Crystallographic preferred orientations and microstructure of a Variscan marble mylonite in the Ossa-Morena Zone (SW Iberia)
a* a b I. Romeo ",R. Capote ,R. Lunar
a Departamento de Geodinamica, Facultad de Ciencias Geol6gicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Departamento de Cristalograjla y Mineralog[a, Facultad de Ciencias Geol6gicas, Universidad Complutense de Madrid, 28IJ4IJ Madrid, Spain
Abstract
The mylonitic marbles of the Cherneca shear zone, a Variscan sinistral strike-slip structure developed in the Ossa-Morena Zone (SW Iberia) related to the emplacement of the Santa Olalla Igneous Complex, were studied in detaiL Different mylonites (from protomylonites to ultramy lonites) were analyzed by Electron Back-Scattering Diffraction (EBSD) in order to determine the crystallographic preferred orientation (CPO) of calcite, The CPOs show very good agreement with experimental torsion work giving suitable interpretations of the operative slip systems and strain quantifications, thus the obtained natural CPOs validate the experimental approaches, A wide range of observed textures and microstruc tures are mainly controlled by the amount of shear strain, the presence of secondary phases, and the development of antithetic subsidiary shears, The effect of secondary phases and the nucleation of antithetic shear bands on texture development are discussed,
Keywords: Crystallographie preferred orientation; Texture; Microstructure; Mylonitc; Shear bands; Variscan Orogeny
1. Introduction a given temperature, and the amount of strain. The geological significance of calcite CPOs is shown by experiments carried The more suitable rocks for accumulating large amounts of out on natural marbles (Griggs et aI., 1960; Handin et aI., ductile strain at the low temperature dominant conditions in 1960; Schmid et aI., 1977, 1980; Rutter, 1974, 1995; Rutter the upper crust are carbonate rocks, Considering their abun et aI., 1994; Casey et aI., 1998; Pieri et aI., 2001a,b; Barnhoorn dance elsewhere, these rocks play a fundamental role during et aI., 2004, 2005), synthetic calcite aggregates (Walker et aI., orogenesis, Thus, advancing the knowledge of the formation 1990; Barnhoorn et aI., 2005), and on simulations performed and evolution of plastic flow in carbonate shear zones is by numerical modelling (Wenk et aI., 1987, 1997, 1998; a main objective for the understanding of orogenic processes Wenk and Christie, 1991; Lebensohn et aI., 1998; Pieri (Pfiffner, 1982; Van der Pluijm, 1991), et al., 2001b). Numerical and experimental results have proven Calcite textures and microstructures in deformed marbles helpful when compared to natural samples. provide information regarding kinematics, deformation mech Deformation experiments on calcite single-crystals (Turner anisms, stress quantification, characteristics of the strain, and et aI., 1954; Griggs et aI., 1960; Turner and Heard, 1965; Borg physical parameters during deformation (e,g, temperature), and Handin, 1967; Paterson and Turner, 1970; Weiss and The type of texture (crystallographic preferred orientation, Turner, 1972; Braillon and Serughetti, 1976; Turner and CPO) developed during ductile deformation of calcite is con Orozco, 1976; Spiers and Wenk, 1980; De Bresser and Spiers, trolled by two factors: active crystallographic slip systems at 1990, 1993) were used to define the crystallographic slip and twin systems active at different temperature, stress and strain rate conditions, and critical activation shear stresses (re
* Corresponding author. Fax: +3491 3944845. views can be found in Paterson, 1979, and De Bresser and Spiers, 1997). Plastic deformation of calcite occurs in low and high temperature regimes. At low temperature deforma ..,. 41·"-/ tion is characterized by twining on e{-1018}<40-4l>, slip 38° � Cambrian melasedimenls and metavolcanites on r{ 10-14}<-202l> (r
4206900
Metasediments � Plutonic rocks 1km �: MetabJffand slate with: � Aguablanca Gabbronorite � f a) marble �e b)skam � Aguablanca Diorite Myllonitic foliation of the Gi cl Rhyolithic porphyry Chemeca ductile shear zone � � Slate. metagraywacke and _ Ni-Cu-PGE orebodies .: � black quartzite Alkaline granite r - - - • f::.::'·.J
• Figure 9 • D SantaOlalla Tonalite
Fig. 2. Geological map of the Olerneca shear zone. The locations of the samples S02l8 and S0235, Fig . 3 and Fig. 9 are indicated. et aI., 1989), the largest p1uton of the complex, consists of am concentration processes and localized softening. Mylonitiza phibole-biotite quartz-diorite in the northern area grading to tion developed an anisotropic and complex shear zone with dominant tonalitic fades in the centre. The Aguablanca stock rock types that range from low-strained protomylonites to is a mafic subcircular pluton composed of phlogopite-rich gab highly-strained ultramylonites. Five mylonitic marble samples bronorite and norite, grading to diorite to the south. This rnafic (S0217A, S0217B, S0217C, S0217D, S0218) were col intrusion has undergone significant endoskarn processes along lected for detailed microstructural and textural characteriza the northern boundary induced by contact with marbles of the tion across the Cherneca shear zone. The results were upper part of the Bodonal-Cala Complex (Casquet, 1980). compared with a marble sample located outside of the shear The intrusion of the Santa Olalla Igneous Complex during de zone (S0235). The latter is considered to represent the starting formation of the Chemeca shear zone induced a sinistral shear material prior to mylonitization. Sample S0218 corresponds strain that is recorded in the NE half of the complex by mag to a low-strained mylonite and is located near the southern matic deformation (Romeo et aI., 2006b). boundary of the shear zone (Fig. 2). Samples S0217C and The geochronology of the Santa Olalla Igneous Complex S0217D are highly-strained ultramylonites located in the cen yielded a Visean age (U-Pb on zircons, Romeo et aI., 2006a), tral part of the shear zone (Fig. 3). Samples S0217A and similar to other plutonic intrusions along the Olivenza Mones S0217B correspond to antithetic subsidiary shears developed terio antiform: Brovales (340 ± 4 Ma obtained by Pb-Pb with on a protomylonite band (Fig. 3); also located in the central the thermal evaporation technique (Kober, 1986, 1987) on area of the shear zone. zircons, Montero et aI., 2000), Valuengo (342 ± 4 Ma obtained Samples S0218, S0217C and S0217D were cut perpendic by Pb-Pb with the thermal evaporation technique on zircons, ular to the mylonitic foliation (XY plane, normal to Z) and Montero et aI., 2000) and Burguillos del Cerro (330 ± 9 Ma parallel to the stretching lineation (X direction) of the main obtained by whole rock Rb-Sr, Bachiller et aI., 1997; 335 Ma sinistral shear belt. Samples S0217A and S0217B were cut obtained by Ar-Ar on amphibole, Dallmeyer et aI., 1995; perpendicular to the foliation plane of the subsidiary dextral 338 ± 1.5 Ma obtained by U-Pb on allanite, Casquet et aI., shear bands (X'Y plane, normal to Z') and parallel to their 1998). The activity of Aguablanca stock has been dated from lineations (X' direction). This second coordinate system has 343 to 338 Ma (U-Pb on zircons, Romeo et aI., 2oo6a). a Y axis parallel to the main shear coordinate system and The study of the Cherneca shear zone has special relevance new X' and ZI axes that are rotated 25° in respect of the X as it was proposed to be the structure that favoured the em and Y axes of the main shear (Fig. 3). The angle between the placement in the Aguablanca stock northern boundary of main shear foliation plane and the antithetic shear bands is 25°. a Ni-Cu-PGE ore deposit (Lunar et aI., 1997; Ortega et aI., The sinistral kinematics of the Cherneca shear zone are in
