Abstract

Achill Island is underlain by a mixed suite of low-grade metamorphic rocks. The lithological units of the northern part, the subject of this paper, are systematically described. The deformation history can be divided into five separate phases. The first phase, D1, produced isoclinal F1 folds and S1 flattening schistosity in mica schists; and an L-tectonite fabric associated with some tight F1 folds in psammitic rocks. The second phase, D2, produced tight to isoclinal reclined F2 folds and S2 strain-slip schistosity in mica schists; and S2 flattening schistosity or an L–S fabric in psammitic rocks. This was followed by the third phase, D3, which produced tight to close upright folds and S3 strain-slip schistosity. The folds formed during these phases plunge gently between north-east and south-east. This variation in the trend of the plunge is the result of open F4 refolding. Finally, the latest deformation, D5, resulted in the development of kink bands. Three tectonic slides, of D1 age, are recognized in the area, one of which separates rocks of two separate stratigraphical successions. This slide can be mapped around the major southward-facing F2, folds of the area. One large dextral strike-slip fault is present. The two stratigraphical successions are compared with the Scottish Dalradian; one with the Islay Succession, and the other with the Moine Series. In North Achill large-scale F2 folding places the equivalents of the Moine Series structurally above the Islay succession in the core of an F2. anticline.