The structure and stratigraphy of the Dalradian rocks of north Achill Island, County Mayo, Ireland

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, D 1, produced isoclinal F 1 folds and S 1 flattening schistosity in mica schists; and an L -tectonite fabric associated with some tight F 1 folds in psammitic rocks. The second phase, D 2, produced tight to isoclinal reclined F 2 folds and S 2 strain-slip schistosity in mica schists; and S 2 flattening schistosity or an L–S fabric in psammitic rocks. This was followed by the third phase, D 3, which produced tight to close upright folds and S 3 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 F 4 refolding. Finally, the latest deformation, D 5, resulted in the development of kink bands. Three tectonic slides, of D 1 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 F 2, 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 F 2 folding places the equivalents of the Moine Series structurally above the Islay succession in the core of an F 2. anticline.