Scaling relationships of joint and vein arrays from The Burren, Co. Clare, Ireland
Introduction
Subvertical fractures such as joints or veins are a common feature of rocks in both highly deformed areas and in regions that are otherwise undeformed. The spatial distribution and scaling of such fractures are highly variable and range from fractures with a small range of sizes and a very regular spacing (e.g. Mastella, 1972; Ladeira and Price, 1981; Huang and Angelier, 1989; Narr and Suppe, 1991) to fractures with very broad size distributions showing a high degree of clustering (Segall and Pollard, 1983; Odling, 1997; Gillespie et al., 1999). The underlying controls on the fracture scaling are likely to be related to both the nature of the host rocks and the conditions of deformation. This article describes the fractures in The Burren, County Clare, Ireland and attempts to provide an explanation for the scaling systematics of the different fracture types. Although the fracture outcrop is of exceptional quality, the area has been left undescribed by structural geologists since the work of King (1875) and Kinahan (1875).
The Burren is a barren area of Carboniferous limestone in the west of County Clare, Ireland (Fig. 1, Fig. 2, Fig. 3). Limestone pavements on the upper parts of hills and along the coast provide superb exposures (Fig. 1, Fig. 3) where two prominent types of fracture occur: barren joints and mineralised veins. This article aims to show that these two fracture sets have contrasting size and spacing characteristics. The veins are clustered and have power-law size distributions, whereas the joints are non-clustered and do not exhibit power-law size distributions. In this respect, The Burren provides a rare opportunity to characterise the systematics of two contrasting mode 1 fracture sets. Furthermore, in The Burren investigation of the causes of their differences can exclude lithological variation, often considered to be a crucial variable. Primarily, the fracture, system mapping was carried out from low level aerial photographs. This facilitated acquisition of high-quality fracture maps and allowed relatively easy recognition and characterisation of the fracture patterns.
Section snippets
Background geology
A sequence of relatively undeformed Asbian to Brigantian age limestones, comprising mostly platform carbonates (Gallagher, 1996), crop out in The Burren (Fig. 1, Fig. 4a). Below we present brief descriptions of the stratigraphic intervals where veins and joints have been studied. The Late Asbian Terraced Member of the Burren Formation forms the well-developed terraces on the upper parts of several of The Burren hills (Fig. 1, Fig. 3, Fig. 4a), such as Cappanawalla (described below). The
Aerial photography
Aerial photographs were commissioned over the areas where fracture-related grykes are best developed. These areas are Sheshymore (Fig. 1), which exposes the Middle Cherty Limestone of the Slievenaglasha Formation (Fig. 4b and Fig. 5), and Cappanawalla, where the Terraced Member of The Burren Formation is exposed (Fig. 4c, Fig. 6 and Fig. 7). Monochrome photographs were taken from a height of 760 m above ground surface, providing a contact scale of 1:3000 and a ground resolution of about 5 cm.
General features
The sub-vertical veins are characteristically planar and parallel-sided. Vein fill is preserved only where grykes are not well-developed, under glacial erratics and on the coast at Gleninagh (Fig. 1). The veins usually form simple parallel arrays but left-stepping en-echelon arrays occur locally, suggesting an element of dextral shear (Fig. 6, Fig. 7). Sigmoidal veins are rare exceptions. Usually, individual veins are relatively regular, planar fractures, which show no signs of forking. In
General features
The joints are typically subvertical and form organised connected networks in plan view. In most areas of The Burren a dominant systematic set of subparallel joints can be identified with individual joint traces up to 100 m long (Fig. 5, Fig. 6). The systematic joints show no evidence for shear displacements and have no tendency to form en échelon alignments. The joints have smooth trajectories and do not bifurcate.
Other joint sets abut against the systematic joints and, therefore, the
Comparison between vein and joint characteristics
A summary of the principal geometric characteristics of the veins and the joints in The Burren is given in Table 4. The crucial difference between the veins and joint arrays is that the vertical propagation of veins was more or less unrestricted whereas most joints are restricted to single limestone mechanical units. The vein characteristics are consistent with a fracture array in a mechanically uniform host. The joints, on the other hand, are characteristic of fracturing in a multilayer, where
General
The spatial distribution of the joints and veins in The Burren can be understood in terms of the stresses around the fractures predicted by linear elastic fracture mechanics. Opening-mode fractures have complex stress distributions in their surrounding volumes. These comprise: (i) a zone of reduced tensile stress (i.e. stress shadow) in the volumes on either side of the fracture, in which fracture growth is inhibited; and (ii) a tip region of increased stress, in which the growth of new
Conclusions
- 1.
The Late Asbian to Brigantian age limestones of The Burren host two types of mode I fractures, veins and joints, with fundamentally different scaling properties.
- 2.
The Burren veins are highly clustered, non-stratabound fractures formed in response to N-S directed compression during the Variscan Orogeny. The veins show probable power-law size and spatial distributions. They are poorly connected in plan view but may be linked by subhorizontal layer-parallel veins.
- 3.
The Burren joints occur in
Acknowledgements
We acknowledge Conor MacDermot for much helpful information about the geology of The Burren. Thanks are also due to Paul Jackson and the late Dave Johnston for help in the field. Additionally, Marie Eales and Dan Ellis of the Fault Analysis Group are thanked for the excruciating task of digitising the fracture maps. BKS Ground Surveys carried out the aerial survey of The Burren. Discussions with George Tuckwell and thorough reviews by Tim Davis, Mark Fischer and Bill Dunne helped to
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- 1
Present address: Norsk Hydro ASA, Sandsliveien 90, N-5020 Bergen, Norway.
- 2
Present address: Fault Analysis Group, Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland.
- 3
Present address: Liverpool University Marine Laboratory, Port Erin, Isle of Man IM9 6AJ, British Isles.