THE IBERIAN VARISCAN OROGEN

Transcription

1 C H A P T E R 1 THE IBERIAN VARISCAN OROGEN Aerial view of the tectonic repetition of limestone beds in the Láncara Formation, Primajas duplex structure of the Esla thrust (photo by J.L. Alonso) Martínez Catalán, J.R. Aller, J. Alonso, J.L. Bastida, F.

2 Rocks of Upper Proterozoic to Carboniferous age - forming the Variscan or Hercynian orogen - crop out widely in the western part of the Iberian Peninsula, in what is called the Iberian or Hesperian Massif. These deformed rocks, often metamorphosed and intruded by different types of granitoids, were witness to the great mountain range formed in the late Paleozoic, basically in the Late Devonian and Carboniferous (between 370 and 290 million years ago), by the convergence and collision of two major continents: Laurasia and Gondwana. The Iberian Massif constitutes a geological framework of global interest. It is unique due to the continuity of its exposures and because it displays excellent records allowing the analysis of continental crust features, the tectonic, metamorphic and magmatic evolution of orogens, and therefore provides enormously relevant data about the lithospheric dynamics during the latest Precambrian and the Paleozoic. The Variscan orogen forms the basement of the Iberian Peninsula and of most of western and central Europe. A crustal basement is the result of an orogeny, that is, the consequence of a deep remobilization of the continental crust caused by the convergence of plates, and is associated to uplift and the creation of relief. The Variscan orogeny is the name given to the whole set of geological processes which deeply modified the continental crust of western Europe and the north and northwest of Africa during the Late Paleozoic (Devonian-Permian). In its long period of activity (approximately 80 million years), this orogeny shortened and intensely deformed those sediments deposited previously along vast continental margins, remobilized the prior basement on which they laid, generated a new crust by partial melting of the former, added fragments of oceanic crust and mantle, eroded and re-sedimented part of the newly formed crust, and deformed the majority of the new sediments. The importance of the Iberian Variscan orogen lies in its location close to three big orogenic belts: Caledonian, Variscan and Appalachian. Given the diversity and quality of the outcrops, it is one of the key geological frameworks recording the latest Precambrian and Paleozoic evolution of the planet, the deformation and generation of structures at different levels of the continental crust, the thermal history and the generation of granitoids, all within the context of an active continental margin (latest Precambrian) and a continental collision (Variscan Orogen). What crops out in the Iberian Peninsula from that basement (Iberian Massif) is part of a much longer orogenic belt, extending to the north and east through Central Europe into Poland, where it is shifted by a tectonic line known as Thornqvist Fault, and continuing towards south Asia. On the opposite side, the belt extends along north and northwest Africa parallel to the Atlantic margin. However, the Atlantic did not exist when the belt was Figure 1. Scheme showing the position of the Iberian Peninsula in relation to the Appalachians and the Caledonian and Variscan belts. Modified from Neuman and Max (1989). formed at the end of the Paleozoic. In its place, what presently is North America (then part of Laurentia) went around the Variscan orogen and a wide belt developed along its eastern margin: the Appalachians. Figure 1 shows a paleogeographic reconstruction of the continental masses at the end of the Paleozoic. It is noticeable how Iberia, the west of France and the south of Great Britain used to occupy a key position within the Variscan belt, at the junction of the three most important Paleozoic orogenic belts: the Appalachians to the southwest, the Caledonides to the northwest, and the Variscan to the northeast. Besides, Iberia shares with west France a marked and striking bend or curvature known as the Ibero-Armorican Arch (Ribeiro et al., 1995). The former reconstruction allows to infer the ultimate cause of the orogeny, since a set of continental crustal blocks are nearby but separated by orogenic belts. It is thus clear that convergence caused the orogenic processes, and continental collision was the final result. We are talking about the convergence of three large tectonic plates: Laurentia (North America), Baltica (Scandinavia, northern belt of Europe and Russia), and Gondwana (Africa, South America, Antarctica, India and Australia). The final result was the creation, by the amalgamation of many continental masses, of a supercontinent, in this case the last Pangea generated by the continuing geodynamics of our planet. Being such an old orogenic belt (its main development finished 290 million years ago), it is to be expected that the continental crust generated is already totally balanced (thermally and isostatically), and the orogenic reliefs should have completely disappeared. However, 14 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

