Pressure solution seams in clastic rocks from southwest Ireland [electronic resource] : their formation, growth, statistical properties and their role in the initiation and development of large-scale faults
Keywords: Geological and Environmental Sciences.

Pressure solution seams (PSSs) are closing mode structures of localised dissolution resulting in a volume reduction. We present new observations constraining the processes by which PSSs initiate and grow in low porosity clastic rocks from southern Ireland. PSSs initiate as inter-granular PSSs at quartz grain to grain contacts. As quartz dissolves, clay remains as a residue along the contacts as well as filling the adjacent pores to form incipient PSSs involving multiple grain boundaries. Further lengthening and thickening occurs by lateral linkage and transverse coalescence of neighbouring segments of incipient PSSs. Multiple PSS segments are observed to concentrate in thin tabular zones that appear as single macroscopic PSSs visible in hand samples. Published numerical modelling results show the ability of both lateral and transverse growth of PSSs due to the presence of asperities on their flanks, which confirm our interpretations based on our laboratory and field observations. In order to gain insight into the statistical properties of PSSs is clastic rocks, we measured the length, height, thickness and spacing of pressure solution seams (PSSs) at outcrop, hand sample and thin section scale from rocks located in the southwest of Ireland. The length and spacing distributions of pressure solution seams have similarly shaped approximately log hyperbolic distributions at outcrop, hand sample and thin section scale suggesting that both length and spacing distributions are scale independent over the range of scales studied with a fractal dimension in the range of 1.4 to 1.6. The mechanism of pressure solution differs substantially from that of jointing, as pressure solution seams, in contrast to joints, are associated with permanent strain and can link through transverse coalescence so that the spacing between two neighbouring pressure solution seams can reduce to zero. The stresses around the tips of pressure solution seams are much lower than that for joints under equal but opposite loading conditions, and do not increase with their length as is the case for joints. The PSS spacing distributions are also expected to be different from that of the joints in that they eventually approach zero. Overall, the statistical trends of PSS arrays are different from those of joints and direct comparisons of these trends should be treated with caution as these structures have some fundamental mechanical differences in terms of their geometrical properties and growth mechanisms. Lastly, we investigated how a PSS array can interact with an orthogonal array of joints/veins to produce complex structural assemblages. The Ross Sandstone in County Clare, Ireland, was deformed by an approximately north-south compression during the end-Carboniferous Variscan orogeny. Orthogonal sets of fundamental structures form the initial assemblage; mutually abutting arrays of 170° oriented set 1 joints/veins (JVs) and approximately 75° pressure solution seams (PSSs) that formed under the same stress conditions. Orientations of set 2 (splay) JVs and PSSs suggest a clockwise remote stress rotation of about 35° responsible for the contemporaneous shearing of the set 1 arrays. Prominent strike-slip faults are sub-parallel to set 1 JVs and form by the linkage of en-echelon segments with broad damage zones responsible for strike-slip offsets of hundreds of meters. Thrust faults with up to 30 metres of offset initiate within shale horizons and follow either the PSSs in the sandstones or high-angle shales within tilted sequences. Within the large thrust fault zones, compartmentalized blocks of rocks are bounded by segments of thrust faults with various dip angles. Strike-slip and thrust faults are contemporaneous and owe their existence to initial weaknesses in the form of JVs and PSSs, as opposed to switching relative stress magnitudes and orientations in conjunction with the Andersonian models of faults and related stress orientations.