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 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 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 Europe. 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 orogeny 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 North America (then part of Laurentia) went modified the continental crust of western Europe and around the Variscan orogen and a wide belt developed the north and northwest of Africa during the Late along its eastern margin: the Appalachians. Paleozoic (Devonian-Permian). 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 France and remobilized the prior basement on which they laid, the south of Great Britain 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 oceanic crust 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), Baltica nental collision (Variscan Orogen). (Scandinavia, northern belt of Europe and Russia), and Gondwana (Africa, South America, Antarctica, India What crops out in the Iberian Peninsula from that and Australia). 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 supercontinent, in this case the last Pangea generated Central Europe into Poland, where it is shifted by a by the continuing geodynamics of our planet. tectonic line known as Thornqvist Fault, and continu- ing towards south Asia. 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 Eurasia in the first fragmentation (Middle Jurassic), but was later iso- The Galicia-Trás-os-Montes Zone of Farias et al. (1987) lated in the Early Cretaceous with the opening of the has also been added, although it is not a belt of the Bay of Biscay 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 Eocene. 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 Pyrenees, 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 Cambrian 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- 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.