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El Complejo de Cabo Ortegal es un grupo de láminas alóctonas que fueron emplazadas sobre el margen occidental de Gondwana durante la Orogenia Varisca . Está formado por una Unidad Tectónica Superior, el manto de Cabo Ortegal, compuesto de rocas que registran un evento metamórfico de alta presión y alta temperatura, y una Unidad Tectónica Inferior, subdividida en tres láminas alóctonas compuestas por rocas equilibradas en facies de las anfibolitas o condiciones de más bajo grado . Las rocas que constituyen estas unidades inferiores provienen de diferentes contextos geodinámicos, tales como son los basaltos E-MORB en la lámina de Purrido, las mélanges tectónicas en la lámina de Moeche y las rocas volcánicas calcoalcalinas en la lámina de Espasante . Este grupo de láminas representan, en su conjunto, la zona de sutura del orógeno.
The Pallaresa massif is a large E-W trending elongated antiformal variscan structure, located in the Axial zone of the central Pyrenees. The studied area is the transitional zone between Pallaresa massif and Aston and Hospitalet gneissic domes. Cambro-ordovician succession outcroping in this sector consists of an unfossiliferous, monotonous alternation of quartzites and shales with some intercalations of marbles in the middle part. A vertical structural zonation was recognized by De Sitter and Zwart (1960) in the Axial zone of the Pyrenees. They distinguished two different structural domains: i) infrastructure, seated deep with medium to high metamorphic grade and main flat-lying foliation and ii) suprastructure, a shallow domain with lower metamorphic grade and main steep foliation. Different interpretations have been proposed to explain this structural zonation. These interpretations may show completely contradictory kinematic and tectonic regime (Carreras & Capellà, 1994). But the main disagreement is the deformation event giving rise to the flat-lying infrastructural foliations. The geological mapping, detailed cross sections and microstructural study have allowed us to identify three deformation events and to establish the deformational sequence for the studying area. So the first structures are slaty cleavage (S1), observed in microscopic scale, associated with south-verging structures. This cleavage is defined by lattice preferred orientation of quartz and mica grains. Under a microscope we can see this slaty cleavage folded by a crenulation foliation (S2). Because of high quality of outcrops in the study area, north-verging structures can be deduced by sedimentary polarity data. We can observe this foliation S2 associated with large recumbent north-verging folds with E-W trend. This foliation is ubiquitous in the study area and can be considered the main foliation. Both the north-verging folds and the main foliation are folded by upright folds with NE-SW to E-W trend. This folding can be observed to all scales. Thin sections show a crenulation foliation associated with upright folds (S3). On the other hand, a lot of thin sections show preferential biotite grain growth in the direction of the S2 cleavage domains and biotite grain folded by S3 foliation. Therefore, at this point we can consider a first option: the metamorphism in this area is approximately simultaneous with main deformation (D2) development and earlier than third deformation event. These new evidences described here are relevant clues for the interpretation of the main flat-lying foliation associated with north-verging recumbent folds in the infrastructure. Moreover similar structures have been observed in other areas in Central Pyrenees (Garona dome). Finally, we think that this structural setting brings into question the models previously presented by others authors.
In the Port of Benasque (Huesca, Spain) we have studied a Zn-Pb deposits, interbedded in the Upper Ordovician series, whose lithology are shales, sandstones, conglomerates and some thin layers of limestone (5 m). The type age (Pb-Pb) of the deposits is equal to that of the host rock, however, the mineralizations are associated with Variscan thrusts. In the Port of Benasque area, Paleozoic rocks are deformed by north-verging, inclined folds of hectometric scale, whose reverse limbs exhibit a subvertical attitude (F1). The main cleavage (S1) of the study area is associated with these F1 folds. The S1 cleavage was originated in subhorizontal attitude and subsequently was deformed by a second generation of folds (F2). The F2 folds have vertical axial plane and, locally, are associated with a S2 rough cleavage. The F2 folds are also associated with subhorizontal and south-directed thrusts. In the reverse limbs of the F1 folds, F2 thrusts are developed subparallel to S1, which is often intensely deformed by F2 folds. The S0 remains vertical and shows only a perpendicular flattening to the beds. In the port of Benasque, the Zn-Pb mineralization is located on the planes of the F2 thrusts developed in reverse limbs of the F1 folds. The Zn-Pb mineralization has been assigned to a SEDEX-type because its upper Ordovician age is the same than that of the host rocks deposited during an extensional episode in the Pyrenees. During the Variscan deformation, the mineralization, located in the reverse limbs of the F1 folds, is remobilized across vertical bedding, and deposited in the F2 thrusts planes.
