The Lower Palaeozoic succession of the Welsh Basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism. Textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development, while mineralogical characters indicate a low pressure facies series. In order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism, in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting. Temperatures remained relatively high during final closure, associated with deformation of the basin sediments and consequent cleavage development. Such a model adequately resolves previously conflicting interpretations of burial- and deformation-related metamorphism.