The Iberian Plate: myth or reality?

Abstract

The plate tectonics theory generally leads us to consider that Iberia was an independent plate separated from Europe by the North Pyrenean Fault (NPF). The NPF has been commonly interpreted as a transform fault associated with a huge counterclockwise transverse and rotational movement that allowed the opening of the Bay of Biscay and the relative eastward motion of Iberia during the Mesozoic. According to some interpretations, this movement may have generated an interplate gap several hundreds of km wide, which led to the creation of an oceanic crust during the Late Jurassic and Early Cretaceous. However, field studies recently carried out in the Pyrenees do not support these interpretations. The North Pyrenean Fault (NPF) of Tertiary age is observed in the central and eastern Pyrenees, where pioneering researchers defined it as separating the North Pyrenean Zone from the Axial Zone. However, this fault cannot be identified in the western part of the range to the west of the Ossau valley. Consequently, the geodynamic evolution of Iberia has always been dependent on Europe, especially during the failed oceanic rifting in the Mid-Cretaceous. Indeed, during this period, a central zone of crustal thinning occupied by turbiditic basins separated the European from the Iberian continental crust, with a very localized mantle exhumation found only in the Mauleon basin. Therefore, far from being an interplate range, the Pyrenees can neither be considered as an intraplate unit. We can define this orogenic belt as resulting from the “Tertiary tectonic inversion of a Mid-Cretaceous rift system”. According to this new interpretation, Iberia would not have been an isolated plate but represented an unstable, outlying part of Europe. Rather than displaying the features of a rigid lithospheric unit with well-defined boundaries, Iberia grouped together different crustal blocks undergoing specific movements at particular times. During the Mesozoic, normal, reverse or strike-slip displacements along their boundary faults generated several en échelon basins (Bilbao, Logroño, Soria and Maestrazgo in Spain; Parentis, Arzacq and the North Pyrenean Flysch Trough in France) whose diachronous development accounts very well for the opening of the Bay of Biscay and the relative W-E sinistral movement of the Iberian crust with respect to the European crust. In this way, we note a more marked paleogeographic shift in the western Basque region than in eastern Catalonia. Therefore, the Pyrenees were not generated by an “interplate collision”, but merely reflect the confrontation of two distended but continuous continental domains during the Tertiary, leading to the incipient underthrusting of the Iberian crust under the European crust. This zone of confrontation/convergence does not correspond in any way to the NPF, but rather comprises a complex imbricated structure revealed by a jump in the Moho, both in the Pyrenees and in the Cantabrian belt, which has recently become known as the “North Iberian Fault”.