1999, 2004; Tornos et aI., 1999; Casquet et aI., 2001; Tornos dicated by mesoscopic (J and o-porphyroclasts and S-C struc et aI., 2001; Pilla et aI., 2006). The mineralization is closely tures on the outcrop. These observations are consistent with associated with a 70-80" N dipping, funnel-like magmatic the sinistral oblique character of the Variscan orogeny else breccia (250-300 m wide in the N-S direction and up to where in the Ossa-Morena Zone (Eguiluz et aI., 2000). 600 m long in the E-W direction). The breccia comprises bar The protomylonite band shown in Fig. 3 is characterized by ren or slightly mineralized rnafic-ultramafic cumulate frag a weak mylonitic foliation parallel to the main shear. Subsid ments enveloped by hornblende and phlogopite-rich iary shears cross-cutting the protomylonitic foliation appears, gabbronorite. The gabbronorite contains disseminated and whose kinematics are indicated by dextral drags of the proto semi-massive Ni-eu-Fe magmatic sulphides. mylonitic foliation. These subsidiary shears deform the proto The Chemeca shear zone was studied in the northern area mylonitic foliation; consequently, they can not correspond to of the Aguablanca stock and the Ni-Cu-PGE mineralization. structures formed before the Cherneca Shear Zone. These sub In this area the Cherneca shear zone is developed on the mar sidiary shears are asymptotic relative to the boundaries of the ble of the Bodonal-Cala Complex, which is locally trans protomylonite band (Fig. 3) in contact with ultramylonites. It formed into garnet-rich skarn. The marble and the skarn are can be established from these structural relationships that the both deformed by sinistral shearing, indicating that deforma subsidiary shears were formed after the protomylonitic folia tion continued after the dated magmatic event. Strain along tion but before or simultaneously with the ultramylonitization the Cherneca shear zone was concentrated in the Bodonal at the boundaries of the protomylonite band. The drags of the Cala marble because of its ductile rheology during deforma protomy lonitic foliation caused by the subsidiary shears tion, rather than on the more fragile Serie Negra. During the clearly indicate a dextral shear sense for these particular struc cooling period after Variscan orogenesis, the marbles became tures. The subsidiary dextral shears are rotated 25° clockwise rheologically stronger and thus the late movements on with respect to the main shear. the shear zone were restricted to the contact between the This structural pattern (Fig. 3) could be interpreted as an Bodonal-Cala Complex and the Serie Negra. S-C' mesoscale structure implying a contradicting dextral shear criterion for the Cherneca shear zone. This interpretation 3. Field kinematic observations and sample location is against all the shear sense indicators on the outcrop, and also against all the microstructural and crystallographic indi The Cherneca shear zone shows typical characteristics of cators presented in this contribution, that clearly established ductile deformation under simple shear, dominated by strain- a sinistral shear sense for the Cherneca Shear Zone. Thus 1m
Fig. 3. A scheme of the location of the samples S0217A, S0217B, S0217C and S0217D, two belonging to ultramylonitic bands, and two from antithetic sub sidiary shears, indicating the 3D geometry and kinematics. The reference frames used for the main shear samples (XYZ) and for the subsidiary shear samples (X'YZ') are indicated. we reject to interpret it as an S-C' structure, considering that misorientation angles between adj acent points were higher the observed subsidiary dextral shears are antithetic structures, than 15°. This method allows accurate and easy measurements which kinematics serves to accommodate transpressive strain. of average grain sizes considering that the spacing of the crys The formation of antithetic shear bands with orientations sim tallographic data was chosen to be at least one-half of the ilar to C' shears was noticed by Behrmann (19S7), indicating finest grains found in each sample. that the interpretation of a subsidiary shears as C' shears has to be done carefully. The interpretation of the dextral subsid 4.1.1. Pre·shearing stage (sample S0235) iary shears as antithetic shears developed into a sinistral shear Bedding of the Bodonal-Cala marble out of the Chemeca zone, accommodating a transpressive component of strain, is shear zone is shown by alternating bands with fine (SO fUll) in good agreement with the transpressional character of the and coarse (250 ftm) grains. Calcite grains are equidimen Variscan collision in the Ossa-Morena Zone (e.g. Eguiluz sional and have abundant triple junctions and straight grain et aI., 2000). boundaries. Although a few twins are present, no significant internal deformation occurred. 4. Results 4.1.2. Low·strained mylonite (sample S0218) The result of the microtectonic study performed on the my The mineralogy of S021S, a sample located close to the lonitic marble of the Cherneca shear zone is divided in two southern boundary of the shear zone, consists of dominant sections: the description of the microstructures determined calcite, with minor rounded quartz grains «1%). The mi by optical microscopy and that of the CPOs determined by crostructure of S021S is typical of low-strained mylonite EBSD. where porphyroclasts (200 ftm) are abundant (31 %) and ap pear completely surrounded by dynamically recrystallized 4.1. Microstructure grains (10 fUll, 69%) producing a core-mantle texture (Fig. 4). A few bent twins were formed in the first stages The observed microstructures of the studied samples are of deformation and were rotated during further strain to described below. The grain sizes were determined by recon wards parallelism with the shear zone boundary. Rotation structing grain boundaries from the crystallographic data, con and stretching of twinned porphyroclasts produced a very sidering the presence of a grain boundary when the strong shape preferred orientation (SPO) with a trend 0.25 mm I 1 mm
Fig. 4. Microphotographs of S0218, S0217C, S0217D, S0217A, S0217B. The sample S0218 is characterized by a core-mantle rectystallized microstruchue, where the elongation of porphyroclasts indicate the presence of a primary oblique foliation SI that was used to calculate the amOlmt of shear strain (y). Note the presence of quartz as secondary phase in the ultramylonite samples S0217C and S0217D. The samples of the subsidiary shear bands (S0217A, S0217B) feahue a core-mantle texhue where the elongation of the porphyroclasts is subparallel to the subsidiary shear plane.
