The influence of inherited extensional structures on the tectonic evolution of an intracratonic chain: the example of the Western Pyrenees

Abstract

A geotraverse, constructed on the basis of surface and subsurface data across the Basque-Béarnais portion of the Pyrenees, east of the Pamplona fault, reveals, from north to south, the existence of three major tectono-sedimentary units: the North Pyrenean Zone (NPZ), the Central Zone (CZ) and the Axial Zone (AZ). These zones are characterized both by the type and age of their sediments and volcanics, and by the style of their Cretaceous synsedimentary structures. Paleogeographically, they correspond to a Cretaceous platform (AZ), margin (CZ) and basin (NPZ), separated by zones of normal faults. During Alpine convergence, the Cretaceous fault system was to varying degrees reactivated in the reverse sense. The NPZ grades into the CZ without any significant hiatus. By contrast, the CZ is separated from the AZ by a major southward overthrust, the Orly-Lakhoura overthrust, which shows up as the most important fault in the cross-section. Its subsurface continuation is constrained by regional microseismicity and other seismic data. All these data lead us to consider the Orhy-Lakhoura overthrust as the surface evidence of the Alpine crustal underthrust of Iberia below the European continental domain. Restoration of the cross-section suggests that this Alpine underthrust follows a previous crustal discontinuity linked to the Cretaceous extensional episode.