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Home , Armorican Massif, Avalonia, Iberian Plate, Sierra Morena

CHAPTER 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. Rocks of Upper Proterozoic to age - forming the Variscan or Hercynian orogen - crop out widely in the western part of the , in what is called the Iberian or Hesperian . These deformed rocks, often metamorphosed and intruded by different types of granitoids, were witness to the great formed in the late , basically in the Late and Carboniferous (between 370 and 290 million years ago), by the convergence and collision of two major : and .

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 Figure 1. Scheme showing the position of the Iberian Iberian Peninsula and of most of western and central Peninsula in relation to the Appalachians and the . A crustal basement is the result of an orog- Caledonian and Variscan belts. Modified from eny, that is, the consequence of a deep remobilization Neuman and Max (1989). of the continental crust caused by the convergence of plates, and is associated to uplift and the creation of relief. The Variscan is the name given to formed at the end of the Paleozoic. In its place, what the whole set of geological processes which deeply presently is (then part of ) went modified the continental crust of and around the Variscan orogen and a wide belt developed the north and northwest of during the Late along its eastern margin: the Appalachians. Paleozoic (Devonian-). In its long period of activity (approximately 80 million years), this orogeny Figure 1 shows a paleogeographic reconstruction of shortened and intensely deformed those sediments the continental masses at the end of the Paleozoic. deposited previously along vast continental margins, It is noticeable how Iberia, the west of and remobilized the prior basement on which they laid, the south of used to occupy a key posi- generated a new crust by partial melting of the former, tion within the Variscan belt, at the junction of the added fragments of and mantle, eroded three most important Paleozoic orogenic belts: the and re-sedimented part of the newly formed crust, Appalachians to the southwest, the Caledonides to the and deformed the majority of the new sediments. northwest, and the Variscan to the northeast. Besides, Iberia shares with west France a marked and striking The importance of the Iberian Variscan orogen lies in its bend or curvature known as the Ibero-Armorican Arch location close to three big orogenic belts: Caledonian, (Ribeiro et al., 1995). Variscan and Appalachian. Given the diversity and quality of the outcrops, it is one of the key geologi- The former reconstruction allows to infer the ultimate cal frameworks recording the latest Precambrian and cause of the orogeny, since a set of continental crustal Paleozoic evolution of the planet, the deformation blocks are nearby but separated by orogenic belts. It is and generation of structures at different levels of the thus clear that convergence caused the orogenic pro- continental crust, the thermal history and the genera- cesses, and continental collision was the final result. tion of granitoids, all within the context of an active We are talking about the convergence of three large continental margin (latest Precambrian) and a conti- tectonic plates: Laurentia (North America), nental collision (Variscan Orogen). (Scandinavia, northern belt of Europe and Russia), and Gondwana (Africa, , , India What crops out in the Iberian Peninsula from that and ). The final result was the creation, by basement (Iberian Massif) is part of a much longer the amalgamation of many continental masses, of a orogenic belt, extending to the north and east through , in this case the last Pangea generated Central Europe into , where it is shifted by a by the continuing geodynamics of our planet. tectonic line known as Thornqvist Fault, and continu- ing towards south . Being such an old orogenic belt (its main development finished 290 million years ago), it is to be expected On the opposite side, the belt extends along north that the continental crust generated is already totally and northwest Africa parallel to the Atlantic margin. balanced (thermally and isostatically), and the orogenic However, the Atlantic did not exist when the belt was reliefs should have completely disappeared. However,

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

THE IBERIAN VARISCAN OROGEN 15 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 The Cantabrian Zone is the most continental part foreland: an area of Paleozoic pre-orogenic sediments of the Paleozoic basin and a magnificent example with relatively small thickness. These are to of foreland thrust belt. In addition, it displays an Devonian shallow sea deposits, and display a wedging extremely tight tectonic bend representing the core of towards the external part as well as some stratigraphic the Iberian-Armorican Arch. discontinuities. This wedging is in part a result of crustal uplift followed by erosion in the late Devonian, Thin-skinned tectonics, the deformation style of the and shows the change from a passive margin to outer crust, features in the Cantabrian Zone a sequen- Variscan collision. This erosion was followed by basin tial development of thrusts and associated folds, con- deepening in the earliest Carboniferous, followed by temporaneous with the classic clastic wedges along synorogenic sedimentation. the front of major thrusts. It is also characterized by

