Recycling of foliations during folding

Bedding on the limbs of early-formed regional folds generally lies oblique to the major directions of bulk shortening in the crust, that is, the horizontal and vertical. During subsequent deformations, matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering. Consequently, the schistosity parallel to bedding (S0//S1) in multiply deformed rocks contains the relics of many deformation events, and the two or three oblique foliations record only the very youngest history. This lengthy early history is preserved as inclusion trails within porphyroblasts. Recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to S0//S1. For some orientations and combinations of shear senses, both limbs of a pre-existing fold can be reactivated from the commencement of a new deformation event. This can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development. For other combinations, one limb of a pre-existing fold will behave in this manner whereas a new foliation does develop fully on the other limb. However, subsequent phases of deformation switch which limb shears vs. develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with S0//S1.