Complex fold patterns developed by progressive deformation

Abstract

Complex folding patterns include folds with curved hinges and/or curved axial surfaces. In simple shear zones, folds commonly nucleate with hinges at a high angle to the shear direction and progressively rotate towards parallelism with the shear/extension direction, giving rise to sheath folds. Axial surfaces also change in attitude with increasing strain, becoming parallel to the shear plane. However, when folds do nucleate with hinges along the extension direction but oblique to a shear component, they are not subjected to hinge rotation, but can experience a variable amount and sense of rotation of axial surfaces, developing into polyclinal folds. Foliated quartzite beds from the Cap de Creus massif in the eastern Pyrenees exhibit complex fold patterns produced during a single transpressional event. While folds in the embedding schists are harmonic and asymmetric, folds in the quartzite are strongly disharmonic and polyclinal, displaying large axial surface rotations. A plausible interpretation is that folds nucleate with hinges closely parallel to the extension direction, developing thickened short limbs that rotate synthetically with regard to the shear component. This may lead to strain partitioning, with localized antithetic rotations enhanced by progressive softening of the quartzite. Thus, fold interference patterns are not necessarily linked to polyphase tectonics but they may result from single-phase progressive deformation. In the here described cases, unfolding during progressive deformation is highly improbable.