The Iberian Massif represents the largest and best exposed portion of the European Variscan orogen, which resulted from the convergence and subsequent collision between an Ibero-Armorican indentor and Laurussia (see [8,21] and references therein). In the southern branch of the Iberian Massif, the contact between the Ossa–Morena and the South Portuguese zones is marked by the high-temperature/low-pressure (HT/LP) Aracena metamorphic belt (Fig. 1a). Two main domains have been distinguished in the Aracena metamorphic belt [4]: an oceanic domain to the south and a continental domain to the north. Both domains present different stratigraphic sequences and record partly separated tectono-metamorphic evolutions [8]. The continental domain is composed of a variety of high- and medium-grade metamorphic rocks, including leucocratic gneisses, and different intermediate to basic intrusive rocks.

The continental domain underwent, during Variscan plate convergence, a tectonic evolution that comprised up to four ductile deformation phases [8]. The first stage (D₁) is related to kilometre-scale recumbent folds (e.g., [2]). However, these structures are not observed in the high-grade rocks of the Aracena metamorphic belt, where the S₁ foliation is only preserved as relic hinges within some S₂ foliation microlithons [8]. The second tectonic phase (D₂) is associated with a widespread extensional event (see below and also [8]). A penetrative metamorphic foliation (S_2me) is present throughout the continental domain, whereas at shear zones, a localised mylonitic foliation (S_2my) and a stretching lineation (L₂) developed. This deformation phase was accompanied by a high-temperature/low-pressure (HT/LP) metamorphism [4,8]. The third stage (D₃) generated symmetric upright folds with variable fold axial trace orientations. The last ductile deformation event (D₄) gave place to kilometre-scale, southwest-verging antiforms, and related reverse shear zones (see below, Fig. 1c and also [8]). Both structures share a NW–SE direction. A penetrative mylonitic foliation (S₄) with a NW–SE trend and nearly down-dip stretching lineations (L₄) developed within these shear zones (Fig. 1b).

The limit between the high-grade and the medium-grade sectors of the Aracena metamorphic belt (Fig. 1b and c) is a high-strain zone, the Cortegana–Aguafría shear zone. The tectonic evolution of the Cortegana–Aguafría shear zone is marked by the presence of kinematic indicators of both normal and reverse shear senses (see below and also [5]). This contribution focuses on the quartz c-axis fabrics of the leucocratic gneisses deformed by the Cortegana–Aguafría shear zone.

The leucocratic gneisses are either granitic (Qtz–Kfs–Pl) or trondhjemitic (Qtz–Pl) in composition. Both types contain minor amounts of biotite, clinoamphibole, and/or clinopyroxene. Occasionally, garnet, epidote, or graphite may be present. The microstructure of the leucocratic gneiss localised in the Cortegana–Aguafría shear zone is essentially the same, irrespective of the dominant deformation phase (D₂ or D₄) or the flow-type recorded by the quartz fabric. This microstructure is characterised by quartz polycrystalline ribbons defining the stretching lineation, which are surrounded by a fine-grained quartz-feldspathic matrix (Fig. 2a). Quartz grains are irregular in shape, rather elongated, with numerous grain and subgrain boundaries, as well as some deformation bands (Fig. 2a). These microstructures suggest the activity of subgrain rotation recrystallization (e.g., [10,19,23]). However, bulging and pinning structures, typical of grain boundary migration recrystallization [10,11], have been recognised as well (Fig. 2b), suggesting that metamorphic conditions during deformation (D₂ and/or D₄) reached, at least, the lower-amphibolite facies (e.g., [23]). Both the ribbons and the matrix quartz grains display a well-developed lattice preferred orientation (LPO), indicating that dislocation creep has been active during quartz deformation (see [19]).

3.1 Methodology

Quartz c-axis fabrics have been measured in the leucocratic gneisses of the Aracena metamorphic belt to estimate qualitatively the amount of strain and to deduce shear senses. Quartz c-axes have been measured in the XZ section (X being parallel to the dominant stretching lineation in each sample, which can be either L₂ or L₄) of seventeen samples using a four-axis universal stage, according to Turner and Weiss’ method [25]. The obtained data were represented in equal-area, lower hemisphere pole-figure projections. Sixteen samples correspond to two cross-sections normal to the main trend of the Cortegana–Aguafría shear zone (Fig. 1b). These are the Cortegana cross-section (samples CO1 to CO8), and the Aguafría cross-section (samples AG1 to AG8). Sample SC corresponds to a minor shear zone located in the high-grade area of the continental domain (Fig. 1b). The quartz fabrics have been classified using an extended Flinn diagram (Fig. 3).