16 Martínez Catalán, J.R. – Aller, J - Alonso, J.L. and Bastida, F. 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- ). 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. Carboniferous age, piled in a series of allochthonous Within the relative autochtonous, a thrust partially structures (Figure 5). Inside this thrust, the frontal and repeats the calcareous succession of the Portilla Fm internal areas of several minor thrust slabs and associ- (lower colored line). ated 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). deposits filling in piggy-back basins located over the thrusted slabs (Marcos and Pulgar, 1982; Rodríguez There are wonderful examples of thrust overstep Fernández et al., 2002). The tectonic structure is fur- geometries, of drape folds and associated fault propa- ther complicated by the arcuate bending of the belt, gation, and of frontal imbrications and duplexes, which seems to be partially contemporaneous with such as those at Primajas, Pardaminos and Pico Jano. thin-skinned tectonics, according to structural data The accumulated minimum shortening of all thrusts (Pérez-Estaún et al., 1988). cropping out in this is considered to be slightly above 92 km. Typical in this zone are the fault rocks As a consequence of Cenozoic Alpine uplift, the developed under low temperature conditions and Cantabrian Mountain Range displays magnificent limited to a 1 to 2 meter thick band at the base of landscapes with outstanding calcareous massifs, the Esla Thrust, which concentrates the deformation some of which are protected areas such as Montaña associated to the displacement, which is calculated to de Covadonga (Picos de Europa) National Park, and be around 19 km (Alonso, 1985, 1987). the natural parks of Somiedo and San Emiliano Valley. Synorogenic deposits are also very interesting, abundant and well preserved, with ages between Several sites with panoramic outlooks allow to see Westphalian and Stephanian B (Figure 8). Their pres- large structures such as the Esla Thrust (Figures 5, 6, ence allows to date the emplacement of thrusts, as well 7 and 8), the front of the Ponga Thrust, and that of as the folds associated to the thrust and the reactiva- the Picos de Europa Thrust, together with associ- tion of bedding as a flexure-folding surface. Five main ated smaller structures such as duplexes, frontal imbri- unconformities have been identified in the synorogenic cations and ramps (Alonso, 1987; Álvarez-Marrón and sediments, although not all of them are discrete and Pérez-Estaún, 1988). some change laterally to progressive unconformities. The first two are related to the emplacement of the The Esla Thrust, in the province of León, has been thrusts, whereas the remaining three are related to selected as a Geosite due to its magnificent outcrops thrust reactivation and to the re-compression of folds of sedimentary formations of early Cambrian to early associated to them (Alonso, 1985).

THE IBERIAN VARISCAN OROGEN 17 Figure 8. Syncline developed upon Stephanian The West Asturian-Leonese Zone is a very impor- conglomerates (thin colored lines) in relation to a tant unit because of its tectonic significance in relation reverse fault (thicker line) thrusting the limestones to middle crust deformation problems and the dynam- of the Alba and Barcaliente formations (Mountain ics of orogenic wedges, as well as for the study of the Limestone). The fault is an example of flexural origin of granitoids and their relation to the orogeny. sliding along the stratigraphic surface during With regard to structural features, this zone forms a tightening of the Peña Quebrada syncline slate belt, not only because that is precisely the most (Alonso, 1985). frequent lithology, but also because it displays a wide belt with vertical folds and steeply-inclined axial-plane cleavage which curves in depth becoming asymptoti- cally 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), Figure 9. Interpretation of the seismic lines is generally characterized by steeply-dipping folds and ESCIN-1 and ESCIN-3.3, placed one next to the cleavage, as well as by the ductile deformation of other to show the large scale structure in this greenschist facies, and includes some thrust-related part of the Iberian Massif, as it is deduced from repetitions. The seismic cross section ESCIN – 3.3 seismic reflection profiles. According to Pérez- (Figure 9) proves that the thrusts join at depth to (or Estaún et al. (1994) and Ayarza et al. (1998). are crossed by) a basal detachment which ends rooted

18 Martínez Catalán, J.R. – Aller, J - Alonso, J.L. and Bastida, F. 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 spec- tacular deformation structures (Bastida, 1980; Pulgar, 1980).

