Kilometre-scale sheath folds in the western Betics (south of Spain)
The three-dimensional exposures of the Sierra Blanca marbles and neighbouring rocks in the western Betic cordillera of the south of Spain show tight to isoclinal F3 folds re-folded by a larger sheath-like structure. The relationships between both structures are as follows: (1) The mean axial surface for Sierra Blanca sheath fold can be defined as the great circle of best fit through the ß-axes of sub-cylindrical F3 folding domains. (2) The asymmetric F3-folds indicate a single mainly eastward normal sense of shear. (3) The mean attitude of the axial-plane crenulation foliation, Sc, is also sub-parallel to the mean axial surface for Sierra Blanca sheath fold. The regional context of the Sierra Blanca sheath fold is finally discussed, and a model of a heterogeneous high-strain extensional zone proposed.