Two vertical different structural domains have been recognized traditionally in the Axial zone of the Pyrenees: i) infrastructure: a deep seated domain with medium to high metamorphic grade and main flat-lying foliation and ii) suprastructure: a shallow domain with lower metamorphic grade and main steep foliation. Different and contradictory interpretations have been proposed to explain this structural zonation. The cross sections constructed have allowed us to observe the features of the transition zone between both domains in the central part of the Axial Zone and propose a new deformational sequence with which the structures of both domains are related.
found associated with it. The D2 event is characterized by E-W trending and north-verging recumbent folds with an associated foliation (S2) that can be observed at all scales. The S2 foliation has a horizontal orientation, and is the main foliation in the northern and central parts of the study area. The D3 deformation event is distinguished by development of E-W trending, upright folds, of centimetric to metric scale. These folds are associated with a steep and rough crenulation cleavage (S3) in the southern part of the study area. Subvertical faults trending E-W are associated with D3 folds. Most of the structures recognized in the eastern part of the Pallaresa massif have developed under low metamorphic grade conditions, although close to the Aston and Hospitalet domes, in the vicinity of some igneous intrusions, the metamorphism is coeval with S2 foliation and may reach high grade conditions. A gradual transition can be observed from zones characterized by structures of type D2 to zones exhibiting a better development of D3 structures. This transition can be explained by a decollement level located in the Cambro-Ordovician succession. This level can be related to faults and D3 folds observed in the eastern part of the Pallaresa massif. Most of the structures can be explained in a contractional geological setting. In addition the relationships between the different Variscan structures of the eastern part of the Pallaresa massif are better described. The different models recently presented by some authors about the Variscan structure of the Pyrenees are discussed in the light of these new results.
A new sequence of Variscan deformations is proposed for the Palaeozoic rocks of the Central Pyrenees. The non-metamorphic units include south-directed thrust systems and related folds with a poorly developed cleavage. In the metamorphic units north-verging, recumbent to inclined folds (D1), associated with a subhorizontal to south dipping cleavage, are refolded by south-verging, upright to inclined folds (D2), with a subvertical to north-dipping axial plane cleavage, and offset by south-directed thrusts approximately coeval with D2. The structural evolution of these units suggests a subdivision of the Variscan Central Pyrenees into two different regions consistent with the zones known for a long time in the core of the Ibero-Armorican or Asturian arc (northern part of the Iberian Variscan Massif). The structure of the Pyrenean non-metamorphic units has foreland affinities and is comparable to that of the Cantabrian Zone, whereas the deformation observed in the Pyrenean metamorphic units is characteristic of the hinterland and is consistent with the features of the West Asturian–Leonese Zone or Central–Iberian Zone. Since the Pyrenean non-metamorphic units are located southwards of the metamorphic ones and the Variscan thrusts are south-directed, we tentatively correlate the Variscan Pyrenees with the northern branch of the Ibero-Armorican or Asturian arc.
Metamorphic reactions, deformation mechanism and chemical changes during mylonitization and ultramylonitization of granite affected by a crustal-scale shear zone are investigated using microstructural observations and quantitative analysis. The Vivero Fault (VF) is a large extensional shear zone (>140Km) in NW of Iberia that follows the main Variscan trend dipping 60° toward the West. The movement accumulated during its tectonic history affects the major lithostratigraphic sequence of Palaeozoic and Neoproterozoic rocks and the metamorphic facies developed during Variscan orogenesis. Staurolite, and locally, andalucite plus biotite grew in the hangingwall during the development of VF, overprinted the previous regional Variscan greenschist facies metamorphism. Andalusite growth took place during the intrusion of syntectonic granitic bodies, such as the deformed granite studied here. The Penedo Gordo granite is coarse-grained two-mica biotite-rich granite intruding the VF and its hangingwall. This granite developed a localized deformation consisting of a set of narrow zones (mm to metric scales) heterogeneously distributed subsequently to its intrusion. Based on pseudosections for representative hangingwall pelites hosting the granite and the inferred metamorphic evolution, the shear zone that outcrops at present-day erosion surface was previously active at 14,7-17 km depth (390-450 MPa). Temperature estimates during deformation reach at least the range 500-600° C, implying a local gradient of 35±6°C/km. Microstructures in the mylonites are characterized by bulging (BLG) to subgrain rotation (SGR) recristallization in quartz with the increasing of deformation. Albitisation, flame-perthite and tartan twining are common in K-feldspar at the early stage of deformation. The inferred dominant deformation mechanisms are: i) intracrystalline plasticity in quartz, ii) cataclasis with syntectonic crystallisation of very fine albite-oligoclase and micas in K-feldspar, and iii) cataclasis with precipitation of K-feldspar in fractures and other dilatational sites in plagioclase. Ultramylonites consist of a matrix mainly containing feldspar, quartz and micas (mainly biotite) with an average grain size below 15 μm, usually featuring some quartz pods and small feldspar porphyroclast. Quartz pods disintegrate into polycrystalline aggregates, and the resultant grains are mixed into the surrounding matrix reaching its average grain size. In the matrix, grain size is uniform and the distribution of mineral phases tends to be homogeneous. Mass balance analysis based on major elements indicates that the deformation process was not isochemical for some elements. Preliminary XRF results show that the mylonitic/ultramylonitic samples are depleted in Na and Mn and enriched in K and Ca respect to the original protolith, while others remains stable (Si, Al or Fe). This data suggests a large-scale transport of some components, and therefore, that fluids were involved during deformation. Similar feldspar microstructures in mylonites, implying cataclasis and neocrystallisation, have been previously reported in natural rocks where the temperature was estimated between 250 to 450°C (see Fitz-Gerald and Stünitz 1993, Hippertt 1998 or Ree et al. 2005). In opposition to this, petrological and mineralogical thermometry data indicate that temperatures during deformation of FV reached at 500-600°C, extending the temperature range previously reported.