Uplifted areas also offer an excellent blend between geology and landscape. The Oscos region of west- ern Asturias, the Ancares Natural Park, and the gra- nitic 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 monzo- granites. They are collectively called early granodior- ites, and are rocks originated deep at the base of the Figure 10. Granitic plutons of the Iberian Massif in continental crust, with mantle contribution. Their relation with Variscan deformation stages (according development was followed by mid-crustal melting to López Plaza and Martínez Catalán, 1987). which generated peraluminous of so-called leucogranites or two mica granites.

Both the early granodiorite massifs and the leucogran- ites are syntectonic with ages between 350 and 305 East-verging isoclinal folds are abundant and show a Ma, broadly coinciding with Variscan deformation. strong scattering of their hinges, very often displaying The most modern group of granitoids is formed by curved hinges and tight shapes belonging to sheath a set of post-kinematic plutons (gabbros, tonalites, folds (Figures 12a, b and c). Folds are also affected granodiorites and monzogranites). In contrast to the by the development of a second axial-plane foliation former, its emplacement is restricted to a short span and by a stretching lineation striking between N 100 of 295-285 Ma, a magmatic pulse suggesting a single and 110º E. Boudinage structures are also common cause which Fernández-Suárez et al. (2000) linked to affecting quartzites, as well as S-C structures, which the delamination of the mantle lithosphere at the end are particularly abundant in Upper Proterozoic rocks at of the orogeny, and its replacement by a hotter asthe- the base of Mondoñedo Thrust, showing an ESE trend nospheric mantle. of displacement (Figure 12d). In addition, the basal thrust crops out at Aureoura beach (As Areouras) and, The Mondoñedo Thrust, and more specifically its north of it, the autochtonous slab of the Mondoñedo basal shear zone in the coastal area of the Lugo Thrust crops out in the tectonic window of Xistral province (Figure 11), has been selected as a Geosite. (Gistral), as a train of regular folds affecting the thick Its Cambrian slate and quartzite outcrops show the homonymous quartzitic formation (Figure 12e). effects of tangential deformation of the second Variscan deformation phase (Bastida and Pulgar, The Central-Iberian Zone comprises the central part 1978; Bastida et al., 1986; Aller and Bastida, 1993). of the Iberian Massif and is the widest of all the belts, Its interest stems from it being an exposure of the around 400 km in the centre of the massif (Figure 2). internal and deeper area of a thrust slab emplaced The structure of the low metamorphic grade under ductile conditions in the beginning stages, and in the upper crustal levels is well defined at the surface fragile in the last stages. Spectacular meso- and micro- by the Armorican Quartzite, which due to its resistance structures are observed in a ductile shear zone related to weathering and erosion forms lined up reliefs bor- to crustal thrusting, with a thickness between 3 and dering the elongated synclines in the southern part of 3.5 km. Some medium-grade metamorphic foliations, this zone, such as Guadarranque, in Toledo, Cañaveral stretching lineations, and fault rocks developed under (cropping out along Monfragüe Natural Park) and a wide temperature range, can also be studied. San Pedro Range, in Cáceres, and Herrera del Duque,

