The Irish Variscides: a fold belt developed within a major surge zone

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Abstract
An arcuate fold belt characterizes the Irish Variscides south of the Dingle-Dungarvan line. The folds are cross-cut by numerous faults which tend to be sinistral in the west and dextral in the east. Clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east, while anticlockwise transections are common in the west. These and other features suggest that north-south dextral and sinistral shearing occurred in the east and west respectively. This is considered to be a consequence of the morphology of the Munster Basin which allowed the development of a surge zone. In the centre of the fold belt the Variscan sole thrust is believed to lie at the base of, or within, the Old Red Sandstone. However to the east, thinning of the Upper Palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into Lower Palaeozoic basement. In the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a Devonian-Carboniferous extensional detachment beneath the basin, while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement. Consequently the tip-line became strongly arcuate and the thrust developed into a surge zone. The sole thrust propagated northwards by stick-slip processes. Much of the deformation within the cover sequence occurred during sticking events, particularly fold tightening and accommodation faulting.