis(in square). Inset shows the location of the studied area within the frame of the northern Iberian margin.
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include the Paleozoic basement (belonging to the Can-tabrian–Asturias Arc, cropping out westwards), Triassicred sandstones (Buntsandstein facies), dolostones andshales (Muschelkalk facies) and marine Jurassic lime-stones and shales, Hettangian to Oxfordian in age.The syn-rift sequence was deposited mainly between theKimmeridgian and Barremian (“Weald facies”) andcomprises the Cabuérniga (Kimmeridgian–Valanginian)and Pas Groups (Valanginian–Barremian) (Pujalte, 1982;García de Cortázar and Pujalte, 1982). The CabuérnigaGroup consists of fluvial, lacustrine and marsh deposits,whereas the Pas Group consists essentially of fluvialdeposits (Pujalte, 1982). The Cabuérniga (E–W) andRumaceo (NW–SE) faults with their associated basementanticlines limit the basin to the North and South, re-spectively (Fig. 1). These faults represent previous late-Variscan structures, first reactivated as normal faultsduring the Mesozoic and later as reverse faults during theAlpine compression (Pujalte, 1982; Rat, 1988; García-Espina, 1997). The present-day geometry of the basin
consists of awideWNW–ESE syncline that progressivelynarrows from East to West. Several parallel minor foldsappear in the central part of the basin (Fig. 1). One of theseminor folds represents an extensional anticline where thePas Group lies unconformably on the older succession(Pujalte, 1989; García-Espina, 1997).
3. Methodology of analysis of anisotropy of magneticsusceptibility
Twenty-three sites were drilled in the Triassic redbeds, Jurassic marine limestones and Upper Jurassic–Lower Cretaceous lacustrine marly limestones, shalesand red beds (Cabuérniga and Pas Groups) distributedalong two N–S sections in the Cabuérniga basin (Fig. 1).Only in sites F18 and CAC9, deformational structuresand cleavage associated with compression were found.Eight to twelve cores with different orientations wereobtained in each site with a portable, water-refrigeratedrock-drill, and oriented in-situ with a magnetic compass.
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All the cores were sliced in standard paleomagneticspecimens (cylinders 25mm in diameter and 22mm inheight) and at least one specimen per core was measured.
The analysis of AMS was carried out with an AGICOKappabridge KLY-4 in the University of Burgos (Spain)and an AGICO KLY-3 in the University of Zaragoza(Spain). The statistical procedure to obtain the direc-tional and tensorial data was based on Jelinek's method(Jelinek, 1977). The magnetic fabrics have been de-scribed using the parameters defined by Jelinek (Jelinek,1981) (Table 1): (i) the corrected anisotropy degree oreccentricity degree, P′ and (ii) the shape parameter, T,varying between T = − 1 (prolate ellipsoids) and T = + 1(oblate ellipsoids).
4. AMS results
4.1. Magnetic properties
The AMS study described here benefits from aprevious palaeomagnetic study in the same rocks(Soto et al., submitted for publication), which in-dicates a syntectonic remagnetization post-datingthe main extensional stage of basin formation andpre-dating the Tertiary compressional stage. This re-magnetization involved all the mesozoic stratigraphicseries, from the Triassic to the Pas Group (Valangi-nian–Barremian).
The bulk susceptibility of the studied rocks variesdepending on the lithology (Table 1). The higher values(ranging from 220 to 550 × 10− 6 S.I.) correspond to theMiddle and Upper Jurassic and Lower Cretaceous marlylimestones. The red beds (Triassic and Lower Creta-ceous in age) show lower values. The bulk susceptibilityof the Triassic red beds ranges between 80 and 150 ×10− 6 S.I., and it varies from 120 to 225 × 10− 6 S.I. inthe Lower Cretaceous red beds (Table 1).
The anisotropy degree P′ is always low (Table 1),with values typical of weakly deformed sediments (P′≤1.13). Graphs of Fig. 2A represent the relation betweenKmean and P′ values for each lithology (Triassic red beds,marine Jurassic limestones, lacustrine Upper Jurassic–Lower Cretaceous limestones and Lower Cretaceous redbeds). Each lithology presents a different behaviour,attesting that rock composition influences on AMSparameters (see also Hrouda, 1982, 1987; Borradaile,1987). Thus, the Triassic red beds show different aniso-tropy degrees with a constant Kmean value (Fig. 2A), thatwe interpret as due to different deformation degrees. Themarine Jurassic limestones show similar low values ofanisotropy but a wide range of Kmean (Fig. 2A), sug-gesting that possible differences in the proportion of
ferromagnetic minerals do not influence on the anisot-ropy degree. Lower Cretaceous red beds (Pas Group)present narrow ranges of both Kmean and P′ values,indicating no significant changes on the mineralogy andanisotropy degree. With respect to the lacustrine UpperJurassic–Lower Cretaceous limestones, except for dataof site CA19, the behaviour is similar to the marineJurassic limestones but with a narrower range of Kmean
(Fig. 2A). The positive correlation between Kmean and P′values in samples of site CA19 probably indicates thatthe increase of the degree of anisotropy is due to a largercontent in ferromagnetic minerals.
