Fastnet, Celtic Sea and St George's Channel

This offshore area broadly comprises two ENE-trending Mesozoic and Tertiary grabens. The northern graben comprises the Fastnet, North Celtic Sea and St. George’s Channel basins, separated from the South Celtic Sea and Bristol Channel basins by the Pembrokeshire Ridge–Labadie Bank (Fig. 17.1). The graben appear to represent the reactivation of Caledonian structures, e.g. the southern margin of the St. George’s Channel Basin was controlled by reactivation of the onshore Bala Fault System (Naylor 2001). The North Celtic Sea Basin locally developed in the hanging wall of a low-angle southerly dipping fault, which may represent the reactivation of the Variscan Front during Mesozoic extension (Gibbs 1987). However, traditionally, the Variscan Front has been taken as a poorly lineated feature extending across southern Ireland, north of the Munster Basin.