Geology Of The Pyrenees

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 Iberian microcontinent and the southwestern promontory of the European plate (i.e. Southern France). The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 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.

Geographic arrangement

The Pyrenees ''sensu stricto'' stretch in a west-northwest-east-southeast-direction (N 110) over 430 km from the Bay of Biscay in the west to the Golfe du Lion and the Golf de Roses in the east, their width across strike varying between 65 and 150 km. They are bounded in the north by the North Pyrenean Front ( French: ''Front nord-pyrénéen'', also ''North Pyrenean frontal fault'' or ''NPFF''), a major thrust fault along which units from the North Pyrenean Zone have been transported over the Subpyrenean Zone, southernmost part of the Aquitaine Basin, their northern foreland. Their southern limit is the South Pyrenean Frontal Fault. Here, thrust slices from the Sierras Marginales and their lateral equivalents are displaced southward over the Ebro Basin.

Yet in a larger, geologically more meaningful sense the Pyrenees continue farther west into the Basque and the Cantabrian mountains (the ''Basque-Cantabrian chain''). They finally disappear along the continental margin of Asturias. Likewise in the east, they do not just vanish in the Mediterranean but rather pursue their course via the nappe units of the Corbières Massif into Bas Languedoc and even into southern Provence. At their far eastern end in Provence, typical Pyrenean fold trends are superimposed by Alpine structures to be finally cut off by the arc of the Western Alps. The Pyrenean chain in the larger sense is nearly 1000 km long.

Structural organisation of the orogen

A profile through the Pyrenees ''sensu stricto'' shows a fan-like, flower-like arrangement. The structure is strongly asymmetric with a steeper and narrower French northern side and a much wider and more gently inclined Spanish southern side.

The double-sided orogen can be divided into several tectonic zones, from north to south, that are bounded by east–west-trending major faults:
* Northern foreland – Aquitaine Basin
* Subpyrenean zone or Subpyrenean Basin
* North Pyrenean zone
* Axial zone
* South Pyrenean zone
* Sierras Marginales
* Southern foreland – Ebro Basin
Along strike, the pyrenean orogen can be split into three distinct domains: an eastern domain reaching from the Mediterranean to the Segre River, a central domain extending from the Segre River to the Pamplona Fault, and a western domain beyond the Pamplona Fault.

Northern foreland

Subpyrenean Zone

The ''Subpyrenean Zone'' is geologically part of the Aquitaine Basin, the northern foreland of the Pyrenees, and was caught up in the Pyrenean orogeny. The zone was folded during the Eocene and overthrust ''en echelon'' by the North Pyrenean Zone along the North Pyrenean Front. These upthrusts change their character in the west and in the east of the orogen, where they become nappe-like, examples being the ''Bas Adour Nappe'' in the west and the ''Corbières Nappe'' in the east. The latter continues farther east via folds and tectonic slices near Saint-Chinian, via the fold near Montpellier to join the ''South Provence Thrust'' near Sainte-Baume, which gradually disappears south of Brignoles.

Within the Pyrenees ''sensu stricto'', the Subpyrenean Zone consists of Upper Cretaceous and very thick Paleogene sediments in surface outcrops. The sediments show simple folds following a WNW-ESE trend.

The subsurface, however, has a far more complicated structure due to Triassic salt diapirs and north-vergent thrusts. Hidden below a more than 6000 metre thick Mesozoic cover are probably more than 6000 m of Paleozoic basement rocks. The Mesozoic cover consists of up to 1500 m of Triassic, well over 500 m of Jurassic and more than 3000 m of Cretaceous sediments.

The up to 500 m thick layer of Lower Triassic (Buntsandstein) comprises conglomerates, breccia, brown sandstones, argillites, shales, and siltstones. The Middle Triassic (Muschelkalk) can attain a thickness of 400 m and shows silty shales, evaporite deposits, and dolomitic micrites. The up to 500 m thick Upper Triassic Keuper deposits are made up of carbonate-rich sediments, salt, siltstones, and intercalated ophitic diabases/olivine dolerites. The lower Lias is a transgressive sequence with up to 200 m of non-marine sandstone, near-shore marine limestone and evaporites. A pelagic fauna at the top suggests open marine conditions. The middle and upper Lias consist of 230 m of shallow marine platform sediments (bioclastic limestone, argillaceous limestone, and micritic limestone). During the Middle Jurassic, an oolitic barrier, made up mostly of argillaceous micrites, separates an outer shelf from an inner shelf. The Upper Jurassic (Malm) deposits are mainly shales and carbonates. Near the end of the Jurassic, restricted environments were established with dolomicrites, banded limestones, and evaporites. The Lower Cretaceous layer starts with sandstones, shales, limestones, and calcareous breccia in the Neocomian, followed by Barremian marls and limestones. During the lower Aptian, sandstones, shales, sandy marls, and limestones were laid down. The upper Aptian and the Albian are mainly marls and limestones. The Upper Cretaceous includes a littoral Turonian with sandstones and sandy limestones. By the beginning of the Senonian (Campanian), a deep trough had formed (the Subpyrenean Basin) receiving a very thick flysch sequence. The Campanian and Maastrichtian flysches comprise 2000 to 3000 m of periodically interlayered fines (marls, calcareous shales, and mudstones) and coarser sediments (conglomerates, sandstones, and greywackes). Near the K/T boundary, the Subpyrenean Basin was filled with continental red deposits in Garumnian facies even including dinosaur eggs in a few places. At this point, the Subpyrenean Basin underwent folding accompanied by a weak metamorphism.

