The structure of the Swansea Valley Disturbance between Clydach and Hay‐on‐Wye, South Wales

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Abstract
The Swansea Valley Disturbance is one of four NE‐SW belts of faulting and folding which cross the northern limb of the South Wales Coalfield syncline at variance with the normal E‐W Variscan structures. The Disturbance extends from Hay‐on‐Wye (Herefordshire) southwestwards to Clydach (near Swansea) and may extend northeastwards to Titterstone Clee Hill (near Ludlow) and southwestwards along the Tircanol Fault to Swansea Bay.The main structural elements of the Disturbance are: impersistent NE‐SW folds; NE‐SW normal faults; and NE‐SW and NNW‐SSE wrench faults. The NE‐SW structures are confined to a narrow zone which seldom exceeds two kilometres in width.It is suggested that this narrow belt of faulting and folding has been controlled mainly by sub‐Devonian basement structures, which involve faulting and/or folding. The effect of the Variscan compression was to reactivate the basement structure, which had the effect of resolving this compression along the disturbed zone to produce sinistral wrench movements. The structure of the Disturbance has been complicated by folding, produced by the Variscan force driving the Upper Palaeozoic rocks against the Lower Palaeozoic block.It is concluded that the main movements are of Variscan age and that vertical movements may have taken place in post‐Carboniferous and post‐Neogene times.

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