4.2. Low-temperature ASM and anisotropy of theanhysteretic remanence magnetization analyses
In the red beds sampled in our study, we interpretthe AMS as predominantly controlled by the preferredorientation of paramagnetic minerals because their bulksusceptibility is less than 250 × 10− 6 S.I. (e.g. Rochette,1987; Hrouda and Jelinek, 1990). With respect to thelimestones, with higher bulk susceptibility values, siteF18 was chosen as a representative site to control therelationships between the paramagnetic and ferrimag-netic sub-fabrics in this lithology. To separate the para-magnetic and ferrimagnetic sub-fabrics, among severalmethods, we chose the Low Temperature AMS (LT-AMS) and the Anisotropy of the Anhysteretic Rema-nence Magnetization (AARM), respectively. Theseanalyses were done in the paleomagnetic laboratory ofthe University of Michigan. The LT-AMS was measuredwith a SI2B susceptibilitymeter with an internal coilfrequency of 19.2kHz (Sapphire Instruments) and thesamples at 77K (liquid nitrogen) using the same methodas Parés and van der Pluijm (2002). In these experi-ments, the enhancement of the paramagnetic fabricat low temperatures follows the Curie–Weiss law (seeParés and van der Pluijm, 2002 for more details) andtherefore this method allows estimating the paramag-netic contribution to the magnetic anisotropy (e.g.Richter and van der Pluijm, 1994). The AARM wascarried out in a SI-4 AF demagnetizer (Sapphire Instru-ments) and a 2G-cryogenic using the method proposedby McCabe et al. (1985) to separate the ferrimagneticsub-fabric.
The steroplots of site F18 show a good overlappingof the LT-AMS (Fig. 3A) and the total AMS (Fig. 4),indicating that the phyllosilicates dominate the total AMS.On the contrary, the AARM measurements (Fig. 3B)show an interchange of Kmax and Kmin with respect to thetotal AMS. The LT-AMS magnitude reaches more than1000 × 10− 6 S.I., four times higher than the magnitude of
CAP2 red bed Valang.–Barremian (Pas Gr.) 8 122.8 (27.4) 1.02 (0.005) 0.451 (0.328) subhorizontal 234, 2 234, 2 13.2°/3.3° Type 2CAP4 red bed Valang.–Barremian (Pas Gr.) 8 171.8 (30.3) 1.012 (0.005) 0.331 (0.413) 287, 32 221, 13 198, 27 30.5°/8.9° Type 2CAP5 red bed Valang.–Barremian (Pas Gr.) 4 224.7 (11.6) 1.029 (0.007) 0.489 (0.369) 290, 50 062, 35 072, 5 – F25 Type 2CAP6 red bed Valang.–Barremian (Pas Gr.) 8 210.7 (27.4) 1.021 (0.009) −0.026 (0.468) 185, 72 065, 13 063, 4 20.7°/13.4° F22 Type 3CAP7 red bed Valang.–Barremian (Pas Gr.) 6 249.6 (39) 1.032 (0.01) 0.424 (0.39) 232, 60 352, 16 016, 3 36.9°/2.9° F21 Type 1CAP8 red bed Valang.–Barremian (Pas Gr.) 8 137.7 (27.1) 1.019 (0.007) 0.188 (0.371) 194, 58 120, 3 116, 11 17.8°/5.4° F20 Type 1–2
n=number of specimens; Km=(Kmax+Kint +Kmin) /3 (mean susceptibility, in 10−6 SI units); Pj=exp {2[(η1−η)2+ (η2−η)2+ (η3−η)2]}1/2 (Jelinek, 1981); Tj=[2(η2−η3) / (η1−η3)]−1 (shape factor; Jelinek, 1981); D, I (Kmax)=
Declination and inclination ofKmax; For each site the line shows the arithmetic means of the individual site mean values (standard deviation in parenthesis) For magnetic ellipsoid Type see text and Fig. 5; E11.1 (e12/e13), e12 and e13are half confidence angles of Kmax from Jelinek's statistics.
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Fig. 2. (A) Mean susceptibility Kmean vs. degree of anisotropy P′ for different sampled rocks. (B) P′–T diagram of the mean shape parameters of theellipsoid of AMS at each site.
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the total AMS (see Table 1). The increase in the valueof the susceptibility at low temperature indicates thepredominance of phyllosilicates as carriers of the fabric
at low temperature. The Flinn diagram of the LT-AMS(Fig. 3A) shows a predominance of oblate shapes coin-ciding with the total AMS (Table 1). The lineation (L) and
Fig. 3. (A) Low temperature AMS (LT-AMS) stereoplot of site F18 showing the paramagnetic sub-fabric and Flinn diagram. (B) Anisotropy of theAnhysteretic Remanence Magnetization (AARM) stereoplot of site F18 showing the ferrimagnetic sub-fabric and Flinn diagram. In situ lower-hemisphere equal-area stereoplots.
Fig. 4. Examples showing four typical magnetic ellipsoids found in the studied area and their P′–T diagrams. Lines correspond to bedding and dashedline to compression-related cleavage. In situ lower-hemisphere equal-area stereoplots.
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foliation (F) of the AARM samples (Fig. 3B) are verysimilar indicating a more triaxial shape of the ellipsoid ofthe remanence but lower anisotropy degreewith respect tothe LT-AMS sub-fabric.