THE IBERIAN VARISCAN OROGEN 19 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) 2nd 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. 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 (France), to refer to a mostly quartzitic lithostratigraphic unit deposited on a wide, stable and homogenous platform exist- ing 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 shal- low marine sedimentary facies and its extraordinary continuity, as well as by the existence of synsedimen- tary normal faults. The Central Iberian Zone provides a magnificent field to study deformation under low to high metamorphic grade conditions. Low-grade meta- morphism is the most widely represented, particularly towards the south and southeast, in and Toledo Mountains, where anchimetamorphic condi- tions are frequent. High-grade metamorphic rocks with extensive partial melting crop out within antifor- Figure 13. El Caurel syncline, folding towards the mal structures and a few domes, where a fast transi- Armorican Quartzite, on the eastern slope of the tion between metamorphic grades is often observed. Ferreiriño river. The transition of metamorphic zones is either thinned or incomplete, so several of these domes have been Figure 14, below. 2nd order anticline in the hinge zone related to extensional detachments, for example in the of El Caurel syncline. The hinge is displayed by the region of Salamanca (Escuder Viruete et al., 1994; Díez Armorican Quartzite (coloured lined marking the top). Balda et al., 1995) and in Toledo (Hernández Enrile, At the summit of the relief, in the middle of the 1991). The natural parks of Sanabria Lake, in Zamora, picture, the quartzite of the reverse flank of El Caurel and Arribes del Duero and Agueda, in Zamora and Syncline. Salamanca, offer excellent exposures of the deeper structural levels of the range, similarly to Guadarrama and Gredos Ranges, where the granitic outcrops are reconstructed thanks to the splendid outcrop condi- also impressive. tions (Matte, 1968). It consists of two structures (Piornal anticline and Caurel syncline) with almost Differing deformation trends are evident within this horizontal axial plane folds in the northern half. Even zone, with a narrow northern domain with recumbent not being the biggest, these are the best outcrops of folds, a wider middle domain with vertical folds and recumbent folds in the northern Iberian Peninsula. The usually not too tight (Díez Balda et al., 1990), and a anticline plunges to the south towards its roots. The very narrow southern domain with recumbent folds overturned limb common to both folds reaches 10 km (Martínez Poyatos, 2002). semi-wavelength in the central part, narrowing to the east and west. The hinge of the anticline is curved, Sierra del Caurel (Serra do Courel in Galician lan- varying between N100ºE and N160ºE from west to guage), in the southern province of Lugo, has been east. The most spectacular views are from the Quiroga selected as a Geosite representative of the great to Seoane do Courel road (El Caurel syncline), at Alto recumbent folds, with spectacular views of the folds do Castro, and from Busto and Froxán. affecting the Armorican Quartzite (Figures 13 and 14), formed during the first Variscan deformational The Ossa-Morena Zone is a key region to deter- phase. mine the significance of the () and therefore to understand end- The geometry of the Sierra del Caurel folds, cropping Precambrian plate dynamics and its continuation out in an area 43 km long and 12 km wide, can be into the Paleozoic. The location of the zone, in the

THE IBERIAN VARISCAN OROGEN 21 southern Iberian Massif (Figure 2) makes its structural that transcurrent faults may have in continental evolution and its magmatism essential to understand deformation. The northern part of the zone is an Late Paleozoic dynamics. Places like Sierra Morena interesting Variscan which crops out along offer a good combination of geology and landscape southern Sierra Morena, most of it inside the Natural all along the Aracena and Picos de Aroche Natural Park of Aracena and Picos de Aroche. In , the Park, and its extension to the east through Sierra coastal sections of Baixo Alentejo and Algarve are Norte Natural Park. worth mentioning.