oblique to the shear zone boundary, implying a sinistral oblique foliation SI) and the shear plane (8') can be directly shear sense for this sample. All the porphyroc1asts of this related to the shear strain ('1) by the expression sample show elliptical shapes with the same orientation of tan(28') � 21'1 (Ramsay and Huber, 1983). In this case 8' their longer axes (Fig. 4). These grains may have originated is 260 and gives a shear strain of '1 � 1.56 (Fig. 4). This from initially rounded grains similar to those of the non estimation is a maximum considering that the Cherneca sheared sample (S0235), which acted as initial subcircular shear zone probably has a transpressive component, as passive shear-strain markers that were transformed to ellip will be discussed later. Flattening perpendicular to the shear ses by internal plastic deformation. Thus, we can infer that zone would have decreased fI' overestimating the measured they are strain ellipses defining an oblique foliation SI (fol 'Y value. This kind of oblique core-mantle microstructure lowing the notation of Barnhoom et aI., 2004). The angle has been successfully reproduced by Pieri et al. (2oo1a,b) between the long axis of the strain ellipse (parallel to and Bamhoorn et al. (2004) during experimental torsion studies at different temperature conditions for values of dynamic recrystallization was more pronounced in S0217B, "I � 1.5-3. allowing a feed-back mechanism that produced strain concen tration in this narrow subsidiary shear. The deformation state 4.1.3. Ultramylonites (samples S0217C, S0217D) of S0217A can therefore be considered as an early stage Hand-specimens of samples S0217C and S0217D are white during the generation of a subsidiary shear. marbles, although large percentage of impurities can be found Despite the fact that the subsidiary antithetic shear bands with microscopic analysis (Fig. 4). Secondary rounded quartz show small displacement in the field (Fig. 3), porphyroclasts
grains are present with an abundance of z 30% in S0217C in both samples, S0217A and S0217B, show a clear elonga
and z 20% in S0217D. The average grain size of S0217C is tion that is oriented subparallel to the subsidiary shear plane. 70 fUllfor quartz and 30 fUllfor calcite, in contrast to the smaller These microstructures contrast with the oblique orientation average sizes of S0217D, 35 fUllfor quartz and 25 ftm for cal of the porphyroclasts of similar size in S0218 with respect cite. In S0217D, boundaries between calcite grains are more to the main shear plane. It is widely observed in experimental sinuous than those of S0217C whose grain boundaries are studies (pieri et aI., 2001a,b; Barnhoorn et aI., 2004) that, dur straighter. S0217D shows thus a complete dynamically recrys ing deformation, porphyroclasts are passively rotated towards tallized mosaic texture except for quartz with slightly coarser the shear plane, reaching almost complete parallelism at very grain size than calcite. The calcite texture of S0217C is more high shear strains. Considering the small displacements ac disturbed by the presence of a larger amount of quartz grains commodated by the subsidiary shear bands that can be ob with bigger grain sizes. The presence of coarser and abundant served in the outcrop (Fig. 3) a very high shear strain must quartz may, therefore, control the average size of calcite grains. be discarded as the mechanism that generated this SPO paral This sample shows straighter calcite boundaries, which proba lel to the shear plane in these samples. Thus a different expla bly indicates that dynamic recrystallization did not play an im nation is needed. One possibility is that this SPO could be portant role in the development of the texture. Texture of sample generated by the general sinistral shear strain in the protomy S0217C was probably controlled by internal deformation lonitic band, where later the subsidiary dextral shear bands during dragging of the calcite crystals between quartz grains. were formed. The observed SPO relative to the main shear ref The development of a stable secondary oblique foliation (S2 erence frame gives a sinistral shear sense that is coherent with following the terminology of Barnhoorn et aI., 2004), equiva mesoscopic indicators. This SPO oblique to the main shear lent to that observed in large-strained and fully recrystallized reference frame (Sl) would have acted as a planar anisotropy samples of the experiments performed by Pieri et al. favouring the nucleation of antithetic subsidiary shears parallel (2001a,b) and Barnhoorn et al. (2004), was not observed in to this previous fabric in order to accommodate the transpres our ultramylonite samples (Fig. 4). The lack of a secondary sive component of the Chemeca shear zone. This interpreta foliation in S0217C and S0217D may have been caused by tion also yields an explanation for the antithetic shear sense the presence of large amounts of quartz grains, inhibiting the of the subsidiary shear bands. formation this microstructure. 4.2. Crystallographic preferred orientations (CPOs) 4.1.4. Subsidiary antithetic shear bands (samples S0217A, S0217B) CPOs were determined by individual orientation data of Two subsidiary antithetic shears developed on a protomylo calcite grains obtained by EBSD during automatic mapping nitic band were studied (S0217A, S0217B; Fig. 3). The mi of previously prepared samples. The oriented samples were crostructure of these samples is very different to that of the cut in thin sections that were used to study the microstructures ultramylonites and the low-strained mylonite. The samples under microscope and were polished for EBSD analysis of the subsidiary antithetic shears also show different textures following the techniques described by Prior et al. (1999). from one another. Both samples feature a core-mantle texture The treatment of orientation data of the mapped area was with abundant porphyroclasts surrounded by recrystallized used to delineate grain boundaries when the misorientation an grains, but S0217A shows coarser grains for both porphyro gles between adj acent points were higher than 15°. Using the clasts and recrystallized grains than S0217B (Fig. 4). misorientation analytical method the grain boundaries of the S0217A has an average size of 350 ftm for the porphyroclasts mapped area were reconstructed and the grain size distribution and 30 ftm for the recrystallized grains, while S0217B shows was obtained, allowing to separate the orientation data of averages of 200 ftm and 25 ftm, respectively. The difference different size classes. This methodology was applied to between both samples can be related to the intensity of dy distinguish the different contribution of porphyroclasts and namic recrystallization. In S0217A only 44% of its surface recrystallized grains to the total CPO. was dynamically recrystallized, contrasting with the 77% of surface dominated by recrystallization in S0217B. Conse 4.2 .1. Pre-shearing stage (sample S0235) quently, it can be inferred that the shear strain accommodated The sample S0235 belongs to the Bodonal-Cala marbles in the subsidiary shear of S0217B was higher than the shear outside the Cherneca shear zone. Bedding (alternating layers strain absorbed by S0217A, taking into account that the per of fine and coarse grains) shows two folding events at meso centage of recrystallized grains increases with shear strain. scale. The CPO of the total data set is approximately random The weakening associated to the grain size reduction during indicating that the amOlll1t pureof shear accommodated by this ZG. c(0001) r(1014) 1(1012) 0(1210) sd(2021) ,
Z'" M"' " N '"0: 0 '" ZW u:
N=182S6 '" z
"' � M '" N 4 0 '"w '" 0: " 0 ()
Fig. 5. Equal area lower hemisphere proj ections of CPOs of S0235, a marble located out of the Chemeca shear zone. The sample is characterized by alternating bands with coarse and fine grains, whose contributions to the total CPO are also shown.