Abstract: The structural study allowed us to recognize three main variscan deformation events (D1, D2 and D3) in the paleozoic succession in the northwest of Andorra. In the metamorphic units studied E-W trending, recumbent to inclined north vergent folds (D1) are dominant. A flat foliation (S1) is associated with these folds. As regards the no metamorphic units are characterized by E-W trending upright or south vergent folds (D2) with an axial plane foliation associated (S2) and E-W trending south-directed thrusts. Some of these thrusts merge into detachment levels located within the Silurian slate and others, out-of-sequence thrusts, with deeper detachment levels located within pre-Caradoc rocks. Moreover, late-variscan shear structures (D3) are observed in the contact areas with the Aston and L’Hospitalet gneissic domes. It is observed a genetic relationship between D2 folds and some of the most important faults and thrusts which affect pre-Caradoc succession, for example the Merens fault. The detachment level of this fault is would place within or bellow pre-Caradoc rocks. The existence of two detachment levels for the D2 thrusts produce a gradual transition from deeper levels where subhorizontal structures are predominant to superficial levels characterized by subvertical structures.
The Vivero fault is the largest extensional shear zone in the Iberian Massif, with a minimum length of 140 km, formed during the Variscan Orogeny. The movement accumulated during its tectonic history affected the Neoproterozoic and Palaeozoic lithostratigraphic sequence of Iberian rocks and the regional metamorphic facies developed during orogenesis. Except for its southern termination, the general structure and geometric features of the shear zone are fairly well established. The minimum vertical offset estimated from petrological and mineralogical geothermobarometry data is 5.5 km. Hangingwall rocks that outcrop along the fault developed a local metamorphism associated with deformation. The most controversial issues of the Vivero fault to date were its age and the evolution and meaning of the metamorphism developed during its activity. Given that the Vivero fault is a major extensional structure playing a key role in the (late) evolution of the Variscan orogen in the Iberian Peninsula, a detailed study on the issues outlined above was carried out and constitute the core of this Ph.D. thesis. So far, the existing data on the microstructure, flow type and deformation mechanisms in the high strain rocks associated with the Vivero fault were scarce, qualitative and in general not systematic. Furthermore, the study of the microstructure is essential for a complete tectonic analysis of the structure and for understanding the behaviour of the continental crust during the late stages of the Variscan Orogeny. One additional aim of this thesis was to carry out a detailed and quantitative analysis of the microstructure developed in high strain rocks in order to be able to infer the operative deformation mechanism during the Vivero fault development. To sum up, the aims of the thesis were: i) The structural characterization of deformed rocks inside and outside the Vivero fault shear zone. ii) A detailed study of microstructure in the high strain rocks related with the Vivero fault development. iii) The study of the metamorphic evolution related with the development of the fault and its tectonic meaning. iv) To constrain the timing of the Vivero fault activity. Geochronological data indicates that the Vivero fault was active between 298 ± 5 and 289.5 ±1.5 Ma, defining period of minimum activity from 2.5 to 15.5 Ma. Its overall activity has been constrained between 314 ± 2 Ma, as an older limit to the onset of the fault development, and the range 289.5 ± 1.5 to 269 Ma for its cessation. The average displacement rate varies between 0.41 and 2.54 mm a-1, limiting the rate of instantaneous shear strain of the fault in the range of 2.68 x 10-13 to 6.50 x 10-15 s-1. The study of the metamorphism shows that the remnants of kyanite, chloritoid, chlorite and andalusite (pseudomorphs) assemblages, which locally appear along the Vivero fault, growth previously to the development of the tectonic foliation related with the Vivero fault. This means that this mineral assemblage is not related to the development of the fault itself. On the other hand, the development of staurolite assemblages along Vivero fault, locally evolved to andalusite and biotite (±staurolite), were contemporaneous with the fault activity. The metamorphic evolution established and the pseudosections drawn suggest a heating and nearly isobaric event for the hangingwall rocks during the Vivero fault development. In general terms, the maximum T reached ranged from 500 to 600 °C in the pressure range 390-450 MPa, equivalent to depths between 14.7 and 17 km; implying