Variscan structures in Southwest Wales.

Abstract

Palaeozoic rocks in the Variscan domain of SW Dyfed, Wales, were deformed under anchimetamorphic conditions during the late Carboniferous. Within the Pembrokeshire Coalfield a 1 km wide zone of conjugate folds and thrusts, external to the northern limit of slaty cleavage, is used to define the Variscan orogenic front. South of the front there is a progressive increase in the average amplitude of macrofolds across three further structural zones. Folding about a WNW axial trend was single phase except within a few narrow belts of refolded folds. Wrench and normal faults which developed during and after folding resulted in the axial elongation and clockwise rotation of the region. Structures in the paratectonic Caledonides controlled the locations of four WSW trending Disturbance Belts of Variscan age to the north of the front. In the Druidston Haven Horst, the westernmost of these Disturbances, Caledonian folds and cleavage were refolded approximately coaxially during the Variscan. The Variscan fold belt is cut by several, laterally impersistent, major strike faults which cannot be regarded as separating allochthonous blocks. Although displacements were dominantly reverse or normal the evolution of the faults during folding probably involved some scissor motion. On the Johnston Thrust northward overthrusting was followed by dextral strikeslip.