The combination of isotopic age and geochemical A set of basic rocks cropping out along the boundary data in the Ossa-Morena Zone supports its evolution between the South Portuguese and Ossa-Morena Zones as an active margin followed by the scattering of the is interpreted as a fragment of oceanic lithosphere peri-Gondwanan terrains. In fact, up to now, this is (Figure 3), represented by the Beja-Acebuches unit. the only zone in the Iberian Massif where ages of South of it, there is a Lower Devonian - Carboniferous Cadomian metamorphism have been obtained. These basement prism, the Pulo do Lobo anti- ages span from 562 to 524 Ma (Blatrix and Burg, form (Munhá et al.,1986; Oliveira, 1990; Quesada et 1981; Dallmeyer and Quesada, 1992; Ochsner, 1993; al.,1994; Onézime et al., 2002). The rest of the South Ordóñez et al., 1997) and even if the youngest ones Portuguese Zone comprises the Iberian Pyrite Belt, one may reflect late- to post-orogenic thermal events, the the most important volcanogenic polymetallic sulphide earlier ones clearly show a latest Proterozoic dynamo- deposits of the planet. The Pyrite Belt volcanism, even thermal metamorphism. if bimodal and dominated by intermediate and acid facies dated in the Devonian-Carboniferous bound- Cadomian or end-Proterozoic magmatism has been ary (347-355 Ma; Quesada, 1999), suggests a relation identified in the West Asturian-Leonese and Central with a magmatic arc. Iberian Zones, but it is particularly well defined in the Ossa-Morena Zone. Here, the geochemistry and ages The South Portuguese Zone displays a south-vergent fit within a long evolution of 120 My, allowing to thin-skinned tectonic overthrust, that is, opposed to identify several stages (Oschner, 1993): anorogenic that in the other structurally comparable belt, the (620-590 Ma), early calcalkaline orogenic magmatism Cantabrian Zone. The genetic significance of these (590-540 Ma), peraluminous orogenic magmatism two external zones of the Iberian Massif is different. (530 Ma), late-orogenic calcalkaline (525-510 Ma) and Whereas the Cantabrian Zone is a Gondwanan over- alkaline (500 Ma) magmatism, and post-orogenic rift thrust foreland belt, the South Portuguese Zone first (490-470 Ma). The last synorogenic stages were con- developed as a foredeep basin, and later as an accre- temporaneous to the detachment and separation of tionary complex, particularly in the northern part. The the first peri-Gondwanan crustal fragments. of oceanic lithosphere under the marginal prism ended with the arrival of continental crust, The northeast and southwest boundaries of the Ossa- when a terrain, most likely peri-Gondwanan, forming Morena Zone are important geological structures, most the basement underlying the South Portuguese Zone, probably related with former plate boundaries. The came under the accretionary prism and developed a Beja-Acebuches ophiolite along the southwest mar- thin-skinned thrust belt on top of it. gin represents an oceanic suture. Furthermore, there are strong differences between the granitoids of the The zone also features mostly sinistral transcurrent Ossa-Morena Zone and those of the rest of the Iberian shearing, and has been proposed to be autochton. The granitic massifs are usually of limited contemporaneous with tangential deformation. extension (Figure 10), lack the typical big plutons of The most abundant stretching lineations in the the Central Iberian Zone, and include more abundant Beja-Acebuches ophiolite are subparallel to the basic rocks, which calls for a bigger mantle contribu- amphibolite band (Crespo Blanc, 1991), although tion. The southern vergence of Variscan structures in the first movements of the ophiolites record the the Ossa-Morena and South Portuguese Zones, and convergent component (Onézime et al., 2002). This the existence of a crustal suture, suggest subduction of suggests that amphibolite shearing represents the the oceanic lithosphere towards the north, underneath transition from a ductile overthrust to a sinistral ductile the Ossa-Morena Zone, which also explains its early tear-fault. Some structures display the kinematics of magmatism. The Ossa-Morena Zone represents the oblique convergence, but it is more common for the complex evolution of a continental margin during the deformation to undergo partitioning into convergent Late Paleozoic, although in this case it is not associated and transcurrent components, each one creating its to a context of Cordilleran orogeny or long-lived active own structures. Both situations point to transpression margin, but to the convergence and collision of two (Sanderson and Marchini, 1984), a deformation large continental masses, Gondwana and Laurasia. regime whose geotectonic significance is that of oblique collision between the plate underneath the The South Portuguese Zone is the southernmost of thrust belt of the South Portuguese Zone, and the the Iberian Massif and holds some of the keys of Iberian autochthonous plate (Silva et al., 1990). Variscan evolution. It is a thin-skinned thrust belt and, from the point of view of orogenic evolution, The Galicia-Trás-os-Montes Zone includes the alloch- the zone is considered an example of the importance thons of Cabo Ortegal, Órdenes and Malpica-Tui in

