Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning

Abstract Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally been insufficiently emphasized because evidence is best preserved where porphyroblasts which contain inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse- and finescale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations, generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of these other minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes rotation or ‘refraction’of axial-plane foliations (forming in the same deformation event causing reactivation) in those beds or zones in which an earlier foliation has been reactivated, and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently ‘wrong sense’, but normally observed ‘rotation’of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and therefore can be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation often reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold, leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated foliation can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts. Geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events. Other aspects of reactivation, however, can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized; for example, D₂-grown porphyroblasts may be dissolved against reactivated S₁ and hence appear to have grown syn-D₁.