sample during folding did not generate a significant preferred 4.2.2. Low-strained mylonite (sample S0218) orientation (Fig. 5). This sample is characterized by alternat Despite moderate shearing as indicated by the micro ing layers with fine and coarse grains whose crystallographic structure of S0218 (Fig. 4), a strong CPO was developed orientation data were separated by grain boundary reconstruc during deformation (Fig. 6). It is characterized by a maxi tion and grain size analysis, which gave very similar random mum of the {c} poles perpendicular to the shear plane ac CPOs for both (Fig. 5). companied with a weaker girdle of {c} poles in the YZ
c(0001) r(1014) 1(1012) 0(1210) sd(2021)
0> � N o '"
0> � N o '"
Fig. 6. Equal area lowerhemisphere projections of CPOs of S0218, a low-strained mylonite. The total CPO was separated showing the preferred OIientation of the rectystallized grains and the porphyroclasts. plane. The axes are distributed within a girdle in the The CPO obtained for S0217D, which is a fully recrystal XY shear plane with a clear maximum in the X direction. lized sample, shows an orthorhombic symmetry with respect This disposition seems to indicate that slip on {c} to the shear plane, with a wide maximum of {c} and {r} poles was active during deformation. The CPO symmetry is normal to the shear plane (XY). The axes are disposed monoclinic (lO° of clockwise rotation) with respect to the within the shear plane with a higher abundance in the X direc shear plane indicating a sinistral sense of shear, in agree tion. This geometry suggests that both c and r could ment with outcrop indicators. have been active during deformation. c is possibly the The CPOs of the dynamically recrystallized grains and the dominant slip system as deduced from this CPO, because porphyroclasts are not completely equivalent. The more ho the {c} maximum is stronger than the {r} one in the Z direc mogeneous data distribution in the CPO of the recrystallized tion normal to the shearing foliation. grains relative to the CPO of the porphyroclasts may be due to the difference of number of grains measured. Some impor 4.2.4. Subsidiary antithetic shear bands tant differences in the CPOs can be observed. Although the (samples S0217A, S0217B) distribution of {c} poles and axes is almost equivalent, A detailed CPO study was performed to unravel the evolu the {r} poles pattern is different. On one hand, the recrystal tion of the texture in subsidiary shear bands. Two subsidiary lized grains show a wide maximum of {r} poles in Z, indicat antithetic shear bands located in a protomylonite band were ing that the slip system r proposed by Pieri et al. analyzed by EBSD. Their CPOs are shown in Fig. 8. All the (2001a,b) and Barnhoorn et al. (2004) could be active during diagrams are referred to the previously described X'YZ' coor deformation. On the other hand, the {r} poles of the porphyr dinate system, which corresponds to the subsidiary shear plane oclasts show two clusters: one rotated 10° with respect to the (X'Y normal to Z') and slip direction (X'). The orientation of Z axis in concert with the sense of shear and the other one ro the coordinate system of the general shear (XYZ) is also tated 50° with respect to the Z axis opposite to the sense of shown in Fig. 8 in order to help the interpretation. shear. Sample S0217B shows a complex CPO which is the mixture of two different CPOs, the orientation of the recrystallized ma 4.2.3. Ultramylonites (samples S0217C, S0217D) trix and the porphyroclasts. The recrystallized grains show The ultramylonite samples, S0217C and S0217D belong a wide maximum of the {c} poles perpendicular to the subsidi to different ultramylonite bands (Fig. 3) and yielded different ary shear plane and a weaker maximum in the same place of the CPOs (Fig. 7), probably controlled by the different amounts of {r} poles. In contrast, the axes are oriented in a girdle within quartz grains in each sample. the subsidiary shear plane (X'Y), with a cluster close to the shear S0217C shows a monoclinic CPO with a maximum of {c} direction (X'). Similarly to S0217D (within the ultramylonites poles giving a sinistral sense of shear for this sample in agree of the main shear zone), the recrystallized grains of S0217B ment with the general kinematics of the shear zone. The show an orthorhombic symmetry that indicates that c and axes are disposed within a weak girdle perpendicular to the to lesser degree r were the active slip systems. main {c} maximum and show three prominent clusters within The orientation of the porphyroclasts in S0217B is not the shear plane (XY). The {r} pole plot displays three maxima, equivalent to that of the recrystallized grains. The disposition one parallel to the maximum of {c} poles and the other two of {c} poles is also around Z' but in this case two different maxima disposed with a monoclinic symmetry with respect maxima can be distinguished, one in Z (note that it is the to the shear plane (XY). The most suitable slip system that axis normal the foliation plane of the main shear) and the other can be deduced from this CPO is c. one between Z and X. The orientation of axes is restricted
'k 0(0001) ,(1014) 1(1012) 0(12'10) sd(2021) ,
4 5
6
Fig. 7. Equal area lower hemisphere projections of CPOs of the ultramylonite samples S0217C and S0217D. c(OOOl) r(1014) 1(1012) a(1210) sd(2021) N=66338
2
3
'" .... N 0 (I) 2
� � � � N=12593 V> l- :IV> '" 0 .... 0 4 N '" 6 0 >- (I) :r 0. 8 0:
0 • 0. ,.
« .... - N o (I)
Fig. 8. Equal area lower hemisphere projections of CPOs of the antithetic subsidiary shear bands, S0217A and S0217B. ill both samples the contribution to the total CPOs of the recrystallized grains and the porphyroclasts was discriminated. The reference frame of the subsidiary shear bands plus the reference frame of the main shear are indicated to help the interpretation of relict fabrics in the porphyroclasts of S0217B.
to a girdle perpendicular to the secondly described {c} maxi due to small error in the orientation of the cutting plane during mum, showing also three clusters inside this girdle. If we an sample preparation. Beside this difference, the epo of this sam alyze this epa relative to the reference system of the general ple is similar to the epo of the recrystallized grains of S02!7B, shear (XYZ), it shows a monoclinic symmetry indicating a si and thus we interpret that the same slip systems were active nistral sense of shear. The most favoured slip systems for this during deformation: c and to lesser degree r. epo are c, r and also r
commodated by twining is very small ('Ymax = 0.68, consider posed for monoclinic versus orthorhombic symmetries in grey ing the optimum single-crystal orientation, Wenk, 1985). This, and white mylonites, respectively, by Herwegh and Kunze together with the observations of undulatory extinction, sub (2002). The presence of nano-scale particles in the grain bound grains and recrystallized grains (core-mantle textures; aries of grey mylonites causes slower grain boundary migration, Fig. 4), indicate that internal plastic deformation of calcite which favours the conservation of a monoclinic symmetry. In was dominantly generated by slip and climb of dislocations. contrast, the continuously recrystallized texture of the white The core-mantle microstructure of S0218 (Fig. 4) shows mylonites generates an orthorhombic symmetry. very similar sizes of subgrains and recrystallized grains, sug gesting that subgrain rotation was the main process leading 5.1.2. Ultramylonites (samples S0217C, S0217D) to dynamic recrystallization. The CPO obtained for S0217C is equivalent to that obtained by Barnhoornet al. (2004), for samples deformed at 500-600 °C and 5.1. Comparison of the epa results with experimental 'I � 2-4. This complex CPO has been reproduced (pieri et aI., works and numerical simulations 2001b) using the numerical model of Wenk et aI., 1987, which assumes that {c} and {r}