3 Figure 2. Zonation of the Iberian Massif according to the subdivision proposed by Julivert et al. (1972). The red lines mark the position of the sections in figure 3. the subsequent history of the Iberian Peninsula originated new reliefs, and that is what makes the Iberian Variscan orogen even more interesting. As a matter of fact, the Iberian Peninsula did not even exist at the end of the Paleozoic, but was a later result of the separation of the African and Eurasian plates. The Iberian crustal block ended up with Eurasia in the first fragmentation (Middle Jurassic), but was later isolated in the Early Cretaceous with the opening of the Bay of Biscay and the Pyrenean line, remaining at the mercy of both plates dynamics from then onwards, which began their convergence in the Paleocene and mid Eocene. This convergence, which is still active, deeply remobilized the Variscan basement in the Betic Range, and with less intensity in the Pyrenees, Iberian Range, Demanda Range, and several other modern mountain ranges. Some of these mountain ranges are aligned from east-northeast to west-soutwest (Cantabrian Range, Ancares, Montes de León, Central System and the Portuguese extension through Serra da Estrela, Sierra Morena and many other minor reliefs) reflecting the north-south convergence of the African and Eurasian plates, and resulting in excellent exposures of the Variscan basement. For a better presentation of the most outstanding features of the Variscan orogen in the Iberian Peninsula, the different areas of the Massif will be described according to the scheme proposed by Julivert et al. (1972) (Figure 2). The five zones parallel and concentric to the Ibero-Armorican Arch are: Cantabrian Zone, West Asturian-Leonese Zone, Central Iberian Zone, Ossa-Morena Zone and South Portuguese Zone. The Galicia-Trás-os-Montes Zone of Farias et al. (1987) has also been added, although it is not a belt of the orogen but an allochthonous crustal block thrusted over the Central Iberian Zone. Figure 3 shows two schematic sections through the Iberian Massif, representative of the northwest (Pérez Estaún et al., 1991) and of the southwest (Simancas et al., 2002) of the Iberian Massif. It can be noticed that we are dealing with a belt with double centrifuge vergence. In the Iberian Massif there are several structures, which due to their geological interest together with the good exposure, have been chosen as Geosites: The Esla River Thrust in the Cantabrian Zone, the Mondoñedo Thrust in the West Asturian-Leonese Zone, Sierra del Caurel in the Central Iberian Zone, and the Cabo Ortegal Complex in the Galicia-trasos-Montes Zone (Figure 4). THE IBERIAN VARISCAN OROGEN 15

4 Figure 3, above. Two schematic geological sections across the Iberian Massif. a) According to Pérez-Estaún et al. (1991). b) According to Simancas et al. (2002). The location of the section is marked in figure 2. Figure 4, left. Geosites described in the text: 1) Esla River Thrust. 2) Sierra del Caurel. 3) Mondoñedo Thrust. 4) Cabo Ortegal Complex. Figure 5, below. Esla River Thrust section showing the main allochthonous units and duplexes, according to Alonso (1985). The Cantabrian Zone represents the Iberian Massif foreland: an area of Paleozoic pre-orogenic sediments with relatively small thickness. These are Cambrian to Devonian shallow sea deposits, and display a wedging towards the external part as well as some stratigraphic discontinuities. This wedging is in part a result of crustal uplift followed by erosion in the late Devonian, and shows the change from a passive margin to Variscan collision. This erosion was followed by basin deepening in the earliest Carboniferous, followed by synorogenic sedimentation. The Cantabrian Zone is the most continental part of the Paleozoic basin and a magnificent example of foreland thrust belt. In addition, it displays an extremely tight tectonic bend representing the core of the Iberian-Armorican Arch. Thin-skinned tectonics, the deformation style of the outer crust, features in the Cantabrian Zone a sequential development of thrusts and associated folds, contemporaneous with the classic clastic wedges along the front of major thrusts. It is also characterized by 16 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

