Thermal modeling of convergent and extensional tectonic settings for the development of low-grade metamorphism in the Welsh Basin

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 syn tectonic metamorphism associated with crustal thickening during basin inver sion and cleavage development. The alternative model links the metamorphism not to traditional ideas of crustal thickening but to thermal anomalies as sociated with the extensional development of a back are basin. These two mo dels are tested by applying quantitative thermal modeling to crustal thicke ning and crustal extension tectonic settings specific to the Welsh Basin. T his basin has a unique and very well constrained record of its stratigraphi c, 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 wit h a pumpellyite-actinolite to greenschist facies transition should be domin ant. 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 compati ble with the lower-pressure metamorphic series that is recorded in metabasi tes as well as pelitic rocks of the region. The metamorphic evolution is re garded as developing initially as a burial style, which is then over printe d by fabric development while at or near peak P-T conditions, thus giving a n apparent syntectonic style of metamorphism.