4.3. AMS data
As shown in the P′–T diagram (Fig. 2B), the shape ofthe AMS ellipsoids is oblate in 70% of sites and prolatein 30% of sites. All sites in red beds except CAP6 areoblate regardless of their age. Four typical shapes ofAMS ellipsoids were found (representative AMS plotsare shown in Fig. 4). The first type (Fig. 4A) shows anoblate magnetic ellipsoid representative of a sedimentaryfabric related to compaction: the minimum susceptibilityaxes are perpendicular to the bedding plane and themaximum and intermediate susceptibility axes are scat-tered within the bedding plane. Only site CAP7 showsthis type of ellipsoid. The second type (Fig. 4B) is atriaxial susceptibility ellipsoid: all three principal sus-ceptibility axes are well grouped and distinct, the mini-mum susceptibility axis Kmin is almost perpendicular tothe bedding plane and a well-defined magnetic lineationappears. In almost 50% of sites this type of magneticellipsoid has been found (Table 1). All Triassic red bedand Cabuérniga Group limestones sites, except sites F18and CAC9 with compression-related cleavage, and sitesJPO5, CAP2, CAP4 and CAP5, correspond to this typeof magnetic ellipsoid. Site CAP8 displays an interme-diate magnetic ellipsoid between Types 1 and 2: anoblate ellipsoid with the minimum susceptibility axisnormal to bedding and the maximum susceptibility axispoorly clustered (Table 1). This fabric suggests an in-cipient deformed state from a sedimentary one. Thethird type refers to a prolate AMS ellipsoid (Fig. 4C):the maximum axes are clearly clustered defining a welldeveloped magnetic lineation while Kint and Kmin formgirdles perpendicular toKmax. Most Jurassic sites, exceptJPO5, and CAP6 show this configuration. Finally, Type4 represents sites with compression-related cleavage(sites F18 and CAC9; Fig. 4D). This magnetic fabrichas Kmin normal to cleavage and Kint and Kmax poorlyclustered and distributed within a girdle parallel to thecleavage plane.
Regarding the magnetic fabric obtained, only threesites (CAP4, CAP5 and CAP7) present a relatively highconfidence angle (e12 N 30°) of Kmax (Table 1) whatindicates a poorly or non-defined magnetic lineation.Since the means of Kmax of sites CAP4 and CAP5 are inagreement with the data in nearby sites, we will considerthe data of these sites for structural interpretations. Thus,only site CAP7, that shows a sedimentary fabric and no
defined magnetic lineation (Type 1 of magnetic ellip-soid), will not be taken into account for structural inter-pretations. All sites with magnetic ellipsoid of Types 1and 2 display a well defined foliation parallel to thebedding plane. The origin of such foliation can be in-terpreted as sedimentary, clearly related to depositionaland compactional processes. We interpret the magneticlineation as a result of a tectonic overprint of a sedi-mentary, compaction fabric. Other possibilities, as thecontrol of magnetic lineation by palaeocurrents couldhave some influence, but do not seem feasible to explainthe largest part of data because: (1) Most of the sites arelocated on limestones and siltstones, probably depositedin a quiet enviroment, and (2) the general trend of themagnetic lineation is constant through sedimentary se-quences that differ in lithological character and age.
4.4. Correlation betweenAMS fabric and structural elements
After simple bed restoration (Fig. 5B), the maxima ofKmax are subhorizontal and better grouped than in thenon-restored stereogram (Fig. 5A), what together withthe constant Kmin orientation perpendicular to bedding,corroborates that the magnetic fabric pre-dated defor-mation and suggests its early origin. Since most partof the present-day dip of beds can be considered as aconsequence of extensional deformation (Soto et al.,submitted for publication), the relationship between themagnetic ellipsoid and bedding allows to ascribe thedeformation associated with the magnetic ellipsoid toextensional deformation, probably acquired during thefirst stages of extensional folding. Considering the totalset of data after tectonic correction (Fig. 5B), the dis-tribution of Kmax from all sites, except CAP7 and siteswith compression-related cleavage, is oriented NE–SW(N045). Analysis of the orientation of Kmax in differentunits shows maxima at ∼ N045 (Fig. 5C,D,) for theTriassic and marine Jurassic sequences that changes to∼N055 in the upper part of the series (Cabuérniga and PasGroups, Fig. 5E). The marine Jurassic limestones showthe narrowest range of Kmax orientation, whereas thelacustrine limestones of the Cabuérniga Gr. and theLower Cretaceous red beds show more variable orienta-tions with two secondary maxima at N015 and N120(Fig. 5E). In the Triassic red beds, a secondary maximumalso appears in a N–S direction (Fig. 5C).
Fig. 6 summarises the orientation of the magneticlineation of all sites after tectonic correction and theorientation of the present structural elements. Mostsites, regardless of their age or lithology, show a simi-lar magnetic lineation orientation, close to N045. Thisorientation is perpendicular to the trend of the Rumaceo
Fig. 5. (A) Lower-hemisphere stereoplot of Kmax before tectonic correction for all sites, except CAP7 and sites with compression-related cleavage,F18 and CAC9. (B, C, D, E) Lower-hemisphere stereoplots, density plots and rose diagrams of Kmax after tectonic correction for all samples (exceptCAP7 and sites with compression-related cleavage), Triassic, marine Jurassic and continental Jurassic and Lower Cretaceous (Cabuérniga and PasGroups) samples, respectively.
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Fig. 6. Map of the studied area showing the orientation of magnetic lineations after bedding correction.
