Synchronous very low-grade metamorphism, contraction and inversion in the central part of the Welsh Lower Palaeozoic Basin

A metamorphic map (white mica crystallinity) based on 663 data points has been generated for the recently published British Geological Survey 1:50 000 geological sheets 178 (Llanilar) and 179 (Rhayader). Epizone, upper anchizone, lower anchizone and sub-anchizone are delineated. Metamorphic cross-sections are drawn and superimposed on structural cross-sections. Assumptions made in order to draw metamorphic cross-sections are: (1) the anchizone spans 100°C; (2) the metamorphic field gradient was 36°Ckm −1 ; (3) metamorphic surfaces were initially generated parallel to the contemporary ground surface. Restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults, development of back-thrusts, footwall short-cuts and limited growth of precursor folds had taken place. Metamorphism continued locally during later stages of the contractional phase as a strain-related phenomenon. Finally, tensional faulting followed the cessation of folding and metamorphism.