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.

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.

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:

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.

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.