Application of the Prandtl–Nadai cell model to a regional scale fault intersection: the Grésigne–Quercy block (SW France)

Abstract

The Grésigne–Quercy region is a regional-scale wedge-shaped block bounded by major conjugate strike-slip faults, where the regional shortening direction bisects the acute angle of the conjugate sets. This block is composed of a Late-Carboniferous, Permian and Mesozoic cover overlying a metasedimentary basement. This region is extensively deformed, whereas the adjacent areas which are comprised of a more rigid granitoid-rich basement are undeformed or slightly deformed. The region is also characterized by the development of a fold and thrust system near the apex and strike-slip conjugate faults in the wide end. The displacement, strain and stress patterns of the Grésigne–Quercy block are inferred on the basis of structural characteristics. These structural characteristics are evaluated in terms of the geometry, kinematics, rheology, strain conditions and stress trajectories of a Prandtl–Nadai cell. This comparison shows good agreement between the natural example and the theoretical model, except that the `plates' constituted by the `rigid' basement outside the cell-bounding faults are immovable, whereas in the theoretical model they move apart. This difference can explain why the fold and thrust system developed in the apical zone. This fold and thrust system propagated backwards, from the apex where the material was first locked, to the wide end of the wedge-shaped block.