a local temperature gradient of 35±6 °C km-1 during the activity of the Vivero fault. The metamorphic evolution established and the petrological geothermobarometry applied discard a strongly decompressive PT-path (~400 MPa) previously inferred by other authors. The syn-tectonic growth of andalusite and biotite in the metamorphic aureoles, the common occurrence of sub-solid deformation microstructures in the plutons affected by the Vivero fault shear zone and the similar emplacement ages of deformed plutons along the fault indicate that the heating PT-path inferred is, at least in part, induced by the development of a magmatism contemporary with the Vivero fault development. It is remarkable the local importance of the Vivero fault in the development of so-called ‘late post-orogenic Iberian magmatism’. In addition, the age matching between the syn-tectonic granitic plutons and the Vivero’s mafic to ultramafic suite involves the participation of mantle-derived magmas during the tectonic activity at the Vivero fault, confirming the importance of the structure as a crustal-scale structure. The fabric and microstructures outlined in the footwall mylonitic rocks show that the dominant deformation mechanism during deformation on quartz and, partially, on feldspar was intracrystalline plasticity. In contrast, the microstructures outlined in hangingwall high strained quartz veins and igneous rocks show that quartz was deformed by intracristalline plasticity while feldspars by cataclasis accompanied by neo-crystallisation. Self-induced by the previous mylonitization of the original protoliths, fine-grained ultramylonites were deformed by dissolution-precipitation creep mechanisms accompanied by grain boundary sliding. The microstructures and deformation mechanism inferred in the high-strain rocks are consistent with the temperatures reached during the metamorphic climax. Structural, metamorphic and geochronological data clearly indicate that the development of Vivero fault took place after the general folding (D1), thrusting (D2) and refolding (D3) deformation phases related with the E-W shortening (in present-day coordinates) in the NW Iberian Massif. The cessation of large transcurrent shear zones affecting the hinterland zones of Iberian Massif and the final closure of the Ibero-Armorican arc in the Cantabrian Zone preceded the movement along the Vivero fault. Furthermore, these relations exclude the possibility that the Vivero fault superposed a previous extensional shear zone as some authors suggested. Lastly, in contrast to some previous interpretations, the orientation of mineral and stretching lineations along the shear zone indicate that the motion of Vivero fault is inconsistent with the hangingwall escape towards the north (in present-day coordinates) during the fault activity.
The uppermost metavolcanic layer of the Cambro-Ordovician Ollo de Sapo Formation, the largest accumulation of pre-Variscan igneous rocks in the Iberian Peninsula, have been dated in its northernmost part using U–Pb SHRIMP-RG zircon age techniques at 479.0 ± 4.7 Ma. The age obtained is the youngest age found so far in the metavolcanic facies of Ollo de Sapo Formation and represents the cessation of the rifting-related Cambro-Ordovician Ollo de Sapo volcanism at the northernmost tip of the Iberian Peninsula. Our results show that the Cambro-Ordovician volcanism in the NW of the Iberian Peninsula is not as short-lived as previously thought and confirm the correlation between the Cambro-Ordovician volcanic sequences that crop out in the Central Iberian Zone and the French Southern Armorican Massif. Finally, our study suggests that the cessation of the Cambro-Ordovician volcanism along the Ibero-Armorican Arc was synchronic or, less probably, slightly diachronic with younger ages towards the north (in present-day geographical coordinates).
The Vivero fault is crustal-scale extensional shear zone parallel to the Variscan orogen in the Iberian massif belt with an associated dip-slip movement toward the hinterland. To constrain the timing of the extension accommodated by this structure, we performed zircon U–Pb LA-ICP-MS geochronology in several deformed plutons: some of them emplaced syntectonically. The different crystallization ages obtained indicate that the fault was active at least between 303 ± 2 and 287 ± 3 Ma, implying a minimum tectonic activity of 16 ± 5 Ma along the fault. The onset of the faulting is established to have occurred later than 314 ± 2 Ma. The geochronological data confirm that the Vivero fault postdates the main Variscan deformation events in the NW of the Iberian massif and that the extension direction of the Late Carboniferous–Early Permian crustal-scale extensional shear zones along the Ibero-Armorican Arc was consistently perpendicular to the general arcuate trend of the belt in SW Europe.