Metamorphism of the Lower Palaeozoic rocks of the Builth Wells district, Wales, 1:50k sheet 196

The pattern of regional metamorphism indicated by illite crystallinity has been used to interpret the tectonic and thermal history of the Builth Wells district. Grade generally increases from the southeast of the district towards the northwest. The Abergwesyn Fault separates an area of mainly late diagenetic rocks in the southeast of the district from high-anchizone and epizonal rocks in the northwest. The lowest grade rocks southeast of the Abergwesyn Fault are largely the result of burial metamorphism under a pre-tectonic overburden 4-5.5 km thick. A slate belt in the northwest of the district was generated by deformation associated with the Acadian Orogeny. Peak metamorphic temperatures of 300°C developed under a tectonic overburden 6-8 km thick.