This paper contributes to the understanding of the offset on the 80 km long E-W striking Mérens fault in the central Pyrenees. Parameters such as amount and sense of offset are a prerequisite for a better understanding of the evolution of the fault, its tectonic significance and for reconstruction of the pre-existing configuration of its wall rocks.
In the western Aston massif, north of the Mérens fault, an over 5 km long, N125E striking steep dextral shear zone occurs. This shear zone is characterized by medium-grade assemblages of metamorphic minerals and is considered to be of late Variscan age. The presence of such a shear zone in this area was deduced from i) published structural data on the Mérens fault, and ii) its cut off relation with an over 40 km long late Variscan shear zone to the south. In this study it is shown that the shear zones north and south of the Mérens fault initially formed a single late Variscan shear zone, cut and displaced by the younger Mérens fault. The exact geographical location of the shear zone in the Aston massif constrains the finite dextral horizontal movement on the Mérens fault to at least 10,4 ± 0,5 km. Finite uplift of the Aston massif exceeds 4 km. Extrapolation of the arguments to the eastern Aston and Hospitalet massifs indicates that the NE terminations of both gneiss cores also lined up along one single steep (W)NW striking shear zone prior to Mérens fault activity. Dextral horizontal offset is 8,5 ± 1 km in that area. Restoration of the late Variscan configuration of the Aston and Hospitalet massifs reveals an elongate WNW trending mantled gneiss antiform, bordered by steep dextral shear zones with similar trend.