ples with 'Y = 2-5 and 727 °C. However in our case the maxi mum are wider and less defined. This softening of the CPO 5.1.3. Subsidiary antithetic shear bands could be related to thermal annealing. Thermal annealing of ex (samples S0217A, S0217B) perimentally deformed samples of Carrara marble (Barnhoorn The recrystallized grains of S0217B (antithetic shear band) et aI., 2005) caused a dispersion of the CPO pattern that is and the complete data set of S0217A (antithetic shear band) very similar to the CPO ofrecrystallized grains in S0218 (Car both yielded a CPO with an orthorhombic symmetry equiva rara marble deformed at 'I � 5 and 1 h of annealing; Barnhoorn lent to that from S0217D (main sinistral shear), although et aI., 2005). Values of 'I � 2-5 are near to the results obtained each sample has its own coordinate system. The dextral sub from the analysis of the microstructure (Fig4) where the sheared sidiary shear bands are developed across a protomylonitic porphyroclasts act as passive markers giving a strain value of band. The CPOs of S0217B and S0217A show deformation 'I � 1.56 (see the microstructure description of S0218). This in these subsidiary shears in a direction that is 25° in clock sample is located close to the southern boundaryof the Cherneca wise sense with respect to the main shear plane. The calcite shear zone where the marbles are moderately sheared. grains are fully reoriented during deformation along the CPOs with monoclinic symmetry indicative of sinistral shear subsidiary shear bands, so in these bands the CPO has an sense have been observed in porphyroclasts and recrystallized orthorhombic symmetry with respect to the subsidiary shear grains of sample S0218. This monoclinic symmetry is more plane. The same comparisons with previous studies as those pronounced in porphyroclasts than in recrystallized grains. done with S0217D can be applied to these samples. This latter CPO is close to an orthorhombic symmetry In contrastwith the orthorhombic symmetry obtained for the (Fig. 6). This discrepancy can be explained by the different de recrystallized grains, the porphyroclasts of S0217B yielded an formation processes that occur in both types of grains. The con asymmetric preferred orientation that, when considered relative sumption of grains by grainboundary migration during dynamic to the main shear (XYZ) , is equivalent to the experimental sam recrystallization generates a continuous resetting of the fabric, ples of Barnhoorn et al. (2004) deformed at 500-600 "C and which allows for the preservation of an orthorhombic symmetry 'I � 4. This sample shows a CPO equivalent to that of of the CPO. However, the porphyroclasts undergo continuous S0217C, indicating the same sinistral shear sense for the main internal deformation and rotation during shearing which shear. 5.2. Main shearing versus antithetic shear bands 2002; Krabbendam et aI., 2003). The rheology of a rock with secondary phases can be weaker or stronger than the In order to understand the role of coarser grains (porphyr pure aggregate. In some cases, growing of grains is reduced oclasts) in antithetic shears, we analyzed the geometry of these by pinning caused by secondary phases. When the final grain subsidiary shear bands. The thickness of the shear bands that size is smaller than the steady state recrystallized grain size of are characterized by a total dynamically recrystallized micro the pure aggregate, the material gets softer in the grain size structure is 1-4 cm. In the surrOlll1dings of the recrystallized sensitive field (Olgaard, 1990; Herwegh and Kunze, 2002). band, a coarser microstructure formed by porphyroclasts dom If secondary phases cause a dragging of the contacts between inates. These coarser grains were strained during sinistral de grains, a reduction of the recrystallization rate occurs, which formation of the main shear on the protomylonite band. The generates a stronger material behaviour (Walker et aI., 1990; mylonitic foliation in this protomylonite band was dragged Herwegh and Kunze, 2002). in the surroundings of the subsidiary dextral shears (Fig. 3). The effect of dragging calcite grain boundaries is increased These folds caused by dragging constitute a flanking structure with the abundance of impurities and also with more aniso (Hudleston, 1989; Passchier, 2001; Grasemann and Stiiwe, tropic shapes of the secondary phases (Herwegh and Berger, 2001) that is classified as an a-type (antithetic sense) flanking 2004). In our samples the main secondary phase present is fold with normal drag, which means a drag coherent with the quartz, which appears as rounded clasts floating in a calcite movement of the subsidiary shear, following the classification pure matrix (see S0217C in Fig. 4). The isotropic shapes of of Grasemann et al. (2003). the quartz grains minimized their effect on rheology, but their S0217A corresponds to the central zone of recrystallized ablll1dance was enough to affect the deformation mechanisms. grains, where porphyroclasts are into this recrystallized band. The effect of the abundance of quartz is evidenced by the dis For this reason S0217A shows for both recrystallized grains crepancy between the CPOs of the samples belonging to the and porphyroclasts an orthorhombic symmetry with respect to ultramylonitic bands (S0217C, S0217D). S0217C with the subsidiary shear reference frame. However, S0217B corre z 30% of quartz grains and an average grain size of 70 fUll sponds to a thilUler subsidiary shear and the thin section com shows a monoclinic CPO. In contrast, S0217D with z 20% prises part of the recrystallized band and part of the external of quartz grains and an average grain size of 35 ).tm shows protomylonite. Consequently, the CPO of the recrystallized an orthorhombic CPO characterized by maxima of {c land grains of S0217B has an orthorhombic symmetry with respect {r} poles parallel to Z. The presence of bigger and more abun to the subsidiary shear plane (X'Y), while the CPO of the por dant quartz grains in S0217C is enough to strongly affect the phyroclasts (that were deformed by the main sinistral shear in texture development. In this sample the role of dynamic the protomylonite) has a monoclinic symmetry with respect to recrystallization is reduced by the presence of quartz, that the main shear plane (XY), comparable with experimental generates drag in the calcite-calcite bOlll1dary migration, results. reducing drastically the recrystallization rate. This process The development of these antithetic subsidiary shears favours the deformation of calcite grains by internal plastic serves to accommodate a component of shortening normal to strain (dislocation creep) causing a monoclinic CPO during the main shear zone. It also allows the stretching of a more rotation of calcite grains towards parallelism with the shear competent band, such as the protomylonite band, where the plane. S0217D, a similar ultramylonite, is less affected by antithetic shears were found. This tectonic pattern can be de the presence of second phases and has undergone a fully dy scribed as an initial stage of asymmetric boudinage, developed namic recrystallization texture which yielded an orthorhombic on this more competent layer, and accomplished by a domino CPO. pattern (Goscombe and Passchier, 2003), coherent with the si Although the presence of different amounts of quartz is the nistral shear displacement of the main shear zone. The pres preferred explanation for the differences observed between the ence of these antithetic subsidiary shears is indicative of an textures of both ultramylonite samples, a different amount of important transpressional component of the Cherneca shear shear strain due to concentration of deformation could also zone, which is in agreement with the general transpressive have played a role. The amount of dynamic recrystallization character of the Variscan orogen elsewhere in the Ossa-Mor increases with strain (Pieri et al., 2001a,b; Barnhoorn et al., ena Zone (Eguiluz et aI., 2000). Deformation across the Cher 2004) causing a subsequent change of the CPO with increasing neca ductile shear zone seems to be partitioned: the sinistral strain. In this case, S0217C would represent a low-strained simple shear component is mainly accommodated by ultramy sample prior to recrystallization and S0217D a high-strained lonite bands where strain is concentrated, while the pure shear fully recrystallized sample. component of strain is accommodated by subsidiary antithetic shears developed on protomylonite bands. 6. Conclusions
5.3. Effe ct of secondary phases The microstructural and textural analyses performed on naturally deformed marble mylonites of the Cherneca Shear The influence of secondary phases on the rheology of rocks Zone indicate a wide range of textures and microstructures was investigated by several authors (e.g. Urai et aI., 1986; within the same shear zone (summarized in Fig. 9). Similar Drury and Urai, 1990; Olgaard, 1990; Herwegh and Kunze, drastic variations in the microstructure and texture at Ultramylonite c(0001) r(1014) a(1210)
.. Subsidiary shears on protomylonite c: � c(0001) r(1014) .(1210) � '" .c.. fJ) '" <.> ..
