A DETERMINATION OF THE BULK ROTATIONAL DEFORMATION RESULTING FROM DISPLACEMENTS IN DISCRETE SHEAR ZONES IN THE HERCYNIAN FOLD BELT OF SOUTH IRELAND

In the Hercynian Fold Belt of South Ireland, near the town of Ardmore, County Waterford, units of openly folded Upper Palaeozoic sandstone and siltstone are traversed by closely spaced arrays of en echelon quartz veins. These arrays mark the sites of shear zones which are bounded by weakly deformed wall rocks. We used the attitude of cleavage and the sigmoidal shapes of veins to calculate the cumulative shear strain across each zone, and then determined the bulk deformation suffered by the outcrop by tensor analysis, with the aid of an off-axis Mohr circle construction. We conclude that, as a consequence of vein formation, the rocks at this outcrop suffered significant bulk deformation with an 8-14 degree clockwise rotation.