The Lower Paleozoic Welsh Basin of the U.K. Caledonides shows a metamorphic transition from zeolite to low-greenschist facies for which there are two radically opposing models of development. The traditional model is of a syntectonic metamorphism associated with crustal thickening during basin inversion and cleavage development. The alternative model links the metamorphism not to traditional ideas of crustal thickening but to thermal anomalies associated with the extensional development of a back arc basin. These two models are tested by applying quantitative thermal modeling to crustal thickening and crustal extension tectonic settings specific to the Welsh Basin. This basin has a unique and very well constrained record of its stratigraphic, sedimentological, and chronologic histories that is used here to provide the framework from which the dynamics of crustal evolution can be modeled. Crustal thickening models suggest that a higher-pressure facies series with a pumpellyite-actinolite to greenschist facies transition should be dominant. In contrast, extensional modeling suggests that a low-pressure facies with a transition from prehnite-pumpellyite to greenschist facies should be dominant. The thermal evolution of the extensional setting is more compatible with the lower-pressure metamorphic series that is recorded in metabasites as well as pelitic rocks of the region. The metamorphic evolution is regarded as developing initially as a burial style, which is then over printed by fabric development while at or near peak P-T conditions, thus giving an apparent syntectonic style of metamorphism. ¿ 1999 American Geophysical Union