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fault (limiting the basin to the South) and the intra-basinal folds, and oblique to the Cabuérniga fault(Fig. 6). Therefore, this trend is consistent with thestretching direction of the basin in its extensional stageduring the Triassic, Jurassic and Lower Cretaceoustime (see Discussion section for more details).
Only 6 sites present an anomalous orientation withrespect to this main trend. Sites CA12 and CAP8 displaya magnetic lineation direction N116, which is almostperpendicular to the trend showed by most sites (N045).The interpretation of this orientation is not straight-forward (see section 5.2 in Discussion). The magneticlineation of sites CAP4 and CAT1 trends N002 andN008, respectively (Fig. 6 and Table 1). In site CAP4,this orientation is parallel to several oblique faults af-
Fig. 7. (A) Conjugate faults with small dip–slip displacement in Jurassic limegashes filled with calcite in Jurassic and Cabuérniga Group limestones, rescutting across shales of the Cabuérniga Group, seen from below. (E) Syn-sedCabuérniga Group. (F) Joints perpendicular to bedding in a sandstone chasiltstones of the Cabuérniga Group, probably resulting from the Tertiary, com
fecting the Cabuérniga fault and located next to this site.Sites with compressional-related cleavage (F18 andCAC9) show a non-horizontal magnetic lineation N060after tectonic correction (Table 1) possibly due to thereorganization of Kmax by the cleavage.
5. Analysis of brittle mesostructures
5.1. Methodology
The analysis of brittle mesostructures is a useful tool todetermine the stress regime after the deposition of rocksor during deformational stages (e.g. Angelier, 1984; Han-cock, 1985). Brittle mesostructures include stylolyticjoints and peaks, tension gashes, tensional and shear
stones associated with bedding-parallel stylolytic joints. (B, C) Tensionpectively. (D) Perpendicularly-trending slickenside striations in faultsimentary or early diagenetic, shallow dipping faults in siltstones of thennel of the Cabuérniga Group lying on shales. (G) Slaty cleavage inpressional, inversion stage of the Cabuérniga basin.
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joints, and faults with slickenside striations. Severalmethods have been developed in the last decades to ob-tain the stress axes from brittle mesostructures. The sim-plest methods represent a semi-quantitative approach: σ1
is usually parallel to the axes of stylolytic peaks, σ3 per-pendicular to tensional joints and to tension gashes, pro-vided that quartz or calcite fibers are perpendicular to thegash walls, although more complex patterns can also befound (see, for example, Hancock, 1985; Arlegui andSimón, 2001). The more sophisticated methods, includingthe possibility of analysing faults with no slickensidestriation, allow, bymeans of statistical techniques, both theorientation of stress axes and the shape of the stressellipsoid to be obtained (e.g. Lisle et al., 2001). From themethods commonly referred to, we have used the RightDihedra method (Angelier and Mechler, 1977) and theStress Inversion method developed by Etchecopar(Etchecopar et al., 1981; Etchecopar, 1984). The firstone provides a qualitative approach to the orientation andshape of the stress ellipsoid. The second one gives theshape ratio of the stress ellipsoid [R= (σ2−σ3)/(σ1−σ3)],together with some complementary tools to check thesolution obtained: the histogram of theoretical striation-measured striation angular deviations, and the position offault planes on Mohr's circle diagram of the stress tensorobtained, to determine themechanical compatibility of thesolution. In general, in our study, the existence ofmonophased fault populations does not make necessarythe separation of the measured faults in several sets todetermine different stress tensors (see e.g., Lisle et al.,2001).
Nevertheless, one of the main drawbacks in mesos-tructural analysis is the difficulty in dating deforma-tional stages and locating them within the frame of thegeological time scale. In other cases, this difficulty hasbeen overthrown by considering cross-cutting relation-ships with igneous intrusions. In other cases, it is pos-sible to constrain the age of deformation by defining therelationships between tectonics and sedimentation for aparticular time bracket. In our case, as we describe in thenext section, the assignment of structures to a particularstage was based on geological criteria.
5.2. Structures
To compare and contrast the results obtained fromthe analysis of AMS, we analysed three types brittlemesostructures from the Cabuérniga basin (Fig. 7):(1) Tension gashes, filled with quartz and calcite, usuallyoccurring in limestones, sandstones and siltstones fromthe Lower Triassic, marine Jurassic and the CabuérnigaGroup. In limestones, tension gashes are genetically re-
lated with bedding-parallel stylolitic joints, (2) tensionaljoints, usually arranged in two perpendicular sets (H-typearrangement), commonly occurring in sandstones of thePas Group, where they are very closely spaced, and(3) faults of metric scale in siltstones and claystones of theCabuérniga Group. The syn-sedimentary or early diage-netic character of these faults can be assessed from thegeometry of slickenside striations, indicating movementin plastic conditions (see, e.g. Guiraud and Séguret,1984), their shallow dip and usually irregular fault surfacegeometry, and their cross-cut relationships with beddingand other fault surfaces (Fig. 7).
The syn-sedimentary or early post-sedimentary char-acter of the first two types of the described structures ismore difficult to check than the third type. However,some geometrical features, such as the perpendicularitybetween fractures and bedding, checked in differentlimbs of the folds, with different dip values and constantthroughout the basin support the early (pre- bed tilting)character of these structures. Some of the extensionalstructures, especially faults, have been re-activated dur-ing the Tertiary compressional stage, and low-pitchslickenside striations can be seen superimposed on theearlier normal striae. A total of more than 600 structures,distributed along two cross-sections, and partly coincid-ing with sites of sampling for AMS analysis, weremeasured in this study.
