• Structure and modelling of an orogen
• Ainsa Basin turbidite systems and 3D structure
• Tectonic inversion in the South-Central Pyrenees
• Graus-Tremp Basin: From inversion tectonics to thrust tectonics
• Pobla de Segur Conglomerates: Temporal constraints on the late evolution of the Pyrenees

   Josep Anton Muñoz

The Pyrenees extend for some 1500 km from the eastern Alps, along the Mediterranean coast, to the Atlantic ocean northwest of the Iberian Peninsula. The Pyrenees resulted from the Mesozoic-Cenozoic interaction between the Afro-Iberian and European plates. Plate kinematics are related with the opening of the Atlantic which progressed from north to south.

Figure 2. Palinspastic reconstruction of the Iberian plate motion from Jurassic to Oligocene times. Click here for a larger image.

The Pyrenees is a mountain range of tectonic inversion which is superimposed on Triassic-Cretaceous extensional to transtensional rift systems. These are associated with the fragmentation of southern Hercynian Europe and western Tethys as a result of the break-up of Pangea, as well as the opening of the Central Atlantic Ocean and the Bay of Biscay, and the resulting rotation of Iberia.

Convergence occurred from Late Santonian to Middle Miocene time as the Afro-Iberian plates moved generally northward against Europe. As a result, the earlier extensional structures were inverted, then incorporated into the thrust system.

The Pyrenees display different characteristics along strike. In the east, the Pyrenees were overprinted by the Neogene extensional features related with the opening of the Gulf of Lions and the drift of the Corso-Sardinian block. The main part of the range between France and Spain corresponds with a continental collisional orogen. Here, the orogen developed over a previously thinned continental crust but without intervening oceanic crust between the two plates.

Further to the west, the oceanic crust of the Gulf of Biscay is involved in the Pyrenean orogen but, unlike most oceanic lithosphere, was only moderately subducted. Instead, deformation was mainly concentrated in the previously thinned continental crust south of the Gulf of Biscay oceanic zone. The northern front of the chain is submerged in the Bay of Biscay along the Cantabrian or North Iberian Margin.

The crustal and lithospheric structure of the Pyrenees has been deduced by different geophysical techniques (deep reflection and refraction seismic profiles, gravity, magnetotellurics, magnetic anomalies, tomography, heat flow. The data that best constrain the Pyrenean crustal structure are from the deep seismic reflection profiles, mainly the ECORS-Pyrenees profile. This profile has been interpreted to show the subduction of the Iberian plate below the European one and has been the basis for the construction of crustal balanced cross-sections. Magnetotelluric information reveals the subduction of the lower crust and its possible partial melting present state.

The central Pyrenees have the characteristics of an asymmetrical tectonic double-wedge of upper crustal rocks. The southern wedge consists of an imbricate stack involving cover rocks and an antiformal stack of basement rocks. Both are southward directed. The northern wedge is formed by a northward directed imbricate stack involving basement and cover rocks.

The southern wedge is much wider than the northern one. Correspondingly, displacement and cumulative shortening is also much greater in the southern wedge. Thrust transport direction was constantly N-S to NNE-SSW through most of the tectonic evolution.

The basement antiformal stack of the southern wedge is bounded to the north by the North Pyrenean fault. This is a major strike-slip fault which developed during the sinistral displacement of Iberia during the Middle Cretaceous.

Lower crustal granulitic rocks as well as ultrabasic upper mantle rocks (lherzolites) are observed embedded between the Early Mesozoic metamorphic rocks along a narrow strip parallel to the North Pyrenean fault. These rocks were carried to upper crustal levels during the strike-slip faulting. Apart from this narrow strip parallel to the North Pyrenean fault neither post-Hercynian metamorphic rocks nor lower crustal rocks are observed at surface in the Pyrenees.

Several different interpretations of the crustal structure of the Pyrenees have been given on the basis of the combined geological and geophysical data. We believe that an explanation in which the orogenic double-wedge involves only upper crustal rocks provides the best geometry in which to integrate all these data. Apparently, the crust was decoupled and the lower crust, below the upper crustal double-wedge, was subducted together with the lithospheric mantle into the mantle. This inferred crustal subduction is compatible with other geophysical data as well as the absence of post-Hercynian metamorphic rocks or lower crustal rocks in the Pyrenean orogenic double wedge.

Balanced and restored cross-sections were constructed not only to integrate geophysical and geological data but also to estimate the amount of orogenic contraction. 165 km.

These shortening calculations are compatible with the estimated separation of the Iberian and European plates as deduced by reconstruction of the past motion of Iberia after paleomagnetic data.

The estimated duration of convergence in the Central Pyrenees is about 60 Ma , which gives a mean shortening rate of 2,5 mm/yr.

The restored cross-section gives an estimate of the geometry of the crust before the Pyrenean collision. The restored cross-section not only shows the geometry of the Early Cretaceous extensional fault system and the North Pyrenean fault but also the geometry of the Hercynian structures (foliation, thrusts and late Hercynian extensional faults). All these structures display a listric geometry above the lower layered crust with most of the discontinuities dipping to the north. The geometry of the Hercynian structures, without the superimposed effects of the Pyrenean collision can also be observed in the crust below the Aquitaine foreland along the ECORS profile. The geometry of the inherited structures at the onset of contraction all of them flattening downwards over the lower layered crust, favoured decoupling of the crust and played a major roll in orogenic development.

Numerical models were designed to reproduce these inherited structures by including weak zones. All the laterally homogeneous models that have been run have given unacceptable results, both with regard to the mass balance and the asymmetry of the orogen. Models require the existence of weak zones as well as an asymmetric distribution of them to reproduce both the final geometry of the orogen and its intermediate stages. Thus, modelling corroborates the idea that the tectonic style of the Pyrenees is strongly controlled by the inversion of previous extensional features, which can also be inferred from the structural pattern along the strike of the chain.

Figure 6. Intermediate and final stages of a numerical model simulating continental collision in the Pyrenees (Beaumont et al, 2000). Click here for a larger image.

• The Pyrenees
• Ainsa Basin turbidite systems and 3D structure
• Tectonic inversion in the South-Central Pyrenees
• Graus-Tremp Basin: From inversion tectonics to thrust tectonics
• Pobla de Segur Conglomerates: Temporal constraints on the late evolution of the Pyrenees