THE SIGNIFICANCE OF STEEP REGIONAL FAULT AND SHEAR ZONES FOR THE DEVELOPMENT OF GNEISS DOMES WITHIN THE VARISCAN CORE ZONE OF THE PYRENEES: STRIKE-SLIP OR REVERSE FAULTS

Large orogen-parallel fault zones are common structural features of the metamorphic Variscan core of the Pyrenees. Ductile shear zones and brittle faulting are evidence for deformation over a wide range of flow types. Unfortunately, the absence of marker horizons makes quantitative determination of offset nearly impossible, even distinction between predominantly lateral or vertical displacement is difficult. Their location within the central part of the Axial Zone and their commonly steep attitude suggest that these fault zones originated during the main Variscan deformation phase, with possible, albeit minor reactivation during the Alpine-age formation of the present-day Pyrenees mountain range. The existence of several fault zones in the Axial zone is most likely related to the development of paired gneiss domes.

A good example is the 70 km long Mérens shear and fault zone (MSFZ), which separates the Aston and Hospitalet domes, consisting of large orthogneiss cores mantled by Cambro-Ordovician mica schists. Core and mantle rocks experienced the same main Variscan deformation during a predominantly compressive regime. The strain observed within the MSFZ is directly related to competence contrast between the rocks involved. High strain mylonites are developed within phyllites and metacarbonates in a narrow zone where the two orthogneiss cores are spaced only 300 m apart. Outside the confinement of the orthogneiss cores, deformation is spread across several kilometres throughout the mica schist of the mantle, and local strain is minimal. The fault zone is reduced to several metre-wide deformation bands without significant offset. Local transition from mylonitic foliation to faulting, as well as similar stress fields inferred for ductile and brittle fabrics indicate a progression from ductile to brittle deformation during the same deformation phase. Shear sense indicators point to overall dextral reverse motion, the Aston dome has been thrusted over the Hospitalet dome, beginning as early as the late Visean, as new geochronological dating of nearby granites suggests. Vertical and strike-slip offset amounts at maximum to a few kilometres.