As pointed out before, the overall evolution of theCabuérniga basin is relatively simple, with two mainstages: i) Triassic to Late Cretaceous, with extensionalbasin formation, and ii) Tertiary compression, involvingbasin inversion, thrust displacement and basement uplifts.Between these two stages the along-strike sinistralmovement of Iberia with respect to Europe broughtabout a transtensional regime with dominant wrenchmovement (García-Mondéjar et al., 1996). We focusedour study on extensional structures, since they can berelated to the extensional stage controlling basin forma-tion. Their extensional and, in some cases, syn-sedimen-tary character can be assessed from the criteria aboveexposed. Therefore, the stress or deformation regime in-ferred from these structures can be referred to the Meso-zoic extensional stage. Locally, the Tertiary compressionformed cleavage and cleavage-related folds, with normaland reverse cleavage fans (Fig. 7). However, cleavage isusually confined to thick pelitic levels or marly levelsinterbedded between more competent limestones.
5.3. Directional analysis
The results obtained from the analysis of tensionaljoints and tension gashes indicate several directional
Fig. 8. Stereoplots and rose diagrams for orientations of brittle structures measured in the Cabuérniga basin, in situ (BTC) and after bedding correction (ATC) (tilting of beds to the horizontal position).Structures measured in all sites are summarised in the diagrams. Rose diagrams represent the trend of lines or the trend of poles to planes in planar structures. N indicates the number of data representedin each stereoplot. (A) Poles to tension gashes. (B) Poles to tension gashes after bedding correction. (C) Joints. (D) Joints after bedding correction. (E) Poles to normal faults. (F) Slickenside striationsin normal faults.
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Fig. 9. Application of the Righ Dihedra method to sites with faults with slickenside striations.
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maxima that can be interpreted in terms of severalextension directions, or deflections of the stress field bymajor faults. Data were represented before and afterbedding correction (rotating data around a horizontalaxis according to the dip of the bedding plane) (Fig. 8).This rotation favours the clustering of poles to joints andgashes, indicating that they probably formed as verticalsurfaces perpendicular to bedding. In an overall view,the variability of directions is stronger for joints than fortension gashes (Fig. 8).
Poles to tension gashes, indicating the extensiondirections, show three directional maxima: NNE, SSE
and N–S (Fig. 8). Relative chronology from outcropobservations indicate that the set corresponding to NNEextension predates the set indicating N–S extension, butno correlation has been established with the other set.
Most of tensional joints were measured in sandstonesof the Pas Group. Although their global directionalpattern is more difficult to infer than in the case of tensiongashes, they are apparently arranged in four sets, formingtwo systems of subperpendicular fractures (Fig. 8): thefirst system consists of a WNW–ESE set (poles withNNE–SSW trend, ranging to N–S), with a secondary,associated NNE–SSW set (poles WNW–ESE). The
Table 2Sites for analysis of brittle mesostructures and orientation of axes of the stress ellipsoid obtained from Etchecopar's method in sites with striated faults
Site So Rock G J Faults n/N α σ1 σ3 R
F1 056,16 Jur. 19 17 3F2 026,24 Jur. 7F3 025,28 Tr. 1 11F4 056.26 Tr. 14 3F5 022.29 Jur. 4F6 030,32 Jur. 2 5F7 020,26 Jur. 2 3F8 020,33 Jur. 4 2F9 041.19 Cab. G. 3F10 020,36 Cab. G. 3 8 14 12/14 10 184,74 093,00 0.41F11 000,28 Cab. G. 2 4 5F12 007,27 Cab. G. 1 8 7/8 10 194,72 328,13 0.25F13 040,26 Cab. G. 4 9 7/9 14 164,59 268,08 0.64F14 030,28 Cab. G. 9 1F15 000,00 Cab. G. 20F16 000,00 Cab. G. 10 110 88/110 10 007,83 250,03 0.09F17 354,20 Cab. G. 7F18 055,17 Cab. G.F19 000,00 Pas G. 24F20 010,30 Pas G. 26F21 045,25 Pas G. 28F22 030,34 Pas G. 21 1F23 342,17 Pas G. 27F24 051,12 Pas G. 31F25 000,00 Pas G. 30F26 105,40 Pas G. 4 2F27 178,42 Jur. 28 1 20 20/20 11 350,40 178,49 0.37F28 185,90 Tr. 22 15/20 12 020,40 132,79 0.94F29 165,25 Jur. 7F30 000,00 Cab. G. 39 35/39 10 298,83 124,07 0.29F31 000,00 Cab. G. 27 24/27 9 195,83 310,03 0.30F32 017,37 Cab. G. 83
So=Bedding orientation in the site (dip direction and dip); G=Number of tension gashes; J=Number of joints;n/N=number of faults explained by the stresstensor/total number of faults; α=mean angular deviation between theoretical and real striations; R=[(σ2−σ3) / (σ1−σ3)], shape ratio of the stress ellipsoid.
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second system consists of two sets, the main one withNNW–SSE direction (poles with NNW–SSE trend) andthe secondary one in ENE–WSW direction (poles withENE–WSW trend).
