Abstract - The 3-D geometry, connectivity and architecture of well exposed fracture systems in the Carboniferous limestones of the Burren, Co. Clare are described. The limestone is cut by subvertical veins and joints, both of which are weathered out to form prominent grykes that are mappable from low level aerial photographs. Kilometre-scale exposures of limestone pavements at stratigraphic intervals of metre-decametre scale provide a basis for 3-D definition of fracture array geometries.
The veins are planar and vertical, and together form a regional N-S striking vein array. Their relationship to folding and bed-parallel slip suggests a probable Variscan age and published fluid inclusion data indicate fluid temperatures of 100º-200ºC. The veins are vertically persistent, showing little evidence of termination at limestone-limestone bed contacts (see below), and typically occur as strongly clustered, sometimes dextral en echelon, arrays characterised in plan view by near zero connectivity. Shale interbeds, though showing evidence of bed-parallel slip contemporaneous with veining, can contain discontinuous bed-parallel veins which may provide linkages between vertical veins.
Later-formed joints are regularly spaced and form complex, but very well connected, networks. Joints are largely confined to individual limestone units, with joint patterns which are similar on a 100m scale, but differ on a kilometre scale. Joint patterns can, however, be remarkably different from one horizon to another; the differences could be due to a number of factors including lithology. The connectivity of systematic joints in map view arises from the gradual rotation of the principal horizontal stress, and hence joint strike directions, and from curving either perpendicular or parallel abutments, as determined by the local stress conditions. The sense of strike rotation of systematic joints is not consistent across the Burren.
The low connectivity and the non-stratabound nature of the veins are attributed to formation at depth under the relatively high differential stresses required to provide a high degree of fracture parallelism and vertical persistence. The high connectivity of the joints is attributed to lower differential stresses, providing a wide range of strike directions.
Abstract of talk given to:
The Woodworth Conference, Tectonic Studies Group, University of Ulster, Ireland, April 1998