5 Figure 6. Esla River Thrust in the Valdoré tectonic window. The colored line shows the thrust plane. On top of it: limestones of the Láncara Fm, slates of the Oville Fm and quartzites of the Barrios Fm (Cambrian- Ordovician). Below, Upper Devonian limestones of the Portilla Fm and a thin succession of Lower Carboniferous. To the left, the layout of the Aguasalio syncline is observed. Figure 7. Esla River Thrust north of Valdoré. The thrust (upper colored line) superimposes with perfect parallelism Cambrian and Devonian layers. Within the relative autochtonous, a thrust partially repeats the calcareous succession of the Portilla Fm (lower colored line). deposits filling in piggy-back basins located over the thrusted slabs (Marcos and Pulgar, 1982; Rodríguez Fernández et al., 2002). The tectonic structure is further complicated by the arcuate bending of the belt, which seems to be partially contemporaneous with thin-skinned tectonics, according to structural data (Pérez-Estaún et al., 1988). As a consequence of Cenozoic Alpine uplift, the Cantabrian Mountain Range displays magnificent landscapes with outstanding calcareous massifs, some of which are protected areas such as Montaña de Covadonga (Picos de Europa) National Park, and the natural parks of Somiedo and San Emiliano Valley. Several sites with panoramic outlooks allow to see large structures such as the Esla Thrust (Figures 5, 6, 7 and 8), the front of the Ponga Thrust, and that of the Picos de Europa Thrust, together with associated smaller structures such as duplexes, frontal imbrications and ramps (Alonso, 1987; Álvarez-Marrón and Pérez-Estaún, 1988). The Esla Thrust, in the province of León, has been selected as a Geosite due to its magnificent outcrops of sedimentary formations of early Cambrian to early Carboniferous age, piled in a series of allochthonous structures (Figure 5). Inside this thrust, the frontal and internal areas of several minor thrust slabs and associated sheets can be studied, as well as the geometrical relationships that the different thrust sheets and faults show amongst themselves and with the bedding (Figures 6 and 7). There are wonderful examples of thrust overstep geometries, of drape folds and associated fault propagation, and of frontal imbrications and duplexes, such as those at Primajas, Pardaminos and Pico Jano. The accumulated minimum shortening of all thrusts cropping out in this region is considered to be slightly above 92 km. Typical in this zone are the fault rocks developed under low temperature conditions and limited to a 1 to 2 meter thick band at the base of the Esla Thrust, which concentrates the deformation associated to the displacement, which is calculated to be around 19 km (Alonso, 1985, 1987). Synorogenic deposits are also very interesting, abundant and well preserved, with ages between Westphalian and Stephanian B (Figure 8). Their presence allows to date the emplacement of thrusts, as well as the folds associated to the thrust and the reactivation of bedding as a flexure-folding surface. Five main unconformities have been identified in the synorogenic sediments, although not all of them are discrete and some change laterally to progressive unconformities. The first two are related to the emplacement of the thrusts, whereas the remaining three are related to thrust reactivation and to the re-compression of folds associated to them (Alonso, 1985). THE IBERIAN VARISCAN OROGEN 17

6 Figure 8. Syncline developed upon Stephanian conglomerates (thin colored lines) in relation to a reverse fault (thicker line) thrusting the limestones of the Alba and Barcaliente formations (Mountain Limestone). The fault is an example of flexural sliding along the stratigraphic surface during tightening of the Peña Quebrada syncline (Alonso, 1985). Figure 9. Interpretation of the seismic lines ESCIN-1 and ESCIN-3.3, placed one next to the other to show the large scale structure in this part of the Iberian Massif, as it is deduced from seismic reflection profiles. According to Pérez- Estaún et al. (1994) and Ayarza et al. (1998). The West Asturian-Leonese Zone is a very important unit because of its tectonic significance in relation to middle crust deformation problems and the dynamics of orogenic wedges, as well as for the study of the origin of granitoids and their relation to the orogeny. With regard to structural features, this zone forms a slate belt, not only because that is precisely the most frequent lithology, but also because it displays a wide belt with vertical folds and steeply-inclined axial-plane cleavage which curves in depth becoming asymptotically parallel to a slightly-tilted mid-crustal detachment, and which may go across the whole crust. The West Asturian-Leonese Zone is divided into two domains: western and eastern. The eastern one, called Navia and Alto Sil domain (Marcos, 1973), is generally characterized by steeply-dipping folds and cleavage, as well as by the ductile deformation of greenschist facies, and includes some thrust-related repetitions. The seismic cross section ESCIN 3.3 (Figure 9) proves that the thrusts join at depth to (or are crossed by) a basal detachment which ends rooted 18 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