The orientation of structures seems to be similarthroughout the studied area, with no specific pattern indifferent areas of the Cabuérniga basin (Fig. 8). Inmeasurement sites where many data were obtained, thethree sets of tension gashes (or even four sets of joints)are clearly represented. In sites with less than 20 mea-surements (usually because of bad outcrop conditions)only one or two sets are clearly represented. No sys-tematic variations seem to be depending on the age ofrocks in which mesostructures were measured (Fig. 8).
5.4. Palaeostress analysis
Striated fault planes allow for a more complete anal-ysis of stress tensors to be done. According to the criteria
exposed in Section 4.2. most faults measured are syn-sedimentary or early diagenetic, thus guaranteeing thatthe stress field corresponds to the Mesozoic stage. In thecase of older rocks (Triassic and Jurassic) the age offaults could only be constrained according to geomet-rical factors: when two of the principal stress axes arewithin the bedding plane (and the third one perpendic-ular to it) in dipping strata we can consider that faultsformed in a relatively early, pre-folding stage.
In an overall view, both faults and slickenside striationson fault planes show a wide variety of orientations, withsome directional maxima for slickenside striations in E toESE, and N to NNE directions (Fig. 8F). The resultsobtained from the Right Dihedra method in most sitesindicate that σ1 is perpendicular to bedding and σ2 and σ3
axes are not well defined within the bedding plane, thusindicating conditions of nearly radial extension (Fig. 9). Insites F10, F30 and F31, the main extension direction canbe defined as NW–SE (Fig. 9). The analysis of faults by
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means of Etchecopar's method (Etchecopar, 1984) allowsfor the main extension direction to be defined (Table 2): inmost of the sites the optimal extension direction iscomprised between E–Wand WNW–ESE, except in thenorthern basin margin where the main extension directionis N–S, perpendicular to the E–W Cabuérniga normalfault, inverted during the Tertiary compression, thatdefines the present-day basin margin. The R relationshipis within the field of triaxial extension (between 0.2 and0.4) (Table 2). However, in the site with more measuredfaults (site F16, 110 faults), the R = 0.09 relationshipindicates conditions of nearly radial extension, possiblyindicating that a biasing towards triaxial extension occurswhen a lower number of faults is measured in the field,depending on the exposure orientation. The mechanicalcompatibility of faults as determined from Mohr'sdiagrams is not the best possible, because of the highangle between σ1 (perpendicular to bedding) and the faultplanes. Probably, the formation of faults under very plasticconditions and water-saturated sediments (Guiraud and
Fig. 10. Geological map with the extension directions (σ3) obtained from paCabuérniga basin.
Séguret, 1984) favoured the movement of faults evenunder poor a priori mechanical conditions.
5.5. Interpretation
The complexity of stress axes pattern obtained fromthe different types of brittle mesostructures precludesthe straightforward interpretation of extension direc-tions obtained within the basin (Fig. 10). Only in thenorthern basin margin (F27 and F28) there is an actualcoincidence between the extension directions obtainedfrom faults and tension gashes, but, even in this case,there are two sets of tension gashes indicating a sec-ondary, SE, extension direction. This SE extension isrepresented throughout the basin from the three kindsof structures, as well as the maximum of trends ofslickenside striations in syn-sedimentary faults. How-ever, the maximum of extension directions obtainedfrom tension gashes in limestones is perpendicular,NNE. Models simulating radial extensional regimes
laeostress analysis, tension gashes and tensional joints recorded in the
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(Simón et al., 1988) indicate that, in these conditions,two sets of perpendicular structures usually form, be-cause of the switching of stress axes during the for-mation of extensional structures. This switching wouldexplain the perpendicularity of structures both in in-dividual sites and globally at the basin scale (Fig. 10).Models shown by these authors (Simón et al., 1988)also demonstrate that stress deflection induced by pre-existing structures is also common under radial ex-tensional regimes. Therefore, extension directionsobtained in the northern basin margin, perpendicular(or parallel) to the main normal faults limiting thebasin, can be interpreted according to this deflectionpattern. The same applies to the southern basin margin(site F32), where the three sets of tension gashes can beinterpreted as the result of a primary N–S extensiondirection combined with extension directions paralleland perpendicular to the NW–SE basement faultlimiting the southern basin margin. The same patternis observed along the southern basin margin (sites F1to F7).
In the Cabuérniga basin, tensional joints are moredifficult to interpret, since the extension directions ob-tained from them are also oblique to the extensiondirections obtained from the other structures. Consideringall the data obtained, the main extension direction ob-tained from tensional joints is parallel to the extensiondirection maximum obtained from tension gashes (com-pare rose diagrams in Fig. 8). Some directional maxima oftensional joints, as the ENE–WSW, show no correlationwith tension gashes, but coincide with a directionalmaximum in the trend of poles of fault planes, probablyindicating that fault planes re-used or re-activated formertensional joints, that are usually the first structure formedduring extensional deformation (see e.g. Angelier andMechler, 1977; Angelier and Colletta, 1983). Obliquestructures (NE–SW) associated with the main E–W faultslocated in the northern basin margin can also explain theexistence of oblique directions of tensional joints in thisarea. The E–Wextension direction is represented both byfaults with slickenside striations and tensional joints, andrepresents an extension direction more or less parallel tothe present-day basin axis.
As a whole, brittle structures are indicative ofa radial extensional regime, pre-dating the tilting ofbeds, and a probable N–S to NNE–SSW primary ex-tension, represented by the three types of structuresanalysed (faults with slickenside striations, tensiongashes filled with quartz and calcite, and tensionaljoints), with an associated E–W to WNW–ESE per-pendicular extension, only represented by faults andtensional joints.
