page_title: Geology of the Pyrenees
section_title: Axial Zone
hierarchical_section_title: Geology of the Pyrenees / Structural organisation of the orogen / Axial Zone
caption_attribution_description: Glaciar de la Maladeta
context_page_description: The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous about 100 million years ago and were consequently colliding during the Paleogene 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.
context_section_description: The Axial Zone, also called Primary Axial Zone, is a huge basement dome of Precambrian and Paleozoic (Primary) rocks folded and metamorphosed during the Variscan orogeny and intruded by late-stage Variscan granitoids. All the highest peaks of the Pyrenees are in the Axial Zone, hence the name.
  Amongst the Variscan granitoids are biotite granites (Canigou, Quérigut Massif), two-mica granites (Caillaouas Massif) and granodiorites (Bassiès, Maladeta). The granitoids are mainly shallow epizonal intrusives, but mesozonal and catazonal rocks are also represented.
  The high elevations of the Axial Zone (generally above 3000 m) are compensated isostatically by an increased thickness of the continental crust. For instance below the Maladeta massif, a root zone formed so that the Mohorovicic discontinuity is encountered there at a depth of 50 km. Likewise over most peaks of the Axial Zone, a negative gravity anomaly can be detected which slowly disappears to the east.
  The basement is traversed by major east–west-striking, late Variscan fracture zones that were reactivated during the alpine orogeny cycle. In the eastern part of the Axial Zone, the fractures are generally upright, a good example being the mylonitic Merens Fault at Pic del Port Vell near Mérens-les-Vals. In the western part, the fractures are more gently dipping to the north and behave as en echelon thrusts arranged in a northwest-southeasterly fashion; along these fractures, the basement of the Axial Zone overthrusts Mesozoic sedimentary units to the south. Good examples are the en echelon thrusts at Eaux Chaudes, Gavarnie and Bénasque—Las Nogueras (referring to the upper reaches of the rivers Noguera Ribagorzana and Noguera Pallaresa). Concomitant with the thrusts, a schistosity developed that affected the basement as well as the sedimentary cover implying an alpine origin. All these fractures account for an overall compression of the Axial Zone by 20% which translates as roughly 10 to 20 km of crustal shortening. As a result, the Axial Zone was squeezed into a south-directed antiformal stack.
  The Axial Zone disappears in the Haut Béarn as a pericline underneath the Upper Cretaceous sedimentary cover only to reappear in the basement uplifts of Aldudés-Quinto Réal, the southernmost of the Basque basement massifs. In the east the Axial Zone becomes downfaulted into Neogene and Quaternary grabens of Northern Catalonia and finally disappears underneath the Mediterranean.
  The central and eastern section of the Axial Zone is bounded in the north by the North Pyrenean Fault, a system of N 110-striking, steeply dipping reverse-faults. The trace of the North Pyrenean Fault becomes more and more diffuse west of Lourdes; near the Basque basement massifs, it seems to be displaced to the south by a wrench fault and then possibly continues into Spain south of the Basque Marble Nappe and south of the Basque Fold Belt. In Cantabria, it finally reaches the Atlantic coast. The southern limit of the Axial Zone runs completely on Spanish territory. It is represented by an alpine reverse-fault along which the sediments of the South Pyrenean Zone are overthrust by the Axial Zone. In the east, the Axial Zone abuts directly against nappes of eastern representatives of the Sierras Marginales.
caption: Maladeta, a granodiorite massif in the Axial Zone, with glacier and Paleozoic cover sediments (front right)
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