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

The Gresigne-Quercy region is a regional-scale wedge-shaped block bounded b y major conjugate strike-slip faults, where the regional shortening directi on bisects the acute angle of the conjugate sets. This block is composed of a Late-Carboniferous, Permian and Mesozoic cover overlying a metasedimenta ry basement. This region is extensively deformed, whereas the adjacent area s which are comprised of a more rigid granitoid-rich basement are undeforme d 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 Gresi gne-Quercy block are inferred on the basis of structural characteristics. T hese structural characteristics are evaluated in terms of the geometry, kin ematics, rheology, strain conditions and stress trajectories of a Prandtl-N adai cell. This comparison shows good agreement between the natural example and the theoretical model, except that the 'plates' constituted by the 'ri gid' basement outside the cell-bounding faults are immovable, whereas in th e theoretical model they move apart. This difference can explain why the fo ld and thrust system developed in the apical zone. This fold and thrust sys tem propagated backwards, from the apex where the material was first locked , to the wide end of the wedge-shaped block. (C) 1999 Elsevier Science Ltd. All rights reserved.