Heterogeneous deformation in the development of the Laxfordian complex of South Uist, Outer Hebrides

Four phases of heterogeneous deformation (f₁ to f₄) are distinguished within the Laxfordian episode of regeneration of the Lewisian gneiss complex of South Uist. The variation in amount of deformation has been examined with respect to three of the Laxfordian fold phases, especially f₃ and f₄, and apparent relationships between the strain heterogeneities and both the original nature of the rock and the syntectonic metamorphic conditions have been observed. The heterogeneities of the f₃ deformation appear to be related to the forms of the large scale f₃ structures that are characterized by broad open antiforms and tight synforms. These owe their form, in part, to the strains set up during the buckling of an interface between competent rocks below and less competent rocks above. Heterogeneities in the f₄ deformation are closely related to variations in the intensity of the syntectonic metamorphism; intense f₄ deformation is closely associated with migmatisation and granitisation. Possibly intense f₄ strain may result from localised variations in ductility caused by a local thermal high.