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

THE IBERIAN VARISCAN OROGEN 23 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 morphic evolution of the upper units suggests a vol- crust, and sometimes even of the underlying mantle. canic arc environment in the Early Ordovician (Abati et The basal units represent the most external margin of al., 1999; Andonaegui et al., 2002; Fernández-Suárez Gondwana, with magmatic evidence for Ordovician et al., 2002). It would be an arc developed along the rifting, and metamorphic evidence for early Variscan Gondwana margin, and its detachment would have subduction (Gil Ibarguchi and Ortega Gironés, 1985; left behind a backarc basin which in time would have Pin et al., 1992; Arenas et al., 1995; Rodríguez Aller, evolved to generate the Rheic Ocean. Figures 15 and 2005). Ordovician rifting continued with the separation 16 show schematically, in a map and a section, a pos- of a peri-Gondwanan terrain and the creation of oce- sible evolution of the plates and terrains involved in anic lithosphere beneath the Rheic Ocean between the Variscan collision, according to the interpretations by terrain and the new border of Gondwana. Fragments Gómez Barreiro et al. (2007) and Martínez Catalán et of oceanic lithosphere with different isotope ages are al. (2007), respectively. present in the suture included in the Galicia-Trás-os- Montes Zone, mostly linked to the Rheic ocean (Díaz The existence of only one oceanic domain between García et al., 1999; Sánchez Martínez et al., 2006, and Gondwana has been taken into con- 2007; Arenas et al., 2007). sideration, although the evolution was undoubtedly complex. The opening of this domain, called Rheic, was The evolution of the upper units indicates their multi- already taking place while the convergence between ple orogenic character, with a first deformation during Avalonia and Laurentia closed the lapetus ocean in the Lower Ordovician and later recycling by Variscan the Early Ordovician. This convergence meant the first collision. The associated chemistry and meta- Taconic orogenic event, 490-480 Ma ago, and ended

24 Martínez Catalán, J.R. – Aller, J - Alonso, J.L. and Bastida, F. with the amalgamation of Avalon 480-430 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 incorpora- tion onto the orogenic prism of a volcanic arc repre- sented 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 subduc- tion in the late Devonian marked the transition to a collisional regime. The subducted basal units were over- Figure 18. Northern end of the eclogite belt of the thrusted onto the external platform of Gondwana in Cabo Ortegal Complex at Punta dos Aguillóns. This is the Early Carboniferous, and a sheet of sediments was part of the allochthonous unit of La Capelada, wedged underneath forming the Schistose Domain. structurally the highest of the complex. Later, intracontinental deformation gradually affected more external areas of the orogen.

The fourth Geosite selected in the Iberian Massif is Figure 17. Geological map and cross section of the the Cabo de Ortegal Complex, in the province of Cabo de Ortegal complex based on Vogel (1967), La Coruña, as it is an excellent representative of the Marcos et al. (1984) and Arenas (1988).

THE IBERIAN VARISCAN OROGEN 25 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 Ortigueira and Espasante offer magnificent structural Zone. All the groups of allochthonous units are present sections, combined with the cliffs on both sides of the in this complex (Figure 17), and one of those units has cape (Figure 18) and with the outcrops of Sierra de la provided the oldest rocks in the Iberian Peninsula (1160 Capelada. Ma; Sánchez Martínez et al., 2006). In addition, this complex stands out for the quality of its exposures and The Cabo de Ortegal Complex includes a great expo- its spectacular landscapes. The estuaries of Cedeira, sure of two catazonal allochthonous units: a deeper