6. Discussion
6.1. Comparison of results
The high quality of the results obtained and therepresentative sampling along the two cross-sectionsdescribed in this paper allow for some interpretationsabout the reliability of palaeostress analysis and AMSstudied to be inferred. Furthermore, the Cabuérnigabasin can be considered as representative of the geo-logical evolution of many Iberian and west-europeanbasins, with an episode of Mesozoic extension fol-lowed by a compressional inversion of the basin duringthe Tertiary.
In an overall view, the results about deformationaxes obtained from AMS analysis are more clusteredthan the palaeostresses obtained from brittle mesos-tructures. Brittle mesostructures are more conditionedby stress deflections due to hectometric-scale, sec-ondary faults linked to the basin margins. Moreover,the number of faults measured at a given site seems toinfluence the results obtained in palaeostress analysis:in sites with a large number of measured faults(i.e. site F10) the stress regime obtained closely ap-proaches radial extension, whereas in other sites thestress regime obtained is triaxial extension, probablyinfluenced by the orientation of exposures. However,regarding the AMS data, our results indicate thatdespite different magnetic ellipsoids are obtained, thegeneral trend of the magnetic lineation is maintainedthrough sequences that differ in lithological characterand age. Therefore this work points out the sensi-tivity of AMS in detecting fabrics in comparisonwith the analysis of brittle mesostructures. Balsleyand Buddington (1960) also showed that magneticmethods are more sensitive in detecting fabrics thantraditional microscopic methods in granites andorthogneisses.
As a whole, the two more representative extensiondirections obtained from brittle mesostructure analysisare NE–SW and NW–SE, whereas AMS resultsindicate a dominant NE–SW stretching direction.The results obtained are consistent with a dominantNE–SWextension related with the activity of the mainnormal faults limiting the basin and linked to theopening of the Bay of Biscay during the Early Cre-taceous. The trend of the Rumaceo fault, located to theSouth of the basin, is almost perpendicular to theobtained magnetic lineation (Fig. 6). This suggeststhat the basin could form linked to a major activity ofthis fault comparing with the Cabuérniga fault to theNorth.
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6.2. Origin of the magnetic fabric
In the Western Pyrenees, south-verging structureslinked to the last compressional tectonic phase andcompatible with N–S compression accommodate a totalshortening of about 25km (Alonso et al., 1996). TheCabuérniga basin represents one of the Mesozoic basinscarried piggyback on a thin-skinned basement thrustlocated 15–20km depth (Alonso et al., 1996; Alonso andPulgar, 2004). This scenario rules out the possibility ofrelating the magnetic fabric to the stretching directionassociated with the tectonic transport direction duringthe compressional stage (e.g. Aubourg et al., 1999) sincethe regional décollement and possible simple shearstructures associated with it are several kilometres depth.
Previous works have invoked an early origin forthe magnetic fabric (i.e. a short time after sedimentdeposition) (e.g. Mattei et al., 1995; Sagnotti et al., 1998,1999; Parés et al., 1999; Coutand et al., 2001; Larrasoañaet al., 2004), which is hardly modified by subsequentbrittle deformational events (Mattei et al., 1997, 1999;Faccenna et al., 2002). In the studied area, the magneticfabric of most sites seems to reflect the extensional forceswithin the basin, which represent the earliest stage ofdeformation. The subsequent compressional tectonicstage during the Tertiary seems only to overprint themagnetic fabric where compression-related cleavageappears (sites CAC9 and F18). The interpretation of themagnetic lineation oriented N116 found in sites CA12and CAP8, perpendicular to the general trend N045, canbe interpreted in two ways: (i) this orientation reflectslocal extensional forces perpendicular to the generaltrend coinciding with the results obtained from the anal-ysis of syn-sedimentary faults, or (ii) the folds locatednear these sites could have influenced their magneticfabric. The anticline located to the North of these sitesand probably the syncline southwards, represent exten-sional folds (Pujalte, 1989; García-Espina, 1997), thatcould suggest that the magnetic lineation trends parallelto the axis of folds even in an extensional stage.
The sensitivity of AMS data for tracking extensionaldeformation within Neogene and Quaternary sedimen-tary basins has been successfully proven (e.g. Sagnottiet al., 1994; Borradaile and Halminton, 2004; Matteiet al., 1997, 1999, 2004; Cifelli et al., 2005). Faccennaet al. (2002) used AMS and structural data to characterisethe tectonic regime that accompanied the deposition ofdifferent sequences between the Eocene and Pliocenedistinguishing between compressional and extensionalepisodes. The present work proves that AMS studiesrepresent a reliable method to analyse the extensionaldeformation in sedimentary basins subsequently in-
verted, due to the difficulty in the re-orientation of themagnetic fabric by subsequent tectonic phases inabsence of important internal deformation.