7 at the Mohorovicic discontinuity much farther to the west. The western one, called Mondoñedo Thrust domain, also shows steep folding and cleavage, although to the west the folds become recumbent and faulted folds, while cleavages turn subhorizontal and metamorphism increases. The coastal section between Cudillero and Viveiro estuary is extremely interesting in this area, as it offers an almost continuous structural section with spectacular deformation structures (Bastida, 1980; Pulgar, 1980). Uplifted areas also offer an excellent blend between geology and landscape. The Oscos region of western Asturias, the Ancares Natural Park, and the granitic massifs of La Tojiza and the surroundings of Viveiro, both north of Lugo province, deserve to be mentioned. The West Asturian-Leonese Zone, and the part of the Central Iberian Zone adjacent to it in Galicia, were key areas in the definition of the different types of granitoids and their interpretation (Capdevila, 1969; Capdevila et al.,1973). The oldest granitoids (Figure 10) are typically biotitic, frequently porphyritic, and include diorites, tonalites, granodiorites and monzogranites. They are collectively called early granodiorites, and are rocks originated deep at the base of the continental crust, with mantle contribution. Their development was followed by mid-crustal melting which generated peraluminous magmas of so-called leucogranites or two mica granites. Both the early granodiorite massifs and the leucogranites are syntectonic with ages between 350 and 305 Ma, broadly coinciding with Variscan deformation. The most modern group of granitoids is formed by a set of post-kinematic plutons (gabbros, tonalites, granodiorites and monzogranites). In contrast to the former, its emplacement is restricted to a short span of Ma, a magmatic pulse suggesting a single cause which Fernández-Suárez et al. (2000) linked to the delamination of the mantle lithosphere at the end of the orogeny, and its replacement by a hotter asthenospheric mantle. The Mondoñedo Thrust, and more specifically its basal shear zone in the coastal area of the Lugo province (Figure 11), has been selected as a Geosite. Its Cambrian slate and quartzite outcrops show the effects of tangential deformation of the second Variscan deformation phase (Bastida and Pulgar, 1978; Bastida et al., 1986; Aller and Bastida, 1993). Its interest stems from it being an exposure of the internal and deeper area of a thrust slab emplaced under ductile conditions in the beginning stages, and fragile in the last stages. Spectacular meso- and microstructures are observed in a ductile shear zone related to crustal thrusting, with a thickness between 3 and 3.5 km. Some medium-grade metamorphic foliations, stretching lineations, and fault rocks developed under a wide temperature range, can also be studied. Figure 10. Granitic plutons of the Iberian Massif in relation with Variscan deformation stages (according to López Plaza and Martínez Catalán, 1987). East-verging isoclinal folds are abundant and show a strong scattering of their hinges, very often displaying curved hinges and tight shapes belonging to sheath folds (Figures 12a, b and c). Folds are also affected by the development of a second axial-plane foliation and by a stretching lineation striking between N 100 and 110º E. Boudinage structures are also common affecting quartzites, as well as S-C structures, which are particularly abundant in Upper Proterozoic rocks at the base of Mondoñedo Thrust, showing an ESE trend of displacement (Figure 12d). In addition, the basal thrust crops out at Aureoura beach (As Areouras) and, north of it, the autochtonous slab of the Mondoñedo Thrust crops out in the tectonic window of Xistral (Gistral), as a train of regular folds affecting the thick homonymous quartzitic formation (Figure 12e). The Central-Iberian Zone comprises the central part of the Iberian Massif and is the widest of all the belts, around 400 km in the centre of the massif (Figure 2). The structure of the low metamorphic grade regions in the upper crustal levels is well defined at the surface by the Armorican Quartzite, which due to its resistance to weathering and erosion forms lined up reliefs bordering the elongated synclines in the southern part of this zone, such as Guadarranque, in Toledo, Cañaveral (cropping out along Monfragüe Natural Park) and San Pedro Range, in Cáceres, and Herrera del Duque, THE IBERIAN VARISCAN OROGEN 19

8 Figure 11. Geological cross section through the northern part of the Mondoñedo Thrust in two versions. The one above shows lithostratigraphic units with folds and major faults. The one below shows regional metamorphism isogrades and main areas of ductile shear. The arrows point to the motion direction of the faults and shears (according to Martínez Catalán et al., 2003). Figure 12. Structures associated to the basal shear zone of the Mondoñedo Thrust: a) 2 nd phase isoclinal folds showing hinge scattering. Cliff between Fazouro and Foz. b) Set of folds with folded hinges and en échelon distribution; the stretching lineation is parallel to the pencil. Cliff north of Nois. c) Cross section of a closed fold. Cliff south of Area Longa beach. d) S-C structures at the base of the Mondoñedo Thrust in the SE limit of Areoura beach. The C surfaces (shear areas) dip around 30º to the east (left) and among them the S surfaces (schistosity) show sigmoidal geometry. e) Anticline on Lower Cambrian quartzites of the Mondoñedo Autochton, underneath the basal thrust. Cliff southeast of Burela. 20 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

