Abstract - The Carboniferous Limestone of The Burren, Co. Clare provides excellent bedding plane exposure of joints and veins in a stable tectonic setting. Use of low elevation aerial photographs combined with examination of cliffs in outcrop allows the three dimensional architecture of the fractures to be described.
In map view the veins form a regional NNE set which is clustered, linear and unconnected. Field evidence suggests that they formed in the Variscan. The later formed joints form variable, well connected patterns. Analysis of digitized maps shows that the joints and veins have different scaling properties. The veins occur in fractal clusters and have power-law length distributions and therefore have no characteristic scale. The joints occur with log-normal length populations and a regular spacing, giving them a characteristic scale.
In cross-section, a clear distinction can be made between veins and joints. The joints are typically contained within mechanical units separated by thin shales and it is the unit thickness which controls the joint spacing. The veins, however, are vertically persistent and so there is no characteristic scale controlling their development.
Given that there are no indications that host rock material properties were significantly different during veining and jointing, the quite distinct systematics of the two kinds of fractures are attributed to the different stress conditions under which they formed. Observation of fracture overlaps allows estimation of the differential stresses (Ds) which occurred when the fractures were growing (Olson and Pollard 1989). The joints curve at overlaps, indicating low Ds, while the veins pass each other without deflection indicating high Ds. The joints, forming at lower Ds, were largely unable to propagate through the shales and the thickness of the mechanical units determined their characteristic scales. The high Ds associated with the veins permitted propagation through shale layers so that vein scaling was unaffected by the scale of the mechanical units.
Reference:
Olson, J. & Pollard, D.D., 1989. Inferring palaeostress from natural fracture patterns: A new method. Geology, 17, 345-348.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting Cardiff, December 1995