Evolution of the Central Pyrenean Mérens fault controlled by near collision of two gneiss domes

Abstract

The Mérens shear and fault zone (MSFZ) is a major orogen-parallel structure within the Variscan crystalline core of the Central Pyrenees, passing through rocks of various strength along its 70 km length. The nature of the MSFZ is strongly dependent on the competence contrast of the rock assemblages transected by it, and the presence of two large gneiss domes, the Aston and Hospitalet. A high-strain mylonite zone is developed in narrow subvertical bands of weak metasedimentary rocks where the spacing of the orthogneiss cores of the domes is minimal (ca. 300 m). In a transition zone west of the orthogneisses, the MSFZ grades into a northerly dipping reverse fault, separating amphibolite-facies mica schist with mylonitic bands to the north from phyllites displaying brittle-ductile fabrics to the south. Sense of shear along the ductile shear zone and the brittle fault indicates an overall dextral reverse motion, thrusting of the Aston dome over the Hospitalet dome to the south. Local shear directions respond to strain partitioning and are in accordance to an overall dextral transpressive regime. Near its western termination, local strain is minimal, deformation spread across several kilometres resulted in south-verging folds. Observed fabrics indicate a progression from ductile to brittle deformation, rather than two separate deformation events resulting in ductile shearing followed by brittle faulting. The development of the high-strain shear and fault zone is the result of the near collision of two large gneiss domes.