9 between Badajoz and Ciudad Real. In the centre of the zone, the Tamames and Peña de Francia synclines crop out in the Batuecas Natural Park (Salamanca). In the north (Galicia) and in the Central System, the Armorican Quartzite also delineates those anticline cores with outcrops of the Ollo de Sapo Formation, a thick volcanic, subvolcanic and volcanoclastic sequence of Tremadoc-Arenig age (Parga Pondal et al., 1964; Díez Montes, 2006). The generic name of Armorican Quartzite is used, by analogy with the Armorican Massif (France), to refer to a mostly quartzitic lithostratigraphic unit deposited on a wide, stable and homogenous platform existing in the Early Paleozoic. These quartzitic facies with Arenig (Lower Ordovician) trace fossils followed by a slate formation with trilobites, are not only present in the Central Iberian Zone, but also in other areas of the Iberian Massif. These facies extend from West Africa to Afghanistan, showing the enormous size of the marine platform. The platform was part of a passive continental margin, as inferred from the mostly shallow marine sedimentary facies and its extraordinary continuity, as well as by the existence of synsedimentary normal faults. The Central Iberian Zone provides a magnificent field to study deformation under low to high metamorphic grade conditions. Low-grade metamorphism is the most widely represented, particularly towards the south and southeast, in Extremadura and Toledo Mountains, where anchimetamorphic conditions are frequent. High-grade metamorphic rocks with extensive partial melting crop out within antiformal structures and a few domes, where a fast transition between metamorphic grades is often observed. The transition of metamorphic zones is either thinned or incomplete, so several of these domes have been related to extensional detachments, for example in the region of Salamanca (Escuder Viruete et al., 1994; Díez Balda et al., 1995) and in Toledo (Hernández Enrile, 1991). The natural parks of Sanabria Lake, in Zamora, and Arribes del Duero and Agueda, in Zamora and Salamanca, offer excellent exposures of the deeper structural levels of the range, similarly to Guadarrama and Gredos Ranges, where the granitic outcrops are also impressive. Differing deformation trends are evident within this zone, with a narrow northern domain with recumbent folds, a wider middle domain with vertical folds and usually not too tight (Díez Balda et al., 1990), and a very narrow southern domain with recumbent folds (Martínez Poyatos, 2002). Sierra del Caurel (Serra do Courel in Galician language), in the southern province of Lugo, has been selected as a Geosite representative of the great recumbent folds, with spectacular views of the folds affecting the Armorican Quartzite (Figures 13 and 14), formed during the first Variscan deformational phase. The geometry of the Sierra del Caurel folds, cropping out in an area 43 km long and 12 km wide, can be Figure 13. El Caurel syncline, folding towards the Armorican Quartzite, on the eastern slope of the Ferreiriño river. Figure 14, below. 2 nd order anticline in the hinge zone of El Caurel syncline. The hinge is displayed by the Armorican Quartzite (coloured lined marking the top). At the summit of the relief, in the middle of the picture, the quartzite of the reverse flank of El Caurel Syncline. reconstructed thanks to the splendid outcrop conditions (Matte, 1968). It consists of two structures (Piornal anticline and Caurel syncline) with almost horizontal axial plane folds in the northern half. Even not being the biggest, these are the best outcrops of recumbent folds in the northern Iberian Peninsula. The anticline plunges to the south towards its roots. The overturned limb common to both folds reaches 10 km semi-wavelength in the central part, narrowing to the east and west. The hinge of the anticline is curved, varying between N100ºE and N160ºE from west to east. The most spectacular views are from the Quiroga to Seoane do Courel road (El Caurel syncline), at Alto do Castro, and from Busto and Froxán. The Ossa-Morena Zone is a key region to determine the significance of the Cadomian orogeny (Neoproterozoic) and therefore to understand end- Precambrian plate dynamics and its continuation into the Paleozoic. The location of the zone, in the THE IBERIAN VARISCAN OROGEN 21