�c: .c.. Ultramylonite U c(0001) r(1014) .(1210) ......
•
low-strained mylonite c(0001) r(1014) .(1210) ......
Mylonite Black Slate 0 � Non-sheared marble 0 Protoylonite f��ta Rhyolithic porphyry c(0001) r(1014) .(1210)
Ultramylonite Skarn .... - 0 M N o Cd Marble )+ ;+ + +I Alkaline granite Vl
Fig. 9. Geological scheme indicating the main results of the CPO study obtained across the Olerneca shear zone. All the CPOs are plotted witilln the main shear reference frame, except S0217B which is plotted in respect of the subsidiary shear reference frame. The location of this map is indicated in Fig. 2.
microscopic scale were observed by Busch and Van der Pluijrn that caused rotation of the reference frame and inversion of (1995) in natural mylonites. The comparison of the observed the sense of shear. The latter caused total resetting of the fabric CPOs of different strained mylonites across the Cherneca in some cases and partial resetting in others (where the initial Shear Zone shows a good agreement with the CPOs obtained deformation of the general shear persists in the porphyroc1asts both in torsion experiments (Casey et aI., 1998; Pieri et aI., preferred orientation), depending on the amount of shear 2001a,b; Barnhoorn et aI., 2004, 2005) and numerical simula accommodated in the subsidiary shear bands. tions (Wenk et aI., 1987, 1997, 1998; Pieri et aI., 2001b), All the studied samples were mainly controlled by slip on which gives support to the application of the experimental ap c, a slip system that is only active at high temperatures. proaches and numerical sirnulations to natural samples. This This observation is supported by high temperatures expected kind of comparisons allows to establish the active slip systems, in the metamorphic aureole of the Aguablanca gabbronorite the shear sense, and to estimate the amount of strain. where these samples were collected. The epos with mono The main factors that control the development of CPOs in clinic symmetry with respect to the shear plane (S0217C in the Cherneca shear zone are: (1) the amount of shear strain ('1) Fig. 7, and S0217B in Fig. 8), indicate that the active slip sys that was irregularly distributed across the shear zone due to tems during deformation were c and r
Eguiluz, L., Carracedo, M., Apalategui. 0.• 1989. Stock de Santa Olalla de Cala (Zona de Ossa-Morena, Espana). Stvdia Geologica Salmanticensia References 4. 145-157. Eguiluz, L., GilIbarguchi, J.I., Abalos, R, Apraiz, A., 2000. Superposed Her Abalos, B., GilIbarguchi, I., Eguiluz, L., 1991. Cadomian subduction/collision cynian and Cadomian orogenic cycles in the Ossa Morena Zone and re and variscan transpression the Badajoz-C6rdoba Shear Belt (SW Spain). lated areas of the Iberian Massif. Geological Society America Bulletin Tectollophysics 199, 51-72. 112. 1398-1413. Bachiller, N., Galindo, C., Darbyshire, D.P.F., Casquet, c., 1997. Geocronolo Exposito, 1., 2000. Evolucion estructural de la mitad septentrional de la Zona gia Rb -Sr de 10s leucogranitos del complejo plut6nico de Burguillos del de Ossa-Morena, y su relacion con el wrote Zona de Ossa-MorenalZona Cerro (Badajoz). Geogaceta 21, 29-30. Centroiberica. PhD thesis, Universidad de Granada, 296 pp. Barnhoorn, A, Bystricky, M., Burlini, L., Kll llze, K., 2004. The role of recrys Exposito, 1., Simancas, l.F., GonzaJ.ez Lodeiro, F., Bea, F., Montero, P., tallization on the deformation behaviour of calcite rocks: larg strin torsion Salman, K., 2003. Metamorphic and deformational imprint of Cambrian experiments on Carrara marble. Journal of Structural Geology 26, Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, 885-903. Spain). lournal of Structural Geology 25, 2077-2087. Bamhoom, A., Bystricky, M., Burlini, L., Kunze, K., 2005. Post-defonnational Goscombe, B.D., Passchier, C.W., 2003. Asymmetric boudins as shear sense annealing of calcite rocks. Tectonophysics 403, 167-191. indicators-an assessment from field data. Jownal of Structural Geology
Bateman. R.• Martin, M.P., Castro, A., 1992. Mixing of cordierite granitoid 25, 575-589.
and piroxene gabbro, and fractionation., in theSanta Olalla tonalite (Anda Grasemann., B., Stiiwe, K.• 2001. The development of flanking folds during 28, 111-131. lucia). Lithos simple shear and their use as kinematic indicators. lournal of Structural Behnnann. 1.H.. 1983. Microstructure and fabrictransitions in calcite tecton Geology 23, 715-724 . ites from the Sierra Alhamilla (Spain). Geologische Rundschau 72, Grasemann, B., Stiiwe, K., Vannay, l.C., 2003. Sense and non-sense of shear in 605-618. flanking structures. lournal of Structural Geology 25, 19-34. BehnnatUl, l.H., 1987. A precautionary note on shear bands as kinematic in Griggs, D.T., Turner, Fl., Heard, H.c., 1960. Defonnation of rocks at dicators. Jownal of Structural Geology 9, 659-666. 500-800e. Memoirs of the Geological Society of America 79, 39-105. Bestmann, M., KlUlze, K., Matthews, A, 2000. Evolution of a calcite marble Handin, l., Riggs, DV, O'Brien, l.K., 1960. Torsion of Yu le marble under shear zone complex on Thassos Island, Greece: microstructural and confining pressure. Memoirs of the Geological Society of America 79, textural fabrics and their kinematic significance. lownal of Structural 245-274. Geology 22, 1789-1807. Herwegh, M., Berger, A., 2004. Defonnation mechanisms in second-phase Borg, 1, Handin, 1., 1967. Torsion of calcite single crystals. lownal of Geo affected microstructures and their energy balance. Jownal of Structural physical Research 72, 641-669. Geology 26, 1483-1498. Braillon, P., Serugherti, J., 1976. Defonnation plastique de monocristaux de Herwegh, M., KlUlze, K., 2002. The influence of nano-scale second-phase par calcite en compression suivant <001>. Physica Status Solidi (a), 36. ticles on defonnation of fine grained calcite mylonites. Jownal of Struc Busch, J.P, Van der Pluijm, BA., 1995. Calcite textures, microstructures and tural Geology 24, 1463-1478. rheological properties of marble mylonites in the Bancroft shear zone, Hudleston., P.J., 1989. The association of folds and veins in shear zones. lour Ontario, Canada. Jownal of Structural Geology 17, 677-688. nal of Structural Geology 11, 949-957. Casey. M., KWlZe, K., Olgaard., DL., 1998. Texture of Solnhofen limestone Kern,H., Wenk, HR., 1983. Calcite texture developmentin experimentallyin defonned to high stains in torsion. lownal of Structural Geology 20, duced ductile shear zones. Contributions to Mineralogy and Petrology 83, 255-267. 231-236.
