SYNCHRONOUS VERY LOW-GRADE METAMORPHISM, CONTRACTION AND INVERSION INTHE CENTRAL PART OF THE WELSH LOWER PALEOZOIC BASIN

A metamorphic map (white mica crystallinity) based on 663 data points has been generated for the recently published British Geological Surve y 1:50 000 geological sheets 178 (Llanilar) and 179 (Rhayader). Epizon e, upper anchizone, lower anchizone and sub-anchizone are delineated. Metamorphic cross-sections are drawn and superimposed on structural cr oss-sections. Assumptions made in order to draw metamorphic cross-sect ions are: (1) the anchizone spans 100 degrees C; (2) the metamorphic f ield gradient was 36 degrees C km(-1); (3) metamorphic surfaces were i nitially generated parallel to the contemporary ground surface. Restor ation of the sections indicates that at the time of metamorphism a con tractional phase had begun because reverse movement on earlier extensi onal faults, development of back-thrusts, footwall short-cuts and limi ted growth of precursor folds had taken place. Metamorphism continued locally during later stages of the contractional phase as a strain-rel ated phenomenon. Finally, tensional faulting followed the cessation of folding and metamorphism.