10 southern Iberian Massif (Figure 2) makes its structural evolution and its magmatism essential to understand Late Paleozoic dynamics. Places like Sierra Morena offer a good combination of geology and landscape all along the Aracena and Picos de Aroche Natural Park, and its extension to the east through Sierra Norte Natural Park. The combination of isotopic age and geochemical data in the Ossa-Morena Zone supports its evolution as an active margin followed by the scattering of the peri-gondwanan terrains. In fact, up to now, this is the only zone in the Iberian Massif where ages of Cadomian metamorphism have been obtained. These ages span from 562 to 524 Ma (Blatrix and Burg, 1981; Dallmeyer and Quesada, 1992; Ochsner, 1993; Ordóñez et al., 1997) and even if the youngest ones may reflect late- to post-orogenic thermal events, the earlier ones clearly show a latest Proterozoic dynamothermal metamorphism. Cadomian or end-proterozoic magmatism has been identified in the West Asturian-Leonese and Central Iberian Zones, but it is particularly well defined in the Ossa-Morena Zone. Here, the geochemistry and ages fit within a long evolution of 120 My, allowing to identify several stages (Oschner, 1993): anorogenic rift ( Ma), early calcalkaline orogenic magmatism ( Ma), peraluminous orogenic magmatism (530 Ma), late-orogenic calcalkaline ( Ma) and alkaline (500 Ma) magmatism, and post-orogenic rift ( Ma). The last synorogenic stages were contemporaneous to the detachment and separation of the first peri-gondwanan crustal fragments. The northeast and southwest boundaries of the Ossa- Morena Zone are important geological structures, most probably related with former plate boundaries. The Beja-Acebuches ophiolite along the southwest margin represents an oceanic suture. Furthermore, there are strong differences between the granitoids of the Ossa-Morena Zone and those of the rest of the Iberian autochton. The granitic massifs are usually of limited extension (Figure 10), lack the typical big plutons of the Central Iberian Zone, and include more abundant basic rocks, which calls for a bigger mantle contribution. The southern vergence of Variscan structures in the Ossa-Morena and South Portuguese Zones, and the existence of a crustal suture, suggest subduction of the oceanic lithosphere towards the north, underneath the Ossa-Morena Zone, which also explains its early magmatism. The Ossa-Morena Zone represents the complex evolution of a continental margin during the Late Paleozoic, although in this case it is not associated to a context of Cordilleran orogeny or long-lived active margin, but to the convergence and collision of two large continental masses, Gondwana and Laurasia. The South Portuguese Zone is the southernmost of the Iberian Massif and holds some of the keys of Variscan evolution. It is a thin-skinned thrust belt and, from the point of view of orogenic evolution, the zone is considered an example of the importance that transcurrent faults may have in continental deformation. The northern part of the zone is an interesting Variscan suture which crops out along southern Sierra Morena, most of it inside the Natural Park of Aracena and Picos de Aroche. In Portugal, the coastal sections of Baixo Alentejo and Algarve are worth mentioning. A set of basic rocks cropping out along the boundary between the South Portuguese and Ossa-Morena Zones is interpreted as a fragment of oceanic lithosphere (Figure 3), represented by the Beja-Acebuches unit. South of it, there is a Lower Devonian - Carboniferous basement accretion prism, the Pulo do Lobo antiform (Munhá et al.,1986; Oliveira, 1990; Quesada et al.,1994; Onézime et al., 2002). The rest of the South Portuguese Zone comprises the Iberian Pyrite Belt, one the most important volcanogenic polymetallic sulphide deposits of the planet. The Pyrite Belt volcanism, even if bimodal and dominated by intermediate and acid facies dated in the Devonian-Carboniferous boundary ( Ma; Quesada, 1999), suggests a relation with a magmatic arc. The South Portuguese Zone displays a south-vergent thin-skinned tectonic overthrust, that is, opposed to that in the other structurally comparable belt, the Cantabrian Zone. The genetic significance of these two external zones of the Iberian Massif is different. Whereas the Cantabrian Zone is a Gondwanan overthrust foreland belt, the South Portuguese Zone first developed as a foredeep basin, and later as an accretionary complex, particularly in the northern part. The subduction of oceanic lithosphere under the marginal prism ended with the arrival of continental crust, when a terrain, most likely peri-gondwanan, forming the basement underlying the South Portuguese Zone, came under the accretionary prism and developed a thin-skinned thrust belt on top of it. The zone also features mostly sinistral transcurrent shearing, and has been proposed to be contemporaneous with tangential deformation. The most abundant stretching lineations in the Beja-Acebuches ophiolite are subparallel to the amphibolite band (Crespo Blanc, 1991), although the first movements of the ophiolites record the convergent component (Onézime et al., 2002). This suggests that amphibolite shearing represents the transition from a ductile overthrust to a sinistral ductile tear-fault. Some structures display the kinematics of oblique convergence, but it is more common for the deformation to undergo partitioning into convergent and transcurrent components, each one creating its own structures. Both situations point to transpression (Sanderson and Marchini, 1984), a deformation regime whose geotectonic significance is that of oblique collision between the plate underneath the thrust belt of the South Portuguese Zone, and the Iberian autochthonous plate (Silva et al., 1990). The Galicia-Trás-os-Montes Zone includes the allochthons of Cabo Ortegal, Órdenes and Malpica-Tui in 22 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

11 Galicia (Spain), and of Bragança and Morais in Trásos-Montes (Portugal), as well as a common underlying thrust sheet which has been called the Schistose or Parautochtonous Domain (Farias et al., 1987; Ribeiro et al., 1990), although it would be more appropriate to call it Lower Allochthon. Due to the variety of terrains comprised by the allochthonous complexes, each with different origin and tectonometamorphic evolution, this zone is a basic reference to understand Paleozoic geodynamics. It is herein described at the end of this chapter because of the exotic character of the units, and because it does not fit the continuous and concentric trend of the rest of the zones making up the Iberian Massif (Figure 3). The units of the allochthonous complexes are grouped into basal, intermediate or ophiolitic, and upper units, according to their structural position within the thrust pile. Many of them have recrystallized under deep Figure 15. Evolution of the peri-gondwanan terrain preserved in the upper allochthonous units, according to Gómez Barrero et al. (2007), based upon the reconstruction of continental masses and oceans by Winchester et al. (2002). In the Silurian, the lapetus ocean almost disappeared, whereas the Rheic ocean is still very wide, although it is being subducted between Gondwana and Laurentia. THE IBERIAN VARISCAN OROGEN 23