Measurement of randomness (isotropy) of crystallographic fabrics can be done with confidence in quartz c-axis fabrics [9]; deviation from isotropy is measured with the parameter S_u [13,15], which is defined as:

Su=(15/2)η∑i=13Si−132

(1)

where n is the number of measurements, and S_i are the normalised eigenvalues of the orientation tensor [18]. The fabric is considered statistically non-isotropic when S_u exceeds a critical value that depends on the chosen significance level. The same parameter S_u can be used to describe the degree of preferred orientation (strength) of the fabric. An approach to the statistical description of the symmetry of a quartz c-axis fabric with respect to the XYZ-axes of the finite-strain ellipsoid (namely, external symmetry) has been given by Fernández-Rodríguez et al. [9]. The acute angle (δ) between each c-axis of the sample and the X-axis of the strain ellipsoid is calculated. The parameter of asymmetry is the arithmetic mean (Am) of the δ values for a given sample. Fabrics showing external symmetry will have Am values statistically equal to 0, and increasing Am values reflect increasing values of external asymmetry in the fabric. Accordingly, both parameters have been determined in the studied fabrics and represented in plots of fabric strength (S_u) and external asymmetry factor (Am) against distance along the studied cross-sections (Fig. 4).

Our quartz c-axis fabrics may be divided into three main types (Fig. 3). Type 0 is weak or random (AG2 and AG5), suggesting low amounts of strain and/or static recrystallization. Type 1 shows symmetric small circles around the maximum shortening direction of the finite strain ellipsoid (Z) with few c-axes plotting parallel to the intermediate axis (Y); the fabric skeleton is symmetrical with respect to the foliation plane, whereas the distribution of the c-axes within the small circles can be either symmetrical (subtype-1A, samples CO1, AG7 and SC) or asymmetrical (types 1B and 1C, samples CO2, CO3, AG3, AG6 and AG8) with respect to the finite-strain axes. Type 2/3 is asymmetric type-I crossed girdles (CO4 to CO7 and AG1). See Fig. 3b for a definition of type-I crossed girdles.

Focusing on the two studied cross-sections, Cortegana and Aguafría, the quartz fabrics, as well as their S_u and Am statistical functions, exhibit significant differences (Fig. 4).

• (1) The two samples, located at the edges of the Cortegana cross-section (CO1 and CO8) show weak fabrics that, presumably, reflect low amounts of strain. Both show a high external symmetry, as evidenced by low Am values. Sample CO4 represents the strongest and the most asymmetric fabric (highest S_u and Am values). The rest of the diagrams show intermediate strengths and asymmetries. The samples located to the southwest of CO4 (CO2 and CO3) display small-circles around the Z-direction, suggesting a top-to-the-south sense of movement. Samples CO4 to 7 show an asymmetric type-I crossed girdles indicating a top-to-the-south sense of movement (Fig. 4a).
• (2) The Aguafría fabrics are diverse, although, in general, their strength and asymmetry are lower than those of the Cortegana cross-section are (Fig. 4b). Fabrics AG2 and AG5 are nearly random. Fabrics AG3, AG4, AG6, AG7, and AG8 tend to small circles around the Z-direction. AG4 and AG7 exhibit statistical external symmetries, whereas AG3, AG6 and AG8 are asymmetrical and display opposed shear senses (top-to-the-south for AG3 and AG8, and top-to-the-north for AG6). Finally, fabric AG1 defines an asymmetric type-I crossed girdle, compatible with a top-to-the-south sense of movement.

Quartz c-axis fabric analysis of leucocratic gneisses from the continental domain of the Aracena metamorphic belt supports a complex tectonic evolution for this sector of the Iberian Massif. These fabrics could record the superposition of two different tectonic phases involving flattening and non-coaxial plane strain (Fig. 4). In both stages, discrete shear zones were generated, with an associated mylonitic foliation and a stretching lineation defined by polycrystalline quartz ribbons formed by dynamic recrystallization. The following interpretation takes into account the geological evolution of the Aracena metamorphic belt [8].

Type-2/3 fabrics (i.e., CO4 to CO7 and AG-1) probably resulted from a non-coaxial plane strain (see [14]). These fabrics invariably indicate a top-to-the-south sense of movement, in accordance with other kinematic criteria (Fig. 2c). In such cases, the Cortegana–Aguafría shear zone is interpreted as a reverse, southwest-verging shear zone that correlates with several thrust structures reported elsewhere in the continental domain, like other reverse shear zones and southwest-verging folds [5,8]. Consequently, all of these structures can be attributed to the D₄ tectonic phase.