6.3. Regional implications
The Mesozoic evolution of the Basque–Cantabrianbasin has been related to the opening of the North Atlanticand Bay of Biscay (e.g. Le Pichon and Sibuet, 1971), andthe left-lateral displacement between Iberia and Europe(e.g. García-Móndejar et al., 1996). This scenario ledto the formation of an extensional framework with majorE–Wand NW–SE faults (Fig. 11A), and some secondaryNNE–SSW and NE–SW faults (Martín-Chivelet et al.,2002), limiting the sedimentary troughs. During the EarlyCretaceous, a NNE–SSW stretching direction, perpen-dicular to the axes of the main NW–SE structural trendsand basins, linked to a rifting episode of simple extensionor normal rifting, has been described (e.g. Montadertet al., 1979; Grimaud et al., 1982; Boillot and Malod,1988; Malod and Mauffret, 1990) (Fig. 11A). Since theAptian, the extension direction changed to NW–SE,linked to left-lateral strike–slip motion along the mainNW–SE faults (e.g. Boillot and Malod, 1988; Mauffretet al., 1989), with oblique rifting throughout the northernmargin of Iberia (Fig. 11A). The extension directionobtained fromAMS results for the Cabuérniga basin, witha dominant NE–SW orientation, is compatible with thedeformation field proposed for the northern Iberianplate margin during the Early Cretaceous (i.e. before theAptian). This tectonic scenario also provides alternativeinterpretations for the orientation of brittle mesostruc-tures: instead of reflecting a radial extensional regime,they could be the result of sequential deformation betweenthe Barremian and Albian times. The analysis of somebrittle mesostructures indicates a dominant NW–SEextension more compatible with the tectonic scenariodescribed for Albian times, and secondary N–S and NE–SWextension directions, reproducing the results obtainedfrom AMS analysis (Fig. 11). According to these inter-pretations, brittle mesostructures could have formedduring a time lasting tens of millions of years, and recordseveral stress directions.
Therefore, the differences between the results obtainedby means of the different methods can be explained con-sidering that brittle mesostructures are more sensitive tofar-field stress conditions imposed by the movement ofthe large faults and can record deformation occurredduring longer intervals of time, whereas the AMS recordsdeformation on the near distance, short-term conditions.Furthermore, AMS results are also influenced by theinternal deformation of beds during extensional folding,
Fig. 11. (A) Sketch showing the position of the Cabuérniga basin within the tectonic scenario of the northern Iberian plate margin during the LateJurassic and Lower Cretaceous. Modified from Mauffret et al. (1989). (B, C) Schematic block-diagrams showing the near and far field conditionsconditioning the magnetic fabric and brittle mesostructures orientation, respectively. See text for details.
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leading to indicate the movement of the most importantfaults during the extensional stage.
All these factors explain the higher complexity in theinterpretation of brittle mesostructures (even when thesestructures, as in the case of “syn-sedimentary” faults,can be considered as syn-sedimentary or early diage-netic), and the variety in the extension directionsobtained from them. Some directions coincide with thestretching direction during rifting, whereas otherscorrespond to the maximum strain axis of the strainellipse linked to transtension.
These results confirm previous interpretations sug-gesting an early origin for the magnetic fabric, when thesediments are soft and only partially lithified (e.g. Matteiet al., 1997; Sagnotti et al., 1998, 1999; Parés et al., 1999;Larrasoaña et al., 2004). Therefore, the time window forfixing AMS is much smaller that the time span duringwhich brittle mesostructures develop. The results ob-tained confirm the effectiveness of AMS in analysingextensional stress regimes, even when basins have
undergone a subsequent inversion history. However,these results should be confirmed by means of detailedsampling and AMS analysis in different sedimentaryunits of the Basque–Cantabrian basin with homoge-neous tectonic evolution.
7. Conclusions
This work demonstrates that the relationship betweenAMS data and classical structural analysis is notstraightforward. Brittle mesostructure analysis seems tobe more sensitive to far-field stress conditions, recordlonger time spans and therefore are more difficult tointerpret, even when they can be ascribed to the Meso-zoic extensional stage, whereas AMS records deforma-tion on the near distance, during shorter intervals of timeand at the earliest stages after deposition of rocks.
The AMS data colleted in Triassic red beds, Jurassicand Lower Cretaceous limestones, sandtones and shalesfrom the Cabuérniga basin defines a compactional
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magnetic foliation, and a magnetic lineation of tectonicorigin. Most sites display a NE–SW magnetic lineationthat is interpreted to represent the stretching direction ofthe extensional stage of the basin occurred between theJurassic and the Barremian. The Cabuérniga basin ex-perimented a transtensional episode during the Aptianand Albian and was inverted later during the Tertiary,but the magnetic fabric was not modified in these sub-sequent tectonic events.
The results obtained from joints and tension gashesshow a dominant N–S to NE–SW with secondary NW–SE extension direction. Paleostresses obtained fromfault analysis (Right Dihedra and Etchecopar's meth-ods) indicate a dominant NW–SE to E–W extensiondirection. These orientations reflect the tectonic scenariooccurred in the northern margin of Iberia during theJurassic and Lower Cretaceous (an extensional stagefollowed by a transtensional event).
Acknowledgements
This work was supported by the projects CGL2006-02514 of the Dirección General de Enseñanza Superior(DGES), SpanishMinistry of Education, and BU002B06of the Junta de Castilla y León (Spain). Financial supportwas also given a research contract for young scientistsfrom the Spanish Ministry of Education and Science toR. Soto (“Juan de la Cierva” program). We are verygrateful to M. Mattei for his comments. Sylvia Graciahelped us in the laboratory work in the University ofZaragoza, A. Carrancho helped us in the field andJ. Parés help us in the laboratory with the AARManalyses.
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