12 Figure 16. Evolution of the Rheic ocean and the Variscan orogen according to Martínez Catalán et al. (2007). conditions, and are thus representative of the lower crust, and sometimes even of the underlying mantle. The basal units represent the most external margin of Gondwana, with magmatic evidence for Ordovician rifting, and metamorphic evidence for early Variscan subduction (Gil Ibarguchi and Ortega Gironés, 1985; Pin et al., 1992; Arenas et al., 1995; Rodríguez Aller, 2005). Ordovician rifting continued with the separation of a peri-gondwanan terrain and the creation of oceanic lithosphere beneath the Rheic Ocean between the terrain and the new border of Gondwana. Fragments of oceanic lithosphere with different isotope ages are present in the suture included in the Galicia-Trás-os- Montes Zone, mostly linked to the Rheic ocean (Díaz García et al., 1999; Sánchez Martínez et al., 2006, 2007; Arenas et al., 2007). The evolution of the upper units indicates their multiple orogenic character, with a first deformation during the Lower Ordovician and later recycling by Variscan collision. The associated magma chemistry and metamorphic evolution of the upper units suggests a volcanic arc environment in the Early Ordovician (Abati et al., 1999; Andonaegui et al., 2002; Fernández-Suárez et al., 2002). It would be an arc developed along the Gondwana margin, and its detachment would have left behind a backarc basin which in time would have evolved to generate the Rheic Ocean. Figures 15 and 16 show schematically, in a map and a section, a possible evolution of the plates and terrains involved in Variscan collision, according to the interpretations by Gómez Barreiro et al. (2007) and Martínez Catalán et al. (2007), respectively. The existence of only one oceanic domain between Avalonia and Gondwana has been taken into consideration, although the evolution was undoubtedly complex. The opening of this domain, called Rheic, was already taking place while the convergence between Avalonia and Laurentia closed the lapetus ocean in the Early Ordovician. This convergence meant the first Taconic orogenic event, Ma ago, and ended 24 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

13 with the amalgamation of Avalon Ma ago, that is, between the Middle Ordovician and Early Silurian. From the Middle Silurian on, deformation progressively started to affect the peri-gondwanan terrains and the Gondwana margin itself. Apparently, an accretionary prism developed on the margin of Laurentia following the amalgamation of Avalonia (Figure 16). The Galicia- Trás-os-Montes Zone records the progressive incorporation onto the orogenic prism of a volcanic arc represented by the upper units, an imbricate pile of oceanic lithosphere of variable ages, and the partial westward subduction of the outermost margin of Gondwana, represented by the basement units (Martínez Catalán et al., 1997, 2007). The blocking of continental subduction in the late Devonian marked the transition to a collisional regime. The subducted basal units were overthrusted onto the external platform of Gondwana in the Early Carboniferous, and a sheet of sediments was wedged underneath forming the Schistose Domain. Later, intracontinental deformation gradually affected more external areas of the orogen. The fourth Geosite selected in the Iberian Massif is the Cabo de Ortegal Complex, in the province of La Coruña, as it is an excellent representative of the Figure 18. Northern end of the eclogite belt of the Cabo Ortegal Complex at Punta dos Aguillóns. This is part of the allochthonous unit of La Capelada, structurally the highest of the complex. Figure 17. Geological map and cross section of the Cabo de Ortegal complex based on Vogel (1967), Marcos et al. (1984) and Arenas (1988). THE IBERIAN VARISCAN OROGEN 25

14 Figure 19. Pictures of the Cabo de Ortegal Complex: a) Recumbent fold syncline in the La Capelada Unit, with granulites of the Bacariza Fm in the core (grey) surrounded by ultrabasic rocks (yellowish). The colored line marks the contact. Southwest of San Andrés de Teixido. b) Closed shaped folds developed during the thrust which emplaced the catazonal units. Unit of Cedeira. c) Shear area at Carreiro, north of Cedeira, limited by two surfaces dipping 45º. It represents the overthrust separating the Cedeira Unit to the east (right), from the Purrido Unit to the west. d) Detail of the upper limit of Carreiro shear, showing a thrust plane under which some folds can be observed. e) Punta Candelaria (o Candieira). The lighter colored rocks are amphibolites, gneisses and ultrabasic rocks of the shear area of Carreiro, while the darker rocks to the left are amphibolites of the Purrido Unit. f) Pillow-lava breccias in the ophiolitic Somozas Mélange, the structurally lower unit of the Cabo de Ortegal Complex. Southern part of the Ensenada de Espasante. exotic terrains making up the Galicia-Trás-os-Montes Zone. All the groups of allochthonous units are present in this complex (Figure 17), and one of those units has provided the oldest rocks in the Iberian Peninsula (1160 Ma; Sánchez Martínez et al., 2006). In addition, this complex stands out for the quality of its exposures and its spectacular landscapes. The estuaries of Cedeira, Ortigueira and Espasante offer magnificent structural sections, combined with the cliffs on both sides of the cape (Figure 18) and with the outcrops of Sierra de la Capelada. The Cabo de Ortegal Complex includes a great exposure of two catazonal allochthonous units: a deeper 26 Martínez Catalán, J.R. Aller, J - Alonso, J.L. and Bastida, F.