Type-1 fabrics are consistent with flattening [20]. Other structures related to flattening in the continental domain are chocolate-tablet boudinage structures developed in marbles [8]. The boudinage structures and the structures of samples showing type-1 fabrics (mylonitic foliation and stretching lineation) were folded during D₃ [8] and, consequently, may be assigned to a previous deformation phase, D₂. Some of these flattening fabrics are symmetrical (CO1, AG4, AG7, and SC); the others are asymmetrical (Fig. 3a), suggesting the influence of a non-coaxial flow component (e.g., [14,20,24]). Asymmetric flattening fabrics (CO2, CO3, AG3, AG6 and AG8) are similar to those generated experimentally in quartzites deformed by a combination of axial compression (ɛ > 30%) and simple shear (γ > 2) (see Fig. 4d–e of Dell’Angelo and Tullis [7]) since both show a high concentration of c-axes close to the foliation pole with an asymmetric distribution on sides of it. These fabrics point to either normal (AG6) or reverse (CO2, CO3, AG3, and AG8) senses of displacement. Several structures (Fig. 2d) indicating also a normal, top-to-the-north sense of movement have been observed near sample AG6, whose fabric could be due to discrete normal shear zones related to vertical flattening. Asymmetric type-I crossed girdles indicating a normal displacement are absent, suggesting that simple-shearing with a normal sense of displacement in the continental domain was always coeval with a component of flattening. Presence of such fabrics throughout the continental domain of the Aracena metamorphic belt [5,8] suggests that flattening was ubiquitous, the extensional non-coaxial deformation remaining concentrated in discrete shear zones (Fig. 4b). Similar fabrics generated because of such a strain partitioning have been described in Sierra Alamilla, Spain [17], and in the Moine thrust, Scotland [12]. The combination of vertical flattening coeval with displacement along localised normal shear zones strongly suggests that extensional collapse or gravity spreading took place to the north of the continental domain of the Aracena metamorphic belt. This event took place during the D₂ phase in association with the HT/LP metamorphic event [4,8]; it can be correlated with a general episode of extension that occurred throughout the southern branch of the Iberian Massif (e.g., [21]).

Assuming that a fabric that forms during flattening can strongly influence the development of subsequent fabrics [14,26], we infer that, in the present case, the fabrics that show combinations of flattening and reverse sense of displacement (CO2, CO3, AG3 and AG8) indicate the superimposition of two tectonic phases: flattening during D₂ and reverse shearing during D₄, which lead to reactivation of discrete shear zones formed during D₂ (Fig. 4c). One of the most important of these shear zones is the Cortegana–Aguafría shear zone, localised at the boundary between the high-grade and the medium-grade zones of the continental domain (Fig. 1b). In areas where deformation produced during D₄ was very intense (such as in the Cortegana sector), the fabrics formed during D₂ were erased and asymmetric type-I crossed girdles developed. As we move away from these highly deformed bands, the asymmetry of the quartz fabrics becomes progressively weaker. In weakly deformed areas (such as in the Aguafría sector), the original D₂ fabric is partly preserved and a weak asymmetry occasionally developed because of D₄. Finally, in areas not affected by D₄, the original D₂ fabric remains unchanged (Fig. 4c). A similar tectonic evolution based on the changes displayed by quartz fabrics has been proposed for the Adra nappe in the Betic Chain [6].

Our quartz fabrics suggest that the tectonic evolution of leucocratic gneisses from the Aracena metamorphic belt comprises two major deformation events: (1) an extensional phase that generated vertical flattening and normal shear zones, and (2) and an episode of shortening giving place to reverse shear zones. Our data support the Variscan tectonic evolution scheme proposed for the contact between the Ossa–Morena and the South Portuguese zones [8], and, more generally, for the southern branch of the Iberian Massif (e.g., [21]), according to which a widespread extensional event was followed by a transpressive one that resulted in southwest-vergent shortening structures coeval with strike-slip and poorly partitioned transpressional deformation zones.

The analysed quartz fabrics have recorded the last deformation event that affected the leucocratic gneisses and, in some cases, part of their deformation path, which includes also a previous phase. As a conclusion of this study, we propose that the superimposition of a non-coaxial strain event over a flattening stage can generate quartz c-axis fabrics where small circles around the Z-axis of the finite-strain ellipsoid are associated with an asymmetric distribution of c-axes within the small circles. This external asymmetry can be used as a kinematic criterion for the last deformation event.

This research was supported by the BTE-2001-2769, BTE2003-0557-CO2-02, CGL2004-06808-CO4-02/BTE and CGL2006-08639/BTE projects (Spanish Ministry of Education and Science), the Junta de Andalucía (groups RNM-120 and RNM-316) and Huelva University. Thanks to G. Lloyd and M. Casey for useful discussions. Careful reviews by Jacques Angelier and an anonymous reviewer are gratefully acknowledged.