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Journal of the Geological Society; 1996; v. 153; issue.2; p. 277-285;
DOI: 10.1144/gsjgs.153.2.0277
© 1996 Geological Society of London

Article

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

B. ROBERTS1, R.J. MERRIMAN2, S.R. HIRONS1, C.J.N. FLETCHER2 & D. WILSON2

1 Geology Department, Birkbeck College, Malet Street, London WC1E 7HX, UK
2 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

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.

Key Words: Wales • metamorphism • low temperature • white mica • tectonics




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