Casquet, C .• 1980. Fenornenos de endornorlismo, metamorfismo y metasoma Kober, B., 1986. Whole-grain evaporation for 207Pb/20Opb-age-investigations tismo en los mrumoles de la Rivera de Cala (Sierra Morena). Doctoral on single zircons using a double- filament thennaJ ion SOUIce. Contributions Thesis, Universidad Cornplutense de Madrid. Spain, Madrid, 290 pp. to Mineralogy and Petrology 93, 482-490. Kober, B., 1987. Single-zircon evaporation combined with Pb+ emitter bed Quesada, c., 1990. Precambrian successions in SW Iberia: their relationship to ding for 2Ct7pbp06Pb_age investigations using thennal ion mass spectrom Cambrian orogenic events. ill: D'Lemos, RS., Strachan, RA, etry, and implications to zirconology. Contributions to Mineralogy and Topley, C.G. (Bds.), TheCadomian Orogeny. Geological Society, London, Petrology 96, 63-71. pp. 353-362. 51. Krabbendam, M., Urai, l.L., Van Vliet, L.l., 2003. Grain size stabilisation by Quesada, c., 1991. Geological constraint on the Paleozoic tectonic evolution dispersed graphite in a high-grade quartz mylonite: an example from of tectonostratigraphic terranes in Iberian Massif. Tectonopysics 185, Naxos (Greece). lOlunal of Struchual Geology 25, 855-866. 225-245. Lebensolm, RA, Wenk, H.R, Tome, CN., 1998. Modelling deformation and Quesada, C., 1997. Evolucion geodin:imica de la Zona Ossa-Morena durante el recrystallization texhues in calcite. Acta Mater 46, 2683-2693. ciclo Cadomiense. ill: Arauj o, A, Pereira, M.F. (Bds.), Estudo sobre a geo Leiss, B., Ullemeyer, K., Weber, K., Passchier, c., 2000. Texhues and physical logia da Zona de Ossa-Morena (Maci�o Iberico).Livro de Homenagem ao properties of rocks. lOlunal of Struchual Geology 22, 1527-1873. Prof. Univ. de Evora, Francisco Gon�alves, pp. 205-230. Liiioin, E., Quesada, C., 1990. Ossa-Morena zone: rift phase (Cambrian). ill: Quesada, C., Dallmeyer, RD., 1994. Tectonothennal evolution of the Badajoz Dallmeyer, RD., Martinez-Garcia, E. (Bds.), Pre-Mesozoic Geology of Cordoba shear zone (SW Iberia): characteristics and 4°ArP9Ar mineral age Iberia. Springer-Verlag, Berlin-Heidelberg. Gennany, pp. 259-266. constraints. Tectonophysics 231, 195-213. Llmar, R, Ortega, L., Sierra, l., GarciaPalomero, F., Moreno, T, Prichard, H., Quesada, C., Fonseca, P.E., Munha, l., Oliveira, l.T., Ribeiro, A, 1994a. The 1997. Ni-Cu (pGM) mineralization associated with mafic and ultramafic Beja-Acebuches Ophiolite (Southern Iberia Variscan fold belt): geological rocks: the recently discovered Aguablanca ore deposit, SW Spain. ill: characterization and geodynamic significance. Boletin Geologico y Minero Papllllen, H. (Bd.), :Mineral Deposits. Balkema, Rotterdam, pp. 463-466. 105, 3-49. Montero, P., Salman, K., Bea, F., Azor, A, Exposito, 1, Gonz:i1ez Lodeiro, F., Quesada, C., Cueto, L.A, Femandez, Fl., Larrea, F.l., 1994b. Mapa Geolo Martinez Poyatos, D., Simancas, l.F., 2000. New data on the geochronol gico de Espana, escala 1:50.000, hoja 895: Encinasola. Instituto Geomi ogy of the Ossa-Morena Zone, Iberian Massif. In Variscan-Appalachian nero de Espana, pp. 104. dynamics: the building of the Upper Paleozoic basement. Basement Ramsay, l.G., Huber, M., 1983. The Techniques of Modern Stmchual Geol Tectonics 15, 136-138. ogy. Academic Press, London, 1307 pp. 1 Munha, l., Barriga, Fl.AS., Kenich, R, 1986. High 80 ore-fonning fluids in Ratschbacher, L., Wenk, HR, Sintubin, M., 1991. Calcite texhues: examples 1 ;16 volcanic hosted base metal massive sulphide deposits: geologic 80 0 form nappes with strain-partitioning. lomnal of Struchual Geology 13, and DIH evi dence for the Iberian Pyrite Belt; Crandon Wisconsin, and 369-384. Blue Hill, Maine. Economic Geology 81, 530-552. Ribeiro, A, Quesada, c., Dalbneyer, RD., 1990. Geodynamic evolution of the Olgaard, D.L., 1990. The role of second phase in localizing deformation, in: Iberian Massif. In: Dalbneyer, RD., Martinez Garcia, E. (Bds.), Premeso Knipe, Rl., Rutter, EH. (Eds.), Defonnation Mechanisms, Rheology zoic Geology of Iberia. Springer, Heidelberg, pp. 339-409. and Tectonics. Geological Society Special Publication 54, 175-181. Romeo, 1, LlUlar, R, Capote, R, Quesada, c., Dll1l1lillg, GR, Pina, R, Ortega, L., Moreno, T, Llmar, R, Prichard, H., Sierra, l., Bomati, 0., Ortega, L., 2006a. UlPb age constraints on Variscan Magmatism and Fisher, P., Garcia Palomero, F., 1999. Minerales del grupo del platino y Ni-Cu-PGE metallogeny in the Ossa-Morena Zone (SW Iberia). lomnal fases asociadas en el deposito de Ni-Cu-(EGP) de Aguablanca, SO Espana. of the Geological Society of London 163, 837-846. Geogaceta 25, 155-158. Romeo, 1, Capote, R, Tejero, R, LlUlar, R, Quesada, C., 2006b. Magma Ortega, L., Lunar, R, Garcia-Palomero, F., Moreno, T, Martin Estevez, l.R, emplacement in transpression: the Santa Olalla Igneous Complex (Ossa Prichard, HM., Fisher, P.c., 2004. The Aguablanca Ni-Cu-PGE Deposit, Morena Zone, SW Iberia). lOlUnal of Struchual Geology 28, 1821-1834. Southwestern Iberia: Magmatic ore-fonning processes and retrograde evo Rutter, EH., 1974. The influence oftemperahue, strain rate and interstitial wa lution. The Canadian Mineralogist 42, 325-335. ter in the experimental defonnation of calcite rocks. Tectonophysics 22, Passchier, C.W., 2001. Flanking struchues. lOlunal of Struchual Geology 23, 311-334. 951-962. Rutter, EH., 1995. Experimental study of the influence of stress temperahue Paterson, M.S., 1979. Defonnation mechanisms in carbonate crystals. ill: and strain on the dynamic recrystallization of Carrara marble. lOlUnal of Borland, D.W., Clarebrough, L.M., Moore, Al.W. (Bds.), Physics of Geophysical Research 100, 24651-24663. Materials. Dept of Mining and MetalllUgy, University of MelbolUne, pp. Rutter, EH., Casey, M., BlUlini, L., 1994. Preferred crystalplastis orientation 199-208. development dlUing the plastic and superplastic flow of calcite rocks. lOlU Paterson, M.S., Tmner, F.l., 1970. Experimental defonnation of constrained nal of Struchual Geology 16, 1431-1446. crystals of calcite in extension. In: Paulitsch, P. (Bd.), Experimental and Sanchez-Garcia, T, Bellido, F, Quesada, C., 2003. Gedynamic setting and Nahual Rock Defonnation. Proceedings illtemational SymposilUll, Dann geochemical signatures of Cambrian-Ordovician rift-related igneous rocks stadt, 1969. Springer, Berlin, pp. 109-141. (Ossa-Morena Zone, SW Iberia). Tectonophysics 365, 233-255.
