Rotation des nappes? — reconsideration de fa configuration des bassins tertiaires sudpyrénéens

Concepts of the Alpine structural evolution of the South Pyrenean thrust zone largely rely upon balancing methods applied to cross-sections at right angles to the strike of the orogen. One of the oldest thrust units involved, the Central South Pyrenean (abbreviated: CSP) thrust sheet complex, has a plan geometry of an inverted trapezium with an almost N-S axis of symmetry. Investigations along the western ramp of the sheet and in its nappe-top basin (Tremp-Ager Basin), combined with published data on the frontal ranges and the eastern flank, reveal however a marked asymmetry. Anticlockwise rotational lateral constriction of the Ainsa Basin and a transpressional, left-stepping tear fault system define the western ramp structure until the Middle Eocene. Dextral wrench faults obliquely dissect the frontal thrust folds after the Priabonian (Mediano High, and the Millà Fault). The eastern sector of the sheet is subdivided by a system of thrust faults(Boixols, Montsec thrusts) fanning from the eastern lateral ramp structures in the Segre Valley. Parallelism of faults and folds gives it an appearance, quite different from that of the opposite lateral ramp.In the nappe-top basin synsedimentary folds and extensional fault bundles diagonally cross the basin axis, while a transversely developing SE-verging hinge structure defines facies patterns during the Late Eocene. All these observations tally with an anticlockwise rotational emplacement of the CSP thrust sheet. From several other areas in the South Pyrenees rotations have been reported on the basis of palaeomagnetic data. Palinspastic basin reconstruction accounting for rotations differs fundamentally from the generally proposed strikeparallel marginal of foreland basin setting. The proposed model shows an en-echelonarrangement of the South Pyrenean basins with axes plunging to the NW. It provides anobvious explanation for basin misfit between the Catalonian and central segments of the South Pyrenean basin, already noted by Roseli and Robles in 1975, but further enhanced by recent restoration of cross-sectionsonboth sides of the Segre Valley thrust zone.It leads to a logic pattern of sandstone distribution and tidal currents for the LowerEocene Ager Group, and explains the Middle Eocene starvation of the Ripoll Basin during full development of the Ainsa-Jaca Basin further to the west, by progressive rotational closure.If anticlockwise rotationof thrust units occurred, this mere fact considerably hampers the application of balancing methods. And also, structural conceptsthat do not account for the observed Tertiary basin misfit have to be looked at with scepticism.