Growth and propagation of buckle folds determined from syntectonic sediments (the Ubierna Fold Belt, Cantabrian Mountains, N Spain)

The geometry of syntectonic Tertiary sediments in the Ubierna Fold Belt can be explained using the buckling folding theory. The geometric features of the fold train in the pre-orogenic succession also agree with a buckling process. Each fold grew with progressive and coeval rotation of both limbs, although rotation was asymmetric and greater on the forelimb than the backlimb. The diachroneity of synrotational sequences (progressive unconformities) through different folds indicates that fold propagation migrated towards the foreland; however, the initial folds were tightened during a later deformation stage synchronous with the development of younger folds.