Above the Albian and before the onset of the Campanian, volcanic rocks occur including basaltic lavas, spilite, and diabase, but also pyroclastic rocks like tuff, lapilli tuff, volcanic breccia, and agglomerate. The volcanic rocks can be cross-cut by lamprophyre dikes.

In Paleocene/Eocene times, the sea transgressed from the Atlantic into the Subpyrenean Basin which behaved as a downwarp to the slowly rising Pyrenees immediately to the south. A very thick (2000 to 3000 m) succession of fine-grained detrital or calcareous sediments was deposited. The sedimentation stopped in the late Eocene due to major compression (Pyrenean main phase).

In the vicinity of the Muret Fault, a left-lateral strike-slip fault and a prolongation of the Toulouse Fault to the south, the Subpyrenean Zone can be divided into two unequal halves. The eastern half between the rivers Garonne and Aude can be separated into three different zones (from north to south):
* a northern foreland.
* a 10 km wide folded zone. Its northern boundary are the ranges of the Petits Pyrénées, which are above a blind thrust. This zone narrows to the east and disappears before reaching the Aude. The sediments comprise a gypsum–bearing Triassic at the bottom followed by an internally thrusted Jurassic and a very thick cover sequence of Upper Cretaceous flysch sediments.
* a narrow band of flysch in the south. This fairly thick flysch sequence was also deposited in the Upper Cretaceous. It was upturned into a nearly upright position by thrusting movements on the North Pyrenean Front and now forms the overturned southern flank of an asymmetric syncline.

In the western half, only the northern foreland is present; it is made up of gently folded, but strongly jointed, epicontinental Mesozoic sediments covered and hidden by Miocene molasse sediments. The east–west and northwest–southeast-striking fold sets interfere and are cut by northeast-trending faults. In the subsurface, Triassic salt diapirs are also present.

Within the northern foreland east of the river Aude, the Paleozoic basement uplift of the Mouthoumet appears, a horst tilted to the south and covered by continental Eocene strata.

The fold trains of the Subpyrenean Zone are disrupted in the Bas Languedoc by the Cevennes Fault, a major left-lateral strike-slip fault.

North Pyrenean Zone

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 from the upper mantle along deep-reaching faults. The lherzolites are associated with amphibolites, pyroxenites, and amphibole-bearing peridotites. All these mantle rocks are arranged in swarms, the biggest outcrop at Moncaup reaching a mere 3 km². They are widely distributed, being found from Béarn all the way to Aude. Their emplacement mode has not been clarified yet, but the following factors are relevant:
* associated Jurassic and lower Cretaceous marbles of the metamorphic band.
* granulites of the basement uplifts in the vicinity.
* migmatitic kinzigites.
* the close spatial association with the North Pyrenean Fault a bit farther south.
* lherzolite sedimentary clasts occur in marbles of the metamorphic strip, so the lherzolites must be older than the metamorphism.
Scattered within the North Pyrenean Zone are also some occurrences of volcanic rocks. They are intercalated in sediments of the Lias and the Upper Cretaceous (Aptian till Campanian) and are found mainly in the west (near Tarbes, Orthez, and in the Basque country). They consist of silica undersaturated spilites, picrites, and nepheline syenites. Associated dike rocks are lamprophyres ( camptonites and monchiquites).

Other features of interest are several different post-metamorphic breccia formations.

The North Pyrenean Zone can be subdivided into three subzones bounded by major faults:
* a northern subzone. Its sedimentary cover has detached from the basement uplifts farther south. It contains flysch from the Upper Cretaceous.
* an intermediate subzone. Here the basement uplifts crop out.
* a southern subzone. It was affected by metamorphism and contains outcrops of ultramafic rocks.
The North Pyrenean Zone is traversed in the west by NNE-SSW-trending, left-lateral strike-slip faults and then changes into the fold-belt of the Basque country. In the east, it continues after a sharp bend in the Corbières right into southern Provence. At the far eastern end, northwest–southeast-striking Miocene fold trains of the Western Alps start interfering and finally completely overwhelm the pyrenean structures.

Axial zone

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

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