page_title: Geology of the Pyrenees
section_title: North Pyrenean Zone
hierarchical_section_title: Geology of the Pyrenees / Structural organisation of the orogen / North Pyrenean Zone
caption_attribution_description: English: Lherzolite Français&#160;: Bloc de lherzolite, roche magmatique isue du manteau. Son nom vient du Massif de Lers (ou Lherz), dans les Pyrénées ariégeoises, où elle fut découverte à la fin du XVIIIe siècle.
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 North Pyrenean Zone is quite narrow, usually only about 10 km wide, but can widen to 40 km. It is characterised by very strong folding. The zone is thrust to the north along the North Pyrenean Front—its northern limit—over the Subpyrenean Zone. This thrusting motion compressed the overthrust foreland and as a result induced folding in the Subpyrenean Zone. The North Pyrenean Zone is itself overthrust by the Axial Zone along the North Pyrenean Fault (NPF), a high-angle reverse-fault forming its southern boundary. The North Pyrenean Fault is marked by highly strained mylonites. The rocks in the vicinity bear horizontal lineations underlining the importance of the fault as a major shear zone. Elsewhere in the North Pyrenean Zone, the strain gradient is also high but the stretching direction is generally vertical.
  The more than 6000 m thick sedimentary package of the North Pyrenean Zone is formed by Mesozoic (Jurassic and Cretaceous) rocks that have been detached above Upper Triassic evaporites and subsequently slid to the north. In contrast with the Subpyrenean Zone, the North Pyrenean Zone contains hardly any Paleogene. Upper Triassic (Keuper) shale and evaporite deposits locally contain interbedded dolomites, tuffs, and diabase (ophites); these deposits behave plastically and commonly form a tectonic mélange with contacts being expressed as decollement surfaces. From the beginning of the Jurassic till the end of the Lower Cretaceous, a shallow-water carbonate platform developed during tectonic quiescence with mainly limestones being sedimented. The Middle Albian witnessed a major facies change to deep marine conditions. This changeover marks the inception of the North Pyrenean Basin, a 400 km long trough of pull-apart origin filled with unconformable, turbiditic flysch sediments during the Upper Cretaceous. By Upper Albian times, this pull-apart basin had split into an internal trough next to the North Pyrenean fault which accommodated the Flysch ardoisier and an external trough farther north filled by the Flysch noir. Later, during the Turonian and the Coniacian, the external flysch trough received the so-called Flysch à fucoides, a very thick succession of interbedded calcareous mudstones/marlstones and sandy calcarenites. This flysch is followed by a regressive series in the Maastrichtian—thick marls (Marnes de Plagne)—platform limestones (Calcaires nankins), as well as lagoonal and lacustrine deposits. Altogether the Coniacian-Maastrichtian series reaches a thickness of 3000 m.
  The Paleozoic basement pierces the sedimentary cover in several almond-shaped, horst-like uplifts, their size ranging from 1 to 300 km². Examples are the so-called massifs satellites nord-pyrénéens (north pyrenean basement uplifts) between Lourdes and Perpignan, amongst them the following uplifts: Agly, Arize, Barousse, Bessède-de-Sault, Castillon, Milhas, Plantach, Saint-Barthélémy, Salvezines, and Rabat-les-Trois-Seigneurs, plus several uplifts in the northern Basque country. These uplifts have a left-lateral shearing origin and are tilted to the north; simultaneously they also exhibit a vertical shearing component. They probably formed in the Variscan orogeny. In the basement uplifts, mainly Precambrian gneisses and granulitic gneisses (in the Agly massif), and Paleozoic igneous and metamorphic rocks are found.
  A small, maximally 5 km wide strip just north of the North Pyrenean Fault experienced dynamic and thermal metamorphism during the Albian/Cenomanian about 110 million years ago (high temperature/low pressure, "HT/LP"-type). Some domains north of the basement uplifts were also metamorphosed (e.g. in the Bigorre and in the southern Corbières). The metamorphism was isochemical without introduction of extraneous elements and affected only the sedimentary cover rocks which were transformed into marble and hornfels. The Paleozoic basement was not affected, probably due to its already dehydrated state.
  Scattered within the metamorphic strip are several occurrences of lherzolites (including their type locality at Lers). They were extruded 
caption: Lherzolite from the North Pyrenean Zone, L'Étang de Lers, Ariège
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