Pfiffner, O.A, 1982. Defonnation mechanisms and flow regimes in limestones Santos, l.F., Mata, l., Gon�alves, F., Munha, l., 1987. Contribui�ao para 0 con from the Helvetic zone of the Swiss Alps. lOlUnal of Struchual Geology 4, hecimento geologico-petrologico da regiao de Santa Suzana: 0 complexo 429-442. Vulcanosedimentar da Toca da Moura. Com. SeIV. Geol. Portugal 73, Pieri, M., BlUlini, L., KlUlze, K., Stretton, 1, Olgaard, D.L., 2001a. Rheolog 29-48. ical and microstruchual evolution of Carrara marble with high shear strain: Schmid, SM., Boland, l.N., Paterson, M.S., 1977. Superplastic flow in fine results from high temperahue torsion experiments. lOlUnal of Struchual grained limestone. Tectonphysics 43, 257-291. Geology 23, 1393-1413. Schmid, SM., Paterson, M.S., Boland, l.N., 1980. High temperahue flow and Pieri, M., KlUlze, K., BlUlini, L., Stretton, 1, Olgaard, D.L., BlUg, l.-P., dynamic recrystallizationin Carrara Marble. Tectonophysics 65, 245-280. Wenk, H.-R, 2001b. Texhue development of calcite by defonnation and Silva, l.B., 1989. Estruhua de lUlla geotransversal da Faixa Piritosa: Zona do dynamic recrystallization at 1000 K dlUing torsion experiments of marble Vale do Guadiana. Univ. Lisboa, Tese de Doutoramento, 450 pp. to large strains. Tectonophysics 330, 119-140. Simancas, l.F., Carbonell, R, Gonz:i1ez Lodeiro, F., Perez Estalin, A, Piiia, R, LlUlar, R, Ortega, L., Gervilla, F., Alapieti, T, Martinez, c., 2006. luhlin, C., Ay arza, P., Kashubin, A, Azor, A, Martinez Poyatos, D., Crystal fractionation and sulphide segregation processes in the SOlUce Ahnodovar, G.R, Pascual, E., Saez, R, Exposito, 1, 2003. Crustal struc chamber of the Aguablanca Ni-Cu-PGE deposit (SW Spain). Economic hue of the transpressional Variscan orogen of SW Iberia: SW Iberia Geology 101, 865-881. deep seismic reflection profile (IBERSEIS). Tectonics 22, 1062. Prior, D.l., Boyle, AP., Brenker, F, Cheadle, M.C., Day, A, Lopez, G., Spiers, C.l., Wenk, HR, 1980. Evidence for slip on r and f in the positive Peruzzo, L., Potts, G.l., Reddy, S., Spiess, R, Timms, N.E., Trimby, P., sense in defonned calcite single crystals. EOS. Transactions American "Wheeler, l., Zetterstrom, L., 1999. The application of electron backscatter Geophysical Union 61, 1128. diffraction and orientation contrast imaging in the SEM to texhual prob Tomos, F, Casquet, C., Galindo, C., Canales, A, Velasco, F, 1999. The gen lems in rocks. American l\1ineralogist 84, 1741-1759. esis of the variscan ultramafic-hosted magmatic Cu-Ni deposit of Aguablanca, SW Spain. ill: Stanley, et al. (Bds.), l\1ineral Deposits: Pro Walker, AN., Rutter, EH., Brodie, K.H, 1990. Experimental study of grain cesses to Processing. Balkema, Rotterdam, pp. 795-798. size sensitive flow of synthetic, hot-pressed calcite rocks. In: Knipe, Rl., Tornos, F, Casquet, C, Galindo, C, Velasco, F, Canales, A., 2001. A new Rutter, E.H (Eds.), Deformation Mechanisms, Rheology and Tectonics .. style of Ni -Cu mineralization related to magmatic breccia pipes in a trans Geological Society Special Publications 54 Geological Society of London, pressional magmatic arc, Aguablanca, Spain. :M:ineralilUll Deposita 36, pp. 259-284. 700-706. Weiss, L.E., Tmner, Fl., 1972. Some obsetvations on translation gliding and Tumer, Fl., Heard, HC, 1965. Defonnation in calcite crystals at different strain kinking in experimentally deformed calcite and dolomite. ill: Geophysical rates. University of California Publications ofGeologicalScience46, 103-126. Monographs 16. American Geophysical Union. 95-107. Turner, Fl., Orozco, M., 1976. Crystal bending in metamorphic calcite, and its Wenk, HR, 1985. Carbonates. ill: Wenk, H.R (Bd.), Preferred Orientation in relations to associated twining. Contributions to l\1ineralogy and Petrology Defonned Metals and Rocks. An illtr oduction to Modem Texture Analysis. 57, 83-97. Academic Press, San Diego, pp. 361-384. Tmner, F.J., Gri ggs, D.T., Heard, H.C, 1954. Experimental deformation of cal Wenk, H.R, Christie, l.M., 1991. Comments on the interpretation of deforma cite crystals. Geological Society of America Bulletin 65, 883-934. tion texhues in rocks. lomnal of Struchual Geology 13, 1091-1110. Urai, l.L., Means, W.D ., Lister, G.S., 1986. Dynamic recrystallization of min Wenk, HR, Takeshita, T., Bechler, E., Erskine, B.G., Matthies, S., 1987. erals. ill: Hobbs, B.E., Heard, H.C. (Bds.), Mineral and Rock Deformation: PlUe shear and simple shear calcite texhues. Comparison of experi Laboratory Studies-The Paterson VollUlle. Geophysical Monographs 36, mental, theoretical and natural data. lOlUnal of StruCtlUal Geology 9, pp. 161-199. 731-745. Van der Pluijm, B.A., 1991. Marble mylonites in the Bancroft shear zone, On Wenk, HR, Canova, G.C., Brechet, Y, Flandin, L., 1997. A defonnation tario, Canada: microstruchues and defonnation mechanisms. lomnal of based model for recrystallization of anisotropic materials. Acta Materia Struchual Geology 13, 1125-1135. 45, 3285-3296. Vauchez, A., 1975. Tectoniques tangeantielles sup erposees dans le segment her Wenk, HR, Matthies, S., Donovan, l., Chateigner, D., 1998. BEARTEX, cynien Sud-Iberique: les nappes et plis couches de la region d'Alconchel a windows-based program system for quantitative texhue analysis. lOlUnal Fregenal de la Sierra (Badajoz). Boletin Geologico y l\1inero 86, 573-580. of Applied Crystallography 31, 262-269.