The Ansó transect of the southern Pyrenees: Basement and cover thrust geometries

A balanced geological section through the western central Pyrenees allows the Alpine geometry of Hercynian basement to be determined. This shows marked differences with other transects of the southern part of the chain. Basement-involved thrust sheets do not form an antiformal stack, but are imbricated and define steps separated by frontal culmination limbs. Basement thrusts propagate upwards and have formed distinct thrust systems in the cover. In this transect, the North Pyrenean Zone is bounded to the south by gently dipping thrusts of large displacement, and the Axial Zone massif, at its western termination area, is a flat-topped culmination caused by a single thrust fault, the Gavarnie thrust. This thrust emerges to the south in the interior of the syntectonic Jaca Basin, the geometry of which is also controlled by underlying basement thrusts. Various generations of structures can be correlated with distinct episodes of syntectonic sedimentation. A sequential restoration of the section illustrates how successive foreland sedimentary wedges were generated and progressively involved in deformation. Tectonic shortening is 48 km, almost half of that estimated further to the east for the southern Pyrenees. The structure deduced at the ECORS-Pyrenees seismic section cannot be translated to this transect.