26 Martínez Catalán, J.R. – Aller, J - Alonso, J.L. and Bastida, F. one, representative of the lower crust and the upper The transpressive orogen in the southwest of the mantle of a , and/or a volcanic arc, corre- Iberian Peninsula, from the Central Iberian Zone to sponding to a peri-Gondwanan terrain (Galán and the South Portuguese Zone, displays peculiar features Marcos, 1997; Santos et al., 2002). These are the in relation to the deeply transpressive nature (oblique Cedeira and La Capelada units (Marcos et al., 1984, collision) under which it formed in the Late Paleozoic 2002), cropping out within the core of a SW-NE (Ribeiro et al., 1990; Quesada, 1991). The reconstruc- elongated synform with a size of 28 by 12 km. The tion of this sector is essential to understand the vast catazonal units are part of the upper part of the circum-Atlantic Paleozoic orogenic domain, presently allochthonous complexes, and consist of rocks which scattered in three continents (Europe, Africa and underwent high pressure and high temperature meta- America) with a among the three of morphism (Vogel, 1967; Mendía Aranguren, 2000). them located precisely in this Iberian southern sec- They include granulites and eclogites structurally tion (Figure 1). overlying ophiolitic units representing fragments of Paleozoic oceanic lithosphere (Figure 19), and base- The spectacular Iberian-Armorican Arch (arcuate struc- ment units representative of the Gondwana margin. ture) is a first-rate imbricate structure of geological These ophiolites and basement units crop out in an units arched or bent around a sub-vertical axis located imbricate zone, structurally emplaced at the base of to the east and with centripetal emplacement direc- the complex, and include an area of tectonic mixture tions. The most external zones of the Iberian foreland known as the Somozas Mélange (Arenas, 1988). The are located in the center of the arch (Cantabrian Zone), imbricate zone continues underneath, affecting the with well-preserved Carboniferous synorogenic depos- Schistose Domain, which is here formed by a series of its. To the west, a complete section of the Variscan sheets comprising metasedimentary and metavolcanic Orogen consists of a piling of E-to-NE-vergent thrusts rocks of Silurian and Ordovician age (Valverde-Vaquero including (from lower-eastern to upper-western), the et al., 2005). Cantabrian, West Asturian-Leonese, Central Iberian and Galicia-Trás-os-Montes zones. The first three The whole set of the complex is key to reconstruct represent progressively deeper units of the Paleozoic the evolution of the Rheic Ocean. It probably includes continental margin of the , whereas the the largest eclogite exposure in the , with high- fourth one comprises the allochthonous exotic com- pressure metamorphosed basic rocks indicative of plexes of Galicia-Trás-os-Montes. Towards the exterior subduction processes. Given the excellent exposure of the arch, there are two more areas, Ossa-Morena conditions of the lower crust, the processes of burial and South Portuguese Zones, where transpressive and exhumation in an orogenic cycle can be studied. deformation may provide some clues about the for- The Cabo Ortegal Complex is also a good example mation of the arch itself. of a deep allochthonous unit emplaced into an epi- zonal crustal domain. The structural sequence shows The models proposed to explain the Ibero-Armorican a spectacular development of structures at all scales, Arch are varied. One of them is that of strict orocli- from recumbent folds, ductile shears and thrusts nal bending, with an initial rectilinear belt which later (Figure 19), to microfolds, boudinage, porphyroclast became bent. A popular model is that of the indenter, systems, lineations and foliations. It constitutes a which suggests the presence of a rigid buttress out of natural laboratory for fabric deformation ranging Gondwana which might have caused the orogen bend from high pressure and temperature conditions to during its collision with Laurentia (Matte and Ribeiro, greenschist facies. 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 mecha- nisms in a collisional orogenic belt at different depths, particularly the large thrust at crustal and even litho- spheric scale. It also turns out to be a paramount refer- ence 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.

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PÉREZ-ESTAÚN, A., PULGAR, J.A., BANDA, SÁNCHEZ MARTÍNEZ, S., ARENAS, R., DÍAZ E., ALVAREZ-MARRÓN, J. and ESCI-N GARCÍA, F., MARTÍNEZ CATALÁN, J.R., Research Group. (1994). Crustal GÓMEZ-BARREIRO, J. and PEARCE, J.A. structure of the external Variscides in (2007). Careón Ophiolite, NW Spain:

30 MARTÍNEZMartínez CATALÁN, Catalán, J.R.J.R. -– ALLER,Aller, JJ -- Alonso,ALONSO, J.L. J.L. and - BASTIDA, Bastida, F.F