15 one, representative of the lower crust and the upper mantle of a continent, and/or a volcanic arc, corresponding to a peri-gondwanan terrain (Galán and Marcos, 1997; Santos et al., 2002). These are the Cedeira and La Capelada units (Marcos et al., 1984, 2002), cropping out within the core of a SW-NE elongated synform with a size of 28 by 12 km. The catazonal units are part of the upper part of the allochthonous complexes, and consist of rocks which underwent high pressure and high temperature metamorphism (Vogel, 1967; Mendía Aranguren, 2000). They include granulites and eclogites structurally overlying ophiolitic units representing fragments of Paleozoic oceanic lithosphere (Figure 19), and basement units representative of the Gondwana margin. These ophiolites and basement units crop out in an imbricate zone, structurally emplaced at the base of the complex, and include an area of tectonic mixture known as the Somozas Mélange (Arenas, 1988). The imbricate zone continues underneath, affecting the Schistose Domain, which is here formed by a series of sheets comprising metasedimentary and metavolcanic rocks of Silurian and Ordovician age (Valverde-Vaquero et al., 2005). The whole set of the complex is key to reconstruct the evolution of the Rheic Ocean. It probably includes the largest eclogite exposure in the world, with highpressure metamorphosed basic rocks indicative of subduction processes. Given the excellent exposure conditions of the lower crust, the processes of burial and exhumation in an orogenic cycle can be studied. The Cabo Ortegal Complex is also a good example of a deep allochthonous unit emplaced into an epizonal crustal domain. The structural sequence shows a spectacular development of structures at all scales, from recumbent folds, ductile shears and thrusts (Figure 19), to microfolds, boudinage, porphyroclast systems, lineations and foliations. It constitutes a natural laboratory for fabric deformation ranging from high pressure and temperature conditions to greenschist facies. The transpressive orogen in the southwest of the Iberian Peninsula, from the Central Iberian Zone to the South Portuguese Zone, displays peculiar features in relation to the deeply transpressive nature (oblique collision) under which it formed in the Late Paleozoic (Ribeiro et al., 1990; Quesada, 1991). The reconstruction of this sector is essential to understand the vast circum-atlantic Paleozoic orogenic domain, presently scattered in three continents (Europe, Africa and America) with a triple junction among the three of them located precisely in this Iberian southern section (Figure 1). The spectacular Iberian-Armorican Arch (arcuate structure) is a first-rate imbricate structure of geological units arched or bent around a sub-vertical axis located to the east and with centripetal emplacement directions. The most external zones of the Iberian foreland are located in the center of the arch (Cantabrian Zone), with well-preserved Carboniferous synorogenic deposits. To the west, a complete section of the Variscan Orogen consists of a piling of E-to-NE-vergent thrusts including (from lower-eastern to upper-western), the Cantabrian, West Asturian-Leonese, Central Iberian and Galicia-Trás-os-Montes zones. The first three represent progressively deeper units of the Paleozoic continental margin of the Iberian plate, whereas the fourth one comprises the allochthonous exotic complexes of Galicia-Trás-os-Montes. Towards the exterior of the arch, there are two more areas, Ossa-Morena and South Portuguese Zones, where transpressive deformation may provide some clues about the formation of the arch itself. The models proposed to explain the Ibero-Armorican Arch are varied. One of them is that of strict oroclinal bending, with an initial rectilinear belt which later became bent. A popular model is that of the indenter, which suggests the presence of a rigid buttress out of Gondwana which might have caused the orogen bend during its collision with Laurentia (Matte and Ribeiro, 1975; Matte, 1986; Ribeiro et al., 1995). Summing up, the Iberian Massif provides the keys to understand the history of the circum-atlantic domain during the Late Proterozoic and the Paleozoic, particularly with regard to the evolution of the northern margin of Gondwana and the Rheic ocean. It also offers excellent exposures and structures to study deformation mechanisms in a collisional orogenic belt at different depths, particularly the large thrust at crustal and even lithospheric scale. It also turns out to be a paramount reference to understand the genesis of granitoids in orogenic collisional contexts. Its importance also lies on the fact that not many times a complete section including both forelands of an old collisional orogen is found preserved. The selected Geosites represent some of those keys but, in addition, the Iberian Massif features two particularly outstanding structural and tectonic characteristics: the transpressive orogen of the peninsular southwest, and the arcuate structure, very tight in the northwest, known as Asturian Knee and Ibero - Armorican Arch at the level of the Variscan orogenic belt. THE IBERIAN VARISCAN OROGEN 27

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