Strain study of the Caledonides in the Islay region, SW Scotland: implications for strain histories and deformation mechanisms in greenschists

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Abstract
Metasediments of the NW foreland have a deformation phase older than the first phase in metasediments of the internal part of the Caledonides to the SE. They also suffered constrictive deformation with NE-SW extension of more than 100%. In contrast, rocks in the adjacent internal zone were extended 70 ± 10% upwards in a NW direction and also thrust in the same direction; they were extended less than 10% along strike. An important concealed boundary separates the foreland from the internal zone. Total strains determined for foreland metasediments (Colonsay Group) and for metasediments of the internal zone (Dalradian Supergroup) allow a partial pretectonic reconstruction. Total cross-strike shortening for both zones averages 55 ± 5%. It is not possible completely to unfold the profile through Islay by removing the measured strains. This is partly because those strain analyses based on grain shapes fail to detect the important component of intergranular slip and partly because mechanisms not involving penetrative rock deformation helped produce the folds. Strain histories for the Dalradian rocks are generally coaxial within the limits of a test applied so that the primary cleavage of the Dalradian rocks and its extension lineation may be mapped as total strain trajectories. The fan of primary cleavage and examples of extreme non-coaxial strain near the hinge of the Islay Anticline may be associated with hinge migration during its growth. Several lines of evidence favour grain boundary sliding as an important deformation mechanism in these low grade metasediments. It may have been enhanced by elevated pore fluid pressures and three classes of grain boundary sliding are proposed, dependent on the prevailing fluid pressure.